SEWAGE TREATMENT IN CONSTRUCTED REED BEDS — DANISH EXPERIENCES

Abstract

The working experiences from twenty-five Danish constructed reed beds are summarised. The removal efficiency with respect to BOD is typically 70-90% after one growing season producing a consistent effluent concentration of less than 20 mg/1. The typical reduction of total-nitrogen and total-phosphorus is 25-50% and 20-40%, respectively. The poor performance with respect to nitrogen and phosphorus can be ascribed to (a) surface run-off (low permeability of the soil) which prevents the sewage from getting into the rhizosphere, and (b) insufficient release of oxygen from the root-system of the reeds to secure a quantitative significant nitrification. The nutrient removal is therefore dependent on the hydraulic loading rate. Only reed beds with loading rates of less than 2 cm/d show a purifying effect in respect of nitrogen and phosphorus better than 50%. The hydraulic permeability of the soil develops slowly, if ever. Even after four growing seasons overland flow predominates, typically forming a pattern covering only a part of the reed bed area. However, as most of the Danish reed beds are constructed on sites with no demand for nutrient removal, the systems function satisfactorily from the point of view of meeting the discharge standards.