Influence of substrate concentration and feed frequency on ammonia inhibition in microbial fuel cells

Excessive amounts of ammonia are known to inhibit exoelectrogenic activities in microbial fuel cells (MFCs). However, the threshold ammonia concentration that triggers toxic effects is not consistent among literature papers, indicating that ammonia inhibition can be affected by other operational factors. Here, we examined the effect of substrate concentration and feed frequency on the capacity of exoelectrogenic bacteria to resist against ammonia inhibition. The high substrate condition (2 g L−1 sodium acetate, 2-day feed) maintained high electricity generation (between 1.1 and 1.9 W m−2) for total ammonia concentration up to 4000 mg-N L−1. The less frequent feed condition (2 g L−1 sodium acetate, 6-day feed) and the low substrate condition (0.67 g L−1 sodium acetate, 2-day feed) resulted in substantial decreases in electricity generation at total ammonia concentration of 2500 and 3000 mg-N L−1, respectively. It was determined that the power density curve serves as a better indicator than continuously monitored electric current for predicting ammonia inhibition in MFCs. The chemical oxygen demand (COD) removal gradually decreased at high ammonia concentration even without ammonia inhibition in electricity generation. The experimental results demonstrated that high substrate concentration and frequent feed substantially enhance the capacity of exoelectrogenic bacteria to resist against ammonia inhibition.