applications thickener tanks

Rod Drayton – Aquatan Lining Systems & Alan Parrock – ARQ Consulting Engineers

The East Rand Water Care Company (ERWAT) recently completed the upgrading of its 45Ml per day Hartebeestfontein wastewater care works near Tembisa and Kempton Park.

Due to the dolomitic subsurface conditions and the possibility of sinkhole formation, the design of various structures required the extensive use of geo-membrane products as underlays to prevent any possible seepage through the structures into the underlying strata.

Balancing Tank
A 1 500m Hyperliner geo-membrane was installed under the entire area of the balancing tank, which was then overlayed with a 200mm thick reinforced concrete slab. This membrane was also installed over the 45 sloping wall areas and under the wall/floor support beam.

Hyperliner, an Ethyl Vinyl Acetate material, was chosen due to its excellent multi-axial deformation characteristics. This allows it to be easily installed into difficult three-dimensional corners.

The construction of this component required close co-operation between Aquatan Lining Systems and the Grinaker/LTA concrete teams to ensure that the membrane was not damaged or ruptured during the difficult concreting operations.

Application Floating Tanks Balancing Tank

Close liaison was also kept with the designers, Messrs ARQ Consulting Engineers, to ensure the watertight sealing of the membrane to the penetrating inlet and outlet structures. These structures had to be modelled due to their intricate nature. Construction traffic was carefully controlled to eliminate any possible damage.

On completion of this tank, thorough leakage-detection testing was carried out with complete success of the system.

Aeration Tanks
A similar underliner was laid under all of the Reactor tanks. These tanks were different in that a matrix of ground beams were first cast and the Hyperliner membrane was laid in panels between the beams. In order to ensure a watertight seal to these beams, a Hyperliner Rearguard water bar was cast into the top edges of the ground beams and the Hyperliner panels were then welded to the cast in Waterbar.

Similar stringent quality control supervision was applied with the same successful results as per the balancing tank above.

AAR4 Tank
The most challenging structure was the lining of the AAR4 tank. This tank was badly eroded and cracked. With the possibility of further movement, ARQ decided to line the entire surface area with a 1500m HDPE Lining. This material was chosen due to the following restrictions:

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The aggressive nature of the contained treated effluent

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The need for ultra-violet protected material where the liner was exposed above the top surface level.

Due to the relative stiffness of the HDPE, it was difficult to install, especially to ensure that the liner conformed to the conical and circular profiles of the tanks.

Application Floating Tanks Aeration Tanks
Application Floating Tanks AAR4 Tank 250

click to view larger image

Each side of the tank had 12 rectangular columns protruding from the sloping walls to support an overhead pipe and access bridge. In order to accommodate the lining system it was decided to modify their rectangular shape into circular stub columns approximately 1.0m high. Special circular pre-formed liner boots were fabricated and sealed to the columns with cascade clamps. A cuspated sheet (Hidrain) was installed over the floor areas which ensured that any leakage could flow unhindered to an under-drainage pipe. The liner also had to be designed and fixed to the concrete at contain positions in order to prevent any movement of the membrane due to hydraulic forces.

When the tanks were tested, much to the dismay of the project team, substantial leakage occurred. Much effort was made to establish the source of the leakage:

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Divers were engaged, who meticulously inspected all areas. This method did not produce any definitive results and minor possible leakage was identified and later repaired.

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The re-installation and re-sealing of all the underwater steel make-offs. The tank included a central division wall to which the liner was sealed below top water level.

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All intricate corner lining areas were removed and reinstalled.

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Approximately 80% of the boot make-offs to the stub columns were redone.

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All pipe flange make-offs were inspected and re-installed if considered at all susceptible.

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Extensive electrical spark testing of all geomembrane joints.

The tanks were then retested with similar results! A lining contractors nightmare!

The tanks were then progressively emptied and carefully monitored. It was established that there was a possibility that the Cascade Clamp make-off to the stub columns could be the source of the problem. After extensive discussions with the clamp suppliers, it was decided to eliminate the ‘step’ between the 2mm HDPE liner and the upper concrete surface by placing a 2mm thick gasketting bandage directly above the top edge of the liner. The clamp was then reinstalled with the new “bandage” ensuring that the clamps were not distorted by the liner “step”.

Hey, Presto! No leaks (and thankfully no more additional costs!)

Aquatan Lining Systems would sincerely like to thank the Project Team of ARQ Consulting Engineers and Grinaker/LTA for their assistance and co-operation and the Client, ERWAT for their patience and understanding during the lengthy investigation period. This was certainly a case of, if at first you don’t succeed, try, try and try again!

2009 END

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 Date of entry: February 2004 | Latest Upload: 06 December, 2013