Synergistic effect of biogas production from co-digestion of fish and vegetable market wastes and kinetic modelling

Fish wastes and vegetable market wastes generated along with other wastes in the urban cities are disposed of in open landfill sites and lead to water, air and soil pollution. It occupies more and more land area for disposal as the generation of waste also increases day by day. In this current study, a novel approach has been identified to recover energy from mixed fish waste (MFW) and vegetable market waste (VMW) by anaerobic co-digestion. In fish waste, more than 75% of the total solids contribute to volatile solids, indicating that waste content is organic and suitable for anaerobic digestion. Further, MFW showed that the protein content is 4 to 5 times higher than carbohydrate content, indicating nitrogen richness will inhibit the AD process. Hence, co-digestion of MFW with carbon-rich VMW to maintain the stability of AD has been carried out. In this batch study, the biogas potential of these organic nitrogen-rich wastes was evaluated for major fish species (Parastromateus niger, Sillago indica and Scomberomorus cavalla) and mixed fish wastes individually and co-digestion of MFW with vegetable market waste (VMW) in different ratios. Based on the results, the specific biogas yields from Parastromateus niger, Sillago indica, Scomberomorus cavalla and mixed fish waste were 0.301 +/- 0.011, 0.291 +/- 0.002, 0.306 +/- 0.008 and 0.289 +/- 0.013 L/g of VS added, respectively. The co-digestion of fish waste with vegetable market waste in different mixing ratio (1:1 to 1:5) showed specific biogas yields of 0.445 +/- 0.02 (1:1), 0.462 +/- 0.01 (1:2), 0.489 +/- 0.013 (1:3), 0.454 +/- 0.015 (1:4) and 0.432 +/- 0.011 (1:5) L/g of VS added. The results showed that there is an increase in biogas production during the co-digestion process confirming the synergistic potential of fish waste and vegetable market as substrates for the anaerobic digestion process. Further, the cumulative biogas production data were modelled using first-order kinetic, modified Gompertz and logistics models. The model results showed that the logistics model fitted well with experimental data (R-2 of 0.99) and the predicted values from this model were less than 1% deviation from experimental biogas production. The results confirmed the suitability of anaerobic digestion for co-digestion of MFW and VMW for enhanced energy recovery for future implementation in an urban area to reduce the waste dumped in landfills.