Chloramines & Nitrification Problems : Aquamonitrix Provides a Solution

Improving the understanding of the risk of nitrous oxide emissions through nitrite monitoring in activated sludge processes and final effluent

Background

Removal of ammonia is a primary aim of wastewater treatment processes. The chemistry and biology of the process is well understood. In summary, it involves a nitrification step in which ammonia is firstly converted to nitrite and then nitrate followed by a denitrification step involving the reduction of nitrate to nitrogen gas. An overview of the nitrogen cycle is depicted in figure 1 below. Nitrite and nitrate are key actors within this cycle and nitrite has a role to play when considering nitrous oxide (N2O) emissions.

Figure 1: Key processes in the N-cycle. N2O highlighted in gray. The formation of nitrite (NO2-) arises as an intermediate along with its oxidation to nitrate (NO3-) (Sabba et al. 2018, Applied Microbiology and Biotechnology, 102, 9815–9829).

Wastewater treatment plants are operated under numerous constraints which can broadly fall into three ‘E’ categories:
❖ Energy (mainly associated with aeration)
❖ Effluent (water quality)
❖ Emissions (of greenhouse gases and toxic gases)
Under the pressures of the drive towards NetZero a holistic approach is taken in which the overall environmental impact of the entire process is minimised.

Process emissions

Over the past 12-24 months there has been a greater emphasis on the role that nitrous oxide process emissions have on NetZero. Water companies and consulting engineers are investigating and exploring new ways to control activated sludge processes to reduce the risk of nitrous oxide production and subsequent emission.

Nitrite and its role in advanced process control

Nitrite accumulation has been put forward as a key indicator and possible analogue of nitrous oxide emission risk. Accurately measuring nitrite and nitrate concentrations on-line in activated sludge processes and final effluents is now possible using the NOx-monitrix on-line, continuous nitrite and nitrate analyser. Here, real- time, continuous nitrite and nitrate concentrations are reported through the direct monitoring of the activated sludge process (ASP) and final effluent of a WWT facility in England. Photographs of the NOx-monitrix continuous nitrite and nitrate analyser deployed to monitor the aeration tank and the final effluent are shown in figure 2 below. The continuous nitrite and nitrate data generated by the analysers over several months within the ASP and final effluent are shown in figures 3 and 4, respectively.

Figure 2: Photographs of NOx-monitrix nitrite and nitrate analysers deployed to continuously monitor the ASP and final effluent of a WWT facility in England.

Figure 3: Continuous and remotely accessible in-situ nitrite and nitrate concentration data from activated sludge process within WWT facility

Figure 4: Continuous and remotely accessible in-situ nitrite and nitrate concentration data from final effluent from WWT facility

Figure 5: Continuous in-situ nitrite and nitrate concentration data from ASP during month of September

Figure 6: Continuous in-situ nitrite and nitrate concentration data from ASP during month of November

Summary

Accurate real time nitrite and nitrate data can be obtained from aeration lanes and aeration tanks in activated sludge treatment processes using NOx-monitrix. The continuous in-situ nitrite data opens the door to advanced process control. The nitrate data allows nitrogen mass balances to be calculated through an aeration lane. Through these accurate continuous data, dissolved set points, ammonia rich digestor centrate feed rates and RAS rates may be adjusted based on nitrite accumulation to minimise the risk of “allowing” the process to go down undesired pathways.