Chemical Oxygen Demand

Chemical oxygen demand (COD) is a laboratory measurement that quantifies how much oxygen would be needed to chemically oxidize all the organic and inorganic oxidizable material in a water sample. It is expressed in milligrams of oxygen per litre of water (mg/L), equivalent to parts per million (ppm). A high COD indicates a high concentration of oxidizable material, which usually means significant organic contamination. In the oil and gas industry, COD is used to monitor the water quality of produced water (the water that comes up with oil and gas), assess the effectiveness of water treatment systems, and demonstrate regulatory compliance before treated water is discharged to the environment or injected into a disposal well.

Key Takeaways

The COD test uses a strong oxidizing agent (potassium dichromate in hot sulfuric acid) to chemically oxidize all oxidizable substances in the water sample. The amount of dichromate consumed is converted to an equivalent oxygen demand. The test takes a few hours and does not require living organisms.
COD differs from biochemical oxygen demand (BOD), which measures only the oxygen consumed by microbial degradation of organic matter over 5 days (BOD5). COD is always higher than BOD because it oxidizes compounds that microorganisms cannot break down (refractory organics). The COD/BOD ratio indicates how biodegradable the organic load is.
Produced water from oil and gas wells typically has COD values ranging from a few hundred mg/L in lightly contaminated water to tens of thousands of mg/L in water from heavy oil or coalbed methane operations with high dissolved organic content.
Regulatory limits for COD in produced water vary by jurisdiction. Environment and Climate Change Canada, the Alberta Energy Regulator (AER), and provincial authorities set discharge limits. In some cases, operators are not permitted to discharge at all and must reinject all produced water into approved disposal wells.
Treatment technologies that reduce COD in produced water include flotation units, biological treatment (where COD/BOD ratio is low enough for biodegradation), advanced oxidation (UV light, ozone, hydrogen peroxide), and reverse osmosis. The choice depends on the initial COD concentration, the treatment target, and the volume of water to be treated.

What Does Chemical Oxygen Demand Measure?

Imagine a pile of dry wood and a fire. The fire consumes oxygen from the air as it burns through the wood. The more wood there is, the more oxygen the fire uses up. Chemical oxygen demand is the same idea in a test tube: it measures how much oxygen would be consumed if all the oxidizable material in a water sample were burned up chemically. More oxidizable material equals more oxygen demand equals higher COD.

The test uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent because it is stronger than oxygen and oxidizes nearly everything organic as well as some inorganic reducing agents (iron(II), sulfide, some chloride). A known amount of dichromate is added to the water sample in strong sulfuric acid, and the mixture is heated for two hours. The amount of dichromate left over is measured. The difference is what the sample consumed, and that consumption is converted to an equivalent mass of oxygen.

The result tells the engineer or environmental scientist how contaminated the water is, without needing to know exactly what specific compounds are present. Whether the contamination is dissolved oil components, organic acids, glycols, or surfactants from stimulation chemicals, the COD test captures all of it in a single number.

Fast Facts

The COD test was developed in the 1940s and 1950s as a faster alternative to the 5-day BOD test for monitoring industrial wastewater. The standard method used worldwide is based on American Public Health Association (APHA) Standard Methods for the Examination of Water and Wastewater, Method 5220. The same method is referenced in Canadian environmental monitoring protocols (Environment and Climate Change Canada Method E3188), Australian National Environmental Protection Measure guidelines, and ISO 6060 for international use. Despite being over 70 years old, the dichromate COD method remains the most widely used water quality indicator in oilfield water management.

COD in Produced Water Management

Every oil and gas well produces some formation water. Shallow conventional wells may produce small volumes with relatively low contamination. Heavy oil wells in Alberta's Lloydminster area, SAGD wells producing bitumen-water mixtures, and coalbed methane (CBM) wells in the Horseshoe Canyon Formation all produce large volumes of water with varying organic content. Managing this water responsibly is both a regulatory requirement and an operational cost driver.

In Alberta, produced water is governed by the AER under the Oil and Gas Conservation Act and AER Directives 047 (Waste Reporting) and 055 (Storage Requirements for Upstream Oil and Gas Facilities). Operators who wish to discharge treated produced water to surface water bodies must demonstrate COD and other parameters below the limits in the discharge approval. Most operators in Alberta reinject all produced water into disposal wells (Class IId wells) rather than treating to discharge standards, because the volume and contamination levels typically make treatment to surface water quality standards uneconomical.

Offshore on the Norwegian Continental Shelf and in the UK North Sea, produced water discharge is tightly regulated by Miljødirektoratet (Norway) and the Environment Agency (UK). The OSPAR Recommendation 2001/1 sets discharge limits for produced water that include total oil content but not directly COD. However, COD is used as an internal monitoring parameter by operators to assess the overall organic load in their produced water treatment systems and to track treatment efficiency.

COD and Drilling Fluid Waste

COD monitoring extends beyond produced water to drilling fluid waste streams. Spent water-base mud, wellbore washouts, and surface runoff from drilling locations contain glycols, surfactants, modified starches, and biocides that contribute to COD. In Canada, the management of these waste streams is covered by AER Directive 050 (Environmental Protection for Upstream Oil and Gas Operations). Operators conducting waste characterization for disposal approval measure COD as one of several parameters to classify the waste and identify the appropriate disposal method (land treatment, injection, or incineration).

Chemical oxygen demand is universally abbreviated COD. Related terms include biochemical oxygen demand (BOD, the amount of oxygen consumed by microbial degradation of organic matter in a water sample over a specified time, typically 5 days (BOD5); always lower than COD because microorganisms cannot degrade all oxidizable material), produced water (water that comes up from the reservoir along with oil and gas; the largest volume of liquid waste produced in oil and gas operations; characterization by COD and other parameters governs treatment and disposal options), total dissolved solids (TDS, the concentration of all dissolved substances in water, typically measured in mg/L; a complementary water quality parameter to COD that captures inorganic salts as well as organic material), water treatment (the set of processes applied to produced water or waste water to reduce contaminant levels before disposal, reinjection, or discharge; COD removal is a primary treatment objective), and disposal well (a wellbore used to inject waste fluids (typically produced water) into an approved subsurface formation; preferred over surface discharge for produced water with high COD in most Canadian and US onshore jurisdictions).

How a COD Monitoring Gap Nearly Cost an Alberta Operator Its Operating License

A mid-size Alberta operator was running a produced water treatment system at a central battery handling water from 18 CBM wells in the Horseshoe Canyon play near Drumheller. The treatment train included a skim tank, dissolved air flotation (DAF) unit, and a polishing filter. The system was designed to treat the produced water to a COD of less than 150 mg/L before reinjection into an approved disposal zone.

Over a four-month period, the DAF unit's performance declined as the air diffuser plates fouled with produced solids. COD of the effluent from the DAF crept from 120 mg/L to 340 mg/L. The operator's internal monitoring protocol called for weekly COD sampling, but a lab backlog meant samples were sitting unanalyzed for two weeks before results came back. By the time the high COD was identified, 180,000 cubic metres of off-spec water had been reinjected into the disposal formation.

AER compliance staff, inspecting the facility as part of a routine audit, reviewed the lab results and found the gap. The operator received a Compliance Action notice requiring an explanation and a remediation plan. The operator cleaned the DAF diffuser plates, resumed on-spec treatment, and implemented daily COD testing with a portable field meter to catch future excursions within hours. The remediation cost approximately CAD 95,000. The alternative, a suspension of injection operations while the disposal approval was reviewed, would have shut down the CBM field's water management system and cost production from all 18 wells. Weekly COD sampling was not frequent enough for a treatment system with a known fouling vulnerability.