DAMS & WATER STORAGE
Restoring the water balance at Setumo Dam
At Setumo Dam, the situation was so severe that the cyanobacterial build-up had formed a series of layers from the surface to the bottom of the dam
BEFORE Situated near Mahikeng, Nor th West, Setumo Dam was constructed in 1997 to meet the water security and recreational needs of the surrounding community. In recent years, with the lack of effective rainfall, the water progressively turned a shade of green as cyanobacterial blooms took hold, killing off aquatic life and making abstraction and potable water treatment increasingly challenging. By Alastair Currie 24
IMIESA January 2022
A dramatic improvement in water quality was achieved at Setumo Dam following its treatment with LakeGuard
A
s in many parts of the world, the growth of these harmful algae blooms is fed by pollution. In Setumo’s case, this was caused by poorly treated sewage over flows from two nearby wastewater treatment works discharging into the Molopo River, which feeds the dam. The situation reached a point where recreational use or abstraction for downstream potable treatment purposes became increasingly hazardous. The dam’s water quality greatly exceeded the levels deemed safe by the World Health Organization. The escalating organic loads at the water treatment plant also significantly increased the cost of chemical dosing. Additionally, there was a concern regarding the bad taste and odour of the treated water.
AFTER Measuring around 1.6 km in length, this earth-fill dam has a surface area of some 447.3 ha (4 km2), with a total design capacity of around 19 600 000 m3. Given its size and the extent of cyanobacterial bloom, many deemed it untreatable.
Countering a parasitic threat Cyanobacteria produce their own food via photosynthesis, which means they need sunlight to survive. In warm, nutrient-rich (high in phosphorus and nitrogen) environments, they can thrive – their cells doubling every 48 hours – progressively forming a scum that robs the water of essential light penetration and oxygen. At Setumo Dam, the situation was so severe that the cyanobacteria buildup had formed a series of layers from the surface to the bottom of the dam.
