Concrete washout water, groundwater from excavations, wheel-wash runoff — most of it can't legally go down a drain without a consent first. Here's which route applies, and what an inspector actually checks.
Why site water isn't just water
A construction site generates several kinds of water that count as trade effluent or a controlled discharge the moment they leave site — not because they're obviously toxic, but because of what they've picked up on the way:
- Concrete washout water — from washing out mixer trucks, pumps and tools — is highly alkaline, commonly pH 11–13, and can be lethal to aquatic life in a watercourse in small quantities.
- Dewatering discharge — groundwater pumped out of excavations — can carry suspended solids, and depending on the site's history, contamination from previous land use.
- Wheel-wash and site runoff — picks up oils, fuels, cement fines and general site sediment.
None of this can legally go straight to a surface water drain, a watercourse, or the foul sewer without the right consent in place first — and which consent you need depends on where the water is actually going.
Two different consent routes
Discharge to surface water or groundwater (a watercourse, a soakaway, or the ground itself) is regulated by the Environment Agency (Natural Resources Wales or SEPA in Wales and Scotland) under the Environmental Permitting Regulations. Depending on volume and risk, this ranges from a low-risk activity that can proceed under a registered exemption, up to a full environmental permit for higher-risk or higher-volume discharges.
Discharge to the foul sewer is regulated by your local water company under trade effluent consent — required under Part IV of the Water Industry Act 1991 for any liquid waste from an industrial or construction process, which explicitly includes concrete washout and similar site water. You apply to the water company, not the Environment Agency, and the consent sets discharge limits and monitoring conditions specific to that sewer network's capacity.
| Discharge consent | Trade effluent consent | |
|---|---|---|
| Regulator | Environment Agency (NRW / SEPA) | Your local water company |
| Destination | Watercourse, soakaway, groundwater | Foul sewer |
| Legal basis | Environmental Permitting Regulations | Water Industry Act 1991, Part IV |
The test isn't "is it dirty" — it's "where is it going." The same washout water needs a different consent depending on whether it's discharged to a watercourse, soaks into the ground, or goes to the foul sewer. Get the destination right before applying; applying for the wrong consent type wastes the time you don't have once the pour schedule is set.
What a consent actually sets
Whichever route applies, expect the consent or permit to specify:
- Discharge limits — for pH, suspended solids, hydrocarbons and other determinands relevant to the site, calibrated to what the receiving watercourse or sewer network can handle.
- Volume and flow rate — how much can be discharged, and how fast, so it doesn't overwhelm the receiving system.
- Monitoring and sampling requirements — regular readings (pH is the most common on-site check for washout water) logged and available on request.
- Pre-treatment requirements — commonly a settlement tank or lagoon system to let solids settle and neutralise pH before water is released, rather than discharging directly from the washout area.
Common mistakes
- Discharging concrete washout to a surface water drain. One of the most frequent causes of pollution incidents on construction sites, and one of the easiest for the Environment Agency to trace back to source.
- Assuming small volumes don't need a consent. Volume affects which tier of permit or exemption applies — it rarely removes the requirement entirely.
- Skipping pH monitoring once a settlement system is installed. A settlement tank reduces solids but doesn't reliably neutralise pH on its own; readings still need to be taken and logged against the consent limit.
- Not knowing the site's discharge route before work starts. Establishing where surface water, groundwater and foul drainage actually go on a site is a first-week task, not something to work out after a discharge has already happened.
The consequences of getting it wrong
Discharging without the required consent, or breaching the limits of one you hold, is a criminal offence under the Environmental Permitting Regulations 2016 or the Water Industry Act 1991 — enforceable by unlimited fines in the Crown Court, and a factor regulators and clients increasingly check against a contractor's environmental track record before awarding future work.
How this connects to your wider environmental reporting
Discharge consent evidence sits in the same place as the rest of your site's water story — consumption, recycling, harvesting and now discharge, one record per site rather than a folder of pH readings nobody can find when an inspector asks. It's the same discipline behind a live Site Waste Management Plan and Environmental Management Plan: evidence kept current through the job, not reconstructed after the fact — and the same monitored data your BREEAM water credits and any water neutrality case will also draw on, all from one water tracking system.