Characterization of domestic wastewater released from ‘green’ households and field study of the performance of onsite septic tanks retrofitted into aerobic bioreactors in cold climate

Abstract

This study aimed to investigate the efficacy of private septic systems retrofitted into aerobic bioreactors with ‘SludgeHammer’ technology. In addition, the study attempted to characterize the strength of domestic wastewater released from ‘green’ households practicing water conservation strategies. Ten retrofitted onsite septic systems were studied in the Edmonton area, Alberta (AB) Canada during winter. These systems could remove BOD5 and TSS by 92 ± 5 and 92 ± 6% respectively which, according to Albertan regulatory standards, were characteristic removal efficiencies of the secondary treatment in the subsequent drain field. These removal efficiencies were remarkable given the strength of the influent wastewater. The raw wastewater carried significantly high pollutant concentrations (1160 ± 350 mg BOD5/L, 1653 ± 1174 mg TSS/L, 99 ± 19 mg NH4+-N/L, 100 ± 56 mg TN/L, and 39 ± 28 mg PO43−-P/L), characterizing it as high-strength domestic wastewater. Mixing provided by the aerator could only suspend 1/34th (3% m/m) of the solids in the bioreactor and consequently released significantly low solid concentrations (195 ± 206 mg TSS/L) into the final treatment component. As such, this technology did not impair the natural function of septic tanks or did not create any unintended excessive solid loading on drain field as a consequence of the added mixing energies provided by the active aeration. Nitrogen balance suggested the possibility of simultaneous nitrification and denitrification (SND) in the aerobic bioreactors. In some cases, PO43−-P removal efficiency was as high as that in enhanced biological phosphate removal (EBPR) process (81–97%). Phosphorus balance estimated that non-assimilative pathways (i.e., EBPR + biologically induced phosphate precipitation (BIPP)) contributed 50–99% to overall phosphorus removal in the system. Long HRTs, high influent BOD5 and anaerobic/aerobic zoning in the bioreactor most likely provided favorable conditions for SND and high phosphorus removal efficiencies in the retrofitted onsite wastewater treatment systems (OWTS).