BOD, COD, TSS and TDS in ETP Design: What Each Parameter Changes

When an industrial ETP fails a laboratory test, four parameters appear repeatedly in discussions with plant teams:

BOD, COD, TSS and TDS.

They are often spoken about together, but they represent very different wastewater characteristics.

A high BOD result may point toward inadequate biological treatment.

High COD may indicate biodegradable load, refractory organics, oils, chemicals, concentrated process wastewater or untreated bypasses.

High TSS usually indicates a solids-separation problem.

High TDS is fundamentally different because dissolved salts cannot be removed by ordinary clarification or biological treatment.

Understanding these differences matters for both ETP design and Pollution Control Board compliance.

An industry whose ETP outlet exceeds its Consent to Operate conditions should not simply increase chemical dosing or install another filter. The correct response is to determine which parameter has failed, what wastewater source creates it, and which treatment stage should control it.

Under India's consent framework, State Pollution Control Boards administer consent requirements under the Water (Prevention and Control of Pollution) Act, 1974 and Air (Prevention and Control of Pollution) Act, 1981. The OCMMS framework notes that Water Act consent applies to industries, processes, treatment/disposal systems and relevant new or altered sewage/trade-effluent discharges.

For industries facing an outlet failure, SARK Engineers & Consultants supports ETP troubleshooting and pollution-control assessment, water balance review, ETP augmentation and compliance-linked technical diagnosis.

Quick Answer: What Do BOD, COD, TSS and TDS Tell You?

BOD indicates the biodegradable organic load that microorganisms may consume.

COD indicates the oxygen-equivalent demand associated with chemically oxidisable substances and is generally broader than BOD.

TSS measures suspended material that can often be removed through settling, clarification or filtration.

TDS represents dissolved salts and other dissolved substances that remain after suspended solids are separated.

In practical ETP design:

Parameter	Main treatment implication
BOD	Biological treatment capacity
COD	Source control + chemical/biological/advanced treatment
TSS	Clarification and filtration
TDS	Membrane recovery, segregation or evaporation
BOD/COD ratio	Biodegradability indication
High TSS + COD	Solids may be carrying organic load
High TDS + COD	Conventional ETP may not be enough

The four parameters should therefore never be treated as four versions of the same pollution problem.

What Is BOD?

Biochemical Oxygen Demand

BOD indicates the oxygen consumed by microorganisms while biologically degrading biodegradable organic matter under specified test conditions.

For industrial wastewater, BOD can originate from:

• Food residues

• Sugars and starches

• Biodegradable process chemicals

• Organic washing streams

• Pulp and paper process water

• Domestic sewage

• Fermentation residues

• Biodegradable oils and organics

A wastewater stream with substantial BOD generally requires biological treatment unless the source can first be eliminated, recovered or segregated.

How BOD Affects ETP Design

Higher BOD can affect:

• Aeration requirement

• Biological reactor volume

• Oxygen-transfer demand

• Biomass concentration

• Sludge generation

• Hydraulic retention time

• Food-to-microorganism ratio

• Nutrient requirement

• Secondary clarifier loading

A biological ETP designed for 500 mg/L BOD cannot automatically be expected to perform satisfactorily if production changes push the actual inlet to 1,500–2,000 mg/L.

The important design quantity is not concentration alone.

It is pollution load.

For example:

Case A

Flow = 100 m³/dayBOD = 500 mg/L

Approximate BOD load:

50 kg/day

Case B

Flow = 50 m³/dayBOD = 1,000 mg/L

Approximate BOD load:

50 kg/day

The concentration is twice as high in Case B, but the total BOD load is similar.

This is why an ETP should be reviewed using both flow and concentration.

What Is COD?

Chemical Oxygen Demand

COD is a broader indicator of oxidisable pollution in wastewater.

Industrial COD may arise from:

• Organic chemicals

• Oils

• Coolants

• Dyes

• Solvents

• Process additives

• Surface-treatment chemicals

• Cleaning compounds

• Pulping chemicals

• Product losses

• Concentrated washings

• Domestic organic matter

Some COD may be readily biodegradable.

Some may biodegrade slowly.

Some may be refractory or toxic to the biological treatment system.

Therefore:

High COD does not automatically mean the aeration tank needs more air.

The source and nature of the COD must first be established.

Why COD Is So Important in Pollution Control Board Compliance

COD is frequently used as an effluent-performance indicator because it provides a broad measure of oxidisable pollution.

When an SPCB laboratory report or consent-monitoring report shows high outlet COD, the industry should investigate:

• Actual inlet COD

• Source-wise COD contribution

• BOD/COD relationship

• Oil and grease

• Batch cleaning

• Concentrated chemical streams

• Biological performance

• Equalisation

• Sludge carryover

• Tertiary treatment

• Bypass or overflow

Do not assume that one numerical COD limit applies to every industrial facility.

Applicable limits may depend on the industry-specific standards, discharge route and the individual consent conditions. CPCB publishes both general and industry-specific effluent/emission standards, while State Boards incorporate applicable requirements through their consent mechanisms.

BOD vs COD: Why the Relationship Matters

One of the most useful diagnostic indicators is the relationship between BOD and COD.

A simple ratio is often used as an indication of biodegradability:

BOD / COD

A comparatively higher ratio may indicate that a meaningful portion of the organic load can be treated biologically.

A very low ratio can suggest:

• Refractory organics

• Toxic chemicals

• Non-biodegradable COD

• Industrial chemical contribution

• High oxidation-resistant load

However, the ratio is a screening indicator rather than a complete treatment design criterion.

For industrial wastewater, actual treatability trials may still be required.

Example

Suppose:

COD = 2,000 mg/LBOD = 1,000 mg/L

BOD/COD = 0.50

Compare this with:

COD = 2,000 mg/LBOD = 200 mg/L

BOD/COD = 0.10

The COD concentration is identical, but the treatment challenge can be completely different.

The second wastewater may require much more emphasis on:

• Source segregation

• Chemical treatment

• Oxidation

• Adsorption

• Recovery

• Specialised treatment

rather than simply increasing biological reactor capacity.

What Is TSS?

Total Suspended Solids

TSS represents suspended particles present in wastewater.

Industrial TSS may include:

• Metal particles

• Fibres

• Pulp

• Process solids

• Precipitated chemicals

• Biological flocs

• Soil

• Grinding solids

• Paint solids

• Chemical sludge

• Product particles

Unlike TDS, suspended solids can often be separated physically.

Treatment may involve:

• Screening

• Settling

• Clarification

• Coagulation

• Flocculation

• DAF

• Tube settlers

• Sand filtration

• Membrane filtration

Why High Outlet TSS Does Not Necessarily Mean Poor Inlet Treatment

An ETP can remove dissolved pollution effectively but still fail TSS because of downstream solids carryover.

Common causes include:

• Poor clarifier settling

• High hydraulic loading

• Excess sludge blanket

• Bulking sludge

• Pin flocs

• Rising sludge

• Improper polymer dosing

• Filter breakthrough

• Excessive backwash interval

• Damaged filter media

High TSS can also increase measured COD because suspended organic solids contribute to the overall pollution load.

Therefore, if both TSS and COD rise together, the first investigation should include the clarification and filtration stages.

What Is TDS?

Total Dissolved Solids

TDS represents dissolved material in water rather than suspended particles.

Typical industrial contributors include:

• Sodium salts

• Chlorides

• Sulphates

• Calcium

• Magnesium

• Dissolved process chemicals

• Acid/alkali neutralisation products

• Cooling-tower blowdown

• Boiler blowdown

• Softener regeneration

• DM-plant regeneration

• RO reject

This creates a fundamentally different treatment challenge.

Why Conventional ETP Treatment Does Not Remove TDS Effectively

Screening does not remove dissolved salts.

Clarification does not substantially remove dissolved salts.

Biological treatment does not remove most inorganic dissolved salts.

Sand filtration does not convert dissolved salts into solids.

In fact, chemical treatment can sometimes increase TDS because acids, alkalis, coagulants and other chemicals add dissolved ions.

High-TDS wastewater may therefore require:

• Source segregation

• Process modification

• Water reuse

• Reverse osmosis

• Nanofiltration in suitable applications

• Evaporation

• MEE

• ATFD/crystallisation

• Approved off-site treatment or disposal, where applicable

This is why an industry considering ZLD should first undertake a proper water balance and ZLD feasibility assessment.

BOD, COD, TSS and TDS: What Each Changes in ETP Design

Parameter	ETP design area most affected	Typical response
BOD	Biological reactor	Aeration, biomass, HRT
COD	Entire treatment train	Source mapping + treatability
TSS	Clarifier and filtration	Settling and solids removal
TDS	Recovery/ZLD system	Segregation, RO, evaporation
Oil & grease	Pretreatment	Skimming, DAF, oil separation
Heavy metals	Chemical treatment	pH-controlled precipitation
Ammonia	Biological treatment	Nitrification
pH	Equalisation/neutralisation	Controlled chemical dosing

How These Parameters Affect State Pollution Control Board Compliance

This section is particularly important for industries searching for answers relating to:

• Pollution Control Board ETP limits

• SPCB effluent standards

• ETP outlet COD limit

• ETP outlet BOD limit

• Pollution Control Board TSS limit

• Pollution Control Board TDS limit

• CTO wastewater conditions

• CTE/CTO wastewater treatment requirements

• failed SPCB sample

• SPCB show-cause notice for ETP

• Pollution Control Board consent renewal

• ETP non-compliance

State Boards issue and administer consent under the statutory framework, while their online procedures, checklists, categorisation systems and consent conditions can differ from state to state. Rajasthan, for example, publishes separate procedures for CTE and CTO as well as checklists and consent fee information; its official CTO guidance states that consent is required before commencement of operation under the Water and Air Acts.

Haryana likewise maintains its own consent policy and consent procedures under the Water and Air Acts.

Therefore:

The applicable effluent standard should always be checked against the unit's current CTO, applicable industry-specific standards and approved discharge route—not copied from a generic online table.

Searching for UPPCB, RSPCB, HSPCB or Other State Board ETP Limits?

The engineering interpretation remains broadly similar even though the administrative authority changes.

An industrial unit may encounter searches such as:

Uttar Pradesh

• UPPCB ETP outlet standards

• UP Pollution Control Board COD limit

• UPPCB BOD limit

• UPPCB CTO effluent condition

• UPPCB ETP failure

• UPPCB show cause notice wastewater

Rajasthan

• RSPCB ETP outlet limit

• Rajasthan Pollution Control Board COD standard

• RSPCB CTO wastewater condition

• RSPCB ETP show cause

• Rajasthan Pollution Control Board consent renewal

Haryana

• HSPCB ETP outlet standards

• Haryana Pollution Control Board COD limit

• HSPCB CTO condition

• HSPCB wastewater compliance

• Haryana ETP consent renewal

Other states

The same search pattern applies to authorities such as:

• Maharashtra Pollution Control Board

• Gujarat Pollution Control Board

• Punjab Pollution Control Board

• Karnataka State Pollution Control Board

• Tamil Nadu Pollution Control Board

• Madhya Pradesh Pollution Control Board

• Bihar State Pollution Control Board

• West Bengal Pollution Control Board

• Odisha State Pollution Control Board

The safest technical approach is always to verify the current consent and official Board requirements applicable to the actual unit.

What Should an Industry Check When an SPCB Sample Fails BOD?

If BOD exceeds the applicable consent limit, investigate:

1. Biological loading

2. Dissolved oxygen

3. Blower operation

4. Diffuser condition

5. Biomass health

6. Hydraulic retention time

7. Sludge age

8. Return sludge

9. Nutrient availability

10. Toxic process streams

Also compare inlet and outlet values.

If inlet BOD has increased substantially since the ETP was designed, the problem may be inadequate design capacity rather than operator performance.

What Should an Industry Check When COD Fails?

Start with source identification.

Check:

• Product loss

• Chemical wash

• Concentrated batch discharge

• Oil/coolant

• Floor washing

• Toxic chemical streams

• RO reject mixing

• Biological-treatment failure

• Bypass

• Sludge carryover

A useful diagnostic sequence is:

Source → Equalisation → Chemical treatment → Biological treatment → Clarifier → Filter → Final outlet

This prevents the common mistake of diagnosing only the final treatment stage.

What Should an Industry Check When TSS Fails?

Inspect:

• Settling test

• Sludge blanket

• Clarifier overflow

• Hydraulic loading

• Return sludge

• Excess sludge removal

• Tube settler

• PSF

• Backwashing

• Filter media

If the outlet looks cloudy, visible solids may provide an immediate clue.

But laboratory measurement is still required.

What Should an Industry Check When TDS Fails?

Start upstream.

Ask:

• Where are salts entering the process?

• Is RO reject being mixed into the ETP?

• Is regeneration wastewater entering the ETP?

• Has chemical dosing increased?

• Is cooling-tower blowdown contributing?

• Has freshwater TDS changed?

• Is recycled water concentrating salts?

• Does the plant actually require ZLD?

Increasing biological treatment will not solve a dissolved-salt problem.

What if the SPCB Laboratory Result Is Much Worse Than the Company's Own Report?

Do not immediately assume either laboratory is wrong.

Investigate:

• Sampling location

• Sampling time

• ETP operating condition

• Production during sampling

• Sample preservation

• Composite vs grab sample

• Laboratory method

• Tank mixing

• Sludge carryover

• Recent chemical dosing

• Rainwater dilution

• Bypass condition

The correct response is technical reconciliation followed by representative resampling, not merely obtaining another favourable report.

What if a Pollution Control Board Issues a Show-Cause Notice?

A show-cause notice linked to wastewater treatment should be treated as an engineering and regulatory problem together.

The response should normally be supported by:

• Failed laboratory report

• Root-cause diagnosis

• Immediate containment action

• Corrective actions completed

• Photographic evidence

• Flow records

• Chemical-dosing records

• ETP operating data

• Independent testing

• Implementation schedule

• Responsible persons

Avoid submitting only a narrative saying:

“The ETP is now operating satisfactorily.”

A stronger response demonstrates what failed, what was corrected and how performance will be verified.

For this type of assignment, see SARK's pollution-control and regulatory support services.

Can a Plant Pass BOD and COD but Still Be Non-Compliant?

Yes.

The outlet may still fail:

• TSS

• TDS

• Oil and grease

• Metals

• Ammonia

• pH

• Chlorides

• Sulphates

• Colour

• Other industry-specific parameters

Conversely, visually clear water is not proof of compliance.

A clear sample can still contain high:

• COD

• TDS

• Chlorides

• Dissolved metals

• Ammonia

Laboratory results must therefore be interpreted together.

Why State Pollution Control Boards Look Beyond One Laboratory Number

Consent compliance is not simply a single COD or BOD result.

State Boards may examine the broader consent and pollution-control arrangement, including matters such as:

• Permitted production

• Manufacturing process

• Water consumption

• Trade-effluent generation

• Sewage generation

• ETP/STP capacity

• Air-emission sources

• Pollution-control equipment

• Waste handling

• Discharge/reuse arrangements

• Monitoring and records

The Water Act consent framework specifically covers industrial processes, treatment/disposal systems and relevant discharge outlets, which is why changes in production or wastewater configuration can become consent issues rather than only treatment issues.

When ETP Design and CTO Data No Longer Match

This is common in operating factories.

The CTO may show:

Trade effluent: 10 KLD

But actual operation may generate:

18 KLD

Or the consent may show one washing line while two additional lines have subsequently been installed.

This can affect:

• Hydraulic load

• BOD load

• COD load

• TDS

• Sludge generation

• Water consumption

• Hazardous waste

• ETP capacity

The correct approach is to reconcile:

Consent → Machinery → Production → Water balance → Wastewater → ETP capacity → Outlet quality

This is far stronger than treating an outlet exceedance as an isolated laboratory failure.

Pollution Load Matters More Than Concentration Alone

For any pollutant:

Pollution load = Flow × Concentration

Consider:

Plant A

Flow = 10 KLDCOD = 4,000 mg/L

COD load ≈ 40 kg/day

Plant B

Flow = 50 KLDCOD = 1,000 mg/L

COD load ≈ 50 kg/day

Plant A has a much higher concentration.

But Plant B actually sends more COD mass to the ETP every day.

This distinction is crucial when evaluating:

• ETP aeration

• Reactor capacity

• Chemical requirement

• Sludge generation

• expansion

• augmentation

Common Mistakes Industries Make With BOD, COD, TSS and TDS

Mistake 1: Treating COD and BOD as interchangeable

They indicate related but different wastewater characteristics.

Mistake 2: Increasing aeration for high TDS

Aeration does not remove dissolved salts.

Mistake 3: Increasing chemicals whenever COD rises

Chemicals may not remove dissolved or refractory COD.

Mistake 4: Ignoring flow

Concentration without flow does not describe total pollution load.

Mistake 5: Ignoring wastewater sources

Treatment should begin with source mapping.

Mistake 6: Using one laboratory result as the complete diagnosis

Trend data is much more useful.

Mistake 7: Copying an internet discharge limit

Use the applicable consent and official regulatory standard.

Mistake 8: Assuming clear water means compliant water

Many pollutants are dissolved and invisible.

Mistake 9: Sending every stream into one ETP

Segregation can substantially improve treatment.

Mistake 10: Modifying the plant without revising the water balance

Treatment capacity must reflect actual wastewater generation.

ETP Diagnostic Matrix

Result	Likely first concern	What to verify
High BOD	Biological failure	DO, MLSS, loading
High COD + high BOD	Organic overload	Source + biology
High COD + low BOD	Refractory load	Source segregation
High TSS	Solids carryover	Clarifier + filters
High TDS	Dissolved salts	Reject/regeneration streams
High COD + oil	Oily wastewater	Oil separation
High TSS + COD	Organic solids carryover	Clarification
High TDS after chemical treatment	Chemical salt addition	Dosing review
Variable COD	Batch discharge	Equalisation
Good outlet but occasional failure	Shock load/bypass	Hourly profiling

What Data Should Be Reviewed Before Changing the ETP?

Collect at least:

Laboratory data

• Inlet BOD

• Outlet BOD

• Inlet COD

• Outlet COD

• TSS

• TDS

• pH

• Oil and grease

• Relevant metals

• Other consent parameters

Flow data

• Average flow

• Peak flow

• Hourly generation

• Batch discharge

Process information

• Production

• Raw materials

• Chemicals

• Cleaning

• Washing

• Product change

ETP operating data

• Chemical dosing

• DO

• MLSS

• blower hours

• pump hours

• sludge withdrawal

• filter backwash

• RO recovery

Regulatory documents

• Consent to Establish

• Consent to Operate

• Latest amendments

• Laboratory reports

• Board inspection reports

• Show-cause notices, if any

How SARK Engineers & Consultants Can Help

SARK Engineers & Consultants undertakes industrial wastewater and pollution-control assessments where laboratory results, ETP performance and consent requirements need to be evaluated together.

Typical scope includes:

• BOD/COD/TSS/TDS interpretation

• Wastewater characterisation

• Source-wise pollution-load mapping

• Water balance

• ETP capacity verification

• Chemical-treatment review

• Biological-treatment assessment

• Aeration review

• Sludge diagnostics

• Tertiary-treatment assessment

• RO reject review

• ZLD feasibility

• Consent-condition reconciliation

• SPCB notice technical response

• ETP augmentation planning

• Vendor design review

For plants already experiencing outlet failure, start with ETP troubleshooting and pollution-control consulting.

For plants where wastewater generation itself is uncertain, first rebuild the industrial water balance.

For significant capacity or process changes, SARK's project assessment services can review process, utilities, wastewater and statutory implications together.

Frequently Asked Questions

What is the permissible BOD limit for an ETP outlet?

There is no single number that should be assumed for every industrial ETP. The applicable requirement depends on the industry's consent conditions, applicable standards and discharge route. Industries should verify their current CTO and the official requirements of the relevant State Pollution Control Board.

What is the permissible COD limit under Pollution Control Board consent?

The applicable COD limit can vary according to industry, discharge route and consent conditions. The value written in the unit's current CTO and applicable official standards should be used for compliance assessment.

What should I do if the Pollution Control Board sample fails COD?

First prevent non-compliant discharge where necessary, verify the result and sampling point, identify the COD source, measure inlet and stage-wise COD and inspect equalisation, chemical treatment, biology, clarification and possible bypasses.

Does high COD mean the ETP needs more aeration?

Not necessarily. High COD may be caused by refractory chemicals, oil, concentrated process streams or bypasses that additional aeration will not solve.

Why can BOD be low while COD remains high?

This commonly indicates that a significant part of the COD is not readily biodegradable. Source-wise wastewater characterisation is needed.

Why does TSS remain high after biological treatment?

The biological process may be functioning while clarification or filtration is failing. Check sludge settling, hydraulic loading, sludge blanket and filter performance.

Can conventional ETP treatment reduce TDS?

Conventional physical, chemical and biological treatment generally has limited ability to remove dissolved salts. High TDS often requires segregation, membrane treatment, recovery or evaporation.

Which State Pollution Control Board decides my ETP outlet limit?

The relevant SPCB or Pollution Control Committee administers consent for the location of the facility, while applicable central/state standards and the specific consent conditions govern the unit.

Where should I check UPPCB ETP limits?

Check the current Consent to Operate issued to the facility and current official UPPCB/CPCB requirements applicable to the industry and discharge route rather than relying on a generic third-party table.

Where should I check RSPCB COD and BOD limits?

Check the facility's current Rajasthan State Pollution Control Board consent and applicable official standards. Rajasthan's official website provides CTE/CTO procedures and consent documentation resources.

What should a company do after receiving an SPCB show-cause notice for ETP failure?

The response should combine immediate containment, root-cause analysis, corrective actions, monitoring evidence, laboratory verification and a time-bound compliance plan.

Final Conclusion

BOD, COD, TSS and TDS are four different engineering signals.

BOD asks: can biological treatment handle the biodegradable organic load?

COD asks: what total oxidisable pollution is entering and leaving the system?

TSS asks: is solids separation working?

TDS asks: how will dissolved salts be controlled?

A successful ETP must answer all four questions.

And for regulatory compliance, one more question matters:

Does the actual plant, wastewater load, treatment system and outlet performance still match the current Consent to Operate?

When the answer is uncertain, increasing chemicals is rarely the best starting point.

• Measure the flow.

• Characterise the streams.

• Calculate the load.

• Trace the failure.

• Then modify the treatment system.