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Case Study - Rippon Lea
De-Watering of Sludge, Melbourne, Australia

Client: National Trust of Australia (Victoria)
Green Waste Environmental Engineering, Melbourne, Australia
Geotextile Supplier
: Permathene Ltd
Date: December 2001

Remediation of a lake in the Rippon Lea Estate in Victoria is a part of National Trust of Australia (NTA) Site Remediation Action Plan. The lake contained approximately 150 m of sludge and the size of the lake is 2-3 acres x 1.2 m deep. The long-term goals of the action plan are to identify contaminated sites, investigate and, if necessary, remediate them within a 40 year period. Several hundred of these sites involve contaminated sediments. Without remedial action the sediment and sludge would have caused problems for many decades. Several investigations and studies were carried out to determine how and under what limitation clean up could be performed. An alternative for the remediation was selected that included vacuum dredging using the high strength geotextile tubes. Dredged material was to be de-watered and disposed of in a landfill.

For this treatment process a work area was set up off site in a neighbouring car park. A pontoon with a pumping unit was designed, constructed and installed in the lake. The pumping system was commissioned and pumps were altered to ensure the most appropriate flow rate of sludge and water mixture was achieved. A flexible pipeline was installed from the
pump to the temporary de-watering area and to the mobile wastewater treatment plant. The sludge was pumped from the lake to the de-watering system; excess water taken from the de-watering system was pumped through the water treatment plant and returned to the lake as treated water.

The de-watering system consisted of two geotextile tubes; 20 m long and 1.4 m wide each fabricated from Syntex 4x4 high strength woven geotextile. This product has sufficient tensile strength to withstand the stresses. The fabric opening size may seem large when compared to the grain size of the dredged material, and might lead to the question of how efficient retention of solids might be. The answer partly lies in the fact that a filter cake forms on the inside of the fabric shell, thus creating the equivalent of a two-stage filter. Filtration efficiencies above 98% are not uncommon for fine grained dredge materials filtered through Syntex 4x4 high strength geotextile.

The dredged material was pumped into the tubes using trash pump through a 150 mm discharge line. The water percolates out through the fabric, leaving a dense sludge and sediment mixture in the tube. The tubes were pumped until full, reaching heights of 1.0 m 1.2 m and widths of 0.8 1.0 m. The 20 m long each tube contained nearly 20 m of dry sludge material. Prior to filling, the de-watering area was lined with a nonwoven geotextile to prevent local erosion, which occurs as water is released from the tube.

De-watering and consolidation in the geotextile tube reduced the volume of the dredged material by a factor of 7 to 8 within 2-4 weeks of filling the tube. Thus, 450 m of material was initially dredged and approximately 56 m to 64 m placed at the final disposal site. The dredged material was highly cohesive and has a high organic content. This stage of de-sludging was carried out in tight and confined conditions and it is expected that greater productivity can be achieved in the larger section of the lake. The ideal process rate for the treatment is between 3 to 5 m per hour.

In conclusion, Syntex tubes provided a cost effective solution to a very difficult dredging project. The tubes de-watered the material at a greatly accelerated rate when compared to open air retention, and eliminated safety issues inherent with disposal pits. The desludging provided a significant increase in the storage volume of the lake to allow for reticulation of the Rippon Lea Estate gardens. This new system provided significant savings and only two tubes were needed for the four-month operation. Although dredge filled geotextile tube technology has been used for many years, recent high profile projects have brought attention to the industry. The technology and the industry are still young, but newer and better protocols are being realised on a daily basis. The future certainly looks bright for dredge filled geotextile tubes.
dewatering tubes