Inhibition of Nitrification in Municipal Wastewater - Sources, Effects, Evaluation and Remedies

Today, as a rule, the most cost-effective way to remove nitrogen from municipal wastewater is the biological method based on nitrification and subsequent denitrification. All biological systems may be affected by toxicants, and biological nitrogen removal is no exception. An investigation of influent wastewaters from 109 Swedish municipal wastewater treatment plants was accomplished. Inhibitory substances were detected in about 60% of the wastewaters. The level of inhibition was in general low, but at 4% of the plants, considerable inhibition was found. The main contribution to inhibitory substances found in the influent of a municipal wastewater treatment plant originates from industrial discharge. It is not possible to point out any special branch that invariably delivers inhibitory wastewater. Internal sources of inhibition should not be neglected. The effect of severe acute accidents is immediate and easy to detect but the, perhaps more important, concealed effects of a suppressed nitrification capacity will only be noticed by chance when the plant is running to capacity, for example during winter operation and peak-loads. A short-term nitrification inhibition test has been used in this study. Short-term inhibition tests do not mirror detailed effects on a wastewater treatment plant, but they are useful tools when identifying/solving problems. A method based on nitrifying organisms is preferable when the purpose of an investigation is to reveal sources that might affect nitrification at wastewater treatment plants. The chosen activated sludge type in the test has an effect on the obtained level of inhibition. The sensitivity of activated sludge from a specific treatment plant remains stable over prolonged periods. The suspended solids concentration used in the test has an effect on the inhibition results for some, but not for all, inhibitors. The heterotrophic respiration rate of the activated sludge used might affect the obtained results if the inhibitor is easily degradable. The ammonium concentration in the test tubes affects the results obtained for competitive inhibitors. No important difference could be distinguished between the inhibition found at 15°C and 20°C, respectively. Neither has any effect on the nitrification rate of the sludge been detected. In order to observe and verify inhibition of nitrification, a comprehensive sampling programme is required since the variation of inhibition is often substantial and irregular and since many sources contribute to the total inhibition. Requirements regarding nitrification inhibition should be set and, consequently, applied when outlet permits for industries are to be issued or renewed. The possibilities for a wastewater treatment plant to neutralise the effects of inhibitory wastewater are limited. Consequently, preventive measures will be a preferable way to control inhibition. Possible measures at industries that have been shown to discharge inhibitory wastewater are to change the properties of the wastewater by altered methods of production, or by detoxification of the wastewater by internal treatment. Nitrification-inhibition measurements on untreated and treated wastewater can be used for judging the detoxification potential of a treatment method. Alternatively industries can show that the inhibitor is easily degradable or that the biological processes in question easily adapts to the wastewater.