Calculate COD (Chemical Oxygen Demand) in mg/L from dichromate titration data using the standard EPA 410.4 formula. Get estimated BOD5, COD:BOD ratio, and automatic wastewater quality classification — from clean river water to industrial effluent.
✓ Verified: EPA Method 410.4 (COD closed reflux) & Standard Methods 5220 — April 2026
Titration: from lab data | Direct: enter COD to classify
FAS used for blank digestionEnter valid blank volume (mL).
FAS used for sample digestionEnter valid sample titrant volume.
Enter your measured COD for classification and BOD estimationEnter a valid COD value (mg/L).
Enter if known to get COD:BOD ratio; leave blank to estimate
Chemical Oxygen Demand
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⚠️ Disclaimer: This calculator is for educational and estimation purposes. Regulatory compliance requires certified laboratory analysis per EPA Method 410.4 or Standard Methods 5220. Do not use calculator results for discharge permit reporting, legal proceedings, or environmental compliance documentation.
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Sources & Methodology
🛡️COD formula and classification thresholds verified against EPA Method 410.4, APHA Standard Methods 5220, and EU Urban Wastewater Treatment Directive limits.
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EPA Method 410.4 — Chemical Oxygen Demand (Colorimetric)
US EPA standard method for COD determination using dichromate oxidation. epa.gov
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Standard Methods for the Examination of Water and Wastewater, 23rd Ed.
APHA/AWWA/WEF Standard Methods 5220 defines COD test procedures, QA/QC requirements, and result reporting. American Public Health Association.
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EU Urban Wastewater Treatment Directive (91/271/EEC)
Sets COD effluent limits: 125 mg/L for treated wastewater from agglomerations >2,000 PE. ec.europa.eu
EPA 410.4 Titration Formula:
COD (mg/L) = [(A − B) × M × 8000] / V × Dilution Factor
A = blank titrant (mL) | B = sample titrant (mL) | M = FAS molarity | V = sample volume (mL)
8000 = equivalent weight of O₂ in mg/meq (8 g/meq × 1000 mg/g)
BOD₅ estimate = COD / 2.1 (typical domestic sewage ratio; industrial may differ)
Understanding COD: Water Quality Measurement Guide
Chemical Oxygen Demand (COD) is one of the most important water quality parameters in environmental engineering and wastewater treatment. It quantifies the total amount of oxygen required to chemically oxidize all organic and inorganic matter in a water sample using a strong oxidant (potassium dichromate). A higher COD indicates greater organic pollution and greater potential harm to receiving water bodies.
COD is measured in milligrams of oxygen per liter (mg O₂/L) and is used across industries including municipal wastewater treatment, industrial effluent monitoring, food and beverage processing, pharmaceutical manufacturing, and environmental compliance. Unlike BOD (Biochemical Oxygen Demand), which requires a 5-day incubation, COD results are available within 2-3 hours, making it the preferred routine parameter for process control.
COD Water Quality Classification
COD Range (mg/L)
Water Quality
Typical Source
Treatment Needed
<20
Clean / Pristine
Natural rivers, groundwater
None
20 – 50
Good
Surface water, treated effluent
Minimal polishing
50 – 125
Fair
Treated wastewater (meets EU limit)
Secondary treatment
125 – 400
Poor
Domestic sewage, light industry
Biological treatment
400 – 2,000
Very Poor
Strong industrial wastewater
Advanced biological + chemical
>2,000
Severely Polluted
Food processing, chemical plants
Pre-treatment + biological + physical
COD vs BOD: Key Differences
The relationship between COD and BOD reveals important information about wastewater treatability. COD measures all oxidizable matter — both biodegradable and non-biodegradable organic compounds, plus inorganic compounds like sulfites and ferrous iron. BOD measures only the oxygen consumed by microorganisms to biodegrade organic matter over 5 days at 20°C.
The COD:BOD ratio is a key diagnostic tool. A low ratio (below 2) indicates highly biodegradable wastewater well-suited for biological treatment. A high ratio (above 3) signals significant refractory (non-biodegradable) organic content that will not respond to conventional biological treatment, requiring chemical oxidation, adsorption, or membrane processes.
💡 COD:BOD Ratio Guide:
<1.5: Excellent biodegradability — biological treatment highly effective
1.5–2.5: Good biodegradability — standard activated sludge or MBBR
2.5–3.5: Moderate — may need longer HRT or co-digestion
>3.5: Poor biodegradability — investigate refractory compounds; consider advanced oxidation (AOP), ozonation, or activated carbon
How the EPA 410.4 Dichromate Test Works
In the standard closed reflux method, a measured water sample is placed in a sealed digestion tube with excess potassium dichromate (K₂Cr₂O₇) and concentrated sulfuric acid. The tube is heated at 150°C for 2 hours. Organic matter reduces the dichromate; the remaining (unreacted) dichromate is then measured by back-titration with ferrous ammonium sulfate (FAS). The more dichromate consumed, the higher the COD.
COD Loading and Treatment Plant Design
For wastewater treatment plant design, the daily COD load (kg/day) is calculated as: COD Load = Flow rate (m³/day) × COD (mg/L) / 1,000,000. This determines the oxygen transfer rate needed in aeration tanks, the sludge production rate, and the reactor volume required for a target treatment efficiency. For typical domestic wastewater (COD ~500 mg/L, flow ~0.3 m³/person/day), each 1,000 people generates about 150 kg COD/day.
Frequently Asked Questions
Chemical Oxygen Demand (COD) is a measure of the amount of oxygen required to chemically oxidize all organic and inorganic matter in a water sample. Expressed in mg O2/L, it is a key water quality indicator used in wastewater treatment, environmental monitoring, and regulatory compliance testing.
The COD formula (EPA 410.4 titration): COD (mg/L) = [(A − B) × M × 8000] / V, where A = blank titrant volume (mL), B = sample titrant volume (mL), M = FAS molarity, 8000 = oxygen equivalent factor, and V = sample volume (mL). Multiply by dilution factor if the sample was diluted before testing.
COD measures the oxygen needed to chemically oxidize all organic and inorganic matter using a strong oxidant. BOD (Biochemical Oxygen Demand) measures only the oxygen consumed by microorganisms to biodegrade organic matter over 5 days. COD is always higher than BOD, provides results in 2-3 hours vs 5 days, and measures all oxidizable matter including non-biodegradable compounds.
Untreated domestic sewage typically has COD of 250-1,000 mg/L. Treated effluent discharge limits are typically below 125 mg/L (EU Urban Wastewater Directive) or 250 mg/L (EPA secondary treatment). Clean river water has COD below 20 mg/L. Industrial wastewater can exceed 10,000 mg/L before treatment.
The COD:BOD5 ratio indicates biodegradability. A ratio below 2 indicates highly biodegradable wastewater suitable for biological treatment. A ratio of 2-3 is moderately biodegradable. A ratio above 3 indicates significant non-biodegradable content requiring chemical or physical treatment beyond standard biological processes.
EPA Method 410.4 is the standard closed reflux method for COD determination. A sample is digested with potassium dichromate and sulfuric acid at 150°C for 2 hours. The remaining dichromate is measured by back-titration with FAS. The oxygen equivalent of consumed dichromate equals the COD.
COD is preferred for industrial wastewater because it takes only 2-3 hours instead of 5 days. Industrial effluents often contain toxic compounds that inhibit the microorganisms used in BOD testing, giving falsely low BOD values. COD gives a complete picture of all oxidizable material regardless of biodegradability.
High COD indicates a large amount of oxidizable organic and inorganic matter. In receiving waters, this leads to dissolved oxygen depletion, eutrophication, and ecosystem damage. High COD in discharge exceeds regulatory limits and can result in enforcement action. It indicates the need for more intensive wastewater treatment.
COD is expressed in milligrams of oxygen per liter (mg O2/L or mg/L). Older texts use ppm (parts per million), which is numerically equal to mg/L for dilute aqueous samples. For daily load calculations, multiply COD (mg/L) by flow rate (m3/day) and divide by 1,000,000 to get kg COD/day.
COD drives treatment plant design: oxygen transfer capacity in aeration tanks, sludge production rates (typically 0.3-0.5 kg VSS per kg COD removed), hydraulic retention time, and effluent compliance. COD removal efficiency across unit processes is the primary measure of treatment performance.