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Wastewater and sludge
valorisation: a novel approach for
treatment and resource recovery
to achieve circular economy
concept
Srujana Kathi
1
, Siril Singh
2
,
3
, Rajni Yadav
3
, Anand Narain Singh
3
*
and Alaa El Din Mahmoud
4
,
5
1
UGC-Human Resource Development Centre, Pondicherry University, Puducherry, India,
2
Department of
Environment Studies, Panjab University, Chandigarh, India,
3
Soil Ecosystem and Restoration Ecology Lab,
Department of Botany, Panjab University, Chandigarh, India,
4
Environmental Sciences Department,
Faculty of Science, Alexandria University, Alexandria, Egypt,
5
Green Technology Group, Faculty of
Science, Alexandria University, Alexandria, Egypt
Global demand for freshwater is rapidly escalating. It is highly essential to keep
pace with the necessities of the increasing population. The effluents of wastewater
are gradually identified as a reservoir of resources for energy generation and
economic boom. Henceforth, most wastewater and sludge have great potential
for reuse and recycling. The re-utilization and valorization of wastewater and
sludge contribute to accomplishing sustainable development goals, combating
water scarcity, and alleviating adverse environmental impacts of wastewater on
the environmental components. The present article highlights the most novel
approaches for wastewater treatment for the waste valorization of different
industrial origins and the generation of value-added products and recovery of
biopolymers, vitamins, enzymes, dyes, pigments, and phenolic compounds. We
highlighted the life cycle assessment and techno-economic analysis. In addition,
we have addressed a critical overview of the barriers to the large-scale application
of resource recovery strategies and economic, environmental, and social
concerns associated with using waste-derived products.
KEYWORDS
resource recovery, SDG 6, value-added products, sustainable development goal, circular
economy, waste valorization
1 Introduction
An essential requirement for human civilisation is having access to clean and fresh water.
The exploitation of non-renewable natural resources has led to their depletion and raised
environmental concerns due to the world’s rapid population growth and economic
development. Water is one of the natural resources that has been impacted by multiple
issues, including poor quality, scarcity, and lack of access to clean water (Scanlon et al., 2023).
The shrinking of freshwater resources has emerged as one of the significant global issues in
the twenty-first century faced by the human race. The ability of the planet’s natural resources
to sustainably meet the rising demand has also grown to be a challenging problem due to
population growth, urbanisation, and industrialisation.
OPEN ACCESS
EDITED BY
Surindra Suthar,
Doon University, India
REVIEWED BY
Daniel Pinto Fernandes,
Federal University of Alagoas, Brazil
Vinay Kumar Tyagi,
National Institute of Hydrology, India
*CORRESPONDENCE
Anand Narain Singh,
dranand1212@gmail.com
RECEIVED 22 December 2022
ACCEPTED 05 April 2023
PUBLISHED 27 April 2023
CITATION
Kathi S, Singh S, Yadav R, Singh AN and
Mahmoud AED (2023), Wastewater and
sludge valorisation: a novel approach for
treatment and resource recovery to
achieve circular economy concept.
Front. Chem. Eng. 5:1129783.
doi: 10.3389/fceng.2023.1129783
COPYRIGHT
© 2023 Kathi, Singh, Yadav, Singh and
Mahmoud. This is an open-access article
distributed under the terms of the
Creative Commons Attribution License
(CC BY). The use, distribution or
reproduction in other forums is
permitted, provided the original author(s)
and the copyright owner(s) are credited
and that the original publication in this
journal is cited, in accordance with
accepted academic practice. No use,
distribution or reproduction is permitted
which does not comply with these terms.
Frontiers in Chemical Engineering frontiersin.org01
TYPE Review
PUBLISHED 27 April 2023
DOI 10.3389/fceng.2023.1129783
In fact, in the 2030 Agenda towards sustainable development
goals of the United Nations, the sixth goal was established as a goal
for clean water and sanitation to guarantee sanitation and water
accessibility and sustainable management worldwide (Mahmoud
et al., 2021a). In the past few years, urbanisation has led to the
generation of a considerable amount of solid waste and wastewater
GRAPHICAL ABSTRACT
FIGURE 1
The circular economy concept of waste valorisation and resource recovery from wastewater and sludge. Adapted from Singh et al., 2022, with
permission from Elsevier.
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Kathi et al. 10.3389/fceng.2023.1129783
effluents disposed of in nearby water bodies without complete
treatment (Markandeya and Shukla, 2022). The release of such
wastes and effluents into the environment has many
repercussions like eutrophication, groundwater leaching,
spreading of pathogens, increase in waterborne diseases, and
deterioration of water body aesthetics (Bashir et al., 2020;Jadon
et al., 2022). The current wastewater treatment plants are now being
examined as a potential reservoir of resources for nutrient recovery
and the generation of value-added products (Singh et al., 2022).
Previously, these facilities were designed to treat wastewater to meet
the essential physical, biological and chemical standards before
disposing it into the environment (Bora et al., 2020). Utilising
sewage sludge and wastewater as a source of nutrients and value-
added goods is essential for the world’s transformation from a linear
to a circular economy when resource availability is constrained to
lessen the mounting stress on our water resources (Puyol et al.,
2017). High-end wastewater treatment systems and advanced waste
management technologies offer multiple ways for nutrient recovery
and transformation to value-added products. Establishing a circular
resource flow can help the water sector alleviate water scarcity and
generate additional revenue (Singh et al., 2022)(Figure 1).
Wastewater and sludge, which are now recognised as valuable
sources of nutrients like phosphorus and ammonium, contain
massive volumes of both organic and inorganic compounds
(Meena et al., 2022). The world’s population is increasing, which
means that more food needs to be produced. To do this, farmers use
fertilisers which contain phosphorous. To make these fertilisers, two
substances called ammonium and phosphate are needed. Studies
have shown that the demand for these two substances is increasing
as more fertilisers are needed. (Tilman et al., 2002;Gong et al., 2022).
Partially or untreated wastewater containing excess
phosphorous and ammonium released in surface waters may lead
to eutrophication and imbalance in the aquatic ecosystem; therefore,
removing these nutrients is essential to maintain healthy aquatic life
(Diaz et al., 2022;el-Maghrabi et al., 2022). Moreover, the depletion
of natural phosphorous resources from agroecosystems has become
a global issue (Sattari et al., 2012;Alewell et al., 2020). Therefore,
wastewater and sludge valorisation have multiple benefits of nutrient
recovery, generation of value-added products, sustainable
wastewater treatment, alleviating water scarcity, additional
revenue generation, and reduction in environmental
contamination (Singh et al., 2022).
Conventional wastewater treatment plants and their integrated
system are primarily designed to treat wastewater to reduce organic
and suspended solid load and meet the basic discharge requirements
(Mahmoud and Kathi, 2022). Traditional physical techniques
mainly use various screens, settling tanks and adsorbents with
enhanced adsorption efficiency and offer a modest process for
wastewater treatment (Kaviya, 2022). Emerging high-end
technologies and advanced membrane systems empowered by
hydraulic and osmotic pressure, thermal and electrical systems
are promising technologies for enhanced treatment and
valorisation, leading to resource recovery and generation of
value-added products (Xu et al., 2017;Ray et al., 2020). For the
oxidative elimination of contaminants, chemical methods utilising
radical chemistry and electrochemical technologies based on the
catalytic electrode have also received considerable attention recently
(Logan and Regan, 2006;Tarpeh et al., 2018). The latter is
predominantly operative for highly saline wastewaters, especially
for brine and tannery wastewaters. Biological techniques such as
bioelectrochemical systems, enzymatic catalysis, and microalgae
cultivation have been employed to remediate wastewater of a
wide range along with the generation of various value-added
compounds (Singh et al., 2022;Yadav et al., 2022). In contrast,
biological approaches are more technologically sound than Physico-
chemical approaches available for wastewater treatment and
valorisation due to their sturdiness, goal-specific, eco-friendly and
resilient characteristics. However, the capital and operational costs
used for wastewater treatment and valorisation are important to
consider while deciding which method to use, as they can have a big
impact on the overall cost of the project.
Therefore, the present article highlights the most novel
approaches for wastewater valorisation and the generation of
value-added products (such as biofuels, biofertilisers, and
biopesticides) and recovery of biopolymers, vitamins, enzymes,
dyes, pigments, and phenolic compounds. In addition, we have
addressed the life cycle assessment and a critical overview of the
barriers to the large-scale application of resource recovery strategies
and economic, environmental, and social concerns associated with
using waste-derived products.
FIGURE 2
Pressure driven membrane systems with their characteristic properties.
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Kathi et al. 10.3389/fceng.2023.1129783
2 Novel approaches for wastewater
treatment
2.1 Membrane technology
Membrane separation is one of the cutting-edge technologies for
wastewater treatment. Here, a permeable membrane is used to let
wastewater pass through it. Any solute larger than the membrane’s
pore size will be restricted, while the remaining solution passes
through the membrane. The nutrients/solutes trapped in the filter
cake are continuously removed throughout this filtration process to
ensure the consistence of filtration process. Based on the size of the
pores present in the membranes, we may categorise the membrane
separation processes (Mahmoud et al., 2022a) into four categories,
(i) RO/FO (Reverse osmosis/Forward osmosis), (ii) NF
(Nanofiltration), (iii) UF (Ultrafiltration), and (iv) MF
(Microfiltration) (Figure 2).
Pore size, structure, and operational pressure are the
fundamental properties of pressure-driven membranes. MF uses a
precise sieving mechanism along with cutting-edge,
environmentally friendly technology. It has extensive industrial
applications in food and dairy industry, textile industry,
biotechnology, and pharmaceutical industry (Foorginezhad and
Zerafat, 2017;Khan and Boddu, 2021). The feasibility of MF was
reported by Marchesi et al. (2019) for the recycling and reuse of
chicken pre-chiller wastewater owing to its high rejection efficiency
(up to 92.5% of COD, 89.1% of TOC, and 100% of microbes).
However, UF membranes work on the principle of size exclusion,
which have pores with a size range of 0.005–0.1 μm. However,
additional elements, including molecular structure, charge, and
hydrodynamic circumstances, can also significantly impact the
rejection efficiency (al Aani et al., 2020). The use of UF for
separating proteins from food industry wastewater has been
reported frequently. NF membranes’pore sizes range from 0.1 to
10 mm. It can display intense permeate flux and significant
macromolecule rejection. While NF has been used to recover
comparatively smaller molecules such as lactose and phenols (Lee
and Stuckey, 2022). According to Pronk et al. (2006), the NF
membrane have potential to discard micropollutants such as
phosphate, diclofenac, propanol, carbamazepine, and ibuprofen
while allowing nitrogen to pass through for its recovery.
NF and RO are high-pressure membrane processes, therefore,
frequently employed to recover ammonium from greywater as they
significantly reject the salts and ions (Adam et al., 2018;Ray et al.,
2020). In the RO process, feed water is propelled through a
semipermeable membrane from a dilute to a concentrated
TABLE 2 Enzymes used in wastewater treatment with removal efficiencies.
Enzymes Form of
enzyme
Pollutants Wastewater Removal
efficiency
References
Laccase Immobilized Phenolic compounds 80% Yadav et al. (2021)
Laccase Free form Dyes textile wastewater 50% Motamedi et al. (2021)
Lipase Immobilised Melanoidins Sugar mill wastewater 84%–86% Wei et al. (2022)
Lacasse Immobilised Meat industry
wastewater
Thirugnanasambandham and Sivakumar
(2015)
Horseradish
peroxidase
Immobilised Reactive Black dye and
Malachite green
70% Jankowska et al. (2021)
TABLE 1 Different membrane technologies with their removal efficiency.
Wastewater type Membrane Material Removal efficiency References
MF Polyether sulfone 92.5% COD, 82.5% TN Marchesi et al. (2019)
Textile wastewater FO Aquaporin based >94% COD, >99% TDS, TSS, Zn
2+
,andSO
4
, 55% water
recovery
Korenak et al. (2019)
Acidic wastewater NF 94% heavy metal ions You et al. (2017)
Digested swine wastewater FO >93% ammonium nitrogen, >50% water recovery Wu et al. (2018)
Molasses distillery wastewater 85.2% COD, 97.3% melanoidins, 65% water recovery Singh et al. (2018)
Activated sludge from wastewater
treatment Plants (WWTP)
FO Flat-sheet CTA 96% ammonium nitrate, 98% phosphate, 99% OC Nguyen et al. (2013)
Municipal primary effluent FO-RO 100% ammonia, fluoride, and phosphate, 90% CaSO
4
and OC, 86%–89% K and Mg
Jiang et al. (2020)
Treated sewage effluent FO Flat sheet TFC 99%, total phosphorus, 97% ammonium Hafiz et al. (2019)
Textile wastewater MF PEG, Clay and Zeolite
composite
5.55% crystal violet, 90.23% methylene blue Foorginezhad, and
Zerafat (2017)
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Kathi et al. 10.3389/fceng.2023.1129783
solution by the difference in osmotic pressure (Ahuchaogu et al.,
2018). Ray et al. (2020), reported successful recovery (64%–90%) of
unionised ammonia from hydrolysed urine by using RO and NF
procedures. However, the high expense of the process of nutrient
recovery from wastewater is a drawback for NF and RO (Xu et al.,
2017). Other factors can further increase operational costs, including
membrane fouling, a decrease in membrane shelf life, and reduced
rejection efficiency (Shi et al., 2014). As a result, effective treatment
and valorisation methods are needed for treatment and resource
recovery, such as FO technology, which is highly operative in the
case of specific target molecules.
In order to recover nutrients from the wastewater and produce
clean water, the nitrogen and phosphorous in primary treated
municipal wastewater can be concentrated by the FO membrane
method (Jafarinejad, 2021). Due to the size-sieving action of FO
membranes, high rejection efficiency for the majority of nutrients,
including the high-in-demand phosphate, has been well
documented in the literature (Linares et al., 2013;Liu et al.,
2019). If the technological and operational shortcomings can be
successfully resolved, membrane technology is anticipated to attract
more interest and profitability in this market segment in the
upcoming years (Table 1).
2.2 Enzymatic catalysis
Enzymes, which are fundamental to all biological activities, are
highly effective biocatalysts that speed up chemical reactions under
physiological conditions by breaking down complex compounds
into simpler ones (Bell et al., 2021). Many wastewaters contain heavy
grease content, dyes, lignocellulosic mass, agrochemicals, blood,
tissues, pathogens, and other emerging contaminants from food
processing industries, restaurants, dairy, oil refineries, meat
processing, tannery, slaughterhouse, cosmetics, and
pharmaceuticals industries (Mahmoud et al., 2016;Mahmoud
et al., 2018;Mahmoud et al., 2021b;Khan et al., 2022;
Mahmoud, 2022). If left untreated, such wastewater can cause
severe environmental pollution, potentially contributing to the
spread of these pollutants into the various environmental
matrices and eventually threatening human health (Singh et al.,
2022).
Treatment and valorisation methods based on enzymatic
catalysis offer numerous advantages over conventional
methods, such as low energy input, low toxicity, the ability to
function under mild conditions, abridged sludge, and byproduct
generation (Rout et al., 2022)(Table 2). Enzymes can be used in
both free form and immobilised forms however, the enzymes’
storage stability and operation ability upsurge upon
immobilisation. The development in enzyme reutilisation and
recoverability, that elevates its productivity and lowers the entire
process’s cost, is the most crucial benefit of the immobilised
enzyme (Chen et al., 2019;Lassouane et al., 2019).
Predominantly, enzymes viz. laccases, tyrosinases, lipases,
cellulases, lignin, and phenol peroxidases hold a promising
role in mitigating any kind of contamination from an
extensive array of wastewater types such as greasy wastewater,
sugarcane industry wastewaters, dairy wastewaters,
pharmaceutical industry, tannery textile, paper and pulp,
mining, etc. in a precise, manageable, and controlled manner
(Brugnari et al., 2018;Chen et al., 2019).
Lipase is a class of hydrolase enzymes with multiple benefits
and high catalytic rates (Nimkande and Bafana, 2022). lipases have
both high catalytic efficiency and substrate specificity, enabling it
to achieve excellent wastewater treatment efficiencies (Ariaeenejad
et al., 2023). For instance, lipase consortium from Pseudomonas
aeruginosa was utilised for the remediation of restaurant
wastewater by Sutar et al. (2023). Another class of enzyme,
laccase, was found to be effective in degradation of phenolic
compounds such as the very unfamous Bisphenol A, an
endocrine disruptor, commonly used in plastic production, and
barely degraded in conventional wastewater treatment systems.
The laccase from Pleurotus ostreatus immobilised onto mono
aminoethyl-N-aminoethyl agarose achieved 90% removal
efficiency of BPA in 15 catalytic cycles (Brugnari et al., 2018).
Additionally, Lassouane et al. (2019) employed immobilised
laccase from Trametes pubescens to remove BPA. at pH 5 and
temperature at 30°C, this biocatalytic system aided the degradation
of BPA with 100% contaminant removal efficiency within 2 hours.
In a different study, Chen et al. (2019) degraded nine pesticides,
owing to immobilised laccase enzyme, using wheat straw and
peanut shells as supports for the reaction system, and
syringaldehyde to enhance the catalytic characteristics of the
laccase. 54.5% decomposition rate was achieved using laccase
immobilised on peanut shells. However, the reaction efficiency
rose to 65.9% when wheat straw was utilised.
Although enzyme technology in wastewater treatment has a
promising role, it has its limitations that need to be addressed. An
efficient immobilisation system can circumvent these challenges by
enhancing enzyme stability, thereby increasing the shelf life of the
enzymes in the reaction system.
2.3 Microalgae cultivation
Urban, agricultural, and industrial activities produce massive
amounts of nutrients rich wastewater, which would inevitably result
in eutrophication in the aquatic environment as well as the threat of
releasing harmful emerging contaminants that can enter the food
chain and pose a risk to human health (Iqbal et al., 2022;Minhas
et al., 2022).
Central and local governments should follow strict laws and
regulations to limit the pollutants released into the environment.
They should also work to reclaim and reuse resources, such as water,
in order to improve the current state of the environment and
promote the use of a circular economy that focuses on reducing
waste and reusing resources (Bolger and Doyon, 2019;Singh et al.,
2022). However, the majority of currently used methods for treating
sewage and wastewater, such as the aerobic activated sludge-based
treatment process, nitrification-denitrification, chemical
phosphorus removal, and coagulating sedimentation, face
difficulties due to their high energy requirements, unstable
treatment effects, lengthy processes, carbon emissions, excess
sludge discharge, and waste of recyclable resources (Sathya et al.,
2022). These challenges are also clear impediments to integrating
wastewater treatment with low-carbon emissions, low-energy
consumption, and resource recycling (Gu et al., 2017).
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One of the most forefront technologies for wastewater treatment is
the microalgae-based treatment process for advanced treatment,
valorisation and nutrient recovery (Figure 3). Microalgae includes
photosynthetic microorganisms that can thrive easily in industrial
and municipal wastewaters, remediate it and produce biomass rich
in nutrients, bioactive compounds, biomolecules, and other value-
added products (Daneshvar et al., 2018;Mishra et al., 2022).
Moreover, these form the raw materials for generating value-added
products, energy, and economy along with the bioremediation process
simultaneously (Michelon et al., 2021). Microalgae’sstrongnutrient
removal efficiency in the advanced treatment of urban, agricultural, and
industrial wastewater have made it possible to use it as a supplement for
tertiary wastewater treatment (Rout et al., 2022). Cultivation of
microalgae from wastewater utilises the nutrients present in the
wastewater to support its growth and allows the reduction of
production costs and greenhouse gas emissions (Mishra et al., 2022;
Dias et al., 2023). To date, a wide range of microalgae species
(Scenedesmus, Botryococcus, Chlorella, Dunaliella, Parachlorella,
etc.) have been reported to be used in valorisation and treatment of
wide range of wastewaters (Amini et al., 2020) as mentioned in Table 3.
Microalgae can tolerate a wide range of water salinity, pH, and
temperature. They can also grow in high concentrations of oxides of
sulfur, carbon and nitrogen, making them suitable for wastewater
cultivation (Ebhodaghe et al., 2022). Scenedesmus quadricauda and
Tetraselmis suecica, two freshwater and marine water strains, have
recently been studied by Daneshvar et al. (2018) for the treatment of
dairy wastewater. While T. suecica strain removed 86.21% of TN,
44.92% of p, and 40.16% of total organic carbon, this strain had an
efficiency of removal of 86.21% TN, 64.47% phosphate, and 42.18%
TOC. Additionally, 295.34 and 56.25 mg g-1 of tetracycline could be
removed from water by each species, respectively (Daneshvar et al.,
2018).
FIGURE 3
Wastewater valorisation and production of value-added products using microalgae cultivation.
TABLE 3 Microalgae cultivation in wastewater treatment with nutrients recovery.
Algae Wastewater COD removal
rate (%)
Ammonium removal
rate (%)
Total N removal
rate (%)
Total premoval
rate (%)
References
Chlorella vulgaris Municipal wastewater 75 70 53 Amini et al. (2020)
Microalgae
consortium
Tannery wastewater 56.7 71.7 97.6 Pena et al. (2020)
Scendesmus Molasses wastewater 87.2 90.5 88.6 Ma et al. (2017)
Ascochlorissps
ADW007
Dairy wastewater 94–96 72–80 80–97 Kumar et al.
(2019)
Chlorella vulgaris Livestock farm
wastewater
62.3 81.2 85.3 Lv et al. (2018)
Chlorella vulgaris Piggery and brewery
wastewater
93 100 96 90 Zheng et al. (2018)
Chlorella
sorokiniana
Dairy wastewater 86.8 80.7 94.6 Kusmayadi et al.
(2022)
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3 Resource recovery from wastewater
and sludge
Wastewater may be beheld as a reservoir of resources as it
contains a wide array of nutrients and components that can be
valorised into value-added products such as fertilisers, biofuels,
biopesticides and many more (Singh et al., 2022). Wastewater
sources, including wastewater and sludge, are rich in nutrients and
are now recognised as important sources of nutrients. Nutrient
recovery such as phosphorus and nitrogen, from wastewater can
make wastewater treatment more sustainable, also reduce the costs
associated with removing these nutrients from the wastewater, and
provide additional fertilisers that can be used to help grow food (Zin
and Kim, 2021;Hui et al., 2022). For the treatment of wastewater and
concurrent resource recovery and production of value-added
products (nutrients, organic acids, bioplastics, bio-pesticides, bio-
surfactants, bio-flocculants etc.) as well as bioenergy, biofuels, and
bioelectricity, more advanced and novel technologies are currently
being identified and fundamentally considered. Numerous
technologies, including conventional ones like chemical
precipitation and adsorption and more cutting-edge ones like bio
electrochemical systems and osmotic membrane bioreactors, have
been successfully examined for their efficacy in nutrient recovery
(Kurup et al., 2019;Nazir et al., 2019;Canadas et al., 2022).
3.1 Recovery of nutrients (phosphate,
nitrates, ammonia, proteins, lipids, vitamins)
A crucial step toward sustainable development is the recovery of
nutrients from wastewaters and sludge. Urban and industrial
wastewater are potential raw materials to recover significant
biologically active components. Sewage sludge from wastewater
treatment facilities provide rich source of biomolecules that
include, nutrients such as NPK, antioxidants, lipids, proteins,
enzymes, carbohydrates, vitamins, nitrogen, phosphates, minerals,
ammonium compounds, and many more (Zhu et al., 2017;Chen
et al., 2020;Sichler et al., 2022).
Due to the commercialization of agriculture and the need to feed
an expanding population, the demand for fertiliser has increased
over the past few decades. By 2022, it has already been predicted that
most regions of the world will experience a shortage of at least one
NPK fertiliser, Moreover, In 2014, the European Commission added
phosphate rock to the list of critical raw materials for the first time
(EC, 2014). Scientists are looking at alternatives to provide these
crucial agricultural nutrients due to the restricted availability and the
high cost of production. Compared to nitrogen and phosphorus,
potassium has not yet had a significant economic impact on
agriculture because, at the pace it is being consumed, there are
more than 330 years of reserves. In contrast, there is no substitute for
phosphorus in agricultural output and its supply is expected to run
out in 100 years, making it a limited resource and a critical
commodity (Cordell et al., 2009).
Municipal wastewater and sludge, offer a promising secondary
phosphorus source, comprising around 40% of the phosphorus in
human excreta (Cordell and White, 2014;Klinglmair et al., 2015).
Several techniques, such as biological release, pH adjustment,
ozonation, mechanical treatment, and heat treatment, have been
used to recover phosphorus from municipal wastewater and sewage
sludge (Xue and Huang, 2007;Quist-Jensen et al., 2018;Wang et al.,
2021). Notably, the total amount of phosphorus found in human waste
(urine and excreta) may meet around 22% of the world’s phosphorus
needs (Cordell and White, 2014). However, due to worries about
possible infections, persistent organic pollutants, heavy metals, and
odor, direct wastewater and sludge application to agriculture is regarded
as unsafe in many places (Cieslik et al., 2015). Therefore, nutrient
recovery is considered the best way to confiscate phosphorous, nitrogen,
ammonium, and other nutrients to support sustainability by putting less
stress on natural gas and other renewable energy sources spent on
producing these nutrients (Theregowda et al., 2019;Ersahin et al., 2023).
Agents like magnesium, calcium, and other alkaline metals are
introduced in the precipitation process to change pH and create the
precipitate (Owodunni et al., 2023). Nutrient recovery from sludge
centrates is largely accomplished commercially through chemical
precipitation and crystallization (Melia et al., 2017). Sludge centrate
is low in volume but high in phosphate and ammonium
concentration when compared to wastewater discharge. The dry
mass of wastewater sludge can contain up to 15% phosphorus, which
is typically coupled to metal ions like calcium, iron, and aluminium
or contained in biomass (Yu et al., 2021). The possibility of
recovering phosphorus from wastewater sludge will be greatly
increased by leaching phosphorus into the aqueous phase. Due to
the dissolution of inorganic phosphorus, acidification is one of the
most successful methods (Quist-Jensen et al., 2020).
In a study, phosphate and ammonium nitrate were recovered
from sewage sludge ash and food industry wastewater using Mg-
Biochar derived from ground coffee bean and palm tree trunk waste.
Palm tree trunk Mg-biochar could recover 92.2% of phosphate and
54.8% of ammonium nitrate, while ground coffee bean biochar
recovered 79.5% of phosphate and 38.6% ammonium (Zin and Kim,
2021). Besides, wastewaters also are a rich source of nutrients such as
proteins, lipids, vitamins, etc. Several industries generate copious
volumes of protein-laden wastewater. Starch processing water from
the potato manufacturing industry is known to have a high content
of protein, where foam separation and membrane technology by
ultrafiltration are used for its recovery (Dabestani et al., 2017).
Similarly, multiple industrial, domestic wastewater, and sewage
sludge are rich source of proteins. Bulk proteins have countless
industrial applications, in manufacturing of food and beverages,
personal care products, medicines, cosmetics and animal feed. The
most common use of bulk proteins derived from waste is in animal
feed (Ritala et al., 2017). Hwang et al. (2008) recovered protein from
sludge to be used as animal feed by the method of sludge
disintegration. Precipitation and drying methods were used to
separate the soluble proteins from the sludge. This concept is
feasible because sludge typically contains beneficial organic
substances such as nucleic acids, enzymes, proteins, and
polysaccharides (Jung et al., 2002). Protein has been estimated to
account for approximately half of the dry weight of bacterial cells
(Shier and Purwono, 1994). Protein is one of the most vital elements
in animal feed, providing energy and nitrogen (Adebayo et al., 2004).
Single-cell protein (SCP) is a type of protein that is made from
microbial biomass and is commonly used as an additive in animal
feed. It is less expensive to produce SCP using microbial
communities, which are groups of different types of
microorganisms, than it is to produce it using a single type of
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organism (Vethathirri et al., 2021). Furthermore, protein recovery
also facilitates wastewater discharge by significantly reducing the
Biological Oxygen Demand (BOD) and COD (Buchanan et al.,
2023). Like proteins, Lipids have a variety of industrial applications
in a variety of products, including manufacturing and production of
biodiesel, agrochemicals, animal feed, personal care items, and
cosmetics (De Luca et al., 2021) to name a few.
Physical methods of extracting and recovering proteins and lipids
from wastewater and sewage sludge involve using adsorption,
microwaves, ultrasonics, heat, centrifuges, and membrane filtration.
Chemical processes involve using thermal alkali, thermal acidic,
pH regulation, and ethylenediamine tetraacetic acid (EDTA). Hui
et al. (2022), conducted a study to isolate proteins from waste-
activated sludge (WAS). Maximum protein concentration
(549.86 mg/L) was recovered with high extraction rate (60.72%)
under alkaline conditions at 70°C. The relative molecular weight of
the sludge proteins was between 6.5 kDa and 16 kDa. Kurup et al.
(2019) demonstrated the use of low-cost Na-lignosulphonate to recover
lipids and proteins from dairy industry effluents. He reported that
change in Na-lignosulphonate concentration (0.002%–0.032%, w/v),
medium’spH(1.0–3.5) and the temperature (22°Cand40
°C) enhanced
the rate of precipitation in the wastewater samples. The procedure was
highly effective (96% of lipids and proteins recovery and 73% BOD
removal at 22°C) over methanol and chloroform-based solvent
extraction of lipids which is neither cost-effective nor
environmentally beneficial. In another study on the extraction of
lipids from synthetic primary sludge by Villalobos-Delgados et al.
(2021), the lipid component was successfully recovered using ethyl
butyrate (13%–23% of the total solids).
3.2 Recovery of phenolic compounds
Phenol is a significant environmental contaminant that may result
in water pollution. Natural occurrencesoranthropogenicactivitiesare
the two critical sources. In nature phenol is produced when dead plants
and animals decompose in water. Humans utilise phenol in their daily
activities in chemical production or petrochemical industries.
Alkylphenols, cresols, and resin are also synthesized using it as a
precursor (Careghini et al., 2015). The building business in the
wood sector mainly provided phenolic resin and appliances to suit
varied needs (Takeichi and Furukawa, 2012). Additionally, phenol is a
key component of explosives, textile industries, and dyes (Stamatelatou
et al., 2011;Patel and Vashi, 2015). Epoxy resins and polycarbonate
plastics are commonly made using phenolic replacements like
bisphenol A (Sirasanagandla et al., 2022).
Some industries including the food industry, paper and pulp
manufacturing, and wood distillation produce phenol, specifically
chloro-phenol, as part of their processes (Olaniran and Igbinosa,
2011). These industrial activities directly or indirectly release phenol-
containing wastewater into water bodies, resulting in water
contamination. Phenol contamination of the water sources would
result from the runoff of the agrochemicals such as pesticides and
herbicides (Badanthadka and Mehendale, 2014). Domestic and
municipal sources of phenol discharge into water sources were also a
factor in addition to the industrial sector (Villegas et al., 2016;Mu’azu
et al., 2017).
Olive Mill Wastewaters (OMWW) are a great source of phenolic
chemicals. These phenolic substances pose a serious threat to aquatic
life, microbes, and plants since they are both extremely phytotoxic
and resistant (Chedeville et al., 2009;Kathi, 2022). Wastewater
toxicity may suppress the microbial population, therefore,
biological treatment facilities are insufficient. Hence, it is essential
that phenolic wastewater be properly treated before its discharge
(Sun et al., 2015). Despite the fact that these techniques have already
been created and proposed, they have not produced satisfying
outcomes (Ochando-Pulido et al., 2013). An integrated approach
is necessary for the practical management of OMWW.
The combination of membrane processes, such as MF, UF, NF,
and RO, is recommended as a good alternative in this regard by a
TABLE 4 Phenolic compounds recovery methods from different wastewaters.
Wastewater
type
Method used Recovered phenols Recovery
percentage
References
Winery wastewater Biobased liquid-liquid extraction
method
Gallic acid, protocatechuic acid, 4-hydroxybenzoic acid,
caffeic acid, vanillic acid, syringic acid
96.46%, 87.44%, 80.82%
92.24% 75.18% 38.19%
Canadas et al.
(2022)
Phenolic wastewater Magnetic ionic liquid aqueous two-
phase separation along with
thermoseparation
Phenol, o-cresol, m-cresol 94.18%, 98.35%, 97.81% Yao et al. (2022)
Municipal
Wastewater sludge
Biochar assisted hydrothermal
liquefaction sorption
5-Hydroxyindoleacetate, 4-Hydroxyphenylacetate, 4-
Hydroxybenzoate, sodium salicylate, scytalone,
maculosin, biphenyl-2,3-diol, biphenyl-2,3-diol,
candicine, chrysosplenetin, abyssinone V, and
hydroxyanthraquinone
66%–99% Wang et al. (2022)
Olive mill
wastewateter
Pomegranate peel and orange waste-
based adsorbent
93.13%, 89.5% Ververi and
Goula (2019)
Winery sludge Ultrafiltration Hydroxycinnamic acids, o-diphenols 81% Galanakis (2013)
Artichoke
wastewaters
Nanofiltration Chlorogenic acid, Apigenin-7-O-glucoside 100% Conidi et al.
(2015)
Orange press liquor Nanofiltration Anthocyanins (cyanidin-3-glucoside chloride, myrtillin
chloride and peonidin-3-glucoside chloride), flavanone
>65% Cassano et al.
(2014)
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Kathi et al. 10.3389/fceng.2023.1129783
number of studies (Zagklis et al., 2015;Bazzarelli et al., 2016;Nazir
et al., 2019)(Table 4). Solvent extraction is popularly used on
OMWW to recover phenolic compounds (Goula and
Gerasopoulos, 2017). In order to get all of the phenolic
compounds out of the OMWW, different solvents are needed
that make the process more complicated, take longer time, and
cost more currency (Tsakona et al., 2012;Galanakis. 2013).
Adsorption is considered the most appropriate method for
recovering OMWW phenolic compounds because it is cost-
efficient and commonly used (Mahmoud et al., 2020;Mahmoud
et al., 2022b).
3.3 Recovery of dyes and pigments
In the production of pharmaceuticals, textiles, food, paper and
pulp, and cosmetics, synthetic dyes are utilised. Water is used
extensively throughout the fixing, dying, and washing stages of
the textile industry’s dyeing process. The dyes are organic soluble
substances that can be categorised as direct, reactive, acidic, or basic.
Due to their great solubility in water, dyes are more difficult to
remove using standard methods. The main carcinogenic and
hazardous pollutants are industrial dyes and their byproducts.
A number of studies demonstrate the harmful effects of azo dyes
on plant growth and germination. The production and growth of
crops may be impacted if such contaminated effluent is used for
irrigation (Rahman et al., 2018). For the treatment of wastewater
from the dye industry, advanced oxidation processes, membrane
filtration, microbial technologies, bio-electrochemical degradation,
and photocatalytic degradation have been documented (Gao et al.,
2023;Iqbal et al., 2023;Verma et al., 2023). In addition to assisting in
the treatment of wastewater, innovative techniques including
microalgae cultivation, biobased adsorbents, and membrane
technologies also aid in the recovery of dyes and pigments that
may be reused and recycled sustainably (Eskikaya et al., 2023;Xiong
et al., 2023). Wastewater phytoremediation represents a promising
circular economy-based approach for wastewater reclamation,
resource recovery, and generation of value-added products. With
unique health benefits, natural pigments are now in high demand in
the market, as natural colouring agents in food industry sector
(Christaki et al., 2015;Villaro et al., 2021). Phycobiliproteins are
photosynthetic pigments found in the microalgae, and are widely
used in the food, cosmetic, and pharmaceutical industries (Sun et al.,
2023). While microalgae are seen to be a promising contender for
the production of natural pigments and the demand for natural dyes
is rising, the high costs associated with large-scale systems
necessitate the use of enormous amounts of water and nutrients.
It also makes it more difficult to produce and market these
bioproducts. Natural pigments’production from microalgal
culture grown over wastewater as growth medium has been
recently foreseen as a potential solution to both environmental
pollution and high cost production of pigments (Blanco-Vieites
et al., 2023). This approach will remediate the wastewater and
produce natural dyes and pigments (Xiong et al., 2023). Further,
the algal biomass may be utilized to produce biogas, biofertilisers,
and other byproducts. Arashiro et al. (2020) cultivated microalgae
consortium on industrial wastewater for the recovery of
phycobiliproteins aiming to bioremediation of wastewater and
sustainable production of value-added compounds. The high
recovery rates were observed for Phycocyanin (103 mg/g dry wt),
allophycocyanin (57 mg/g dry wt), and phycoerythrin (30 mg/g dry
wt), from algal biomass respectively. Furthermore, microalgae
culture successfully removed the COD, inorganic nitrogen, and
phosphate up to 98%, 94%, and 100%. Stepnowski et al. (2004)
used fish scale waste as a natural adsorbent for the recovery of
astaxanthin, a carotenoid pigment from seafood industry
wastewater. Astaxanthin is widely utilised as a coloration agent
for farmed salmonids, including the natural or synthetic pigment in
the fish diet (Hatlen et al., 1995). The pigment is also considered in
medical and biomedical studies and applications due to its biological
function as a vitamin A precursor and its high antioxidative effects.
The results show that astaxanthin bound onto the scales-based
adsorbent was 88%–95% of the total sorbed pigment in its
esterified form. The maximum loading capacity of astaxanthin
onto the scales was 362 mg/kg dry wt.
3.4 Recovery of enzymes
Most commercial enzymes are derived from bacterial biomass.
Additionally, they are extracted of other fungi, including yeast made
in a commercial fermentation method. Specific culture mediums are
used in the synthesis of industrial enzymes. These culture media are
capable of offering all the nutrients required for microbial growth,
consolidation, and purification. By disrupting the biomass, enzymes
are extracted, then combined with stabilisers to create various solid/
liquid products that can be sold. Particularly synthetic culture
mediums are among the most expensive components of
commercial enzyme synthesis, accounting for approximately to
30%–40% of the overall cost of production. Therefore, cost-
effective approaches to producing hydrolytic microbial biomass,
avoiding the need for synthetic growth media, might offer significant
commercial advantages for industrial enzyme production.
Cheng et al. (2015) studied how to extract an enzyme called
polyphenol oxidase (PPO) from potato starch wastewater. PPO has
many uses but is expensive to produce commercially, so it is not used
as much as it could be. The researchers adjusted the pH of the
wastewater to 3.5 to cause a chemical reaction that would cause a
protein called PPO to separate from the wastewater. They then
added 50% acetone to the wastewater to cause a different chemical
reaction that would cause a different protein called beta-amylase to
separate from the wastewater. Finally, they added 80% acetone to the
wastewater to cause a reaction that would cause a type of
carbohydrate called sporamins to separate from the wastewater.
After the acetone was recovered, the researchers were able to obtain
4.3 × 105 units of PPO, 4.0 × 106 units of beta-amylase, 8.70 g of
sporamins, and 20.2 g of SMNs from 1 kg of sweet potato
wastewater. Libardi et al. (2017) studied the possibility of using
wastewater from toilets containing toilet paper as a source of
cellulose for producing cellulases. Using wastewater as a source
of cellulose makes it possible to produce an added-value product
while also reducing the pollution charge of the wastewater by
consuming it as a source of carbon and nutrients. In another
study Libardi et al. (2019) conducted a study to produce cellulase
enzymes from domestic wastewater. They used Trichoderma
harzianum as the culture medium and tested three bioreactor
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Kathi et al. 10.3389/fceng.2023.1129783
systems; a bubble column bioreactor (BCB), column-packed bed
reactor (CPBR), and stirred tank reactors (STR). The BCB system
was the most successful, producing the highest cellulase activity and
productivity of 31 UFP/mL and 645 UFP/mL.h without needing
immobilisation. The fermented broth was then microfiltered and
ultrafiltered, resulting in a cellulase recovery of 73.5% using a 30 kDa
membrane, which increased the activity concentration by 4.23 times.
Additionally, the study found that the COD and nitrogen
concentration in the wastewater was reduced by 81.37% and
52.9%, respectively. The production of cellulase enzymes from
domestic wastewater and the subsequent development of an
added-value process with minimal environmental impact are
both highly anticipated opportunities. Moreover, accounting
about 30% of the global enzyme market, amylase is a vital
industrial enzyme. (Gomez-Villegas et al., 2021). It has significant
role in multiple industries, such as pharmaceuticals, textiles,
chemicals, paper, and food. Yusree et al. (2022) used a Liquid
biphasic system (LBS) to extract and purify a type of amylase
called beta-amylase from sweet potato slurry. This LBS is a green
extraction technology that combines polyethylene glycol and citrate
to separate the beta-amylase from the sweet potato slurry. The
results demonstrate an upward trend in recovery yield (78.24%),
purification factor (3.5087), and selectivity when salt and polymer
concentrations upsurged.
4 Valorisation of wastewater and sludge
4.1 Production of biofuels
Wastewater and sewage sludge are no longer considered a waste
because it may be used to recover, recycle and produce valuable
products (Singh et al., 2022). Biofuels like biogas, biodiesel,
biohydrogen, bioethanol, etc., can be produced using wastewater
and sludge. Anaerobic digestion methods are already being used on
an industrial scale to effectively valorise the sludge as these provide
various social and environmental benefits in addition to green
energy and manure (Pasciucco et al., 2023). Biogas production
from organic waste helps reduce the negative impacts associated
with organic wastes in landfills causing groundwater and soil
pollution, the emission of air pollutants such as dioxins and
furans, methane, and more substantial greenhouse gases.
However, the amount of methane gas produced from sludge by
anaerobic digestion is lower than other types of organic waste. Thus,
pretreatment and co-digestion contribute to improving the
degradability of organic matter and methane production potential
of sludge, respectively (Kumar et al., 2017;Lewis et al., 2017;Elalami
et al., 2019;Yadav et al., 2023).
Municipal wastewater sludge, pulp, and paper wastewater
containing cellulose combined with lignin could be used as raw
materials for bioethanol production (Bon et al., 2022). Using
cellulases enzymes from Trichoderma reesei, cellulose components
are hydrolyzed into sugar (saccharification), and the resulting sugar
is used as a raw ingredient for the production of bioethanol by
Saccharomyces cerevisiae (el-Zawawy et al., 2011). Further, in a study
led by Dufreche et al. (2007), Lipids were used as a cheap source of
raw material for the production of biodiesel after being extracted
from municipal wastewater sludge using solvents such n-hexane,
methanol, acetone, and supercritical CO
2
. Additionally, dried sludge
was reported to be used to produce 6.23% biodiesel using in situ
transesterification process (Dufreche et al., 2007). Owing to its clean
product and high energy production (122 kJ/g), hydrogen is an
attractive energy source. Water electrolysis, steam reforming of
hydrocarbons, and auto-thermal processes are well-known ways
of producing hydrogen gas, but these are expensive due to high
energy needs. Environment-friendly anaerobic biohydrogen
fermentation from organic wastes is gaining popularity because
of its minimum waste and bioenergy produced. A low hydrogen-
yielding rate is essential in biohydrogen synthesis from the garbage.
Several aspects must be addressed to maximise hydrogen generation
throughout the fermentation process, such as minimising hydrogen
loss to hydrogen-consuming anaerobes like methanogens. Other
elements, such as inoculum, substrate, temperature, nitrogen
sparging, and early startup, have all been investigated by
researchers to increase hydrogen generation (Li et al., 2008;Tang
et al., 2008). However, biofuel production from wastewater and
sludge is still in its infancy. Therefore, further large-scale studies are
required to realise the benefits of this new technology at an industrial
scale.
4.2 Production of biopolymers
To build a sustainable wastewater treatment facility, resource
recovery from wastewater sludge is emphasised. Wastewater sludge
is thought of as a viable source for recovering high-value products
like biopolymers and biofertilizers. (Sharma et al., 2022). Activated
sludge is a matrix of microbial cells entrenched in an extracellular
polymeric substance (EPS) released by microbes. About 10%–40%
dry weight of total activated sludge accounts for the EPS. EPS is
considered as rich source of polysaccharides, nucleic acid, lipids,
proteins, and humic acids (Basuvaraj et al., 2015;Boltz et al., 2017).
In a recent study, EPS mined from wastewater sludge has been
characterised to measure physical and chemical properties (Lin et al.,
2013;2018;Pronk et al., 2015). The matrix’s biopolymers provide a
distinctive structure that can be salvaged to create potentially useful
biomaterials. (Sheng et al., 2010;More et al., 2014). For instance,
alginate-like extracellular polymers (ALE) can potentially be
extracted from aerobic granular sludge (AGS) (Rehm, 2010;Lin
et al., 2015;Lin et al., 2018). Due of their widespread use in the food,
paper, medicinal, construction, and textile industries, the recovery of
TABLE 5 The chemical composition and characteristics of untreated and
digested sewage sludge.
Chemical composition Untreated sludge Digested sludge
Total dry solids (TS), % 2.0–8.0 6.0–12.0
Nitrogen, (N, % of TS) 1.5–41.6–6.0
Phosphorus, (P
2
O
5
, % of TS) 0.8–2.8 1.5–4.0
Potash, (K
2
O, % of TS) 0.0–1.0 0.0–3.0
Protein, (% of TS) 20–30 15–20
pH 5.0–8.0 6.5–7.5
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these ALE polymers from sludge is receiving a lot more attention
lately (Kim et al., 2020;Li et al., 2021).
By using solution casting, EPS recovered from surplus granular
sludge was successfully integrated into polyvinyl alcohol (PVA)
(Kim et al., 2022). Polyvinyl alcohol (PVA) is a type of material
that has many uses. It is known for its strong resistance to chemicals
and its physical qualities, making it useful for various applications.
These applications include textile production, paper sizing, and the
manufacture of filaments (Mohareb et al., 2011). In the building
industry, it is common to use a certain substance as a binding and
thickening agent for latex paints and seals. This substance helps to
hold the paint and seal together, making it thicker and more durable
(Marten, 2002) and as a modifier to improve the strength and
durability of the material in fibre reinforcement and cement-based
composite materials (Thong et al., 2016).
4.3 Production of biofertilisers
The valorisation of waste into byproducts by applying a
circularity principle is a central and essential issue to foster
sustainable economic development. In this context, municipal
wastewater has a high potential for nutrient valorisation and
sustainable production chain development. An example is sewage
sludge as a source of nutrients for crops (Table 5). Dewatered sludge
containing nutritional and organic content is utilised as agricultural
fertiliser. Sludge’s organic matter, nitrogen, and phosphorus content
promotes plant development and improves agricultural soil quality.
Majority of farmers from West African and South Asian countries
view untreated sewage as a useful fertiliser, despite the fact that using
untreated sludge as a fertiliser is unacceptable (Cofie et al., 2005;
Verhagen et al., 2012). Farmers use untreated dried sewage as free
fertiliser, including nitrogen (322 kg N/ha/year) and phosphorus
(64 kg P/ha/year). The chemical composition and characteristics
of untreated and digested sewage sludge are shown in Table 1.
The demand for fertilisers for agricultural production is increasing
due to the world population (Mahmoud et al., 2022c). At current
demand for nitrogen is more than 130 million tonnes, and the
demand for phosphorus is about 16 million tonnes. Recovery of
nutrients from wastewater sludge could meet the local requirement
of fertilisers for agricultural production (FAO, 2008;FAO, 2012;
Balasubramanian and Tyagi, 2017).
Recent advancement in sewage sludge treatment technology
allows the recovery of nutrients such as nitrogen and
phosphorus. These recovered nutrients can be utilised as bio-
fertilisers by replacing synthetic fertilisers (Herzel et al., 2016).
Rebah et al. (2002) and Singh et al. (2013) conducted
experiments to explore the utilisation of sludge as a medium to
grow Rhizobium bacteria which has been applied as a bio-fertiliser
directly onto the soil. Paliya et al. (2019) also experimented on
sludge ash. They concluded that sludge ash could be utilised as a
sustainable raw material to produce bio-fertilisers. This conclusion
was made possible by incorporating it in a definite amount with the
inoculum of different microbial species. Industrial and municipal
wastewater sludge (primary and secondary sludge from wastewater
treatment plants) was used as a raw material to grow various
rhizobial strains such as Sinorhizobium meliloti, and Rhizobium
leguminosarum bv. viciae, Bradyrhizobium japonicum, and
Bradyrhizobium elkanii. Among all these strains S. meliloti strain
grew well compared to other strains in the sludge. Sludge
characteristics also influenced bacterial growth, cell yield, and
fermentation time. Suspended solid particles increased in sludge
significantly reduced the oxygen transfer and the rhizobial growth
(Rebah et al., 2001). Romero-Garcia et al. (2022) used wastewater
sludge as a raw material to form a microalgal bio fertiliser high-
pressure homogenisation process. This process converts the
microalgal biomass produced in the wastewater treatment plant
into a bio-fertiliser used in agriculture to improve crops and reduce
traditional fertilisers. Subsequently, progress toward a sustainable
food system for the twenty-first century under the United Nations’
2030 Agendas of Sustainable Development Goals. Microalgae grown
in wastewater can be utilised as a bio-fertiliser for soil amendment.
Microalgal biomass can enhance soil nitrogen and phosphorus levels
and numerous other plant-required trace elements (Ca, K, Fe, and
Mn). When combined with soil-based phosphorus-solubilizing
organisms, microalgal fertilisers have boosted phosphorus release
in the soil. The landfill leachate biomass possesses great potential to
generate biofertilizers (Tighiri and Erkurt, 2019;Solovchenko et al.,
2021). A study led by Umamaheswari and Shanthakumar (2021),
reported 30% increase in the dry weight and biochemical content of
wheat crop upon input of filamentous microalgae strains as bio-
fertilizers. Microalgae and bacteria symbiotically transform organic
and inorganic compounds into smaller biomolecules that plants can
absorb, making them efficient biofertilizers (Ferreira et al., 2019).
4.4 Production of biopesticides
Biological agents might be used to manage and target insects
in a specific and selective manner, reducing the need for
hazardous chemical pesticides while simultaneously improving
environmental quality and human health (Sabbahi et al., 2022).
To attain a high yield of biopesticide manufacturing, wastewater
sludge has been proven an economically viable solution (Butu
et al., 2022). Wastewater sludge is considered a global problem,
and its sustainable management via value addition is an efficient
alternative to other disposal methods, such as landfilling and
incineration (Sachdeva et al., 2000). Tyagi et al. (2002) achieved
the production of Bacillus thuringiensis, a potential biopesticide
by using both industrial and municipal wastewater sludge as raw
materials. Both municipal and industrial wastewater sludge has
been proven viable for biopesticide synthesis; however, the
entomotoxicity potential of the biopesticide produced was
found to rely on the nutrient composition of the sludge (Tyagi
et al., 2002;Yezza et al., 2005). Utilisation of sludge waste for Bt
biopesticide has potential to can considerably cut the production
costs while contributing significantly to sustainable sludge usage
and management. Brar et al. (2006) used submerged
fermentation in the bioconversion of wastewater sludge into
Bt-based biopesticides. However, extraction of microorganism
was found to be challenging due to the low concentration of
products in submerged fermentation, resulting in complex
downstream processing (Brar et al., 2006) and necessitating
pricy and high end equipments such as a high-speed cooling
centrifuge and spray dryer (el-Bendary, 2006). In contrast to
traditional submerged fermentation, solid-state fermentation
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technology has several biotechnological advantages, including
low to medium cost and energy consumption, little effluent
output, and decent product stability (Pandey, 2003;Holker
and Lenz, 2005;Singhania et al., 2009).
Therefore, producing Bt-based biopesticides from sludge or
wastewater could be a novel and ingenious way to clean up the
environment and an addition to the path for commercialising
affordable biopesticides (Tiwari and Awasthi, 2022). Biopesticide
production from the starch industry wastewater and sludge has been
thoroughly studied including pretreatment, medium amelioration
agents, optimising process parameters, scaleup, and formulation and
development studies (Tyagi et al., 2002;Brar et al., 2004).
Wastewater sludge and starch industry wastewater proved to be
very effective media for the cultivation of Bt (Ndao et al., 2021). The
shift from linear to circular economies, in which the value of
resources, materials, and products is retained throughout the
value chain, is an essential step towards establishing a more
sustainable, waste free, and competitive economy. Bacillus
thuringiensis (Bt) is the most frequently employed microbial
biopesticide, producing crystal proteins with biopesticide activity
against insects from the orders Coleoptera, Lepidoptera, Diptera,
Hymenoptera, Hemiptera, and Orthoptera, as well as
phytopathogenic nematodes and terrestrial gastropods
(Malovichko et al., 2019). The sporulation process is linked to
the synthesis of this protein (Cry protein) (Chandler et al., 2011).
Nowadays, 90% of commercialised biopesticides are produced from
this entomopathogenic bacteria. These biopesticides are more
effective to target pests even in a small dose, and are fast
decomposable without leaving hazardous residues as compare to
chemical conventional pesticides (Kumar and Singh, 2015;Damalas
and Koutroubas, 2018;Yadav et al., 2022). Biopesticides,
bioherbicides, biofertilizers, bioplastics, and enzymes derived
from wastewater sludge are low-cost biological alternatives that
can compete with chemicals or other cost-intensive biological
goods in present market. The bioconversion of wastewater sludge
into biopesticides or other bio-control agents, has been
contemplated, with promising results. Trichoderma, Rhizobium,
and Bacillus sp. are the more capable of growing in wastewater
and wastewater sludge and producing microbial derivatives utilised
in biocontrol and bio-bleaching processes. (Verma et al., 2007a;b;
Subramanian et al., 2008). Unfortunately, Trichoderma sp. has not
been widely used due to lack of economically feasible procedures, as
most existing techniques uses expensive raw ingredients and have
lower spore production, which is critical for their performance as a
bio-control agents. Alam and Fakhrul-Razi (2003) discussed the use
of Trichoderma sp. for wastewater and wastewater sludge treatment
to decompose organic matter and hazardous organic compounds,
but they did not use the waste as a raw material for the
manufacturing of value-added products. According to Verma
et al. (2007a),Verma et al. (2007b),Trichoderma viride is a
promising bacterium that forms laccases in wastewater sludge
and has the ability to breakdown or detoxify organic
contaminants such as BPA. Cells of Rhizobium sp. and
Sinorhizobium meliloti are utilised as microbial inoculants in
leguminous cultures and as biological control agent in
agricultural crops. Laccases generated by S. meliloti have the
capacity to breakdown or detoxify organic chemicals, particularly
phenolic groups contained in wastewater sludge, while also bio-
converting wastewater sludge into value-added products (Rosconi
et al., 2005). Bacillus sp. can generate enzymes that breakdown
organic contaminants in wastewater and wastewater sludge, such as
oxidases, dehydrogenases, and peroxidases. Bacillus thuringiensis
generates tyrosinases, which may breakdown phenolic substances
(Donova et al., 2005;Quan et al., 2005). Biopesticides based on B.
thuringiensis have been demonstrated to breakdown an
Endocrine disrupting chemical, Dimethyle patholate,
concurrently with biopesticide synthesis (Brar et al., 2009).
Consequently, value-added product-generating microorganisms
like Rhizobium sp., Bacillus sp., and Trichoderma sp. have an
enzyme system that can efficiently break down organic chemicals
(Mohapatra et al., 2010). Several researchers have utilised SSF
technology to create Bt-derived biopesticides from soy waste,
sugar beet pulp, sesame meal, wastewater sludge and wasted
mushrooms. Zhang et al. (2013) achieved a successful
fermentation procedure with 35 kg of fermentation media that
was mostly made of kitchen garbage. When the spore count was
increased from 4 to 8 kg, the ultimate spore counts increased.
When the procedure was expanded up to 35 kg, spore output
reduced marginally. When 40 kg of substrate was used, this value
significantly reduced (from 9.6108 CFU/g to 6.4106 CFU/g),
owing to insufficient ventilation and heat removal as the
primary hypothetic factors. The feasibility of utilising digestate
as a fermentation substrate for B. thuringiensis var kurstaki (Btk)
culture and spore generation was investigated by Rodriguez et al.
(2019). This proof of concept demonstrated that the procedure
was achievable at lab and bench-scale (10-L), with scanning
electron microscopy pictures confirming the existence of
protein crystals. Nevertheless, scaling up to 22 and 100-L
proved that Bt was unable to develop, revealing only its
capacity to sporulate. As a result, the procedure for producing
biopesticide must be improved to improve Bt’s capacity to
colonise solid material at these sizes. A successful operational
strategy for B. thuringiensis spore production was achieved using
a digestate-biowaste mixture (62.5%–37.5%, wet weight basis)
and a novel aeration strategy divided into two stages: a micro
aeration phase during the first hours of fermentation, followed by
a high-rate aerated period after 22 h. Under these circumstances,
Bt spore production ranged between 5 × 10
7
and 1.5 × 10
8
spores/
g dry matter, with a maximum production yield of 5–7.3 spores/
CFU. Biodegradability has emerged as a critical issue in the
feasibility of both Bt growth and two-stage aeration strategies.
This research established the operational framework for carrying
out the developed method on a demonstration scale (Mejias et al.,
2020). Therefore, using sludge for biopesticide production will
minimise the environmental problem of sludge disposal and the
problems encountered with chemical pesticides.
5 Life cycle assessment and techno-
economic analysis
The term “life cycle assessment”(LCA) refers to the process of
measuring a product’s environmental impact throughout the course
of its whole life cycle, from conception to disposal. LCA is a
methodical process that uses ISO 14040 (2006) standards to
translate a system’s inputs and outputs into corresponding
Frontiers in Chemical Engineering frontiersin.org12
Kathi et al. 10.3389/fceng.2023.1129783
environmental consequences. LCA is divided into four stages—goal
and scope definition, data acquisition and life cycle inventory, life
cycle impact assessment, and outcome interpretation (Cortes et al.,
2020).
Sangma and Chalie (2023) performed LCA and techno-economic
analysis (TEA) to demonstrate that wastewater treatment with
microalgae can significantly lessen adverse environmental effects in
comparison to conventional methods. The system also provided the
benefit of low operating costs, scope for nutrient recovery, waste
valorisation into value added products, and the ability to reduce
emissions by absorbing CO
2
from flue gases. A more comprehensive
environmental assessment was also recommended in the review by
Lee and Jepson (2021) which compared the LCA studies of RO against
emerging technologies, such as forward osmosis and capacitive
deionization. This review highlighted the trade-off between carbon
emissions and chemical treatment and concluded emerging
technologies that aim to reduce energy consumption have the
potential to increase other environmental burdens such as
chemical usage; Hand and Cusick (2021) performed LCA for
electrochemical oxidation (EO) technology and reported that EO is
a promising technique for wastewater treatment and purification, but
it may pose unintended negative consequences on the environment
due to the consumption of resources and energy during the
electrodes manufacturing and operational stages. Sun et al.
(2023) quantitatively assessed the LCA of the EO method’s
environmental effects from the lab to the industrial scale. As
per their findings EO was found to be efficient in destroying
organic pollutants in wastewater, however, the benefits were
outweighed by the environmental burdens related to the
manufacturing of anode materials, use of electrolytes, and
energy consumption during various operational stages. This
obligated the attention to the LCA by taking reactor design,
anode materials, electrolyte, and flow pattern into consideration,
as well as demanded decentralized location with a significant
share of renewable power source and stringent contamination
control policies for WWTPs.
Juneja and Murthy (2017) applied an engineering course model
on a renewable diesel (RD) production facility with a processing
capacity of 60 Mgal wastewater/day to conduct a TEA of RD
production on a commercial scale. Algal RD yields were
estimated as 10.18 MML/year, with a production cost per RD of
$1.75. According to the LCA, the manufacturing of RD resulted in
GHG emissions that were 6.2 times lower than those of regular
diesel. Sensitivity study suggested that using high lipid algae or
expanding the production facility could lower the cost of producing
ethanol. The long-term viability of RD in an ecologically friendly
path was found to be helped by the integrated TEA and LCA
evaluations. Gholkar et al. (2021) used microalgal biomass grown
on steel mill wastewater to perform TEA and LCA on hydrogen and
methane generation, and they found life cycle climate change impact
was 7.56 kg CO
2
eq./kg hydrogen generated, which was 36.47% less
than steam reforming of methane. It was not economically possible
to produce methane. Yet, with a life cycle climate change impact of
1.18 kg CO2 eq./kg methane generated, it was more environmentally
friendly. Sensitivity analysis was done to find areas where costs and
life cycle impacts could be further reduced.
Mu et al. (2014), evaluated the environmental performance of
wastewater based algal biofuels and reported that environmental
performance of wastewater based algal biofuels was much better
than freshwater based algal biofuels however, the characteristics
of wastewater and conversion technologies played key role in
performance and LCA. Arashiro et al. (2022) conducted LCA to
compare microalgae systems for wastewater treatment and
bioproducts for treating industrial wastewater (food sector)
and urban wastewater, with the goal of recovering bioproducts
and bioenergy. Findings showed that microalgal based WWT
systems assisted bioproduct recovery can minimise the
environmental impacts up to five times in comparison to
conventional systems owing to the lower chemical consumption
for microalgae cultivation. Because of its higher quality compared to
urban wastewater, which also permits the growing of a single species of
microalgae, food industry effluent emerged as the most efficient set-up
for the recovery of bioproducts from microalgae treated
wastewater. In conclusion, the LCA advocated the microalgae
wastewater treatment systems a promising alternative for
wastewater treatment and nutrient recovery. Castro et al.
(2020) used LCA to compare the environmental effects of
producing microalgae biomass-based phosphate biofertilizer to
triple superphosphate. Phosphorus recovery was made from the
wastewater of the meat processing sector using microalgae.
Climate change impact was found to be 3.17 kg CO
2
eq.
Microalgae biofertilizer had higher environmental effect than
conventional fertilizer in all impact categories. To get around
this, a seamless set-up was established that used photovoltaic
panels as the power source and a drying bed for biomass drying
instead of thermal drying; Styles et al. (2018) used LCA to
evaluate the production and usage of digestate biofertilizer
(DBF) derived from liquid digestate (LD), accounting for SFS
efficacy, with the conventional management of LD from food
waste. More judicious use of nutrients in the DBF product could
result in benefits in terms of environment and economy by
preventing methane, nitrogen oxides, and ammonia gas
emissions from LD and SFS. They came to the conclusion that
DBF extraction was economically advantageous and sustainable
after averaging these results against larger environmental
loadings.
Henceforth, the use of LCA is extremely helpful in determining
the energy products and co-products which will demonstrate higher
economic viability and environmental performance (Collet et al.,
2015). Since there is a large variability in assessments done by
multiple studies, Quinn and Davis (2014) emphasized the need for
integrated Resource assessment (RA), Techno economic analysis
(TEA), and LCA constructed on a foundation of experimentally
validated technological system models.
6 Challenges in resource recovery and
wastewater valorisation
Wastewater valorisation is a potential method to reuse and
recycle water and nutrients for economic generation and
environmental protection. However, the current status and
technological bottlenecks still inhibit the effective use of this
concept. Different countries have different obstacles that they
need to overcome. The difficulty of these obstacles can vary
depending on the situation.
Frontiers in Chemical Engineering frontiersin.org13
Kathi et al. 10.3389/fceng.2023.1129783
1. The main challenge with Membrane separation technology is
figuring out which membrane is best suited for the desired
outcome. It is important to choose the right membrane
suitable for the process to be successful. To compete with the
existing conventional technologies, membrane clogging and
fouling are still considered the primary stumbling blocks for
membrane-based separation.
2. Enzymatic catalysis and recovery of products depend upon the
physicochemical properties of wastewater, such as pH; since
enzymes are heat and pH labile and are easily denatured in
highly basic or acidic solutions, this may limit their functioning,
increasing the cost of treatment and valorisation.
3. Microalgae-based WWT is one of the most optimistic
technologies for waste’s advanced treatment and nutrient
recovery. Most studies have been conducted at pilot or bench
scale and under controlled conditions. Thus, cultivation under
outdoor conditions and scaling up of reactors are still needed to
evaluate the direct interferences on processes involved in the
valorisation of wastewater. However, large-scale cultivation
under outdoor conditions is still a challenge.
4. Economic, technical, political, institutional, and social challenges
are frequently encountered in implementing small and large-
scale valorisation operations.
5. Another big challenge is the lack of trust and awareness of
recycled/recovered and valorised products. Many consumers
may be skeptical about the quality of valorised and recycled
products. The consumer may perceive the products based on the
Circular economy as high-risk and uncertain.
6. Additionally, if the price of non-circular products is higher than
circular/valorised products, then taxation on virgin resources
(resources that have not been used before) could be used to make
the circular/valorised products more attractive to consumers.
Thus, taxation on virgin resources might provide an advantage.
7. A key barrier often presents unavailable and irregular
infrastructure. in developing countries, these systems and
services are often not up to the same standards as in more
developed countries.
7 Conclusion and recommendations
Adopting the circular economy concept is crucial to meet global
challenges. These global challenges include increasing demand for
water supply, water scarcity, and declining non-renewable resources.
Sludge and wastewater are no longer considered to be trash, but rather
a source of valuable products that can benefit environmentally and
social-economically. In this review, authors have explained the
wastewater and sludge valorisation for multiple aspects such as
water resource management, environmental protection, resource
recovery, and generation of value-added products. With the aid of
the cutting-edge technologies, wastewater can be efficiently treated,
resulting in effluents of a calibre that meets the standards of industry
and agriculture. Water reuse is not the only way to value wastewater.
Recognizing that wastewater contains a variety of valuable resources
that can be recovered is vital. In order to reduce pollution, protect the
environment, wastewater treatment processes must recover nutrients.
Traditional approaches by themselves are insufficient to extract
nutrients in high quantities and of desired quality until they are
united with membrane technology. The use of the unique hybrid
systems in nutrient recovery is highlighted. To increase the
technological viability and lower operational expenses, further
work must still be done to improve the recovery system’s usability.
Finally, apart from this, governments should put into place relevant
legislation, policies, and regulations to support and encourage nutrient
recovery strategies.
Nutrients such as phosphorous, nitrogen, phenolic compounds,
antioxidants, vitamins, proteins, enzymes, and other value-added
products can be potentially recovered from wastewater from various
industrial sectors. High-end and novel separation and purification
technologies, such as membrane technology, enzyme-mediated
separation, chromatography, microalgae cultivation, and novel
adsorbents, enable efficient recovery of nutrients and treatment
of wastewater. Various strategies can be applied to valorise this
waste related to excess sludge produced during wastewater
treatment. Value-added substances made from anaerobic sludge
digestion, such as bio-fertilizers, bioplastics, biofuels, and
biocompost, are already used extensively in large-scale processes.
Yet, incorporating the water sector into the circular economy has
increased collaboration between the public sector, academia, and
industry. Furthermore, comprehensive cost-benefit analyses (CBA)
and LCA may support the implementation of the suggested
technologies as workable substitutes. Moreover, valorisation
procedures are more dependable as compared to conventional
disposal methods because they generate value and minimise
pollution, even when the market value of the recovered materials
may not be high enough to justify their application.
Author contributions
SK compiled and edited the manuscript. SS and RY framed the
concept and worked on the figures and tables. AS supervised the
various concepts distributed across the institutions. AM completed
the final editing. All authors contributed to the article and approved
the submitted version.
Funding
The authors gratefully acknowledge the reviewers for their
insightful comments and suggestions. SS acknowledges financial
assistance in the form of a research project under Women Scientist
Scheme-B, WISE-KIRAN DIVISION, Department of Science and
Technology, Ministry of Science and Technology, Government of
India (Project Grant No. DST/WOS-B/2018/1589).
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Frontiers in Chemical Engineering frontiersin.org14
Kathi et al. 10.3389/fceng.2023.1129783
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
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