Moving-bed biofilm reactor (MBBR) alleviates the negative effect of methylisothiazolinone on the nitrification-denitrification process

Methylisothiazolinone (MIT) is an emerging contaminant used as a biocide in housecleaning and skin care products. Therefore, MIT can reach civil wastewater treatment plants due to domestic usage, thus potentially inhibiting the biological process. This study aims to investigate, for the first time, the effect of MIT on the nitrification-denitrification process in a moving-bed biofilm reactor (MBBR). After a preliminary study performed in a fed-batch reactor, two MBBRs were simultaneously operated in semi-continuous mode for 136 days evaluating a concentration of MIT in the influent of 0.1 and 1.0 mg/L. A parallel study was also conducted under anaerobic conditions to evaluate the effect of MIT on methane production and volatile fatty acid accumulation. At a concentration of 0.1 mg MIT/L, the nitrification process experienced an initial complete inhibition. Subsequently, nitrifying bacteria gradually recovered, reaching approximately 49 % oxidation of ammonia nitrogen, a removal improved up to 64 % at 1.0 mg MIT/L. Although denitrification dealt with a temporary inhibition, the process ultimately achieved a 100 % decrease in nitrite and nitrate concentrations when the MIT concentration was increased from 0.1 to 1.0 mg/L. The cumulative methane yield reached approximately 134 mL CH4/g VS for the control group and was similar to the methane production obtained in the presence of MIT. A MIT removal of 100 % was achieved under aerobic, anoxic, and anaerobic conditions.