Environmentally and economically attractive

Advanced control and operation of waste water treatment plants in Aarhus, Denmark

The Danish Environmental Action plan set the effluent standard to 10 mg BOD/1, 8 mg N/1 and 0.5 - 1 mg P/1 from January 1993 onwards. Altogether, the municipality of Aarhus runs 20 modern waste water treatment plants which clean sewage of approximately 500.000 PE.

Computerized on-line control of waste water treatment plants optimizes their operation.

To fulfill the effluent standards, the treatment plants are surveyed and operated by computers that optimize their operation. In addition, the measuring equipment has also been improved, and today it is possible to measure ammonia, nitrate and phosphorous continuously. New on-line control system The results of the new on-line control system have been very convincing. Compared to a more locked time control of nitrogen removal, the on-line control system is able to variate the time used for nitrification and denitrification according to need. This ensures that the biological volume of the plant is used as efficiently as possible. On-line sensors are used at plants with biological phosphorous removal. At these plants, the bacteria removes nearly all the phosphorus from the waste water, but in situations where the load is too high the phosphorous sensor controls assist in dosing ferrous salts, minimizing the use of chemicals. Both environmentally and economically attractive In both cases the introduction of an on-line sensor system is not only a favourable option with respect to optimization of the operation of a plant, but it is also economically attractive. The price for an on-line control system inclusive of a filtering system is about DKr 400.000, but compared with the construction price for a new biological tank, this on-line system is feasible. Besides the operational advantages mentioned, the computer systems also improve the collection of data related to these processes. Modern computer technology has therefore afforded many advantages in the operation of waste water treatment plants in Aarhus, and it has therefore been possible to fulfill the new effluent standards set by the Action plan. In general, measured effluent values reach only about 60 % of the effluent standards. For more information, please contact Mr Jørn Andersen, Chief Engineer, Environmental Department, Municipality of Aarhus, Denmark, fax: + 45 8940 4520 Back to the Table of Contents