Separation of wastewater split streams and utilization of the resources contained therein are an important prerequisite for a sustainable wastewater treatment. In areas with existing infrastructures, this leads to transition states during which current systems must continue to operate. A simple mass-flow based method was developed to assess the impacts of an incremental transition (co-digestion of blackwater and sewage sludge) upon plant operation. The results proved that blackwater co-digestion could be easily integrated into existing wastewater infrastructures; however, beyond 8% transition thickening was required due to hydraulic limitations in the digester. Additionally, nitrogen recovery was necessary beyond 35% transition to avoid unfavorable C:N ratios. The investigated concept also led to a power gain of 11 kWh/(PE•a) due to enhanced biogas production and aeration savings. The developed Excel-based method was considered suitable for evaluating the benefits and probable tipping points for plant operation during transition to source-separated sanitation systems.

a b s t r ac tThe performance of two pilot-scale anaerobic reactors for blackwater co-digestion was studied as an attempt to investigate the transition of current wastewater infrastructures to source-separated sanitation. The focus of this study was to assess the feasibility of blackwater co-digestion at conventional wastewater treatment plants. Two scenarios were investigated; in scenario one, blackwater was co-digested with municipal sewage sludge in a 630 L continuous stirred-tank reactor (CSTR). In scenario two, blackwater was digested alongside high-strength municipal wastewater (concentration peak) in a 720 L upflow anaerobic sludge blanket (UASB) reactor. For CSTR operation, increasing methane yields from 222 to 332 L CH 4 kg/COD removed were achieved by enhancing the blackwater fraction at the reactor inlet from 0% to 35% (% total influent load as COD BW /COD tot ). The observed COD removal ranged from 60% to 78% at 0.9-1.6 kg COD/(m³·d). For UASB operation, COD removals of 57%-67% were reported at COD loading rates of 6.1-8.4 kg/(m 3 d). Removal of organic matter was successfully carried out in both reactors, yet blackwater co-digestion alongside raw sludge (CSTR) proved to be more advantageous to the plant in terms of overall biogas production. The results also indicate that municipal digesters can be successfully integrated in transition strategies for resource-oriented sanitation, thus potentially increasing energy utilization in the plant.