Dewatering Bags for a Municipal Water Treatment Contractor

A municipal water treatment contractor handles lagoon sludge that had to be hauled away wet, which put most of the cost into water rather than solids. They ordered dewatering bags for a trial campaign on one lagoon: fill, drain on the existing concrete pad, then lift the drained cake. The bags have shipped and are awaiting the site's drainage trial.

The contractor was hauling lagoon sludge at its full water content, so most of every tanker load was water. They wanted to drain the material on site before haulage, but had no room for a settlement tank or a filter press, and the only available area was an existing concrete pad with a surface water drain at one end that they could not discharge into.

Two risks decided the specification. First, the drained water: if the fabric released solids with the water, the drain at the pad's end would carry them off site, which is exactly what the contractor's own permit conditions forbid. Second, the drained weight: the site lifts bags with a telehandler on the loop pair, so a bag that still holds too much water cannot be lifted safely. Fine solids in the sludge also risk blinding the fabric and stopping drainage partway through the campaign.

We proposed geotextile dewatering bags sized for one lagoon cell, filled by pump through a top fill port and drained on the existing pad, with the fill port closed after each batch so rain does not re-wet the cake. The fabric opening size was selected against the fines the contractor expects in the lagoon rather than against a generic sludge figure, and the bags were laid out with the drain end of the pad bunded by the contractor so any carry-over can be seen and captured.

A settlement tank or filter press was rejected by the contractor on cost and footprint, and a lined roll-off container was rejected because it would not remove any water at all. A dewatering bag does the one job the site actually needs — removing water before haulage — and it does it on the pad they already have. The fill port was specified over an open top because rain re-wetting the cake would have undone the drainage they are paying for.

Validation so far covers the order and the dispatch, not the site: fabric specification against the agreed opening size, seam stitch counts, bag dimensions and the fill port attachment are recorded on the batch sheet, and a fabric sample was retained with the batch. A trial bag with the contractor's own sludge is the next step and will be the first real measure of drainage and retention.

Bags produced, inspected and shipped against the confirmed specification. Drainage rate, cake solids and the condition of the drained water are not yet confirmed — they can only be measured on the contractor's own material once the trial campaign starts.

Shipped, awaiting site drainage trial

The trial bag result: how long drainage takes, whether the fabric blinds, and how the drained water reads against the contractor's permit conditions. Those three answers decide the fabric for any follow-on order.

For sludge work, the fabric has to be chosen against the fines in the material, not against a generic sludge figure, and the fill port matters more than buyers expect: a bag left open in the rain gives back part of the water it drained. The pad layout also decides the project as much as the bag does, because the drained water has to go somewhere the site is allowed to send it.

The lagoon's fines content, which varies between cells and campaigns; the drained water route and the contractor's permit conditions; and the drained weight per bag, which sets the lifting arrangement. None of these carry over from one campaign to the next without checking.