The short answer: washing on a lot is legal. Discharging the resulting water into a storm drain without a permit is not, and that is a distinction most people only discover after a notice of violation.
A storm drain is not a sewer. Sanitary sewer goes to a treatment plant; storm drainage goes essentially untreated to the nearest bayou, creek or bay. So anything entering a storm inlet is entering surface water directly.
Why commercial wash water counts
Under the Clean Water Act, discharging a pollutant to waters of the United States from a point source without a permit is prohibited. Commercial vehicle wash water is a pollutant in that sense — it carries detergent, oil, grease, heavy metals from brake dust and sediment. A parking lot draining to a storm inlet is a point source.
Residential car washing on grass is generally treated differently from commercial fleet washing on impervious pavement, which is the source of most of the confusion. Volume, contaminant load and the drainage path are what change the answer.
Who actually holds the liability
The discharge occurs at your property, from your drainage, under your site's stormwater authorisation. The vendor leaves. The site remains identifiable and enforcement follows the site.
Consequences range from a notice of violation and a corrective action requirement through to administrative penalties — and for sites where environmental performance is contractually audited, which is most of the Houston petrochemical corridor, the loss of a contract.
How to do it compliantly
- Seal every storm inlet in the work area before any water moves, using drain seals and booms
- Recover the wash water rather than letting it run off — a vacuum system rather than a squeegee
- Transfer it into a holding tank and remove it from the property for appropriate disposal
- Or wash on a purpose-built pad with an interceptor connected to sanitary sewer under the right authorisation
- Keep a record of how the water was handled, because that record is what answers the question later
This is not legal advice
This describes the regulatory framework and the process built around it. Your site may carry specific permit conditions, and your environmental manager should read them against whatever process a vendor proposes. Primary sources for every claim here are cited on the compliance page.