Dewatering: silt bags and discharge to watercourse

Almost every excavation fills with water, and getting it out dewatering is routine. What isn't optional is where that water goes. Pump silty, muddy water straight into a drain or a river and you've caused a pollution incident, with serious consequences. Silt bags are how pumped water is filtered before discharge. Here's how to dewater a site without polluting.

Why You Can't Just Pump It Away

Water pumped from an excavation is typically full of suspended silt and fine sediment. Discharged directly into a surface water drain or a watercourse, that silt clouds the water, smothers riverbeds and spawning gravels, and harms or kills fish and aquatic life a serious pollution incident that can bring enforcement and prosecution. Sediment is one of the most common construction pollutants of Irish watercourses, and dewatering is a frequent cause. So the water has to be treated before it goes anywhere sensitive.

How Silt Bags Work

A silt bag (or dewatering bag) is a large, permeable geotextile bag. You pump the silty water into it, and the fabric retains the sediment while the filtered water seeps out clean enough to discharge. It's a simple, effective way to take the sediment out of pumped water on site. The bag fills with the trapped sediment over time and is managed and disposed of appropriately. Silt bags are often combined with settlement (letting sediment drop out in a settlement area) for heavier loads.

Discharge to Watercourse and Consent

• 1
  Discharging to a watercourse or drain isn't automatically allowed it generally requires the appropriate consent or licence and must meet conditions.
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  The discharge must meet water-quality requirements clean enough not to harm the receiving water.
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  Inland Fisheries Ireland and the planning conditions set expectations for protecting watercourses and fisheries.
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  Never discharge silty water, oil-contaminated water or concrete-contaminated water to a watercourse or surface drain.

Getting It Right

Frequently Asked Questions

• Can you pump silty water from a site into a drain or river?
  No. Silty water clouds the water, smothers riverbeds and spawning gravels and harms fish discharging it directly is a pollution offence. The water must be filtered first (with a silt bag and/or settlement) so it's clean, and discharge to a watercourse or drain requires meeting water-quality conditions and any necessary consent.
• What is a silt bag for dewatering?
  A silt (dewatering) bag is a large permeable geotextile bag you pump silty site water into. The fabric retains the sediment while the filtered water seeps out clean enough to discharge. It's a simple, effective way to remove sediment from pumped water, often combined with settlement for heavier loads.
• Do I need permission to discharge water to a watercourse?
  Generally yes — discharging to a watercourse or drain isn't automatically allowed and typically requires the appropriate consent or licence and must meet water-quality conditions. Inland Fisheries Ireland and the site's planning conditions set expectations, and silty, oily or concrete-contaminated water must never be discharged.