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Microbial fuel cell application for sludge remediation and minimization
Microbial fuel cell application for sludge remediation and minimization
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Gauravarapu Navlur Nikhil Department of Biotechnology, Dr. B.R. Ambedkar National Institute of Technology Jalandhar, Punjab, India
1
Introduction
Sewage sludge is a by-product of municipal wastewater treatment, generated from primary and secondary sedimentation. Sewage denotes both black water and gray water at the household level, where black water refers to wastewater generated in toilets and gray water to the wastewater generated in kitchen, bathroom, and laundry (Demirbas et al., 2017). Sewage sludge is the semisolid precipitate produced in wastewater treatment plants (WWTPs). A survey analysis by the nonprofit Centre for Science and Environment (CSE) reported that India generates a staggering 1.7 million tons of sewage waste every day and about 78% of the sewage generated remains untreated and is disposed of in rivers, groundwater, or lakes. The CSE analysis points out that the amount of sewage generated in 2009 was 38,255 million liters daily (MLD), while India only had the capacity to treat 11,788 MLD—a mere 30% of the total. However, the actual sewage treated was even less, at 8251 MLD, which was about 22%. It has been reported that in India only 33% of houses are connected to sewer systems, only around 38% use septic tanks. These septic tanks do not treat waste and have to be emptied periodically. The services are offered to clean septic tanks, pump out the sludge from the tanks and empty it into drains, fields—just about anywhere. Also, cities are not able to recover the high costs of supply and therefore have no money to invest in sewage treatment (Chandramouli and General, 2011). Sewage sludge generation in India is increasing at a faster rate as more and more sewage treatment plants (STPs) are required; in addition, alternative ecotechnologies are been developed to relieve this serious issue. At the moment, sewage sludge and effluents from these STPs are frequently disposed of on agricultural land for irrigation or manure (Singh and Agrawal, 2008). In India, wastewater disposal systems are usually managed by local bodies. In a few specific cases, they are managed by departments/statutory boards set up by state governments. This service facility falls into the water supply and sanitation sector. In municipal WWTPs, the treatment and disposal of sewage sludge can comprise up to 50% of the operation costs (Appels et al., 2008).
Industrial and Municipal Sludge. https://doi.org/10.1016/B978-0-12-815907-1.00032-5 © 2019 Elsevier Inc. All rights reserved.
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1.1 Scope and objectives Sewage treatment is a process that removes the majority of the contaminants from wastewater or sewage and produces both a liquid effluent suitable for disposal in the natural environment and sludge. Estimates from the Ministry of New and Renewable Energy (MNRE) in India suggest that there is a potential of about 226 MW to be generated from treated and untreated sewage sludge (Vaish et al., 2016). There are several approaches for treating sludge to reduce solid content and to stabilize biomass; however, anaerobic digestion (AD) is generally preferred because of its costeffectiveness and bioenergy production (Ge et al., 2013). Digested sludge can be further composted for agricultural use, and biogas can be converted to electricity and/or heat through combustion, thus compensating for some of the energy use in a WWTP (Rulkens, 2007). Because of a large amount of organic content, primary sludge contains about 66% of the energy content of wastewater (Ting and Lee, 2007), and about 81% of biodegradable organic energy may be converted to methane (CH4) (McCarty et al., 2011). Despite the great energy potential of biogas production, several issues limit successful AD application; for instance, electric generators and their maintenance are costly, and biogas may need pretreatment to remove contaminants such as hydrogen sulfide (Appels et al., 2008). In addition, energy will be lost during CH4 conversion because the common efficiency of CH4-to-electricity conversion is about 33% (Ge et al., 2013). Efficient, noninvasive techniques are desired for restoring organic-contaminated sludge. Therefore, it is of great interest to explore alternative technologies for sludge treatment and energy recovery. This chapter comprehensively describes microbial fuel cell (MFC) technology. Considering the MFC research trends, the following objectives are pursued in this chapter: (1) to highlight MFC for sludge remediation and to provide insights into how to extend its application to a large scale; (2) to update the current status of MFC technology in India and (3) to present a vision for the future of sludge management and remediation.
2
MFCs—Construction and working mechanism
Bioelectrochemical technology, typically involving MFCs, has been widely used as an environmental remediation approach. Its applications include wastewater, sludge, sediment, and soil remediation (Butti et al., 2016). MFCs directly produce electricity from organic wastes, in which bacteria oxidize organic and inorganic compounds in the anode chamber and generate protons and electrons that travel to the cathode to reduce oxygen to water. Electron flow from the anode to the cathode generates electric current or power if a load is connected (Fig. 1). The potential of contamination removal has numerous aspects. First, an inexhaustible anode is introduced as the electron acceptor, which solves the problem of lack of electron acceptors. Second, the biocurrent stimulates the growth and activity of functional microbes and symbiotic microflora, which in turn accelerates the biodegradation of organic pollutants. Third, the coupling effect derived from the biocurrent boosts the oriented transfer of free electrons; this speeds up the rate of the oxidation-reduction reaction. Finally, the method of synchronous power production
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Fig. 1 Schematic diagram of microbial fuel cell during process operation.
from MFCs is the opposite of that of electrokinetic remediation, which needs power input; to some extent, this reduces the cost of environmental governance. Additionally, bioelectrochemical technology has no secondary pollution and is more specifically suited to clear low-concentration contaminants with the advantages of a high energy conversion rate, operation under normal temperature and pressure and in mild conditions, and easy to control (Wang and Ren, 2013). Theoretically, MFCs are able to utilize any kind of biodegradable organic substance. The degradation kinetics are different and dominated by the structural characteristics of substances and their operational conditions (Pant et al., 2010). MFCs reduce the consumption of energy and the production of by-products, which are the main advantages of their application. Compared with more complicated wastewater treatment by bioelectrochemical systems, simpler sediment MFCs have been under development in the last decade. Based on a previously successful bioelectrochemical methodology in dealing with wastewater and sediment, sludge remediation has recently emerged. MFC remediation belongs to the category of bioelectrochemical remediation, which is one of the biostimulation technologies (Lu et al., 2014; Rozendal et al., 2008). When an air-cathode MFC is used, the pollutant is biodegraded by microbial catalysis as the electron donor. Electrons are exported from the intracellular space and (1) hop to the anode directly (Fig. 2A), (2) are transferred by the mediators (e.g., flavin or thionine) to the anode (Fig. 2B), and (3) reach the anode by flowing through the nanowires (i.e., pili) (Fig. 2C). Subsequently, electrons are conveyed to the air cathode through the external circuit, and react with oxygen (terminal electron acceptor) to form water (Venkata Mohan et al., 2014). In MFCs for sludge remediation, the existence of anodes accelerates the capability of the electrogenic microorganism to provide more electrons to promote the metabolic reaction rates of anaerobic bacteria that degrade the contaminants. The main advantages of using MFCs in wastewater treatment come from the savings of aeration energy and sludge disposal (Oh et al., 2010). For traditional activated sludge systems, aeration can amount to 45%–75% of plant energy costs, so the conversion of aeration tank to MFC units is very beneficial because not only does it eliminates aeration energy consumption, but studies also showed that MFCs can
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Fig. 2 Electron transfer mechanisms involved during microbial fuel cell operation where (A) direct electron transfer, (B) an electron shuttle, and (C) a solid conductive matrix (nanowires).
produce 10%–20% more energy that can be used for other processes (Huggins et al., 2013; Pant et al., 2010). The reported maximum power density from lab-scale aircathode MFCs has reached 2.87 kW/m3, making them promising for commercialization development (Fan et al., 2007), even though the system scale-up remains a major challenge. Another main benefit of MFC systems is the low biomass production. The MFC is a biofilm-based system, and the cell yield of electrochemically active bacteria (0.07–0.16 gVSS/gCOD) is much less than the activated sludge (0.35–0.45 gVSS/ gCOD), so it can reduce sludge production by 50%–70% (Fan et al., 2007; Huggins et al., 2013), which in turn may reduce 20%–30% of the plant operation costs. Other benefits may include nutrient removal and the production of value-added products, such as caustic solutions for disinfection, or H2 and biogas for energy, which will be discussed more extensively in the following sections. Diverting some organic compounds to direct electricity generation in MFCs could reduce biogas processing and conversion, resulting in some potential economic benefits, but we also need to understand the challenges of MFC application. For example, MFCs generally have much more complex structures and higher capital cost than anaerobic digesters (Pham et al., 2006). The use of high-surface-area electrodes and high-solid substrates like sludge can create problems such as reactor clogging. Unlike anaerobic digesters, which can be constructed in a single reactor with a large volume, MFCs are expected to be built in small-scale modules to form an MFC assembly; a single MFC with a very large volume will have a greater distance between the anode and the cathode electrodes, thereby increasing the internal resistance and decreasing electricity generation. With multiple small-scale MFC modules, the heating and feeding of the anolytes will be very challenging (Kondaveeti et al., 2018).
3
Applications of MFC for sludge remediation
In recent decades, the development of MFC applications has attracted increasing attention. A vast number of studies were performed in the fields of wastewater, sediment, sludge, and soil treatment. The use of MFCs is a promising approach for the
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direct production of electric energy or other energy carriers, such as hydrogen gas, from various organic substrates (Logan and Regan, 2006; Pant et al., 2010). Sewage sludge also has been studied in MFCs for electricity generation. A single-chamber MFC with a baffle inside its anode compartment generated low power from anaerobic sludge due to large internal resistance from the baffle (Hu, 2008). Hydrolysis is considered to be a limiting step in AD (Halalsheh et al., 2005), therefore appropriate pretreatment of solid organic waste is expected to improve the contents of soluble and small-particle organics which can be better used by microorganisms. Chandrasekhar and Venkata Mohan (2014) have reported bioelectrohydrolysis as a potential strategy for pretreatment of solid waste with simultaneous power generation. The ultrasonic and alkaline pretreatment of sludge improved its degradability and resulted in a higher power output of 12.5 W/m3, with 61.0% and 62.9% reduction of total chemical oxygen demand (TCOD) and volatile solids (VS), respectively ( Jiang et al., 2010; Jiang et al., 2009). Likewise, improved power output and total solid production were observed in an MFC after pretreatment with sterilization and alkalization (Xiao et al., 2011). When an MFC was linked to an anaerobic digester to form an integrated recirculation loop, it was found that CH4 production was higher than the digester alone (Inglesby and Fisher, 2012) because a high concentration of ammonium/ammonia will inhibit methanogenic activity (Sung and Liu, 2003). The improved biogas production, resulting from the use of a recirculation loop, was likely due to the migration of ammonium ions from the digester to the cathode compartment of the MFC driven by electricity generation in the MFC, which was also demonstrated previously (Kim et al., 2008). A recent study reported the performance of MFCs in treating a fermentation solution from primary sludge, in which higher power production was obtained when treating a mixture of fermentation supernatant and primary effluent because of elevated concentrations of soluble COD and volatile fatty acids (VFAs) after the fermentation process (Yang et al., 2013). In another study, long-term investigation (almost 500 days) of MFCs treating sewage sludge for energy production, organics removal, and solid reduction were reported (Ge et al., 2013). The experiment consisted of two phases: In Phase I, two tubular MFCs were operated with primary sludge and digested sludge, respectively, for more than 10 months; in Phase II, both MFCs were operated as a two-stage system to treat primary sludge for about 6 months. Biogas production in the MFCs was examined and compared energy production between MFCs and anaerobic digesters. The study concluded that the MFCs fed with the primary sludge acted mostly as modified anaerobic digesters. Alternatively, MFCs could function as a posttreatment process to polish the supernatant of digested sludge. Recently studies on MFCs for sludge remediation and energy recovery are being reported and a few are listed in Table 1.
4
Current status
As the global demand for renewable energy and organic matter increases, organic wastes, including sewage sludge, could be one of the available resources to use for this purpose. This substrate can be used as an energy resource for power and heat
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Table 1 Comparison of MFC performances in systems that employed different inoculum, substrate, and designs MFC configuration
Inoculum/ pretreatment
Two-chamber Singlechamber
Anaerobic sludge Effluent from MFC
Singlechamber
Effluent from MFC
Tubular MFC
Raw sludge from a primary sedimentation tank
Two-chamber
Anaerobic microbial consortium
Substrate Pretreated activated sludge Fermented primary sludge with 15.5 g/L soluble COD Fermented primary sludge with 15.5 g/L soluble COD Primary sludge and digested sludge from anaerobic digestors Secondary sludge obtained from a sewage treatment plant
Power density 5.6 μW/cm2 32.0 μW/cm2
References Xiao et al. (2011) Yang et al. (2013)
103 μW/cm2
Yusoff et al. (2013)
1.43 and 1.80 kWh/ m3, respectively
Ge et al. (2013)
13.5 μW/cm2
Passos et al. (2016)
via emerging technologies. Moreover, sewage sludge can be considered as a substrate for soil fertilization and remediation if the applied technology allows high-quality products to be obtained. Such reuses of sewage sludge are economically viable and environmentally sustainable compared to waste-handling and land-filling (Rulkens, 2007). The economic potential is found in offsetting costs related to traditional waste treatment methods and decreased energy costs by the generation of biogas and biofuels, which can partially replace the usage of traditional fuels (Kacprzak et al., 2017). To achieve these goals, wastewater treatment operations require careful management of sewage sludge. The management of sewage sludge in an economically and environmentally acceptable manner is one of the critical issues facing modern society, due to the rapid increase in sludge production. In India, sludge management is governed by Municipal Solid Waste (MSW) Handling Rules, 2000 if the sludge is categorized as nonhazardous and the sewage sludge mostly falls into this category. However, if the sludge is characterized as hazardous for any reason, the disposal and management is governed by Hazardous Waste Handling and Management rules, as stipulated by an amended version of the regulations in 2003. All these regulations are administered by the Ministry of Environment and Forests (MoEF) (Chandrappa and Das, 2012).
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A paradigm shift in waste remediation and waste management has been noticed in India since the launch of the Swachh Bharat Mission (SBM), an official campaign initiated by the government of India in October 2014, which aims to clean up the streets, roads, and infrastructure of the cities, smaller towns, and rural areas by the end of 2019. The government has a clearly defined path for achieving open defecation-free (ODF) cities and districts/villages. More critically, there is also a well-defined process, for the different phases of the mission, across the sanitation value chain called build, use, maintain, and treat (BUMT). A survey conducted by the Government of India in 2017 declared Indore as cleanest city based on various sanitation and cleanliness parameters, including waste collection, ODF status, and feedback from citizens. The survey is part of the government’s initiative toward a cleaner India. Its focus on sanitation, preventing open defecation, and waste collection is significant, considering their impact on the environment and on the health of city dwellers. The southern Indian city of Bangalore is extolled as India’s "Silicon Valley." It has experienced a growth of 45% of its urban population in a span of five years, crossing the 12 million population mark in 2016. The Water Supply and Sewerage Board of Bangalore has selected the French multinational corporation SUEZ to support the city’s efforts to build sludge recovery plants and improve the wastewater infrastructures for two million inhabitants of Vrishabhavathi Valley. SUEZ has operations primarily in water, electricity, and natural gas supply, as well as waste management. The SUEZ group will be in charge of building a new WWTP of 150,000 m3/day, besides, rehabilitation of an existing plant of 150,000 m3/day, and also, building of a sewage sludge recycling and recovery plant coming from these two plants. This rapid urban rise, combined with the development of business activities, has led to an explosion in the demand for drinking water supply and sewage disposal. The common sludge treatment plant for the two treatment plants will be equipped with SUEZ’s Degremont technologies (Sedipac and Digelis), which will convert into electricity the biogas produced in order to make the plant self-sufficient. During the first three years, the group will design and build a new plant that uses activated sludge process (i.e., a bacterial treatment) to treat effluents to achieve an optimum discharge quality into the Vrishabhavathi River in accordance with the most recent standards of the Indian authorities, requiring in particular a total nitrogen content of less than 10 mg/L.
5
Future scope and perspective
The world is undergoing a new wave of urbanization, especially in developing countries, leading to multiple impacts on natural resources and the environment. A review of the research literature suggests that waste management poses various challenges and opportunities at an urban and rural scale (Reddy et al., 2018). In this context, biorefinery has emerged as a potential alternative to petroleum-based refineries, where biomass of nonedible feedstocks/biogenic wastes are used as raw materials and a range of products, such as biofuel, industrial biochemicals, and biomaterials including commercially important biopolymers, are produced (Clark and Deswarte, 2015).
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Concepts such as the biorefinery create aspirations toward increasingly integrated technologies (Aresta et al., 2012). The burgeoning amount of waste (specifically sewage sludge) has tremendous potential, but to date, humankind has not exploited even a fraction of that potential. Waste as a prime target substance in the biorefinery, with a wide range of opportunities for mercantile interests rather than for utilitarian reasons (Tuck et al., 2012; Zondervan et al., 2011). A vivid, articulate review by Venkata Mohan et al. (2016) presented the possibilities of biorefinery models that could be explored for the utilization of biogenic wastes as a resource in the framework of a circular economy. The most popular microbial and photosynthetic processes are widely deployed for many technological solutions. Integrating various models involving unit operations and bioprocesses gives a much more holistic outcome through an updated “product versus energy” approach explored in the bioeconomy (Fig. 3). An expression of interest in modern energy and material recovery technologies using waste feedstock often attempts to integrate processes and remediation. Therefore, this paradigm transition from a linear economy toward a circular economy is obligatory; the futuristic biorefinery platform should have an ambitious vision to promote a switch from the consumption of fossil reserves to the use of renewable waste as
Fig. 3 Conceptualized flow sheet of sludge biorefinery that includes: Photo-synthetic process—bioelectrochemical process (microbial fuel cells) and thermo-chemical process. In the figure, the direction and color of arrows infer—bi-directional yellow bold arrows indicate formation and usage of excess sludge—uni-directional grey bold arrows indicate bioprocess yielding products—uni-directional dashed arrows indicate conditional channeling or by-products of that bioprocess.
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resources. This will also lead to incredible employment opportunities in industry and academia, especially in the sectors of agriculture, food, chemicals, healthcare, pharmaceuticals, and logistics (Amulya et al., 2016). However, the sustainable biorefinery is viewed as a way to move fuels and other valorized coproducts onto much greener footprints, with life-cycle analysis providing the rigor needed to make the best decisions on feedstock utilization, upscaling process technology, and value-chain conformation (Liu and Shonnard, 2014) Gottumukkala et al. (2016) reviewed opportunities and prospects of biorefinerybased valorization of pulp and paper sludge. The paper and pulp industry (PPI) generates a large amount of solid waste with high moisture content. Numerous opportunities exist for the valorization of waste paper sludge, although this paper focused on primary sludge with high cellulose content. The most mature options for paper sludge valorization are fermentation, AD, and pyrolysis. In this review, biochemical and thermal processes are considered individually and also as an integrated biorefinery. The objective of an integrated biorefinery is to reduce or avoid paper sludge disposal by landfilling, water reclamation, and value addition. The assessments of selected processes for biorefinery vary from a detailed analysis of a single process to high-level optimization and integration of the processes that allow the initial assessment and comparison of technologies. This data can be used to provide key stakeholders with a road map of technologies that can generate economic benefits and reduce carbon waste and pollution load. Crutchik et al. (2018) recently reported on a biorefinery of cellulosic primary sludge toward targeted short-chain fatty acids (SCFAs), phosphorus, and CH4 recovery. Cellulose from used toilet paper is a major untapped resource embedded in municipal wastewater, of which recovery and valorization to valuable products can be optimized. Cellulosic primary sludge (CPS) can be separated by upstream dynamic sieving and anaerobically digested to recover as much as 4.02 m3/capitayear of CH4. On the other hand, optimal acidogenic fermenting conditions of CPS allows the production of targeted SCFAs to reach as much as 2.92 kg COD/capitayear. Here, propionate content can be more than 30% and can optimize the enhanced biological phosphorus removal (EBPR) processes or the higher valuable copolymer of polyhydroxyalkanoates (PHAs). In this work, the optimized conditions were applied in the long term to a sequencing batch fermentation reactor where the highest propionate production (100–120 mg COD/g TVSfedd) was obtained at 37°C and adjusting the feeding pH at 8. At the same time, around 88% of the phosphorus released during the acidogenic fermentation was recovered (amounting to as much as 0.15 kg of struvite per capitayear). Finally, the potential market value was preliminarily estimated for the recovered materials, which can triple over the conventional scenario of biogas recovery in existing municipal WWTPs. Zhang et al. (2018) reported on value-added products derived from WAS in a biorefinery scenario. Substantial research has been carried out on sustainable WAS management over the last decade to endorse WAS as a feedstock for the production of bioproducts such as amino acids, proteins, short chain fatty acids, enzymes, biopesticides, bioplastics, bioflocculants, and biosurfactants. In this review, a critical
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assessment of the production process for a wide range of value-added products from WAS, their current limitations, and recommendations for future research to help promote more sustainable management of this underutilized and ever-growing waste stream has been discussed.
6
Conclusions
In India, management of solid wastes in general, and sewage sludge in particular, is going to be a big challenge in the near future due to rapid urbanization and economic growth. Unless technoeconomically viable alternatives are developed, sewage sludge management is going to be a formidable task in environmental management. MFCs are a promising alternative and renewable technology for the current water-waste conundrum. Based on the current literature review, MFCs have satisfactorily reduced both organics and suspended solids in sewage sludge. The total energy production from primary sludge in the two-stage MFC system was comparable to that of anaerobic digesters; however, direct electricity generation made a minor contribution, while energy from biogas still dominated the overall energy production. It will be very challenging to apply MFC technology to treating primary sludge; but MFCs may be used to polish the digested effluent from anaerobic digesters, offering potential benefits in energy savings compared with aerobic treatment. Alternatively, the proposed biorefinery approach could be a potential sludge management option, along with value addition.
Acknowledgments The author would like to sincerely thank the editors of this book for giving this opportunity to contribute a chapter. I also give my heartfelt gratitude to Dr. S. Venkata Mohan (Book Editor) for his kind support and valuable suggestions.
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Chandrappa, R., Das, D.B., 2012. Solid Waste Management: Principles and Practice. Springer Science and Business Media. Chandrasekhar, K., Venkata Mohan, S., 2014. Bio-electrohydrolysis as a pretreatment strategy to catabolize complex food waste in closed circuitry: function of electron flux to enhance acidogenic biohydrogen production. Int. J. Hydrog. Energy 39 (22), 11411–11422. Clark, J.H., Deswarte, F., 2015. Introduction to Chemicals from Biomass. John Wiley and Sons. Crutchik, D., Frison, N., Eusebi, A.L., Fatone, F., 2018. Biorefinery of cellulosic primary sludge towards targeted short chain fatty acids, phosphorus and methane recovery. Water Res. 136, 112–119. Demirbas, A., Edris, G., Alalayah, W.M., 2017. Sludge production from municipal wastewater treatment in sewage treatment plant. Energ Source Part A 39 (10), 999–1006. Fan, Y., Hu, H., Liu, H., 2007. Enhanced Coulombic efficiency and power density of aircathode microbial fuel cells with an improved cell configuration. J. Power Sources 171 (2), 348–354. Ge, Z., Zhang, F., Grimaud, J., Hurst, J., He, Z., 2013. Long-term investigation of microbial fuel cells treating primary sludge or digested sludge. Bioresour. Technol. 136, 509–514. Gottumukkala, L.D., Haigh, K., Collard, F.-X., Van Rensburg, E., G€ orgens, J., 2016. Opportunities and prospects of biorefinery-based valorisation of pulp and paper sludge. Bioresour. Technol. 215, 37–49. Halalsheh, M., Koppes, J., Den Elzen, J., Zeeman, G., Fayyad, M., Lettinga, G., 2005. Effect of SRT and temperature on biological conversions and the related scum-forming potential. Water Res. 39 (12), 2475–2482. Hu, Z., 2008. Electricity generation by a baffle-chamber membraneless microbial fuel cell. J. Power Sources 179 (1), 27–33. Huggins, T., Fallgren, P., Jin, S., Ren, Z., 2013. Energy and performance comparison of microbial fuel cell and conventional aeration treating of wastewater. J. Microb. Biochem. Technol. 6(2). Inglesby, A.E., Fisher, A.C., 2012. Enhanced methane yields from anaerobic digestion of Arthrospira maxima biomass in an advanced flow-through reactor with an integrated recirculation loop microbial fuel cell. Energy Environ. Sci. 5 (7), 7996–8006. Jiang, J., Zhao, Q., Wang, K., Wei, L., Zhang, G., Zhang, J., 2010. Effect of ultrasonic and alkaline pretreatment on sludge degradation and electricity generation by microbial fuel cell. Water Sci. Technol. 61 (11), 2915–2921. Jiang, J., Zhao, Q., Zhang, J., Zhang, G., Lee, D.-J., 2009. Electricity generation from biotreatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 100 (23), 5808–5812. Kacprzak, M., Neczaj, E., Fijałkowski, K., Grobelak, A., Grosser, A., Worwag, M., Rorat, A., ˚ ., Singh, B.R., 2017. Sewage sludge disposal strategies for sustainBrattebo, H., Alma˚s, A able development. Environ. Res. 156, 39–46. Kim, J.R., Zuo, Y., Regan, J.M., Logan, B.E., 2008. Analysis of ammonia loss mechanisms in microbial fuel cells treating animal wastewater. Biotechnol. Bioeng. 99 (5), 1120–1127. Kondaveeti, S.K., Seelam, J.S., Mohanakrishna, G., 2018. Anodic electron transfer mechanism in bioelectrochemical systems. In: Microbial Fuel Cell. Springer, pp. 87–100. Liu, J., Shonnard, D.R., 2014. Life cycle carbon footprint of ethanol and potassium acetate produced from a forest product wastewater stream by a co-located biorefinery. ACS Sustain. Chem. Eng. 2 (8), 1951–1958. Logan, B.E., Regan, J.M., 2006. Microbial fuel cells-challenges and applications. Environ. Sci. Technol. 40 (17), 5172–5180.
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Lu, L., Huggins, T., Jin, S., Zuo, Y., Ren, Z.J., 2014. Microbial metabolism and community structure in response to bioelectrochemically enhanced remediation of petroleum hydrocarbon-contaminated soil. Environ. Sci. Technol. 48 (7), 4021–4029. McCarty, P.L., Bae, J., Kim, J., 2011. Domestic Wastewater Treatment as a Net Energy Producer – Can This Be Achieved? ACS Publications. Oh, S.T., Kim, J.R., Premier, G.C., Lee, T.H., Kim, C., Sloan, W.T., 2010. Sustainable wastewater treatment: how might microbial fuel cells contribute. Biotechnol. Adv. 28 (6), 871–881. Pant, D., Van Bogaert, G., Diels, L., Vanbroekhoven, K., 2010. A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresour. Technol. 101 (6), 1533–1543. Passos, V.F., Aquino Neto, S., Andrade, A.R.D., Reginatto, V., 2016. Energy generation in a Microbial Fuel Cell using anaerobic sludge from a wastewater treatment plant. Sci. Agric. 73 (5), 424–428. Pham, T., Rabaey, K., Aelterman, P., Clauwaert, P., De Schamphelaire, L., Boon, N., Verstraete, W., 2006. Microbial fuel cells in relation to conventional anaerobic digestion technology. Eng. Life Sci. 6 (3), 285–292. Reddy, C.N., Sudhakar, M., Min, B., Shanmugam, P., 2018. Future perspectives on costeffective microbial fuel cells in rural areas. In: Microbial Fuel Cell Technology for Bioelectricity. Springer, pp. 283–302. Rozendal, R.A., Hamelers, H.V., Rabaey, K., Keller, J., Buisman, C.J., 2008. Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol. 26 (8), 450–459. Rulkens, W., 2007. Sewage sludge as a biomass resource for the production of energy: overview and assessment of the various options. Energy Fuel 22 (1), 9–15. Singh, R., Agrawal, M., 2008. Potential benefits and risks of land application of sewage sludge. Waste Manag. 28 (2), 347–358. Sung, S., Liu, T., 2003. Ammonia inhibition on thermophilic anaerobic digestion. Chemosphere 53 (1), 43–52. Ting, C., Lee, D.-J., 2007. Production of hydrogen and methane from wastewater sludge using anaerobic fermentation. Int. J. Hydrog. Energy 32 (6), 677–682. Tuck, C.O., Perez, E., Horva´th, I.T., Sheldon, R.A., Poliakoff, M., 2012. Valorization of biomass: deriving more value from waste. Science 337 (6095), 695–699. Vaish, B., Srivastava, V., Singh, P., Singh, A., Singh, P.K., Singh, R.P., 2016. Exploring untapped energy potential of urban solid waste. Energy Ecol. Environ. 1 (5), 323–342. Venkata Mohan, S., Nikhil, G.N., Chiranjeevi, P., Reddy, C.N., Rohit, M., Kumar, A.N., Sarkar, O., 2016. Waste biorefinery models towards sustainable circular bioeconomy: critical review and future perspectives. Bioresour. Technol. 215, 2–12. Venkata Mohan, S., Velvizhi, G., Modestra, J.A., Srikanth, S., 2014. Microbial fuel cell: critical factors regulating bio-catalyzed electrochemical process and recent advancements. Renew. Sust. Energ. Rev. 40, 779–797. Wang, H., Ren, Z.J., 2013. A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnol. Adv. 31 (8), 1796–1807. Xiao, B., Yang, F., Liu, J., 2011. Enhancing simultaneous electricity production and reduction of sewage sludge in two-chamber MFC by aerobic sludge digestion and sludge pretreatments. J. Hazard. Mater. 189 (1-2), 444–449. Yang, F., Ren, L., Pu, Y., Logan, B.E., 2013. Electricity generation from fermented primary sludge using single-chamber air-cathode microbial fuel cells. Bioresour. Technol. 128, 784–787.
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Yusoff, M.Z.M., Hu, A., Feng, C., Maeda, T., Shirai, Y., Hassan, M.A., Yu, C.P., 2013. Influence of pretreated activated sludge for electricity generation in microbial fuel cell application. Bioresour. Technol. 145, 90–96. Zhang, W., Alvarez-Gaitan, J.P., Dastyar, W., Saint, C.P., Zhao, M., Short, M.D., 2018. Valueadded products derived from waste activated sludge: a biorefinery perspective. Water 10 (5), 545. Zondervan, E., Nawaz, M., de Haan, A.B., Woodley, J.M., Gani, R., 2011. Optimal design of a multi-product biorefinery system. Comput. Chem. Eng. 35 (9), 1752–1766.
32
Gauravarapu Navlur Nikhil Department of Biotechnology, Dr. B.R. Ambedkar National Institute of Technology Jalandhar, Punjab, India
1
Introduction
Sewage sludge is a by-product of municipal wastewater treatment, generated from primary and secondary sedimentation. Sewage denotes both black water and gray water at the household level, where black water refers to wastewater generated in toilets and gray water to the wastewater generated in kitchen, bathroom, and laundry (Demirbas et al., 2017). Sewage sludge is the semisolid precipitate produced in wastewater treatment plants (WWTPs). A survey analysis by the nonprofit Centre for Science and Environment (CSE) reported that India generates a staggering 1.7 million tons of sewage waste every day and about 78% of the sewage generated remains untreated and is disposed of in rivers, groundwater, or lakes. The CSE analysis points out that the amount of sewage generated in 2009 was 38,255 million liters daily (MLD), while India only had the capacity to treat 11,788 MLD—a mere 30% of the total. However, the actual sewage treated was even less, at 8251 MLD, which was about 22%. It has been reported that in India only 33% of houses are connected to sewer systems, only around 38% use septic tanks. These septic tanks do not treat waste and have to be emptied periodically. The services are offered to clean septic tanks, pump out the sludge from the tanks and empty it into drains, fields—just about anywhere. Also, cities are not able to recover the high costs of supply and therefore have no money to invest in sewage treatment (Chandramouli and General, 2011). Sewage sludge generation in India is increasing at a faster rate as more and more sewage treatment plants (STPs) are required; in addition, alternative ecotechnologies are been developed to relieve this serious issue. At the moment, sewage sludge and effluents from these STPs are frequently disposed of on agricultural land for irrigation or manure (Singh and Agrawal, 2008). In India, wastewater disposal systems are usually managed by local bodies. In a few specific cases, they are managed by departments/statutory boards set up by state governments. This service facility falls into the water supply and sanitation sector. In municipal WWTPs, the treatment and disposal of sewage sludge can comprise up to 50% of the operation costs (Appels et al., 2008).
Industrial and Municipal Sludge. https://doi.org/10.1016/B978-0-12-815907-1.00032-5 © 2019 Elsevier Inc. All rights reserved.
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1.1 Scope and objectives Sewage treatment is a process that removes the majority of the contaminants from wastewater or sewage and produces both a liquid effluent suitable for disposal in the natural environment and sludge. Estimates from the Ministry of New and Renewable Energy (MNRE) in India suggest that there is a potential of about 226 MW to be generated from treated and untreated sewage sludge (Vaish et al., 2016). There are several approaches for treating sludge to reduce solid content and to stabilize biomass; however, anaerobic digestion (AD) is generally preferred because of its costeffectiveness and bioenergy production (Ge et al., 2013). Digested sludge can be further composted for agricultural use, and biogas can be converted to electricity and/or heat through combustion, thus compensating for some of the energy use in a WWTP (Rulkens, 2007). Because of a large amount of organic content, primary sludge contains about 66% of the energy content of wastewater (Ting and Lee, 2007), and about 81% of biodegradable organic energy may be converted to methane (CH4) (McCarty et al., 2011). Despite the great energy potential of biogas production, several issues limit successful AD application; for instance, electric generators and their maintenance are costly, and biogas may need pretreatment to remove contaminants such as hydrogen sulfide (Appels et al., 2008). In addition, energy will be lost during CH4 conversion because the common efficiency of CH4-to-electricity conversion is about 33% (Ge et al., 2013). Efficient, noninvasive techniques are desired for restoring organic-contaminated sludge. Therefore, it is of great interest to explore alternative technologies for sludge treatment and energy recovery. This chapter comprehensively describes microbial fuel cell (MFC) technology. Considering the MFC research trends, the following objectives are pursued in this chapter: (1) to highlight MFC for sludge remediation and to provide insights into how to extend its application to a large scale; (2) to update the current status of MFC technology in India and (3) to present a vision for the future of sludge management and remediation.
2
MFCs—Construction and working mechanism
Bioelectrochemical technology, typically involving MFCs, has been widely used as an environmental remediation approach. Its applications include wastewater, sludge, sediment, and soil remediation (Butti et al., 2016). MFCs directly produce electricity from organic wastes, in which bacteria oxidize organic and inorganic compounds in the anode chamber and generate protons and electrons that travel to the cathode to reduce oxygen to water. Electron flow from the anode to the cathode generates electric current or power if a load is connected (Fig. 1). The potential of contamination removal has numerous aspects. First, an inexhaustible anode is introduced as the electron acceptor, which solves the problem of lack of electron acceptors. Second, the biocurrent stimulates the growth and activity of functional microbes and symbiotic microflora, which in turn accelerates the biodegradation of organic pollutants. Third, the coupling effect derived from the biocurrent boosts the oriented transfer of free electrons; this speeds up the rate of the oxidation-reduction reaction. Finally, the method of synchronous power production
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Fig. 1 Schematic diagram of microbial fuel cell during process operation.
from MFCs is the opposite of that of electrokinetic remediation, which needs power input; to some extent, this reduces the cost of environmental governance. Additionally, bioelectrochemical technology has no secondary pollution and is more specifically suited to clear low-concentration contaminants with the advantages of a high energy conversion rate, operation under normal temperature and pressure and in mild conditions, and easy to control (Wang and Ren, 2013). Theoretically, MFCs are able to utilize any kind of biodegradable organic substance. The degradation kinetics are different and dominated by the structural characteristics of substances and their operational conditions (Pant et al., 2010). MFCs reduce the consumption of energy and the production of by-products, which are the main advantages of their application. Compared with more complicated wastewater treatment by bioelectrochemical systems, simpler sediment MFCs have been under development in the last decade. Based on a previously successful bioelectrochemical methodology in dealing with wastewater and sediment, sludge remediation has recently emerged. MFC remediation belongs to the category of bioelectrochemical remediation, which is one of the biostimulation technologies (Lu et al., 2014; Rozendal et al., 2008). When an air-cathode MFC is used, the pollutant is biodegraded by microbial catalysis as the electron donor. Electrons are exported from the intracellular space and (1) hop to the anode directly (Fig. 2A), (2) are transferred by the mediators (e.g., flavin or thionine) to the anode (Fig. 2B), and (3) reach the anode by flowing through the nanowires (i.e., pili) (Fig. 2C). Subsequently, electrons are conveyed to the air cathode through the external circuit, and react with oxygen (terminal electron acceptor) to form water (Venkata Mohan et al., 2014). In MFCs for sludge remediation, the existence of anodes accelerates the capability of the electrogenic microorganism to provide more electrons to promote the metabolic reaction rates of anaerobic bacteria that degrade the contaminants. The main advantages of using MFCs in wastewater treatment come from the savings of aeration energy and sludge disposal (Oh et al., 2010). For traditional activated sludge systems, aeration can amount to 45%–75% of plant energy costs, so the conversion of aeration tank to MFC units is very beneficial because not only does it eliminates aeration energy consumption, but studies also showed that MFCs can
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Fig. 2 Electron transfer mechanisms involved during microbial fuel cell operation where (A) direct electron transfer, (B) an electron shuttle, and (C) a solid conductive matrix (nanowires).
produce 10%–20% more energy that can be used for other processes (Huggins et al., 2013; Pant et al., 2010). The reported maximum power density from lab-scale aircathode MFCs has reached 2.87 kW/m3, making them promising for commercialization development (Fan et al., 2007), even though the system scale-up remains a major challenge. Another main benefit of MFC systems is the low biomass production. The MFC is a biofilm-based system, and the cell yield of electrochemically active bacteria (0.07–0.16 gVSS/gCOD) is much less than the activated sludge (0.35–0.45 gVSS/ gCOD), so it can reduce sludge production by 50%–70% (Fan et al., 2007; Huggins et al., 2013), which in turn may reduce 20%–30% of the plant operation costs. Other benefits may include nutrient removal and the production of value-added products, such as caustic solutions for disinfection, or H2 and biogas for energy, which will be discussed more extensively in the following sections. Diverting some organic compounds to direct electricity generation in MFCs could reduce biogas processing and conversion, resulting in some potential economic benefits, but we also need to understand the challenges of MFC application. For example, MFCs generally have much more complex structures and higher capital cost than anaerobic digesters (Pham et al., 2006). The use of high-surface-area electrodes and high-solid substrates like sludge can create problems such as reactor clogging. Unlike anaerobic digesters, which can be constructed in a single reactor with a large volume, MFCs are expected to be built in small-scale modules to form an MFC assembly; a single MFC with a very large volume will have a greater distance between the anode and the cathode electrodes, thereby increasing the internal resistance and decreasing electricity generation. With multiple small-scale MFC modules, the heating and feeding of the anolytes will be very challenging (Kondaveeti et al., 2018).
3
Applications of MFC for sludge remediation
In recent decades, the development of MFC applications has attracted increasing attention. A vast number of studies were performed in the fields of wastewater, sediment, sludge, and soil treatment. The use of MFCs is a promising approach for the
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direct production of electric energy or other energy carriers, such as hydrogen gas, from various organic substrates (Logan and Regan, 2006; Pant et al., 2010). Sewage sludge also has been studied in MFCs for electricity generation. A single-chamber MFC with a baffle inside its anode compartment generated low power from anaerobic sludge due to large internal resistance from the baffle (Hu, 2008). Hydrolysis is considered to be a limiting step in AD (Halalsheh et al., 2005), therefore appropriate pretreatment of solid organic waste is expected to improve the contents of soluble and small-particle organics which can be better used by microorganisms. Chandrasekhar and Venkata Mohan (2014) have reported bioelectrohydrolysis as a potential strategy for pretreatment of solid waste with simultaneous power generation. The ultrasonic and alkaline pretreatment of sludge improved its degradability and resulted in a higher power output of 12.5 W/m3, with 61.0% and 62.9% reduction of total chemical oxygen demand (TCOD) and volatile solids (VS), respectively ( Jiang et al., 2010; Jiang et al., 2009). Likewise, improved power output and total solid production were observed in an MFC after pretreatment with sterilization and alkalization (Xiao et al., 2011). When an MFC was linked to an anaerobic digester to form an integrated recirculation loop, it was found that CH4 production was higher than the digester alone (Inglesby and Fisher, 2012) because a high concentration of ammonium/ammonia will inhibit methanogenic activity (Sung and Liu, 2003). The improved biogas production, resulting from the use of a recirculation loop, was likely due to the migration of ammonium ions from the digester to the cathode compartment of the MFC driven by electricity generation in the MFC, which was also demonstrated previously (Kim et al., 2008). A recent study reported the performance of MFCs in treating a fermentation solution from primary sludge, in which higher power production was obtained when treating a mixture of fermentation supernatant and primary effluent because of elevated concentrations of soluble COD and volatile fatty acids (VFAs) after the fermentation process (Yang et al., 2013). In another study, long-term investigation (almost 500 days) of MFCs treating sewage sludge for energy production, organics removal, and solid reduction were reported (Ge et al., 2013). The experiment consisted of two phases: In Phase I, two tubular MFCs were operated with primary sludge and digested sludge, respectively, for more than 10 months; in Phase II, both MFCs were operated as a two-stage system to treat primary sludge for about 6 months. Biogas production in the MFCs was examined and compared energy production between MFCs and anaerobic digesters. The study concluded that the MFCs fed with the primary sludge acted mostly as modified anaerobic digesters. Alternatively, MFCs could function as a posttreatment process to polish the supernatant of digested sludge. Recently studies on MFCs for sludge remediation and energy recovery are being reported and a few are listed in Table 1.
4
Current status
As the global demand for renewable energy and organic matter increases, organic wastes, including sewage sludge, could be one of the available resources to use for this purpose. This substrate can be used as an energy resource for power and heat
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Table 1 Comparison of MFC performances in systems that employed different inoculum, substrate, and designs MFC configuration
Inoculum/ pretreatment
Two-chamber Singlechamber
Anaerobic sludge Effluent from MFC
Singlechamber
Effluent from MFC
Tubular MFC
Raw sludge from a primary sedimentation tank
Two-chamber
Anaerobic microbial consortium
Substrate Pretreated activated sludge Fermented primary sludge with 15.5 g/L soluble COD Fermented primary sludge with 15.5 g/L soluble COD Primary sludge and digested sludge from anaerobic digestors Secondary sludge obtained from a sewage treatment plant
Power density 5.6 μW/cm2 32.0 μW/cm2
References Xiao et al. (2011) Yang et al. (2013)
103 μW/cm2
Yusoff et al. (2013)
1.43 and 1.80 kWh/ m3, respectively
Ge et al. (2013)
13.5 μW/cm2
Passos et al. (2016)
via emerging technologies. Moreover, sewage sludge can be considered as a substrate for soil fertilization and remediation if the applied technology allows high-quality products to be obtained. Such reuses of sewage sludge are economically viable and environmentally sustainable compared to waste-handling and land-filling (Rulkens, 2007). The economic potential is found in offsetting costs related to traditional waste treatment methods and decreased energy costs by the generation of biogas and biofuels, which can partially replace the usage of traditional fuels (Kacprzak et al., 2017). To achieve these goals, wastewater treatment operations require careful management of sewage sludge. The management of sewage sludge in an economically and environmentally acceptable manner is one of the critical issues facing modern society, due to the rapid increase in sludge production. In India, sludge management is governed by Municipal Solid Waste (MSW) Handling Rules, 2000 if the sludge is categorized as nonhazardous and the sewage sludge mostly falls into this category. However, if the sludge is characterized as hazardous for any reason, the disposal and management is governed by Hazardous Waste Handling and Management rules, as stipulated by an amended version of the regulations in 2003. All these regulations are administered by the Ministry of Environment and Forests (MoEF) (Chandrappa and Das, 2012).
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A paradigm shift in waste remediation and waste management has been noticed in India since the launch of the Swachh Bharat Mission (SBM), an official campaign initiated by the government of India in October 2014, which aims to clean up the streets, roads, and infrastructure of the cities, smaller towns, and rural areas by the end of 2019. The government has a clearly defined path for achieving open defecation-free (ODF) cities and districts/villages. More critically, there is also a well-defined process, for the different phases of the mission, across the sanitation value chain called build, use, maintain, and treat (BUMT). A survey conducted by the Government of India in 2017 declared Indore as cleanest city based on various sanitation and cleanliness parameters, including waste collection, ODF status, and feedback from citizens. The survey is part of the government’s initiative toward a cleaner India. Its focus on sanitation, preventing open defecation, and waste collection is significant, considering their impact on the environment and on the health of city dwellers. The southern Indian city of Bangalore is extolled as India’s "Silicon Valley." It has experienced a growth of 45% of its urban population in a span of five years, crossing the 12 million population mark in 2016. The Water Supply and Sewerage Board of Bangalore has selected the French multinational corporation SUEZ to support the city’s efforts to build sludge recovery plants and improve the wastewater infrastructures for two million inhabitants of Vrishabhavathi Valley. SUEZ has operations primarily in water, electricity, and natural gas supply, as well as waste management. The SUEZ group will be in charge of building a new WWTP of 150,000 m3/day, besides, rehabilitation of an existing plant of 150,000 m3/day, and also, building of a sewage sludge recycling and recovery plant coming from these two plants. This rapid urban rise, combined with the development of business activities, has led to an explosion in the demand for drinking water supply and sewage disposal. The common sludge treatment plant for the two treatment plants will be equipped with SUEZ’s Degremont technologies (Sedipac and Digelis), which will convert into electricity the biogas produced in order to make the plant self-sufficient. During the first three years, the group will design and build a new plant that uses activated sludge process (i.e., a bacterial treatment) to treat effluents to achieve an optimum discharge quality into the Vrishabhavathi River in accordance with the most recent standards of the Indian authorities, requiring in particular a total nitrogen content of less than 10 mg/L.
5
Future scope and perspective
The world is undergoing a new wave of urbanization, especially in developing countries, leading to multiple impacts on natural resources and the environment. A review of the research literature suggests that waste management poses various challenges and opportunities at an urban and rural scale (Reddy et al., 2018). In this context, biorefinery has emerged as a potential alternative to petroleum-based refineries, where biomass of nonedible feedstocks/biogenic wastes are used as raw materials and a range of products, such as biofuel, industrial biochemicals, and biomaterials including commercially important biopolymers, are produced (Clark and Deswarte, 2015).
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Concepts such as the biorefinery create aspirations toward increasingly integrated technologies (Aresta et al., 2012). The burgeoning amount of waste (specifically sewage sludge) has tremendous potential, but to date, humankind has not exploited even a fraction of that potential. Waste as a prime target substance in the biorefinery, with a wide range of opportunities for mercantile interests rather than for utilitarian reasons (Tuck et al., 2012; Zondervan et al., 2011). A vivid, articulate review by Venkata Mohan et al. (2016) presented the possibilities of biorefinery models that could be explored for the utilization of biogenic wastes as a resource in the framework of a circular economy. The most popular microbial and photosynthetic processes are widely deployed for many technological solutions. Integrating various models involving unit operations and bioprocesses gives a much more holistic outcome through an updated “product versus energy” approach explored in the bioeconomy (Fig. 3). An expression of interest in modern energy and material recovery technologies using waste feedstock often attempts to integrate processes and remediation. Therefore, this paradigm transition from a linear economy toward a circular economy is obligatory; the futuristic biorefinery platform should have an ambitious vision to promote a switch from the consumption of fossil reserves to the use of renewable waste as
Fig. 3 Conceptualized flow sheet of sludge biorefinery that includes: Photo-synthetic process—bioelectrochemical process (microbial fuel cells) and thermo-chemical process. In the figure, the direction and color of arrows infer—bi-directional yellow bold arrows indicate formation and usage of excess sludge—uni-directional grey bold arrows indicate bioprocess yielding products—uni-directional dashed arrows indicate conditional channeling or by-products of that bioprocess.
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resources. This will also lead to incredible employment opportunities in industry and academia, especially in the sectors of agriculture, food, chemicals, healthcare, pharmaceuticals, and logistics (Amulya et al., 2016). However, the sustainable biorefinery is viewed as a way to move fuels and other valorized coproducts onto much greener footprints, with life-cycle analysis providing the rigor needed to make the best decisions on feedstock utilization, upscaling process technology, and value-chain conformation (Liu and Shonnard, 2014) Gottumukkala et al. (2016) reviewed opportunities and prospects of biorefinerybased valorization of pulp and paper sludge. The paper and pulp industry (PPI) generates a large amount of solid waste with high moisture content. Numerous opportunities exist for the valorization of waste paper sludge, although this paper focused on primary sludge with high cellulose content. The most mature options for paper sludge valorization are fermentation, AD, and pyrolysis. In this review, biochemical and thermal processes are considered individually and also as an integrated biorefinery. The objective of an integrated biorefinery is to reduce or avoid paper sludge disposal by landfilling, water reclamation, and value addition. The assessments of selected processes for biorefinery vary from a detailed analysis of a single process to high-level optimization and integration of the processes that allow the initial assessment and comparison of technologies. This data can be used to provide key stakeholders with a road map of technologies that can generate economic benefits and reduce carbon waste and pollution load. Crutchik et al. (2018) recently reported on a biorefinery of cellulosic primary sludge toward targeted short-chain fatty acids (SCFAs), phosphorus, and CH4 recovery. Cellulose from used toilet paper is a major untapped resource embedded in municipal wastewater, of which recovery and valorization to valuable products can be optimized. Cellulosic primary sludge (CPS) can be separated by upstream dynamic sieving and anaerobically digested to recover as much as 4.02 m3/capitayear of CH4. On the other hand, optimal acidogenic fermenting conditions of CPS allows the production of targeted SCFAs to reach as much as 2.92 kg COD/capitayear. Here, propionate content can be more than 30% and can optimize the enhanced biological phosphorus removal (EBPR) processes or the higher valuable copolymer of polyhydroxyalkanoates (PHAs). In this work, the optimized conditions were applied in the long term to a sequencing batch fermentation reactor where the highest propionate production (100–120 mg COD/g TVSfedd) was obtained at 37°C and adjusting the feeding pH at 8. At the same time, around 88% of the phosphorus released during the acidogenic fermentation was recovered (amounting to as much as 0.15 kg of struvite per capitayear). Finally, the potential market value was preliminarily estimated for the recovered materials, which can triple over the conventional scenario of biogas recovery in existing municipal WWTPs. Zhang et al. (2018) reported on value-added products derived from WAS in a biorefinery scenario. Substantial research has been carried out on sustainable WAS management over the last decade to endorse WAS as a feedstock for the production of bioproducts such as amino acids, proteins, short chain fatty acids, enzymes, biopesticides, bioplastics, bioflocculants, and biosurfactants. In this review, a critical
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assessment of the production process for a wide range of value-added products from WAS, their current limitations, and recommendations for future research to help promote more sustainable management of this underutilized and ever-growing waste stream has been discussed.
6
Conclusions
In India, management of solid wastes in general, and sewage sludge in particular, is going to be a big challenge in the near future due to rapid urbanization and economic growth. Unless technoeconomically viable alternatives are developed, sewage sludge management is going to be a formidable task in environmental management. MFCs are a promising alternative and renewable technology for the current water-waste conundrum. Based on the current literature review, MFCs have satisfactorily reduced both organics and suspended solids in sewage sludge. The total energy production from primary sludge in the two-stage MFC system was comparable to that of anaerobic digesters; however, direct electricity generation made a minor contribution, while energy from biogas still dominated the overall energy production. It will be very challenging to apply MFC technology to treating primary sludge; but MFCs may be used to polish the digested effluent from anaerobic digesters, offering potential benefits in energy savings compared with aerobic treatment. Alternatively, the proposed biorefinery approach could be a potential sludge management option, along with value addition.
Acknowledgments The author would like to sincerely thank the editors of this book for giving this opportunity to contribute a chapter. I also give my heartfelt gratitude to Dr. S. Venkata Mohan (Book Editor) for his kind support and valuable suggestions.
References Amulya, K., Dahiya, S., Venkata Mohan, S., 2016. Building a bio-based economy through waste remediation: innovation towards sustainable future. In: Bioremediation and Bioeconomy. Elsevier, pp. 497–521. Appels, L., Baeyens, J., Degre`ve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34 (6), 755–781. Aresta, M., Dibenedetto, A., Dumeignil, F., 2012. Biorefinery: From Biomass to Chemicals and Fuels. Walter de Gruyter. Butti, S.K., Velvizhi, G., Sulonen, M.L., Haavisto, J.M., Koroglu, E.O., Cetinkaya, A.Y., Singh, S., Arya, D., Modestra, J.A., Krishna, K.V., 2016. Microbial electrochemical technologies with the perspective of harnessing bioenergy: maneuvering towards upscaling. Renew. Sust. Energ. Rev. 53, 462–476. Chandramouli, C., General, R., 2011. Census of India 2011. Provisional Population Totals. Government of India, New Delhi.
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