Introduction
In wastewater treatment across India, BOD, COD, and TSS are the three most critical parameters used to assess pollution load, treatment efficiency, and regulatory compliance. Whether for an industrial Effluent Treatment Plant (ETP) or a municipal Sewage Treatment Plant (STP), these indicators define the “strength” of wastewater and determine whether treated effluent is fit for discharge or reuse.
Indian regulatory authorities, including the Central Pollution Control Board and State Pollution Control Boards (SPCBs), mandate strict limits on BOD, COD, and TSS for industries, urban local bodies, and Common Effluent Treatment Plants (CETPs). Understanding how these parameters are reduced through physical, chemical, and biological treatment stages is essential for compliance and sustainable water management.
What Are BOD, COD, and TSS in Wastewater?
Biochemical Oxygen Demand (BOD)
BOD represents the amount of dissolved oxygen required by aerobic microorganisms to biologically decompose organic matter in wastewater over a fixed time period, typically five days at 20°C.
It is an indirect measure of biodegradable organic pollution. High BOD indicates a high concentration of biodegradable organics, which can rapidly deplete oxygen in receiving water bodies and harm aquatic life.
Chemical Oxygen Demand (COD)
COD measures the oxygen equivalent required to chemically oxidise both biodegradable and non-biodegradable organic matter using a strong oxidising agent.
Because it includes almost all organic compounds, COD values are always higher than BOD. COD is particularly important in Indian industrial wastewater, where non-biodegradable organics are common.
Total Suspended Solids (TSS)
TSS refers to the solid particles suspended in wastewater that can be physically filtered. These include silt, grit, fibres, organic flocs, and microbial biomass. High TSS reduces light penetration in natural water bodies, interferes with disinfection, and contributes to sludge accumulation in treatment systems.
Why BOD, COD, and TSS Reduction Is Important in India
In India, untreated or inadequately treated wastewater poses serious environmental and public health risks. Excess BOD and COD consume dissolved oxygen in rivers and lakes, while high TSS causes turbidity and sedimentation.
From a compliance perspective, exceeding CPCB or SPCB limits can lead to penalties, withdrawal of consent to operate, or plant shutdowns. From a sustainability standpoint, effective reduction enables water reuse in cooling, gardening, flushing, and process applications.
How BOD, COD, and TSS Are Reduced in ETP and STP Systems
Wastewater treatment plants reduce these parameters in a step-by-step manner, with each stage targeting specific pollutants.
Preliminary and Primary Treatment
Main Focus: TSS and Partial BOD/COD Removal
In the initial stages, physical processes remove large and settleable solids. Screening systems such as bar screens trap plastics, rags, and debris to protect downstream equipment. Grit chambers allow heavy inorganic particles like sand and gravel to settle by gravity. Primary sedimentation tanks slow the wastewater flow, allowing suspended solids to settle. This stage typically removes 60–70 percent of TSS and 30–40 percent of BOD, with some reduction in COD. In Chemical Enhanced Primary Treatment (CEPT), coagulants such as alum or ferric chloride are added to destabilise colloidal particles. This improves settling efficiency and can achieve 80–90 percent TSS removal and 50–80 percent BOD reduction, especially in industrial ETPs.
Secondary Treatment
Main Focus: BOD and COD Removal
Secondary treatment is the biological heart of an ETP or STP.
In the aeration tank, wastewater is mixed with activated sludge containing aerobic microorganisms. Compressed air supplies oxygen, allowing bacteria to metabolise organic matter. Organic pollutants are converted into carbon dioxide, water, and new microbial biomass.
This stage typically achieves up to 90 percent reduction in BOD and COD, depending on design and operating conditions.
The mixed liquor then flows to a secondary clarifier, where biological flocs settle. A portion of the settled sludge is recycled to maintain microbial concentration, while excess sludge is removed for further treatment.
Tertiary and Advanced Treatment
Main Focus: Polishing Residual BOD, COD, and TSS
Tertiary treatment improves effluent quality to meet stringent discharge or reuse standards.
Pressure Sand Filters (PSF) remove residual suspended solids, while Activated Carbon Filters (ACF) adsorb remaining organic compounds, colour, and odour, further reducing COD.
Advanced membrane systems such as Membrane Bioreactors (MBR) replace conventional secondary clarifiers with ultrafiltration membranes. MBRs produce effluent with near-zero TSS and very low BOD and COD, making them ideal for reuse and Zero Liquid Discharge (ZLD) systems.
Disinfection using chlorine or ultraviolet radiation destroys remaining pathogens and stabilises residual biological oxygen demand.
Typical Performance Levels in Indian STPs and ETPs
Regulatory Perspective in India
CPCB and SPCB norms specify discharge limits for BOD, COD, and TSS depending on industry type, discharge location, and reuse intent. Many urban local bodies and industrial parks now mandate tertiary treatment for water reuse.
Meeting these limits consistently requires proper design, operation, and monitoring of ETP and STP systems.
Frequently Asked Questions (FAQs)
What is the difference between BOD and COD?
BOD measures biodegradable organic pollution, while COD measures total organic pollution, including non-biodegradable compounds.
Why is COD always higher than BOD?
COD includes both biodegradable and non-biodegradable organics, whereas BOD includes only biodegradable matter.
What causes high TSS in wastewater?
Suspended solids such as silt, fibres, organic matter, and microbial flocs contribute to high TSS.
How much BOD reduction is required for CPCB compliance?
Typically, treated effluent BOD must be below 30 mg/L, though stricter norms apply for reuse or sensitive discharge areas.
Can primary treatment alone meet discharge norms?
No. Primary treatment only removes a portion of BOD and TSS and is insufficient for compliance.
Which process removes the maximum BOD?
Secondary biological treatment removes the majority of BOD and COD.
Are MBR systems better than conventional STPs?
Yes. MBRs provide superior TSS and BOD removal and support high-quality water reuse.
How is TSS measured in an ETP?
TSS is measured by filtering a known volume of wastewater and weighing the retained solids.
Does tertiary treatment reduce COD?
Yes. Activated carbon and membranes significantly reduce residual COD.
Why are BOD, COD, and TSS monitored daily?
They indicate treatment performance and ensure ongoing regulatory compliance.