Generally, the answer is that yes, aerobic digestion systems are expected to have high NO3 content. However, it is dependent on the type and concentration of waste in the water and the design of the system. If you are not seeing a good mass-balance of nitrogen in your system, then the missing nitrogen may have been assimilated into microbial biomass. This is especially true if the wastewater has a high concentration of carbohydrates. The missing nitrogen may also be off-gassed as N2 due to anaerobic areas in an otherwise aerobic system. The center of a biosolid clump is almost always anaerobic, even if the surface of the clump is in right in an aerator bubble stream. In intensive fish farming, if the fish require a high protein diet, then we will see high NO3 after aerobic treatment. However, if the fish are fed a high carbohydrate diet, the effluent may have relatively low NO3.

Dear Sir. Concerning your issue about the range of NO3 in the Aerobic digester. Investigations were undertaken to study the occurrence and progress of nitrification during aerobic digestion of activated sludge in a wide range of initial concentrations of total solids (1000 to 80 000 mg litre−1, initial pH range of 4·5 to 10·4 and digestion temperature range of 5° to 60°C. Batch aerobic digestion studies on activated sludge grown on wastewater (enriched with organic solids from human excretal material) indicate that almost complete elimination of the ‘biodegradable’ matter of the activated sludge was one of the essential prerequisites to initiate nitrification. Favourable ranges of temperature and pH for nitrification were observed to be 25° to 30°C and 6·0 to 8·3, respectively. With all favourable conditions, a minimum period of about 2 days was necessary for population build-up of genera Nitrosomonas and Nitrobacter, and to initiate nitrification. Nitrate formation invariably lagged behind nitrite formation, but under certain conditions both phases of nitrification were observed to progress hand in hand. I think the following below links may help you in your analysis:

https://www.sciencedirect.com/science/article/pii/0269749189902376

http://aerobicdigestion.solutions/wp-content/uploads/2015/11/Aerobic-Digestion-Workshop-vol.-1.pdf?x97417

Thanks

Some species could grow at a nitrate content as high as 2800 mg l−1.

High NO3 levels in aerobic digesters indicate wasting aeration energy.

There is no reason why NO3 levels would have to be higher than 5 - 10 ppm. The NO3 level can easily be controlled by modulating the aeration while maintaining complete mixing independent from the air input.