Reducing the environmental footprint of wastewater screenings through anaerobic digestion with resource recovery

Screenings produced as the first stage of wastewater treatment and currently disposed of to landfill, are rich in volatile organic solids, nitrogen and phos- phorus which could be recovered through anaerobic digestion. Biochemical methane potential (BMP) tests on screenings demonstrated a methane yield of 0.33 m3 methane/kg volatile solids (VS) and a VS destruction of 50%. Con- sequently, the effect of a range of hydraulic retention time (HRT) and organic loading rates (OLRs) was evaluated in lab-scale continuously fed mesophilic digesters. The highest methane yield of 0.416 Nm3 methane/kg VS added was observed with an HRT of 15 days and an OLR of 2.5 kg VS/m3/day, when up to 65% of the VS were destroyed. If treated by anaerobic digestion, every dry tonne of screenings digested would divert 466 kg from landfill, save 4.6 tonne equivalent carbon dioxide (CO2 eq) and deliver 3.4 MWh of renewable energy.