Managing conflicting needs in BNR treatment of high strength nitrogenous effluents

Biological nitrogen removal in wastewater is now seen as one of the essential treatment requirements prior to discharge to a receiving environment for many municipalities and industrial wastes generators. For municipal wastewaters, the level of nitrogen concentration is generally low and therefore the impact of nitrogen removal on bioprocess is not apparent.

When high strength nitrogenous effluents are subjected to biological nitrogen removal (BNR), the wastewater composition, environmental controls and the process microbiology start to impose significant constraints to the success of the BNR treatment system.

A rigorous characterisation of the incoming wastewater needs to be undertaken to determine how the nitrogen removal could be impacted by substrate concentration and the controls required to manage efficient treatment as well as ensuring that the bioflocculation of the activated sludge is maintained.

At a a case study site, operational difficulties were encountered for biological nitrogen removal. The operational controls and the process constraints resulted in poor nitrogen removal. The rate of rise of ammoniacal nitrogen concentrations within the BNR reactor was unacceptable and resulted in significant ammoniacal nitrogen excursion.

In addition bioflocculation was negatively impacted with a significant change in the cation balance. An examination of the cation balance showed a disproportionate amount of monovalent cations compared to divalent cations. The imbalance resulted in very poor solids separation in the clarifier. Remedial measures were undertaken by adding hydrated lime, but this in turn resulted in uncontrolled nitrification.

In this paper, the process fundamentals are examined to provide guidance on the BNR treatment processes for high strength nitrogenous effluents. A description of the process constraints and the methods used to manage efficient nitrogen removal is discussed.