Improving methane productivity of foodwaste by enzymatic pretreatment and electrode modification in mec-ad hybrid system

Abstract

Hydrolysis has been identified as a rate-limiting stage in anaerobic-digestion. While it’s been widely used in biomethane production, biomethane only accounts for 50-60%. A Therefore, an integrated AD-MEC system was developed to increase the biomethane content using food waste. However, high electrode’s cost in the hybrid system poses an economical challenge to the market. Moreover, the microbial community plays a crucial role in the system, yet, minimal studies address the enhancement of microbial community and diversity. Hence, the characterization of food waste was performed in terms of carbohydrates, lipids, proteins, chemical oxygen demand, moisture content, solids, and volatile solids. The enzymatic hydrolysis of food waste was conducted to obtain the hydrolysate by one-factor-at-a-time (OFAT) through various factors including reaction time, temperature, enzyme loading, substrate concentration, and pH. The results showed that the optimum pH of 7, substrate concentration of 6%TS, the temperature of 50oC, and time of 16h gave the best release of reducing sugars. followed by the statistical optimization using faced centred central composite design (FCCCD) of selected factors, namely enzyme loading and substrate concentration,. The optimum conditions were enzyme loading of 6% (w/v) and a substrate concentration of 10% as the total solids (TS). Another pre-treatment, the acidic-enzymatic treatment using different concentrations of acids were performed. An acid concentration of 0.5% (v/v) showed the best hydrolysis effect achieving a value of 20 g/L reducing sugar,34.2% solids reduction, and 90 g/L soluble chemical oxygen demand (SCOD). However, the biogas production and free amino nitrogen release from acidic-enzymatic treated samples were lesser than only enzymatically treated samples. For MEC system, the effect of electrode modification using multiwall carbon nanotubes (MWCNT) and microbial growth into the electrodes was monitored using scanning electron microscope (SEM) images. The MWCNT growth was in-between the carbon felt fibres and the stainless steel mesh strands. The effectiveness of the electrodes was tested by inserting them into the hybrid system with glucose as the main substrate. Stainless steel mesh- modified cathode showed the highest biogas and methane production with a value of 14.4 ml CH4/g glucose. In addition, carbon-felt modified electrodes showed a maximum substrate degradation value of 93% and a current density of 4.5 mA/m2. The SEM imaging of the microbial growth on the electrodes showed that the microbes followed a different growth behaviour in modified and unmodified electrodes. In addition, MWCNT-modified Stainless steel mesh(SSTM) showed a potential hydrogenotrophic growth selectivity, unlike unmodified SSTM, which had a more syntrophic microbial community. Hybrid systems showed a higher hydrolysis efficiency especially modified systems, with a percentage of 39.4% by the 48th hour, followed by unmodified systems. The acidogenesis efficiency results showed that the hybrid systems were dominated by the acetic acid pathway, which is favourable in the hybrid system, unlike the conventional digester, which was dominated by a different pathway. Mixing the original inoculum obtained from a previous AD with cow manure has enhanced and increased the competitiveness of the microbial community. Thus, it was positively reflected on the biomethane production potential and rate, with a value of 38ml/g COD and 1.2 ml/h, respectively. In this study, we successfully enhanced the hydrolysis rate, improved the selectivity of microbes in the system, and introduced a set of commercially available electrodes. Our findings also provided compelling evidence that increasing microbial diversity significantly enhances the overall performance of the system.