Phosphorus Removal in Different Constructed Wetlands

To make the constructed wetlands more acceptable for sewage treatment, these systems need to guarantee high purification rates for both organic substances as well as plant nutrients (N, P). However, in most constructed wetlands, the P elimination rate remains relatively low. By comparing the purification rates for P (and P fractions in soil after fractionated extraction) among different systems, it was determined that the addition of iron filings to the filter material is more effective in ensuring a sustainable high removal capacity than the use of Ca‐rich soils. Long‐term studies on a horizontal flow wetland (HFW) show that an iron‐rich soil filter with Phragmites australis removes 97% of P from sewage even after nine years at pH values of between 4.6 and 4.9. About 60% of the P is bound to Fe (or Al) in a predominately amorphous form. In a vertical flow wetland (VFW), the P removal rate remains much lower (27%) at pH values of between 5.8 and 6.4. About 30% of the P is bound to iron, 30% to Ca and 20% as refractory P. Altogether, HFWs seem to be more effective in P elimination than VFWs because of the longer flowing distance and treatment time. Removal of plants from the soil filter leads to a significant decrease of 50% in the P elimination rate. This result indirectly shows the role of plants in microbiological P transformation processes and in the direct elimination of P by binding it to humic substances.