Development of black coating on filtering media and its effect on filter performance at Kpong headworks of Ghana

Abstract

Kpong water Treatment plant is one of the two major Water treatment plants supplying the Accra/ Tema Metropolitan areas. The filtering media at Kpong water works has been observed to develop a black coating after three to four years of commissioning. This appearance results in more water than necessary being used for backwashing the filters. The source and toxicity of the black coating material as well as the possibility of it leaching into the treated water have not been determined. The main objective of this study was to characterize and investigate the development of black coating on the Kpong filtering media and its effect on the filter performance. Another focus of the study was to conduct water quality analysis of effluents from the riparian industries, and the raw water at strategic points within the study area in order to trace the source of the black coating material. The effect of the black coating on filter performance with respect to iron removal was investigated using a laboratory based batch reactor. The study showed the chemical components of the black coating of the Kpong filter media to be mainly iron, manganese, sulphates and sulphides. Other components like copper, lead, mercury occurred in trace concentrations. The blackening of the filter media at Kpong is probably due to occurrence of iron sulphates and sulphides, hydroxides and / oxides of iron and manganese ions in the raw water. The black coating has a limiting effect on the iron removal capacity of the coated (Kpong) filter media. The adsorption phenomenon on the coated media of the Volta River Authority and the fresh filter media was mainly chemisorption. Keywords : Filter media, black coating, filter performance, iron