Tarbes is a French town with about 42,000 inhabitants and the administrative centre of the Hautes-Pyrénées department in the Midi-Pyrénées region of southwestern France. To the south of Tarbes is the border with Spain. The Pyrenees mountains, lying along the border between France and Spain, can be seen from the town. The northeast and northernmost region belongs to the mid-range mountains of the Massif Central whereas the central and northwestern part of the region is plain. The region is characterized by the river Garonne and its tributaries which feed into the Atlantic Ocean.

The expansion of the sewage treatment plant from 46,700 PE to 55,700 PE could be finished in summer 2014. A maximum wastewater flow of 855 m³/h can now be treated with the Zenon membranes. Three screens of the type HUBER ROTAMAT® Star liquid, size 1400 with 1.5 mm perforated plate, were installed and put into operation already in September 2014. They are installed downstream of the grit trap and protect the Zenon membranes. As each of the HUBER screens is designed for a flow of 470 m³/h it is sufficient to operate two screens at a time to handle the wastewater volumes on site. The third screen is used as standby redundancy unit. 

A 6 mm step screen is installed upstream of the grit trap to protect the HUBER fine screen against potentially occurring coarse material.

When looking for the best yet most cost-effective technical system for the protection of the membrane plant the main contractor of the project, Degrémont, selected a solution with HUBER ROTAMAT® Star liquid screens for several reasons.

High throughput capacity and high mechanical stability even with small perforation sizes

"Star" type screens have a star-shaped screen basket which is made of a specially folded perforated plate. This special design significantly increases the mechanical stability of the screen basket so that higher pressure losses are permissible in practical operation with the result that higher throughputs can be achieved. The "Star" design is a must especially with 1 mm perforation as the necessary rigidity of the screening elements with their small 1 mm wall thickness can only be achieved with the zig-zag folded perforated plate.

Another advantage of the "star-shaped" screen basket is the resulting 30% increase in surface area which allowed for economical layout and dimensioning of the screens in this project. Conventional screens are far from being able to effect the guaranteed hydraulic performance with the same perforation size and the same screen drum diameter.

An additional increase in throughput capacity has been achieved through the reduced inclination angle of the screens. We selected an inclination angle of 25° which provides additional screening surface compared to drum systems with discharge screw and the typical inclination angle of 35°. The increase in screening surface means even more increase in throughput capacity.

Screenings removal by pump or gravity for efficient screenings transport over long distances

Another advantage of the HUBER screens is that the separated screenings can be removed in liquid form as a water-solids mixture either by pump or by gravity where suitable height conditions are present.

In the project Tarbes Ouest each of the three screens is equipped with a pump which delivers the solids-water mixture into a collection line. The mixture flows by gravity over a long distance of 40 m to a downstream HUBER Wash Press WAP liquid where the water-solids mixture is dewatered to a DR higher than 30%.

Innovative treatment of screenings from fine screens

The HUBER Wash Press WAP liquid is the youngest member of the HUBER Wash Press family and has been developed especially for the treatment of screenings from fine screening systems.

The requirements the technical design of a wash press has to meet depend directly on the type of screening system that is used upstream of the wash press. The finer the screen separates, the more important it is to ensure that no solids flow back into the wastewater stream along with the press liquor from the wash press.

For manufacturing and process-technical reasons wash presses are usually designed with 3-5 mm apertures in the trough area. The use of such a wash press would therefore be not at all suitable for this project with a 1.5 mm perforated fine screen as also bigger solid particles would be passed back to the wastewater flow along with the press liquor and end up in the membrane plant. This would take the purpose of using a fine screen ad absurdum. It is neither a solution to return the press liquor to upstream of the fine screens as this would lead to an increasing concentration of solids and as a consequence impair the hydraulic efficiency of the screens.

We developed the HUBER Wash Press WAP liquid to have a best possible technical solution at hand for this purpose. A great advantage the HUBER Wash Press WAP liquid provides is that its aperture size in the trough area is adjusted to suit the upstream fine screen. The trough of the HUBER Wash Press WAP liquid for the Tarbes Ouest project is equipped with a perforated plate of also 1.5 mm to ensure that the high separation degree the fine screens achieve is not ruined by the downstream screenings treatment system.

Summary

At Tarbes Ouest HUBER's newest solids/liquid separation solutions for the protection of membrane systems have operated to the customer's full satisfaction for a year now.

Due to its unique design the HUBER ROTAMAT® Star liquid screen offers both high separation efficiency and high hydraulic throughputs.

As the screenings can be removed as "liquid" it is possible to efficiently transport them even if the distances between the screen and screenings treatment plant are very long.

The especially developed HUBER Wash Press WAP liquid allows for a screenings treatment that can be adjusted to optimally suit the fine screen system.

Innovation and quality win through

Due to the convincing benefits the new HUBER ROTAMAT® Star screen provides 50 machines of this type have already been sold. Many of them are already in operation, some have proven well in practice for as long as two years.

We are currently in the middle of the preparations for the manufacture of 12 HUBER ROTAMAT® Star screens with 2 m drum diameter and 1 mm perforated plate each. These screens will protect Europe's presently biggest membrane plant on STP Achères in France where the customer also relies on the high quality of HUBER machines!