Comparative evaluation of the macrophytes in the constructed wetlands for the treatment of combined wastewater (greywater and septic tank effluent) in a sub-tropical region

Abstract

The role of macrophytes in constructed wetlands (CW) has been studied extensively in the last three decades. The remediating effect of macrophytes in CWs depends on whether the species is suitable for the given climatic and operational conditions and on the morphological traits of the species. This study experimentally investigated and compared the contribution of three macrophytes, namely Phragmites karka, Iris kashmiriana, and Sagittaria latifolia in removing pollutants from combined wastewater (greywater and septic effluent) using horizontal sub-surface flow type constructed wetlands (HSSF-CW) under sub-tropical conditions. The study monitored the standard wastewater parameters (COD, Nitrogen, Phosphorus, and Solids) in the inlet and outlet of the CW microcosms, the fate of nutrients through the planted microcosms, and the growth rate of the selected species. Phragmites karka showed better removal efficiencies than the other two species, with organic loading removal rates of 10 ± 1.3 gm−2d−1, nitrogen removal rates of 2.6 ± 0.4 gm−2d−1, phosphorus removal rates of 1.3 ± 0.2 gm−2d−1, and solids removal rates of 3.6 ± 1.2 gm−2d−1. Mass balance showed that the nutrient uptake by species Sagittaria latifolia (TP – 9.3 g and TN – 48.2 g) was the most, followed by Phragmites karka (TP – 8.6 g and TN – 47.9 g) and Iris kashmiriana (TP – 4.4 g and TN – 29.1 g). Biomass analysis revealed that Phragmites karka had the maximum biomass growth rate with an increment of 1383 gm−2: 524 gm−2 in the above-ground: below ground dry biomass weight, while as Sagittaria latifolia and Iris kashmiriana showed an increase of 1299 gm−2: 866 gm−2 and 663 gm−2: 1312 gm−2, respectively. The macrophytes with well-developed root systems (Phragmites karka) had a better pollutant remediating effect. The nutrient uptake by macrophytes depended on the total dry biomass increase and the nutrient content of the biomass. The CW beds of the three macrophytes need nutrient pre- or post-treatment to comply with the irrigation reuse and conventional sewage treatment plant (STP) discharge regulations in India.