Where pressure washing wash water can go in Wisconsin
Where can a Wisconsin pressure washing crew let its wash water go?
A storm sewer is not a path to a treatment plant
A storm sewer carries water untreated to a lake, river or wetland. Wash water sent down one is a discharge to a water of the state, not a way of getting rid of it.
Wisconsin's low-impact discharge permit defines a surface water discharge as any confined conveyance, and it names the storm sewer in that list. The permit treats the inlet at the edge of the lot as the start of the discharge.
EPA makes the same point for municipalities. Its outreach sheet on washing tells residents that storm drains lead directly to local waterways, not to treatment facilities. A commercial crew is held to that fact more strictly than a homeowner is.
The Wisconsin low-impact discharge permit counts a storm sewer as a conveyance that carries wastewater to surface water. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
EPA's vehicle washing fact sheet says storm drains lead directly to local waterways, not to treatment facilities. — US EPA, Stormwater Best Management Practice fact sheet EPA-832-F-21-032F, retrieved 2026-09-29
The sanitary sewer route and who has to say yes
Sending wash water to a sanitary sewer moves it to a treatment plant, and Wisconsin's permit does not require coverage for that route. The utility still gets a say.
The permit lists two routes as outside its reach. One is a holding tank that is pumped and hauled to a public treatment works. The other is a sanitary sewer that conveys the water to a treatment works.
Local rules sit on top. Madison tells anyone whose wastewater contains chemicals to ask the Madison Metropolitan Sewerage District first, and to get a city sanitary discharge permit if it agrees. A crew that uses degreaser or a cleaning agent should call before it plumbs anything to a drain.
The permit excludes discharges to a holding tank hauled to a public treatment works, and to a sanitary sewer that conveys wastewater to a treatment works. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
Madison says wastewater containing chemicals needs a request to the sewerage district first, and a city sanitary discharge permit if approved. — City of Madison Engineering, retrieved 2026-09-29
Grass, soil and gravel as the first choice
The permit's washing rules start with the ground. Wash on grass, soil or gravel where you can, and keep as much water as possible soaking in.
Its groundwater rules say the same about the discharge point. The water should reach grass, soil, gravel or a seepage area to the extent possible, with infiltration maximized.
One route looks like soil but is not. Wash water sent to a septic tank and drain field is listed as not applicable to this permit, so a home septic system is not a place to put a crew's water.
Washing activities shall occur on grass, soil or gravel areas to the extent possible so infiltration of washwater is maximized. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
Settling solids or blocking the inlet on paved jobs
Most commercial jobs sit on pavement. On impervious surfaces the permit asks for one of three steps before you start washing.
You can send the water to a settling basin, tank or similar device. You can block, barricade or plug channels such as storm sewers and let solids settle. Or you can direct the water to grass, soil or gravel where it can soak in.
All three aim at solids first. Dirt, algae and mulch are what a paved lot gives up, and they are what settles out. The permit walk-through covers when coverage applies at all.
On impervious surfaces the permit requires settling the washwater, blocking channeled flow such as storm sewers, or directing it to grass, soil or gravel. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
Match the destination to the job before you load the truck
Pick the destination when you write the quote, not when the water starts to run. That decides whether you need a tank, a berm, a hose to a lawn or a call to the sewer utility.
Ask three questions on the walk-through. Where is the nearest inlet? Is there grass or gravel downhill? Will chemicals go into the water? The answers point to a route in the table below.
Then read the scenario about a lot behind a strip mall and use the job-site checklist to record what you chose.
| Destination | How the permit treats it | What to confirm first |
|---|---|---|
| Storm sewer | A surface water discharge | Coverage letter, or block the inlet and settle solids |
| Sanitary sewer | Outside the permit's coverage | Utility approval, especially with chemicals |
| Hauled tank | Outside the permit's coverage | A treatment works that accepts the load |
| Grass or gravel | Groundwater discharge, infiltration maximized | No spill, no septic system, no oil |
A DNR coverage letter is required before discharging to waters of the state under the low-impact general permit. — Wisconsin DNR, Low-Impact Discharge general permit WI-0066575-01-0, retrieved 2026-09-29
Questions
Is it legal to let wash water run into a street gutter?
Not by default. A street gutter usually feeds a storm sewer, and the DNR permit treats a storm sewer as a conveyance to surface water. Whether a given job needs permit coverage depends on the water, so ask the DNR reviewer for your region.
Can wash water go down a customer's sink or floor drain?
That is a sanitary sewer route, and the DNR permit does not require coverage for it. The local sewer utility can still refuse or set conditions, so ask before you plumb a hose to any indoor drain.
Does a septic system count as soil infiltration?
No. The permit lists washwater sent to a subsurface soil absorption system such as a septic tank and drain field as not applicable to it. Keep crew wash water away from private septic systems.