Effectiveness of purification processes in removing algae from Vaal Dam water at the Rand Water Zuikerbosch treatment plant in Vereeniging

Abstract

The aim of this study was to investigate the efficacy of purification processes at the Rand Water Zuikerbosch treatment plant near Vereeniging. Raw water is abstracted via a canal and gravity pipeline from the Vaal Dam (in the upper Vaal River) and purified to ensure it meets the stringent standards set for drinkable water. The first step was to determine the ecological status of the raw water and it was done by measuring chemical, physical and biological variables as well as to identify and enumerate the different algal groups that occur in the raw water. The turbidity of the raw water was low but the phosphorous and ortho–phosphate levels were high. The Cyanophyceae (blue–green bacteria) especially Anabaena species were dominant in the raw water for the duration of the study. Potential problems such as relatively high alkalinity, chemical oxygen demand (COD) and total dissolved solids (TDS) as well as potentially hazardous chemicals such as cadmium and lead were observed in the raw water. The Zuikerbosch Water Treatment Plant (ZWTP) is a conventional water treatment plant which involves the following stages: coagulation, flocculation, sedimentation, sand filtration and chlorination. The use of pre–treatment chemicals ensures better water quality and effective removal of particles from the water. Only five of the variables (methylisoborneol (MIB); geosmin; chlorophyll–a; chlorophyll–665 and total organic carbon (TOC) were measured before filtration, after filtration and in the final water. Samples from the raw water, before and after filtration, as well as final water were collected weekly for a period of two years to measure the environmental variables as well as to do algal identification and enumeration. The purification processes at ZWTP were not able to remove MIB, geosmin, chlorophyll–a and TOC from the final water. Algal concentration was reduced but not totally removed by the purification processes. Although some variables were not totally removed by the purification processes, ZWTP produce potable water that complies with the Rand Water guidelines.