The Difference Between Stormwater and Process Water: NPDES Regulations 101

Understanding Stormwater and Non-Stormwater (Process Water) Under NPDES Permits

If your facility handles any type of industrial activity, you’ve probably heard the terms “stormwater associated with industrial activity” and “process wastewater” (also called non-stormwater or process water). They sound similar, but in the world of environmental compliance, they’re not interchangeable - and getting them wrong can lead to violations and fines.

At RMA, we hear this question all the time: “How do I know if what’s leaving my site is stormwater or process water?” The simplest way to tell is by using what we call the Dry Day Test.

Here’s how it works: ask yourself - did it fall from the sky? If the water comes from rain, snow, or sleet, it’s stormwater. If you see water being used, handled, created, or discharged on a dry day, when it hasn’t rained, that’s non-stormwater (or process water).

In short: if it didn’t fall from the sky, it’s part of your process, and it’s likely regulated differently.

If you’re not sure what’s running off your site, start with the Dry Day Test. It’s simple and free, and it can help you spot potential compliance issues before they become real problems (and there’s also a good chance you should be doing dry day inspections under your NPDES permit anyway!). And of course, if you need clarity, reach out to RMA, we do this every day.

Table of Contents

• What Is Stormwater Associated with Industrial Activity?
• What Is Non-Stormwater (Process Water)?
• Why Confusion Happens with Stormwater and Non-Stormwater (and How to Avoid It)
• When the Line Gets Blurry: Stormwater Turning to Process Water
• Snowmelt: The Forgotten Stormwater Source
• Quick Comparison Table: Stormwater vs. Process Water
• Getting Help with Stormwater and Process Water
• Additional NPDES Permitting Information

What Is Stormwater Associated with Industrial Activity?

Under most state and federal NPDES (National Pollutant Discharge Elimination System) permitting programs, stormwater associated with industrial activity refers to rainwater or snowmelt runoff that comes into contact with industrial materials or operations that are capable of imparting pollutants (of any kind) to that runoff.

Does it rain or snow on your site? Of course it does. Does that precipitation touch any machinery, equipment, stockpiles, or other aspects of your operation (“contact”)? Of course it does! And, is there any chance that the resulting runoff might carry away some dirt, oil, chemicals, colors, debris – you name it. Almost guaranteed, the answer is yes!

THAT is what is regulated as “stormwater associated with industrial activity” under the NPDES discharge regulations.

This can include:

• Equipment storage or maintenance areas
• Loading and unloading zones
• Scrap and waste storage
• Outdoor processing or material handling areas

When precipitation hits these surfaces, it can pick up pollutants like oil, grease, or metals and carry them into nearby waterways. That’s why many facilities are required to maintain coverage under an NPDES Industrial Stormwater Permit, usually some type of “general permit”.

And don’t forget - snowmelt counts as stormwater too. Once that snow pile starts to melt, the runoff is regulated just like rainfall. Many facilities in northern states overlook this during the winter, only to find themselves out of compliance come spring.

This “classic” view - that stormwater means only what falls from the sky - is the standard interpretation used by the USEPA and by most states with delegated NPDES authority.

However, some states apply their own spin. For instance, several states classify runoff from aggregate stockpiles (like sand or stone) as process water, even when the runoff results entirely from rainfall. At RMA, we believe this is an incorrect interpretation of the Clean Water Act, but if that’s your state’s official position, there’s usually little choice but to comply as written.

What Is Non-Stormwater (Process Water)?

Non-stormwater, or process water, is any water that’s used, handled, or generated by your operations - not water that fell from the sky.

This includes:

• Equipment or floor wash water
• Cooling water or boiler blowdown
• Truck or tank washdowns
• Leachate, condensate, or sump discharges
• Water from air compressors or production processes

Because process water usually contains higher levels of pollutants, it cannot be discharged under a standard NPDES stormwater permit (since it’s not stormwater), unless that specific permit explicitly allows it.

Most general industrial stormwater permits cover only stormwater, nothing else. But some states issue broader NPDES general permits that allow both stormwater and certain types of process water discharges.

For example, in North Carolina, the NCG19 General Permit (which applies to concrete facilities) is more than just a stormwater permit. It allows the discharge of certain types of process water, such as wash water from mixer drums or equipment rinse water… but only under specific conditions and treatment requirements.

This is why it’s so important not only to understand the difference between stormwater and process water, but also to understand what that difference means for your particular operation and which types of NPDES permits are available in your state.

If you discharge process water under a permit that doesn’t allow it, even accidentally, you could be violating federal law.

Why Confusion Happens with Stormwater and Non-Stormwater (and How to Avoid It)

It’s easy to see why facilities get this wrong. Stormwater and process water can both move across your site, enter the same drains, and look nearly identical. The key difference is where the water came from. The source determines how it’s regulated.

Here are some of the most common problem areas:

• Washdown areas: Hosing off equipment or pavement generates process water, not stormwater.
• Leaking containment: Rainwater that mixes with oil or chemicals becomes contaminated and is no longer stormwater.
• Condensate or cooling discharges: Even if the water looks clean, it’s process water.
• Rooftop units: Drains or blowoffs from HVAC systems or compressors are typically non-stormwater discharges.

A simple rule: If water’s flowing on a sunny day, it’s not stormwater.

Documenting your dry day inspections in your SWPPP (Stormwater Pollution Prevention Plan) is an easy way to stay ahead of compliance issues - and it’s something regulators appreciate seeing during inspections (and, it may even be a requirement of your permit!).

When the Line Gets Blurry: Stormwater Turning to Process Water

Even under the standard definition, some situations aren’t straightforward. A common example is stormwater that collects in a secondary containment area.

If rain or snowmelt gets trapped inside containment and becomes contaminated with petroleum or chemicals, it’s no longer considered stormwater. It’s non-stormwater, and depending on the contaminants, it might even qualify as hazardous waste.

In those cases, the water must be tested, documented, and disposed of properly - not simply pumped out.

Snowmelt: The Forgotten Stormwater Source

Snowmelt is one of the biggest compliance blind spots we see. When snow builds up for weeks and finally melts, it can flush out a concentrated mix of oil, grease, sediment, and road salt that’s accumulated all winter.

Your stormwater program should account for seasonal changes by:

• Conducting inspections in winter
• Storing snow away from drains and materials
• Checking secondary containment before spring thaw

Remember: snowmelt is stormwater, and it can trigger your monitoring or sampling obligations just like a rain event.

Quick Comparison Table: Stormwater vs. Process Water

When in doubt, check your permit. If your discharge isn’t listed as authorized, it’s probably not allowed.

Getting Help with Stormwater and Process Water

Knowing the difference between stormwater and process water doesn’t have to be complicated. The Dry Day Test is your first line of defense - if it didn’t fall from the sky, it’s probably regulated differently.

But because every state handles this a little differently, it’s critical to understand not just what the definitions say, but how they apply to your specific operations and permits.

At RMA, we help facilities across the country navigate these gray areas - reviewing site conditions, interpreting permit language, and ensuring full compliance.

If you’re unsure what type of discharge you have or whether your permit covers it, don’t wait until an inspector shows up.
Reach out to RMA and we’ll help you make sure your water management program is accurate, compliant, and fully covered.