The impact of solids retention time on Greenhouse Gases Emissions from biological wastewater treatment processes

Biological wastewater treatment is linked to direct and indirect greenhouse gases (GHGs) emissions, thus new approaches on design and operation of wastewater treatment plants (WWTPs) must be adopted aiming towards GHGs emissions’ mitigation. This study evaluates the impact of solids retention time (SRT) on GHGs (CO2 and N2O) emissions of activated sludge (AS) wastewater treatment processes with nutrients removal operating at 10, 30 and 180 days SRT. The 180 days SRT represents the complete solids retention AS process (CRAS), which introduces a novel WWTP design and operation approach for excess sludge reduction. The evaluation of GHGs emissions and process performance via ASDM model and Bridle methodology showed that the total N2O emissions increased together with increasing dissolved oxygen and SRT. CRAS process achieved 97,4% and 96,2% excess sludge reduction compared to AS process with SRT of 10 d and 30 d respectively, with total direct emissions in CRAS process being respectively higher by 20,0% and 12,2%. The lower total indirect emissions in CRAS process compensate for the higher direct emissions. At high influent loads, the balance in GHGs emissions tilts in favour of CRAS process due to management of increased excess sludge quantities, rendering CRAS process a more sustainable choice.