Field scale evaluation of seasonal wastewater treatment efficiencies of free surface- constructed wetlands in ICRISAT, India

The disparity between volume of wastewater generated and treated has resulted in severe water pollution and eutrophication of the water bodies in most Indian cities. Constructed wetlands (CWs) present a low-cost wastewater treatment option; however, field scale studies with real life wastewater are limited. Eichhornia crassipes (water hyacinth), Typha latifolia (Typha) and Pistia stratiotes (water lettuce) grow abundantly in eutrophicated water bodies, and are known for their nutrient uptake ability. In the present study, the wastewater of a nearby urban residential colony was treated by two-field scale free water surface CWs operating under identical hydraulic loading. The first treatment cells, in each of these two CWs were vegetated with Typha. The second treatment cells were vegetated with water hyacinth (CW-1) in one of the CWs and with water lettuce (CW-2) in the other. Wastewater treatment efficiencies of these free water surface CWs were evaluated, in terms of the removal efficiencies for key parameters, viz. chemical oxygen demand (COD), ammoniacal and nitrate nitrogen, phosphate, sulphate and total suspended solids (TSS). The CW-1 showed greater seasonal variation in performance. A steady removal efficiency of 35–40% was observed for ammoniacal nitrogen in both the free water surface CWs throughout the year, though removal efficiency of nitrate nitrogen reduced significantly during the winter. Plant sample analysis showed that the N, P and K uptake capacities of water lettuce were 1.53, 1.55 and 1.34 times higher than that of water hyacinth, for identical wastewater loading. The dry weight of the harvested biomass for water lettuce, during summer months, was much higher at 5.63 g/m/d compared to 3.8 g/m/d for water hyacinth.