Novel electro-ion substitution strategy in electrodialysis for ammonium recovery from digested sludge centrate in coastal regions

Membrane fouling has been the primary challenge limiting the application of electrodialysis (ED) technologies in wastewater treatment, particularly the wastewater containing abundant dissolved organic matter, such as the digested sludge centrate. This study proposed an electro-ion substitution modified electrodialysis (EIS-ED) system that can recover NH4+ from the sludge centrate in coastal wastewater treatment plants (WWTPs), with negligible membrane fouling and scaling formed. In this system, the sludge centrate flowed between two cationexchange membranes (CM), alongside the seawater providing Na+ as the substitution of NH4+. According to the experimental data and modeling, EIS-ED recovered more than 70% of NH4+ from the sludge centrate with an energy consumption of 2.03 kWh/kg NH4+-N, which was 14% lower than the conventional ED. Membranes, solutions and electrodes were the three major contributors of the linear ohmic resistance in the EIS-ED process. EIS-ED significantly resisted membrane fouling by means of electrostatic repulsion between the CM and negatively charged compounds, including particles and dissolved organic matter, and membrane scaling was also mitigated. After a treatment of 20 L sludge centrate, no significant decrease of membrane ion-exchange capacity was observed in the EIS-ED, while a decrease of 5.3% was found in the conventional ED. Accordingly, the potential application prospect of EIS-ED was proposed, which can recover at least 70% of NH4+ from the sludge centrate and reduce 10-20% of the NH4+ loading to the mainstream processes in coastal WWTPs.