Energy and Economic Assessment of an Integrated FO-AD-RO System Combining Wastewater Treatment, Brine Valorization, and Biogas Generation

Isolated regions face significant challenges in managing water, energy, and waste due to their inherent geographic and resource constraints. This study evaluates the energy and economic viability of an integrated forward osmosis, anaerobic digestion, and reverse osmosis (FO-AD-RO) process for the simultaneous treatment of wastewater, valorization of brine streams, and generation of biogas, using the Canary Islands as a representative case study. Three real draw solutions (DS) were assessed, brine from a brackish water desalination plant (BWB), seawater (SW), and brine from a seawater reverse osmosis plant (SWB), across a wide range of FO and RO recovery rates. The results show that the combined use of FO and RO, when BWB and SW are used as DS, reduces the specific energy consumption of the RO stage by 14 to 28% relative to the baseline scenario, depending on the FO recovery rate. For BWB, the integrated process achieves an overall recovery rate close to 97%, approaching zero liquid discharge, while for SWB, overall seawater recovery above 70% is reached. The incorporation of an AD stage to treat the FO concentrate allows part of the RO energy demand to be offset through biogas-based energy recovery, an effect that is more pronounced for BWB and diminishes with increasing recovery rates. From an economic standpoint, the levelized cost of water remains below or close to 0.8 €/m3 for RO recovery rates below 50% across all scenarios, which is competitive with other brine minimization strategies. Overall, the integrated FO-AD-RO process proves to be a technically and economically viable strategy for reducing desalination energy consumption, valorizing waste streams, and supporting the decarbonization of water and wastewater treatment in isolated, energy-dependent regions.