Sewage suction

Gravity sewerage collections systems are popular due to the large number of tall buildings in the Middle East. However when the land is flat and reclaimed, like the Palm Jumeirah, another solution is necessary. Peter Ward visits the Palm to find out more.
Sewage suction
By Peter Ward
Sun 19 Oct 2008 04:00 AM

Gravity sewerage collections systems are popular due to the large number of tall buildings in the Middle East. However when the land is flat and reclaimed, like the Palm Jumeirah, another solution is necessary. Peter Ward visits the Palm to find out more.

Gravity systems for sewerage management have been used across the world for hundreds of years. However when faced with flat terrain in a coastal area this method can be less than ideal.

In a gravity system large trenches must be dug in order for the wastewater to flow downhill and on an island such as the Palm Jumeirah it can be a huge task.

After spending two years reclaiming the land from the sea to build the Palm, it is understandable that developer Nakheel did not want to immediately dig huge trenches into it, which meant the need for another solution.

Corodex Concorde Group business development manager Mohaned Awad describes the task that was faced: "The challenge was: how do you transport wastewaters from the villas on the Palm Jumeirah to the sewage treatment plant in an ecologically sensitive area?"

The system also needed to meet sustainable goals with a similar contract for Palm Deira recently awarded, this example reveals how vacuum sewerage can fit into the Middle East. The solution was to install a vacuum system with shallow trenches and one of the world's largest vacuum stations.

The system, which currently serves 2,000 villas, will be hooked up to many more as buildings are completed on the site. Over 40km of pipeline has been installed on the Palm Jumeirah by the Corodex Concorde Group, the firm that won the contract to design, supply, install, operate and maintain the system.

Gravity's limitations

There were several reasons why a gravity system was not possible for the project as Awad explains: "Gravity sewers transport wastewater by a slope; this didn't meet Nakheel's stringent criteria [for the Palm development].

First of all the manholes are usually nine to ten metres deep - that was the first violation. Usually the manholes are very smelly because they have stagnant water in them, also these pipes are usually made of concrete which over time is prone to exfiltration and infiltration."

The vacuum station is located towards the base of the trunk in an underground utility building and serves the entire island. Differential air pressure generates an air stream that drives wastewater towards the station, it is then pumped to a wastewater treatment facility that is independent of the Corodex system.

Collection chambers are located outside the villas, with some serving a single villa and others hooked up to two.

Awad explains: "Basically all of the wastewater that is collected from the bath tub and toilets just comes down by simple slope into a sump and from there it is transported immediately into the vacuum station."There are 16 vacuum pumps in the station, each with a capacity of 1,000m3. The wastewater is pumped from a collection chamber to the station at approximately 6m/s.

On the scale of the station, Awad explains: "The vacuum station that exists on the Palm Jumeirah is the world's largest. The benchmark is the actual number of vacuum vessels. This is basically a way of transporting sewage by air - these vacuum vessels are the heart of the system."

Sewage success

Using the vacuum system brings with it a host of advantages with reduced construction time being a major one. Again due to the depth of the trenches, the time taken to install the pipework is reduced.

In using the vacuum system, deep excavation was avoided and this overcame the challenge of the rocky terrain and high ground water table presented by the area.

The maximum depth of the pipe is 1-1.5m, meaning trench-supporting equipment was unnecessary. A lack of slope on the Palm Jumeirah was another obstacle that would have hindered the use of a gravity sewerage system.

Corodex reports that the average time to set up the full system is eight months to one year, compared to three years for a comparable gravity system.

"We have the advantage of the depth of the pipes; the diameter of the pipes was small; we are using HDPE and it is cheaper than the components used in the gravity lines," explains service, operations and maintenance team leader Ibrahim Abdallah.

"It is easier for installation and faster: you save time, money and power." The firm predicts that over the long-term there can be a 30-40% cost saving when using vacuum instead of gravity systems.

The system has been in operation for around 18 months although when it was first being used it serviced far fewer villas. Eventually the vacuum system will serve all of the villas on the Palm Jumeirah, although it will not handle the sewerage from the hotels on the island.

Corodex readily admits that this sewerage solution is not compatible with buildings that are higher than five or six storeys. Despite the numbers of skyscrapers in Dubai the firm still believes that the city is an ideal location for its application.

Abdallah states: "We can't avoid using gravity [systems...because we are not using this system for the high buildings, it is not possible. It is suitable for use on properties with two, maximum three, floors or small villas with two storeys. In Dubai and the UAE they are going on to build very large areas with villas and this system is very good for this."

In terms of limitations this seems to be the biggest faced by the system, although since its deployment there have been challenges as Abdallah points out: "Sometimes we have found very strange things dropped down the toilet. One day we found a cleaning machine with a long hose and it reached our sewage pump. Sometimes people drop plastics and when some tissue or something attaches to this it will block the entrance." Pressurised piping

Chances of exfiltration and infiltration in the system were greatly reduced by the use of the high density polythene (HDPE) pipes. The pipe diameters used range from 90-250mm and the valves are manufactured from ductile iron. No stainless or metallic steel components are in contact with either ground water or wastewater.

Awad affirms: "This system is actually certified by the German wastewater board to pass through wetlands. Wetlands are ecologically very sensitive areas. Imagine passing a gravity system through that - it would be a disaster. By ensuring that there is no exfiltration of sewerage into the seawater you are actually protecting the eco-system.'

The vacuum system allows wastewater to be transported uphill, which makes it much easier for obstacles to be avoided during the installation of the pipework. Maintaining the system also becomes easier with the vacuum technique.

Theoretically there is no need for operators to have contact with the wastewater and a sensor in the collection chamber can send an SMS message to a maintenance worker should a blockage occur. Inspection pipes along the sewer lines enable manual aeration and a place for pressure gauges to be connected.

Corodex has not released the value of the work involved in the Palm Jumeirah sewerage system but Abdallah comments: "In comparison with the gravity system it is cheaper; in the short term you are saving three things: the materials that you use, the time and the manpower. Three or four pipefitters can pipe lay 500m per day, I can't imagine how many days you would need to finish this in the gravity line system."

With the majority of building projects in the Middle East getting taller, it may be surprising that a system incompatible with skyscrapers can been such a success in the region.

However with its flat terrain and high water table, the vacuum sewerage system on the Palm Jumeirah seems to be very much the right system in the right place.

Vacuum facts

• Over 2,000 villas and 23,000 people are connected to the system.

• There are 900 collection chambers along the 40km of vacuum sewer line, which is made up of high density polythene pipes (HDPE).

• Diameters of the pipes used range from 90-250mm. Division valves on the sewer lines are capable of sustaining a vacuum pressure of 80KPa.

• Three or four pipe fitters can pipe lay 500m per day.

• The valves used are made from ductile iron with internal and external epoxy coating.

• The vacuum station contains four vacuum tanks, 16 vacuum pumps, six non-clog sewage pumps and two control panels.

• Controllers only open the interface valve if there is a minimum vacuum of 22kPa available and the air admission time is adjustable by up to 15 seconds.

• The pumps have a capacity of 1,000m3 and incorporate automatic and manual duty change-over facilities.

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