The general Danish standards for discharge from municipal waste water treatment plants larger than 5000 PE require extended nitrogen and phosphorus removal to 8 mg/l total-nitrogen and to 1.5 mg/l phosphorus. In many cases the local authorities require that the same standards have to be met by smaller plants down to 500 PE. In some cases even tighter standards are set. As plants for extended nitrogen and phosphorus removal for smaller communities should not necessarily look like the large plants, new concepts have been developed. Combined nitrogen and phosphorus removal based on one activated sludge tank and one clarifier has proved to be advantageous both technically and economically in order to meet the above mentioned standards given under the Danish climate conditions with water temperatures of 38°C during winter. Six plants ranging from 600-2,500 PE and further three plants from 4,000-7,300 PE are now in operation. This paper gives a general description of the design and the operation of these plants. The plants operate by means of simultaneous precipitation and a special intermittent operational strategy or nitrification/denitrification. Operational results from the five plants with up to 4 years of operation show average concentrations of nitrogen and phosphorus of 5-6 mg/l and 0.4-0.8 mg/l, thereby leaving a rather large margin to the general Danish standards. As regards competition, the concept has proven to be very competitive in the range of 1,000-10,000 PE.

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