Performance Evaluation of Constructed Wetlands and Conventional Wastewater Treatment Systems in Selected Kenyan Tea Factories

Abstract

Environmental pollution due to discharge of untreated or poorly treated industrial waste waters has become a major source of concern with respect to the safety of our environment. An assessment of the performance of wastewater treatment systems in treating tea factory effluent was carried out on selected tea factory treatment plants which use constructed wetlands and conventional treatment systems. Selected factories were Eberege tea factory and Chinga tea factory using conventional and constructed wetlands systems respectively. A conventional treatment plant consists of chemical, biological and physical processes combined which aid in removal of contaminants while a constructed wetland is a natural biological system comprised of various methods which utilize natural biological processes for effluent treatment. In Kenya, the National Environmental Management Authority (NEMA) regulates discharge to the environment and therefore every industry is required to obtain a license for releasing their waste water in to the environment upon satisfaction of specified requirements. As per NEMA guidelines, parameters of interest in tea factory effluent include; Biological Oxygen Demand, Total Suspended Solids, pH, fecal coliform, Chemical Oxygen Demand, color, organic nitrogen, flow, copper, zinc and surfactants. These parameters were therefore analyzed in both treated and untreated wastewater from the two factories using approved US/EPA testing methods for wastewater. The results obtained from the untreated effluent in both factories had high levels of >48.6mg/L BOD, >150.3mg/L COD and >29.3mg/L TSS while the treated effluent registered a major decrease of pollutants levels with readings of <25.5mg/L BOD, <70.4mg/L COD and <30.2mg/L TSS. Color in the untreated effluent gave readings of >15 hazen units while the treated effluent recorded <12 hazen units with the conventional treatment plant giving the clearest effluent of <6 hazen units. Organic nitrogen levels in the untreated effluent were >7.8 mg/L while the treated effluent levels were <2.8 mg/L with the constructed wetland registering the lowest values. Copper and zinc levels were below the detection limit in all untreated effluent samples with an exception of one sample which recorded zinc levels of 0.543mg/L but was effectively treated since the result of the treated effluent was <0.001mg/L. Surfactants were found to be below detection level in both untreated and treated effluent samples. Fecal coliform bacteria were also absent in all samples. Both systems of treatment were effective in treating the factory effluent since the values obtained were below the permissible NEMA limits even though the degree of treatment varied with the conventional plant giving the least toxic treated effluent. Conventional wastewater treatment plants are more expensive to install and maintain compared to constructed wetlands hence not highly recommended for treating less toxic effluent.