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Chemical Oxygen Demand Calculator

Chemistry

Calculate the theoretical chemical oxygen demand (ThCOD) of a compound from its molecular formula, or compute COD from dichromate titration data (mg/L of O₂).

6
12
6
180
0
0
0.25
20

Theoretical COD (mg O₂/L)

191.8
O₂ per Mole (g/mol)
191.99
Titration COD (mg O₂/L)
0
Wastewater Class
Low COD — clean water or lightly contaminated

This calculator computes your Theoretical COD (mg O₂/L), O₂ per Mole (g/mol), Titration COD (mg O₂/L), Wastewater Class from the values you enter.

Inputs
Calculation ModeCarbon Atoms (C)Hydrogen Atoms (H)Oxygen Atoms (O) in compoundCompound Concentration (mg/L)Blank Titration (mL dichromate)Sample Titration (mL dichromate)Dichromate Normality (N)Wastewater Sample Volume (mL)
Outputs
Theoretical COD (mg O₂/L)O₂ per Mole (g/mol)Titration COD (mg O₂/L)Wastewater Class

What is a COD?

The Chemical Oxygen Demand Calculator computes the theoretical COD (ThCOD) of an organic compound from its molecular formula and concentration, or the measured COD from dichromate titration data. ThCOD is calculated as the oxygen required to completely combust the compound; titration COD is calculated from the standard formula (B − S) × N × 8000 / V.

COD is the most widely used parameter for characterising organic pollution in industrial and municipal wastewater. In India, it is the primary effluent quality parameter monitored under CPCB standards — every industrial Effluent Treatment Plant (ETP) must achieve COD ≤ 250 mg/L before discharge to inland surface waters. The Detention Time Calculator and Hydraulic Retention Time Calculator are used alongside COD to design the treatment systems that achieve these limits.

How to use this COD calculator

  1. Select Calculation Mode — Theoretical (from formula) or Titration.
  2. For Theoretical: enter C, H, O atom counts (e.g., glucose C₆H₁₂O₆: C=6, H=12, O=6) and Concentration (mg/L).
  3. For Titration: enter blank and sample mL of Mohr's salt or dichromate, normality, and sample volume.
  4. Read ThCOD or Titration COD in mg O₂/L.
  5. Compare to CPCB limit (250 mg/L for inland discharge) and the Wastewater Class interpretation.

Formula & Methodology

Theoretical COD:

CₓHᵧOᵤ + (x + y/4 − z/2) O₂ → x CO₂ + (y/2) H₂O O₂ per gram = (x + y/4 − z/2) × 31.998 / M_compound  [g O₂/g compound] ThCOD (mg/L) = O₂_per_gram × concentration_mg_L

Titration COD (standard APHA 5220C):

COD (mg/L) = (B − S) × N × 8000 / V_sample B = blank titration (mL); S = sample titration (mL) N = normality of titrant; 8000 = equivalent weight of O₂ × 1000

Worked example — glucose in distillery effluent:

A distillery's primary effluent contains 5000 mg/L glucose (C₆H₁₂O₆, M=180.16 g/mol).

O₂ coefficient = 6 + 12/4 − 6/2 = 6 mol O₂/mol glucose.

O₂ per gram = (6 × 31.998) / 180.16 = 191.988/180.16 = 1.066 g O₂/g glucose ThCOD = 1.066 × 5000 = 5330 mg O₂/L

This 5330 mg/L COD exceeds the CPCB limit by 21×. A UASB (upflow anaerobic sludge blanket) reactor can remove 80–90% of COD, reducing effluent to 533–1066 mg/L — still above CPCB limit, requiring secondary treatment. Most Indian distilleries use UASB + biomethanation + composting to achieve compliance while generating biogas for boiler fuel. India has over 700 distilleries (alcohol, ethanol, and molasses-based) under CPCB monitoring.

Frequently Asked Questions

Chemical oxygen demand (COD) is the amount of oxygen (in mg/L or g/m³) required to chemically oxidise all organic matter in a water sample, expressed as mg O₂ per litre. COD measures the total oxygen-consuming organic load in water — higher COD means more pollutants. It is determined by digesting the sample with a strong oxidant (potassium dichromate K₂Cr₂O₇ in H₂SO₄ at 150°C for 2 hours) and back-titrating to find how much dichromate was consumed. COD is the primary parameter for characterising industrial wastewater in India under CPCB (Central Pollution Control Board) standards.
Theoretical COD (ThCOD) is the COD calculated from the stoichiometry of complete combustion: ThCOD = (O₂ moles per mole of compound × 32 g/mol × concentration) / molar mass. For a compound CₓHᵧOᵤ: O₂ required per mole = x + y/4 − z/2 (from balanced combustion CₓHᵧOᵤ + a O₂ → x CO₂ + y/2 H₂O). ThCOD (mg O₂/mg compound) = (x + y/4 − z/2) × 31.998 / M_compound. For glucose C₆H₁₂O₆: O₂ = 6 + 12/4 − 6/2 = 6 + 3 − 3 = 6 mol per mol glucose; ThCOD = 6 × 32/180.16 = 1.067 g O₂/g glucose.
BOD (Biochemical Oxygen Demand) measures the oxygen consumed by microorganisms (bacteria) decomposing biodegradable organic matter over 5 days at 20°C — BOD₅. COD measures the oxygen consumed by chemical oxidation of all oxidisable organic matter (biodegradable + non-biodegradable). COD is always ≥ BOD. The ratio BOD/COD indicates biodegradability: BOD/COD > 0.5 indicates easily biodegradable waste (amenable to biological treatment); < 0.3 indicates resistant/toxic compounds needing chemical treatment. For domestic sewage: BOD/COD ≈ 0.5; for textile effluent: BOD/COD ≈ 0.1–0.2 (dyes are not biodegradable).
Select Calculation Mode: 'Theoretical COD from formula' (enter molecular formula and concentration) or 'COD from dichromate titration' (enter blank and sample titration volumes, dichromate normality, and sample volume). For theoretical: enter C, H, O atom counts in the compound and concentration (mg/L). The calculator gives ThCOD (mg O₂/L) and wastewater classification. Default: glucose C₆H₁₂O₆ at 180 mg/L → ThCOD ≈ 192 mg O₂/L.
India's Central Pollution Control Board (CPCB) and Ministry of Environment, Forest and Climate Change (MoEF&CC) set effluent standards by industry type. General effluent standards (inland surface water): COD ≤ 250 mg/L. Textile industry (ZLD zones): COD ≤ 150 mg/L. Pharmaceutical industry: COD ≤ 250 mg/L. Sugar mills: COD ≤ 250 mg/L. Tanneries: COD ≤ 250 mg/L. These are enforced by State Pollution Control Boards; non-compliant industries face closure orders under the Water (Prevention and Control of Pollution) Act, 1974. Treatment plants must achieve these limits before discharge.
Domestic sewage: COD 250–500 mg/L. Municipal wastewater: COD 300–600 mg/L. Dairy industry: COD 1,000–5,000 mg/L (from whey). Sugar mill (distillery): COD 10,000–100,000 mg/L (molasses fermentation). Pulp and paper: COD 2,000–10,000 mg/L. Textile dyeing: COD 1,000–5,000 mg/L. Pharmaceutical synthesis waste: COD 5,000–50,000 mg/L. High-COD effluents (distillery, pharma) typically require biological treatment (UASB + activated sludge) followed by tertiary treatment (ozonation, UV) to meet CPCB limits.
Standard APHA method 5220C: COD (mg/L) = (A − B) × N × 8000 / V, where A = mL of Fe(NH₄)₂(SO₄)₂ (Mohr's salt) used for blank titration, B = mL for sample titration, N = normality of Mohr's salt, 8000 = equivalent weight of O₂ (8 g/equivalent) × 1000 mL/L, V = volume of sample (mL). This calculator uses dichromate normality directly: COD = (B_mL − S_mL) × N_dichromate × 8000 / V_sample. Higher dichromate consumed by blank vs sample means more oxidisable matter in sample.
TOC (total organic carbon) measures carbon mass directly (by catalytic combustion + CO₂ detection). COD measures oxygen demand. They are related by: COD/TOC ≈ 2.7 for completely oxidised organic matter (since oxidising 1 g C requires ~2.67 g O₂, from C + O₂ → CO₂). For partially reduced compounds (containing O in the molecule), COD/TOC < 2.67. For highly reduced compounds (methane, fats), COD/TOC > 2.67. Example: methane CH₄ (ThCOD = 4 g O₂/g; TOC content = 75% C) → COD/TOC = 4/0.75 = 5.33. TOC is now preferred in many Indian ZLD monitoring programmes due to faster measurement (minutes vs 2-hour COD digestion).
Primary treatment: settling, screening — removes particulate COD (20–30% reduction). Secondary biological treatment: activated sludge, SBR (sequencing batch reactor), UASB (upflow anaerobic sludge blanket) for high-COD effluents — removes 70–90% of biodegradable COD. UASB reactors are widely used in India for distillery and food industry effluents (biogas is captured as fuel). Tertiary treatment: ozonation, UV/H₂O₂ (advanced oxidation processes — AOP) for non-biodegradable COD in pharmaceutical and chemical effluent. Effluent Treatment Plants (ETPs) are mandatory for CPCB Red Category industries in India.
In anaerobic treatment, organic matter (measured as COD) is converted to biogas (CH₄ + CO₂). Theoretical methane yield: 0.35 Nm³ CH₄ per kg COD removed (at STP). This relationship is used to estimate biogas yield from wastewater COD: a dairy plant with 10 m³/day of 5000 mg/L COD effluent can theoretically produce 0.35 × 50 kg/day = 17.5 Nm³/day CH₄ ≈ equivalent to 9.4 kg LPG per day. India has over 5,000 biogas plants treating agricultural and food industry wastewater, converting high-COD effluents into renewable energy under the National Biogas Mission.