BOD and COD tests explained: methods, calculations and limits

What is the difference between BOD and COD?

BOD is a biological test that takes days; COD is a chemical test that takes hours. BOD tells you how much oxygen bacteria will consume, while COD tells you how much oxygen a strong chemical oxidant consumes.

Because dichromate oxidises many substances that bacteria cannot break down, COD is normally higher than BOD for the same sample. That is why IS 3025 (Part 44) suggests estimating the expected BOD from the COD before choosing dilutions.

Which methods apply to BOD and COD tests in India?

The Indian methods are IS 3025 (Part 44) for BOD and IS 3025 (Part 58) for COD. The BIS portal shows that both were revised in 2023: IS 3025 (Part 44):2023 supersedes the 1993 edition, and IS 3025 (Part 58):2023 supersedes the 2006 edition.

The procedural details below are taken from the 1993 and 2006 editions, which are publicly available. If your NABL scope cites the 2023 editions, check each step against the current text before you change your SOP.

Many Indian labs also use APHA Standard Methods. APHA 5210 B measures "oxygen consumed in a 5-d period", and APHA 5220 lists three dichromate methods: 5220 B and C are titrimetric, and 5220 D is the closed reflux colorimetric method.

For compliance reports, note that the E(P) Rules were amended in 1996 (G.S.R. 176) so that "BOD 5 days 20 °C" is read as "BOD (3 days at 27 °C)". A 5-day, 20 °C result is not the parameter Schedule VI sets a limit for. The foreword to IS 3025 (Part 44) explains the choice: "The average aquatic temperature in our country is around 27°C."

What is the BOD test procedure?

The BOD test procedure dilutes the sample with aerated, nutrient-rich water, measures dissolved oxygen (DO), incubates sealed bottles and measures DO again. The steps below follow IS 3025 (Part 44):1993.

1. Prepare dilution water: aerate distilled water for 8 to 12 hours, let it stabilise for about 4 hours at about 27 °C, and add 1 mL each of phosphate buffer, magnesium sulphate, calcium chloride and ferric chloride solutions per litre.
2. Seed the dilution water if the sample needs it, using 2 to 5 mL of treated sewage per litre or a commercial microbial seed (Amendment 1). The standard says seeding is not required for domestic sewage or surface water.
3. Neutralise the sample to about pH 7.0.
4. Choose dilutions from the expected BOD: mL of sample per litre = X / expected BOD × 1000, with X of 2.5 and 4.0 for two dilutions, or 3.0 to 3.5 for one.
5. Fill BOD bottles with each dilution and with blanks, and measure the initial DO.
6. Incubate at 27 ± 1 °C for 3 days.
7. Measure the final DO and calculate.

APHA 5210 B uses 5 days at 20 ± 1 °C, incubated in the dark. A North Carolina DEQ checklist for SM 5210 B-2016 asks that only dilutions that "deplete at least 2.0 mg/L DO and have at least 1.0 mg/L DO remaining" are averaged. The 1993 IS text does not state this rule, so check what your method edition requires.

How is BOD calculated?

IS 3025 (Part 44):1993 gives three cases, where D1 and D2 are the DO of the diluted sample before and after incubation, B1 and B2 are the same for the seed control, f is the seed ratio and P is the mL of sample diluted to 1 litre:

• Undiluted sample: BOD = DO before incubation minus DO after incubation.
• Unseeded dilution water: BOD, mg/L = (D1 − D2) / P × 1000.
• Seeded dilution water: BOD, mg/L = [(D1 − D2) − (B1 − B2) f] / P × 1000.

Report BOD as "mg/L, 3 days at 27 °C". The 1993 edition reports values up to 10 to one decimal and values above 10 as whole numbers.

How do you check the BOD method?

Run a glucose-glutamic acid check with each batch. In IS 3025 (Part 44):1993, a mixture of 150 mg/L glucose and 150 mg/L glutamic acid is run at a 2 percent dilution; "If the BOD value of the check is outside the range of 200 ± 37 mg/l, reject any BOD determinations". For the 5-day APHA test, US regulations (40 CFR 136.3, footnote 85) allow acceptance limits no wider than 198 ± 30.5 mg/L.

What is the COD test procedure?

The COD test procedure refluxes the sample with a known excess of potassium dichromate in strong sulphuric acid, then titrates the dichromate left over with ferrous ammonium sulphate (FAS). The steps below follow IS 3025 (Part 58):2006 for samples with COD of 50 mg/L or more.

1. Put about 0.4 g of mercuric sulphate into the reflux tube to complex chloride ("About 0.4 g HgSO4 is adequate to complex 40 mg Cl ions").
2. Add 20 mL of sample, then 10 mL of 0.25 N potassium dichromate.
3. Add 30 mL of sulphuric acid containing silver sulphate (22 g per 4 kg of acid), slowly and with mixing.
4. Reflux for 2 hours at 150 ± 2 °C with a condenser.
5. Cool, dilute and titrate the excess dichromate with FAS using ferroin indicator.
6. Run a blank with distilled water in place of the sample through every step.

For low-COD samples up to 160 mg/L, the 2006 edition uses 0.05 N dichromate and 0.02 N FAS.

How is FAS standardised?

IS 3025 (Part 58):2006 says of the FAS solution: "Standardize this solution daily against standard K2Cr2O7." Titrate 10 mL of 0.25 N dichromate with FAS, then:

Normality of FAS = 10 × 0.25 / V, where V is the mL of FAS used.

If 10 mL of dichromate takes 25.05 mL of FAS, the normality is 2.5 / 25.05 = 0.0998 N.

How is COD calculated? A worked example

COD, mg/L = (V1 − V2) × N × 8000 / V0, where V1 is the FAS used for the blank, V2 the FAS used for the sample, N the FAS normality and V0 the sample volume. The 2006 edition reports COD to the nearest whole number.

1. Net titre = 24.6 − 16.6 = 8.0 mL.
2. 8.0 × 0.0998 = 0.7984.
3. 0.7984 × 8000 = 6387.2.
4. 6387.2 / 20 = 319.36, reported as 319 mg/L.

If the effluent was diluted before analysis, multiply by the dilution factor. Against the Schedule VI limit of 250 mg/L for inland surface water, this sample does not comply.

Now see why standardisation matters. Suppose the FAS bottle is labelled 0.1 N but has weakened to 0.0950 N. With the label value the result is 8.0 × 0.1 × 8000 / 20 = 320 mg/L; with the true value it is 304 mg/L. Every sample on that worksheet carries the same error.

What are the BOD and COD limits for effluent discharge?

Schedule VI of the Environment (Protection) Rules, 1986 sets general standards by discharge point. The COD limit for effluent is 250 mg/L for inland surface water and marine coastal areas, with no COD limit for public sewers or land for irrigation.

Two cautions apply. Under rule 3(3A) of the E(P) Rules, Schedule VI applies to industries not covered by an industry-specific standard in Schedule I. And under rules 3(2) and 3(3A), the CPCB or a State Board "may specify more stringent standards" for specific industries or locations, so a unit's consent order can carry tighter limits than this table.

How long can BOD and COD samples be held?

Start BOD as soon as possible, because the sample changes while it waits. IS 3025 (Part 1):1987 gave BOD 24 hours when cooled to 2 to 5 °C in the dark. For COD, it gave 2 days when acidified to pH below 2, and 1 month frozen at −20 °C.

That standard has been superseded by IS 17614 (Part 1):2021, so confirm the current values in your method. For comparison, US regulations (40 CFR 136.3, Table II) allow BOD 48 hours cooled to 6 °C or below, and COD 28 days cooled and acidified with sulphuric acid to pH below 2.

Whatever limit you adopt, record the date and time of sampling on the inward record. Without it, you cannot show that the holding time was met.

What mistakes cause wrong BOD and COD results?

Most wrong BOD and COD results come from a handful of avoidable slips:

• Using the label strength of FAS instead of today's standardised normality.
• Skipping mercuric sulphate on high-chloride samples, so chloride is counted as COD.
• A sample titre too close to the blank, which means the dichromate was nearly used up; repeat with a smaller volume or a dilution.
• Forgetting the dilution factor after diluting a strong effluent.
• BOD dilutions that deplete too little or too much oxygen, so no dilution gives a valid result.
• Not seeding industrial effluents that lack a microbial population.
• Reporting 5-day, 20 °C BOD against a Schedule VI limit written for 3 days at 27 °C.
• Skipping the glucose-glutamic acid check, so a weak seed goes unnoticed.
• Missing the holding time because the sampling time was never recorded.
• Judging against the wrong discharge point, for example a sewer discharge against the inland surface water limit.