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Trace metal elements vaporization and phosphorus recovery during sewage sludge thermochemical treatment - A review
Abstract
Phosphorus (P) plays essential roles in crops growth. Natural mineral sources of phosphate are non-renewable, overexploited and unevenly distributed worldwide, making P a strategic resource for agricultural systems. The search for sustainable ways to secure P supply for fertilizer production has therefore become a critical issue worldwide. Sewage sludge (SS) is an organic waste material considered as a key alternative source of P. Switzerland and the European Union are is about to make it mandatory to recover P from SS or its treatment residues. Among the many technical options to achieve this objective, SS thermochemical treatments spiked with Cl-donors appear as a promising approach to recover P from SS and separate it from mineral pollutants such as trace metal elements (TME). The purpose of Cl-donor additives is to fix P within the mineral residues, possibly in bioavailable P species forms, while promoting TME vaporization by chlorination mechanisms. This review paper compares the various thermochemical treatments investigated worldwide over the past two decades. The influence of process conditions and Cl-donor nature is discussed. The presented results show that, except for nickel and chromium, most TME can be significantly vaporized during a high temperature treatment (over 900 °C) with Cl addition. In addition, the fixation rate and solubility of P is increased when a Cl-donor such as MgCl2 is added.
... Wet-chemical methods have gained much attention due to their high efficiency of P extraction, simplicity of operation and low expense (Baldi et al., 2021b;Boniardi et al., 2021). But in most cases, this method results in co-leach of HMs from SSA, which will limit further use of recovered P as crop fertilizer (Chilian et al., 2022;Galey et al., 2022;Luyckx and Caneghem, 2022). In order to solve the puzzle of HMs, P purification of the above-mentioned leachate is usually required by chemical precipitation, ion exchange or adsorption. ...
... Due to the different characteristics of SSA, the corresponding optimal P extraction conditions are also different. Optimizing the conditions of P extraction is extremely important to obtain a P-rich solution with less HMs Galey et al., 2022;Havukainen et al., 2016). ...
... The core of thermochemical process is to utilize high temperature (900-2000 • C) to gasify or liquefy HMs and their compounds in order to realize the separation of HMs and P by gas phase separation (or density separation) devices (Galey et al., 2022;Husek et al., 2022;Krüger et al., 2014;Luyckx and Caneghem, 2022;Zhu et al., 2022). Therefore, thermochemical methods can simultaneously extract and purify P in hot environment. ...
Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5~6 €/kg P, while the cost of recovering P by thermochemical methods is about 2~3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions.
... Mono-incineration of sludge is a well-established thermochemical treatment technology for sludge disposal and has great potential to recovery P. Incineration can achieve a 90% of sludge volume reduction to enrich P content in the range of 9-13.1%, and simultaneously decontaminate pathogens and organic pollutants (Galey et al., 2022). Sewage sludge ash (SSA) is in the form of dry powder and relatively easy for P recovery. ...
... Similar as SS, wet-chemical washing is the most popular method for P extraction from SSA . However, SSA has high levels of heavy metals than SS due to the volume and mass reduction (Galey et al., 2022). Series of acid extraction and alkali precipitation cycles followed the treatment by cation exchange resins have been applied to recover high grade struvite from SSA . ...
... Summary graph comparing the impact on trace metal vaporization and P bioavailability of a classic sludge incineration and a thermochemical process with MgCl 2 and/or PVC (for As, Cr, and Ni removal) as Cl-donors. The circle size is related to the content in the different phases (adapted fromGaley et al. (2022)). ...
Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.
... In single step SS thermochemical processes, the sludge is directly treated with additives either by incineration, pyrolysis or gasification, with the dual purpose of recovering energy and P. Despite advantages of single industrial installation, only few studies focus on the single step SS thermochemical treatments with Cl-based compounds (Liu et al., 2015;Saleh Bairq et al., 2018;Xia et al., 2020;Yang et al., 2019). Galey et al. recently wrote a review article on the topic (Galey et al., 2022). ...
... It appears that the contents of Zn and Pb in the ashes are greatly decreased thanks to MgCl 2 addition (from 3011 to less than 350 mg/kg for Zn and from 59 to less than 1 mg/kg for Pb, Fig. 5). Cl addition therefore promotes Zn and Pb removal thanks to ZnCl 2 (boiling point of 732 • C) and PbCl 2 (boiling point of 950 • C) volatile compounds formation, as reported elsewhere (Fraissler et al., 2009;Galey et al., 2022;Liu et al., 2015;Xia et al., 2020). According to (Galey et al., 2022), the chosen calcination conditions (combustion under air, at 1000 • C, 4 h residence time and with MgCl 2 as chlorine donor) are the best to promote both Zn and Pb removal through the gas phase. ...
... Cl addition therefore promotes Zn and Pb removal thanks to ZnCl 2 (boiling point of 732 • C) and PbCl 2 (boiling point of 950 • C) volatile compounds formation, as reported elsewhere (Fraissler et al., 2009;Galey et al., 2022;Liu et al., 2015;Xia et al., 2020). According to (Galey et al., 2022), the chosen calcination conditions (combustion under air, at 1000 • C, 4 h residence time and with MgCl 2 as chlorine donor) are the best to promote both Zn and Pb removal through the gas phase. It probably explains the low contents observed in the ashes after calcination. ...
Phosphorus (P) plays an essential role in crops growth. Due to the overexploitation of non-renewable phosphate rock deposits and their uneven distribution worldwide, P has turned into a strategic resource for the modern agricultural system. Finding sustainable ways to secure P for fertilizer applications has become critical. Sewage sludge (SS) is a waste material containing P in high amounts. Among several options, SS thermochemical treatment with Cl-donors addition such as MgCl2, CaCl2 or NaCl allows to separate P from inorganics pollutants such as trace metal elements (TME). The additives goal is to help fix and form bioavailable P species in solid residues while promoting TME vaporization through the gas phase via chlorination mechanisms. This paper studies the impact of MgCl2 addition on TME vaporization and P speciation during SS direct calcination treatment at 1000 °C for 4 h. Four different preparation methods for the “SS + MgCl2” mixtures were compared, with a variation of either the SS (raw or crushed) or the MgCl2 form (powder or solution). The samples were characterized by ED-XRF, FTIR, XRD, TGA/DSC and the P speciation was obtained thanks to Williams-Saunders method. The use of 0.06 g MgCl2/g SS as a Cl-donor divides the TME contents in the solid residues by a factor of more than 10 for Zn and Pb, about 2 for As and Ni but less than 1.25 for Cu. Concerning P, Ca associated P species (mostly Mg3Ca3(PO4)4) are formed with MgCl2 addition. The results showed that the impact of different preparation methods on the obtained solid residues characteristics is limited.
... In another approach to P-recovery, WAS-incinerated ash (5-10 P% or 11-23 P 2 O 5 %) have a high potential in the P-recovery efficiency, up to 90% (Fang et al., 2020;Jupp et al., 2021;Liu et al., 2021). Furthermore, incineration has been identified as an ultimate approach to disposing WAS (Galey et al., 2022;Hao et al., 2020a;Ma and Rosen, 2021). As a result, P-recovery from WAS-incinerated ash would become a mainstream approach to handling WAS, especially under the current condition that European counties generally emphasize to cover at 80% influent P-load from wastewater treatment (Jupp et al., 2021;Liu and Qu, 2016). ...
... In practice, P-recovery techniques from WAS-incinerated ash have become relatively mature. Among others, wet-chemical approaches seem the most sustainable methods (Fahimi et al., 2021), followed by thermochemical methods (Fang et al., 2020;Galey et al., 2022;Liu et al., 2021). However, each of these techniques involves metal-removing processes. ...
Wet-chemical approach is widely applied for phosphate recovery from incinerated ash of waste activated sludge (WAS), along with metals removed/recovered. The high contents of both aluminum (Al) and iron (Fe) in WAS-incinerated ash should be suitable for producing coagulants with some waste anions like Cl- and SO42- With acid (HCl) leaching and metals' removing, approximately 88 wt% of phosphorus (P) in the ash could be recovered as hydroxylapatite (HAP: Ca5(PO4)3OH); Fe3+ in the acidic leachate could be selectively removed/recovered by extraction with an organic solvent of tributyl phosphate (TBP), and thus a FeCl3-based coagulant could be synthesized by stripping the raffinate with the original brine (containing abundant Cl- and SO42-). Furthermore, a liquid poly-aluminum chloride (PAC)-based coagulant could also be synthesized with Al3+ removed from the ash and the brine, which behaved almost the same in the coagulation performance as a commercial coagulant on both phosphate and turbidity removals. Both P-recovery from the ash and coagulant production associated with the brine would enlarge the markets of both 'blue' phosphate and 'green' coagulants.
... In recent years, intensive anthropogenic activities have resulted in rapid industrialization, agricultural modernization and urbanization. While promoting economic growth, these activities have also released multitudes of trace metal(loid)s (TMs) into the environment (Galey et al., 2022;Gobert et al., 2022), exacerbating soil TM pollution. Due to their universality, high toxicity, cumulative nature, and persistent bioavailability (Ali et al., 2019;Jia et al., 2020), soil TMs can not only damage the structure and function of ecosystem , but also threaten the health of animals and humans through bioaccumulation (Yan et al., 2022). ...
... The purpose of this conversion was to produce species with reduced boiling points and increased vapor pressures [60]. During co-pyrolysis, chlorine can react with metals in F1 fraction to form stable chemicals such as F4 fraction [61]. ...
Transforming municipal sludge (MS) into biochar via co-pyrolysis with calcium salts enhances phosphorus (P) recovery and reduces the risk of heavy metal contamination. In this study, the performance and mechanism of bioavailable P conversion and heavy metal stabilization were systematically compared under co-pyrolysis of MS with CaCO3, CaO, and CaCl2 at mass ratios of 2.5–10 % at 700 °C. The results showed that CaCl2 had the best ability to convert P from non-apatite inorganic P to apatite P, followed by CaO and then CaCO3. CaO and CaCO3 addition induced more Ca5(PO4)3OH formation, while CaCl2 induced more Ca2PO4Cl and Ca5(PO4)3Cl formation. The degree of stabilization of heavy metals (e.g., Cd, Cr, Cu, Ni, Pd, and Zn) was significantly improved after co-pyrolysis of MS with these salts, thus reducing the potential environmental risk. In particular, because of the dual action mechanism of chloride ions and calcium ions, co-pyrolysis with CaCl2 exhibited the lowest risk of heavy metal contamination compared with CaCO3 and CaO. These findings suggest that addition of 2.5 % CaCl2 can optimize the production of bioavailable P and the stabilization of heavy metals in biochar. This treatment is recommended for the safe utilization of P resources derived via MS pyrolysis.
... SL is a source of organic matter, nitrogen, phosphorus, calcium, magnesium, and other macro-and microelements necessary for plants growth and development (Tomczyk et al., 2020). However, it is still forbidden to use raw SL directly as soil fertilizer, due to the fact that SL may also contain many hazardous substances (Galey et al., 2022;Gao et al., 2020). Among many hazardous substances that can be found in SL, polycyclic aromatic hydrocarbons (PAHs) are a group that in recent years has been the subject of interest (Premnath et al., 2021;Stańczyk-Mazanek et al., 2019). ...
... In recent years, the supply of high-quality coal such as bituminous coal and anthracite in China had become increasingly Responsible Editor: Guilherme Luiz Dotto tight, while low-rank coal resources such as lignite were still abundant and cheap, which caused the low-rank coal to be of value for utilization. Galey et al. (2022) found that sewage sludge had higher heavy metals and lower chloride compared to coal. SS-coal co-combustion resulted in poor volatility of the target heavy metals. ...
Combustion experiments of sewage sludge (SS) blended with low-rank coal were conducted through a drop tube furnace (DTF) to explore the effects of low-rank coal type, blending ratio, and steam on the transformation of Zn and Cr. The results showed that the retention rates of Zn and Cr in ash increased from 24.35% and 71.49% for sludge combustion alone to 53.77% and 117.49%, respectively, for coal blended to SS with a mass ratio of 7:3. The greater the proportion of low-rank coal in the fuel, the greater the residual rate of heavy metals in the ash. Meanwhile, rapid diffusion of vapor occupied adsorption sites on metal mineral surfaces, reducing the retention of Zn and Cr in the co-combustion ash. The leaching toxicity analysis of ash showed that the co-combustion ash of SS with coal was free from leaching toxicity hazards in simulated scenarios. The extraction rate of Zn in co-combustion ash increased from 90.72% with hydrothermal acid leaching to 95.46% with microwave-assisted in 2 mol/L H2SO4 extract. The extraction rate of Cr in hydrothermal acid leaching was 62.80%, which was much higher than that in microwave-assisted extraction (31.76%).
... Phosphorus could be recovered from sewage sludge or the ash of incinerated sewage sludge (Cornel & Schaum 2009;Ciesĺik & Konieczka 2017). Enhanced phosphorus recovery can be carried out using the released phosphorus from sewage sludge with anaerobic-based treatment (Yu et al. 2021), thermal (Meng et al. 2019), and thermochemical treatment (Galey et al. 2022), or using the extracted phosphorus from sewage sludge ash with the wet chemical extraction (Boniardi et al. 2021) and the electrodialytic process (Guedes et al. 2014). The major challenge for phosphorus recovery from sewage sludge or sewage sludge ash is the higher concentration of metallic ions than that in sewage caused by bio-adsorption of sewage sludge, and the pretreatment with ion-selective nanofiltration could be an effective way to remove multivalent metal cations (Schütte et al. 2015). ...
In this study, behaviors of metals and their effects on phosphorus recovery by calcium phosphate were investigated by the laboratory and pilot experiments as well as by the modified thermodynamic model. Batch experimental results indicated that the efficiency of phosphorus recovery decreased with the increase in metal content and more than 80% phosphorus can be recovered with a Ca/P molar ratio of 3.0 and a pH of 9.0 for the supernatant of an anaerobic tank in the A/O process with the influent containing a high metal level. The mixture of amorphous calcium phosphate (ACP) and dicalcium phosphate dihydrate (DCPD) was assumed to be the precipitated product with an experimental time of 30 min. A modified thermodynamic model was developed using ACP and DCPD as the precipitated products, and the correction equations were incorporated to simulate the short-term precipitation of calcium phosphate based on the experimental results. From the perspective of maximizing both the efficiency of phosphorus recovery and the quality or purity of the recovered product, the simulation results showed that a pH of 9.0 and a Ca/P molar ratio of 3.0 were the optimized operational condition for phosphorus recovery by calcium phosphate when the influent metal content was at the level of actual municipal sewage.
HIGHLIGHTS
With a short reaction time, the main recovery product obtained was not HAP but DCPD and ACP.;
Modified thermodynamic model to simulate the inhibition effect of metals and carbonate.;
Co-precipitation of other metals either pH or molar Ca/P increased.;
Optimized operational conditions: molar Ca/P = 3.0 and pH = 9.0.;
More than 80% of P was recovered for the anaerobic supernatant containing metals under optimized conditions.;
... The composition of sewage sludge plays a crucial role in biomethane production [5]. Sewage sludge comprises organic carbon compounds as well as toxic organic and inorganic compounds such as heavy metals, pesticides, sulfonates, pathogens, silicates, aluminates, calcium, and magnesium [6]. The content of heavy metals is significant, both in terms of the final quality of the produced excess sewage sludge as well as the overall performances of WWTPs [7]. ...
The anaerobic digestion of sewage sludge is highly sensitive to high zinc concentrations. Although sulfate-reducing bacteria (SRB) activity can negatively affect methanogenesis, SRB-mediated metal sulfide precipitation can alleviate zinc toxicity. A series of mesophilic anaerobic batch experiments was performed for the biomethane potential of three different sewage sludge samples for 74 days using the background sludge zinc content, alone or in combination with the external addition of 200, 300 and 400 mg Zn/L. The highest biomethane production was 165 ± 1 mL CH4/g VS using activated sludge (AS) with a background concentration of 93 mg Zn/L. A slight decrease in the biomethane yield (i.e., 157 ± 1, 158 ± 1 and 159 ± 1 mL CH4/g VS) was obtained in the presence of 293, 393 and 493 mg Zn/L, respectively. The potential reason for the high methanogenic activity at high inlet Zn concentrations could be that the AS used in this study was already acclimated to those conditions. Zinc was likely removed from the system by sulfide precipitation, and a removal efficiency above 99% was achieved under all zinc concentrations. A sulfate reduction efficiency of 99% was also obtained. Overall, this study details the potential utilization of biogenic sulfide as a metal detoxifying agent without detrimental effects on methane production from sewage sludge.
... To recover energy and materials from sewage sludge, thermochemical conversion methods like incineration, pyrolysis, gasification, and hydrothermal are typically employed (as shown in Fig. 2). There have been many excellent reviews on the thermochemical conversion processes of sewage sludge towards value-added energy and materials recovery (Galey et al., 2022;Gao et al., 2020;Gopinath et al., 2021). However, thermochemical conversion processes are multiphases and usually require higher temperature, which brings many challenges to the large-scale application (as shown in Table 2). ...
Because of the rapid growth of global population and economic activity, sewage sludge management is a global conundrum. Conventional landfilling and land-farming of sewage sludge receives increasing critiques due to the negative impacts on environmental sustainability and economic viability. Recently, the thermochemical conversion processes have attracted more and more attentions in sewage sludge treatment, which aims to reduce the volume of sewage sludge and recover of value-added energy and materials. However, the thermochemical conversion involves multiple processes and usually requires a high temperature, which brings many challenges to the large-scale application. Therefore, comprehensive reviews of thermochemical conversion of sewage sludge towards enhanced value-added energy and materials recovery is conducted. Firstly, the composition of sewage sludge and its recovery potential are briefly introduced. Then, a unique analysis of thermochemical conversion of sewage sludge towards value-added energy and materials recovery is elucidated with a focus on the challenges. Ideally, approaches for further sewage sludge management should allow achieving both environmental and economic sustainability. In this instance, varied value-added energy (e.g., thermal energy, combustible gases, H2-rich gas, and bio-oils) and materials (e.g., biochar-based functional materials and ash to value-added materials) recovery from sewage sludge are analyzed. Meanwhile, the environmental assessment of thermochemical conversion technologies is evaluated. The further perspectives and recommendations are also outlined. These insights provide further perspectives and recommendations for enhanced sewage sludge management.
Phosphorus (P) recovery from sewage sludge (SS) (or its ash/char) is challenging due to the presence of heavy metals (HMs) in SS. This study proposed the integration of SS, alum sludge (AS), sequential wet extraction, and pyrolysis for P recovery without simultaneous HM extraction. SS + AS mixture (P:Al molar ratio = 3:1) was first acid pretreated to obtain amended SS, then pyrolyzed, and finally, P was extracted from the char using an alkali. Pyrolysis was conducted at low (400 °C), medium (600 °C), and high (900 °C) temperatures. The highest alkaline P recovery efficiency was 88 % from the amended SS char, while 32 % from the unamended SS char (both prepared at 400 °C). Overall, the highest P recovery in the amended process was 48 % higher than the unamended process despite 11 % P loss during acidic pretreatment. Standards measurements and testing (SMT) protocol and solid-state NMR (31P and 27Al) analyses revealed that acidic pretreatment of SS with AS induced apatite phosphate (AP) to non-apatite inorganic phosphate (NAIP) conversion in SS. As NAIP is more soluble in alkali than AP and HMs, higher alkaline P recovery was achieved for amended SS char than unamended SS char, without HM contamination. However, higher pyrolytic temperatures impaired P recovery due to the immobilization of P in the char carbon matrix and the formation of recalcitrant P phases. These findings were corroborated by complementary characterization techniques such as XPS, XRD, and FTIR. Some major HMs in SS and AS were concentrated in char, except Zn, which partially volatilized above 600 °C. This study demonstrated the applicability of AS for extracting P from SS, offering a potential pathway for the synergistic management of sludge waste streams.
Urbanization, Portland cement production, and urban waste have caused concern about increasing environmental pollution. Sewage sludge ash (SSA) comes from the incinerating plant of sewage sludge, which is a by-product of wastewater treatment. SSA can cause metal pollution if improperly handled. Recycling SSA into cement composite such as concrete (i.e., the most widely used material after water) can obviate landfilling and mitigate CO2 and metal pollutants from cement production and SSA. Chemicals called accelerators, such as CaCl2 and Ca(NO3)2 are used to overcome the relatively slow hydration of SSA cement pastes (CPs). In this study, we found the capability of CaCl2 and Ca(NO3)2 to enhance Mo and Cr immobilization by CPs with SSA (i.e., mixed with Portland and blast furnace types of cement). The mechanistic investigation was performed by employing nitrogen gas adsorption, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. The role of the chemical properties rather than physical characteristics of the hydrates was more significant for the immobilization. CaCl2 and Ca(NO3)2 enhanced more formation of aluminate ferrite tri-sulfate (AFt), increasing the host sites for MoO42- and CrO42-. Density functional theory (DFT) simulations showed the likelihood of SO42- in AFt substituted by MoO42- and CrO42-. The unveiled capabilities and mechanism of CaCl2 and Ca(NO3)2 to enhance the MP immobilization of SSA-CPs disclosed new knowledge for the mitigation strategy of pollution from increasingly generated SSA.
Phosphorus (P) is a non-regenerative and finite raw material. Due to its decreasing availability, and to protect the environment, recycling methods are needed. With the focus on closing nutrient cycles, sewage sludge (SS) is a potential source for P recovery. The objective of this study was to produce a mineral P-reach fertilizer. For this purpose, the treatment of SS in a multi-stage process, consisting of a hydrothermal carbonization (HTC) and thermochemical post-treatment was examined and compared with a direct thermochemical treatment. The focus was on the transformation of P and the migration of the heavy metals during the processes. In addition, the role of MgCl2 as an additive was examined. During the HTC, most of the P remained in the HTC-char, so that the P content increased in the HTC-char compared with the SS. The addition of MgCl2 to the process resulted in lower transportation rates of P in the liquid phase and higher P solubilities in water, citric acid, and alkalic ammonium citrate out of the solid phase. The thermochemical treatment of SS and the HTC-chars further concentrated P in the ash. Retention rates of >97% were achieved, and P2O5 contents in the ash were as high as ∼16 wt-%. The presence of the additive resulted in (i) higher retention rates of P in the ashes (ii) higher P-solubility and (iii) higher removal rates of easily volatile heavy metals such as Pb and Zn, and the treatment of HTC-char favored these effects compared with the direct treatment of SS.
Olive mill solid waste (OMSW), also called olive pomace, is a biomass waste available in many Mediterranean countries, including developing countries. In this study, OMSW has been tested as a fuel in an original pilot-scale downdraft gasifier with three-stage air supply, and extensive metrology was implemented. After pelletizing and characterization of the biomass waste, gasification was investigated by operating a 2-day campaign using air as a gasifying agent. The evolution over time of operating parameters such as flowrates, temperatures and pressure drop were recorded. The syngas (including tars) and solid residues were analyzed. The fate of inorganic elements was investigated. Mass and energy balances were performed and the performance of the gasifier was evaluated.
The syngas contained 14 % v/v H2 and 17 % v/v CO (dry and tar-free basis) and its lower heating value was 4.7 MJ/Nm3 (wet basis). The solid residues were rich in potassium and phosphorus and could be considered as an organic amendment. Cold gas efficiency was 68 % and carbon conversion efficiency was 89 %. The points of caution concerned the low melting point of the ash, the relatively high tar content in the syngas and the high NOx content of the flue gases after syngas combustion in a boiler. The energy potential of OMSW is substantial and could help reduce the pressure on fossil fuels.
Sewage sludge represents a renewable source of organic carbon and nutrients such as nitrogen (N), potassium (K), and phosphorus (P) that can be valorised through the recovery of energy carriers (e.g. biofuels) and fertilizers (N, K, and P precipitates). This review analyses>60 recent studies that have investigated P recovery potential from sewage sludge by hydrothermal processes. The effect of process parameters such as temperature, residence time, pressure, solid-to-liquid ratio, and addition of additives on P conversion pathways has been investigated by a critical discussion of the results published in the literature. Results show that temperature is the most influential parameter for P speciation and repartition: the increase in temperature appears to promote the increase in solid P recovery yield, the mineralization of organic P, and the conversion of non-apatitic P into apatitic P. The increase in reaction time has similar effects as temperature, but to a lesser extent. Solid P recovery yield and apatitic P fraction can be enhanced by increasing the medium alkalinity and by adding Ca-containing reactants. Non-apatitic P fraction can be increased by lower medium alkalinity, and by the addition of Fe- and Al-containing reactants. The results of this review provide to researchers and practitioners in the field of sewage sludge management key elements for the best operation of hydrothermal reactors to improve the recovery of P and biofuels. Finally, some new research perspectives and technical challenges are proposed to improve the knowledge and the scaling up of the technology.
Two interrelated problems exist: the non-renewability of phosphate rock as a resource and the excess phosphate in the water system lead to eutrophication. Removal and recovery of phosphorus (P) from waste streams at wastewater treatment plants (WWTPs) is one of the promising solutions. This paper reviews strategies for P recovery from waste streams in WWTPs are reviewed, and the main P recovery processes were broken down into three parts: enrichment, extraction, and crystallization. On this basis, the present P recovery technology was summarized and compared. The choice of P recovery technology depends on the process of sewage treatment and sludge treatment. Most P recovery processes can meet the financial requirements since the recent surge in phosphate rock prices. The safety requirements of P recovery products add a high cost to toxic substance removal, so it is necessary to control the discharge of toxic substances such as heavy metals and persistent organic pollutants from the source.
Due to the lack of performance- and cost-effective processing techniques, a large proportion of sewage sludge is not effectively treated or utilized, resulting in environmental pollution and a waste of resources. In this study, plasma-electrolytic technology was successfully employed for the electrified refinery of sewage sludge into bio-crude and fly ash-resembling residue. Reactive plasma species with strong oxidation capacity (·O and ·OH), were beneficial to the precipitation of metal ions in the raw sludge. With the addition of NaOH, the thus-obtained crude oil contained only 0.11% N and 0.02% S under the optimized conditions and featured a high heating value (HHV) of 28.04 MJ/kg. Analysis of the residue confirmed that the metal oxides and their contents were similar to fly ash, which is widely used as a performance-enhancing admixture for cement and concrete. Using the residue as the admixture for paving block preparation, with the residue addition of 15%, the compressive strength of the blocks exceeded 35 MPa (C35), and the water absorption was <6.5%, thus meeting the China's national standard (GB 28635–2012). This plasma-enabled power-to-X approach may provide a viable option for biomass or waste valorization and comprehensive utilization leading to substantial socioeconomic and environmental benefits.
A variety of sludge originating from wastewater treatment is increasingly accumulating worldwide, especially in the developed regions. Fortunately, phosphorus (P) in sludge can potentially be recovered via thermal treatment, and P forms are crucial to the quality of the recovered products. This review provides an overview related to the transformation of P and its regulation strategies in the different thermal treatment processes, highlighting the current knowledge regarding the influence factors and mechanisms of transformation. First, the current researches with P recovery by thermal treatments (i.e., incineration, pyrolysis, and hydrothermal carbonization) were illustrated and critically discussed, which indicates that the selection of appropriate method significantly affects the recovery effects, such as the proportion of Apatite Phosphorus (AP) or Ca-P (P combing with Ca). Furthermore, the effects of various factors (e.g., treatment techniques, temperature, additives, the composition of sludge, pH and metals ions) on the transformation or regulation of P forms were critically discussed, suggesting that temperature and Ca/Cl-based additives are the two crucial factors, which have the considerable effects on the emergence of Ca-P. Especially the Ca-based additives, which have the highest combining ability with P and the uppermost impact on hydroxyapatite formation. In addition, an overall comparison of incineration, pyrolysis, and hydrothermal carbonization technologies in practical applications is provided, focusing on the recovery efficiency, advantages, and disadvantages, demonstrating that HTC will be more predominant for P recovery from sludge in the future. Finally, challenges and potential development directions regarding the transformation mechanisms of P forms and regulate strategies are identified. This review will become the foundation for future research devoted to improving the quality of the recovered products.
As an attractive method to recover energy and reduce heavy metals in the resultant biochar, co-pyrolysis of sewage sludge with polyvinyl chloride (PVC) plastics may increase the ecological risk of heavy metals. Considering the sound performance of CaO in dechlorination and immobilization of heavy metals, pyrolysis experiments were performed on four feedstocks and combined with the analysis of heavy metals to investigate the effects of temperature and added PVC/CaO on the behavior of As, Cr, Cu, Ni, Pb, and Zn. The results indicated that, increased temperature and added PVC could reduce the content of As, Pb, and Zn in biochar while CaO had the opposite effect, and the temperature set here had limited effect on the volatilization of Cr, Cu, and Ni with more than 90% of them remaining in biochar. Besides, compared with untreated sewage sludge, higher temperature and extra CaO could reduce the ecological risk of heavy metals in biochar by transforming them into more stable forms while the added PVC inhibited the transformation. When both PVC and CaO were present in pyrolysis system, although the ecological risk of heavy metals in biochar caused by PVC could be partially offset by increasing temperature and introducing CaO, it was still higher than that of other biochar. This study provides valuable insights for the treatment of heavy metals during sludge pyrolysis.
The proper recycling of anaerobically digested sludge (ADS) is a great challenge, and the recovering and harvesting of the dissolved organic substances (DOS) in ADS to prepare innovative adsorbents may be an attractive option. This study uses ADS as the raw material to develope the DOS based amyloid-fibrils-like adsorbent (DOS-AFs) for heavy metal removal, and the mechanisms involved in DOS fibrosis and heavy metals adsorption are illustrated. Results indicate that the molecular weight (MW) of the DOS precursors greatly affect the fibrosis extent and the DOS-AFs surface morphology. DOS with low MW tends to form the rigid and smooth rod-like DOS-AFs with long-chain fibrous structure, whereas the macromolecular DOS is more likely to form DOS-AFs with rougher and more complicate branching structure. The formation of β-sheet structured DOS-AFs is largely formed from low-MW DOS rather than high-MW ones. The as-prepared DOS-AFs with different DOS fractions as precursors show different affinities towards heavy metals. The adsorption sites on macromolecular DOS-AFs are more likely to bind silver ion, whereas those small molecule DOS-AFs exhibit stronger affinity towards copper ion and nickel ion. The humic acid fraction in DOS plays two different roles in terms of heavy metals adsorption. The strong ligand of humic acid within DOS-AFs favors heavy metals adsorption, and the fibrosis extent of low-MW fraction is rather weak and the formation of soluble heavy metals complexes adversely inhibits adsorption performance. This study provides an alternative to harvest organic substances in ADS and to prepare valuable adsorbents towards heavy metals, although further studies are necessary to advance it in engineering practice.
Dewatering of sewage sludge (SS) was the prerequisite for saving its drying energy and sustaining its stable combustion. Hydrothermal treatment (HT) has been a promising technology for improving SS dewaterability with high energy efficiency. However, the knowledge of phosphorus (P) transformation and heavy metals (HMs) behaviors in the combined HT and incineration process was still lack. P fractions, P-bioavailability, HMs speciation, and their environmental risk in the ash samples from this combination process were evaluated and compared with those from the co-incineration of SS and CaO. The combination process was superior to the latter one in the light of P and HMs. CaO preferred to enhance the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP) initially with enriched P and increased P-bioavailability in the resultant ash samples. The combination process further reduced the values of risk assessment code and individual contamination factor with the increment of the stable F4 fraction in HMs. Significant reduction of potential ecological risk was observed with the lowest global risk index of 43.76 in the combination process. Optimum CaO addition of 6% was proposed in terms of P and HMs. The work here can provide theoretical references for the potential utilization of P from SS to mitigate the foreseeable shortage of P rocks.
A new approach to analysing resilience reveals changes in China’s phosphorus system and suggests ways to improve it.
Für die zukünftig obligatorische Phosphorrückgewinnung aus Klärschlamm werden besonders nasschemische und thermochemische Verfahren diskutiert. Die Kopplung der Phosphorrückgewinnung mit der Verbrennung in stationären Wirbelschichtfeuerungen, der dominierenden Monoverbrennungstechnologie, bietet hinsichtlich des Nachbehandlungsaufwandes und der Wirtschaftlichkeit potenzielle Vorteile. In Versuchen konnte durch Einstellung der Verbrennungsbedingungen und Zugabe von Additiven eine Asche hergestellt werden, die der Düngemittelverordnung entspricht und somit potenziell als Phosphor‐Rezyklat einsetzbar ist. Currently, wet‐chemical and thermochemical processes in particular are being discussed for the future obligatory recovery of phosphorus from sewage sludge. The coupling of phosphorus recovery with incineration in stationary fluidized bed incinerators, the dominant mono‐incineration technology, offers potential advantages in terms of aftertreatment costs and economic efficiency. Tests have shown that by adjusting the incineration conditions and adding additives, an ash could be produced that corresponds to the fertilizer ordinance (DüMV) and can, therefore, potentially be used as phosphorus recyclate. Die thermochemische Phosphorrückgewinnung aus Klärschlamm, direkt integriert in eine stationäre Wirbelschichtfeuerung, bietet hinsichtlich des Nachbehandlungsaufwandes und der Wirtschaftlichkeit potenzielle Vorteile. Voraussetzung zur Nutzung der erzeugten Asche als Phosphor‐Rezyklat ist die Einhaltung der Düngemittelverordnung.
The chaotic distribution and dispersal of phosphorus (P) used in food systems (defined here as disorderly disruptions to the P cycle) is harming our environment beyond acceptable limits. An analysis of P stores and flows across Europe in 2005 showed that high fertiliser P inputs relative to productive outputs was driving low system P efficiency (38 % overall). Regional P imbalance (P surplus) and system P losses were highly correlated to total system P inputs and animal densities, causing unnecessary P accumulation in soils and rivers. Reducing regional P surpluses to zero increased system P efficiency (+ 16 %) and decreased total P losses by 35 %, but required a reduction in system P inputs of ca. 40 %, largely as fertiliser. We discuss transdisciplinary and transformative solutions that tackle the P chaos by collective stakeholder actions across the entire food value chain. Lowering system P demand and better regional governance of P resources appear necessary for more efficient and sustainable food systems.
The development of highly concentrated phosphorus (P) fertilizers, such as triple superphosphate, by the Tennessee Valley Authority helped mark the beginning of a revolution in the way we manage food crop nutrition. Since then, scientists, with the help of farmers, have made great advancements in the understanding of P fate and transport across many environments but largely have failed to produce a new generation of products and/or application techniques that are widely accepted and that vastly improve plant acquisition efficiency. Under certain conditions, important advancements have been made. For example, applying liquid formulations of phosphates in lieu of dry granules in some highly calcareous soils has dramatically reduced precipitation as sparingly soluble calcium phosphate minerals, but other attempts, such as the co‐application of humic substances, sorption to layered double hydroxides, or use of nanoparticles, have not generated the kind of results necessary to continue economically increasing crop yields without further environmental cost. New sources of fertility will need to be affordable to produce, transport, and furnish P to soil solution in a manner well synchronized with crop demand. This paper provides a review of recent literature on cutting‐edge phosphorus fertilizer technology. The goal is that this synthesis will be used as a starting point from which a larger discussion on responsible nutrient management and increased P use efficiency research can be built.
Core Ideas
Reaction with some soil constituents limits P availability and crop yield.
A variety of approaches to improve fertilizer use efficiency are being explored.
Ideally, P availability should be well synchronized to crop demand.
More innovation along with mechanistic and field‐scale trials is required.
The effects of interactions between and among chlorine (Cl), sulfur (S), phosphorus (P), and minerals on migration, transformation, and volatilization of zinc (Zn) were numerically simulated in sludge co‐incineration using the chemical thermodynamic equilibrium method. Our results showed that all the minerals of Fe2O3, Al2O3, Fe2O3, and TiO2 except for CaO in the sludge co‐incineration system reacted with Zn which inhibited the Zn volatilization. The presence of S and P was beneficial to the formation of ZnSO4(s) and Zn3(PO4)2(s). Cl weakened the chemical reactions between the minerals and Zn, thus increasing the Zn volatilization. Changes in Zn transformation and migration induced by the coupling of Cl + S were mainly controlled by Cl, S, and the minerals, while those induced by Cl + P and S + P were mainly controlled by P and S + P. The presence of P + Cl, S + Cl, S + P, S + Cl + P, Cl, and Al2O3 in the coexisting mineral system controlled the reactions between the minerals and Zn.
Phosphorus is an essential nutrient for every organism on the Earth, yet it is also a potential environmental pollutant, which may cause eutrophication of water bodies. Wastewater treatment plants worldwide are struggling to eliminate phosphorus from effluents, at great cost, yet current research suggests that the world may deplete the more available phosphorus reserves by around 2300. This, in addition to environmental concerns, evokes the need for new phosphorus recovery techniques to be developed, to meet future generations needs for renewable phosphorus supply. Many studies have been, and are, carried out on phosphorus recovery from wastewater and its sludge, due to their high phosphorus content. Chemical precipitation is the main process for achieving a phosphorus-containing mineral suitable for reuse as a fertilizer, such as struvite. This paper reviews the current status and future trends of phosphorus production and consumption, and summarizes current recovery technologies, discussing their possible integration into wastewater treatment processes, according to a more sustainable water-energy-nutrient nexus.
Using the computation method of thermodynamic equilibrium, effects of sewage sludge (SS) co-combustion conditions and interactions with Fe2O3, SiO2, CaO and Al2O3 on migration and transformation of arsenic (As) were simulated in oxy-fuel (CO2/O2) and air (N2/O2) atmospheres. Arsenic mainly existed as As(s), As4(g), As2O5(s), As4O6(g) and AsO(g) and volatilized more easily in reducing than oxidizing atmosphere. Increased O2 concentration slowed down the formation rate of AsO(g), thus reducing the volatilization rate of As. With the increased pressure, the conversion rate of As2O5(s) into As4O6(g) accelerated. In the multi-chemical system of SiO2, Al2O3 and CaO, As reacted with CaO and Al2O3 to form AlAsO4(s) and Ca3(AsO4)2(s) which inhibited As volatilization. SiO2 prevented As from reacting with CaO to generate Ca3(AsO4)2(s). Fe2O3 affected reactions between Al2O3(CaO) and As which inhibited As volatilization. In the whole SS co-combustion system, As reacted with O2 but had a weak affinity with Cl and with no arsenic chlorides observed.
The removal of phosphorus (P) from domestic wastewater is primarily to reduce the potential for eutrophication in receiving waters, and is mandated and common in many countries. However, most P-removal technologies have been developed for use at larger wastewater treatment plants that have economies-of-scale, rigorous monitoring, and in-house operating expertise. Smaller treatment plants often do not have these luxuries, which is problematic because there is concern that P releases from small treatment systems may have greater environmental impact than previously believed. Here P-removal technologies are reviewed with the goal of determining which treatment options are amenable to small-scale applications. Significant progress has been made in developing some technologies for small-scale application, namely sorptive media. However, as this review shows, there is a shortage of treatment technologies for P-removal at smaller scales, particularly sustainable and reliable options that demand minimal operating and maintenance expertise or are suited to northern latitudes. In view of emerging regulatory pressure, investment should be made in developing new or adapting existing P-removal technologies, specifically for implementation at small-scale treatment works.
Sludge or waste activated sludge (WAS) generated from wastewater treatment plants may be considered a nuisance. It is a key source for secondary environmental contamination on account of the presence of diverse pollutants (polycyclic aromatic hydrocarbons, dioxins, furans, heavy metals, etc.). Innovative and cost-effective sludge treatment pathways are a prerequisite for the safe and environment-friendly disposal of WAS. This article delivers an assessment of the leading disposal (volume reduction) and energy recovery routes such as anaerobic digestion, incineration, pyrolysis, gasification and enhanced digestion using microbial fuel cell along with their comparative evaluation, to measure their suitability for different sludge compositions and resources availability. Furthermore, the authors shed light on the bio-refinery and resource recovery approaches to extract value added products and nutrients from WAS, and control options for metal elements and micro-pollutants in sewage sludge. Recovery of enzymes, bio-plastics, bio-pesticides, proteins and phosphorus are discussed as a means to visualize sludge as a potential opportunity instead of a nuisance.
Thermodynamic equilibrium calculations were performed to reveal effects of interactions among Cl, S, P and other minerals on Cu migration. Our results showed that HCl(g), SO2(g) and (P2O5)2(g) were released from the sewage sludge co-incineration. Cl was found to weaken adsorption of Cu by Al2O3, CaO and Fe2O3, while S delayed reactions of Fe2O3 and Al2O3 with Cu, with P having no effect on reactions between the minerals and Cu. Among the coupled systems of Cl, S and P, the co-existences of Cl and S, and Cl, S and P were determined to inhibit Cu volatilization, and the co-existence of Cl and P had an enhancing effect. Cu migration was affected only by S in the S and P system. With the SiO2, CaO and Al2O3 system, both Cl alone and Cl and P led to failed reactions between the minerals and Cu. In the systems of S, S and Cl, S and P, and S, Cl and P, the migration behavior of Cu was mainly affected by S at low temperatures and by Cl at high temperatures, whereas P had no effect on Cu migration during the entire process.
Municipal solid waste incineration residues from one of the Polish incineration plants were sampled to characterize the distribution of metallic elements among them. Those elements previously dispersed within the waste mass, as a result of incineration processes were unequally distributed to bottom ash (BA), fly ash (FA) and air pollution control residues (APC) due to their physical properties, form of occurrence and characteristics of the incineration system. Fe, Cu, Cr were concentrated in BA; Mg, Ti in FA; Pb, Sn in APC. Beside them several elements were equally distributed among BA and FA (Al, Mn), and FA and APC (Zn).
The impact of global phosphorus scarcity on food security has increasingly been the focus of scientific studies over the past decade. However, systematic analyses of alternative futures for phosphorus supply and demand throughout the food system are still rare and provide limited inclusion of key stakeholders. Addressing global phosphorus scarcity requires an integrated approach exploring potential demand reduction as well as recycling opportunities. This implies recovering phosphorus from multiple sources, such as food waste, manure, and excreta, as well as exploring novel opportunities to reduce the long-term demand for phosphorus in food production such as changing diets. Presently, there is a lack of stakeholder and scientific consensus around priority measures. To therefore enable exploration of multiple pathways and facilitate a stakeholder dialog on the technical, behavioral, and institutional changes required to meet long-term future phosphorus demand, this paper introduces an interactive web-based tool, designed for visualizing global phosphorus scenarios in real time. The interactive global phosphorus scenario tool builds on several demand and supply side measures that can be selected and manipulated interactively by the user. It provides a platform to facilitate stakeholder dialog to plan for a soft landing and identify a suite of concrete priority options, such as investing in agricultural phosphorus use efficiency, or renewable fertilizers derived from phosphorus recovered from wastewater and food waste, to determine how phosphorus demand to meet future food security could be attained on a global scale in 2040 and 2070. This paper presents four example scenarios, including (1) the potential of full recovery of human excreta, (2) the challenge of a potential increase in non-food phosphorus demand, (3) the potential of decreased animal product consumption, and (4) the potential decrease in phosphorus demand from increased efficiency and yield gains in crop and livestock systems.
The distribution behaviors of metals in bio-oils derived from sewage sludge (SS) by liquefaction with different solvents (ethanol, methanol, or acetone) and by pyrolysis at different temperatures (550-850 °C) were investigated. The concentrations of crust metals (K, Na, Ca, Mg, Fe, and Al) in bio-oils were much higher than those of the anthropogenic metals (Cu, Zn, Pb, Cd, Cr, Ni, V, Mn, Ba, Co, Ti, Sn, As, and Hg), but the anthropogenic metals were more inclined to distribute in bio-oil phase compared with crust metals. The anthropogenic metals in bio-oils can be divided in three groups in terms of the distribution similarities according to Cluster analysis: (A) Cu, Co, Ni, V, and Sn; (B) Cr, Ti, Mn, and Ba; (C) Pb, Cd, As, Hg, and Zn. Cu, Cr, Hg, Cd, V, Co, and Sn distributed in the liquefaction/pyrolysis bio-oils accounted for as high as 5-20 % of the metals in SS and were evaluated "moderate enrichment" by the enrichment factors method. According to the potential ecological risk index (PERI) method, Hg presented very high risk, Cu presented moderate risk, and Cd presented low to moderate risk; and the overall risk levels of these bio-oils were very high risk (except P550, presented considerable risk).
Pb, Cu, Zn, Mn, and Cr were removed from municipal solid waste fly ash through chloride volatilization method using poly(vinyl chloride) (PVC) as the chlorinating agent. To investigate the effective utilization of PVC, chloride volatilization at 700–900 °C was conducted for 0–120 min under three conditions: elevated heating (~ 40 °C/min), isothermal heating, and isothermal heating with Ca(OH)2 as a HCl trapping agent. PVC was a better chlorinating agent than CaCl2 and removed ~ 100% Pb and Zn and ~ 50% Cu and Mn by isothermal heating at 900 °C for 120 min. Ca(OH)2 reduced Pb removal at 700 °C by ~ 20% and Cu removal at 900 °C by ~ 50%; however, it promoted Zn volatilization by 10–20% at all temperatures. The effects of co-existing elements on the chloride volatilization behaviors of these heavy metals were determined by conducting thermodynamic simulations under equilibrium conditions. The combined experimental and thermodynamic approach suggested that Pb and Zn were mainly volatilized as metal chlorides via reaction with the HCl produced by PVC pyrolysis, whereas Mn was volatilized by both CaCl2 and Na2O. Thus, CaO, CaCO3, MgO, and residual carbon in fly ash could inhibit the chlorination of Pb, Cu, and Cr.
The increasing levels of sewage sludge production demands research and development to introduce more commercially feasible options for reducing socio-economic and environmental problems associated with its current treatment. Sewage sludge may be processed to produce useful products or as a feedstock for energy generation. Initially, the characteristics of sewage sludge are discussed in terms of composition and the current options for its treatment with the associated environmental impacts. Processes to val- orize sewage sludge are discussed, including heavy metal removal from sewage sludge, production of bio-char, production and use of activated carbon and use of sewage sludge combustion ash in cement and concrete. Thermochemical processes i.e., pyrolysis, co-pyrolysis and catalytic pyrolysis, also gasifica- tion and combustion for process intensification, energy and resource recovery from sewage sludge are then critically reviewed in detail. The pyrolysis of sewage sludge to produce a bio-oil is covered in rela- tion to product bio-oil composition, reactor type and the use of catalysts. Gasification of sewage sludge focusses on the characteristics of the different available reactor types and the influence of a range of pro- cess parameters and catalysts on gas yield and composition. The selection and design of catalysts are of vital importance to enhance the selectivity of the selected thermochemical pyrolysis or gasification pro- cess. The catalysts used for sewage sludge treatment need more research to enable selectivity towards the targeted desired end-products along with optimization of parametric conditions and development of innovative reactor technologies. The combustion of sewage sludge is reviewed in terms of reactor tech- nologies, flue gas cleaning systems and pollutant emissions. In addition, reactor technologies in terms of technological strength and market competitiveness with the particular application to sewage sludge are compared for the first time for thermochemical conversion. A critical comparison is made of the drying techniques, co-feedstocks and catalytic processes, reaction kinetics, reactor technologies, operating condi- tions to be optimized, removal of impurities, fuel properties, their constraints and required improvements. The emphasis of this review is to promote environmental sustainability for process intensification, energy and resource recovery from pyrolysis, gasification and combustion involving the use of catalysts.
Sewage sludge from municipal wastewater treatment plants in Germany is currently disposed of via thermal treatment and agricultural utilization or used for landscaping. Increasing focus on hygiene, soil protection and most recently on phosphorus recovery combined with the associated legal changes leads to an increased relevance of thermal sewage sludge treatment processes. This article reviews existing technologies for thermal treatment of sewage sludge with a view to the situation in Germany. Thermal sewage sludge treatment can be divided into different processes: drying reduces high water contents of mechanically dewatered sewage sludge and often precedes subsequent treatment processes. Today, most of the sewage sludge in Germany is incinerated, about half in mono-incineration, mostly in stationary fluidized beds, and the other half in co-incineration, in particular in coal-fired power plants, cement kilns or, to a lesser extent, waste incineration plants. Some alternative thermal processes, mainly pyrolysis and gasification, but also metallurgical approaches, are tested in bench or pilot scale. Recent amendments to the German Sewage Sludge Ordinance will restrict the disposal route of co-incineration in future. Consequently, a significant increase in mono-incineration capacity is expected. These processes should enable the combination of environmentally friendly disposal and phosphorus recovery.
Co-incineration is promising for disposal of sewage sludge (SS) and municipal solid waste (MSW) with energy recovery, although mercury (Hg) emission in this process is non-negligible. In this work, a series of experimental and simulation works was conducted to provide knowledge about the distribution, ecological toxicity, and transformation of Hg during co-incineration of SS and MSW. The experimental results show that co-incineration can increase the Hg content in incineration ash and reduce the amount of Hg released into the atmosphere. The optimal blending ratio for Hg enrichment was 54 wt% SS and 46 wt% MSW. A modified three-step sequential extraction method was used to investigate the chemical species of Hg in incineration ash. It was found that when SS accounted for 50 wt% ∼75 wt% of the feedstock, the direct toxicity risk of Hg in incineration ash was reduced because of the presence of less hazardous chemical forms of Hg. Thermodynamic equilibrium simulation was also conducted on incineration flue gas. The results show that co-incineration caused more gaseous Hg⁰ to be oxidized to Hg²⁺ during the cooling process, leading to less environmental risk to the atmosphere. It is hoped that this work can help develop better co-incineration strategies and guide downstream Hg control technology, to dispose of and recover energy from SS and MSW with a cleaner approach.
Phosphorus (P) is an indispensable element of living organisms and plays an irreplaceable role in the growth of crops. As a non-renewable element, the reserves of phosphorus rock, the primary source of phosphorus in nature, are facing the danger of exhaustion. As a phosphorus-rich solid waste, sewage sludge has gradually become a main renewable phosphorus resource. The combination of effective recycling of phosphorus and innocuous disposal of sewage sludge can not only alleviate the crisis of phosphate rock resources shortage but also reduce the environmental hazards of sewage sludge. This study reviewed the application of thermal treatment in sewage sludge disposal. Besides the advantages of reducing waste volume, decomposing organic pollutants, generating valuable byproducts, it can also significantly promote the recycling of phosphorus. Studies have shown that thermal treatment (incineration, pyrolysis, and hydrothermal) can enrich phosphorus in the products and transform the speciation of phosphorus to increase the bioavailability. The physical and chemical properties of different thermal treatment products and the speciation of phosphorus are different. The transformation and migration of phosphorus affect the efficiency of subsequent phosphorus recovery and reuse. At the same time, this study compared several general phosphorus recovery methods (wet extraction, thermochemical, and electrochemical methods), and further summarized the advantages and disadvantages of various methods and application conditions. This review summarizes recent advances in phosphorus recovery from sewage sludge, identifies challenges and knowledge gaps, and provides the foundation for future research aimed at achieving efficient, economic, and eco-friendly reclamation of phosphorus in sewage sludge.
Increasing attention has been paid on the application of sewage sludge-derived biochar as soil amendments, but is always limited by heavy metals. This study conducted experiments on heavy metal removal by adding chlorinating agents (PVC, NaCl, MgCl2, CaCl2) during sludge pyrolysis. The chlorides addition can largely remove heavy metals by achieving the highest removal efficiency with dosage of 80 g(Cl)/kg(dry sludge) at 700 °C. The most effective removal effect was observed for Zn, Mn, Cu and Pb, with removal efficiency from 37.44% to 99.45%, 5.24% to 93.64%, 9.11% to 86.15% and 16.57% to 90.75%, respectively for the sludge before and after chlorination. Furthermore, the P-solubility in neutral ammonium citrate (Pnac) was enhanced after chlorination and the maximum P-solubility can be obtained at 700 °C for each series. After 700 °C pyrolysis, the P-solubility was significantly increased from 40.08% of the sludge biochar to 72.07%, 74.05%, 74.00% and 76.57% of the biochar obtained after adding PVC, NaCl, CaCl2, and MgCl2, respectively. The highest P-solubility was observed in samples with MgCl2 due to the formation of Mg3(PO4)2. This study proposed a novel method to use the sludge biochar as potential P-fertilizer with effective heavy metal removal, finally achieving a "waste-to-resource" strategy for integrated management of sewage sludge.
Phosphorus (P) is an essential nutrient for all organisms, and the recovery of P from sewage sludge (SS) has been the focus of recent research. The effect of temperature and chlorine-based additives (MgCl2 and CaCl2) on P recovery and heavy metal (Cu, Zn, and Cr) removal during SS incineration were investigated. The results showed that temperature and chlorine-based additives increased the P fixation and improved the P-bioavailability. The fixation rate of P reached a maximum of 98.5% in SS using 3% Mg (MgCl2) at 900 °C and 97.8% in SS using 5%Ca (CaCl2) at 800 °C. Additionally, the mechanism of P associated with aluminum oxides/hydroxides (Al-P) transferring to that of magnesium oxides/hydroxides (Mg-P) and calcium oxides/hydroxides (Ca-P) was also investigated. The results indicated that Mg3(PO4)2 was formed via the reaction between AlPO4 and MgCl2 during incineration, which increased the P solubility in 2% citric acid. When AlPO4 was incinerated with CaCl2, Ca2PO4Cl which has a higher P-bioavailability than AlPO4 was formed. In addition, the mass fraction and leaching concentration of Cu and Zn in treated SS greatly decreased during SS incineration, while no reduction in Cr. These findings suggest that chlorine-based additives can be used to treat SS at high temperatures to obtain an ideal material for P-fertilizer production.
The scarcity of phosphorus worldwide led researchers to develop indicators of national vulnerability to phosphorus scarcity, however this has not been applied at a national level so far. A systematic approach is described here to identify country-specific indicators of such vulnerability, in this case for India, based on literature and
stakeholder engagement. The indicators are then aggregated to compute a phosphorus vulnerability index (PVI) to help policymakers formulate strategies towards building national resilience towards scarcity of phosphorus. For India, the key indicators include import dependence, soil fertility, purchasing power of farmers, and their
access to credit. A PVI of 37.29 puts India in the ‘highly vulnerable’ category. Existing schemes targeted for agricultural sector in India are assessed for their ability to manage phosphorus vulnerability and for building resilience. Other developing countries with similar challenges can easily replicate the approach.
The concentration of a high number of elements in the solid residue after the thermal conversion of dried sewage sludge was measured, mainly targeting the change in concentration of heavy metals under different thermochemical conditions. Experiments were conducted in a laboratory fluidized bed and in a packed bed reactor under conditions relevant for pyrolysis, gasification and combustion. The concentration of twenty elements in the solid residue was measured as a function of temperature, reaction time, oxidizing atmosphere (O2 and H2O), presence of Cl in the gas and type of reactor gas-solid contact (fluidized vs packed bed). The temperature and the oxidizing conditions in the gas were shown to play a dominant role, whereas the effect of reaction time was negligible. The concentrations of most of main elements increased with temperature, whereas those of most of minor elements followed the opposite trend. The presence of chlorine in the gas resulted in higher concentration of Fe, Mg, Cu, Ni, Zn Co, Sb and V in the residue, while it promoted almost complete volatilization of Pb and Sn. The concentration of minor elements in the residue was lower in packed bed than in fluidized bed, as a result of the higher dilution caused by the lower degradation of the fuel sample in packed bed conditions. The enrichment of the elements in the solid residue decreased with the temperature, being more significant in pyrolysis than in combustion and gasification.
This study elucidated complex interactions during the chloride volatilization of heavy metals (Pb, Cu, Zn, Mn, and Cr) from municipal solid waste fly ash by combining thermodynamic and kinetic approaches. Chloride volatilization tests under HCl flow at 900 °C and subsequent rinsing with water achieved almost complete removal of Pb, Zn, and Mn. In contrast, almost 100 % of Cr and ∼40 % of Cu were not removed by either volatilization or rinsing processes. Kinetics indicated that the chlorination of Pb, Zn, and Mn followed a pseudo second order reaction and their apparent activation energies were 96.3, 89.2, and 43.5 kJ/mol, respectively. Further thermodynamic calculation revealed that the components contained in fly ash greatly influenced the chlorination of each heavy metal. Unburned carbon facilitated the chlorination of Pb, Zn, and Mn, while it inhibited Cu chlorination. MgO immobilized Cr and inhibited chlorination. KCl and NaCl promoted Zn and Mn chlorination, respectively. The revealed chloride volatilization behavior and effects of co-existing elements could be useful in the design of high-efficiency recovery process of heavy metals from fly ash and the utilization of residues as raw materials for cement. Furthermore, these findings could guide the realization of a recycling-oriented society in terms of reducing waste disposal.
Phosphorous (P) is an essential element for living organisms and is predicted to be depleted within the next 100 years. Across the world, significant phosphorous losses due to its low utilization efficiency become one of the main reasons for water pollution. Struvite crystallization has been found to be a promising recovery technique to mitigate these problems, as the recovered precipitate can be used as a slow release fertilizer or raw material for chemical industry. Although this technique has been widely investigated over the past two decades, there are currently few real applications in industry. This paper addresses this issue by reviewing key aspects relevant to process design to pave the way for future application. It will help to narrow down struvite process design options and thus reduce the voluminous calculations for a detailed analysis. Struvite process development, research trend, product application and process economics are reviewed and a conceptual process design is provided. This analysis provides comprehensive information that is essential for future industrial struvite crystallization process design.
In the near future, phosphorus (P) recycling will gain importance in terms of decreasing primary resources. Sewage sludge (SSL) is an adequate secondary P-resource for P-fertilizer production but it is also a sink for heavy metals and organic pollutants. The present study is an investigation on thermochemical P-recycling of SSL. Various temperatures and amendments were tested regarding their performance to remove heavy metals and polycyclic aromatic hydrocarbons (PAH) and simultaneous increase of the plant-availability of P. The investigations were carried out on two types of SSL originating from wastewater treatment plants with chemical P-precipitation and enhanced biological P-removal, respectively. The results show that thermochemical treatment with chlorine donors is suitable to remove the majority of heavy metals and that a combination of a gaseous chlorine donor (HCl) and sodium additives leads to both high heavy metal removal and high plant availability of P. Furthermore, plant experiments show that almost all investigated thermochemical treatments can significantly reduce the bioavailability and plant uptake of heavy metals. Furthermore, PAHs are secondarily formed during low-temperature treatments (400–500 °C), but can be significantly reduced by using sodium carbonate as an additive.
This chapter will explore whether the differences in the process development for phosphorus recovery and recycling and its implementation in Europe and Japan are linked to the waste flows and the regulative framework. The main waste flows and their qualities are summarized for the two geographical areas. Then a comparative overview of the full-scale applications and their importance in relation to the potential is presented. The drivers for phosphorus recycling and the expected further development in Europe and Japan are described.
Thermal processing of sewage sludge (SS) has received increasing attention in recent years. Thermal processes valorise the carbon rich organic fraction of SS, while effectively reducing SS volume. However, the fate and distribution of heavy metals (HMs) during thermal processing of SS is an important issue to address because it has impact on the generation of secondary pollutants and the environmental acceptability of the residues for reuse and reclamation. The refractory metals (thermally stable, i.e. Cr, Mn and Ni) are less volatile at typical temperature ranges (200–1100 °C) of thermal processes, and they are enriched in the residues. On the contrary, HMs with lower thermal stability (i.e. Hg, Cd, As and Pb) are prone to volatilisation. However, volatilisations and enrichments of HMs in the residues strongly depend on the characteristics of SS and nature of the thermal process. This review article discusses the volatilisation, enrichment and speciation (or stabilisation) of HMs in the residues formed during thermal processing of SS (incineration, pyrolysis, gasification and hydrothermal treatment). First, it summarises the fundamental aspect of SS in each thermal process. The influencing factors on the fate and distribution of HMs are discussed in terms of process principles, reactor types, operating conditions, pre-treatment of SS, use of additives and co-processing with secondary feedstocks. The use of advanced analytical techniques and modelling tools to analyse the complexity of HMs redistribution during thermal processing is described. Practical and economic challenges associated with HMs in SS during operation of full-scale thermal processing facilities are also addressed. Finally, a brief comparison of HMs redistribution and stabilisation during SS incineration, gasification, pyrolysis and hydrothermal treatment is provided.
Controlling the removal of heavy metals such as Copper (Cu), Zinc (Zn), lead (Pb) and cadmium (Cd) during the sewage sludge incineration and the recovery of phosphorus (P) from sewage sludge ash (SSA) remain challenging. Herein we aim to investigate the effect of the temperature, retention time and chlorinating agent additives (MgCl2 and KCl) on both the behavior of selected heavy metals and the fixed rate of phosphorus (P) during sewage sludge incineration. Dry sewage sludge was mixed with various amounts of chlorinating agent and treated by a laboratory scale furnace in the temperature range of 800-1000oC for different retention times (30, 60 and 120 minutes). The results demonstrated that the removal efficiency of heavy metals exhibited an increasing tendency with the addition of chloride, especially in the cases of Cu, Zn and Pb. Moreover, the temperature and retention time demonstrated significant effects on the promotion of heavy metals removal.
In the case of Pb 96% was removed at 800oC within 120 minutes, while 86.6% of Cd was effectively removed at 1000oC within 30 minutes. MgCl2 proved to be more effective than KCl in improving the removal efficiency of heavy metals, such that up to 81.6% of Cu, 84.9% of Pb and 73.5% of Zn was removed with the addition of 15%wt Cl-/sewage sludge (SS), At 960oC, the boiling point of cadmium chloride CdCl2 and CdO.Al2O3.2SiO2 retention time and chlorinating agent had no impact on Cd removal. It was also observed that high temperature was beneficial for the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP).
At 900oC this transformation is efficient for Phosphate fixation, which is evidence that apatite phosphorus (AP) has the ability to be more stable than non-apatite inorganic phosphorus (NAIP) at high temperature.
Keywords: Sewage sludge; Heavy metals; Phosphorus; Thermal treatments;
Removal efficiency
The effect of chlorine on arsenic (As) release dynamics during municipal solid waste (MSW) incineration in a fluidized bed was studied on the basis of an on-line analysis system. This system can continuously and quantitatively measure the concentrations of trace elements in flue gas. Chlorine addition increases obviously the concentration of arsenic in flue gas, indicating a promoting effect of chlorine on arsenic release during MSW incineration. Based on the temporal concentration of arsenic in flue gas, the overall kinetic parameters of arsenic release during MSW incineration were calculated. A second-order kinetic law r(x) = 81.6e-66.9/RT (-1.05x2 - 0.01x + 1.03) was ascertained for arsenic release during MSW incineration without chlorine addition, and r(x) = 177.3e-65.3/RT (-0.68x2 - 0.43x + 1.08) for arsenic release with chlorine addition. Thermodynamic calculations were performed to predict the partitioning behavior of arsenic during MSW incineration. The addition of chlorine can not only compete with gaseous arsenic to react with mineral, but is also able to increase the volatilization of arsenic by forming volatile arsenic chlorides, thereby affecting the release kinetics of arsenic during MSW incineration.
Direct application of sewage sludge ash (SSA) to agriculture is mostly prohibited due to its heavy metals and limited bioavailability. Thermochemical treatment of SSA with Cl-donors were carried out to remove heavy metals and enhance P bioavailability. MgCl2 and CaCl2 removed more heavy metal than other Cl-donors. Pb was completely removed followed by Cd, Zn, Cu. They also increased P bioavailability by the conversion of Al-P to Ca/Mg-P which was confirmed by chemical fractionation. XRD confirmed the conversion of AlPO4 to Ca5(PO4)3Cl, Ca3(PO4)2, and Mg3(PO4)2 by the thermochemical treatment.
Global food production and security rely heavily on finite reserves of newly mined phosphate for fertilizers. However, systemic inefficiencies result in the deposition in aquatic ecosystems of much of the phosphorus mined for food production causing costly eutrophication problems that damage aquatic ecosystems and human health. The Sustainable Phosphorus Alliance (SPA, formerly named North American Partnership for Phosphorus Sustainability) was created to implement sustainable phosphorus solutions through active engagement of stakeholders in both the private and public sectors. This paper describes a conceptual model of transformative change to a sustainable phosphorus system for the North American region. The model emerged from discussions at a series of formal and informal meetings held in conjunction with a ‘Future of Phosphorus’ event (National Science Foundation’s Phosphorus Sustainability Research Coordination Network) and an inaugural SPA Board meeting. Model development drew on the multi-level perspective of socio-technical transitions to develop a series of pathways to a transformed phosphorus system. The uses of the model and transition pathways are discussed in terms of their potential to form an important first step towards the development of a regional vision for improved phosphorus sustainability. The process provides an example of how research in sustainability science can contribute to action on environmental improvement.
Sewage sludge, the inevitable by-product of municipal wastewater treatment plant operation, is a key issue in many countries due to its increasing volume and the impacts associated with its disposal. Thermochemical processing offers a new way of managing sewage sludge, not only by providing effective volume reduction, but also enabling transformation of carbon-rich organic fraction into valuable energy and fuel. Owing to some unique properties, sewage sludge differs from other solid fuels such as lignocellulosic biomass and coal, making its thermochemical conversion application somewhat complicated and challenging. This paper reviews the options of converting sewage sludge to energy and fuel via three main thermochemical conversion processes namely pyrolysis, gasification and combustion. The fundamental aspects of sewage sludge and its behaviour in each of thermochemical process are summarised. The challenges in adopting thermochemical conversion technology in sewage sludge management are addressed, and various alternative approaches deserving further consideration, such as the incorporation of pre-processing and co-utilisation, are discussed.
The study is part 2 of 2 in an investigation of gasification and co-gasification of municipal sewage sludge in low temperature gasifiers. In this work, solid residuals from thermal gasification and co-gasification of municipal sewage sludge were investigated for their potential use as fertilizer. Ashes from five different low temperature circulating fluidized bed (LT-CFB) gasification campaigns including two mono-sludge campaigns, two sludge/straw mixed fuels campaigns and a straw reference campaign were compared. Experiments were conducted on two different LT-CFBs with thermal capacities of 100 kW and 6 MW, respectively. The assessment included: (i) Elemental composition and recovery of key elements and heavy metals; (ii) content of total carbon (C) and total nitrogen (N); (iii) pH; (iv) water extractability of phosphorus after incubation in soil; and (v) plant phosphorus response measured in a pot experiment with the most promising ash material. Co-gasification of straw and sludge in LT-CFB gasifiers produced ashes with a high content of recalcitrant C, phosphorus (P) and potassium (K), a low content of heavy metals (especially cadmium) and an improved plant P availability compared to the mono-sludge ashes, thereby showing the best fertilizer qualities among all assessed materials. It was also found that bottom ashes from the char reactor contained even less heavy metals than cyclone ashes. It is concluded that LT-CFB gasification and co-gasification is a highly effective way to purify and sanitize sewage sludge for subsequent use in agricultural systems.
Phosphorus deposits around the world are rapidly depleting, therefore phosphorus recovery methods are gaining more and more interest both in science and industry. This article presents the main methods of phosphorus recovery from sewage sludge. The described approaches are divided in two groups: phosphorus recovery from sewage sludge and leachate, and recovery of phosphorus from sewage sludge ashes. The latter seems to have more advantages connected with both ecological and economical aspects. The need for development of “no solid waste generation” strategy is becoming more and more urgent. The concept of comprehensive management of all solid residues after what is currently considered the most ecological process of sewage sludge incineration connected with phosphorus recovery based on acidic extraction, is described in the article. Solid residues after phosphorus recovery from sewage sludge ashes by means of acidic extraction can be stabilized with solid residues after sewage sludge incineration exhaust gas treatment. Such an approach may enable production of phosphoric raw material together with stabilized construction material. Advantages and disadvantages of the discussed approaches are given. An analysis of the composition of ashes produced in different sewage sludge treatment plants indicates that the proposed technology could be successfully applied in most of such units, especially because the concentrations of elements such as K, Mg, Na, P are sufficiently high, respectively 1.5–12.1 g/kg; 9.9–14.9 g/kg; 3.6–13,3 g/kg and 27,4–99,0 g/kg. However, a phosphorus recovery method should be developed separately for each treatment plant. Only then all comprehensive management methods will be ecologically and economically justified. Analytical methods which could be of use at every step of designing a proper phosphorus recovery process are described.
All creature of nature needs metals for subsistence. The plants and trees contain this element in a different amount. During biomass combustion and energy production this element could cause environmental and health problems, thus, the cognition of burning and determination of material flow is a very important question. The high amount of metals steps out from the combustion chamber in gaseous form. To determine the metal distribution inside the burning equipment system, we have to get samples from the solid combustion remains and flue gas, and we have to measure the material flows. At the first stage, we made a biomass combustion experiment using ligneous plants with high metal content. The experiment was carried out with a Binder RRK 500 type wood chips-, briquette- and bark-fueled heating system. The fuel was wood chips, and the operational parameters were automatically set by the temperature control so that the circulating water was kept at a constant temperature. The amount of flue gas was calculated based on the theoretical estimate. The analysed metals were Zn, Cu, Cd, Pb, Cr, Ni, Co, and Fe. The metal content of the wood chips, ash and fly ash was determined by ICP spectrometry, and the ash content and fly ash concentration in the flue gas was measured. The material flow was calculated in the matter of metals. The results show that under the experimental circumstances, the solid burning residues (bottom ash, fly ash) contain the following percent of metals referred to the fuel metal concentration: Zn - 17–18%, Cu - 2–7%, Cd - 4–20%, Pb - 14–15%, Cr - 0.5%, Ni - 2%, Co - 10–11%, and Fe - 5–7%. The results proved that the metals are very volatile in the combustion temperature (800 °C) and flue gas temperature (250–300 °C).
Global food security is a priority for the future development agenda of the United Nations. Given the high dependence of the modern global food production system on the continuous supply of commercial phosphorus (P) fertilizers, the goal of achieving global food security could be hampered by any form of paucity of the global P resource. P is a finite, non-substitutable, non-renewable, and geographically restricted resource. The anthropogenic influences on this critical resource are likely to pose a number of challenges to its sustainability. Based on an in-depth and systematic review of recently published literature, this study presents a detailed, methodical, and up to date assessment of the key sustainability challenges for the global P resource, and highlights their implications for global food security. The assessment indicates that the demand for P fertilizer to produce enough food for the growing global population will substantially increase by the middle of this century. There are also significant discrepancies in estimates of the lifetime of global phosphate rock (main source of chemical P fertilizer) reserves that range from a few hundred to a thousand years; however, as global reserves deplete, exploitation and exploration of resource will replenish reserves although the global P resource is finite. The signs of geopolitical constraints regarding available phosphate rock reserves are already evident, and are likely to be more intense in future. The combined impact of increasing demand, dwindling reserves, and geopolitical constraints could result in a substantial decline in the production and supply of chemical P fertilizer in the global market and lead to increased P price, and this situation may eventually restrict the access of farmers of many countries to P fertilizers for food production. Moreover, there is evidence of significant P wastage and loss as well as high P discharge to water bodies from various systems at different geographical scales. The wastage and loss of P in this manner, if not checked, will not only increase the global demand for P but also result in the increased depletion of available global phosphate rock reserves. On the other hand, the continuing high discharge of P to water bodies will accelerate harmful processes such as algal bloom, hypoxia and eutrophication that deplete fish and other aquatic foods, the signals of which are already apparent, and may cause irreparable damage to aquatic ecosystems. Shortage in supply of fish and sea foods in turn will restrict the access of many people to a protein based diet, and eventually result in hunger and malnutrition; however, this important fact is often overlooked while addressing the significance of sustainable P management for global food security. We recommend that any policy at the international, national, and local levels that aims to achieve global food security and to protect aquatic ecosystems should incorporate necessary measures to overcome the key sustainability challenges for the global P resource. An overview of the recent advances in the knowledge of sustainable P management strategies as presented in this study could be effectively utilized to mitigate these challenges for achieving global phosphorus security, food security, and environmental sustainability.
The heavy metal problem during phosphorus recovery from sewage sludge cannot be ignored. Accordingly, this study investigates the effect of the form and content of phosphorus on the behavior of heavy metals (specifically, Cu, Zn, Pb, and Cd) in sewage sludge incineration to determine the optimum condition for heavy metal enrichment in ashes. The experiment is performed in a laboratory-scale furnace with phosphate additives at 750–950°C to analyze the heavy metal content in sewage sludge ash (SSA). The crystalline phase is analyzed using X-ray diffraction to explore the mechanism of phosphorus immobilized heavy metals. The morphology is analyzed by scanning electron microscopy to investigate the effect of surface structure difference caused by different temperatures on heavy metal volatilization. Phosphorus significantly affects Zn, Pb, and Cd but not Cu. P2O5 and KH2PO4 have more obvious effects than (NH4)3PO4 and Ca3(PO4)2 on heavy metal immobilization. The additive (e.g., Ca3(PO4)2) could increase SSA melting point and observably improve volatilization of Zn at 950°C. © 2016 American Institute of Chemical Engineers Environ Prog, 2016
Significance
The biogeochemical cycle of phosphorus (P) has been massively altered in China, challenging its food security and causing eutrophication of freshwaters. This study shows, for the first time to our knowledge, how P cycling in China was intensified in the past four centuries to sustain the increasing population and its demand for animal protein. Our analysis also reveals the spatial disparity of its concomitant eutrophication impact. The findings advance the knowledge base needed for closing the P cycle to sustain future food production and maintain healthy rivers, lakes, and oceans.
Permanent link to Lappeenranta University of Technology depository:
http://urn.fi/URN:NBN:fi-fe2016102825620
All life forms require phosphorus (P), which has no substitute in food production. The risk of phosphorus loss from soil and limited P rock reserves has led to the development of recycling P from industrial residues. This study investigates the potential of phosphorus recovery from sewage sludge and manure ash by thermochemical treatment (ASH DEC) in Finland. An ASH DEC plant could receive 46–76 kt/a of sewage sludge ash to produce 51–85 kt/a of a P-rich product with a P2O5 content of 13–18%, while 320–750 kt/a of manure ash could be supplied to produce 350–830 kt/a of a P-rich product with a P content of 4–5%. The P2O5 potential in the total P-rich product from the ASH DEC process using sewage sludge and manure ash is estimated to be 25–47 kt/a, which is significantly more than the P fertilizer demand in Finland’s agricultural industries. The energy efficiency of integrated incineration and the ASH DEC process is more dependent on the total solid content and the subsequent need for mechanical dewatering and thermal drying than on the energy required by the ASH DEC process. According to the results of this study, the treated sewage sludge and manure ash using the ASH DEC process represent significant potential phosphorus sources for P fertilizer production.
Synthetic bottom ash of burnt urban waste containing heavy metal oxides, CdO, Cr 2O 3, Cu 2O, PbO and ZnO, is reacted with polyvinyl chloride, PVC. The synthetic ash is prepared from chemical reagents of heavy metal oxides, Al 2O 3, CaO and SiO 2. It is revealed that the heavy metal oxides except Cr 2O 3 are removed as metal chloride from the synthetic ash by PVC. It is considered that the heavy metals in bottom ash of burnt urban waste can be removed by use of end of life PVC.
This work aims to study the mechanism of heavy metals vaporization by MgCl2⋅6H2O. Firstly, the decomposition mechanism of MgCl2⋅6H2O was investigated by thermodynamic equilibrium calculations, XRD and TG. Upon heating, MgCl2⋅6H2O went through the processes of dehydration and hydrolysis simultaneously accompanied by the release of HCl between 150 and 500°C. At temperature higher than 500°C, Mg(OH)Cl gradually release part of HCl. MgCl2⋅6H2O followed the similar processes of decomposition at both oxidative and reductive atmospheres. In oxidative atmosphere, vaporization of Zn and Cu was significantly accelerated by MgCl2⋅6H2O. However, in inert atmosphere, vaporization of Cu was not promoted since copper chloride was only stable in oxidative atmosphere. Under slow heating condition, vaporization of heavy metals were close to that under fast heating condition. This may be partially attributed to that most heavy metals already reacted with HCl forming metal chlorides below 500°C, which can be vaporized at higher temperature. Moreover, the Mg(OH)Cl contributed to release HCl up to 800°C. At such high temperature, the metal chlorides continue to be formed and then vaporized. After treatment, the leaching concentration of heavy metals from treated fly ashes were much lower than that from raw fly ash and met the regulatory limit of leachate. Since a large amount of MgSiO3 were formed during thermal treatment, the fly ash treated with MgCl2⋅6H2O can be used as raw materials for glass-ceramics production.
Heavy metal emission is a great environmental concern for the development of municipal solid waste (MSW) thermal treatment techniques. In this study, both experimental investigations and theoretical simulations are carried out to identify the partitioning of heavy metals between the gaseous phase and solid fractions during pyrolysis, gasification, and incineration of simulated MSW. Two types of incinerators are used. A tubular furnace is applied to evaluate the evaporation of metals from residues, whereas the metal distribution among bottom ash, cyclone fly ash, and filter fly ash is further examined in a fluidized bed. Six target metals (Cd, Pb, Zn, Cu, Cr, and Ni) are studied. Results show that a reductive atmosphere favors the evaporation of Cd and Zn but refrains Cu, Ni, and Cr volatilization, because metals are mainly reduced to their elemental form or sulfide, according to thermodynamic equilibrium calculation. Oxides are the dominant species under oxidizing condition due to the abundance of alkalis. Pb behaved differently, most probably by forming stable metal-matrix compounds such as Pb3Ca2Si3O11 and PbZnSiO4. The cyclone ash is then separated into different sizes. The metal concentrations recorded reveal that most of the vaporized metals are transferred to the cyclone at its working temperature of 350-600 °C by an evaporation and condensation process; however, entrainment is also a determining factor for the transfer of less-volatile metals. Overall, parameters determining the transfer of heavy metals during MSW thermal treatment can be summarized as (i) metal speciation affected by redox atmosphere, temperature, and the presence of alkalis, chloride, sulfur, and other mineral substances; (ii) system characteristics, such as furnace type and cyclone temperature; and (iii) mechanical entrainment of particles caused by gas velocity.
Without phosphorus, we could not produce food. Farmers need access to phosphate fertilizers to achieve the high crop yields needed to feed the world. Yet growing global demand for phosphorus could surpass supply in the coming decades, and the world currently largely relies on non-renewable phosphate rock that is mined in only a few countries. Morocco alone controls 75% of the remaining reserves, including those in the conflict territory of Western Sahara. While some argue that the market will take care of any scarcity, the market price of phosphate fertilizers fails to account for far-ranging negative impacts. Drawing on multi-stakeholder supply chain risk frameworks, the article identifies a range of negative impacts, including the exploitation and displacement of the Saharawi people, the destruction of aquatic ecosystems by nutrient pollution, and jeopardizing future generations' ability to produce food. This paper fills a crucial gap in understanding phosphorus impacts by mapping and discussing the nature of phosphorus supply chain risks, and the transmission of such risks to different stakeholder groups. It also identifies a range of potential interventions to mitigate and manage those risks. In addition, the paper highlights that while risks are diverse, from geopolitical to ecological, those groups adversely affected are also diverse – including the Saharawi people, farmers, businesses, food consumers and the environment. Potential risk mitigation strategies range from resource sparing (using phosphorus more sparingly to extend the life of high quality rock for ourselves and future generations), to resource diversification (sourcing phosphorus from a range of ethical sources to reduce dependence on imported phosphate, as a buffer against supply disruptions, and preferencing those sources with lower societal costs), and sharing the responsibility for these costs and consequences.
Phosphorus (P) for fertilizer use can be recovered from sewage sludge ash (SSA). To enhance the bioavailability of P and reduce the heavy metal content of SSA, it can be treated thermochemically with Na2CO3 or Na2SO4 at 950 °C in a rotary kiln using dry sewage sludge or lignite as reducing agent. These processes were investigated by thermogravimetry/differential thermal analysis coupled with gas analysis. Reducing conditions in this experimental setup were provided by 2 % hydrogen in the Ar carrier gas. During SSA + Na2CO3 treatment CO2, CO and water were detected in the off-gas. During SSA + Na2SO4 treatment SO2, some CO2 and water were detected. Heavy metal removal was more efficient for SSA + Na2CO3 compared to the sulfate variant. A SSA + Na2SO4 + lignite variant which also formed CO shifted the heavy metal removal to the results obtained with Na2CO3 which was obviously due to the additional reduction potential. However, Zn evaporation was not achieved with the Na2SO4 variants which were most probably due to immobilization as ZnS.
Sewage sludge incineration is extensively practiced in some European countries such as the Netherlands, Switzerland, Austria and Germany. A survey of German sewage sludge ash showed that the recovery potential is high, approx. 19,000t of phosphorus per year. However, the survey also discovered that the bioavailability of phosphorus in the sewage sludge ash is poor and that more than half of the ashes cannot be used as fertilizers due to high heavy metal content. A new thermochemical process for sewage sludge ash treatment was developed that transforms the ash into marketable fertilizer products. Sewage sludge ash was thermochemically treated with sodium and potassium additives under reducing conditions, whereby the phosphate-bearing mineral phases were transformed into plant available phosphates. High P-bioavailability was achieved with a molar Na/P ratio >1.75 in the starting materials. Sodium sulfate, carbonate and hydroxide performed comparably as additives for this calcination process. Potassium carbonate and -hydroxide have to be added in a molar K/P ratio >2.5 to achieve comparable P-solubility. The findings of the laboratory scale investigations were confirmed by an industrial demonstration trial for an ash treatment with sodium sulfate. Simultaneously, the volatile transition metal arsenic (61% removal) as well as volatile heavy metals such as cadmium (80%), mercury (68%), lead (39%) and zinc (9%) were removed via the off-gas treatment system. The product of the demonstration trial is characterized by high bioavailability and a toxic trace element mass fraction below the limit values of the German fertilizer ordinance, thus fulfilling the quality parameters for a P-fertilizer.
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This study aims to vaporize heavy metals and alkali/alkaline earth metals from two different types of fly ashes by thermal treatment method. Fly ash from a fluidized bed incinerator (HK fly ash) was mixed with one from a grate incinerator (HS fly ash) in various proportions and thermally treated under different temperatures. The melting of HS fly ash was avoided when treated with HK fly ash. Alkali/alkaline earth metals in HS fly ash served as Cl-donors to promote the vaporization of heavy metals during thermal treatment. With temperature increasing from 800 to 900°C, significant amounts of Cl, Na and K were vaporized. Up to 1000°C in air, less than 3% of Cl and Na and less than 5% of K were retained in ash. Under all conditions, Cd can be vaporized effectively. The vaporization of Pb was mildly improved when treated with HS fly ash, while the effect became less pronounced above 900°C. Alkali/alkaline earth metals can promote Cu vaporization by forming copper chlorides. Comparatively, Zn vaporization was low and only slightly improved by HS fly ash. The low vaporization of Zn could be caused by the formation of Zn2SiO4, ZnFe2O4 and ZnAl2O4. Under all conditions, less than 20% of Cr was vaporized. In a reductive atmosphere, the vaporization of Cd and Pb were as high as that in oxidative atmosphere. However, the vaporization of Zn was accelerated and that of Cu was hindered because the formation of Zn2SiO4, ZnFe2O4 and ZnAl2O4 and copper chloride was depressed in reductive atmosphere.
Copyright © 2015. Published by Elsevier Ltd.
Phosphorus security is emerging as one of the twenty-first century's greatest global sustainability challenges. Phosphorus has no substitute in food production, and the use of phosphate fertilizers in the past 50 years has boosted crop yields and helped feed billions of people. However, these advantages have come at a serious cost. Mobilizing phosphate rock into the environment at rates vastly faster than the natural cycle has not only polluted many of the world's freshwater bodies and oceans, but has also created a human dependence on a single nonrenewable resource. The 2008 phosphate price spike attracted unprecedented attention to this global situation. This review provides an updated and integrated synthesis of the biophysical, social, geopolitical, and institutional challenges and opportunities for food security. Remaining phosphorus resources are becoming increasingly scarce, expensive, and inequitably distributed. All farmers require fertilizers, yet a sixth of the world's farmers and their families are too poor to access fertilizer markets. Inefficient use of this fossil resource from mine to field to fork calls for substantial reduction in demand through efficiency and recycling. Phosphorus governance at global, regional, and local scales is required to stimulate and support context-specific sustainable strategies to ensure all the world's farmers have sufficient access to phosphorus to feed the world and ensure ecosystem integrity and farmer livelihoods.