Chemical Oxygen Demand Calculator

About the Chemical Oxygen Demand Calculator

Chemical Oxygen Demand (COD) is a critical parameter in environmental and water chemistry that measures the total amount of oxygen required to chemically oxidize both organic and inorganic substances in a water sample. It's expressed in milligrams of oxygen per liter (mg O₂/L) and serves as a key indicator of water pollution and organic load.

The standard COD test uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent in a strongly acidic solution, heated under reflux conditions. The excess dichromate is then determined by back-titration with ferrous ammonium sulfate (FAS), and the amount consumed is used to calculate the COD. This calculator automates the computation from your titration data, converting between COD, BOD₅, and TOC values.

Engineers and environmental scientists use COD measurements to assess wastewater treatment efficiency, design treatment plants, monitor industrial discharge compliance, and evaluate natural water body health. This calculator helps you quickly process lab data and compare results against regulatory discharge standards from EPA, EU, and other jurisdictions.

When This Page Helps

This calculator eliminates manual COD computation errors from titration data, quickly converts between COD/BOD/TOC parameters, and provides regulatory context with discharge standards — essential for environmental labs processing multiple samples daily.

How to Use the Inputs

1. Enter the FAS titrant volume used for the blank sample (mL).
2. Enter the FAS titrant volume used for your water sample (mL).
3. Input the exact normality of the FAS solution (typically 0.025 or 0.25 N).
4. Enter the sample volume in mL (typically 2.5, 10, or 20 mL depending on expected range).
5. Use presets for common test configurations (low-range, mid-range, high-range COD).
6. Review the calculated COD, estimated BOD₅, and TOC equivalents.
7. Compare your results against the discharge standards table.

Example Calculation

Tips & Best Practices

• Always run a blank with each batch of COD tests to account for reagent oxidant demand.
• Use accurately standardized FAS — restandardize weekly or whenever preparing fresh solution.
• Chloride interference is significant above 2000 mg/L Cl⁻; add mercuric sulfate to complex chlorides.
• If the sample turns green during digestion, it may have consumed all the dichromate — dilute and repeat.
• For samples with low COD (<50 mg/L), use the low-range method with 0.025 N FAS for better precision.
• Store digested samples at room temperature; do not refrigerate before back-titration.

Understanding COD in Wastewater Treatment

Chemical Oxygen Demand is used at every stage of wastewater treatment — from characterizing raw influent to monitoring treated effluent. In the primary treatment stage, COD values help size settling tanks. In biological treatment (activated sludge, trickling filters), the BOD/COD ratio determines whether the waste is amenable to biological degradation. Tertiary treatment requirements are often triggered by COD thresholds in discharge permits.

The Dichromate Reflux Method (EPA 410.4 / ISO 6060)

The standard method involves adding a known excess of potassium dichromate to an acidified water sample, refluxing at 150°C for 2 hours, then titrating the unreacted dichromate with ferrous ammonium sulfate using ferroin indicator. The color change from blue-green to reddish-brown marks the endpoint. Closed (sealed tube) methods are now common and produce less hazardous waste than open reflux.

Regulatory Standards and Compliance

Discharge limits vary by jurisdiction: US EPA typically requires <125 mg/L COD for secondary treatment effluent, the EU Urban Wastewater Directive sets 125 mg/L (or 75% removal), and many developing countries allow higher limits. Industrial pretreatment programs may set site-specific limits. Regular COD monitoring with proper calibration ensures compliance and avoids regulatory penalties.

Frequently Asked Questions

• What is the difference between COD and BOD?

  COD measures total chemically oxidizable matter (2-3 hour test); BOD measures only biologically degradable matter (5-day test). COD is always ≥ BOD. The BOD/COD ratio indicates biodegradability.

• What is a typical BOD/COD ratio?

  For domestic wastewater: 0.3–0.8. For highly biodegradable waste: >0.5. Values <0.3 indicate poor biodegradability, suggesting chemical or advanced treatment may be needed.

• Why is the dichromate method preferred?

  Potassium dichromate oxidizes about 95-100% of organic compounds, providing a more complete and reproducible result than other oxidants like permanganate. This keeps planning practical and lowers the chance of preventable errors.

• What are typical COD values for different water types?

  Clean rivers: <20 mg/L, domestic wastewater: 250-1000 mg/L, industrial wastewater: 1000-100,000+ mg/L depending on the industry. This keeps planning practical and lowers the chance of preventable errors.

• How do I convert COD to TOC?

  The general relationship is COD/TOC ≈ 2.67 for fully oxidized organic carbon. Actual ratios vary from 2.0 to 4.0 depending on the chemical nature of the organics.

• What sample volume should I use?

  For low-range COD (<150 mg/L): 20 mL sample with 0.025 N FAS. For mid-range (100-900 mg/L): 10 mL with 0.25 N FAS. For high-range: dilute first.