Anaerobic co-digestion of abattoir and winery solid waste for enhanced biogas production

Abstract

The thesis investigated the potential application of abattoir and winery solid waste as co-substrates for enhanced biogas generation as well as stabilizing the AD process by using standard laboratory biochemical methane potential (BMP) techniques. Various input parameters on biodegradability, overall AD efficiency and bioenergetic kinetics were evaluated. The inoculum used was locally synthesized from zebra dung and ruminal content. Abattoir waste is rich in fats and proteins which makes it highly sought to produce good quality biogas with a high methane content. However, several challenges arise from sole processing of this type of wastes due to its low carbon-to-nitrogen (C/N) ratio. Thus, a supplemental substrate was used to mitigate issues associated with mono-digestion, namely winery solid wastes. BMP tests were conducted in batches under mesophilic temperature (38±0.5 oC) conditions, utilizing abattoir solid (As), cow blood (Cb) and winery solid (Ws) wastes in mono- and co-digestion modes for a period of 30 days. The parameters evaluated were substrate-ratio (1:1:21, 1:1 and 2:3), food-to-microorganism ratio (F/M) (0.5-2) and volatile solids (VS) concentration (5-20 gVS/L). For anaerobic co-digestion (AcoD) experiments binary blends (1:1 and 2:3) of AsWs and CbWs and a ternary blend (1:1:2) of AsCbWs were used to determine the effects of the simultaneous processing on specific methane production (SMP) and the overall digestion efficiency. Bioenergetic kinetics and parameter estimation were conducted using curve-fitting and least squares nonlinear regression techniques of experimental data points to the first order, and Gompertz, Logistic and Richard’s model(s). The optimization of biogas production was also evaluated in two-lock steps. Firstly, a screening ABCD mixture design was developed to determine the optimal mixture blend composition, and to assess individual, synergistic and or antagonistic effects of each substrate within the mixture. The last step was evaluation of optimal conditions for methane production using identified optimal mixture blends from the previous step employing Response surface methodology (RSM) to a Central composite rotatable design (CCRD). Herein, the effects of organic load, food-to-microorganisms (F/M) ratio and initial reactor pH were investigated with SMP and maximum specific methane production rate (Rmax) as the response(s) variables. After optimization, biogas production from the anaerobic co-digestion of five mixture blends consisting of AsCb (1:1), AsWs (2:3), CbWs (2:3), and AsCbWs (1:1:1 and 1:4:1), was studied using up-scaled two 5L acrylic custom built laboratory digesters equipped with a pH-control, gas-scrubbing and metering units and an automated data logging/control system for pH and temperature control. The digesters were run under mesophilic conditions to compare the operational efficiency of batch, single-stage semi-continuous and two-stage semi-continuous mode(s). For batch experiments, short retention periods of 5-15 days and semi-continuous experiments were run with varying hydraulic retention time (HRT) of (19 - 450 days), organic load (0.3-1.4 gVS/Ld-1). The results from the BMP studies determined the highest SMP from mono-D of As and Cb to be 192 and 110 NmLCH4/gVSadded. They also revealed that AcoD of 2:3 binary mixture blends of AsWs and CbWs yielded the highest SMP’s of 370 and 354 NmLCH4/gVSadded, while ternary blends yielded poorest results where a SMP of 22 NmLCH4/gVSadded was recorded. All kinetic models sufficiently simulated SMP with coefficient of determination (R2) values above 90%. Furthermore, an increase in F/M and VS concentration negatively impacted the overall digestion performance. This led to the conclusion that various factors play a significant role in the efficiency of the overall AD process, and that most particularly mixture compositions, organic load, F/M ratio and pH were identified as more relevant in the AD process. The optimal mixture blend was determined to be of (1:4:1) AsCbWs and AsCb (1:1) with a SMP of 112 and 104 NmLCH4/g VSadded and maximum SMP rate (Rmax) of 11 and 14 CH4/gVS day-1, respectively. Overall, 21.3 % and 29.3 % improvements in SMPs were respectively recorded from ternary and binary blends, as the results of synergistic effects prompted by mixture blending. The antagonistic effects were only recorded for mixture of AsWs. The RSM optimization results showed organic load of 1.59 g VS/L, F/M of 0.25 gVS/gVS and initial pH of 6.5 as optimal values. A maximum SMP of 309 NmLCH4/gVSadded was predicted by the special cubic model for the AsCbWs mixture blend. The experimental data was a close fit with SMP of 316 NmLCH4/gVSadded and maximum SMP rate (Rmax) of 18 CH4/gVSday-1, respectively which was slightly higher than the predicted values as indicated by the coefficient of determination (R2) of 96.7 %. The RSM was therefore successfully implemented in the optimization of SMP for AcoD of abattoir and winery wastes. Primary emphasis was given to AsCbWs (1:4:1) mixtures which resulted in the highest methane efficiency and stable operation. The highest methane yield of above 500 NmL was obtained in batch mode. Abattoir and winery waste can be successfully co-digested to improve biomethane production by optimizing AD process input parameters