Effects of anaerobic digestion centrate pre-treatment on further nutrient recovery by membrane contactors

Hollow fibre membrane contactors (HFMCs) technology emerges as a promising solution for efficient nitrogen recovery from anaerobic digestion centrates. However, full-scale implementation requires an efficient pre-treatment stage. This pre-treatment includes the increase of pH over 8.5 to maximize ammonia recovery and suspended solids removal to prevent membrane fouling thus extending its lifespan. This study compares different pre-treatment strategies, demonstrating its critical role in HFMC performance. Two pH-raising reagents (caustic soda and lime), the addition of coagulant (FeCl3) and flocculant (polyelectrolyte), and three solids removal methods: sedimentation, centrifugation and ultrafiltration through a ceramic membrane were evaluated by means of five sets of laboratory experiments. The most effective approach involved dosing lime to achieve pH values close to 12, followed by a 30-min settling period and a filtration process through a ceramic ultrafiltration membrane, resulting in a permeate free of suspended solids. Pre-treatment improved ammonia mass transfer and enabled ammonium recovery efficiencies in the HFMC around 90 %. An economic assessment confirmed the viability of scaling-up the process. The total cost of pre-treatment and nitrogen recovery was estimated at 2.12 € per m3 of centrate treated, while generating a revenue of 0.53 € per m3 of centrate treated. Integrating this pre-treatment process into a full-scale urban wastewater treatment plant would increase the overall cost of treating the influent flow by only 2.8 %, highlighting its viability as a cost-effective solution for nitrogen recovery.