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High-rate anaerobic co-digestion of kraft mill fibre sludge and activated sludge by CSTRs with sludge recirculation
Abstract
Kraft fibre sludge from the pulp and paper industry constitutes a new, widely available substrate for the biogas production industry, with high methane potential. In this study, anaerobic digestion of kraft fibre sludge was examined by applying continuously stirred tank reactors (CSTR) with sludge recirculation. Two lab-scale reactors (4L) were run for 800 days, one on fibre sludge (R1), and the other on fibre sludge and activated sludge (R2). Additions of Mg, K and S stabilized reactor performance. Furthermore, the Ca:Mg ratio was important, and a stable process was achieved at a ratio below 16:1. Foaming was abated by short but frequent mixing. Co-digestion of fibre sludge and activated sludge resulted in more robust conditions, and high-rate operation at stable conditions was achieved at an organic loading rate of 4 g volatile solids (VS) L−1 day−1, a hydraulic retention time of 4 days and a methane production of 230 ± 10 Nm L per g VS.
... Another treatment method that includes energy recovery is anaerobic digestion (AD) of the paper sludge. Although this potential treatment has been addressed in several studies (Amare et al., 2019;Bayr and Rintala, 2012;Bokhary et al., 2021;do Carmo Precci Lopes et al., 2018;Ekstrand et al., 2016;Karlsson et al., 2011;Kinnunen et al., 2015;Lin et al., 2013;Meyer and Edwards, 2014;Puhakka et al., 1992;Thompson et al., 2001), sludge processing by AD in the pulp and paper industry is still a developing technology due to the low methane production potential of the cellulosic material and the digestate disposal problem (Bentancur et al., 2020;Kamali and Khodaparast, 2015;Mohammadi et al., 2019). ...
... analyzing the AD capability of BS in a CSTR pilot plant, methane yields of 205 to 248 NL CH 4 kg − 1 VS of BS were achieved with HRTs of 15 to 30 d. In addition, Ekstrand et al. (2016) determined methane yields of about 230 NL CH 4 kg − 1 VS in a long term study of fiber sludge AD with a relatively short HRT of 4 days and a high OLR of 4 kg VS / m 3 d. ...
... They concluded that primary sludge is more suitable for biogas production than secondary sludge. Since the HRT can have a significant impact on the microbial community in the AD reactor and thus significantly reduce the methane yield, shortening the HRT should only be done cautiously or with concurrent recirculation of the reactor sludge, as suggested by Ekstrand et al. (2016). However, Karlsson et al. (2011) found that high viscosity was a problem in semicontinuous CSTR reactors and limited the OLR. ...
In this work, the feasibility of the anaerobic digestion of paper sludge as a co-substrate in anaerobic digestion mechanical–biological treatment (MBT) plants is investigated. In the first phase, the biochemical properties, biomethane potential (BMP), and pollutant contents of 20 different industrial paper sludges are determined. Following the general evaluation in the BMP tests, the second phase of the project involves the semi-continuous co-digestion of six paper sludges in continuous stirred reactors (CSTR). Paper sludges are categorized according to their origin within the pulp and paper mills: Deinking Sludge (DS), Primary Sludge (PS) and Biological Sludge (BS). The analysis of potentially inhibiting elements shows that the concentrations of chlororganic compounds, mineral oil and some heavy metals are highest in DS, while the mean heavy metal loads in all paper sludges are relatively low compared to other industrial sludges. Large differences in total solids (TS) and volatile solids (VS) contents are observed among the different paper sludges investigated, with DS having the highest TS due to the high inorganic contents. The BMP of the investigated sludges ranges from 90 to 355 NL CH4 kg⁻¹ VS. In subsequent semi-continuous co-digestion experiments simulating MBT conditions, three DS and two fiber sludges (a mixture of PS and BS) show good methane generation rates, while one fiber sludge causes inhibition and indicates an increase in viscosity. In general, co-digestion of paper sludge in anaerobic digestion MBT plants can be a viable option for energy production and also facilitates a safe disposal of the paper sludge digestates.
... Two lab-scale CSTRs (R1 and R2) with working volumes of 4 L were operated for 800 days at 37 °C, as described by [26]. The substrates used were fibre sludge from the primary clarification and activated sludge from the secondary treatment of two kraft pulp and paper mills. ...
... pH was controlled by additions of Ca(OH) 2 , and from day 49, part of the Ca(OH) 2 was replaced by MgO to increase the concentration of magnesium in the reactors. For details on the feeding procedures, see Ekstrand et al. [26]. Nitrogen and phosphorus were supplied at a ratio of 350:5:1 for COD:N:P, where COD is the chemical oxygen demand of the substrate. ...
... The samples for extraction of EPS/SMP were kept frozen at − 20 °C until time of analysis, when they were thawed at + 4 °C over-night. TS, volatile solids (VS), pH and volatile fatty acids (VFA) were determined as described by [26]. Elemental analysis was carried out once a month for the reactor sludge samples and for every new delivery of substrate by Eurofins Environment Testing Sweden AB. ...
The production processes of the pulp and paper industry often run in campaigns, leading to large variations in the composition of wastewaters and waste sludges. During anaerobic digestion (AD) of these wastes, the viscosity or the production of extracellular polymeric substances (EPS) and soluble microbial products (SMP) may be affected, with the risk of foam formation, inefficient digester mixing or poor sludge dewaterability. The aim of this study was to investigate how viscosity and production of EPS and SMP during long-term AD of pulp and paper mill sludge is affected by changes in organic loading rate (OLR) and hydraulic retention time (HRT). Two mesophilic lab-scale continuous stirred tank reactors (CSTRs) were operated for 800 days (R1 and R2), initially digesting only fibre sludge, then co-digesting fibre sludge and activated sludge. The HRT was lowered, followed by an increase in the OLR. Reactor fluids were sampled once a month for rheological characterization and analysis of EPS and SMP. The production of the protein fraction of SMP was positively correlated to the OLR, implicating reduced effluent qualities at high OLR. EPS formation correlated with the magnesium content, and during sulphate deficiency, the production of EPS and SMP increased. At high levels of EPS and SMP, there was an increase in viscosity of the anaerobic sludges, and dewatering efficiency was reduced. In addition, increased viscosity and/or the production of EPS and SMP were important factors in sludge bulking and foam formation in the CSTRs. Sludge bulking was avoided by more frequent stirring.
... These are less well investigated than sewage water, but they are characterized by a high content of carbon and a low content of other nutrients such as nitrogen, phosphorus, trace elements, etc. Therefore, they should be supplemented with other materials or additives to ensure an optimal digestion process , Ekstrand et al 2016, Meyer & Edwards 2014. The content of dry solids is higher than for wastewater, so it is possible to use CSTR processes. ...
... This can be improved through pre-treatment to break up the structures (Meyer & Edwards 2014). Methane potentials have been reported to be similar to those obtained for wastewaters (Meyer et al 2014, Ekstrand et al 2016. ...
The microbiology of the biogas process
This handbook describes the biogas process (anaerobic digestion), the different groups of microorganisms that are active in degradation, their nutritional and environmental requirements, and how the operating parameters of biogas plants can be adapted to them. The handbook also describes the various organic materials (substrates) that are suitable for digestion, the strategies that can be pursued to achieve an efficient and stable biogas process, and the production and use of the organic residues or digestate (bio-manure). In addition, methods for monitoring and control of the biogas process are described, as well as ways to avoid and remedy common problems.
Each chapter contains a number of study questions/exercises with answers. The information and knowledge summarized in this handbook is supported by full references to the relevant scientific publications.
... They are commonly used owing to their simple design, easy adjustment of operating conditions (pH, temperature), and their stirring action, which facilitates homogeneous conditions of the medium and provides a good contact between the microorganism and substrate. 61 Ekstrand et al. 105 studied the codigestion of paper and pulp wastewater sludge and activated sludge in two CSTRs in series with sludge recirculation. The group showed that the codigestion of the nutrient-poor fiber sludge with activated sludge demonstrated a higher stability of the AD and high biomethane yield of 230 N mL/gVS. ...
Alternatives in biofuels today include three main sources of renewable energy derived from biomass: biodiesel, bioethanol, and biogas. Biofuels can be first generation (produced from food crops; using sugars, starches, and vegetable oils) or second generation (produced from all forms of lignocellulosic biomass) fuels. Renewable energy production from energy crops and agricultural or industrial waste or wastewater; for example, crops and agricultural residues, domestic wastewater, industrial wastewater, sewage sludge, and animal manure, is a viable solution because it provides the correct destination for various types of waste and facilitates energy production. This chapter presents an overview of biogas production, as well as applications for an efficient bioenergy recovery from anaerobic systems. Aspects regarding renewable energy sources, anaerobic processes, and biogas production, including some characteristics of biogas purification and upgrading, are presented. Overall, this text compiles critical information to better understand the potential of anaerobic biodigestion as a robust processing step for bioenergy production.
... However, this efficiency comes at high capital and operational costs, as well as advanced technology requirements [1,2]. High-rate anaerobic digestion biotechnology has proven to be an excellent process and is considered by many authors to be the core of sustainable waste management techniques [1,[3][4][5][6][7]. ...
The up-flow anaerobic sludge blanket (UASB) process has emerged as a promising high-rate anaerobic digestion technology for the treatment of low-to high-strength soluble and complex wastewaters. Sewage, a complex wastewater, contains 30-70% particulate chemical oxygen demand (COD P). These particulate organics degrade at a slower rate than the soluble organics found in sewage. Accumulation of non-degraded suspended solids can lead to a reduction of active biomass in the reactor and hence a deterioration in its performance in terms of acid accumulation and poor biogas production. Hydrolysis of the COD P in sewage prior to UASB reactor will ensure an increased organic loading rate and better UASB performance. While single-stage UASB reactors have been studied extensively, the two-phase full-scale treatment approach (i.e., a hydrolysis unit followed by an UASB reactor) has still not yet been commercialized worldwide. The concept of treating sewage containing particulate organics via a two-phase approach involves first hydrolyzing and acidifying the volatile suspended solids without losing carbon (as methane) in the first reactor and then treating the soluble sewage in the UASB reactor. This work reviews the available literature to outline critical findings related to the treatment of sewage with and without hydrolysis before the UASB reactor.
... Paper-mill sludge is the most abundant by-product in the pulp and paper industry, and its disposal is a serious ecological and economical problem (Battaglia et al. 2003;Geng et al. 2007a). It has been predicted that the global production of paper-mill sludge will rise over the next 50 years by 48-86% from the current levels (Mabee and Roy 2003) and the increasing costs prompted research activities for alternative solutions (Geng et al. 2007b;Migneault et al. 2011;Yan et al. 2011;Chen et al. 2014;Ekstrand et al. 2016). ...
The growth of oyster mushroom (Pleurotus ostreatus) on pulp and paper industry wastes was studied. Specifically, the question was investigated whether solid-state fermentation of P. ostreatus on paper-mill deinking sludge and primary sludge substrates is appropriate for production of enzymes, relevant to the pulp and paper industry. Following fermentation, extracellular protein was extracted and the specific activities of four enzymes were determined, namely, the cellulase, xylanase, lipase and peroxidase. Furthermore, the effects of the pH of the extraction buffer on these enzyme activities were determined, along with the effects of the incubation time. The data show that P. ostreatus can grow on solid wastes from the pulp and paper industry, which could help to minimize the waste volume and to decrease the ecological impact. Furthermore, the solid wastes in focus are good substrates for the production of commercially interesting enzymes.
Food waste have become a growing concern worldwide with raising population and economic growth. Wastewater discharged from food industries contains many valuable and toxic components that have a negative impact on the ecological system. Large amounts of wastewater are discharged from the food industry, which necessitates the creation of effective technologies. Wastewater from the food industry can be seen as a rich source of energy and a primary source for generating valuable products. Waste disposal and resource recovery are sustainably valued by anaerobic digestion of wastewater from the food sector. The characteristics, composition, and nature of wastewater produced from various food sectors are elaborated upon in this review. An overview of the anaerobic digestion process for wastewater treatment in the food industry is included. Enhancement strategies for the anaerobic digestion process have been discussed in detail. In addition, various types of reactors utilized for performing anaerobic digestion is illustrated. Though anaerobic digestion process possesses advantages, the challenges and future scope are examined for improving the outcome.
Lignocellulosic biomass is a promising renewable feedstock for the production of bioenergy and platform chemicals; however, its use would not be economically viable without adopting effective transformation technologies. In this work, long-term anaerobic treatment (370 days) of pulp and paper primary sludge (PS) was performed by a thermophilic anaerobic membrane bioreactor (AnMBR) for the first time to investigate the effect of various solid retention times (SRTs) on biogas production and solids reduction ratios. The tested STRs (32–55 days) have shown varying biogas productivity, and it can be concluded that the longer the SRT, the higher is the biogas yield. At the optimum solids retention time of 55 d, an organic loading rate of 2.15 ± 0.10 kg-TSS/L.d, and hydraulic retention time of 5 d, average biogas yield of 106.4 ± 8.5 m³ biogas/tonne TSS added (approximately 72.7 m³ CH4/t VSadded) was achieved with an average methane content of 56 ± 4% and the solids reduction ratio ranged between 47% and 54.9%. Besides biogas yield, the quality of the permeates, solids reduction, and digestates characteristic/dewaterability changed by changing the SRT. The mixed liquor suspended solids concentrations and solids reduction increased with increasing SRT, while the effluent COD concentration and sludge dewaterability decreased. The Fourier Transform Infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) findings exhibited that the digestate contains more lignin and cellulose than the other substances, while the nitrogen and carbon concentration decreased with increasing SRT.
During the process of pulp-and papermaking, large volumes of fibre-rich primary sludge are generated. Anaerobic digestion of primary sludge offers a substantial potential for methane production as an alternative approach to the inefficient energy recoveries by commonly used incineration techniques. However, a systematic study of the importance of upstream process techniques for the methane potential of pulp fibres is lacking. Therefore, biochemical methane potentials were determined at mesophilic conditions for 20 types of fibres processed by a variety of pulping and bleaching techniques and from different raw materials. This included fibres from kraft, sulphite, semi-chemical, chemical thermo-mechanical (CTMP) and thermo-mechanical pulping plants and milled raw wood. The pulping technique was clearly important for the methane potential, with the highest potential achieved for kraft and sulphite fibres (390-400 Nml CH 4 g VS − 1). For raw wood and CTMP, hardwood fibres gave substantially more methane than the corresponding softwood fibres (240 compared to 50 Nml CH 4 g VS − 1 and 300 compared to 160 Nml CH 4 g VS − 1 , respectively). Nuclear magnetic resonance characterization of the organic content demonstrated that the relative lignin content of the fibres was an important factor for methane production, and that an observed positive effect of bleaching on the methane potential of softwood CTMP fibres was likely related to a higher degree of deacetylation and improved accessibility of the hemicel-lulose. In conclusion, fibres from kraft and sulphite pulping are promising substrates for methane production irrespective of raw material or bleaching, as well as fibres from CTMP pulping of hardwood.
Anaerobic digestion is a versatile biotechnology to treat waste activated sludge (WAS), the main by-products of biological wastewater treatment, because it can achieve simultaneously energy recovery (biogas) and pollutant reduction (organic matter, pathogens). However, the potential of biogas production from mono-digestion of WAS is usually limited by the imbalance carbon to nitrogen (C/N) ratio of WAS and ammonia accumulation. Anaerobic co-digestion, simultaneous digestion of two or more substrates, should be a feasible option to resolve these disadvantages. The abundant organic wastes from municipal, industrial, and agricultural field have been the ideal co-substrates because they not only can balance the substrate nutrient to obtain the optimal C/N ratio, but also can adjust pH and dilute the toxic materials to mitigate the inhibition to methanogens, consequently improving the yield of biogas, especially methane. This paper classified the main organic co-substrates according to their source and reviewed their application in anaerobic co-digestion of WAS. Then the influence of temperature, pH, organic loading rate, hydraulic retention time, C/N ratio, digester type and pretreatment method on biogas production was extensively discussed. Finally, this review brought forward the challenges and outlooks of anaerobic co-digestion in the future.
The pulp and paper industry is a large producer of wastewater and sludge, putting high pressure on waste treatment. In addition, more rigorous environmental legislation for pollution control and demands to increase the use of renewable energy have put further pressure on the pulp and paper industry’s waste treatment, where anaerobic digestion (AD) and the production of methane could pose a solution. Kraft pulping makes up 80% of the world production of virgin wood pulp, thus, the wastewaters from this sector represent a large unused potential for methane production.
There are three main types of substrates available for AD at pulp and paper mills, the wastewaters, the primary sludge/fibre sludge, and the waste activated sludge. AD treatment of these streams has been associated with several challenges, such as the presence of inhibiting compounds or low degradability during AD. The aim of this thesis was to experimentally address these challenges and potentials, focusing on wastes from kraft mills.
Methane potential batch tests showed that many wastewater streams still posed challenges to AD, but the alkaline elemental chlorine-free bleaching stream and the condensate effluents had good methane potentials. Further, the methane potential of kraft mill fibre sludge was high, and co-digestion of kraft mill fibre sludge and waste activated sludge was feasible in stirred tank reactors with sludge recirculation. By increasing the organic loading in a pilot-scale activated sludge facility and thereby lowering the sludge age, the degradability of the waste activated sludge was improved. The higher wastewater treatment capacity achieved by this method provides an opportunity for the mills to increase their pulp and paper production. Further, by dewatering the digestate after AD and returning the liquid to the activated sludge treatment, costs for nutrient supplementation can be reduced.
In conclusion, the thesis shows that AD of wastes from the kraft pulp and paper industry was feasible and carried many benefits regarding the generation of methane as a renewable energy carrier, improved wastewater treatment and reduced costs. Different strategies on how AD may be implemented in the kraft pulp and paper industry were formulated and discussed.
Agricultural and centralized biogas plants are facing a fast development, leading to the production of huge amounts of digestates. Whereas the solid fraction of digestates is used as fertilizer, the liquid fraction can be valorized by the recovery of mineral materials, although the presence of organic compounds may be problematic for the treatment and disposal of this effluent. The objectives of this thesis are: i) to characterize the liquid fraction of digestates and to provide guidelines for its further treatment, ii) to explain the variability of residual compounds in the liquid fraction of digestates in relation with substrate origin, process parameters and the type of solid-liquid separation. In a first part, eleven digestates from full scale codigestion plants were thoroughly characterized. Their liquid fractions were fractionated by nine successive filtrations, which allowed quantifying the contribution of suspended particles, coarse and fine colloids and dissolved matter on several physico-chemical and biological parameters. Organic compounds were mainly found in suspended particles (> 1.2 μm) and presented low aerobic biodegradability. To enlarge the data base, eighteen more digestates from codigestion and one digestate from waste activated sludge were characterized; their liquid fractions were fractionated by two successive filtrations (1.2 μm and 1 kDa). Principal component analysis, hierarchical clustering analysis and correlation matrix carried out on the 30 digestates set, highlighted the major impact of the solid-liquid separation process on the composition of the liquid fraction of digestates. In the high performance separation process group, subgroups allowed separating digestates from sewage sludge, digestates from pig manure and digestates from plug-flow thermophilic processes. In the low performance separation process group, COD and total solids concentration in the liquid fraction of digestates were correlated to the percentage of energy crops and cow manure in the feed. Finally, SUVA parameter which accounts for aromatic compounds content and the stabilization of organic matter, was correlated to the retention time in digester for the whole digestates set. To understand the origin of residual compounds in the liquid fraction of digestates from co-digestion of high proportions of cattle manure, four CSTR reactors fed with wheat straw, cow dung and cow manure were operated for 48 weeks. Anaerobic digestion performances showed that cow manure had undergone some aerobic degradation during its storage at the farm. Therefore, the liquid fraction of manure digestate had the highest concentrations in organic compounds. In addition, these organic compounds had the highest complexity measured by 3D fluorimetry. The low biodegradability of organic compounds and their high percentage in suspended particles suggest the development of physico-chemical separation process such as coagulation for the treatment of the liquid fraction of digestates.
RESUME
Le secteur de la méthanisation à la ferme ou territoriale connait un fort développement, entrainant la production de grandes quantités de digestats. Si la fraction solide de ces digestats est généralement utilisée en tant qu’amendement organique, la fraction liquide peut être valorisée par la récupération des éléments minéraux bien que la présence de composés organiques pose des problèmes de traitement et d’élimination. Les objectifs de cette thèse sont i) de caractériser la fraction liquide des digestats et donner des premiers éléments pour leur futur traitement et ii) d’expliquer la variabilité des composés résiduels présents dans cette fraction liquide en relation avec l’origine des substrats, les paramètres du procédé de méthanisation et le type de séparation solide/liquide. Dans une première partie, nous avons caractérisé de manière détaillée onze digestats issus d’installations industrielles de co-digestion. Le fractionnement par neuf filtrations successives de la fraction liquide des digestats a permis de quantifier la contribution des matières en suspension, des colloïdes grossiers et fins et des matières dissoutes aux différents paramètres physico-chimiques et biologiques. Il a été montré la faible biodégradabilité aérobie des composés organiques dont la majeure partie se trouve dans la matière en suspension (> 1,2 μm). Pour élargir la base de données, nous avons caractérisé dix-huit digestats supplémentaires et un digestat de boues de station d’épuration dont les fractions liquides ont été fractionnées par deux filtrations successives (1,2 μm et 1 kDa). L’analyse en composantes principales, la classification hiérarchique et la matrice de corrélation réalisées sur l’ensemble des 30 digestats ont mis en avant l’impact majeur des techniques de séparation solide/liquide sur la composition de la fraction liquide des digestats. Dans le groupe des techniques de séparation hautement performantes, des sous-groupes ont permis de distinguer les digestats à base de boues d’épuration, ceux à base de lisiers porcins et ceux issus de procédés piston thermophiles. Dans le groupe des procédés de séparation à faible performance, les teneurs en DCO et matières sèches de la fraction liquide des digestats ont été corrélées aux teneurs en cultures énergétiques et fumiers bovins dans l’alimentation. Finalement, pour l’ensemble des digestats, le paramètre SUVA, lié à la teneur en matière aromatique et à la stabilisation des composés, a été corrélé au temps de séjour des digesteurs. Pour comprendre l’origine des composés résiduels observés dans la fraction liquide des digestats issus de la co-digestion avec de fortes proportions de fumier bovin, quatre réacteurs CSTR alimentés avec respectivement de la paille de blé, de la bouse de vache et du fumier ont été conduits pendant 48 semaines. Les performances de méthanisation ont mis en évidence le caractère âgé du fumier qui avait été partiellement dégradé pendant le stockage à la ferme. Ainsi la fraction liquide du digestat du fumier a présenté les plus fortes concentrations en composés organiques qui en outre présentaient une complexité, mesurée par fluorimétrie 3D, plus élevée que les autres substrats. La fraction liquide du digestat issue de la méthanisation de la paille a révélé les plus fortes proportions de DCO dans les fractions colloïdales et dissoutes. La faible biodégradabilité des composés organiques et leur forte proportion dans les matières en suspension suggèrent le développement de procédés physico-chimiques de séparation tels que la coagulation pour le traitement de la fraction liquide des digestats.
The activated sludge process within the pulp and paper industry is generally run to minimize the production of waste activated sludge (WAS), leading to high electricity costs from aeration and relatively large basin volumes. In this study, a pilot-scale activated sludge process was run to evaluate the concept of treating the wastewater at high rate with a low sludge age. Two 150 L containers were used, one for aeration and one for sedimentation and sludge return. The hydraulic retention time was decreased from 24 hours to 7 hours, and the sludge age was lowered from 12 days to 2-4 days. The methane potential of the WAS was evaluated using batch tests, as well as continuous anaerobic digestion (AD) in 4 L reactors in mesophilic and thermophilic conditions. Wastewater treatment capacity was increased almost four-fold at maintained degradation efficiency. The lower sludge age greatly improved the methane potential of the WAS in batch tests, reaching 170 NmL CH4/g VS at a sludge age of 2 days. In addition, the continuous AD showed a higher methane production at thermophilic conditions. Thus, the combination of high-rate wastewater treatment and AD of WAS is a promising option for the pulp and paper industry.
Agricultural and centralized biogas plants are facing a fast development, leading to the production of huge amounts of digestates. Whereas the solid fraction of digestates is used as fertilizer, the liquid fraction can be valorized by the recovery of mineral materials, although the presence of organic compounds may be problematic for the treatment and disposal of this effluent. The objectives of this thesis are: i) to characterize the liquid fraction of digestates and to provide guidelines for its further treatment, ii) to explain the variability of residual compounds in the liquid fraction of digestates in relation with substrate origin, process parameters and the type of solid-liquid separation. In a first part, eleven digestates from full scale codigestion plants were thoroughly characterized. Their liquid fractions were fractionated by nine successive filtrations, which allowed quantifying the contribution of suspended particles, coarse and fine colloids and dissolved matter on several physico-chemical and biological parameters. Organic compounds were mainly found in suspended particles (> 1.2 µm) and presented low aerobic biodegradability. To enlarge the data base, eighteen more digestates from codigestion and one digestate from waste activated sludge were characterized; their liquid fractions were fractionated by two successive filtrations (1.2 μm and 1 kDa). Principal component analysis, hierarchical clustering analysis and correlation matrix carried out on the 30 digestates set, highlighted the major impact of the solid-liquid separation process on the composition of the liquid fraction of digestates. In the high performance separation process group, subgroups allowed separating digestates from sewage sludge, digestates from pig manure and digestates from plug-flow thermophilic processes. In the low performance separation process group, COD and total solids concentration in the liquid fraction of digestates were correlated to the percentage of energy crops and cow manure in the feed. Finally, SUVA parameter which accounts for aromatic compounds content and the stabilization of organic matter, was correlated to the retention time in digester for the whole digestates set. To understand the origin of residual compounds in the liquid fraction of digestates from co-digestion of high proportions of cattle manure, four CSTR reactors fed with wheat straw, cow dung and cow manure were operated for 48 weeks. Anaerobic digestion performances showed that cow manure had undergone some aerobic degradation during its storage at the farm. Therefore, the liquid fraction of manure digestate had the highest concentrations in organic compounds. In addition, these organic compounds had the highest complexity measured by 3D fluorimetry. The low biodegradability of organic compounds and their high percentage in suspended particles suggest the development of physico-chemical separation process such as coagulation for the treatment of the liquid fraction of digestates
Since the 1980s, the pulp and paper industry in Finland has resulted in the accumulation of fibres in lake sediments. One such site in Lake Näsijärvi contains approximately 1.5 million m³ sedimented fibres. In this study, the methane production potential of the sedimented fibres (on average 13% total solids (TS)) was determined in batch assays. Furthermore, the methane production from solid (on average 20% TS) and liquid fractions of sedimented fibres after solid-liquid separation was studied. The sedimented fibres resulted in fast methane production and high methane yields of 250±80 L CH4/kg volatile solids (VS). The main part (ca. 90%) of the methane potential was obtained from the solid fraction of the sedimented fibres. In addition, the VS removal from the total and solid sedimented fibres was high, 61−65% and 63−78%, respectively. The liquid fraction also contained a large amount of organics (on average 8.8 g COD/L), treatment of which also has to be considered. The estimations of the methane production potentials in the case area showed potential up to 40 million m³ of methane from sedimented fibres.
A produção de suínos no Brasil ocorre em escala industrial, com animais confinados e de
forma estratificada, ou seja, têm-se grandes unidades produtoras, especializadas em etapas
específicas da produção. Geralmente, essas unidades concentram grandes quantidades de
animais em pequenas áreas territoriais e geram efluentes ricos em matéria orgânica, que
precisam ser corretamente manejados. Este estudo foi divido em duas etapas, sendo que na
primeira objetivou-se determinar a influência da estratificação da produção de suínos na
capacidade de produção de biogás, estudando também estratégias de separação de sólidos
que contribuam para maior rendimento de metano. Para isso, foram coletadas amostras de
duas unidades de gestações (GS-a e GS-b), duas unidades de maternidade (MS-a e MS-b),
uma unidade de crechário (CS) e uma unidade de terminação (TS). Os testes de potencial
bioquímico de biogás (PBB) foram realizados de acordo com procedimento padrão (VDI
4630). A fração de lodo sedimentado correspondeu de 20 a 30% do volume do efluente
bruto, produzindo de 40 a 60% do volume de metano. A produtividade de metano da fração
de lodo sedimentado é aproximadamente duas vezes maior do que a fração sobrenadante.
Verificaram-se diferenças no rendimento de biogás entre o dejeto bruto proveniente das
diferentes etapas da criação de suínos (GS-a: 326,4 e GS-b: 577,1; MS-a: 860,1 e MS-b:
479,2; CS -970,2; TS 474,5 mLN biogás.gSV-1
). As diferenças apresentadas são relativas às
diferentes estratégias adotadas nas diferentes fases de crescimento do suíno (nutrição,
tecnificação da produção, etc.), mas também relacionadas ao manejo dos efluentes dentro
das instalações. Na segunda etapa, buscou-se estudar a digestão anaeróbia em reator
CSTR (do inglês “Continuous Stirred Tank Reactor”) com o objetivo de verificar a influência
de parâmetros do processo na capacidade de recuperação de metano e suas influências
nas arqueas metanogênicas. Foram realizados experimentos em batelada (utilizando
metodologia padronizada – VDI, 4630) e contínuos (em reator CSTR), com objetivo de
atingir a maior produtividade e menor emissão de metano (gás de efeito estufa). Foram
estudados parâmetros operacionais, como carga orgânica volumétrica (COV), tempo de
retenção hidráulica (TRH), amônia livre e comunidade de arqueas. Observou-se que a COV
de 1,06 kgSV.m-3
.d-1
e TRH de aproximadamente 20 dias contribuem para melhor
recuperação de metano, o que resulta em um ganho de produtividade em uma planta de
biogás e redução nas emissões de metano. Os resultados de concentrações de amônia livre
não apontam evidências de inibição ao processo de digestão anaeróbia, mas provavelmente
influenciam a rota metanogênica preferencial (rota hidrogenotrófica). Adicionalmente,
observou-se uma correlação entre a estabilidade do processo e o aumento na concentração
de arqueas metanogênicas. Os resultados podem servir de subsídio para plantas de biogás
em alcançar maior produtividade e menor emissão de metano.
The effect of adding four minerals as micronutrient sources to stimulate the anaerobic process of
microcrystalline cellulose (MCC) was investigated. Three mineral doses (5, 10 and 15 mg/L) were evaluated
and the response to trace metal addition was monitored by the methane yield (YCH4) and specific methanogenic activity (SMA). A clear stimulation of the YCH4 and SMA on MCC due to the presence of minerals with a high content of trace elements was observed, respectively: the YCH4 and SMA of the sludge with 5 mg/L of high-Fe-Mg mineral increased to 397 NmLCH4/gVSsubstrate and 0.267 gCODNmLCH4/gVSinoculum/d compared to 303 NmLCH4/gVSsubstrate and 0.205 gCODNmLCH4/gVSinoculum/d in a medium without adding mineral. An increase in the doses of high-Ca mineral highly decreased methane production and process stability, due to a possible inhibition of the anaerobic digestion by calcium ions.
Non-productive cellulase adsorption onto lignin has always been deemed to negatively affect the enzymatic hydrolysis of lignocellulosic feedstocks. Therefore, understanding enzyme-lignin interactions is essential for the development of enzyme mixtures, the processes of lignocellulose hydrolysis, and the genetic modification of lignocellulosic biomass and enzymes. In this work, we examined the properties of six lignins from diverse types of lignocellulosic biomass (aspen, pine, corn stover, kenaf, and two Arabidopsis lines, wild-type and SALK mutant of fah1) to determine the mechanism of differences in their adsorption of enzymes.
We found that lignin sources affected enzyme adsorption using structural features, such as functional groups and lignin composition. Guaiacyl (G) lignin had a higher adsorption capacity on enzymes than syringyl (S) lignin. The low S/G ratio and high uniform lignin fragment size had good correlations with high adsorption capacity. A higher content of phenolic hydroxyl groups and a lower content of carboxylic acid groups resulted in stronger adsorption affinity for corn stover lignin (CL) than for kenaf lignin (KL) and aspen lignin (AL). The lower amount of aliphatic hydroxyls that reduced hydrophobic interactions could explain the higher adsorption capacity of pine lignin (PL) than CL. Enzyme activity assays, as well as the hydrolysis of Avicel, phosphoric acid-swollen cellulose (PASC), and holocellulose, were performed to study the behaviors of mono-component enzymes that resulted in adsorption. We found that cellobiohydrolase (CBH) and xylanase were adsorbed the most by all lignins, endoglucanase (EG) showed less inhibition, and beta-glucosidase (BG) was the least affected by lignins, indicating the important role of carbohydrate-binding module (CBM) in protein adsorption.
Lignin sources affect enzyme adsorption using structural features and lignin composition, such as S/G ratio, carboxylic acid, aliphatic hydroxyl, and phenolic hydroxyl. For mono-component enzymes, the adsorption capacity decreased in the order CBH, xylanase > EG > BG. These investigations revealed the difference in lignin properties between diverse biomass and adsorption capacity of enzymes to lignins, and the possible underlying mechanism. The results can also serve as a reference for the genetic modification of lignocellulosic biomass and enzymes.
The elemental composition of 10 methanogenic species was determined by inductively coupled plasma emission spectrometry and by a C-H-N-analyzer. The 10 species were representative of all three orders of the methanogens and were cultivated under defined conditions. Special emphasis was given toMethanosarcina barkeri, represented by 5 strains and cultivated on various substrates. The following elements with the lowest and highest values in parentheses were determined: C (37-44%, w/w), H (5.5-6.5%), N (9.5-12,8%); Na (0.3-4.0%), K (0.13-5.0%), S (0.56-1.2%), P (0.5-2.8%), Ca (order I: 85-550 ppm; order II: 1000-4500 ppm), Mg (0.09-0.53%), Fe (0.07-0.28%), Ni (65-180 ppm), Co (10-120 ppm). Mo (10-70 ppm), Zn (50-630 ppm), Cu (<10-160 ppm), Mn (<5-25 ppm). The biggest variations were found with respect to N and K, which both seem to have important physiological functions. Although it is unknown whether zinc and copper are essential trace elements for methanogens, all investigated species contained remarkably high zinc contents, whereas copper seemed to be present only in some species.
This paper presents an inclusive approach with focus on energy use and recovery in wastewater management, including wastewater treatment (WWT) and sludge handling. Process data from three Swedish mills and a mathematical model were used to evaluate seven sludge handling strategies. The results indicate that excess energy use in WWT processes counters the potential energy recovery in the sludge handling systems. Energy use in WWT processes is recommended to aim for sufficient effluent treatment, not for sludge reduction. Increased secondary sludge production is favourable from an energy point of view provided it is used as a substrate for heat, biogas or electricity production.
A methanogenic bacterium, commonly seen in digested sludge and referred to as the "fat rod" or Methanobacterium soehngenii, has been enriched to a monoculture and is characterized. Cells are gramnegative, non-motile and appear as straight rods with flat ends. They form filaments which can grow to great lengths. The structure of the outer cell envelop is similar to Methanospirillum hungatii. The organism grows on a mineral salt medium with acetate as the only organic component. Acetate is the energy source, and methane is formed exclusively from the methyl group. Acetate and carbon dioxide act as sole carbon source and are assimilated in a molar ratio of about 1.9:1. The reducing equivalents necessary to build biomass from these two precursors are obtained from the total oxidation of some acetate. Hydrogen is not used for methane formation and is not needed for growth. Formate is cleaved into hydrogen and carbon dioxide. Coenzyme M was found to be present at levels of 0.35 nmol per mg of dry cells and F420 amounted to 0.55 microgram per mg protein. The mean generation time was 9 days at 33 degrees C.
The conversion of lignocellulosic biomass to fuel ethanol typically involves a disruptive pretreatment process followed by enzyme-catalyzed hydrolysis of the cellulose and hemicellulose components to fermentable sugars. Attempts to improve process economics include protein engineering of cellulases, xylanases and related hydrolases to improve their specific activity or stability. However, it is recognized that enzyme performance is reduced during lignocellulose hydrolysis by interaction with lignin or lignin-carbohydrate complex (LCC), so the selection or engineering of enzymes with reduced lignin interaction offers an alternative means of enzyme improvement. This study examines the inhibition of seven cellulase preparations, three xylanase preparations and a beta-glucosidase preparation by two purified, particulate lignin preparations derived from softwood using an organosolv pretreatment process followed by enzymatic hydrolysis. The two lignin preparations had similar particle sizes and surface areas but differed significantly in other physical properties and in their chemical compositions determined by a 2D correlation HSQC NMR technique and quantitative 13C NMR spectroscopy. The various cellulases differed by up to 3.5-fold in their inhibition by lignin, while the xylanases showed less variability (< or = 1.7-fold). Of all the enzymes tested, beta-glucosidase was least affected by lignin.
With the final aim of reducing the energy consumption and increase the methane production at Swedish pulp and paper mills, the methane potential of 62 wastewater effluents from 10 processes at seven pulp and/or paper mills (A–G) was determined in anaerobic batch digestion assays. This mapping is a first step towards an energy efficient and more sustainable utilization of the effluents by anaerobic digestion, and will be followed up by tests in lab-scale and pilot-scale reactors. Five of the mills produce kraft pulp (KP), one thermo-mechanical pulp (TMP), two chemical thermo-mechanical pulp (CTMP) and two neutral sulfite semi-chemical (NSSC) pulp. Both elementary and total chlorine free (ECF and TCF, respectively) bleaching processes were included. The effluents included material from wood rooms, cooking and oxygen delignification, bleaching (often both acid- and alkali effluents), drying and paper/board machinery as well as total effluents before and after sedimentation.
Anaerobic digestion often generates ‘biogas’ – an approximately 3:1 mixture of methane and carbon dioxide – which has been known to be a ‘clean’ fuel since the late 19th century. But a great resurgence of interest in biogas capture – hence methane capture – has occurred in recent years due to the rapidly growing spectre of global warming. Anthropogenic causes which directly or indirectly release methane into the atmosphere, are responsible for as much as a third of the overall additional global warming that is occurring at present. Hence the dual advantage of methane capture – generating energy while controlling global warming – have come to the fore.
The impact of lignins of various origins on filter paper hydrolysis by fungal cellulase was evaluated. Powdered lignins were added to enzyme incubations, either as isolated or after thorough hydroxypropylation of phenolic sites. Extent of cellulose hydrolysis was reduced by 14−60% by the addition of up to 15% lignin to the substrate. Unmodified lignins were more detrimental to cellulose hydrolysis than hydroxypropylated lignins. The inhibitory effect of lignin addition was only partially overcome by a 10-fold increase in cellulase activity, suggesting inhibitory lignin interactions with both substrate and enzyme. Preincubation of cellulase with underivatized lignins resulted in reduced enzyme activity and soluble protein concentration in the supernatant, suggesting protein precipitation with lignin rather than reduced activity of a lignin−enzyme complex as the inhibitory mechanism. Two further experiments showed that the negative impact of lignin on cellulose hydrolysis can be counteracted by addition of various N compounds and by ammoniation. Keywords: Lignin; cellulose degradation; cellulase inhibition; free phenolic group; ammoniation; PEG; PVP; N compounds
Anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge were studied for the first time in semi-continuously fed continuously stirred tank reactors (CSTR) in thermophilic conditions. Additionally, in batch experiments, methane potentials of 210 and 230 m³CH₄/t volatile solids (VS)(added) were obtained for primary, and 50 and 100 m³CH₄/tVS(added) for secondary sludge at 35 °C and 55 °C, respectively. Anaerobic digestion of primary sludge was shown to be feasible with organic loading rates (OLR) of 1-1.4 kgVS/m³d and hydraulic retention times (HRT) of 16-32 d resulting in methane yields of 190-240 m³CH₄/tVS(fed). Also the highest tested OLR of 2 kgVS/m³d and the shortest HRT of 14-16 d could be feasible, if pH stability is confirmed. Co-digestion of primary and secondary sludge with an OLR of 1 kgVS/m³d and HRTs of 25-31 d resulted in methane yields of 150-170 m³CH₄/tVS(fed). In the digestion processes, cellulose and hemicellulose degraded while lignin did not. pH adjustment and nitrogen deficiency needs to be considered when planning anaerobic digestion of pulp and paper mill wastewater sludges.
The effects of different concentrations of Mg2+, Ca2+, or Na+ on the morphology and growth of Methanosarcina thermophila TM-1 growing on acetate at concentrations comparable with those found in anaerobic digestors was studied. At 30 mm Mg2+ or less, M. thermophila grew as large aggregates that settled rapidly. At 100 mm Mg2+ or more, the bacteria grew as single cells or a mixture of single cells and small aggregates is suspended culture. Mg2+ was necessary for growth and could not be substituted by addition of either Ca2+ or Na+. The optimal Mg2+ concentration was 30 mm and no growth was observed at 400 mm Mg2+. Cultures could be adapted to 300 mm Mg2+ without a change in growth rate. Added Ca2+ was not required for growth and had no effect on cell morphology. Inhibition by Na+ was directly related to the Mg2+ concentration. When the Mg2+ was 0.05 mm or less, 0.35 m Na+ completely inhibited growth. However, more Na+ was required for inhibition at higher Mg2+ concentrations. The same inhibitory effect of Na+ was observed when the temperature was 52C or 45C. The potential for disaggregation of Methanosarcina aggregates in anaerobic digestor environments was discussed.
When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant. Although different disposal routes are possible, anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas (60–70 vol% of methane, CH4), as thereby it also reduces the amount of final sludge solids for disposal whilst destroying most of the pathogens present in the sludge and limiting odour problems associated with residual putrescible matter. Anaerobic digestion thus optimises WWTP costs, its environmental footprint and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source has long been widely recognised and current techniques are being developed to upgrade quality and to enhance energy use. The present paper extensively reviews the principles of anaerobic digestion, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active cures, and the recent developments to reduce the impact of the problems. After having reviewed the basic principles and techniques of the anaerobic digestion process, modelling concepts will be assessed to delineate the dominant parameters. Hydrolysis is recognised as rate-limiting step in the complex digestion process. The microbiology of anaerobic digestion is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. As will be shown, these groups are sensitive to and possibly inhibited by several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, sodium, potassium, heavy metals, volatile fatty acids and others. To accelerate the digestion and enhance the production of biogas, various pre-treatments can be used to improve the rate-limiting hydrolysis. These treatments include mechanical, thermal, chemical and biological interventions to the feedstock. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species. Possible techniques to upgrade the biogas formed by removing CO2, H2S and excess moisture will be summarised. Special attention will be paid to the problems associated with siloxanes (SX) possibly present in the sludge and biogas, together with the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or eliminate the SX from the biogas by adsorption or other techniques. The reader will finally be guided to extensive publications concerning the operation, control, maintenance and troubleshooting of anaerobic digestion plants.
Anaerobic digestion is an attractive waste treatment practice in which both pollution control and energy recovery can be achieved. Many agricultural and industrial wastes are ideal candidates for anaerobic digestion because they contain high levels of easily biodegradable materials. Problems such as low methane yield and process instability are often encountered in anaerobic digestion, preventing this technique from being widely applied. A wide variety of inhibitory substances are the primary cause of anaerobic digester upset or failure since they are present in substantial concentrations in wastes. Considerable research efforts have been made to identify the mechanism and the controlling factors of inhibition. This review provides a detailed summary of the research conducted on the inhibition of anaerobic processes. The inhibitors commonly present in anaerobic digesters include ammonia, sulfide, light metal ions, heavy metals, and organics. Due to the difference in anaerobic inocula, waste composition, and experimental methods and conditions, literature results on inhibition caused by specific toxicants vary widely. Co-digestion with other waste, adaptation of microorganisms to inhibitory substances, and incorporation of methods to remove or counteract toxicants before anaerobic digestion can significantly improve the waste treatment efficiency.
Achievement of efficient enzymatic degradation of cellulose to glucose is one of the main prerequisites and one of the main challenges in the biological conversion of lignocellulosic biomass to liquid fuels and other valuable products. The specific inhibitory interferences by cellobiose and glucose on enzyme-catalyzed cellulose hydrolysis reactions impose significant limitations on the efficiency of lignocellulose conversion - especially at high-biomass dry matter conditions. To provide the base for selecting the optimal reactor conditions, this paper reviews the reaction kinetics, mechanisms, and significance of this product inhibition, notably the cellobiose and glucose inhibition, on enzymatic cellulose hydrolysis. Particular emphasis is put on the distinct complexity of cellulose as a substrate, the multi-enzymatic nature of the cellulolytic degradation, and the particular features of cellulase inhibition mechanisms and kinetics. The data show that new strategies that place the bioreactor design at the center stage are required to alleviate the product inhibition and in turn to enhance the efficiency of enzymatic cellulose hydrolysis. Accomplishment of the enzymatic hydrolysis at medium substrate concentration in separate hydrolysis reactors that allow continuous glucose removal is proposed to be the way forward for obtaining feasible enzymatic degradation in lignocellulose processing.
Anaerobic digestion foaming has been encountered in several sewage treatment plants in the UK. Foaming has raised major concerns for the water companies due to significant impacts on process efficiency and operational costs. Several foaming causes have been identified over the past few years by researchers. However, the supporting experimental information is limited and in some cases absent. The present report aims to provide a detailed review of the current anaerobic digestion foaming problem and to identify gaps in knowledge regarding the theory of foam formation in anaerobic digesters.
Retention mechanisms of an unmodified and a hydroxylated polystyrene-divinylbenzene polymer were studied by solid-phase extraction of o-phthalic acid and some of its mono- and diesters from purified water and then analysing by GC-MS. The monoesters and phthalic acid were retained only when protonated (i.e. acidified with HCI to pH 0.9). Of all elution solvents tested, ethyl acetate gave the best overall recoveries (61-89%) with both polymers. Applicability to complex matrixes (e.g. acidogenic landfill leachates) was examined by introducing a washing step with acetone in acidified water (pH 0.9) to eliminate volatile fatty acids (C2-C6) from the cartridge. Finally, the method was tested on real samples.
Pulp and paper mills generate varieties of pollutants depending upon the type of the pulping process. This paper is the state of the art review of treatability of the pulp and paper mill wastewater and performance of available treatment processes. A comparison of all treatment processes is presented. Combinations of anaerobic and aerobic treatment processes are found to be efficient in the removal of soluble biodegradable organic pollutants. Color can be removed effectively by fungal treatment, coagulation, chemical oxidation, and ozonation. Chlorinated phenolic compounds and adsorable organic halides (AOX) can be efficiently reduced by adsorption, ozonation and membrane filtration techniques.
Eleven million tonnes of waste are produced yearly by the European pulp and paper industry, of which 70% originates from the production of deinked recycled paper. Wastes are very diverse in composition and consist of rejects, different types of sludges and ashes in mills having on-site incineration treatment. The production of pulp and paper from virgin pulp generates less waste but the waste has similar properties to waste from the production of deinked pulp, although with less inorganics. Due to legislation and increased taxes, landfills are quickly being eliminated as a final destination for wastes in Europe, and incineration with energy recovery is becoming the main waste recovery method. Other options such as pyrolysis, gasification, land spreading, composting and reuse as building material are being applied, although research is still needed for optimization of the processes. Due to the large volumes of waste generated, the high moisture content of the waste and the changing waste composition as a result of process conditions, recovery methods are usually expensive and their environmental impact is still uncertain. For this reason, it is necessary to continue research on different applications of wastes, while taking into account the environmental and economic factors of these waste treatments.
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