Treatment of Wastewater in the Rhizosphere of Wetland Plants – The Root-Zone Method

The present paper describes the theoretical basis of wastewater treatment in the rhizosphere of wetland plants, the so-called “root-zone method”, along with the first working experiences from eight treatment plants in Denmark. Mechanically treated wastewater is led horizontally through the rhizosphere of wetland plants. During the passage of the wastewater through the rhizosphere, the wastewater is cleaned by microbiological degradation and by physical/chemical processes. The wetland plants supply oxygen to the heterotrophic microorganisms in the rhizosphere and stabilize the hydraulic conductivity of the soil. Nitrogen is removed by denitrification and phosphorus and heavy metals are bound in the soil. The first working experiences from Denmark show, that as far as BOD is concerned root-zone treatment plants are very nearly up to conventional secondary treatment standards already from the first growing season (removal efficiency: 51-95%). For the nutrients nitrogen and phosphorus the results vary (total-N removal: 10-88%; total-P removal: 11-94%). The removal efficiencies depended mainly on the composition of the soils and the degree of surface runoff in each treatment plant. It is concluded that root-zone treatment plants seem to be a viable alternative to conventional wastewater treatment technology, especially suitable for single households and small to medium sized communities. There is, however, still very little information on the removal processes for nitrogen (denitrification), on the effect of soil type and on the required surface area to load ratio,