Treatment plants may be exposed to a whole range of toxic organic and inorganic compounds that may inhibit the removal of organic matter and nitrogen. In order to secure maximum treatment efficiency, the plant manager has to monitor the toxicity of the influent sewage. With regard to the receiving water the manager also has to make sure that toxicity in the influent is significantly reduced during treatment. Because a whole range of chemicals may be present, chemical analysis may be insufficient and expensive as a control instrument. Instead, direct toxicity measurements are preferable to capture the complexity of the wastewater. The monitoring methods have to be relevant and sensitive for the processes in the treatment plant, i.e. removal of organic matter and nutrients. The methods also have to be simple and inexpensive. The paper reports on recent results from the application of nitrification, algae and Biotox tests, and summarises the experience with monitoring of toxicity. Although the sensitivity of the tests varies with respect to individual chemicals or group of chemicals, the application of a combination of the tests gives a high likelihood of detecting toxic impacts on treatment plants and receiving waters.

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