Your Crew Is Dumping Degreaser Down the OWS

This pattern repeats at facilities across the country. A manufacturer calls because their oil/water separator is not working right. Water is not draining. There is a sheen on the stormwater discharge. The publicly owned treatment works (POTW) or the state environmental agency has sent a notice of pretreatment violation. The maintenance crew has been pouring degreaser into the floor drains for months without anyone realizing the consequences.

The crew was trying to help. The shop floor gets greasy, drains get slow, and someone reasoned that degreaser would cut through the buildup. That reasoning is correct in a kitchen sink. It is exactly wrong in an oil/water separator. The same chemistry that makes degreaser work on a counter or floor makes it actively defeat the function of every gravity separator in the system downstream.

How an Oil/Water Separator Actually Works

An oil/water separator is a piece of environmental compliance equipment, not a drain. Almost all separators operate on Stokes' Law, the physical principle that immiscible oil droplets in water rise at a rate determined by their diameter and the density difference between the two fluids. The two most common designs at industrial facilities are:

• American Petroleum Institute (API) separators: Large gravity separators sized to API Publication 421 specifications. Common at refineries, terminals, and large industrial sites. They rely on long residence times and low flow velocities to allow oil droplets to rise to the surface.
• Coalescing plate separators (CPS or PPI/CPI units): Compact separators with parallel inclined plates that shorten the rise distance for oil droplets, effectively concentrating Stokes' Law into a smaller footprint. Common at vehicle maintenance shops, parking structures, and equipment yards.

Both designs depend on the same fundamental requirement: oil droplets must be large enough to rise and aggregate at the surface where they can be skimmed or pumped off. Modern coalescing plate units are typically designed to remove free oil droplets of 60 microns or larger; high-performance units can target 20 microns. Below those thresholds, the design assumptions no longer hold.

Crucially, no gravity separator removes emulsified oil. Emulsified droplets are too small and too stable to rise. They flow through the unit and out the effluent line in the same form they entered, regardless of residence time, plate area, or unit size.

Why Degreaser Destroys Your OWS

Industrial degreasers are formulated to emulsify oil. That is the active mechanism, not a side effect. The surfactants in degreaser reduce the surface tension at the oil-water interface, allowing the oil to break into microscopic droplets dispersed throughout the water column. Once that emulsion forms, it is stable for hours to days depending on the surfactant chemistry.

Pouring even moderate volumes of degreaser into a drain that feeds an OWS produces three immediate consequences:

• Existing accumulated oil in the separator gets re-suspended. Months or years of trapped oil that should be skimmed and disposed of as waste oil instead emulsifies and is discharged from the effluent line. The OWS becomes a source of contamination rather than a treatment unit.
• Coalescing plates lose effectiveness. Surfactants coat the plates and reduce the contact angle that supports droplet aggregation. Even after the surfactant has been flushed through, plate performance may be permanently degraded depending on the chemistry.
• The downstream discharge contains emulsified petroleum. The effluent shows elevated total oil and grease, total petroleum hydrocarbons, and chemical oxygen demand. If the discharge enters the sanitary sewer, the POTW sees a sudden loading event that may trigger pretreatment violation notices. If the discharge enters stormwater, an NPDES violation is occurring.

The Regulatory Framework

Two distinct regulatory frameworks apply depending on where the OWS effluent goes.

Discharges to a POTW (Sanitary Sewer)

Industrial users that discharge to a POTW are subject to the federal pretreatment regulations at 40 CFR Part 403. Local POTWs administer the program under EPA-approved pretreatment programs and issue industrial user permits with site-specific limits. Common pretreatment limits applicable to OWS effluent include:

• Total oil and grease (typically 100 mg/L, often lower)
• pH range (typically 5.0 to 11.0)
• Total petroleum hydrocarbons
• Flammability (no discharges with closed-cup flash point below 60°C per 40 CFR 403.5(b)(1))
• Local limits for specific metals, COD, BOD, and total suspended solids

Exceeding a pretreatment limit triggers enforcement under the local industrial user permit. Most pretreatment ordinances start at $1,000 to $5,000 per violation per day, with significant escalation for repeated violations or significant noncompliance (SNC) status, as defined at 40 CFR 403.8(f)(2)(viii).

Discharges to Stormwater or Surface Water

An OWS that discharges to a stormwater system, a separate sewer, or a surface water is regulated under the Clean Water Act NPDES program (33 U.S.C. § 1342, 40 CFR Part 122). The discharge must be authorized by an individual NPDES permit, the Multi-Sector General Permit (MSGP), or a state equivalent. Unauthorized discharges, or discharges that exceed permit limits, are violations of CWA Section 301 (33 U.S.C. § 1311).

Civil penalties for CWA violations reach $64,618 per day per violation for violations occurring after November 2, 2015 and assessed on or after January 8, 2025, per 40 CFR 19.4 Table 1. Criminal penalties under CWA Section 309(c) apply to knowing violations and false reporting.

What a Violation Actually Looks Like

POTW pretreatment violations typically progress through a documented enforcement sequence:

1. Notice of Violation (NOV): Issued by the POTW following an exceedance detected during routine industrial user monitoring. May require a written response with corrective action plan within 30 days.
2. Compliance Schedule: A formal agreement to achieve compliance by a fixed date, with milestone deadlines. Failure to meet milestones is a separate violation.
3. Significant Noncompliance (SNC) designation: Triggered by chronic violations, technical review criteria exceedances, or violations of the compliance schedule. Required to be published in a local newspaper at least annually per 40 CFR 403.8(f)(2)(viii).
4. Civil action: POTWs can pursue civil penalties under their enabling ordinance. Federal CWA penalties may be pursued by EPA or the state agency for serious or sustained violations.

Stormwater violations escalate differently. State NPDES authorities typically issue administrative orders or compliance schedules first, but a documented unauthorized discharge can move directly to formal enforcement, particularly if it reaches a waters-of-the-United-States location or impairs aquatic life.

What to Do Right Now

The five immediate steps when a degreaser-down-the-drain pattern is discovered:

1. Stop the source. Walk every drain on the property and confirm what each one connects to. Stop any practice of pouring solvents, degreasers, or cleaning chemicals into any drain that feeds the OWS, the sanitary sewer, or stormwater. Floor mop water containing degreaser must be collected and managed as waste, not poured down a drain.

2. Get the OWS pumped and inspected. Schedule a service to pump out the separator, clean the coalescing plates, and inspect the unit. The pumped contents may require characterization as hazardous waste depending on what is in the emulsion. Manifest the waste under 40 CFR Part 262 if it tests hazardous.

3. Sample the effluent. Before resuming discharge to the sewer or stormwater, collect grab samples and analyze for oil and grease, pH, and any parameters listed in the industrial user permit or NPDES permit. The result establishes a baseline and helps demonstrate that corrective action was effective.

4. Notify the POTW or NPDES authority proactively. Self-reporting before the agency discovers the violation typically results in significantly lower penalties under EPA's audit policy (60 FR 66706) and equivalent state policies. The penalty mitigation for prompt self-disclosure can exceed 75 percent of the gravity-based component.

5. Update the SWPPP and pretreatment program. Document the incident, the corrective actions, and the prevention measures going forward. The SWPPP under MSGP Part 5 must reflect actual site practices, not an idealized version.

Drain Labeling and Crew Training

Two recurring sources of unauthorized discharge are unlabeled drains and uninformed crews. Both are fixable.

Label every floor drain. Each drain should have a permanent, weatherproof sign or stencil indicating its destination: "OIL/WATER SEPARATOR. No chemicals, solvents, paints, or degreasers" or "STORMWATER ONLY. No discharges other than uncontaminated rainwater" or "SANITARY SEWER ONLY. No industrial wastewater". Painted color-coding around drains in some facilities adds visual reinforcement.

Train every employee whose job touches drains. Maintenance, sanitation, contractors, equipment operators, and shift supervisors. Training should include:

• What the OWS does and how it works (5 minutes with a cross-section diagram)
• What is allowed in each drain category (clear, written list)
• What to do with mop water, parts-washer fluids, paint waste, and floor scrubber recovery
• What to do when an accidental discharge happens (immediate notification of supervisor, do not flush or dilute)
• Where the spill kits are and how to use them

Document the training with sign-in sheets, agendas, and content materials. SPCC plans and stormwater permits both require documented annual training, and the documentation is what protects a facility during an enforcement investigation. Annual refresher training is the minimum; new-employee training plus topical refreshers after any incident is better practice.

Approved Alternatives for Floor Cleaning

The reason crews reach for degreaser is real. Floors do get greasy. Several approaches handle the floor-cleaning problem without compromising the OWS:

• Auto-scrubbers with recovery tanks: Walk-behind or ride-on scrubbers apply detergent solution, scrub the floor, and recover the dirty water into an on-board tank for proper disposal as wastewater. No drain involvement. Capital cost $3,000 to $20,000 depending on size.
• Dry absorbent for spot cleanup: Granular absorbent (clay, calcium silicate, cellulose-based) applied to oil drips, swept up, and managed as solid waste. For larger spills, absorbent pads, socks, and pillows handle the same job.
• Steam cleaning with vacuum recovery: Mobile steam cleaners with integral wet-vac recovery for parts and equipment cleaning that would otherwise go through the parts washer.
• Closed-loop parts washers: Replace open-tray parts washers with closed-loop bioremediation units or aqueous-based washers that recirculate cleaning solution rather than disposing of it. The cleaning solution is replaced periodically as a managed waste stream.
• OWS-compatible biological cleaners: Some bioremediation products break down oil without emulsifying it, allowing use in environments where the cleaner will eventually reach an OWS. Verify product compatibility in writing from the manufacturer before purchase.

The Larger Lesson

The crew-dumping-degreaser pattern is not a labor problem. It is an information problem. Facilities that have not communicated how the OWS works, what each drain connects to, and what alternatives are available end up with crews making reasonable-sounding decisions that produce enforcement actions. The infrastructure to prevent this is inexpensive (labels, an annual training session, an auto-scrubber). The infrastructure to recover from it (legal fees, lab analysis, OWS replacement, civil penalties) routinely runs 20 to 50 times the prevention cost.

Need your OWS cleaned or inspected? Find qualified service providers in our provider directory, or learn about OWS maintenance services. For related compliance topics, see our industrial wastewater permits guide and stormwater sampling compliance guide.