Evaluation of enhancement methods for the production of biogas for anaerobic codigestion of sewage sludge with industrial wastewater

Abstract

The worldwide move towards a sustainable, equitable future faces two major obstacles: unsustainable waste management and access to clean energy (Kwietniewska and Tys, 2014). Alternative renewable energy sources are needed to curb global warming and the consumption of non-renewable fuels (Akinbami et al., 2021). Industrial and municipal wastewater with high organic matter content has increased due to rapid industrialisation in many emerging nations. If appropriately treated, wastewater may produce biogas through anaerobic digestion to generate green energy (Chrispim et al., 2021). While anaerobic digestion is a mature technology, challenges around process efficiency remain. Thus, much research has examined ways of enhancing anaerobic digestion (AD) efficacy. This research evaluated the suitability and investigated the effects of intermediate municipal landfill leachate and sugar industry wastewater as co-substrates in the AD of sewage sludge. The effects of biochar addition synthesised from sugarcane bagasse at three pyrolysis temperatures (3500C, 4500C and 5500C) together with ultrasonic pretreatment as a potential enhancement method were evaluated with respect to four key performance indicators: (I) biogas yield, (II) biogas quality, (III) COD and (IV) Volatile Solid (VS) removal. Response surface methodology (RSM) was employed to evaluate the following specific objectives: Five objectives were evaluated: (i) Characterization of primary sewage sludge, inoculum, and industrial wastewater. The total solids (TS) and volatile solids (VS) for PS were 39.02 (gTS/l) and 29.72 (gVS/L), respectively, falling within the expected range. The results also suggest that the sludge exhibits excellent biodegradability, evidenced by a VS/TS ratio of> 50%. Sugar industry wastewater (SIWW) exhibited a VS/TS ratio of 0.68, indicating a greater presence of organic substances than insoluble ones. The inoculum displayed the lowest VS/TS value, measuring 0.48, indicating a high concentration of microorganisms vs organic materials, confirming its potential use as an inoculum rather than as a substrate. (ii) Production and characterisation of biochar (BC) derived from sugarcane bagasse using energy dispersive x-ray, scanning electron microscopy (EDX/SEM), and Fourier transform infrared spectroscopy (FTIR). The BC synthesised at 5500C was the most alkaline at a pH of 9.1, followed by 9.0 (BC 4500C) and 7.0 (BC 3500C). While all the synthesised BC displayed carbon contents > 50 %, BC 5500C exhibited the highest at 78.33%. SEM analysis found BC 4500C exhibited greater formation of surface micropores. FTIR analysis of the BCs confirmed the presence of carboxyl, carbonyl, carboxyl, hydroxyl, C=N bond, and ether group. (iii) Investigate the effect of Municipal Intermediate Landfill Leachate (ILL) and Sugar Industry Wastewater (SIWW) as co-substrates and optimisation of process parameters with RSM. This was executed in two stages; the first was employed to identify the best-performing IWW. ILL produced the highest biogas yield at (54,35 mL/gVSadded) vs (12.55ml/gVSadded) for SIWW; ILL achieved the highest COD removal (46.84%). ILL was found to increase the COD removal by 5.36% compared to the CNTRL. Hence, ILL was selected as the best-performing co-substrate for process optimisation. Optimisation facilitated by (RSM) employing Box–Behnken design with cosubstrate loading between (1:20 – 1:5), ISR (1:2– 1.5:1) and temperature (25 – 550C), the optimum co-substrate loading of (1:20), ISR of (1.5:1), and a temperature of 370C, achieved desirability of 90.10%. The RSM-BBD models exhibited a significant correlation (0.9 < R2 < 1) with projected outcomes that aligned well with the experimental data. (iv) Investigate the effect of Biochar as an additive on the AD process. This was executed in two stages; the first was employed to identify the best-performing BC. BC 450 produced the highest biogas yield at (50.38mL/gVSadded) vs (46.88 and 26.57mL/gVSadded) for BC 3500C and BC 5500C, respectively; BC 4500C also achieved the greatest COD and VS removal of 50.86% and 35.11%, respectively. Hence, BC 450 was selected as the best-performing BC for process optimisation. Optimisation with co-substrate loading between (1:20 – 1:5), BC loading (2.5 – 10 g/L) and temperature (25 – 550C), the optimum co-substrate loading of (1:20), BC loading of (6.7 g/L), and a temperature of 54.990C, achieved desirability of 94.10%. The RSM-BBD models exhibited a significant correlation with projected outcomes that aligned well with the experimental data. (v) Investigate the effect of ultrasonic pre-treatment of industrial wastewater on the anaerobic digestion process. ILL was subjected to ultrasonic pretreatment and codigested employing the optimum process parameters from (iv) Process efficiency was assessed with respect to biogas yield, COD and VS removal and biomethane content and achieved a COD and VS removal of 46.18% and 49.21, respectively. The biomethane content peaked at 78.23% CH4. Representing a marginal decrease in COD and VS removal compared to the untreated sample.