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 t reatment options for domestic and industrial wastew ater streams in places where land is available. They nee d more land than conventional wastewater treatment plants but occupy less space when compared to waste stabil ization ponds. They are generally affordable in ope rational and maintenance costs while offering effective and reliable service. Constructed Wetlands are manmade wastewater treatment systems that consist of shallo w ponds and channels that have been planted with macrophytes. They rely on natural, microbial, biolo gical, physical and chemical processes to treat was tewater. They normally comprise of impervious clay liners cl ay liners and engineered structures to control the flow direction, wastewater retention times and water lev els. Research wok was conducted on a tropical const ructed wetland to establish its capability to treat wastew ater during the wet season. A comparison of its eff icacy with that of conventional wastewater treatment plants wa s made on the basis of measured water quality param eters. Temperature, pH, dissolved oxygen, and conductivity were measured in situ. Total suspended solids (TSS ), total dissolved solids (TDS), biochemical oxygen demand ( BOD 5 ), chemical oxygen demand (COD) phosphorus, ammonia, and nitrites were analyzed in the laborato ry. 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 2 4 mg/l at the effluent point, depicting a reduction of 79. 31%. Influent TDS averaged 847 mg/l, while the effl uent averaged 783mg/l. Wet season BOD 5 levels were reduced from an average of 472 mg/l at the inlet point to 24 mg/l at the outlet, depicting a reduction efficienc y of 94.9%. COD levels were reduced from a mean of 2174.2 mg/l to 71mg/l, representing a removal efficiency o f 96.7%. Phosphorus was reduced from a mean of 14 m g/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 in let point to 7.99 at the outlet of the system. Temp erature and dissolved oxygen measurements showed a diurnal vari ation. 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 detecte d in the wastewater samples. Overall, tropical cons tructed wetlands are effective in treating wastewater strea ms and they perform a lot better than the popularly used waste stabilization ponds. This paper recommends their wi despread use within the tropics as the prevalence o f warm temperatures all the year round enhances their perf ormance