An innovative approach to dealing with elevated iron concentrations at the Knysna Hospital

In response to the drought which started in 2017, the Western Cape Government set about securing water supplies to key facilities across the province, including the Knysna Hospital. Drilling and testing of two boreholes at the facility indicated it to be viable to establish a groundwater supply of 66 KL/d from the underlying Table Mountain Group Aquifer. Iron concentrations were low and the initial water chemistry analyses pointed to concentrations below the SANS 241 aesthetic limit. However, further to the implementation and operationalization of the groundwater supply schemes, significantly elevated iron concentrations of up to 6 mg/L were observed. This contributed to the difficulty in getting the Knysna Hospital’s alternative water supply operational. Best practice requires that as little oxygen as possible gets introduced into the groundwater system; and this can be achieved by pumping the borehole continuously at the lowest rate possible. It is not always possible to do this under operational conditions when the water demand varies. To counter the iron problem in the potable water and to prevent or retard an increase in the iron concentration in the abstracted groundwater, iron treatment was added to the treatment train and a dual pumping regime was adopted. Using the variable speed drives that had been installed with the pumps, two pumping rates were adopted – with the rate controlled by the level in the treated water storage tank. When the tank level is low, the borehole is pumped at a rate of 0.9 L/s. However, when the level fills to 70%, the pumping rate is reduced to 0.35 L/s and continues pumping even if the tank is full. The modified system was brought into operation in August 2019 and has continued to meet the water demand of the hospital.