ANNOUNCEMENTS
India generates 62 million tonnes of municipal waste every day. Roughly half of this is organic waste, consisting largely of food waste including vegetables and fruit (V&F) waste. The trend of waste generation shows that the quantity of waste would be around 165 million tonnes by 2031 and 436 million tonnes by 2050. The most promising method of treating V&F waste is anaerobic digestion.
The effectiveness of hydrolytic bacteria in the pre-treatment was assessed at different concentrations of salt and at different values of pH. Consortia of bacteria were developed and their abilities to hydrolyse FW and VW at different concentration of total solids (TS) was tested to identify the most efficient consortia for biogas production. Hydrolysis using the identified bacteria is particularly significant because this method is not only simple, robust, and cheap but also produces more biogas.
The first objective of the study was to identify the bacterial strains and testing their abilities to hydrolyse the V&F waste. About 2400 strains of bacteria (identified by IGIB, CSIR Laboratory, Delhi) 1600 from the MBG series and 800 from the MBGC series (Bacillus cereus and B. thuringiensis strains), were screened to identify the more efficient hydrolytic bacteria. Based on their relative enzymatic activity, 11 strains were selected from each series. The selected strains were mixed to prepare 11 consortia for each series (the consortia from the MBG series were labelled C1 to C11 and those from the MBGC series were labelled BC1 to BC11). Each of the 22 consortia was tested at different concentrations (3%, 5%, and 7%) of TS on both FW and VW. Hydrolytic ability of these consortia was measured in terms of the amounts of volatile fatty acids (VFA) produced, chemical oxygen demand, (COD), and pH of the leachate and the slurry. Of the MBG series, four consortia, namely C1, C5, C7, and C9, were the most efficient for pre-treatment of VW and four more, namely C8, C9, C10, and C11, for FW. These consortia were tested further for biogas production at the three concentrations of TS.
The second objective was to test the selected consortia for the biogas generation. Of the MBGC series too, four, namely BC6, BC7, BC8, and BC10, were chosen for the pre-treatment of VW and another four, namely BC1, BC6, BC8, and BC9, for FW. The pre-treatment also reduced the HRT for FW and VW to 4 days compared to 6 days without any pre-treatment. Biomethanation of VW pre-treated with BC7 proved to be the most efficient for producing methane (CH4): the production was 72.2 m³/t of VW at 3% TS, three times the production without pre-treatment.
Biomethanation of FW pre-treated with BC8 proved to be the most efficient, producing 65.8 m³/t in 8 days at 5% TS, nearly double of that without pre-treatment.
The third objective of the study was to evaluate the effects of heating as a pre-treatment on hydrolysis of FW and VW for producing biogas. Heat treatment is desirable in colder regions because biogas production is low at low temperatures. The heat treatment consisted of heating the substrate to 25 °C, 35 °C, 45 °C, or 55 °C for 6 days before biomethanation. The highest temperature and 5% TS proved the most productive for both VW and FW, producing, in 20 days, 36.6 m³/t (86.7% more than that from the control, that is without heating) from VW and 137.8 m³/t (163% more) from FW. The net energy gain was also been calculated by subtracting the energy required for heating the digester from the total energy generated in the form of biogas.
The fourth objective of the study was to estimate the emissions of CO2, NH3, and SO2 from the digestate (the residue after anaerobic digestion) under field conditions as affected by the composition of waste, the method of pre-treatment, and the season. On average, emissions from digestate from 1 kg of raw waste (FW or VW) pre-hydrolysed with the defined consortia were estimated at 93.2 mg of CO2, 35.9 mg of NH3, and 47.7 mg of SO2 whereas neither CH4 nor N2O was emitted from the digestate, whether derived from untreated waste or from waste treated with the consortia of bacteria. The amounts of emissions from digestate derived from untreated waste were greater (98.3 mg of CO2, 38.8 mg of NH3, and 51.8 mg of SO2). Thus the consortia also have the potential to reduce gaseous emissions from digestate.
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