Many waste water treatment plants in Sweden have been using pre-precipitation, which has considerably decreased the load on the existing activated sludge process. In many cases it has been possible to introduce predenitrification, with minor changes or increase of volume, in order to reach the required effluent values, which in Sweden varies between 10 and 15 mg Ntot/l as annual average value.

In treatment plants with anaerobic digestion of sludge, some 15–20% of the nitrogen load is recirculated with the return liquors from dewatering. Separate treatment of ammonia-rich return liquors has successfully been using SBR-technique for nitrogen removal, adding external carbon source, e.g. methanol or ethanol. The overall result is, however, not sufficient to meet the requirement, without additional action.

The use of SBR-reactors, designed for nitrification/denitrification of return liquors from dewatering of digested sludge, and part of the influent raw waste water, 20–30%, as a carbon source, has been proven most cost-effective, both from investment and operation cost point of view. The result is a considerably decreased load on the existing biological stage, making it possible to introduce pre-denitrification without any extension of reaction volumes. The paper presents the result from full-scale operation of some plants in Sweden.

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