THE new state-of-the-art desalination plant in Sedgefield has only been operating for a month, but already Knysna’s municipal officials are sold on the idea that the purification of sea water offers a long-term and environmentally friendly solution to the Garden Route’s ongoing drought problems.

The “reverse osmosis” purifying process used at the plant to desalinate sea water is not a new concept, but a number of the technological innovations employed in the Sedgefield operation are considered groundbreaking.

Foremost is the new plant’s ability to harness and re-use the pressure energy produced in the reverse osmosis process, a bit like a turbocharger recycles energy in a car. This results in a saving of a up to 40% of the electrical energy required to run the plant.

As a result, the cost of producing quality potable water has been cut to about R3 per kilolitre – only slightly more than the R2 to R3 per kilolitre cost to harvest and purify water using traditional methods.

Also, desalination will become even cheaper in the future as new energy technology is developed.

Reverse osmosis works by forcing sea water through many layers of a specialised membrane under high pressure, leaving the salt concentrate on one side of the membrane and desalinated water on the other.

Concerns about the environmental impact of the Sedgefield plant have been largely allayed since the production of potable water started on December 23.

The ratio of waste water to purified water produced at the Sedgefield plant is roughly 55:45, which means that the waste or “brine” returned to the ocean has slightly less than double the salt content of normal sea water.

Testing of the area around the discharge wells buried on the pristine Myoli Beach at Sedgefield started a month before the plant was put into operation to obtain a comparative assessment of the environmental impact, and so far the results had been “highly satisfactory”, said Melissa Mackay, of Cape Environmental Assessment Practitioners.

“We have not yet completed the testing cycle, but so far the environmental impact has been minimal and extremely localised. In fact, no increased salinity has been detected beyond 10m of the discharge wells, which indicates that the higher salt levels in the discharge are dispersing rapidly.”

The impact of the desalination plant on marine plant and animal life was being monitored, and so far no changes had been observed, Mackay said.

The initial environmental assessment is expected to be completed at the end of the month.

There is also zero aesthetic impact on the area of Myloi Beach from where the salt water is being abstracted and the brine discharged via two intake wells and five discharge wells, all buried at least 1,5m below the beach surface, along with all the piping leading to and from the wells.

“When we started with the construction of the plant a number of residents expressed concern that the infrastructure was going to be an eyesore and would ruin the beach. Now that the plant is completed and running, it is impossible to tell that water is being taken and brine discharged in the area, even if you walk up and down the beach,” Knysna Municipality project management head Rodney Nay said.

The system of burying the intake wells in the beach has also eliminated the need for biocides to rid the water of bacteria because the sand acts as a natural filter.

Another innovation at Sedgefield has been the ability to place each of the two reverse osmosis modules inside 12m sound-proofed containers, cutting the noise emissions down to negligible levels.

Reverse osmosis can be used to remove any impurities from water, not just salt. Thus it can also be used to produce potable water by recycling effluent waste.

The Sedgefield plant is the biggest of its kind in South Africa, producing up to 1,5 megalitres a day. But Knysna had already sealed the deal on another, bigger plant to recycle the town’s effluent waste to produce two megalitres of potable water a day, according to infrastructure and development committee chairman Andrew Finn.

“This is the future of water management for towns. Installation of this plant has significantly reduced the risk of Sedgefield running out of water, which happened a year ago when we had to truck water in from George,” he said.

The new plant, at the Knysna sewerage works is due to start operating around the end of March.

The Sedgefield plant cost about R10-million, plus a further R6-million for the 2,5km pipeline.

“This is a fraction of what it would cost to build a new dam,” Mackay said.

Only one employee is required to run both reverse osmosis modules at the Sedgefield plant, and he mainly monitors the pumps to make sure they are running smoothly.

The plant is also monitored remotely from Cape Town by the manufacturers, who can shut it down if problems occur.

Mossel Bay, George and Bitou are all at various stages of acquiring reverse osmosis plants.