Assessment of Biomethane Production from Animal and Crop Waste Through Anaerobic Digestion Process

Abstract

Biomethane being a mixture of gases a possible solution for the energy crisis worldwide. The biomethane comes from anaerobic digestion (AD) process that can be run with the widely available lignocellulosic biomass like agricultural and municipal waste which otherwise would have been wasted in raw. In the past, the common usage of manure was to be applied as a fertilizer, which can have an adverse environmental impact in terms of terrestrial eutrophication and emission of greenhouse gases. These days, a solution applied for wastes and residues is to treat them with the simultaneous recovery of energy through anaerobic digestion (AD). The present study was designed to increase the methane production by using codigestion and photo-catalytic oxidation with aim to increase the methane production by synergism/biodegradability and lignin oxidation respectively. In the first step, the substrates were separately analyzed for mono-digestion where wheat straw (WS) produced 255 mLCH4/gVS, meadow grass (MG) produced 301 mLCH4/gVS and cattle manure (CM) produced 307 mLCH4/gVS. Furthermore, the co-digestion results revealed a maximum of 25 per cent increase in methane production which had a significance level of p<0.05 over mono-digestion of cattle manure (CM), when 25 per cent of the manure contents were replaced with lignocellulosic biomass. The over-all biodegradation of the above mentioned combination also reached to 84 per cent in co-digestion from 67 per cent in monodigestion. In the second step the wheat straw (WS) which produced least methane in mono-digestion experiments was subjected to photo-catalytic oxidation with titanium oxide TiO2 and ultraviolet light of 200-400 nm (UV). Specifically, four different TiO2 concentrations (0.0, 0.5, 1.0, 1.5, and 2.0 per cent (w/w) were tested xvii at three different irradiation times (0, 1, 2, and 3 hours). Among the products from the oxidative degradation of lignin under TiO2/UV catalyst system, vanillic acid and ferulic acid were detected at a maximum value of 91.18±2.00 mg/L and 1.67±0.01 mg/L, using 2.0 per cent (w/w) TiO2 for 3 hours. Moreover, the most effective pretreatment strategy (1.5 per cent (w/w) TiO2 and 3 hours) was found to increase the biodegradability of wheat straw up to 37 per cent compared to untreated biomass. The positive impact of photo-catalytic pretreatment was also observed in continuous trials, when methane production was increased by 25 per cent. The photo-catalytic oxidation of lignin-rich substrates is a promising method to disrupt the non degradable organic fraction under mild conditions. It is expected that thorough study of this process can still increase methane production to many folds at industrial scale. Not only high methane yield but also the direct conversion of lignocellulosic biomass can be attained e.g vanillic and ferulic acid. Therefore, it is also expected that application of this methodology for production of biomethane in future will not only help to overcome shortfall of energy but also will provide environmental benefits