Anaerobic digestion of organic waste for biogas production

Abstract

Anaerobic digestion (AD) is a robust technology, to mitigate sustainable treatment of organic waste. AD has gained tremendous importance within last three decades for biological conversion of solid waste and waste water into biogas a renewable form of energy. Despite the several advantages, deprived operational parameters and presence of inhibitors in system leads to process instability and prevent wide commercialization of AD process. The presence of high ammonia concentration in feed stock and to maintain a balance in the acidogenic and methanogenic population in anaerobic digestion make a process stability challenging. Usually acetoclastic methanogens are considered more susceptible to operating conditions, environmental variations, and process inhibitors. Presence of ammonium up to certain level is beneficial for the microbial growth, above threshold level ammonia inhibit methanogenesis, leading to low methane yield and high volatile fatty acids (VFA), indicator of process instability. However high ammonia concentration is a primary cause of digester failure. The anaerobic co-digestion is a strategy to balance the C/N ratio for stable reactor performance. The specific objectives of present research were to 1) enhance biogas production by anaerobic co-digestion of cow manure (CM) with agricultural waste (AW) and fruit-vegetable waste (FVW) in batch and continuous reactor (CSTR): Effect of temperature and organic fraction 2) characterisation of microbial consortia in anaerobic digester treating organic waste for enzyme production 3) Analysis of the tolerance level or inhibition of the biomass in the anaerobic digester against different concentration of the inhibitor ammonium. 4) Effect of natural zeolite on biogas production at mesophilic and thermophilic temperature. The biotechnological systems including molecular and biochemical methods help us in understanding the complexity of microbial system, ecology and evolution to have an insight picture of microbial process and effective operation strategy in different environmental pressures, assist to understand the microbial ecology. In both batch and semi-continuous bench-scale testing, the greatest increase in biogas production and methane content was achieved when cow manure was co-digested with Wheat straw (WS) and fruit vegetable waste (FVW) in the ratio of 3 CM/WS (6 g-VS CM /L+2 g-VS WS /L) and 1.33 CM/FVW (6g-VS CM /L+4.5 g-VS FVW /L) respectively. At these optimum ratios, the methane yield from 4.5g-VS FVW /L was 493ml/g-VS at thermophilic conditions and 418ml/g-VS at mesophilic conditions. During the semi-continuous bench-scale testing, 10g-VS CM /L/day produced a Anaerobic digestion of organic waste for biogas production methane yield of 301 ml/g-VS during monodigestion and the total methane content increase by 57% during co-digestion of CM with FVW. However, a maximum methane content of 53% was achieved during co-digestion of CM with wheat straw (WS) at a higher OLR of 2.6g-VS/L/day. The addition of a co-substrate helps to balance the C/N ratio, enhances the buffer capacity and enables the substrate to be digested at higher OLR. The morphological, biochemical and molecular techniques were used to identify the microbial flora present in the high yielding reactor. The abundance of Bacillus, Clostridium and Enterobacter spp were observed along with Methanomicrobia and Methanosarcina. The- amylase enzyme was purified by ammonium sulphate precipitation and column chromatography. Theα-Amylase molecular weight estimation was confirmed by SDS-PAGE with a band of 52 kDa for B. subtilis (RAS-1) and 73kDa of C. perfringens (RAS-4). Specific methanogenic activity (SMA) was determined as the slope of a linear line fitted to the methane data over time (expressed in units of chemical oxygen demand or COD equivalents, normalized with respect to the amount of VS in the sludge added to each vial). All the experiments were run in triplicate and the error in SMA was estimated at the 95% confidence level (seven degrees of freedom). The SMAs were plotted against NH 3 and KI-50 was estimated by linear interpolation, corresponding to the NH 3 content at which SMA had been reduced to 50% of the highest measured SMA. As expected, increasing NH 3 decreased measured microbial activity/ SMA likely due to inhibition. The estimated KI-50 gTAN/L (given with error at 95% confidence level) was the threshold concentration for NH 3 inhibition of the particular sludge sample being tested. The background NH 3 (776 mgN/L) was noted. Further, the shape of the inhibition profile showed a gradual decrease in SMA with increasing NH 3 , indicating that the microbes were reasonably tolerant to increases in NH 3 , albeit with some decrease in SMA. A stronger threshold-type response was observed for other sludge, with decrease in activity being more drastic around the KI-50 value. These different shapes of the SMA curves suggested differences in tolerance to NH 3 . The results illustrated how inhibition test data can be used to estimate a threshold inhibitor concentration (KI-50) as well as to obtain a measure of microbial tolerance to increases in inhibitor concentration. The performance of anaerobic treatment was found better with zeolite doses at 7 and 14 g/l than without zeolite. The treatments with natural zeolite also have shown Anaerobic digestion of organic waste for biogas production higher COD and volatile solid removal. This could be due to the ammonia adsorption and pH regulating ability of zeolite. These research findings will provide knowledge about the optimum mixture ratio of organic substrates for high methane yield, the resilience of microbial community against ammonia concentration present in different substrates and will help to develop strategies to monitor and control the process operation in the presence of ammonia inhibitor. The need of present time is to understand the composition and dynamics of microbial population and to develop a relationship between microbial community shift s and ammonia tolerance in anaerobic digester.