Primary sludge to valuable chemicals, hydrogen peroxide (H_2O_2), in microbial electrochemical cells - H_2O_2 production and in-situ sludge treatment

摘要

Organic-rich wastes such as wastewater sludges have been used for production of methane (CH4) in anaerobic digestion (AD). As an alternative anaerobic technology, microbial electrochemical cells (MxCs) have been studied for electron recovery from waste organics while detouring electron flow to electrode (anode) instead of CH4. Thus, MxC represents a promising, anaerobic biotechnology having the same advantages of AD including energy source production, low energy usage, and small excess cell production, which is associated in disposal cost. On top of that, the recovered electrons in the anode can be utilized to produce electrical power, hydrogen, or hydrogen peroxide, etc. at the cathode. Hydrogen peroxide (H_2O_2) production in MxCs, called microbial hydrogen peroxide producing cell (MPPC), is attractive because of the variety of uses for this chemical, especially for on-site wastewater-treatment such as tertiary treatment, pre- or post-treatment of sludge, or cleaning membranes. In this study, we conducted two experiments with different designs of MPPCs fed with primary sludge (PS) as a sole feeding substrate: dual-chambered and single-chambered MPPCs. First, in the dual-chambered MPPC (dMPPC), we aimed to produce H_2O_2 in the cathode chamber when feeding PS in the anode chamber. PS-COD and -VSS removals were 49 and 43%, respectively, and maximum current density was ～1 A/m~2. A maximum H_2O_2 concentration of -230 mg/L was achieved in 6 hours of batch cathode operation. This is the first demonstration of H_2O_2 production using PS in the well-designed MPPC and indicated that the energy requirement for H_2O_2 production was low (-0.87 kWh per kg H_2O_2). This energy requirement compared with that of previous studies using real wastewaters (up to 78 kWh per kg H_2O_2). However, we observed the H_2O_2 in the cathode chamber operated in batch mode gradually decayed with time due to the diffusion of H_2O_2-scavenging carbonate ions from the anode. To avoid H_2O_2 decomposition, several future studies should follow: ⅰ) continuous cathode operation, ⅱ) addition of H_2O_2 stabilizer, and ⅲ) direct application of H_2O_2 for wastewater disinfection. Next, in the single-chambered MPPC (sMPPC), we employed direct usage of H_2O_2 for sludge treatment by removing membranes and cathode chambers used in the dMPPC so that H_2O_2 can move into the anode chamber for enhancing sludge stabilization. We tested two different types of filters as separators between anodes and cathodes: glass fiber and polyester stitchbond fabric. In the sMPPC test, we increased PS loading, ～5 g COD/L/day, which was over 5-fold higher than that of dMPPC. Even in the higher loading condition, we achieved better COD and VSS removals, 52 and 49%, respectively, as well as higher maximum current density, -3.5 A/m~2. Especially with the polyester stitchbond fabric filter, which has larger pore size than glass fiber and thus better transport of H_2O_2, we achieved the important requirements for land application of biosolids: Class B biosolids disinfection and the vector attraction reduction (VAR) (< 2 million most probable number (MPN) per gram biosolids, dry-weight basis and > 38% volatile solids reduction); our results were 52% VSS removal and 1.2 × 10~5 MPN per g dry-weight PS. Also, with open circuit test (no current flowing), we differentiated the H_2O_2 contribution to PS organic removal by 13% on top of the electron recovery by anode respiration. Based on the electrochemical analysis with linear sweep voltammetry, ～0.5 g H_2O_2/L/day can be produced in the sMPPC with no energy input (energy neutral) and the assumption of 100% cathodic efficiency. As a proof-of-concept, we demonstrate H_2O_2 production in dMPPC fed with real wastewater sludge. More importantly, the results of sMPPC were very promising with ⅰ) the stable operation with high PS loading rates (5 gCOD/L/day and 2.7 gVSS/L/day), ⅱ) the achievement of a biosolids quality having Class B disinfection and the