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Dewatered sludges from twenty-two sewage treatment facilities were characterized to develop the quality control indices in India. This study used fertilizer index (FI) and clean index (CI) as a tool for categorizing sludge utilization into different classes (A, B, C and limited use classes LU-1, LU-2, LU-3) by their fertilizing potential, toxicity level, pathogen presence, and vector attraction reduction criteria. The findings revealed that total P (as P2O5), total N, and K (as K2O) in the sludge samples ranged from 0.9 to 5.7%, 1.2 to 3.8%, and 0.1 to 1.5%, respectively. Moreover, all sludges belong to Class C and lower category due to the presence of toxic metals and pathogens. The helminth eggs were found in the range of 25–1450 Numbers/4gTS in sludge samples. The highest number of helminth eggs of 1450/4 g of TS was found in the fecal sludge. The specific oxygen uptake rate (SOUR) values in the sludge samples varied from 0.3 to 4.9, with a median value of 1.3 and an RSD of 86% is way too high. It indicates that the sludge samples need further treatment to be less attractive as a food source for vectors and rodents. However, sludge samples have fertilizing potential and FI values ranges from 4.1 to 4.9, and CI value ranges from 2.5 to 5.0. It indicates that compost is best in quality and has high-fertilizer potential and low heavy-metal content, which is suitable for high-value crops such as organic farming. Further sludge treatment using typical composting, aerobic or anaerobic digestion, and solar or thermal drying could bring the sludges into the Class A and B categories. [ABSTRACT FROM AUTHOR]

Untreated sewage sludge is one of the biggest problems in India and around the globe. In the current study, we try to figure out an efficient technique that uses sewage sludge (SS) food waste (FW) to generate biogas. In this work, FW and SS are co-digested in single-stage anaerobic digestions (SSAD) and two-stage anaerobic digestions (TSAD) at various processing parameters, such as temperature, carbon-to-nitrogen (C/N) ratio, volatile solid (VS) percentage content, retention time (RT), and pH value. The high-solid digestion of FW and SS in the SSAD process reduces the biogas production rate compared to the TSAD process with 35 days of retention time at mesophilic temperature due to the oily substance from FW accumulating in the scum floating layer in the SSAD process. By altering the mass ratio between FW and SS (based on VS%), the TSAD process could be more effective. This ratio of 60:40 between FW and SS would produce more biogas with maximum efficiency in TSAD compared to the SSAD process. The results highlight an increase in biogas production and VS removal efficiency of the TSAD process 41.90% and 9.8%, respectively, compare to the SSAD process. The cumulative biogas yield in the TSAD process was 1914.8 ml and in the SSAD process was 1349.4 ml when the ratio of FW and SS was taken to be 60:40. In the TSAD process, Methane (C H 4 ) concentrations increased on average from 51% to 75%, due to better substrate hydrolysis and more readily accessible volatile fatty acids in the second stage. The analysis shows the co-digestion approach and TSAD process might work together to overcome the limitations of the conventional SSAD procedure for FW or SS when used separately. • Proposed a modified model of two stage anaerobic digestion for maximum bio energy production. • An experimental study to analyze the performance of the proposed working model. • Superior performance in comparison to conventional two stage and single stage anaerobic digestion model. • Can solve the problems of conventional two stage and single stage anaerobic digestion process. [ABSTRACT FROM AUTHOR]