It is possible to have high TKN and low ammonia-N in a municipal wastewater treatment plant solids stream. This may occur due to the presence of organic nitrogen compounds in the influent, such as proteins and amino acids, which are hydrolyzed and converted to TKN during the treatment process. However, the ammonia-N fraction may be low due to nitrification and denitrification processes that occur in the treatment process, which convert ammonia-N to nitrate and nitrogen gas, respectively.

One possible explanation for this phenomenon is that the SBRS (sequencing batch reactor system) in the treatment process is effectively removing the ammonia-N through nitrification and denitrification processes, while the organic nitrogen compounds remain in the solids stream as TKN. This can occur if the SBRS is designed and operated for efficient nitrification and denitrification, which would result in low levels of ammonia-N in the effluent.

Another possibility is that the digesters in the treatment process are converting a significant portion of the organic nitrogen compounds to TKN, while the ammonia-N is being removed through nitrification and denitrification processes. The specific microorganisms present in the digester, as well as the operating conditions (e.g., temperature, pH, retention time), can influence the extent and rate of conversion of organic nitrogen to TKN.

It is important to note that the presence of high TKN in the solids stream may have implications for the disposal and reuse of the biosolids, as TKN can contribute to nitrogen loading in receiving waters or soil if not properly managed.

I am sorry I do not have a specific reference paper to point you to, but you may be able to find more information by searching for studies or reports on the treatment of municipal wastewater with high TKN and low ammonia-N.