Phosphorus removal in different constructed wetlands

To make the constructed wetlands more acceptable for sewage treatment, these systems need to guarantee hi-fi 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.