Occurrence of nitrification inhibition in Swedish municipal wastewaters

The occurrence of substances inhibiting nitrification in Swedish municipal wastewaters was investigated using three methods: a screening method based on activated sludge and two pure culture methods based on Nitrosomonas and Nitrobacter. Influent samples from 109 wastewater treatment plants collected every day during one specific week were investigated. The three test methods were also compared. The results of the screening method showed that about 60% of the plants received wastewater containing inhibitory substances, although only 4% had considerable inhibition (>20%). With the two pure culture methods, inhibition was found at about 45% of the plants investigated, with considerable inhibition found at 13% and 20% of the plants with the Nitrosomonas and the Nitrobacter methods, respectively. The limit of detection was determined to be 5% inhibition for the screening method, 11% inhibition for the Nitrosomonas method and 13% inhibition for the Nitrobacter method. The pure culture methods found more samples strongly inhibitory or stimulating than the screening method. The highest correlation between the inhibition results from the three methods was found between the screening method and the Nitrosomonas method. It was also shown for sludge from several activated sludge treatment plants, that they were adapted to the toxic compounds present in the influent. At high inhibition this acclimatisation was less pronounced. No correlation was found for any of the methods between the inhibition and parameters such as the size of the plant, the geographic location, the content of ammonia, COD or conductivity in the influent, the presence of leachate or the percentage of industrial wastewater in the influent, or types of industries in the catchment area. However, the constantly highest inhibition was found at a plant with a large number of different industries connected. Any clear pattern for the variation of inhibition during the week was not found.