Performance evaluation of constructed wetland for tertiary treatment

Abstract

The textile industry in Sri Lanka is a leading apparel manufacturer worldwide, with a state-of-the-art dye house and a well-equipped automated laboratory with physical and washing test facilities. The dye plants inside the factory deploy high-tech methods such as exhaust dyeing (hank and high pressure) and piece dyeing (continuous dying) for polyester, nylon, and cotton fabrics. A chemical wastewater treatment plant (1,000 m3/day) has been installed to treat the factory's wastewater (process wastewater and domestic wastewater). The quality of the treated effluent meets the tolerance limits for industrial wastewater discharge into inland surface waters, reducing environmental harm. As requested by the Central Environmental Authority (CEA), a constructed wetland and fishpond have been incorporated into the existing wastewater treatment facility to improve the treated wastewater quality and ensure that no harmful chemicals remain in the treated wastewater discharged into a nearby waterway. The wetland is a sub-surface flow type where no water column is maintained, and the wetland area is around 1acre. Phragmites spp. (Phragmites karka) has been planted. The medium in which plants are grown comprises broken burnt clay disposed of from tile factories. A 50% void ratio was maintained for the easy flowing of wastewater through the wetland. The zigzag configuration is maintained throughout the wetland to avoid the short- circuiting phenomenon. The research study showed removal efficiencies of 72.3%, 51.8%, 47.2% 25%, and 73% for the contaminants COD, BOD, TSS, TKS, and TP, respectively. The temperature and pH almost remain the same with little variations. data showed faecal coliform levels were less than 2 MPN/100 ml throughout the data collection period. The study suggests that, if appropriately operated, constructed wetlands might have been effectively utilized for tertiary wastewater treatment under local circumstances. As a result, constructed wetlands can be included in the treatment process to modify existing underperforming wastewater treatment plants as well as a sustainable green concept.