The Efficacy of a Tropical Constructed Wetland for Treating Wastewater during the Wet Season: The Kenyan Experience

Abstract

Constructed Wetlands are among the most promising treatment options for domestic and industrial wastewater streams in places where land is available. They need more land than conventional wastewater treatment plants but occupy less space when compared to waste stabilization ponds. They are generally affordable in operational and maintenance costs while offering effective and reliable service. Constructed Wetlands are manmade wastewater treatment systems that consist of shallow ponds and channels that have been planted with macrophytes. They rely on natural, microbial, biological, physical and chemical processes to treat wastewater. They normally comprise of impervious clay liners clay liners and engineered structures to control the flow direction, wastewater retention times and water levels. Research wok was conducted on a tropical constructed wetland to establish its capability to treat wastewater during the wet season. A comparison of its efficacy with that of conventional wastewater treatment plants was made on the basis of measured water quality parameters. Temperature, pH, dissolved oxygen, and conductivity were measured in situ. Total suspended solids (TSS), total dissolved solids (TDS), biochemical oxygen demand (BOD₅), chemical oxygen demand (COD) phosphorus, ammonia, and nitrites were analyzed in the laboratory. Faecal coliforms were enumerated and Escherichia coli counts were determined. The TSS values were reduced from a mean of 116 mg/l at the influent point to 24 mg/l at the effluent point, depicting a reduction of 79.31%. Influent TDS averaged 847 mg/l, while the effluent averaged 783mg/l. Wet season BOD₅ levels were reduced from an average of 472 mg/l at the inlet point to 24 mg/l at the outlet, depicting a reduction efficiency of 94.9%. COD levels were reduced from a mean of 2174.2 mg/l to 71mg/l, representing a removal efficiency of 96.7%. Phosphorus was reduced from a mean of 14 mg/l to 11 mg/l representing a percentage removal of 21.4%. Levels of ammonia reduced from an influent mean of 61 mg/l to an effluent mean of 48 mg/l representing a percentage reduction of 21.3%. There were a 99.99% reduction for both the faecal coliforms and E.coli counts. Conductivity of wastewater increased from 1.08mS to 1.98mS, while the p H increased from 6.23 at the inlet point to 7.99 at the outlet of the system. Temperature and dissolved oxygen measurements showed a diurnal variation. The wet season wastewater heavy metal concentrations were in the following ranges: Pb (7.9-11.9ppm), Cd (1.0-3.8ppm), Cr (1.4-8.8ppm), Zn (0.1-10.4ppm), Ni (2.2-8.3ppm) with Cu not being detected in the wastewater samples. Overall, tropical constructed wetlands are effective in treating wastewater streams and they perform a lot better than the popularly used waste stabilization ponds. This paper recommends their widespread use within the tropics as the prevalence of warm temperatures all the year round enhances their performance.

Keywords: Constructed Wetlands, Microbial, Physical, Chemical, Heavy metals, Tropics