Due to the extension of wastewater treatment plants to nutrient removal and the development towards reuse of sludge in agriculture, new guidelines for regulating industrial discharges in Denmark were needed. The paper describes how a concept for regulating the discharge of specific organic substances and heavy metals has been developed during the past two years. The concept is based on guidelines that are made according to considerations of the environment and the treatment plant system, and that encourage the introduction of a cleaner technology and integrated preventive measures.

For most organic substances, present knowledge of fate and effects in biological treatment plants is too scarce to underpin the setting of general standards. Therefore, it has been decided to base the developed priority system on the data used in the EEC-system for classification of hazardous chemicals. This includes ready degradability, defined by the OECD-test, bio-sorption and bio-accumulation, defined by the octanol/water distribution coefficient and toxic effects on water organisms.

Several potential effects of seven heavy metals have been evaluated, and the most critical effects were found to be the quality criteria for sludge intended for use in agriculture, and the quality criteria for the aquatic environment. Proposals for general guidelines have been calculated using a simple mass balance model combined with water quality criteria and the Danish limit values for use of sludge in agriculture.