Chemical Oxygen Demand (COD) – Definition, Measurement, Importance

Chemical Oxygen Demand (COD) is a way to measure how much oxygen is needed to chemically oxidize organic and some inorganic substances in water that is acidic and has a strong oxidant, like potassium dichromate. It is usually shown in mg O₂ per liter. To find out how much oxidizable material is in a sample, you mix it with an oxidant and measure how much of it is used up (or left over) by titration or spectrophotometry.

COD is a quick and high-throughput way to measure the amounts of organic pollution in surface waters or wastewater. It usually gives larger values than Biochemical Oxygen Demand because it contains both biologically and chemically degradable compounds. The main use of COD is to keep an eye on how well treatment is working and whether it is following the rules. High COD levels can mean that oxygen levels in receiving aquatic systems may drop, which can be harmful to aquatic life.

What is Chemical Oxygen Demand (COD)?

• Oxygen in Chemicals Demand is a way to figure out how much oxygen is used up when compounds (both organic and some inorganic) in water are oxidized with a strong oxidant.
• It’s commonly written as mg of oxygen per liter of water.
• For COD testing, you add too much of a chemical oxidant, say potassium dichromate in acid, to the water sample and heat it up to oxidize the molecules.
• To find out how much oxygen was needed, the residual oxidant is titrated to see how much was utilized. Wikipedia
• It includes both biodegradable and non-biodegradable things, while BOD just looks at biodegradable things.
• COD is used to measure pollution levels, figure out how well wastewater treatment is working, and guess how it will affect oxygen levels in water.

Measurement of COD

• Chemical Oxygen Demand is measured by oxidizing all organic material in a water sample using a strong chemical oxidant like potassium dichromate under acidic conditions, with the sample heated during a digestion step often at 150 °C for about two hours
• an excess of oxidant ensures complete oxidation, and after digestion the leftover oxidant is quantified either by titration with ferrous ammonium sulfate using an indicator like ferroin or by colorimetric measurement of chromium ions
• in the titrimetric method the amount of titrant used to reduce excess dichromate is used in a formula (COD = 8000·(b – s)·n / sample volume) to calculate COD in mg/L
• in the colorimetric method the absorbance change at specific wavelengths (600 nm for Cr³⁺ or 420 nm for Cr⁶⁺) is measured in a photometer or spectrophotometer, and related to COD via calibration
• chlorine ions and other inorganic substances can interfere with COD measurement, so mercury sulfate is often added to remove chloride, and sulfamic acid may be used to eliminate nitrite interference
• colorimetric methods are widely used because they’re faster and less labor‑intensive than titration, though both are EPA‑approved

Methods to Reduce COD in Wastewater

• Biological treatment is common and effective– Aerobic activated sludge or microbial consortia are used to degrade organics; this can remove up to 75–85 % COD in primary + secondary stages, but more is often needed afterward
• Advanced anaerobic + aerobic systems – Anaerobic pre‑treatment (biomethane) followed by aerobic polishing achieves > 99 % BOD/COD removal, proven in sugar refineries
• Moving bed biofilm reactors (MBBR) -Plastic carriers support fixed biofilms in aeration tanks; these systems boost biomass retention, fit retrofit projects, & improve COD removal efficiency
• Constructed wetlands – Engineered vegetated systems mimic nature, combining microbial biofilms and plant uptake; they remove organics & nutrients cheaply but need space
• Chemical coagulation & flocculation – Coagulants like alum, ferric chloride, etc clump suspended/colloidal organics for removal by sedimentation/filtration; simple but costly
• Electrocoagulation (EC)– Electrodes generate coagulant in situ without added chemicals—effective for emulsified oils, refractory organics, polishing COD
• Dissolved air flotation (DAF)– Tiny bubbles in pressurized treated water float suspended organics to be skimmed; often used after coagulation in industrial streams
• Advanced oxidation processes (AOPs)– Using OH radicals via ozone, UV, H₂O₂, Fenton, TiO₂ to mineralize recalcitrant organics; tertiary step that can give > 99 % removal though expensive
• Vacuum evaporation– Evaporates water leaving behind pollutants; good for high‑strength wastewater but energy‑intensive and often requires pre/post‑treatment
• Chlorination or H₂O₂ oxidation -Chemical oxidation of COD; chlorine removes inorganic COD nearly completely, organic COD depends on dose and contact time
• Electrochemical + activated carbon hybrid– 3D electrochemical flow reactors with GAC fields, combining adsorption and electro‑oxidation improve persistent organics removal & reduce COD
• Photocatalytic beads (new research)– Concrete beads with photocatalyst under sunlight reduced > 82 % COD, re‑use > 90 % efficiency over cycles—emerging low‑energy tech

Importance of COD

• COD is a key water‑quality index– it measures the amount of oxidizable organic (and some inorganic) material in water by quantifying the oxygen required for chemical oxidation and serves as an indicator of pollution impact on receiving bodies
• COD predicts dissolved‑oxygen depletion– high COD means more oxygen demand, which lowers dissolved‑oxygen (DO) and can drive water bodies anaerobic, harming aquatic life
• COD vs BOD: faster & broader -COD test takes ~2–4 hours vs BOD’s 5 days, and measures all oxidizable substances not just biodegradable ones
• Essential for treatment‑plant monitoring– tracking influent/effluent COD helps gauge treatment efficiency, spot process upsets, and optimize performance in real time
• Regulatory compliance tool– many regulations set maximum COD limits for discharged effluent, so measurement is crucial to ensure legal compliance
• Useful for toxic or industrial streams -COD works even when BOD fails—eg toxic industrial effluent where microbes can’t survive, so chemical oxidation is still measureable
• Enables rapid process control– real‑time or online COD sensors enable quick adjustments to treatment systems, reducing costs and improving effluent quality
• Helps distinguish organic fractions – COD can be fractionated (e.g., total vs dissolved), enabling better control over nutrient and organic removal strategies

Disadvantages of Chemical Oxygen Demand (COD)

• Is time‑consuming – The standard COD method requires a 2 h reflux digestion, or even longer cooling, so it slows down sample throughput and hinders quick decision‑making
• Relies on hazardous reagents – Uses potassium dichromate (a carcinogenic, toxic Cr⁶⁺ compound) and often mercury salts as chloride‑masking agents
• Generates toxic waste – Produces chromium and mercury‑containing waste that needs specialized disposal, causing environmental and safety concerns
• Poor selectivity – Oxidizes inorganic species (e.g., chlorides, nitrites, sulfides), which overestimates organic load, and needs chemical masking
• Doesn’t distinguish organics – Cannot differentiate between biologically degradable organic matter and inert ones, giving only total oxidizable content
• Requires bulky lab equipment – Needs large reflux apparatus and fume hoods, which makes on‑site or high‑throughput testing impractical
• High operational cost – Involves expensive reagents (silver sulfate, etc.), energy consumption during heating, and consumable vials or chemicals
• Risk of errors and explosions – Vials can explode under high pressure; interferences from matrix components (e.g. halides, peroxides) can distort results
• Limited for real‑time monitoring – Though faster than BOD, still not ideal for continuous or instant measurements without advanced analyzers

Challenges Associated with COD Monitoring

• Instrument and calibration errors are common challenge– systems may drift due to probe aging, inconsistent calibration or reagent degradation which leads to unreliable COD readings 
• Sample handling variability influences accuracy– inaccurate sampling volume, poor mixing, contamination or delay change results significantly 
• Environmental interferences reduce precision– factors such as temperature, turbidity, chloride or heavy metals cause inaccurate oxidation demand measurement 
• Hazardous chemicals pose safety risk -regular lab methods use dichromate & strong acids that need careful handling and disposal, risk for operators and environment 
• Matrix complexity causes readings distortion– industrial effluents can contain interfering substances or inorganics that react with reagents disrupting test results 
• Lack of real‑time data complicates process control– traditional methods take hours and lab analysis delays corrective treatment actions; sensors offer realtime but need validation 
• Sensor tech needs frequent calibration and maintenance– on‑site COD probes need recurring calibration cycles, cleaning, sensor replacement to maintain accuracy 
• Data processing and interpretation error risk– complex COD data from online sensors requires proper software and trained operators, else misinterpretation occurs 
• Low‑cost sensors need robust calibration -affordable sensors vary with design and environment; without standardized calibration they produce uncertain results 
• Regulatory pressure adds challenge– strict compliance demands frequent and accurate COD measurement; poor reliability could lead to fines or process shutdown
• High wastewater load variability stresses sensors– fluctuations in influent organics make breakthrough detection difficult and may overwhelm sensor linear range 

Determination of Chemical Oxygen Demand of Wastewater

The COD test is based on the idea that nearly all organic compounds in wastewater can be chemically oxidized to carbon dioxide and water when treated with a strong oxidant under acidic conditions. In practice, a known excess of potassium dichromate (K₂Cr₂O₇) in sulfuric acid is added to a measured water sample, often with catalysts like silver sulfate to enhance oxidation and mercury sulfate to neutralize interfering chloride ions. The mixture is heated—typically under reflux—for about two hours to ensure complete oxidation . After digestion, the remaining unreacted dichromate is quantified—either by titrating with a ferrous ammonium sulfate solution using ferroin as an indicator or via colorimetric measurement, to determine how much oxidant was consumed. The difference between the initial and residual oxidant corresponds to the oxygen equivalent of the oxidized organic matter, giving the COD value in mg/L.

This method provides a rapid and reliable assessment of the total oxidizable organic load in wastewater.

Requirement for COD test

• Apparatus commonly needed
  • burette, pipettes, measuring cylinders and conical or Erlenmeyer flasks for sample handling
  • reflux or digestion unit (heating block) capable of ~150 °C for 2 h
  • condenser (Allihn or reflux type) if sample contains volatiles
• Reagents required
  • potassium dichromate (K₂Cr₂O₇) solution (≈ 0.25 N, dried before use)
  • concentrated sulfuric acid often mixed with silver sulfate catalyst
  • silver sulfate to aid oxidation of refractory organics
  • mercuric (or mercury) sulfate to neutralize chloride interference
  • ferrous ammonium sulfate standardized titrant (or FAS)
  • ferroin indicator solution (1,10‑phenanthroline‑Fe²⁺ complex)
• Blank and standards
  • reagent blank (DI water through all steps) in each batch
  • calibration standards (e.g., potassium hydrogen phthalate solutions 100–1000 mg/L)
• Sample preparation
  • homogenize samples with solids via blending (~500 mL sample)
  • acidify and preserve (optional), store at 4 °C, test within 28 days
• Measurement instrumentation
  • titrimetric: burette/manual endpoint detection via ferroin color change
  • colorimetric: spectrophotometer or photometer (λ = 420 nm for hexavalent, 600‑620 nm for trivalent chromium)
• Safety and waste handling
  • corrosive acids, toxic and carcinogenic reagents (Cr⁶⁺, Hg²⁺, Ag⁺) require fume hood, PPE
  • collect and dispose of hazardous waste properly, don’t pour down drain
• Interference controls
  • add HgSO₄ for samples with ≥ 2000 mg/L Cl⁻ (or adjust)
  • remove nitrite with sulfamic acid if present
  • correct for inorganic oxidizable species if necessary

Procedure of COD test

• The test starts by homogenizing a representative sample—shake or blend the wastewater to evenly suspend solids—and then preserving it to pH < 2 with concentrated sulfuric acid, storing it at about 4 °C until analysis, ideally within 28 days.
• Next, prepare your apparatus: use a reflux setup consisting of an Erlenmeyer or round-bottom flask, an Allihn condenser, and a digestion block preheated to ~150 °C.
• Into the flask or pre‑measured vial, pipette a known volume (usually 2 mL) of sample alongside a known excess of 0.25 N potassium dichromate, a measured amount of concentrated sulfuric acid mixed with silver sulfate catalyst, and add mercuric sulfate to neutralize chloride interference.
• Seal the flask or vial, heat it under reflux at 150 °C for two hours to fully oxidize organics, then allow it to cool slowly to room temperature in a fume hood—rapid cooling risks glass breakage and hazardous release.
• After cooling, titrate the residual dichromate with standardized ferrous ammonium sulfate using ferroin indicator until a color endpoint indicates the amount of oxidant consumed, or measure absorbance spectrophotometrically at 600 nm for colorimetric methods like EPA 410.4.
• Run a reagent blank and calibration standards (e.g., potassium hydrogen phthalate at known COD) alongside samples to ensure quality control.
• Finally calculate COD in mg/L using the volume of titrant consumed relative to the blank or from a calibration curve—remember to correct for any inorganic oxidizable substances like chlorides or nitrite if needed

Calculation

Formula for COD determination

COD (mg/L) is calculated using COD = ((A – B) × N × 8000) / V

Where;

• A – volume of FAS used for blank (mL)
• B – volume of FAS used for sample (mL)
• N – normality of FAS (eq/L)
• V – volume of sample (mL)
• 8000 – equivalent weight factor (O₂ = 8 × 1000 for mg/L unit conversion)

Explanation of variables

• A: volume of ferrous ammonium sulfate used for the reagent blank
• B: volume of FAS used for the digested sample
• N: normality of FAS titrant
• V: volume of sample aliquot used

Conversion factor 8000

represents equivalent weight O₂ (8 mg per meq) × 1000 mL/L, so units convert correctly to mg/L

Example calculation

if blank = 12.0 mL, sample = 4.2 mL, N = 0.1 eq/L, V = 10 mL
then COD = ((12.0 − 4.2) × 0.1 × 8000) ÷ 10 ≈ 624 mg/L

Alternative stoichiometric method

for known compound concentration C (mg/L): COD = (C ÷ FW) × RMO × 32 mg/L, where FW is mol wt and RMO is moles O₂ per mole compound

Spectrophotometric method

use calibration curve: measure absorbance of ferrous or dichromate at ~600 nm or 420 nm after digestion, then convert to COD value against standards

Corrections & quality control

• always subtract blank to correct for reagent demand
• apply chloride or nitrite interference corrections when needed
• ensure proper standardization of FAS daily

Key units consistency– ensure A, B, V in mL; N in eq/L; result yields mg O₂ per litre.