Improving contaminant removal and inhibiting CH4 and H2S emissions from septic tanks: Nitrified human urine as a source of electron acceptor.

Septic tanks are widely adopted in decentralized household wastewater treatment systems serving billions of people globally. Due to the lack of effective electron acceptors, insufficient nutrient removal and the emission of harmful gases, e. g. H2S, CH4, etc., are the common drawbacks. In the present work, we attempted to supplement nitrite into septic tanks as an electron acceptor, via nitrifying human urine source-separated from blackwater, to overcome these drawbacks. Partial or complete nitritation of source-separated urine was achieved in a sequencing batch reactor. The addition of nitrified urine into septic tanks improved organic and nitrogen removals in blackwater up to 90 % and 70 %, respectively. The emission of harmful gases from the septic tanks was stably diminished, with more than 75 % of CH4, CO2 and H2S reductions. Nitrite addition significantly reduced the abundance of hydrogenotrophic methanogens in septic tanks. Though the activity of sulfate-reducing bacteria recovered after the initial inhibition upon nitrite addition, the bio-generated H2S was retained in water since the increased wastewater pH after nitrite addition promoted the disassociation of H2S in aqueous solution.