After a period of severe drought in the catchment area of the De Bos Dam in the Western Cape Province of South Africa, the water content of the dam dropped to below 18% of its full supply capacity by May 2012.
As the dam is the main water source for the greater Hermanus area, the Overstrand Local Municipality addressed the critical situation by developing a series of groundwater wellfields to supplement the surface water being sourced from the De Bos Dam.
The groundwater, however, is rich in soluble iron and manganese, which creates problems ranging from the staining of fabrics to the clogging of the water distribution network with gelatinous bacterial organisms that use iron and manganese in their metabolism.
The Overstrand Local Municipality appointed Zutari to provide engineering services to increase the water supply and treatment capacity of the Preekstoel water treatment works (WTW). This involved integrating the treatment of the pumped water from the various wellfields and designing a treatment process tailored to ensure reliable achievement of appropriate water quality.
A process of removing iron and manganese by biological oxidation and filtration was chosen in lieu of conventional chemical precipitation. Although biological oxidation had been demonstrated internationally, it was the first application of its kind in South Africa. An area of concern was that test samples of the raw water iron and manganese concentrations were far higher than any of the international case studies that had been reported.
“The project provided a cost-effective and innovative means to increase the water supply and treatment capacity of the Preekstoel WTW, which secured the greater Hermanus water supply requirements, as well as protected against future droughts”
Still considered to be the most promising solution, a biological plant was constructed adjacent to the existing plant at the Preekstoel WTW for treating a maximum of 10 Ml/d of borehole water. After an extensive period during which the team monitored and tuned the process, the biology had established sufficiently to deliver treated water that meets the South African National Standard for quality requirements of such a plant (SANS 241:2011).
“The project provided a cost-effective and innovative means to increase the water supply and treatment capacity of the Preekstoel WTW, which secured the greater Hermanus water supply requirements, as well as protected against future droughts,” says Geoff du Toit, Zutari Associate for Water Engineering.
In 2014, Preekstoel WTW was the winner in the category for Water and Wastewater at the Institute of Municipal Engineering of Southern Africa (IMESA) Biennial Project Excellence Awards.
The project was also the Overall Winner in the Professional Services category at the 2015 Construction World Best Projects Awards.
*The Aurecon Africa business has been officially renamed Zutari as at 21 July 2020. Zutari acquired Aurecon Middle East on 20 November 2020.