950
mg/L O2
9.5
mL
19
mg O2
50
x
950
mg/L O2
9.5
mL
19
mg O2
50
x
The Chemical Oxygen Demand (COD) Calculator determines the amount of oxygen required to chemically oxidize organic and inorganic matter in a water sample, expressed in mg/L of O₂. COD is one of the most important parameters in wastewater characterization and treatment plant design, providing a rapid measure of the total oxidizable matter present. Unlike BOD (which measures biologically degradable matter over 5 days), COD measures all chemically oxidizable substances — including refractory organics and certain inorganic compounds — in just 2-3 hours. The standard method uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent in acidic solution with silver sulfate catalyst, followed by back-titration with ferrous ammonium sulfate (FAS). COD testing is mandated in discharge permits, treatment efficiency monitoring, and industrial wastewater compliance.
COD is calculated from the dichromate titration results:
$$COD \,(mg/L) = \frac{(V_{blank} - V_{sample}) \times N_{FAS} \times 8000}{V_{sample}}$$
Where:
The factor 8000 comes from the equivalent weight of oxygen (8 g/eq) and the conversion to mg/L. The blank represents the total oxidizing capacity of the dichromate, and the sample reading shows the unreacted dichromate. The difference represents the amount of dichromate consumed by the sample's oxidizable matter, converted to oxygen equivalents.
COD values vary enormously by water type. Clean rivers typically have COD below 20 mg/L. Domestic wastewater ranges from 250-800 mg/L. Industrial wastewater can exceed 10,000 mg/L. For discharge compliance, typical limits are 75-250 mg/L depending on the receiving water body and regulations. The COD/BOD ratio is informative: ratios of 1.5-2.5 indicate readily biodegradable waste, while ratios above 3-4 indicate significant refractory (non-biodegradable) organic content. Treatment plants typically achieve 85-95% COD removal through combined biological and chemical treatment.
Inputs
Results
The difference of 9.5 mL between blank and sample titrations gives COD of 950 mg/L — typical for strong municipal wastewater requiring biological treatment.
Inputs
Results
Small difference of 0.8 mL yields COD of 32 mg/L — indicating effective treatment. Most discharge standards would accept this level.
COD measures all chemically oxidizable matter using a strong chemical oxidant (dichromate), taking 2-3 hours. BOD measures only biologically degradable matter using microorganisms, taking 5 days (BOD₅). COD is always greater than or equal to BOD because it oxidizes substances that bacteria cannot. The COD/BOD ratio indicates the biodegradability of the waste.
Potassium dichromate is preferred because it is a strong, stable oxidant that achieves 95-100% oxidation of most organic compounds. It has a well-defined color change for titration endpoint, is stable in solution, and has a known equivalent weight. Silver sulfate catalyst ensures oxidation of straight-chain compounds, while mercuric sulfate suppresses chloride interference.
Ferrous ammonium sulfate (FAS), also known as Mohr's salt, is used to back-titrate the excess (unreacted) dichromate. FAS is preferred because it has a stable normality, is easy to standardize, and the ferroin indicator provides a sharp color change from blue-green to reddish-brown at the endpoint. FAS normality should be checked daily.
Chloride ions are oxidized by dichromate (Cl⁻ → Cl₂), consuming oxidant and giving falsely high COD readings. This interference is suppressed by adding mercuric sulfate (HgSO₄) before the test, which complexes chloride as insoluble HgCl₂. Typically, 1 g HgSO₄ per 50 mL sample is used for chloride levels up to 2000 mg/L.
Open reflux uses standard reflux apparatus with larger sample volumes (50 mL) and is suitable for heterogeneous samples. Closed reflux uses sealed culture tubes with small volumes (2.5-10 mL) heated in a digestion block. Closed reflux is more economical, generates less hazardous waste, and is widely used in modern laboratories. Both give equivalent results.
Yes, after dichromate digestion, COD can be determined by measuring the absorbance of the remaining Cr₂O₇²⁻ (at 420 nm for low COD) or the produced Cr³⁺ (at 600 nm for high COD). Pre-made reagent vials for spectrophotometric determination are commercially available and widely used in water treatment laboratories for rapid results.
Waters with COD above 20-50 mg/L typically require treatment before discharge. Most environmental discharge permits specify COD limits of 75-250 mg/L depending on the receiving water body. Industrial facilities may need pretreatment to reduce COD before discharge to municipal sewer systems, which often set limits of 500-1000 mg/L for sewer discharge.
COD/BOD ratios below 2.5 indicate readily biodegradable waste suitable for biological treatment. Ratios of 2.5-4 suggest mixed waste requiring combined biological and chemical/physical treatment. Ratios above 4-5 indicate predominantly non-biodegradable waste requiring advanced oxidation, activated carbon, or chemical treatment rather than biological processes.
COD testing involves hazardous chemicals: concentrated sulfuric acid (corrosive), potassium dichromate (toxic, carcinogenic hexavalent chromium), mercuric sulfate (toxic). Use fume hood, wear acid-resistant gloves, safety goggles, and lab coat. Waste containing chromium and mercury must be collected and disposed of as hazardous waste according to local regulations.
Yes, COD is excellent for process control because results are available in 2-3 hours versus 5 days for BOD. Online COD analyzers provide continuous monitoring. COD measurements at various treatment stages (influent, primary effluent, secondary effluent, final effluent) track treatment efficiency and help operators optimize chemical dosing and aeration.
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