Pilot scale study: use of subsurface flow constructed wetlands for domestic wastewater treatment in Sri Lanka

Abstract

There is no any environmentally acceptable standard method practiced to treat domestic grey water in Sri Lanka. Many methods of wastewater treatment have been researched and employed by responsible nations around the globe. An alternative approach in addressing this crisis is the adoption of low cost sanitation facilities, such as an engineered reed bed treatment system. A reed bed typically consists of a substrate confined within an impermeable containment supporting macrophytes such as reeds or rushes. Treatment occurs within the bed as a result of a number of interrelated physical, chemical and biological processes that occur during the water’s passage through the bed. This treatment technology is already being applying in Europe, Australia and North America where temperature greatly fluctuates during summer and winter seasons. But for being a tropical country with relatively high ambient temperature throughout the year, it is not economical to directly apply the designs developed for those countries as they require relatively high retention time. Major objectives of this research were to find out the optimum retention time needed for the sub-surface horizontal flow constructed wetlands, removal efficiencies of major pollutants present in the domestic grey water and comparison of three types of commonly available reed varieties (Common reed, narrow leaf cattail and bulrush) found in Sri Lanka for the removal efficiencies and their tolerance/ propagation in the artificial wetland conditions. Out of three reed varieties, highest propagation was observed in Common Reed while lowest was observed in Bulrush. It was observed that 2.6 days retention time is sufficient to treat the wastewater up to the limits prescribed by the National Standards for discharging effluent into inland surface waters. The treated effluent was clear, colourless and odourless and 93-97% COD, 87-90% BOD, 99% TSS reductions were observed in the tested beds while in Coliforms and E.coli analyses, reductions were within 83-97% and 91-99% respectively.