Metsä-Serla has studied the possibilities of controlling phosphorus and nitrogen discharges from its five activated sludge plants for several years in cooperation with various research programmes.

Phosphorus in waste waters has been found to originate mainly from wood, nitrogen mainly from process chemicals.

Efficient sludge separation is crucial for nutrient removal, but is usually not sufficient to meet the new stringent limits. The levels of soluble nutrients need to be lowered as well.

With few exceptions control of nitrogen discharges is dependent on the daily adjustment of nitrogen dosage to the activated sludge plant This is also the case for phosphorus control at paper mills. Efficient control of phosphorus discharges at pulp mills, on the contrary, can only be achieved by measures within the pulp production process.

Optimization of the nutrient dosages requires a rather large amount of analytical measurements of the purification process at the mill, which can be problematic especially for small production units.

The nutrient control strategies for pulp and paper mills, as well as the problems of secondary wastes produced by the purification processes are discussed.