Geotextile Dewatering Bags Used for Desludging Wastewater Ponds

When a wastewater treatment pond fills with sludge, the operator has two choices: build new capacity or empty the existing pond. Desludging is usually the cheaper route, but strict environmental rules on how sludge is removed and where it goes can make even a simple upgrade hard to plan.

Geotextile dewatering bags offer a low-cost way to do it. They work in a small footprint next to the pond, hold the solids that need to be removed, and release clean water, so less material has to be trucked away.

Geotextile Dewatering Bags in South African Wastewater

Wastewater treatment turns contaminated water into an effluent that is safe to return to the environment. The solids (sludge) are separated out and sent to a disposal site or to a tailings storage facility. Doing that separation cheaply is a problem for treatment and tailings facilities everywhere.

A series of wastewater treatment sites in South Africa shows how dewatering bags handle it. Several of the sites had reached full storage capacity, mainly because sludge had built up in the ponds.

Sludge Removal and Dewatering Choice

Dredging is the most common way to remove sludge, followed by mechanical removal, for example from a tailings dam. The sludge is lifted out, dried, and sent to a landfill or spread on land. Dredging is slow and expensive, though, and the equipment can tear the pond’s lining, which weakens the pond and lets water seep out.

On the South African sites, tight budgets and short windows for the upgrade work pushed the project teams to look for an alternative. Dewatering bags placed on site proved cheaper and needed less space.

Designing with Dewatering Bags

The dewatering area was first lined with a 1 mm geomembrane. The liner stopped local erosion and collected all the water draining from the bags, which was then returned to the dam or sent for further treatment.

The bags were made from UV-stabilised woven polypropylene (PP) geotextile with high-strength seams, and they captured the dredged sludge as it was pumped in. Seam strength decides whether a bag survives filling, because pumping sludge in under pressure puts the seams under the most stress. On these projects the sewing thread had a higher breaking strength than the geotextile itself.

The bags work as filters. Water passes out through the pores of the fabric and the solids stay inside. As filling goes on, a filter cake builds up on the inner face of the fabric and traps particles smaller than the fabric’s openings, while still letting water through. The fabric and the cake together act as a two-stage filter, and woven, high-strength geobags can retain more than 98% of fine-grained solids this way.

Each bag had two standard 200 mm inlets, which took pipes up to 200 mm in diameter. When a bag was full, the pipe was disconnected and the inlet tied off.

Pumping was kept below 40 m³/h. A manifold split the flow and slowed it at the inlet in case a pump ran above the design rate. The pumping rate did not change how fast water drained out of the bags, because the opening size of the geotextile controls the outflow.

No flocculants were needed: the sludge dewatered fast enough under gravity alone, which kept project costs down.

The final disposal site for the filled bags should be as close to the pond as possible to keep haulage costs low. The dried solids can be disposed of safely or, in some cases, reused, for example as fertiliser. The bags cannot be recycled, so their disposal has to be planned along with the solids.

In the selection of a disposal site, one must:

• Make sure the site is not in a sensitive area where disposal is not permitted.
• Place the sludge disposal site as far as possible from the point where the final effluent is discharged, so it cannot contaminate the effluent or the receiving water.
• Leave the widest buffer possible, more than 400 m from surface water.
• Grade the site to minimise run-off, erosion and ponding.
• Keep the site outside the 1:100-year flood line.