Ethanol has been known for a long time, being perhaps the oldest product obtained through traditional biotechnology. It is an attractive, sustainable energy source for fuel additives. Based on a four-level central composite design (CCD) involving the variables substrate composition (20-100%) X 1 , pH (4.5-6.5) X 2 , incubation temperature (28 -36 C) X 3 , and fermentation time (12-60 h) X 4 , a response surface methodology (RSM) for the production of ethanol using waste cashew apple juice as substrate by Zymomonas mobilis MTCC 090 was standardized. The design contains a total of 31 experimental trials with the first 16 organized in a fractional factorial design and 25 to 31 involving the replications of the central points. Data obtained from RSM on ethanol production were subjected to the analysis of variance (ANOVA) and analyzed using a second-order polynomial equation, which resulted in the optimized process conditions of 62% (v=v) as substrate concentration, pH of 5.5, temperature of 32 C, and fermentation time of 37 h. Maximum ethanol concentration (12.64 g=L) was obtained at the optimized conditions in an anaerobic batch fermentation.
The effect of anaerobic co-digestion of vegetable market waste and sewage sludge was studied extensively. The effects of co-digestion were compared with the separate digestion of vegetable market waste and sewage sludge. The batch studies were carried out using three bench scale reactors having 1.5 L working volume. The cumulative biogas production shows that the organic waste available from the vegetable waste contains easily biodegradable organic matter compared with the sewage sludge. First order reaction kinetics is maintained throughout the methanation fermentation. The reductions in volatile solids (VS) in the three reactors were in the range of 63 -65 %. The specific gas production for vegetable waste was higher (0.75 L biogas/g VS in and 1.17 L biogas/g VS des ) than for the sewage sludge (0.43 L biogas/g VS in and 0.68 L biogas/ g VS des ). Consequently, the specific gas production for the co-digestion of the mixture of vegetable waste and sewage sludge (0.68 L biogas/g VS in and 1.04 L biogas/g VS des ) was considerably higher than for the sewage sludge only. Batch kinetics of anaerobic digestion is useful in predicting the performance of digesters and for the design of appropriate digesters.
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