On several occasions fish died in Hjälmaren due to, among other things, elevated nitrite levels in the lake. A contributing factor turned out to be the high nitrite content that leaves the wastewater treatment plant Skebäck, located in Örebro. The wastewater is purified from nitrogen through nitrification and denitrification where nitrite occurs as an intermediate in several reaction steps. The report contains theory round the formation of nitrite and the results from samplings where nitrite concentration was determined. The objective was to increase nitrite conversion and thereby decrease the content of nitrite in the effluent water, by studying the increase of nitrite.   Samplings started in March and no changes in the process where made, the first samples were analyzed to determine where the nitrite accumulation occurred. The samplings were taken in the intermittently aerated basin, wish includes a aerated period and a nonaerated period.   The initial samples showed that nitrite accumulates when there is oxygen in the basin. The content of nitrite decreases when there is no oxygen in the water. The ammonium limits were changed in one basin because it was suspected that the bacteria were becoming stressed by the intermittent aerated start-stop process. This resulted in a longer aerated period for the nitrified bacteria to grow and due to the change it was expected that the bacteria would thrive. This proved insufficient for nitrite oxidation to take place, but the concentration of nitrite at this time was at least lower.  These changes did not show any reduction in the nitrite concentration. Suggestions for future work are addition of an extern carbon source and optimizing the aeration.