Influence of toilet flushing water when using an on-site faecal sludge treatment combining electrocoagulation-flotation and anaerobic digestion

Abstract

Addressing the need for sustainable sanitation in public transport and tourism, this study evaluates electrocoagulation-flotation (ECF) combined with anaerobic digestion (AD) for treating synthetic human faecal sludge (FS). Especially the effects of influent aeration and the use of either rainwater or tap water for flushing on the efficiency of pollutant removal and methane production were investigated. In detail, flushing with rainwater combined with influent aeration resulted in the highest removal efficiency of total COD (tCOD), non-soluble COD (nsCOD), total nitrogen (TN), and total phosphate (TP) of 55 %, 27 %, 22 %, and 75 %, respectively, during ECF treatment, compared to 48 %, 15 %, 14 %, and 63 %, respectively, when tap water was used. As such, post-treatment is mainly necessary for the removal of TN. Combining flushing with rainwater and influent aeration also resulted in the highest faradeic efficiency (107 %) and the most insoluble iron coagulant formation. In contrast, the application of influent aeration during ECF of FS produced using rainwater flushing led to a decrease in methane production from 354 mL/g VS (when no aeration is applied) to 281 mL/g VS during subsequent AD. In conclusion, this study demonstrates that ECF can be applied as a good pre-treatment for sanitation water due to the high nsCOD and TP removal, by which ECF could replace, e.g., a septic tank. Furthermore, the potential of combining ECF with AD is demonstrated in view of energy recovery.