Industrial Pollutants Explained: COD, BOD, TSS & PM (A Practical Guide)

Based on 25+ years of field experience from Perfect Pollucon Services and practical insights for junior EHS professionals.

Introduction: “The Four Numbers That Decide Whether Your Plant Is Compliant or Not”

Whether you run an ETP, STP, boiler, furnace, DG set, or any industrial operation, you will always find these four parameters in your Consent to Operate (CTO):

• COD (Chemical Oxygen Demand)
• BOD (Biochemical Oxygen Demand)
• TSS (Total Suspended Solids)
• PM (Particulate Matter – PM10/PM2.5/SPM)

These are not just “lab values.” They are the vital signs of your industry - like blood pressure, pulse, and oxygen saturation for the human body.

If these four parameters are under control:

✔ Your ETP/STP performs smoothly.

✔ SPCB monitoring visits become stress-free.

✔ You avoid non-compliances, show-cause notices, and penalties.

✔ Your management sees you as a reliable EHS professional.

But when they spike, even by a small margin, the entire plant feels the pressure - operators panic, management questions begin, and compliance fear spreads.

This guide explains it all - in simple, Indian English, with real experience, real failures, and real solutions.

The Cheat Sheet: Pollutants at a Glance

Note: Limits vary by industry category, receiving body, and CTO conditions. Always follow your consent limits first.

COD, BOD, TSS & PM Full Forms (Quick Definitions)

COD (Chemical Oxygen Demand): Total oxygen required to chemically oxidize all oxidizable pollutants in wastewater.
BOD (Biochemical Oxygen Demand): Oxygen required by microorganisms to break down biodegradable pollutants.
TSS (Total Suspended Solids): Undissolved particles floating in water that can be removed by settling/filtration.
PM (Particulate Matter): Fine solid particles suspended in air (PM10 and PM2.5 are the most tracked sizes).

EHSShala Tip: If you can explain these four lines to a plant manager in 60 seconds, you are already ahead of most EHS officers.

Read more about EHS Glossary - Key Environmental Health & Safety Terms

1. COD (Chemical Oxygen Demand)

COD is one of the first parameters every industry looks at during ETP design, SPCB sampling, and daily monitoring. But most beginners only know the textbook line: "COD is the amount of oxygen required to chemically oxidize organic and inorganic matter."

This definition is correct - but useless for real-world troubleshooting.

1.1 What COD Actually Means in Real Life

Think of COD as “Total food load (good + bad) in the wastewater.”

Your bacteria in the aeration tank can only “eat” the good part. COD includes everything, even the tough, non-biodegradable pollutants.

• COD includes: Biodegradable organics, non-biodegradable organics, toxic chemicals, and refractory compounds.
• BOD includes: Only biodegradable material.

1.2 Why SPCB Cares So Much About COD

Because COD tells the State Pollution Control Board:

1. How polluted your effluent is.
2. Whether you are shocking your biological treatment.
3. If upstream process controls are working.

Note: If COD fluctuates wildly, SPCB suspects batch discharge or improper equalization. COD = Pollution Load = Risk Indicator.

1.3 Why COD Suddenly Becomes High (Troubleshooting Checklist)

This is the section junior EHS professionals search for when panic hits.

• 🔴 Sudden process discharge: Dye, pharma, resin, or solvent contamination causes COD to skyrocket.
• 🔴 Poor equalization: Without proper mixing, high-COD pockets hit the biological tank and shock the microbes.
• 🔴 Refractory chemicals: Dyes, Phenolics, Surfactants, or Pesticides do not degrade easily.
• 🔴 Low Dissolved Oxygen (DO): If DO < 2 mg/L, bacteria slow down, and COD remains untreated.
• 🔴 Toxic shock load: Acids, alkalis, or biocides can kill MLSS instantly.

• What is the Difference Between COD and TSS?

• COD measures pollution load (chemical oxygen demand) - even if water looks clear.
• TSS measures solid particles - even if COD is under control.
• A plant can have:
• Low TSS but high COD (clear water but chemically polluted), or
• Low COD but high TSS (chemically treated, but solids carryover/clarifier problem).

1.4 Senior-to-Junior Summary

"COD tells you the total pollution your plant is throwing out - good + bad. If COD is high, everything collapses: aeration, MLSS, clarity, BOD. Control COD at the source, and half your ETP headache disappears."

ETP & STP Failures and Troubleshooting from Industry Experts

2. BOD (Biochemical Oxygen Demand)

If COD is the total load, BOD is the biodegradable part - the part your microbes can actually “eat.”

2.1 What BOD Really Represents

Think of BOD as “The portion of pollution your bacteria WANT to eat.”

This is why BOD is the heartbeat of your ETP.

• Consistent BOD: Aeration works, MLSS is healthy, sludge settles well.
• Spiking BOD: Shock loads, foam formation, MLSS death.

2.2 BOD Testing in India (Important Clarification)

There is massive confusion here.

• IS 3025 (Part 44) specifies BOD5 at 20°C (International standard).
• Indian CPCB Manuals traditionally use BOD3 at 27°C (Tropical standard).
• Tip: Almost all Indian labs follow BOD3 at 27°C unless specified otherwise.

2.3 Why BOD Spikes Suddenly

• 🔴 High organic load: Food waste, starch, sugar, or pharma intermediates.
• 🔴 Toxic chemicals: Solvents or biocides kill the bacteria, stopping BOD removal.
• 🔴 Nutrient Imbalance: Bacteria need Nitrogen (N) and Phosphorus (P). If you don't add Urea/DAP, BOD removal fails.

3. The COD/BOD Ratio - The "Golden Ratio" of ETPs

This is the most important diagnostic tool in ETP operations. Most blogs never mention this, but every experienced operator depends on it.

3.1 What the Ratio Tells You

It acts like an ECG for your plant. It tells you if biological treatment is even possible.

3.2 Ideal COD/BOD Ratios

3.3 What to Do If the Ratio Is High?

If you have a ratio of 5 or 6, stop looking at the aeration tank. The problem is upstream.

• Add chemical pre-treatment (coagulation/oxidation).
• Identify and segregate toxic solvent streams.
• Use advanced oxidation (Fenton/Ozonation) for refractory loads.

Senior Note: "The COD/BOD ratio tells you whether the ETP will ever work - before you even switch on the aeration blower."

How COD, BOD and TSS Are Related (The Plant Reality)

In real plants, these three parameters move like a chain:

• COD and BOD tell you how much pollution load is present (COD total, BOD biodegradable).
• TSS tells you how well solids are being removed (settling + filtration discipline).

The common pattern you’ll see:

• If COD/BOD is high, the wastewater is harder to treat biologically → COD stays high even after aeration.
• If TSS is high, your treated sample looks cloudy → even if BOD is controlled, your outlet still fails visually.

Think like this: COD/BOD is “treatment difficulty,” TSS is “clarity & discipline.” Both decide compliance.

4. TSS (Total Suspended Solids)

TSS looks simple... until it chokes your filters and ruins your compliance report.

What is TSS in Wastewater? (Simple Meaning)

TSS in wastewater means the total amount of undissolved solid particles present in the water. These particles can be biological (sludge), physical (sand/grit), or chemical (precipitates).
If your treated effluent is cloudy, TSS is usually the first suspect.

4.1 What TSS Actually Means

“Whatever you can see as cloudiness or turbidity in water is mostly TSS.”

It includes sludge flocs, sand, grit, fiber, and chemical precipitates. SPCB officers look at clarity first - if the sample is cloudy, you are already in trouble.

4.2 TSS vs TDS (The Beginner Confusion)

• TSS (Total Suspended Solids): Particles NOT dissolved. Visible. Removable by settling/filtration.
• TDS (Total Dissolved Solids): Salts/minerals dissolved. Invisible. Removable only by RO/Evaporation.

4.3 Why TSS Suddenly Increases (Troubleshooting)

• 🔴 Clarifier Malfunction: Poor settling, rising sludge, or hydraulic overload.
• 🔴 Sludge Bulking: Filamentous bacteria cause loose sludge that floats out.
• 🔴 Filter Bypass: Sand/Carbon filters are choked or channeling.
• 🔴 Pin Flocs: Wrong coagulant dosing in primary treatment.

• What is the Relationship Between BOD and TSS?

• BOD and TSS are linked through biological treatment:
• If your aeration tank biology becomes unhealthy (shock load, low DO, nutrient imbalance), sludge settles poorly → TSS increases in the outlet.
• If TSS increases, it can carry untreated organic matter with it → BOD and COD may also rise.
• In simple terms:
  Unstable biology → poor settling → high TSS → higher risk of BOD/COD failure.

4.4 The Simple "Beaker Test"

Take a 1-liter beaker of treated effluent and let it sit for 30 mins.

• Water stays cloudy? TSS issue (Check Coagulation/Flocculation).
• Solids settle to bottom? Clarifier/Filter issue.
• Crystal clear but salt content high? TDS issue.

5. PM (Particulate Matter – PM10 / PM2.5)

We now switch from Water (ETP) to Air (Stacks & Ambient).

PM affects public health and visible emissions. If your PM spikes, the notices come fast because the public can see it.

5.1 What is PM?

• PM10: Dust smaller than 10 micrometers (Respirable).
• PM2.5: Dust smaller than 2.5 micrometers (Fine dust, enters bloodstream).

Getty Images

5.2 Real Industrial Sources

• Stack Emissions: Boiler ash, coal combustion, DG set exhaust.
• Process Dust: Grinding, crushing, powder mixing.
• Fugitive Emissions: Coal handling yards, road dust, loading areas.

5.3 Why PM Spikes (The Checklist)

• 🔴 Leaking Bag Filters: Even one torn bag releases a visible plume.
• 🔴 Hopper Jams: Dust re-enters the gas stream if hoppers aren't emptied.
• 🔴 Pulse-Jet Failure: Bags get clogged, pressure drops, and dust escapes.
• 🔴 Poor Combustion: Unburnt carbon (black smoke) increases PM.

5.4 Senior-to-Junior Summary

"PM is the parameter everyone can see. If your bag filter, housekeeping, and combustion are in control, you will never fear air monitoring again."

6. Common Mistakes & EHSShala Tips

These tips will save you from 90% of violations.

Common Mistakes

1. Not mixing the EQ Tank: Causes shock loads downstream.
2. Not preserving BOD samples: Samples degrade if not kept at 4°C immediately.
3. Wrong Stack Nozzle: Using the wrong nozzle size during stack monitoring leads to non-isokinetic sampling and false high readings. (Learn Isokinetic Sampling Stack Monitoring)
4. Diluting Samples: Never, ever dilute samples to "pass" a limit. SPCB labs can detect it, and it destroys your credibility.

Top 5 EHSShala Tips

• Tip #1: Maintain DO above 2 mg/L. Below this, bacteria sleep, and BOD rises.
• Tip #2: Check the COD/BOD Ratio. Do this before you try to fix the aeration tank.
• Tip #3: Backwash Filters. Most TSS violations are due to lazy filter maintenance.
• Tip #4: Watch the Road Dust. Ambient PM10 often spikes because of dirty plant roads, not the stack.
• Tip #5: Record Everything. SPCB respects documentation.

• CPCB General Standards: https://cpcb.nic.in
• MoEFCC Notifications: https://moef.gov.in
• BIS Standards Index: https://bis.gov.in

COD/BOD/TSS/PM Quick Notes (Printable Section)

If you want a quick revision before an inspection, use this:

• COD: Total pollution load (biodegradable + non-biodegradable).
• BOD: Biodegradable load (bacteria food).
• COD/BOD ratio: Shows whether biological treatment will work.
• TSS: Solids carryover/settling/filtration discipline indicator.
• PM: Dust emissions from stacks + fugitive sources (bag filters/housekeeping decide outcomes).

Tip: Many EHS teams print this section and keep it near the ETP logbook.

7. Frequently Asked Questions (FAQs)

Q: What is BOD, COD and TSS?
A: COD shows total pollution load, BOD shows biodegradable load, and TSS shows undissolved solids/clarity. Together they decide ETP performance and compliance.

Q: What is TSS and COD?
A: COD measures chemical pollution load, TSS measures solid particles in water. A sample can fail due to either chemical load (COD) or solids carryover (TSS).

Q: What is TSS in wastewater?
A: TSS is the amount of undissolved solids in wastewater (sludge, grit, precipitates). High TSS makes water look cloudy and often indicates clarifier/filter issues.

Q: What is the relationship between BOD and TSS?
A: Poor biological health causes poor settling, which increases TSS. High TSS can also carry organics, increasing the risk of higher BOD/COD in treated effluent.

Q: What is the difference between COD and BOD?

A: COD is the total pollution (everything oxidizable). BOD is only the biodegradable pollution. COD is always higher.

Q: Why is my COD high even after aeration?

A: You likely have "refractory" (non-biodegradable) pollutants, or your bacteria are dead due to toxic shock.

Q: Can I meet PM limits without a Bag Filter?

A: For coal or biomass boilers in India? No. You need an ESP or Bag Filter to meet the strict 50–100 mg/Nm³ limits.

Q: What causes PM10 to increase in ambient air?

A: Usually road dust, raw material handling, and fugitive emissions-not just the boiler stack.