The proper operation of the activated sludge process is endangered by toxic spills in the wastewater that has to be treated. Therefore there is a need for so called biological early warning systems (BEWSs) that can detect toxicity for the activated sludge process and allow operators to take protective actions.

Two independently developed BEWSs are described and evaluated. Both systems are based on the on-line measurement of the respiration rate of activated sludge taken from the plant to be protected. Their practical implementation however is different with respect to the way the suspicious wastewater is brought into contact with the sludge. It is illustrated what the consequences of the design choices are with respect to the sensitivity, the response time and the positioning at the treatment plant of these BEWSs.

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