Enhancing biomethanation performance through co-digestion of diverse organic wastes: a comprehensive study on substrate optimization, inoculum selection, and microbial community analysis

A blend of organic municipal solid waste, slaughterhouse waste, fecal sludge, and landfill leachate was selected in different mixing ratios to formulate the best substrate mixture for biomethanation. Individual substrates were characterized, and the mixing ratio was optimized with the help of a response surface methodology tool to a value of 1:1:1:1 (with a C/N ratio of 28±0.769 and total volatile fatty acid (VFA) concentration of 2500±10.53 mg/L) to improve the overall biomethanation. The optimized blend (C/N ratio: 28.6, VFA: 2538 mg/L) was characterized for physicochemical, biological, and microbial properties and subjected to anaerobic digestion in lab-scale reactors of 1000 mL capacity with and without the addition of inoculum. The biogas yield of individual substrates and blends was ascertained separately. The observed cumulative biogas yield over 21 days from the non-inoculated substrates varied between 142±1.95 mL (24.6±0.3 ml/gVS) and 1974.5±21.72 mL (270.4±3.1 ml/gVS). In comparison, the addition of external inoculation at a 5% rate (w/w) of the substrate uplifted the minimum and maximum cumulative gas yield values to 203±9.9 mL (35.0±1.6 mL/gVS) and 3394±13.4 mL (315.3±1.2 mL/gVS), respectively. The inoculum procured from the Defence Research and Development Organisation (DRDO) was screened in advance, considering factors such as maximizing VFA production and consumption rate, biogas yield, and digestate quality. A similar outcome regarding biogas yield and digestate quality was observed for the equivalent blend. The cumulative gas yield increased from 2673±14.5 mL (373.7±2.2 mL/gVS) to 4284±111.02 mL (391.47±20.02 mL/gVS) over 21 days post-application of a similar dosage of DRDO inoculum. The 16S rRNA genomic analysis revealed that the predominant bacterial population belonged to the phylum Firmicutes, with the majority falling within the orders Clostridiales and Lactobacillales. Ultimately, the study advocates the potential of the blend mentioned above for biomethanation and concomitant enrichment of both biogas yield and digestate quality.

A comparative assessment of biomethane potential of fresh fecal matter and fecal sludge and its correlation with malodor

Comprehensive and proper management of fecal sludge (FS) is an ongoing concern in many nations. Decentralized fecal sludge treatment plants (FSTPs) are effective in this regard; however, many have experienced strong public opposition based partly on suspicion of malodor. Fecal sludge and freshly generated fecal matter (FM) samples from various FSTPs were collected, characterized, and investigated for biomethane potential. The homogenized samples were anaerobically digested for 28 days. Digestion successfully reduced total suspended solids, biochemical oxygen demand, and threshold odor number values of 97,350-97,420 mg/l, 43,230-43,260 mg/l, and 130-150 for FM, to 49,500-49,650 mg/l, 23,760-23,850 mg/l, and 3338, respectively, for FS samples. The comprehensive gas yield from Bhongir, Boduppal, and Shadnagar FS samples was 40, 55, and 31 ml, respectively. In contrast, cumulative gas generation from the FM was 26,361 ml. Digestion of FS samples also reduced concentrations of volatile solids and coliforms by 66-72% and 99%, respectively. Characterization of gas samples revealed methane and carbon dioxide concentrations as 56% and 22% for FM, and 0.4% and 61% for FS samples, respectively. Hydrogen sulfide and ammonia gas were absent in FS samples, dispelling common societal misconceptions of FSTPs being associated with malodor.

Feasibility study of faecal sludge treatment by Geotube and jute tube-based technologies

The decentralized standalone treatment of faecal sludge is a generally cumbersome, energy-intensive, and expensive process. Particle size enlargement, dewatering, and disinfection are the crucial unit operations that contribute to the major portion of the operational expenditure. Therefore, the present study took up the task to investigate and yield a natural alternative to each of the above-delineated unit operations. Crushed seed powders of Moringa, Guilandina bonduc, Nirmali, tamarind, soap nut, nutmeg, jackfruit, and custard apple were experimented as natural coagulants, while a double-stitched cow dung-lined jute tube was utilized as the media for solid-liquid separation. Finally, the powder of Neem and custard apple seed and leaf were considered herbal disinfectants for the separated liquid and solid fractions, respectively. The findings of the physicochemical and biological analyses of the above unit operations were compared with a conventional polymer-based Geotube-driven treatment scheme. The natural alternatives registered some promising outcomes. However, the combined treatment efficiency of 79.32% (mean value of TS, TSS, TDS, COD, and BOD percentage removal) for pollutant removal and disinfection could not exceed the value of 97.98% offered by the conventional method. Further, the financial comparison for processing a 10 KL batch of FS indicates that the conventional scheme is considerably cheaper (USD 7.95) than the herbal alternate (USD 22.1). Therefore, though the present findings depict promising alternatives for sustainable and eco-friendly faecal sludge treatment, the authors advocate conducting further research to address the existing challenges to facilitate their seamless implementation on a field-scale level.

Bioremediation of reverse osmosis concentrate generated from the treatment of landfill leachate

The moisture content of municipal solid waste (MSW) and local precipitation events lead to the leachate generation from MSW landfills. The high concentration of organic pollutants in landfill leachate (LL) makes it hazardous, requiring treatment before disposal into the environment. LL is most commonly treated by reverse osmosis (RO), which generates large volumes of concentrate known as RO concentrate. This investigation aims to stabilize the RO concentrate through an inexpensive and effective bioremediation strategy. A bench-scale aerobic suspended growth reactor study was conducted using three commercial conversion agents, namely EM.1, Bokashi powder, and coir pith powder. Overall bench-scale efficiency of 63% was achieved in this study. The onsite studies were conducted in 7.5-m3 artificial ponds with 46% efficiency amid atmospheric influences and constraints. The overall efficiencies of both bench and field-scale studies were derived by ascertaining the arithmetic mean of the individual efficiency of the following parameters: chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total dissolved solids (TDS). In contrast, the control pond with no conversion agents showed an increase in pollution concentration over the 100 days of retention time. The findings revealed that the investigated technology had a marginally lower evaporation rate and performed relatively well compared to traditional solar evaporation ponds. Moreover, the technology can be easily scaled-up and readily applied for RO concentrate treatment in MSW landfills.

Reactive extraction of lactic and acetic acids from leached bed reactor leachate and process optimization by response surface methodology

The present work focused on extracting lactic and acetic acids from the leachate collected from leached bed reactor (LBR) during acidogenesis of food waste using the reactive extraction (RE) process. A wide range of diluents was screened either alone by physical extraction (PE) or in combination with extractants using RE to extract acids from the VFA mix. Aliquat 336-Butyl acetate/MIBK extractants in RE demonstrated higher distribution coefficients (k) and extraction yield (E %) than PE. The response surface methodology (RSM) was used to optimize the extraction of lactic and acetic acids from the synthetic acid mix, using three variables (extractant concentrations, solute/acid concentration and time). Consequently, these three variables were optimized for LBR leachate. The RE was promising, and extraction efficiencies of 65% (lactate), 75% (acetate), 86.2% (propionate) and almost 100% for butyrate and medium-chain fatty acids (MCFA) were achieved after 16 h of extraction. The RSM optimization predicted a maximum E % of 59.60% and 34.67% for lactate and acetate in 5.5 and 1.17 min, respectively. In the leachate experiment, an increase in E% and k was observed with increasing extractant concentration and lactate and acetate concentrations over time. Using a 1M reactive extractant mix and 1.25 and 12 g/L of solute concentrations, the maximum E % of acetate and lactate were 38.66% and 61.8% in 10 min. The results could contribute to developing a rapid in-situ product recovery system integrated with food waste acidogenesis for lactate and acetate recovery, contributing to the bio-economy.

Sustainable enzymatic treatment of organic waste in a framework of circular economy

Enzymatic treatment of food and vegetable waste (FVW) is an eco-friendly approach for producing industrially relevant value-added products. This review describes the sources, activities and potential applications of crucial enzymes in FVW valorization. The specific roles of amylase, cellulase, xylanase, ligninase, protease, pectinase, tannase, lipase and zymase enzymes were explained. The exhaustive list of value-added products that could be produced from FVW is presented. FVW valorization through enzymatic and whole-cell enzymatic valorization was compared. The note on global firms specialized in enzyme production reiterates the economic importance of enzymatic treatment. This review provides information on choosing an efficient enzymatic FVW treatment strategy, such as nanoenzyme and cross-linked based enzyme immobilization, to make the process viable, sustainable and cheaper. Finally, the importance of life cycle assessment of enzymatic valorization of FVW was impressed to prove this approach is a better option to shift from a linear to a circular economy.

A comprehensive investigation of toxicity and pollution potential of municipal solid waste landfill leachate

It is evident from the literature that research on the treatment of leachate generated from municipal solid waste (MSW) landfills has been a focus area of environmental management. However, the available information is discrete because most studies have reported only one or a couple of aspects of either closed or active MSW landfill leachate treatment. Hence, this investigation has focused on comprehensive attributes of both closed landfill leachate (CLL) and active landfill leachate (ALL), including generation, characterisation, and toxicity assessment to quantify and establish their pollution potential. The results indicated that CLL generation is higher (188.59 m³/d) than ALL (49.53 m³/d). The concentrations of principal physical, chemical, and biological constituents and concomitant leachate pollution index were higher in CLL (33.20) than in ALL (26.65). Furthermore, the germination indices of CLL (57.48) and ALL (79.14) and tail DNA damage of CLL (56.49%) and ALL (23.8%) ratified greater phytotoxicity and genotoxicity potential, respectively of CLL over ALL. The reasons for the variations in the generation, characteristics, and toxicity of CLL and ALL were discussed in detail. Evaluation of the commonly used landfill leachate treatment methods through the analytical hierarchy process confirmed that the activated sludge process and Fenton oxidation process are the most and least preferred treatment methods. The comprehensive investigation of CLL and ALL have established their pollution potential and the inevitable necessity for their treatment. The findings of this investigation will serve as a ready reference for researchers from academia and industry who work on the monitoring, treatment, and management of landfill leachate.

Interactions between microplastics and unit processes of wastewater treatment plants: a critical review

Microplastics are classified as emerging pollutants of the aquatic environment, necessitating a comprehensive understanding of their properties for successful management and treatment. Wastewater treatment plants (WWTPs) serve as point sources of microplastic pollution of the aquatic and terrestrial (eco)systems. The first part of this review explores the basic definitions of microplastics, sources, types, physical and chemical methods of identifying and characterizing microplastics in WWTPs. The next part of the review details the occurrence of microplastics in various unit processes of WWTPs and sewage sludge. Followed by this, various methods for removing microplastics from wastewater are presented. Finally, the research gaps in this area were identified, and suggestions for future perspectives were provided.

Detection of TiO2 Nanoparticles in Municipal Sewage Treatment Plant and Their Characterization Using Single Particle ICP-MS

Establishment of analytical methods for detection and characterization of nanoparticles in the environment are gaining prominence across the globe. The present study was designed to quantify titanium (Ti) and to characterize titanium dioxide nanoparticles (TNP) from a municipal sewage treatment plant, by inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of Ti & TNP were 1085 & 13.6 mg/kg in the influent sewage and 298 & 3.3 mg/kg in the aeration tank contents, respectively. The size of TNP ranged between 71-145 nm in the sludge fraction. Determining environmentally realistic concentrations of TNP could serve as a tracer material for characterization of those nanomaterials with similar size and aggregation properties. Furthermore, inference of Ti and TNP in municipal sewage in the study will also help in environmental risk assessment of nanomaterials.