High efficiency screw compressors make the difference

The Aquafin water treatment station in Dendermonde (Belgium) handles the water of 72,000 IE (resident equivalents) in the basin of the 'Beneden Schelde'. The water is discharged via a polder canal and pumped into the River Scheldt one kilometre farther on. Built thirty years ago (1985), over the years the WWT plant has been extended and updated with new technologies, relying on Robuschi products in almost every stage of the treatment process. Surely the Robuschi high efficient screw compressors make the difference!

Old biological water line

In the Dendermonde water treatment plant, the waste water is pumped from the intermediate elevator gulley of the pre-sedimentation tanks to the central feed culvert of the aeration basins. The waste water is biologically cleaned by the active sedimentation process in the two aeration tanks.

Wastewater treatment plant

How the WWT Plant is running

Deep-aeration was opted for the Dendermonde treatment plant and thanks to the injector-aerator it is ensured a very fine air diffusion. Due to the depth of the basin (17 meters), a high water pressure on the bed is obtained so that the bubbles will cover a longer distance. Thus, together with the diffusion of the bubbles, this leads to optimum oxygen absorption. Practically, the plug flow is run continually, so untreated water and return sediment go through different compartments in succession.


In this way, the mixture covers the longest possible distance. The level of the mixed liquid in the aerator arrives below the rim of the overflow gullies, so that the sediment water mixture flows first upwards and then downwards through all compartments. The air feed is intermittent, so that nitrifying and denitrifying processes alternate with one another. Then the water flows to the post-sedimentation tanks.

Robox Screw Compressor at Work

Screw Compressors

The deep-aeration is supplied by two WS 85 Robuschi ROBOX SCREW compressors, with a power motor of 110 kW, a capacity of 2,080 m3/h and a delta P of up to 1.5 barg. In 2012, this new "screw technology" replaced the existing compressors, which were responsible for aerating the tanks with approximately 15 meters water level. The tanks are located fully beneath the ground level.

These screw compressors link low energy consumption to low-noise operation (thanks to their low maximum speed of 6,000 revolutions per minute) and all the advantages of compact systems. Over-sized bearings, seals and a heavy-duty shaft guarantee extreme robustness. This means that ROBOX SCREW packages are highly suitable for tough applications, have long service life and their maintenance costs remain very low.

The clever design vouches for high efficiency with which maximum capacity and peak volumetric efficiency is already achieved at low speed.

ROBOX SCREW are equipped with Premium Efficiency IE3 motors with self-tensioning Poly-V toothed belt and an electronic control panel (Sentinel Pro). The electronic control panel guarantees optimum safety and considerably facilitates maintenance; in fact, the display automatically shows the maintenance intervals for the different components. The control functions of Sentinel Pro and data communication MODBUS enable alarm signals to be sent via modem to operators' mobile phones.

Biological Water Line

Three Robuschi Blowers (type S 125/4P, 132 kW each with a capacity of 3,750 m3/h at 750 mbarg service pressure) are also located in the part upgraded in 2005 of the biological water treatment plant. All blowers are controlled by frequency control, according to the oxygen demand of the water basins.

Biogas plant

In Dendermonde, the sediment fermentation plant built very compactly on top of the aeration system not only processes the sediment from its own water treatment plant, but also the sediment from neighboring water treatment plants. The sediment is dewatered and fermented and a biogas motor cogenerates electricity. This biogas plant and a CHP plant enable the Dendermonde water treatment station to provide for its own power needs.

At this site Aquafin works with two Robuschi type KRVS 23 TM-CT liquid ring compressors with stainless steel impellers and equipped with water separator and all accessories. These compressors are a special biogas version with Atex certificate. They are replacing the previous compressors, which required much maintenance. The Robuschi compressors work contactless and are far less susceptible to contamination.

Robox Screw Compressor