Where the wash water goes in Boise

Soap is the line.

Boise's storm drains do not go to a treatment plant. Whatever enters one arrives in surface water more or less as it left the driveway, which is why the city writes rules about what is allowed to go in. The city's stormwater management and discharge control ordinance is what sits behind the guidance below.

The city's own Resource Guide on Non-Stormwater Disposal Best Management Practices states it in one line: wash water with soap of any kind is not allowed into storm drains. Direct small amounts soapy wash water to landscaped areas for infiltration or collect and dispose into the sanitary sewer.

Now the other half, which is the part almost nobody knows. The same document's list of authorized non-stormwater discharges includes routine external building wash down which does not use detergents. Street and pavement wash waters are on the list too, where no detergents are used and no spills or leaks of toxic or hazardous materials have occurred.

So the plain water rinse off the side of a house is something Boise has already decided is fine. Add detergent and the same job becomes what the city defines as an illicit discharge: any discharge to the MS4 that is not entirely composed of stormwater, except the authorized non-stormwater discharges. MS4 is the storm drain system.

What the city asks a crew to actually do

Sweep the paved area before wet cleaning and put the debris in trash or landscaping. Keep the water volume down, and small discharges can be directed onto adjacent landscaped areas. Nothing entering a storm drain should show a visible sheen.

Hot water does not buy an exemption. High pressure hot water cleaning, including steam cleaning, is subject to all the same discharge control requirements.

Paint changes the job completely

Washing loose paint off a building is a different category in the city's guide, and the rule tightens hard. No wash water or debris to be left in the street and no discharge to storm drains. The guide points at dry cleaning methods instead, and notes that if paint contains lead, assistance is available from the EPA Lead Program.

That matters on older Boise housing stock more than anywhere else. Where old paint may contain lead, it is a lead question before it is a cleaning question, and a pressure washer is an efficient way to turn a stable surface into dust in a yard where kids play. The city's guide points at the EPA Lead Program for that case.