Measuring nitrification inhibition by metals in wastewater treatment systems: Current state of science and fundamental research needs

Wastewater treatment is an important step within the water continuum as it reduces the risks associated with microorganisms as well as organic and inorganic compounds. From a chemical standpoint, treatment effectiveness is generally linked to carbon and nutrient (e.g., nitrogen, phosphorus) removal. A critical step in nitrogen removal is initiated by the conversion of ammonia to nitrate, a process that is carried out by nitrifying microorganisms. Nitrification is considered a key step as it removes the toxic effect of ammonia, while the resulting nitrate can be further converted into N2 gas (by autotrophic or heterotrophic nitrifying bacteria), which is not harmful to the environment. As nitrification is susceptible to a wide range of inhibitory substances, measuring the effect of potential inhibitors on nitrification rates is relevant to maintaining the performance of treatment plants. In this review we discuss the primary methods used to measure nitrification inhibition, their applications, and potential limitations. The authors also identify the research gaps that need to be addressed to better assess inhibition, with special attention on the effect of metals on nitrification in engineered wastewater treatment systems.