BMP DEEP DIVE VOL. 6

The Pollution You Can’t See: ​Why COD Matters More Than Ever

Chemical Oxygen Demand (COD) is one of the most misunderstood (and most dangerous) stormwater pollutants for industrial facilities. Unlike sediment or trash, you can’t always see COD in runoff. But when COD levels spike, dissolved oxygen in receiving waters drops, aquatic life is harmed, and regulators take notice fast.

If your site handles oils, organics, chemicals, food waste, or wash water, COD is a parameter you cannot afford to ignore.

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What is COD and why regulators care

COD measures the amount of oxygen required to chemically break down organic and inorganic material in water. The higher the COD, the greater the pollution load in your runoff.

High COD discharges can:

• Deplete oxygen in rivers and stormwater systems
• Harm fish and aquatic organisms
• Signal uncontrolled industrial pollutants
• Trigger benchmark exceedances and corrective actions
• Lead to increased inspection frequency and enforcement
  

Because COD represents a total pollution load, regulators use it as a powerful indicator of overall site cleanliness and BMP performance.

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Common industrial sources ​of COD

COD is generated at many industrial sites, often from routine operations, including:

• Oils, grease, and hydrocarbons from equipment, loading areas, and maintenance

• Food residues, sugars, and organics at food and beverage facilities

• Chemical residues and solvents from manufacturing and processing

• Washdown water from floors, trucks, and outdoor equipment

• Waste handling and recycling operations

• Material stockpiles exposed to rain
  

Even small releases spread across large paved areas can accumulate into elevated COD during storm events.‍

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Why high COD is a mid-season compliance risk

Once the rainy season is underway, COD problems tend to compound quickly. Pollutants build up on paved surfaces between storms, then wash off in concentrated pulses during rain. If BMPs aren’t actively reducing organic and chemical loading, your samples can fail even if your site “looks clean.”

Unlike some metals, COD often reflects ongoing operational practices, not just legacy contamination. That makes it a frequent trigger for Level 1 and Level 2 corrective actions under many industrial permits.

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BMPs that effectively reduce COD

COD is best controlled through a layered BMP strategy that targets both source control and treatment:

Source Control BMPs

• Strict housekeeping and spill prevention
• Covered waste and organic material storage
• Dry cleanup before washdown
• Designated wash areas routed to treatment
• Regular paved surface sweeping

Stormwater BMPs

• Media-filled drain filters for hydrocarbons and organics
• Oil absorbents and inlet protection for maintenance areas
• Catch basin inserts designed for high organic loading
• Secondary containment for chemical storage
• Covered loading and transfer zones
  

Reducing COD starts with keeping organics and chemicals off the pavement and out of runoff at the source.

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TREATMENT SYSTEMS FOR PERSISTENT COD ISSUES

When COD exceedances persist despite standard BMPs, a stormwater treatment system may be required. These systems provide active removal of oxygen-demanding pollutants before discharge.

Common treatment approaches include:

• Multi-stage media filtration systems
• Oil-water separation with polishing media
• Solid flocculant systems to bind organics
• Cartridge filtration for dissolved pollutants
• Automated pump-and-treat systems for high-flow sites
  

Treatment systems are especially effective for facilities with:

• Routine washdown operations
• Food processing residues
• High hydrocarbon loading
• Dense industrial activity over large paved areas
  

While they require upfront investment, treatment systems often deliver long-term compliance stability and reduced enforcement risk.

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Sampling & Monitoring Tips for COD

• Monitor COD trends across multiple storm events — not just one sample
• Compare COD results with visual site conditions and housekeeping logs
• Track rainfall intensity and antecedent dry days
• Document all BMP maintenance and cleanouts
• Pair COD data with oil & grease and TSS results for a full picture
  

Consistent monitoring helps identify whether COD is driven by episodic spills or ongoing site practices.

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The bottom line

COD is a powerful regulator indicator of overall stormwater pollution. You may not always see it, but regulators will. Facilities that actively control oils, organics, chemicals, and wash water through proper BMPs and treatment systems are far more likely to maintain clean samples and avoid enforcement actions.

Controlling COD isn’t just about one parameter. It’s about proving your entire stormwater program is working.