Thermal energy recovery from a sequencing batch biofilter granular reactor (SBBGR) on a pilot scale: Evaluation of the effects of energy extraction on the depuration process, process effectiveness, and results scalability

Urban wastewater is a valuable source of clean energy available for both building conditioning and hot sanitary water production, thus reducing primary energy demand and greenhouse gas emissions. In the present study, the integration of a highly efficient solar-assisted fully off-the-grid water-source heat pump (SHP) with a sequencing batch biofilter granular reactor (SBBGR) is tested on a pilot scale for recovering and reusing thermal energy generated during the depuration process. The prototype was designed to simulate wastewater production (240 L/d), domestic hot water (DHW) (152 L/d at 40 °C), and space heating (20-25 °C) energy demand for a one-person equivalent. Three set temperatures for heat extraction from the SBBGR were tested: 20, 14, and 10 °C. Heat extraction had limited effects on the average SBBGR performances. The SBBGR ensured a removal efficiency close to 90% for total suspended solids (TSS), chemical oxygen demand (COD), and ammonia, whereas a decrease in total nitrogen (TN) removal efficiency, namely from 75% to 71%, was observed with the operating temperature decrease. Energy recovery data suggested that the energy extracted from the SBBGR might cover the energy demand for DHW production or space heating from April to October. Thus, the collected energy data was modeled with the following purposes: highlighting the key parameters for optimizing energy recovery, quantifying the share of recoverable energy derived the microbial metabolism, and supporting or rejecting the scalability of the results. The model outcomes confirmed that the temperature difference between the sewage and heat extraction set point temperatures was the key parameter for energy recovery and succeeded in estimating the contribution of microbial metabolisms (i.e. about 3.2 kWh/m3 × d). However, the estimation of the full-scale recoverable energy was partially biased by the impact of the environmental conditions on the pilot.