Water is the source and basis of all life. As a solvent and transporting agent, it carries not only vital minerals and nutrients, but also, increasingly, harmful pollutants which accumulate in aquatic or terrestrial organisms.

Because of the associated health risks, the World Health Organization (WHO) has issued guideline values for about 200 substances found in water, including nitrogen compounds such as ammonia (NH₃), nitrate (NO₃^-), and nitrite (NO₂^-). Therefore, the treatment of wastewater is a critical topic and compliance with the legal limits is very important to help safeguard the environment from these mobile pollutants.

This Process Application Note deals with online measurements of ammonia, nitrite, and nitrate in wastewater treatment plants. These nitrogen compounds are analyzed simultaneously using a drift-free colorimetric measurement in a multi-parameter process analyzer from Metrohm Process Analytics.

The importance of nutrient removal in the form of nitrogen and phosphorus in industrial and municipal wastewater has increased significantly over the past decade to minimize water pollution and avoid eutrophication of water bodies. Nitrogen is present in wastewater as mostly organic nitrogen, ammonium/ammonia, and nitrate. This wastewater must be properly treated to meet stringent effluent requirements imposed by legislation.

Biological nutrient removal (BNR) processes are put in place at modern wastewater treatment plants (WWTP) to treat and remove the nitrogen compounds, and these involve both nitrification and denitrification process steps. Nitrification (Reaction 1A) takes place in the aeration basin (Figure 1) where ammonium is oxidized to nitrate via nitrite using oxygen and different species of nitrifiers (bacteria). During subsequent denitrification (Reaction 1B), the nitrate is further converted to nitrogen gas (N₂) using specialized heterotrophic bacteria in the absence of oxygen (i.e., anoxic zone). The N₂ formed is harmlessly released into the atmosphere. It is therefore very important to measure the ammonia, nitrate, and nitrite concentrations continuously throughout both process steps to ensure complete nitrogen oxidation and subsequent nitrogen gas conversion.

An increase in ammonia and nitrite levels in the effluent indicate that either an insufficient aeration step or a change in toxicity or pH is disturbing the nitrification process. On the other hand, an increase of nitrate in the effluent can indicate that the anoxic zone is not developing correctly or the Biochemical (or Biological) Oxygen Demand (BOD) food source in the effluent is lower than usual.

The ammonia and nitrate/nitrite content must be constantly monitored in wastewater effluent to ensure environmental compliance. Traditionally, these parameters can be measured by laboratory analysis. However, this methodology does not provide «real-time» results and requires constant maintenance and human intervention to adapt to current operation conditions. Robust self-cleaning online process analyzers are the only reliable measurement solution for industrial, sewage, and municipal wastewater applications with high concentrations of solids and bacteria.

Metrohm Process Analytics offers a multi-parameter process analyzer solution for the simultaneous analysis of ammonia, nitrate, and nitrite over a wide concentration range covering all influent and effluent requirements—the 2060 TI Process Analyzer (Figure 2). This process analyzer can also monitor nitrogen compounds online, guaranteeing high nutrient treatment efficiency and low operating and energy costs.

NH₃, NO₃^-, and NO₂^-  can be analyzed simultaneously using a drift-free photometric measurement. The 2060 TI Process Analyzer is able to measure a wide range of NO₂^-, NO₃^-, and NH₃/NH₄⁺ concentrations, from μg/L to mg/L (Table 1).

Ammonia, nitrite, and nitrate are important parameters to measure in wastewater treatment plants. For increased analysis frequency, the 2060 TI Process Analyzer from Metrohm Process Analytics can prevent out-of-specification data of wastewater streams and help to guarantee compliance with governmental regulations.