- Check influent TKN (Total Nitrogen)
Most systems only run influent ammonia. However organic N is converted to ammonia in the wastewater treatment process and should be considered part of your total loading.
- Look at the aeration basin environment
Alkalinity, pH, Temperature, D.O. - did anything change from when the system was working correctly?
- Acute toxicity
Not as common as you would think -> Examples sulfides, phenols, organics with thiocarbonyl functional group (>C=S), cyanide
- Organic loading and combination of factors causing loss of nitrifier populations
Remember that both AOB & NOB cultures grow at slower rates than heterotrophic cultures. An old rule of thumb is that 80 of BOD/COD needs to be removed before nitrifiers can compete for nutrients and grow. This means that you must have hydraulic residence time for the nitrifiers to grow and develop. Also consider low temperatures or non-ideal pH factors that can slow nitrifier growth. All of this can lead to a loss of nitrifiers as a % of biomass.
Most of us learned nitrifiers as Nitrosomonas (AOB) and Nitrobacter (NOB) - but in practice wastewater treatment systems we see a different mix of organisms than were isolated and grown in the lab. Our use of next generation sequencing has identified that most systems rely on a mix of Nitromonas and Nitrospira (which can be NOB & COMAMMOX). After running over 5 years of sequencing data, we now have developed primer sets to run qPCR on wastewater samples to get rapid quantitative data on AOB & NOB populations. The qPCR nitrifier test gives information if you have a true toxicity event that killed the nitrifers or if you have conditions inhibiting their normal metabolism which eventually leads to washout. This can save money and time in addressing nitrification problems.