Prerotation pumping system for Abu Dhabi

The Abu Dhabi Sewerage Projects Committee chose Hidrostal's prerotation pumping system, as it offered several advantages over conventional centrifugal pumping installations.

One of the major problems in the Middle East when pumping sewage is the region's very high ambient temperature.

Consequently sewage quickly becomes septic, generating excessive amounts of hydrogen sulphide (H2S) which in turn leads to rapid corrosion, if sewage pumps are made from unprotected cast iron or nodular iron.

The odour problems resulting from this excess hydrogen sulphide make the atmosphere around pumping stations very unpleasant.

In addition, in areas near to the coast, the sewers can suffer from seawater ingress, adding still further to potential corrosion problems.

It is against this background that the Government of Abu Dhabi Sewerage Projects Committee decided to build just two pumping stations for a new housing development at Madinat Rahbah, which lies alongside the Abu Dhabi highway, near to the coast.

The Committee planned to reduce the probability of odour nuisance incidents in the housing area when it built the sewage pumping stations in the middle of densely populated areas.

Owing to the area's level terrain, the pumping stations were placed in very deep wells, at 21 and 16 metres depth respectively, to allow for sufficient gravity feed of sewage to the pumping stations.

The smaller pumping station feeds into the larger one, which in turn pumps the sewage to a treatment works, situated 15 miles away in the desert - far from any populated areas.

In order to further reduce the possibility of odour nuisance from the pumping stations, the Sewerage Projects Committee chose Hidrostal's prerotation pumping system, as it offered several advantages over conventional centrifugal pumping installations.

The system uses fixed speed immersible pumps, which operate well in both wet and dry conditions.

The pump hydraulic end and immersible motor have all wetted parts manufactured from Duplex stainless steel which ensures that the whole pump is protected against corrosion from any hydrogen sulphide gas produced by the sewage.

Consequently, although the initial cost is higher where these better quality materials are used, the maintenance costs are greatly reduced over the pump's long life.

This is a significant consideration when considering maintenance requirements for the pumps situated at such great depths.

Both the pumping stations use two prerotation cells, duty/assist, with another as a standby, and the whole prerotation system matches sewage inflow and outflow.

Prerotation works by placing the pump in a special basin set in the sump floor, which allows the sewage to flow directly to the pump at high flows, but at low flows causes the sewage to prerotate prior to entering the pump.

The prerotating flow automatically causes a screw centrifugal impeller pump to reduce its output to match inflow.

At high sewage in-flows, the floating material remains on the surface of the wastewater, but, as the flow reduces, this solid material is entrained and enters the pump, and thus cleans the sump.

This action means there is no need for maintenance engineers to clean the sump separately, which is another major benefit as there are no associated waste disposal problems.

This feature, together with Hidrostal's highly reliable and efficient pumps means the whole system costs less to operate and maintain than a conventional system, again an important point when sited in deep wells.

The stored volume of sewage is greatly reduced, as is the residence time of sewage in pumping wet wells.

As a result of these two features, the incidence of odour nuisance is further reduced.

The Hidrostal pump's impeller design provides excellent rag handling properties.

Its geometry ensures that rags and other solids easily pass through the pump maintaining an unblocked passage.

Maintenance engineers often need to remove and unblock conventional pumps, whereas Hidrostal pumps rarely block - another important factor for deep-sited wells, such as in the Madinat Rahbah site, where access at such a deep level is difficult and unpleasant.

The Hidrostal pump's flat power demand also makes it more energy efficient, and the low NPSH characteristics provide a high degree of safety too.

Hidrostal designed the system in conjunction with Parsons Engineering Science of Pasadena, California who was a consultant to the Government of Abu Dhabi.

The installation works at the site were carried out by Eagle Electro-Mechanical Limited, which is an Emirates-based contracting company working extensively in the field of sewerage and water services in the United Arab Emirates.

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