Clean and inexpensive drinking water for an entire region

Phil Black - PII Editor 27/09/2022

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Slide gates optimize Belgium's largest Reverse Osmosis drinking water treatment plant

In Ostend, Belgium, Veolia Water Technologies has built an ultramodern installation for the production of drinking water. Thanks to a filtration process comprising several stages, the local water supplier now produces drinking water of excellent quality, well beyond the legal requirements. At crucial points in the process – during reverse osmosis, filtration with activated carbon and remineralization of water – slide valves from Schubert & Salzer Control Systems provide pressure and flow regulation.

The water supplier FARYS produces drinking water for the city of Ostend and its surroundings with brackish water from the Bruges-Ostend canal. In the current extension phase of the water production plant built by Veolia Water Technologies, it is possible to inject up to 1,200 cubic meters of drinking water per hour directly into the distribution network. The leading water treatment specialist's facility is Belgium's largest drinking water production plant using reverse osmosis technology. Its production rate is one of the fastest in the world. The special feature of the installation is also that it can be used very flexibly depending on the variety of qualities of the canal waters, the whole process is carried out with significantly reduced energy costs.

Frederik Debaillie, Project Manager at Veolia Water Technologies Belgium, describes the process as follows: “The canal water is treated in eight stages. During coarse, fine and micro filtration, all suspended particles, microbiological substances and pathogenic microorganisms are first removed. In the subsequent reverse osmosis, fine-pored, semi-permeable membranes filter out micropollutants down to particles as small as 0.1 nanometers, as well as minerals and salts. What remains are water molecules. The water passes through activated carbon filters and after injection of carbon dioxide, it is remineralized with limestone. Finally, it is disinfected using UV rays, then chlorinated. The result: drinking water of the highest quality, which is fed into the distribution network via buffer storage.

“The operator of the water treatment plant wanted a plant that operated economically. Energy efficiency therefore had to be maximized everywhere, including for the control valves,” explains Tristan Lejeune, International Sales Manager at Schubert & Salzer Control Systems. “At the same time, reverse osmosis and subsequent process steps are demanding applications. There are special challenges here in terms of the control accuracy and the response speed of the valves used. »

Slide valves effectively prevent damage

“For reverse osmosis, precise and rapid pressure control is very important,” underlines Mr. Debaillie, engineer at Veolia. “The highly sensitive filter layers are rolled up in tubes under pressure. Water hammer and excessive flow rates must be safely avoided. Even a smaller oscillation during regulation could damage expensive membranes. That is why in each of the 12 reverse osmosis units we use a DN125 slide valve and a DN50 slide valve from Schubert & Salzer Control Systems to regulate the pressure. » They guarantee the precise control necessary to balance the osmotic pressure of the brackish water and to keep the reverse osmosis in operation.

The specific design of the slide valves is decisive for their high precision and extremely short response times. Slide valve technology allows flow rates to be controlled in milliseconds: two slotted discs arranged perpendicular to the direction of flow are slid over each other. The pneumatic actuator only has to overcome the dynamic friction between the two discs. The force required for adjustment is thus up to 90% less than that of other types of valves. Consequently, the dimension of the actuators as well as the consumption of control air is significantly reduced. At the same time, the short operating strokes of only a few millimeters and the low moving masses of the shutter protect the actuator with the control rod and its sealing.

Material and energy efficiency supports the overall economy

“The innovative slide valve design has a doubly positive effect on weight and dimensions. On the one hand, the valves are smaller and lighter thanks to the wafer construction and more compact actuators. On the other hand, the significantly better flow characteristics due to the particularly high KVS values also allow the use of smaller nominal diameters, which makes the valves used even more compact and lightweight than the common alternative solutions”, explains Mr. Lejeune . Thus, the 45 sliding gates of the installation together weigh only 1,100 kilograms. In comparison, seat valves would have represented a weight of about 5 tons on the scale. This difference is significant and has a positive impact on the entire life cycle of the valve, from manufacturing to operation in the factory, including transport, thanks to the savings in resources and CO2 achieved. Maintenance requirements and therefore operating costs are also reduced thanks to the more compact dimensions and low weight.

“The high service life of the slide valves was also a decisive point. This stems, for example, from the fact that it neutralizes the harmful effects of cavitation,” explains Mr. Lejeune. In reciprocating poppet valves, imploding cavitation bubbles often cause costly erosive wear. “Thanks to the special design of the slide gate valves without diverting the flow, the cavitation bubbles implode one to two meters after the valve in the pipeline. This can be easily designed so that the cavitation does not have a deleterious effect. All that is needed is to straighten the pipe a short distance after the valve”, adds Berdien Uytterhaegen, responsible engineer at Schubert & Salzer Control Systems.

“Even in the event of water hammer, the control valves are not really affected,” explains Debaillie. The force of a possible water hammer in the pipe network is not transmitted to the slide valve actuator, so that it cannot be damaged by excess pressure.

Uniform workload thanks to high-precision positioners

“Before the treated water is injected into the region's distribution network, we use DN150 slide valves during activated carbon filtration and limestone and CO2 remineralisation,” adds Debaillie. Here too, the high-precision positioners from Schubert & Salzer, combined with the slide valves, guarantee extremely precise flow regulation, so that the 8 activated carbon filters and 13 remineralization tanks are used evenly. In this application, a linear flow characteristic is particularly suitable for controlling flow rates to maintain process stability.

A guaranteed safe and regional water supply

With an average production of 24,000 cubic meters per day, the installation makes an important contribution to the safe and economical supply of drinking water to the inhabitants of the Ostend region. Phases of water scarcity – such as Belgium has experienced in recent summers and which will be even more frequent due to climate change – will have to be avoided in the future. For this reason, FARYS is already planning a second similar installation in Nieuwpoort.