Ministry of Environment and Energy, Denmark

6.4. Municipal sewage treatment plants

There are approx. 1,500 municipal sewage treatment plants in Denmark, varying in size from 1 million PE (the Lynetten plant in Copenhagen) to very small village plants of 30 PE. More than 90% of households discharge their sewage to these plants. As a result, the municipal sewage treatment plants receive sewage equivalent of almost 5 million PE from Danish households. In addition they receive a further 4 million PE from industry.

The EC Directive concerning urban waste water treatment, 91/271, requires Member States to ensure:

that agglomerations of more than 15,000 PE are provided with collecting systems for urban waste water by 31 December 2000 at the latest, and those of between 2,000 and 15,000 PE by 31 December 2005 at the latest;

that urban waste water entering collecting systems shall before discharge be subject to secondary treatment (biological treatment) by 31 December 2000 for agglomerations of more than 15,000 PE, by 31 December 2005 for agglomerations of between 10,000 and 15,000 PE, and by 31 December 2005 for discharges to freshwater and estuaries from agglomerations of between 2,000 and 10,000 PE;

that sewage treatment plants function efficiently measured in terms of either degree of treatment (70%-90%) or by the concentration of BOD5, COD, suspended matter, nitrogen and phosphorus.

As far as concerns Denmark, the EC sewerage requirements have long been met.

Danish sewage treatment requirements as stipulated in the 1987 Action Plan on the Aquatic Environment are in concert with those of the EC Directive. After a few minor adjustments to the Directive the requirements to sewage treatment plants are as follows:

Large treatment plants (over 15,000 PE) have to comply with fixed requirements with respect to COD, BOD5, nitrogen and phosphorus;

Medium-sized treatment plants (5,000-15,000 PE) have to comply with fixed requirements with respect to COD, BOD5 and phosphorus;

Small treatment plants (2,000-5,000 PE) that discharge into watercourses and lakes have to comply with fixed requirements with respect to COD and BOD5.

The fixed requirements are under 75 mg COD/l, under 15 mg BOD5/l, under 8 mg N/l and under 1.5 mg P/l, measured as annual means. Denmark has defined all its aquatic areas as sensitive to eutrophication in the sense of the Directive, and Danish treatment requirements are slightly more stringent than those of the Directive.

In addition to the general requirements as to COD, BOD5, N and P, the Counties stipulate individual requirements to municipal sewage treatment plants that depend on their size and the contents of the discharge.

Treatment leads to the annual production of approx. 1,000,000 tonnes sludge (dry weight 160,000 tonnes). In Denmark it is considered the best solution that this sludge is exploited as agricultural fertilizer. At present 55% of sludge is used in this way, 25% is incinerated, 15% is deposited and 5% is used in some other way12. The percentage that is incinerated remains constant. Since 1987, however, there has been a shift from deposition to utilization as agricultural fertilizer; in 1987, 30% was deposited. The trend has been influenced by the fact that a levy (DKK 195 per tonne) is imposed on deposited sludge. Consideration has been given to whether or not the sludge that is spread on agricultural land should be sterilized (heat treatment), but this has not been found necessary.

The heavy metal content of sewage sludge used for agricultural purposes is governed by Directive 86/278, which stipulates the following limit values per kg dry weight:

Cadmium	20-40 mg/kg
Copper	1,000-1,750 mg/kg
Lead	750-1,200 mg/kg
Mercury	16-25 mg/kg
Nickel	300-400 mg/kg
Zinc	2,500-4,000 mg/kg

Danish limit values for sewage sludge spread on agricultural land are in line with the Directive in the case of copper and zinc, but considerably more stringent in the case of the other metals. For example, the limit value as of 1 July 1995 is 0.8 mg/kg for cadmium and 120 mg/kg for lead.

Lead emissions from traffic,average lead content of petrol and percentage of unleaded petrol

Progress with respect to the treatment of urban sewage as a result of the major investments made in recent years has been rather satisfactory; pollution corresponding to 9 million PE enters the treatment plants, but 93% of the BOD5 content is removed by treatment such that pollution from the discharges only corresponds to 650,000 PE14. Treatment also removes 64% of the nitrogen and 76% of the phosphorus. When all the sewage treatment plants have been upgraded in accordance with the Action Plan on the Aquatic Environment by around 1995, the figures will be even lower.

	that agglomerations of more than 15,000 PE are provided with collecting systems for urban waste water by 31 December 2000 at the latest, and those of between 2,000 and 15,000 PE by 31 December 2005 at the latest;
	that urban waste water entering collecting systems shall before discharge be subject to secondary treatment (biological treatment) by 31 December 2000 for agglomerations of more than 15,000 PE, by 31 December 2005 for agglomerations of between 10,000 and 15,000 PE, and by 31 December 2005 for discharges to freshwater and estuaries from agglomerations of between 2,000 and 10,000 PE;
	that sewage treatment plants function efficiently measured in terms of either degree of treatment (70%-90%) or by the concentration of BOD5, COD, suspended matter, nitrogen and phosphorus.

	Large treatment plants (over 15,000 PE) have to comply with fixed requirements with respect to COD, BOD5, nitrogen and phosphorus;
	Medium-sized treatment plants (5,000-15,000 PE) have to comply with fixed requirements with respect to COD, BOD5 and phosphorus;
	Small treatment plants (2,000-5,000 PE) that discharge into watercourses and lakes have to comply with fixed requirements with respect to COD and BOD5.