For biological treatment of nutrient deficient industrial wastewaters, such as those of the pulp and paper industry, the addition of nitrogen and phosphorus is essential. As a certain surplus is necessary, both elements will be found in the effluent in varying concentrations.

For the often used activated sludge treatment 5 parts of N and 1 part of P are said to be required for elimination of 100 parts of BOD. In-plant optimization generally leads to about 3.5 parts of N and 0.6 parts of P for 100 parts of BOD. In most plants N is added as urea and P as phosphoric acid.

Optimized nutrient dosage aimed at stable operating conditions in the treatment plant generally gives average concentrations of 1 mg/l of both ammonia N and phosphate P in the treated effluent. However, due to fluctuations in loading and efficiency, variation coefficients of more than 100 % result in maximum concentrations in 24 hours mixed samples of more than 10 mg/l for N and P. Three examples of operational results are evaluated and discussed in detail.

Water quality requirements will impose general limitations on N and P concentrations in treated effluents. Depending on the concentration limits and on the type of limited substance - i.e. ammonia N, total inorganic N or total N, phosphate P or total P - different strategies have to be developed.

Nutrient dosage depending on the wastewater amount will become state of the art in the near future. The next step would be a loading dependent dosage. No results of technical operation are known which show the effluent concentrations obtainable with this technique.

For cases in which limits cannot be met with this strategy the possibilities of nitrification, denitrification and biological P removal are discussed for nutrient deficient wastewaters. Results show that nitrification will work at low ammonia concentrations, however no steady nitrification will be obtained. Denitrification, on the other hand, seems to be difficult with low nitrate concentrations. At the present stage, no technical process meeting stringent total N or total inorganic N limits is known to exist for this type of effluent.

Low P concentrations in the effluent can only be achieved by tertiary treatment, preferably final flocculation filtration processes. However, these will give rise to special problems in the treatment of pulp mill wastewaters, which are discussed in the paper.

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