About half of the properties in rural areas of Denmark discharge their mostly ordinary domestic sewage directly into watercourses, lakes, or the sea. This discharge of poorly treated sewage is responsible for many watercourses and lakes not presently meeting their quality objectives (Ministry of Environment and Energy, 2000). Therefore, national regulations have recently been adopted which define the permissible discharge of organic matter and nutrients from these properties (Ministry of Environment and Energy, 1997). The regulations stipulate four treatment classes that have to be met in different areas depending on the quality objective of the receiving water body. Removal of organic matter, measured as BOD5, is always required to a level of 90 or 95% removal. Where the effluent is discharged into a lake or into a watercourse which later discharges into a lake 90% removal of phosphorus is required, and where the wastewater effluent is discharged into a watercourse with a water quality objective for salmon fish 90% nitrification is obligatory. There is no general requirement for the removal of nitrogen because of the high ‘background’ levels of nitrate in the aquatic environment as a consequence of drainage water from agricultural fields.