How to Calculate BOD and COD for Your ETP Design — A Practical Guide

If you are building a new ETP or upgrading an existing one, the two numbers that will drive every design decision are BOD and COD. Get them wrong and your entire plant is sized incorrectly — too small means non-compliance, too large means wasted capital. Yet most factory owners rely on a single grab sample taken on a day when production was light, and then wonder why their ETP struggles six months later.

This guide explains what BOD and COD actually measure, how to test them correctly, what the typical values are for different Indian industries, and — most importantly — how to use these numbers to choose the right ETP technology and size it properly.

What BOD and COD Actually Measure

Both BOD and COD measure organic pollution in wastewater, but they do it in fundamentally different ways. Understanding the difference is critical before you hand over test results to your ETP designer.

BOD — Biochemical Oxygen Demand

BOD measures how much oxygen microorganisms need to break down the biodegradable organic matter in your wastewater. Think of it this way: if you dumped your factory’s wastewater into a river, BOD tells you how much dissolved oxygen the bacteria would consume while digesting the organic waste. The higher the BOD, the more food for microorganisms, and the more oxygen they consume — which is exactly what suffocates fish and kills aquatic life.

The standard test is BOD&sub5; — measured over 5 days at 20°C. The sample is incubated in the dark, and the dissolved oxygen is measured at the start and at the end. The difference is your BOD. It only captures the biodegradable fraction of organic matter, which is why it is the single most important number for sizing biological treatment systems like activated sludge, MBBR, or SBR.

COD — Chemical Oxygen Demand

COD measures the total oxygen required to chemically oxidise all organic matter in the wastewater — both biodegradable and non-biodegradable. It uses a strong chemical oxidant (potassium dichromate in sulphuric acid) to break down everything. The result is always higher than BOD because it captures compounds that bacteria cannot easily break down — oils, greases, certain synthetic chemicals, and recalcitrant organics.

COD is faster to measure (2–3 hours vs. 5 days for BOD), which makes it useful for daily process monitoring. It also tells you how much of your wastewater’s pollution load is non-biodegradable — a critical factor in deciding whether you need chemical treatment or advanced oxidation in addition to biological treatment.

Testing Methods

BOD&sub5; Test (IS 3025 Part 44)

1. Sample collection — Collect a composite sample over 8–24 hours (not a grab sample) in a clean glass bottle with no air headspace
2. Dilution — Dilute the sample with aerated dilution water containing phosphate buffer, MgSO&sub4;, CaCl&sub2;, and FeCl&sub3; as nutrient supplements
3. Seed — If the wastewater lacks microorganisms (e.g. highly acidic or toxic effluent), add a seed culture from settled domestic sewage or acclimatised sludge
4. Incubation — Seal the bottles and incubate at 20°C ± 1°C for 5 days in the dark
5. Measurement — Measure dissolved oxygen (DO) on Day 0 and Day 5 using the Winkler method or a DO meter
6. Calculation — BOD&sub5; (mg/L) = (DO&sub0; − DO&sub5;) × Dilution Factor

COD Test (IS 3025 Part 58 / Closed Reflux Method)

1. Sample preparation — Homogenise the sample, filter if required to remove large solids
2. Digestion — Add known excess of potassium dichromate (K&sub2;Cr&sub2;O&sub7;) and concentrated sulphuric acid with silver sulphate catalyst. Heat at 150°C for 2 hours in a COD reactor (closed reflux method)
3. Titration or spectrophotometry — After digestion, determine the unreacted dichromate by titrating with ferrous ammonium sulphate (FAS), or read absorbance on a spectrophotometer
4. Calculation — COD (mg/L) = (Blank FAS − Sample FAS) × Normality of FAS × 8000 / Sample Volume (mL)

Typical BOD and COD Values by Industry

These are the ranges we encounter in practice across Indian installations. Your actual values will depend on production processes, raw materials, water usage, and housekeeping practices. Always verify with your own composite sampling before design.

Why the BOD:COD Ratio Matters for ETP Design

The ratio of BOD to COD is arguably more important than either number alone. It tells your ETP designer how much of the organic pollution can be removed by biological treatment — and how much will need chemical or advanced treatment.

How to Use BOD and COD Data for Technology Selection

Once you have reliable BOD and COD numbers, here is how to translate them into design decisions:

Step 1: Determine the Organic Load

Calculate the daily organic load in kilograms:

• BOD load (kg/day) = BOD (mg/L) × Flow (m³/day) ÷ 1000
• COD load (kg/day) = COD (mg/L) × Flow (m³/day) ÷ 1000

Example: A dairy generating 50 KLD with BOD of 1,800 mg/L has a BOD load of 1,800 × 50 / 1000 = 90 kg BOD/day. This number directly determines the size of your aeration tank and the blower capacity.

Step 2: Check the BOD:COD Ratio

If the ratio is above 0.5, a straightforward biological system will work. If below 0.3, you need to plan for pre-treatment (physico-chemical) to remove the non-biodegradable COD before the biological stage. If you are between 0.3 and 0.5, extended aeration or a two-stage biological system may be required.

Step 3: Select the Treatment Technology

Step 4: Size the System

For biological systems, the key sizing parameters derived from BOD include:

• Aeration tank volume = BOD load ÷ volumetric loading rate (typically 0.5–1.5 kg BOD/m³/day for ASP)
• Air requirement = approximately 50–80 m³ of air per kg of BOD removed (for diffused aeration)
• Sludge production = approximately 0.3–0.5 kg TSS per kg of BOD removed
• Nutrient requirement = BOD:N:P ratio of 100:5:1 (check if your wastewater is nutrient-deficient)

Sampling Best Practices

Your ETP design is only as good as your wastewater data. Here is how to collect representative samples:

• Always composite, never grab. Collect samples every 2–4 hours over a full production cycle (minimum 8 hours, ideally 24 hours). Mix proportionally. A single grab sample on a slow production day will undersize your ETP.
• Sample during peak production. If your factory has seasonal peaks (e.g. mango processing season, festival-period textile orders), sample during the peak. Design for the worst case, not the average.
• Use proper containers. Glass bottles for BOD (no air bubbles), polyethylene for COD. Clean, acid-washed containers only.
• Preserve and transport correctly. BOD samples must be analysed within 24 hours and kept at 4°C during transport. COD samples can be preserved with sulphuric acid (pH < 2) for up to 28 days.
• Sample at the right point. Collect from the final combined drain before any treatment, not from individual process streams unless you are troubleshooting a specific process.
• Test at least three times. One set of results is unreliable. Sample on three different production days to establish a range, then design for the 90th percentile value.

Common Testing Errors to Watch For

1. Chloride interference in COD — If the lab does not add mercuric sulphate to suppress chlorides, COD values for textile and tannery wastewater can be 30–50% higher than actual. Ask your lab about their chloride masking protocol.
2. Nitrification in BOD — If the sample contains nitrifying bacteria, BOD&sub5; includes nitrogenous oxygen demand, inflating the value. Use a nitrification inhibitor (2-chloro-6-trichloromethyl pyridine) if you want carbonaceous BOD only (CBOD).
3. Toxic inhibition in BOD — Wastewater containing heavy metals, biocides, or high concentrations of certain chemicals may suppress microbial activity. This gives falsely low BOD values and a deceptively low BOD:COD ratio. If your COD is high but BOD seems unusually low, consider a toxicity-adjusted BOD test.
4. Improper dilution — Too little dilution means all DO is consumed before Day 5 (reported as “BOD exceeded range”). Too much dilution means the DO change is within measurement error. Experienced labs use multiple dilutions.
5. Temperature control failure — BOD incubation must be at 20°C ± 1°C. Indian labs without proper BOD incubators sometimes run the test at ambient temperature (25–35°C), which accelerates microbial activity and inflates BOD values.
6. Sample deterioration — Sending samples by courier without ice packs or leaving them at room temperature for 48 hours before testing renders BOD results meaningless. Always confirm sample receipt conditions with your lab.

CPCB Discharge Standards for BOD and COD

The general discharge standards set by the Central Pollution Control Board (CPCB) under the Environment (Protection) Rules, 1986 are:

* Not specified in general standards; check industry-specific norms and state PCB conditions. Many SPCBs impose COD limits of 150–250 mg/L for inland discharge.

For specific industries, CPCB has notified stricter standards. For example:

• Sugar industry: BOD 30 mg/L, COD 250 mg/L
• Textile industry: BOD 30 mg/L, COD 250 mg/L (revised 2016 norms)
• Tannery: BOD 30 mg/L, COD 250 mg/L, with additional Cr(VI) limits
• Pharma: BOD 30 mg/L, COD 250 mg/L (bulk drug sector has specific limits)
• Dairy: BOD 30 mg/L, COD 250 mg/L for discharge to inland surface water

Cost of BOD and COD Testing in India

When to Hire a Consultant vs. Testing In-House

Do It In-House If:

• You already have a running ETP and need routine monitoring (daily COD checks)
• Your wastewater composition is stable and well-characterised
• You have a trained chemist or operator who can handle the test procedures
• You only need COD (the quick test) for process control — portable COD kits cost ₹15,000–40,000 and can give results in 2 hours

Hire a Consultant If:

• You are designing a new ETP or expanding an existing one — accurate baseline data is critical
• Your wastewater has complex or variable composition (multiple process streams, batch operations)
• You need a treatability study (jar tests, pilot plant trials) to confirm technology selection
• You are dealing with a SPCB closure notice and need emergency characterisation and remediation
• You need to identify which process stream is contributing the most to pollution load

A good consultant will not just hand you test results. They will interpret the data, identify opportunities for source reduction (reducing pollution at the process level before it reaches the ETP), and help you choose the most cost-effective treatment technology. The cost of a proper wastewater audit (₹50,000–2,00,000 depending on complexity) is a small fraction of the capital cost of the ETP it informs.