Effectiveness of vegetated constructed wetland in the reducation of BOD₅ in pre-treated wastewater

Abstract

This paper describes the performance of a Horizontal Sub-Surface Flow Constructed Wetland (HSSFCW) as a post-treatment unit to reduce BOD5 from Upflow Anaerobic Sludge Blanket (UASB) pre-treated domestic wastewater in a pilot treatment system at the University College of Lands and Architectural Studies (UCLAS), Dar es Salaam. Two HSSFCW units (A and B) were designed and operated at a hydraulic loading rate of approximately 0.1m3/m2/day and a Hydraulic Retention Time (HRT) of approximately two days. Unit A was used as control with no macrophytes while the second (unit B) was planted with Phragmites mauritianus. The control wetland unit exhibited a lower BOD5 removal (66.9±2.25%) than the vegetated unit (72.3±1.75%). Similarly, the BOD5 loading removal rate in unit A was observed to be 91.42 kg/ha/day, while in unit B it was 98.9 kg/ha/day. Better performance of the planted wetland unit can be attributed to the metabolic activities of the micro-organisms and oxygen transfer, which might have been enhanced by the roots of the wetland plants. Also, the root zone increased the surface area thereby increasing the contact between micro-organisms and the pollutants to be destroyed. The removal of BOD5 in the unvegetated CW unit A suggests that in CW systems, gravitational settling of solids is also a predominant removal mechanism of BOD5.

Journal of Building and Land Development Vol. 13 (2) 2006: pp. 74-83