Ammonium sulfate production from wastewater and low-grade sulfuric acid using bipolar- and cation-exchange membranes

Abstract

Conventional electrodialysis can be used to recover ammonia from dewatering centrate (i.e., down-stream wastewater from digested sludge dewatering). However, ionic impurities of the recovered ammonia solution are still a limiting factor for broad applications of ion-exchange membranes (IEMs) in wastewater treatment and resource recovery. In this study, an electrodialysis stack with bipolar membranes (BPMs) and cation exchange membranes (CEMs) but without anion exchange membranes (AEMs) was examined under various operation conditions to demonstrate high-purity ammonium sulfate production using low-grade sulfuric acid and dewatering centrate. Two significant benefits of removing AEMs from the bipolar membrane electrodialysis (BMED) are no more membrane fouling problems on AEMs and complete exclusion of impurity anions (e.g., chloride ions) in the recovered ammonium sulfate solution. A higher applied voltage condition (30 V over 7 pairs of CEM and BPM) resulted in a substantially high ammonia recovery (88.4%) and concentration (4.34 g-N/L) in 90 min. The ammonia recovery and concentration were also improved by increasing the flow rate through the BMED stack. The lowest electric energy consumption in the membrane stack was 9.6 kW h/kg-N, indicating energy efficient production of high-purity ammonium sulfate from wastewater. The amount of divalent cation scales accumulated in the BMED stack was linearly proportional to the average electric current, implying that the scaling problem can be controlled by reducing the applied voltage. Even with the scale accumulation, the electric current generation (i.e., separation performance) was hardly affected during the experiment. These findings demonstrated the strong potential of AEM-lacking BMED for practical ammonia separation from wastewater with reduced impurities.