Until now, the removal of unwanted salts from groundwater using reverse osmosis incurred a water loss of approximately twenty percent. By regularly reversing the direction of flow, however, this loss can be reduced to seven percent. This saves not only water, but also energy and chemicals. Research into another method of reducing water loss is ongoing.
“Reverse osmosis is a highly suitable means of desalinating our raw water,” says process technologist and membrane expert Bas Reitman. “This technique consists of us pumping water at a pressure of seven to eight bar through membranes mounted in tubes that retain over 98 percent of dissolved salts. This produces clean permeate – water from which the salts have been removed – and a concentrate flow of brine. During the process, the concentration of salts on the feed side increases to such an extent that they begin to precipitate on the membrane surface. This causes such a decline in permeability that it is no longer efficient to continue with the production of permeate. Although we can briefly prevent precipitation of salts by adding a kind of liquid Calgon to the feed water, we do eventually have to dispose of the membrane concentrate.”
“We began to consider the issue of minimising water loss, as some twenty per cent of the feed water is lost in discharging the concentrate”
Minimising water loss
“Some twenty per cent of the feed water is lost in discharging the concentrate,” process technologist Nico Wolthek explains. “Not only is this a waste of the water itself, but also the energy required to extract and pressurise it, and the chemicals we use to treat the feed water. We therefore began to consider the issue of minimising water loss. This has led to the launch of two research projects. The first – which we have since completed – investigated whether we might reduce water loss by improving the entire membrane filtration process. The second research project is examining the possibility of our reducing water loss by post-treating the concentrate stream.”
Nico continues: “During the first project, we investigated the possibilities of feed flow reversal. The principle of this technique is that you reverse the direction of flow of the feed water shortly before signs of salt scaling appear on the surface of the membrane. This scaling always starts in the end section of the membrane tubes (see ROTEC diagram). The salt concentration is highest there, as it is furthest away from the point where the feed water enters. Reversing the direction of flow and directing the feed water through the pipe where the concentrate flow originally exited causes the salt concentration in this section of the pipe to decline once more.”
“We built a pilot set-up next to the existing membrane system at our Het Engelse Werk production site, which is fed with the same raw water. Trials carried out with this system show that we can reduce the water loss to seven per cent. This is a highly favourable result. We are nevertheless also investigating another means of reducing water loss. After all, we would have to install many additional valves and complex control technology in our existing systems in order to apply feed flow reversal. That would call for not only substantial investment, but also increased maintenance.”
“The advantage of this technique is that one can continue to use the existing systems, which implies the investment costs are much lower”
Concentrating concentrate flow
Bas: “In the second project, we are examining the possibilities of further concentrating the concentrate flow. In particular, we are considering a technique called closed circuit reverse osmosis. It involves the additional extraction of water from the concentrate flow, while regularly expelling the thick concentrate, as it were. We hope to complete this study in the course of 2022.”
Nico adds: “The advantage of this technique is that one can continue to use the existing systems, and need only install a small additional unit for post-treatment of the concentrate flow – which has a relatively small volume. The investment costs are therefore considerably lower than those that would be incurred in rendering the membrane systems suitable for feed flow reversal.”
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