The use of vertical flow constructed wetlands for on-site treatment of domestic wastewater: New Danish guidelines

Official guidelines for the on-site treatment of domestic sewage have recently been published by the Danish Ministry of Environment as a consequence of new treatment requirements for single houses and dwellings in rural areas. This paper summarises the guidelines for vertical constructed wetland systems (planted filter beds) that will fulfil demands of 95% removal of BOD and 90% nitrification. The system can be extended with chemical precipitation of phosphorus with aluminium polychloride in the sedimentation tank to meet requirements of 90% phosphorus removal. The necessary surface area of the filter bed is 3.2 m2/person equivalent and the effective filter depth is 1.0 m. The filter medium must be filtersand with a d10 between 0.25 and 1.2 mm, a d60 between 1 and 4 mm, and a uniformity coefficient (U = d60/d10) less than 3.5. The sewage is, after sedimentation, pulse-loaded onto the surface of the bed using pumping and a network of distribution pipes. The drainage layer in the bottom of the bed is passively aerated through vertical pipes extending into the atmosphere in order to improve oxygen transfer to the bed medium. Half of the nitrified effluent from the filter is recirculated to the first chamber of the sedimentation tank or to the pumping well in order to enhance denitrification and to stabilise the treatment performance of the system. A phosphorus removal system is installed in the sedimentation tank using a small dosing pump. The mixing of chemicals is obtained by a simple airlift pump, which also circulates water in the sedimentation tank. The vertical flow constructed wetland system is an attractive alternative to the common practice of soil infiltration and provides efficient treatment of sewage for discharge into the aquatic environment.