Removal of organic matter from domestic wastewater in a tropical subsurface horizontal flow constructed wetlands

Pilot scale studies were conducted to determine the performance of a subsurface horizontal flow constructed wetland in the tropics in the period April to July 2003. . The wetland located at Jomo-Kenyatta University of Agriculture and Technology (JKUAT) sewage treatment works consisted of four cells set up in parallel, each of 22.5m2. Three of the cells had gravel and in two of the cells with gravel the tropical macrophyte Cyperus Papyrus was introduced. The wetland received a continuous feed of settled sewage from a primary facultative pond at the treatment works. Performance of the wetland in terms of removal of organic matter was evaluated. This was done under different operating conditions of hydraulic loading rates. Average removal rates of up to 50 % for organic matter namely Biochemical oxygen demand (BOD5) and Chemical oxygen demand (COD) were obtained with an areal BOD5 removal rate constant of 0.0485 m/d. The background or residual BOD5 determined for the vegetated wetland cells was 11mg/L. The loading rate deduced from the study for BOD5 was 373 Kg/Ha.d. A retention time of 2 days was determined to be optimum for BOD5 removal. Keywords: Alternative wastewater treatment technology, Constructed wetlands, Subsurface horizontal flow constructed wetland (SSF), Tropical conditions, Macrophyte, Residual BOD5