The experience with the upgrading of 14 treatment plants to N and P removal is presented. For all plants it has been possible to fulfil the effluent standards. Through process optimization it has been possible to achieve a stable biological P-removal. The introduction of control systems based on nn-line sensors for ammonia, nitrate and phosphate has resulted in a more stable effluent quality and an increased capacity. The experience also shows that it can be very difficult to predict and control the changes in the industrial load. From this it is concluded that instead of spending a lot of time on fine-tuning the design criteria in the design phase, it may be better to incorporate a considerable flexibility into the design, allowing the processes to be adjusted to the changing operational conditions during time.

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