Passive separation of recovered ammonia from catholyte for reduced energy consumption in microbial electrolysis cells

Abstract

Ammonia recovery using microbial electrolysis cells (MECs) is of great interest due to potentially high energy efficiency. However, separating the recovered ammonia from MEC catholyte demands energy input via gas stripping or solution mixing. In this study, a passive separation method has been investigated to greatly reduce energy input for ammonia recovery. By exposing the cathode electrode directly to the gas phase, the passive separation led to comparable current generation to that with the active aeration, through the mutual influence of catholyte pH and conductivity. With the active aeration, the percentage of nitrogen in the catholyte (1–4%) was much lower than that in the MEC catholyte with the passive separation (21.1 ± 4.8%). Although the active aeration could achieve a higher ammonia recovery efficiency of 90.1 ± 1.3%, the energy consumption with the passive separation was only 1.3 kW h per kg N recovery, significantly lower than 2.3 kW h per kg N recovery with the active aeration. These results have demonstrated the effectiveness of passive separation as a simple method to collect the recovered ammonia with high energy efficiency.