Long-term electrodialysis nutrient concentration from anaerobic membrane bioreactor effluents: Challenges for calcium sulfate precipitation

Fertilizer requirements keep rising while phosphorus reserves diminish, and Haber-Bosch process contributes significantly to the greenhouse effect. In this sense, transforming conventional Wastewater Treatment Plants (WWTP) into resource recovery facilities (RRF) seems an attractive option. Electrodialysis enables the separation of ions from solution, generating dilute and concentrated effluents. Hard waters with high sulfate concentrations, such as Spanish Mediterranean basins, generate gypsum during the process, hindering nutrient concentration. Therefore, different long-term approaches were tested with AnMBR effluent to address this challenge. Although gypsum precipitation could not be avoided, pH control in the concentrated stream with HCl delayed the phenomenon. However, approximately 500 mg·L −1 NH 4 -N and 60 mg·L −1 PO 4 -P were obtained in the concentrate when bulk precipitation was observed. In contrast, using softened water yielded concentrations above 1500 mg·L −1 NH 4 -N and 135 mg·L −1 PO 4 -P, corresponding to 24.3- and 39.5-fold increases over their initial values and confirming that ED is suitable for soft AnMBR effluents. Energy consumption ranged from 0.2 to 0.6 kWh·m −3 , requiring further reduction to facilitate the process scale-up. Nonetheless, nitrogen production showed similar values (7.2 to 17.6 kWh·kgN −1 ) to those of the Haber-Bosch industrial process (8.9–19.3 kWh·kgN −1 ). Regarding the effluent, all the hard water experiments met the least restrictive requirements of DIRECTIVE (EU) 2024/3019 of 10 mg·L −1 NH 4 -N and 0.7 mg·L −1 PO 4 -P. Therefore, ED appears an interesting option for transforming WWTP into RRF.