Effect of oxidation reduction potential on methane emission from anaerobic septic systems

• Septic systems contribute to emissions of greenhouse gases, such as methane. • Monitoring oxidation reduction potential (ORP) in septic systems is proposed. • Biodegradation experiments were performed with ORP change by nitrate. • Methane emission is suppressed at ORP of −350 to −450 mV vs. Ag/AgCl. Septic systems are major on-site sanitation facilities used in many developing countries to treat domestic wastewater. Climate change concerns have prompted efforts to reduce greenhouse gas (GHG) emissions; nonetheless, septic systems contribute to emissions of GHGs, such as methane. The present study investigated modifications to improve the operating conditions of septic systems to minimize methane emissions by evaluating the oxidation reduction potential (ORP) as an operating parameter using laboratory-scale biodegradation experiments. To investigate the influence of ORP on methane emissions, dog food and potassium nitrate were used as representatives of blackwater and alternative electron acceptors to oxygen, respectively, under various biodegradation conditions. The experimental results suggest that methane emission is suppressed at a critical ORP level (−350 to −450 mV vs. Ag/AgCl). They also showed that ORP can be used as a monitoring signal to better understand methane-producing conditions in septic systems. The proposed modifications to improve septic system operating conditions are to shorten the desludging period and provide sufficient oxygen to the septic tank, considering the critical ORP to prevent anaerobic conditions.