David Shapshak
South African scientists have developed a low-cost, environmentally friendly water- stabilisation system which could revolutionise the way water is treated.
The system’s benefits are untold in a country were water is scarce:providing safer drinking water, especially for rural areas, and potentially saving the country a fortune in maintaining the underground water pipes in urban areas. It has been successfully installed in Stellenbosch, were it is a “perfect solution,” says town engineer Eddie Delport.
The system, devised by research engineers at the Stellenbosch offices of the Council for Scientific and Industrial Research’s (CSIR) division of water, environment and forestry technology, is a simple, robust and cheap way to neutralise “aggressive and corrosive” water.
Given that some 40% of South Africa’s water is soft and acidic – which means it has a low pH and very low calcium and alkalinity – the system has great potential.
Pipe maintenance and repair due to corrosion is one of the hidden costs in water reticulation, say the developers. Soft, acidic water attacks concrete and metal, corroding and destroying underground and household pipes, geysers and water supply reservoirs. House owners have to replace pipes and geysers, it stains laundry and water goes to waste. Water supply authorities have to repair damage to the reticulation network.
In 1992 it was shown that in the Durban townships of KwaMashu, Ntazuma and Ulundi some 50% (or R12-million worth) of water entering the network was lost. In Stellenbosch in 1995 an average of 50 pipes burst each month, costing about R250 000 a month.
The water-stabilisation system is the brainchild of Grant Mackintosh, Hendrik de Villiers and Gerhard du Plessis. The system builds on the notion that naturally occurring calcium carbonate (limestone) can be used to make water less corrosive.
Conventional modern stabilisation methods are not only expensive but potentially problematic. The most commonly used system uses lime. But, says Mackintosh, “lime is produced from limestone in an energy- intensive process which also produces the greenhouse gas, carbon dioxide, as a by- product. Lime dosing is one of the most troublesome processes in water treatment, [lime] being difficult to introduce in the water stream – it is a fine, messy powder.”
Alternate conventional methods, using industrially produced sodium carbonates or sodium hydroxide, are even more expensive and can be dangerous if overdosing occurs.
“The question begged itself,” says Mackintosh, “why not use limestone? It dissolves easily and uses the natural driving force of the water to take up only as much calcium carbonate as it needs to, thereby naturally controlling the upper limit of alkali, calcium and pH.” It requires little or no operator skills and the limestone only needs to be replenished monthly or weekly, depending on the size of the stabilisation plant. This makes it ideal for rural areas, where some of the country’s most acidic water is found.
After encouraging laboratory scale studies, Mackintosh and his team investigated numerous limestone deposits to find the right kind of limestone. A suitable deposit in the Western Cape is now commercially available as water- treatment grade chemical, and sold as Aquastab pebbles. Limestone is sold at about R150 a ton, compared to R750 for the same amount of lime.
Stabilisation systems were developed in three sizes for different users: the small groundwater system, Spraystab; the town or municipal-sized Calcostab; and the largest, the Sidestream Stabilisation Process, for cities.
The Spraystab, prototype units of which treat about 2 000 litres per hour, is excellently suited for rural dwellings or small villages and is designed to stabilise the water and remove iron. The device is just 2m high and has a diameter of 60cm. And while a conventional iron-removal system would take three hours to purify water, the Spraystab takes 10 to 15 minutes.
In addition, the capital savings are enormous, says Mackintosh. It is envisaged that this system will be available off the shelf, and it is designed to be easily installable and inexpensive. Interest in Spraystab has been expressed by water boards and consulting engineers.
The Calcostab was installed initially as a pilot project in Stellenbosch. Studies showed that with a 10-megalitre-a-day (10-million litres) Calcostab unit, running costs would be about 25% of those of the lime process. “Technically it was almost a perfect solution, due to the cost-effectiveness of the plant,” says Delport. “It works perfectly. We’re quite satisfied. In the future it will be the way to go.” Stellenbosch now has three Calcostab units.
Stabilisation, says Delport, is lessening the deterioration of Stellenbosch’s pipes. The asbestos cement pipes now have a lifespan of 30 to 40 years, where previously it was 12 to 15 years.
Similarly, pilot plant estimates at Bredasdorp in 1997 showed a Calcostab unit would cost just R5 000 a year to run, compared to R90 000 for the sodium and R45 000 for the lime options over the same period. These findings resulted in the installation of a 10-megalitre-a-day unit at Bredasdorp. Twelve full-scale Calcostab units are now operational in the Western Cape.
The success of the Spraystab and Calcostab systems led to the development of a modified system for stabilisation of city-size volumes of water. The Sidestream Stabilisation Process has been patented locally and internationally.
Pilot plant findings have indicated similar advantages over the lime process, and have raised considerable interest both locally and in the United Kingdom and the United States. The CSIR has entered into a partnership with Technifin, a technical licensing company, to license the technology locally and internationally.