Water stabilisation through a limestone mediated sidestream process

Abstract

Approximately 40% of the surface waters of South Africa, and the surface waters of Lesotho, are soft and acidic with low calcium, alkalinity and carborrate species concentration. Distribution of such waters results in aggressive attack of cement concrete pipes and linings, and corrosive attack of metal pipes. valves, etc. within the distribution network. The impact of such attack is usually significant, and usually includes the loss of water, the need for expensive repairs and deterioration in drinking-water quality. Water conditioning to prevent aggression and/or corrosion (termed stabilization) is conventionally achieved by the addition of lime (Ca(OH)2) and carbon dioxide (C02). Although this process is well documented and understood, it has a number of drawbacks which include the use of troublesome lime, high chemical operating costs, and the need for high quality white lime which is currently in short supply in South Africa. An alternative stabilization process is stabilisation via contact with limestone (solid calcium carbonate), which has been shown to have many advantages over Iime-mediated stabilization. However, the use of limestone mediated stabilization has hitherto been limited to smaller water treatment works as a result of the large contact tanks required. Use of limestone for larger water treatment works was made viable by the development by CSIR of the limestone mediated Sidestream Stabilisation Process (SSP). The SSP consists of taking a sidestream of approximately 2 - 5% of the unstabilised water, dosing high levels of C02, and then contacting the COr acidified stream with limestone. The acidified sidestream takes up considerable amounts of calcium carbonate (CaC03), increasing the alkalinity and the calcium concentration.