During nitrification, ammonia-oxidizing microbes oxidize free ammonia to nitrite, and nitriteoxidizing bacteria further oxidize nitrite to nitrate. Nitrification deteriorates water quality by destroying chloramine residuals, releasing free ammonia, promoting microbial (re)growth, and producing toxic nitrite and nitrate. Therefore, monitoring and controlling nitrification in engineered water systems is critical to ensuring drinking water quality and protecting public health.