Anaerobic digestion of tannery solid waste by mixing with different substrates

Abstract

Leather is one of the most prospective industries in Bangladesh .Out of the total 206 tanneries in Bangladesh 192 of them are located in Hazaribagh of Dhaka city. A large amount of solid waste (even in lean season 69.1 ton/day according to 2006 survey of UNIDO) is generated daily by leather manufacturing process in this area and crude disposal of this waste are creating serious hazard to the environment. An attempt has been made to analyze the present status of Hazaribagh in terms of solid waste management. Now it is prerequisite to develop an appropriate waste management system of the tannery waste and for this it is obvious to manage each fraction of the waste in a cost effective and environment friendly way. With rapid depletion of conventional energy sources, the need to find an alternative, preferably renewable, source of energy from waste is becoming increasingly important for the sustainable development. A major portion of the solid wastes from leather industry is fleshing, originates from animal tissue, which contains mainly fat and protein, and residual chemicals such as lime and sulphide used in the unhairing process of beam house operation. Experimental results showed that fleshing contains a significant quantity of volatile solids (including non-biodegradable fraction of volatile matter such as lignin, 82.57%VS) amenable for biodegradation. Further the characterization of fleshing showed that it contains low C/N ratio (2.64) and high pH (10.99), these characteristics were not suitable for using fleshing alone as substrate for anaerobic digestion,for this reason different proportion of cowdung and domestic sewage were used as co-substrate for optimum results. The study was carried out by using different proportions of waste fleshing, cowdung and domestic sewage in four different laboratory scale reactors carrying 6% total solid concentration. In all the reactors fleshing and domestic sewage were mixed in 1:1ratio. Anaerobicdigestion was carried out for 60 days of digestion period at room temperature 30°c ±3 and the pH of the slurry was maintained by the process itself. The inoculums were synthesized by the cowdung and water in the laboratory in the same environmental condition taken for the study and sufficient active biomass was present there.For thatthe start-up of the reactor was achieved easily. The reactors were subjected to anaerobic digestion with an aim of developing an appropriate technology for recovery of bioenergy from the tannery solid waste fleshing. From the study it was observed thatamended fleshing with domestic sewage when mixed with cowdung, the significant increase of gas production was remarkable. The performances of the reactors were analyzed on the basis of volatile solids destruction efficiency, specific gas yield, COD reduction rate and on methane yield. Within four reactors the percentages of volatile solid destruction were between 42% to 52%, specific gas production were 0.401, 0.476, 0.239 and 0.288 l/g respectively according to volatile solids fed and methane yieldwere achieved72%, 73%, 70% and 72% by volume respectively.COD reduction achieved in the test reactors 48.02% 50.67%, 44.80% and 44.61% respectively. Further, the kinetic analysis of the data revealed that first order kinetic model is adequate to describe the co-digestion of animal tissue with cowdung and domestic sewage. On the basis of analysis of the performance of the reactors, it was evident that amended fleshing is highly potential substrate for biogas generation and it was also found that R2 reactor, where on the total solids (6%TS) of the slurry 75% of fleshing with domestic sewage and 25% of cowdung were used, showed the optimum results in terms of VS reduction (52%), specific gas yield (0.476 l/g) and methane yield (73% by volume). For this reason, the standards of the reactor R2 were chosen as optimum condition for gas generation from fleshing. The criteria of the reactor R2 can be used for further study or design purposes. Based on the principles of R2 reactor, the economic feasibility of a fleshing based pilot scale biogas plant was also studied. The economic return was found to be 3.5 years for that type of plant.