Comparing continuous and batch operation for high-rate treatment of urban wastewater

The water-energy nexus has changed the concept of wastewater treatment plants (WWTPs), which should move from energy consumers into energy neutral or even energy positive facilities. The A/B process aims at achieving self-sufficient energy WWTPs: organic matter is removed in the first step (A-stage) and derived to biogas production whereas autotrophic nitrogen removal is implemented in a second step (B-stage). This work compares two high-rate systems that can be used as A-stage in view of organic matter removal: a continuous high rate activated sludge (HRAS) reactor and a high-rate sequencing batch reactor (HRSBR). Both systems were operated with real urban wastewater at a short hydraulic retention time (2.5 h) and at short sludge retention time (SRT) of 1-2 d to minimize COD mineralization and to maximize organic matter diversion to methane production and, hence, energy recovery. The HRAS showed higher COD removal efficiencies and better energy recovery. On the other hand, the HRSBR was better to avoid undesired nitrification and provided lower COD mineralization for all the SRTs tested (ranging 20-48% for the HRSBR, and 41-58% for the HRAS). Then, the energy as methane recovered per unit of COD degraded was higher in the HRSBR. The HRSBR seems to be a good option, because the solids content in the effluent was similar for both systems and its COD removal efficiency can be further improved by optimizing the SBR cycle configuration.