COD Testing Methodology: IS 3025 Part 58 Explained for ETP Operators

COD — Chemical Oxygen Demand — is the parameter that keeps ETP operators informed between BOD₅ test cycles. While BOD₅ takes five days to produce a result, a COD test using IS 3025 Part 58 gives you a reliable, quantitative measure of organic load in under three hours. This guide walks through every step of the closed reflux titrimetric method: the reagents, their roles, the digestion and titration procedure, how to handle the most common interferences, and how to use the number once you have it.

Why COD Is the Central Parameter in ETP Monitoring

COD is the workhorse parameter of ETP operations. While BOD₅ takes 5 days and is used for compliance reporting, COD gives results in 2–3 hours and is used for daily operational decisions: Has the inlet load increased? Is the biological treatment performing? Is the treated effluent likely to meet discharge limits? All of these questions can be answered from COD without waiting 5 days.

The COD result also enables the BOD:COD ratio to be calculated instantly (using yesterday's BOD₅), which guides treatment strategy. A sudden rise in inlet COD with no corresponding rise in BOD suggests a change in wastewater character — perhaps a cleaning chemical discharge or a new production batch — rather than a simple load increase. That distinction changes how you respond.

The CPCB General Standard for inland surface water discharge is COD ≤250 mg/L. Many state PCBs have stricter limits — industries required to achieve Zero Liquid Discharge (ZLD) must treat to virtually zero COD before the evaporation stage. At the other end, discharge to a public sewer typically allows COD ≤600 mg/L. Always check your specific Consent to Operate (CTO) conditions, as these override General Standards.

For practical process control, COD is monitored at four points: inlet (raw wastewater), post-primary treatment (after DAF or primary clarifier), post-biological treatment (after MBBR, ASP, or SBR), and final effluent. Each step tells you something different about where load is being removed and where the system is underperforming.

IS 3025 Part 58: Closed Reflux Titrimetric Method

IS 3025 Part 58 (2006) — Methods of Sampling and Test (Physical and Chemical) for Water and Wastewater: Determination of Chemical Oxygen Demand — specifies the closed reflux titrimetric method for COD determination. IS 3025 Part 58 aligns with APHA Standard Methods 5220C (Closed Reflux, Titrimetric Method), which is the internationally recognised reference procedure.

The "closed reflux" approach is an improvement over the older open reflux method. In closed reflux, sealed digestion tubes heated in a block digester replace the open round-bottom flask and condenser apparatus. The advantages are significant: reduced toxic dichromate vapour exposure for laboratory staff, smaller sample volumes (typically 2.5 mL versus 10–50 mL), faster results, and substantially reduced chemical waste to dispose of.

The IS 3025 Part 58 method works on the following chemical principle: organic matter in the sample is oxidised by excess potassium dichromate (K₂Cr₂O₇) in concentrated sulphuric acid at 148–150°C for 2 hours. During oxidation, hexavalent chromium (Cr⁶⁺) is reduced to trivalent chromium (Cr³⁺). The dichromate is added in excess — more than enough to oxidise all organic matter in the sample. The remaining, unreacted Cr⁶⁺ is then measured by back-titration with ferrous ammonium sulphate (FAS). The difference between the initial Cr⁶⁺ (measured via the blank) and the remaining Cr⁶⁺ (measured via the sample titration) represents the oxygen equivalent consumed by organic matter — that is the COD value.

The method is applicable over a COD range of approximately 40–2,000 mg/L using standard reagent concentrations. Samples with higher COD must be diluted before testing. For very low COD samples (below 40 mg/L), alternative micro or ultra-low COD procedures with modified reagent concentrations are used.

Reagents and Their Roles in COD Testing

Understanding what each reagent does — not just how to prepare it — is essential for troubleshooting when results are anomalous. Six reagents are used in the IS 3025 Part 58 method:

Potassium Dichromate (K₂Cr₂O₇), 0.04167 M: The primary oxidising agent. Cr⁶⁺ in the reagent oxidises organic compounds during digestion; the excess remaining after digestion is measured to determine COD. The standard solution must be prepared from primary-standard grade K₂Cr₂O₇ dried at 103°C for 2 hours before weighing — moisture absorbed during storage will dilute the actual concentration and give low COD readings.

Sulphuric Acid (H₂SO₄) with Silver Sulphate (Ag₂SO₄): Concentrated sulphuric acid provides the strongly acidic conditions required for oxidation at elevated temperature. Silver sulphate dissolves in the concentrated acid to act as a catalyst — it promotes the oxidation of straight-chain aliphatic compounds that potassium dichromate alone oxidises incompletely. Without Ag₂SO₄, acetic acid and other short-chain fatty acids are under-recovered. The Ag₂SO₄/H₂SO₄ reagent is pre-prepared by dissolving Ag₂SO₄ in concentrated H₂SO₄ (5.5 g Ag₂SO₄ per 1 kg H₂SO₄) and allowing it to dissolve over 1–2 days.

Mercury Sulphate (HgSO₄): Added to the digestion tube to precipitate chloride ions as mercuric chloride (HgCl₂), which is non-reactive with dichromate. Chloride ions are oxidised by Cr⁶⁺ and would give false high COD readings if not removed. HgSO₄ is critical for seawater-contaminated samples, brine discharges, or coastal industrial effluents. Environmental note: HgSO₄ is a mercury compound — handle with appropriate PPE and dispose of digestion tubes and their contents as hazardous mercury waste, not as ordinary chemical waste.

Ferroin Indicator: Tris(1,10-phenanthroline)iron(II) complex — a sharp, reliable redox indicator used at the endpoint of the back-titration. The solution is blue-green (excess Cr⁶⁺ present) during titration and switches sharply to red-brown when all Cr⁶⁺ has been reduced and excess FAS is present. The endpoint is reversible — do not over-titrate by more than one drop.

Ferrous Ammonium Sulphate (FAS), 0.25 M: The back-titrant. FAS (Fe²⁺) reduces unreacted Cr⁶⁺ back to Cr³⁺ during titration. The volume of FAS consumed by the sample titration represents the Cr⁶⁺ remaining after digestion (oxygen NOT consumed by the sample). FAS must be standardised against the K₂Cr₂O₇ solution on each day of use — FAS concentration decreases on standing due to air oxidation of Fe²⁺ to Fe³⁺. Stale FAS gives high COD results.

Blank solution: The same reagent volumes with Type 2 or Type 3 distilled water in place of the sample. The blank goes through the identical digestion and titration procedure. The blank titration volume (B) represents the total initial Cr⁶⁺ in the system — it is the reference against which the sample titration volume (A) is compared. A blank COD above approximately 5 mg/L indicates reagent contamination or impure water and invalidates the batch.

Step-by-Step COD Test Procedure

The following procedure follows IS 3025 Part 58 for the closed reflux titrimetric method using a block digester and 16 mm × 100 mm COD digestion tubes.

Step 1 — Sample homogenisation: If suspended solids are present, homogenise the sample for 30 seconds using a blender or probe sonicator. Suspended solids contribute to COD and must be included in the analysis — do not filter the sample before testing. For samples with very high SS, ensure the homogenised sample is pipetted promptly before settlement.

Step 2 — Sample preparation: If the sample COD is expected to exceed 900 mg/L (typical for raw inlet from food, distillery, or tannery), dilute with distilled water to bring it within range. Record the dilution factor. If the sample chloride concentration exceeds approximately 2,000 mg/L, add HgSO₄ at a ratio of 10:1 (HgSO₄ mass to Cl⁻ mass) to each tube before adding other reagents.

Step 3 — Tube preparation: Add 1.5 mL of potassium dichromate reagent to a clean, dry COD digestion tube. Carefully add 2.5 mL of the sulphuric acid/Ag₂SO₄ reagent by running it down the inside wall of the tilted tube so it layers below the dichromate. The tubes become hot immediately — use tube holders or heat-resistant gloves. Prepare one blank tube alongside each batch of samples using 2.5 mL of distilled water instead of sample.

Step 4 — Sample addition: Add 2.5 mL of the prepared sample (or diluted sample) to the tube. Cap tightly with the PTFE-lined cap and mix by carefully inverting the tube three times. The contents will be hot — grip firmly. Prepare the blank in the same way with 2.5 mL distilled water.

Step 5 — Digestion: Place the capped tubes in a pre-heated block digester set at 150°C. Digest for exactly 2 hours. Do not open the digester during digestion. The block digester must be at operating temperature before tubes are inserted — cold start gives incomplete oxidation and low COD results.

Step 6 — Cooling: Remove tubes from the digester and place in a tube stand. Allow to cool to room temperature — approximately 20–30 minutes. Do not cool under running water or in an ice bath; rapid cooling creates negative pressure inside the tube and can draw liquid past the cap seal, contaminating the exterior and causing errors.

Step 7 — Transfer and titration: Transfer the entire contents of the cooled digestion tube to a 250 mL Erlenmeyer flask. Rinse the tube three times with distilled water, adding all rinsings to the flask (total volume approximately 150 mL after rinsing). Add 2–3 drops of ferroin indicator. Titrate with standardised FAS solution from a burette, adding FAS dropwise as the endpoint approaches, until the colour changes sharply from blue-green to red-brown (brick-red). This endpoint is permanent — do not continue titrating. Record the FAS volume consumed for the sample (A, in mL) and for the blank (B, in mL).

Step 8 — Calculation: COD (mg/L) is calculated as:

Formula: COD (mg/L) = (B − A) × M × 8,000 / V

Worked example: B = 8.20 mL, A = 3.60 mL, FAS molarity = 0.25 M, sample volume = 2.5 mL.

COD = (8.20 − 3.60) × 0.25 × 8,000 / 2.5 = 4.60 × 0.25 × 8,000 / 2.5 = 3,680 mg/L.

This result represents a raw inlet sample — for such high-COD wastewater (typical of food processing or distillery inlet), the sample would have been diluted (e.g., 1:10) before testing, and the dilution factor applied to give the final result. At a 1:10 dilution, a measured COD of 368 mg/L on the diluted sample × 10 = 3,680 mg/L actual inlet COD.

Interferences and How to Handle Them

The COD test is susceptible to several interferences that cause results to be falsely elevated or, less commonly, falsely depressed. Knowing these is essential for valid results across the range of industrial wastewaters encountered in ETP operation.

Chloride (Cl⁻) — most significant interference: Chloride is oxidised by dichromate under acidic conditions, consuming Cr⁶⁺ and giving falsely high COD. The correction is mercury sulphate (HgSO₄), which precipitates Cl⁻ as insoluble, non-reactive HgCl₂. Add HgSO₄ at 10:1 (mass ratio HgSO₄:Cl⁻) before adding other reagents. At 0.4 g HgSO₄ per tube, interference is eliminated up to approximately 1,000 mg/L Cl⁻. For very high chloride samples such as seawater (Cl⁻ ≈ 19,000 mg/L) or brine, dilute the sample substantially before testing in addition to adding HgSO₄.

Nitrite (NO₂⁻): Nitrite is oxidised by dichromate and contributes to apparent COD. The correction is sulphamic acid — add 10 mg sulphamic acid per mg of NO₂⁻-N present before beginning the digestion. This is relevant for partially nitrified effluents where nitrite accumulates (e.g., during nitritation in an SBR or when a nitrifying system is under stress).

Reduced iron (Fe²⁺): Ferrous iron consumes dichromate directly and contributes to measured COD without representing organic pollution. Wastewaters from steel pickling, electroplating, and acid mine drainage may contain significant Fe²⁺. Reporting the ferrous iron content alongside COD allows the inorganic contribution to be estimated and, if required, subtracted.

Volatile fatty acids (VFAs): Short-chain VFAs (acetic, propionic, butyric acid) are volatile and can partially escape as vapour during digestion if tubes are not properly sealed. This results in slightly low COD for anaerobic digester effluents and high-VFA streams. Ensure caps are tightened firmly and check for cap degradation (PTFE liners should be replaced regularly). VFA loss is one reason COD of anaerobic digester samples can appear systematically lower than expected from the theoretical calculation.

Inorganic sulphide (S²⁻): Sulphide is oxidised by dichromate and contributes to COD. Tannery wastewater, seafood processing effluent, and anaerobic digester overflow can contain significant dissolved sulphide. For samples where inorganic sulphide contribution is a concern, a separate sulphide analysis (IS 3025 Part 29) allows the inorganic COD contribution to be quantified.

Interpreting COD Results for Process Control

A COD number alone means little without context. Here is how to use COD data at each monitoring point across the treatment train:

Inlet COD: Track daily and plot as a trend. Sudden spikes of more than 20% above baseline indicate production upsets, batch discharge events, or cleaning chemical discharge (CIP chemicals can have extremely high COD). Alert the biological treatment team to adjust loading, recycle rate, or HRT before the slug load reaches the bioreactor. If inlet COD consistently exceeds design values, the biological system is being asked to do more than it was designed for.

Post-primary treatment COD: After DAF or a primary clarifier, COD should drop 30–50% for typical food industry wastewater (more if the wastewater is FOG-heavy and DAF is well-operated). If primary COD removal is much less than expected, check DAF polymer dose, float blanket thickness, and whether FOG is entering the primary stage in emulsified form that the DAF cannot capture.

Post-biological treatment COD: After MBBR, ASP, or SBR. For wastewater with BOD:COD ≥0.5, overall COD removal from inlet to post-biological should be 85–95%. If post-biological COD is higher than expected, systematically check: MLSS (target 2,500–4,000 mg/L in an ASP or SBR), dissolved oxygen in the aeration zone (target >2 mg/L), hydraulic retention time (compare actual HRT against design value using daily flow data), and sludge age (SRT). Each of these parameters has a measurable COD consequence when it deviates from target.

Final effluent COD: Compare against your CTO discharge limit (typically ≤250 mg/L for inland surface water, stricter for certain industries and states). If approaching the limit, identify at which treatment stage the shortfall occurs rather than guessing. A post-biological COD of 200 mg/L with a limit of 250 mg/L leaves almost no margin; polishing filtration or activated carbon adsorption may be needed.

COD:BOD₅ ratio of treated effluent: As wastewater moves through treatment, the biodegradable fraction is progressively removed. The residual COD after biological treatment is predominantly non-biodegradable (refractory) organic matter. The treated effluent BOD:COD ratio is therefore typically lower than the inlet ratio — sometimes substantially so. If this residual refractory COD still exceeds the discharge limit, biological treatment has done all it can; advanced treatment (activated carbon adsorption, ozonation, or Fenton oxidation) is required to reduce the refractory fraction.

A useful operational heuristic: if your final effluent COD is persistently above the limit and your biological system is well-operated (correct MLSS, DO, HRT, SRT), the problem is almost certainly refractory COD — not biological underperformance. Optimising biology further will give diminishing returns. At that point, characterise the residual COD (what compounds remain?) and evaluate appropriate advanced treatment options.