We have long known that deficient macronutrients often impacts floc formation and the filament/floc forming organism balance. But what happens with the actual microbial makeup in the biomass. Molecular testing allows for conducting a total microbial census and we can compare similar systems with and without added macronutrients.
Diazatrophs are microbial organisms that use N2 gas - present at approximately 80% of our atmosphere - to provide nitrogen needed for cell growth. This reaction is most described in agriculture with legume crops where the microbes grow in root nodules. In wastewater, diazotrophs include a diverse mix of organisms including Rhizobia, Rhodopseudomonas, Frankia, Klebsiella, Azotobacter, Paenibacillus, and many cyanobacteria genera. Some of these organisms are floc forming and desirable in wastewater but others like cyanobacteria can create water quality issues.
In systems adding macronutrients, operators want to add sufficient N & P to keep a healthy, floc forming microbial population but not overdose nutrients. In the past, nutrient dose was based on finding N & P residuals in the effluent and maintaining these at target residuals. With molecular testing, we are now able to monitor for the presence of AOB/NOB (feeding too much nitrogen) and diazotroph populations (if in excess, you have insufficient nitrogen).