Industrial wastewaters with high ammonia concentrations, high toxicity and/or unfavourable COD/N ratio call for special attention and concern with regard to design and operation of the wastewater treatment system. Two cases are described: wastewater from a rendering plant and from a coke oven plant.

The precondition for obtaining and maintaining a nitrification process in both a rendering plant and a coke oven plant is close control and regulation of pH in the process tanks. Both high and low values of pH cause inhibition of the nitrifiers. pH should be kept in the range 6.5-7.0.

The organic fraction at the rendering plant mainly consists of easily biodegradable organic acids. The BIO-DENITRO concept has been chosen for this application and proven to be very efficient to obtain low effluent values for (NH3 + NH4)-N due to the flexibility in adjusting the phase length under variation in loadings and wastewater composition. Complete denitrification could be obtained with COD/N ratio larger than 5.

Wastewater from a coke oven plant contains compounds that are extremely toxic to the nitrifiers as for example phenol, cyanide, thiocyanate, sulphide and ammonia. Sufficient upstream buffering is essential to prevent shock loads. In order to obtain complete denitrification at the coke oven plant by using an internal carbon source, it is necessary to have an efficient ammonia stripping and phenol must not be removed from wastewater. COD/N ratio must be larger than 8 to obtain complete denitrification.