A novel membrane-free ion enrichment electrolyzer recovers ammonia from digestate

The widespread adoption of anaerobic digestion in manure management necessitates handling discharged ammonia–nitrogen (NH 4 + -N)-rich digestate, where electrochemical ammonia recovery is a viable solution but is limited by the cost of membrane fouling. Here, we present a membrane-free ion enrichment electrolyzer (MIEE) engineered with an electrode spatially decoupled structure, enabling NH 4 + and OH – ion enrichment in the anaerobic digestate for further ammonia recovery. Through in-situ   measurements, we identify   electrochemically induced relative concentration gradients of ions   as the pivotal mechanism driving ion enrichment. MIEE maintains electroneutrality by migrating ions, and the effect is close to electrodialysis below ∼50 % migration of NH 4 + ions. Cathode effluent treated by MIEE achieves ∼80 % ratio of NH 4 + -N recovery in the stripping process, coupled with   carbon mitigation. The cost of OH – ions supplied by MIEE is ∼50 % compared to directly dosing NaOH. The anode effluent neutralizes the stripping effluent pH from 9.3 to 8.6 and optimizes the organic carbon/nitrogen molar ratio from 1.5 to more than 3, enhancing compatibility with the bioprocess. This study presents a novel strategy for enriching ions without ion exchange membranes, which reduces the cost of membranes and stripping pretreatment in the ammonia recovery.