Municipal wastewater treatment in horizontal and vertical flows constructed wetlands

Two large-scale subsurface constructed wetland pilot plants were designed, implemented and operated for almost three years for the treatment of a real municipal wastewater. One unit was running in a horizontal flow (HFCW) with a surface area of 654.5 m2, while the other one was a sub-tidal vertical flow (VFCW) with 457.6 m2 surface area. The two wetland units were operated at a hydraulic load of 20 m3/day for each unit and at temperature ranged from 15 to 30 °C. The organic loading rate was 2.02 kg BOD/day. The two pilot units were planted with three kinds of plants namely; Canna, Phragmites and Cyperus. Monitoring and evaluation of the performance of the two pilot units were carried out through regular physico-chemical as well as biological analysis of the inlet and outlet wastewater. In addition, nutrient uptake in the plants was also measured. The results indicated significant removals of different pollutants in both HFCW and VFCW in terms of COD, BOD and TSS. The average removal efficiencies of COD, BOD and TSS in HFCW were 91.5%, 92.8% and 92.3%, while it reached 92.9%, 93.6% and 94% in VFCW. However, VFCW proved to be more efficient than HFCW not only in COD, BOD removals but also for nitrification because of the tidal vertical flow, which allows the penetration of more oxygen, beside its small size and long detention time. The removal rate of ammonia due to nitrification reached 62.3% in VFCW, while in HFCW it reached 57.1%. In addition, the results indicated that most of the plants survived for almost 12 months and their growth depend on the type of constructed wetlands (CW) used. In conclusion VFCW proved to be more effective for wastewater treatment than HFCW due to its smaller size, high quality effluent and better nitrification.