Removal of ammonia is one of the primary aims 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 converted to first nitrite and then nitrate followed by a denitrification step involving the reduction of nitrate to nitrogen gas.
Wastewater treatment plants are operated under various constraints which 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 process is minimised.
Process emissions
Over the past 3 years there has been more emphasis on the role that nitrous oxide process emissions have on Net Zero. Water companies and consulting engineers are investigating 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 key indicator, and possible analogue, for the production of nitrous oxide.
Measuring nitrite and nitrate concentrations online in activated sludge processes is now possible using the NOx-
Monitrix online nitrite and nitrate monitor.
Real time data Concentration limits and guidelines
Figures 1 and 2 show a weeks’ worth of data from an activated sludge plane at different times of the year.
Figure 1: September 2022
Figure 2; November 2022
Discussion and observations
There are clear differences between the data sets, understanding of why the differences occur would need complementary data sets, dissolved oxygen, ammonia concentrations, RAS rates, temperature etc.
In the September data set nitrite is seen to accumulate on a 12-hour cycle in the daylight hours between
approximately 7am and 7pm. The average during the baseline period is around 10ppm with peaks up to
30ppm. The nitrate cycle does not feature a flatline period, it cycles on a 24 cycle with the nitrate peak
around 13 hours behind the nitrite peak. The nitrate cycles between approximately 70 and 130 ppm.
The November data set does not feature clear diurnal behaviour in either parameter. Nitrite does have a 24- hour cycle between approximately 4ppm and 7ppm with nitrate cycling between 50 and 70ppm.
Questions
Does the September data confirm that nitrite accumulation risk is higher in warmer months?
Does the data show the time relation between nitrite production and nitrate production as nitrite is acted upon by NOB’s.
Summary
Real time nitrite and nitrate data can be obtained from aeration lanes in activated sludge treatment
processes. The nitrite data opens the door to advanced process control. The nitrate data allows nitrogen mass balances to be calculated through an aeration lane. Dissolved set points, ammonia rich digestor centrate feed rates, RAS rates etc can be adjusted based on nitrite accumulation to minimise the risk of “allowing” the process going down undesired pathways.
