BOD, COD and TSS: How to Read a Wastewater Report

Your laboratory report has arrived. BOD is one number, COD is another, and TSS is a third. Which tells you whether the treatment plant is working? Which should go into a quotation request? And why can water that looks clear still have a high result?

For a factory owner around Lahore, Sheikhupura or Raiwind, or a housing developer planning an STP, these questions come before choosing equipment. Students and operators need the same foundation: BOD and COD describe oxygen demand. TSS describes suspended material. None is a complete verdict on water quality.

This guide explains the numbers, then walks through a hypothetical report. The examples are teaching calculations, not measurements from a Pakistani facility or recommended discharge limits.

What do BOD, COD and TSS actually measure?

Do not add the three results together. They are not separate buckets of pollution. An organic particle can contribute to TSS and also exert BOD and COD. Adding them would mix different measurements and count overlapping material.

BOD: what microorganisms consume oxygen to break down

BOD means biochemical oxygen demand. In the commonly reported BOD₅ test, microorganisms consume oxygen over five days at 20°C. This is a controlled laboratory temperature, not the temperature your tank must operate at. The instrument manufacturer VELP explains this five-day measurement and its purpose in assessing biodegradable pollution.

A BOD₅ result of 200 mg/L means an oxygen demand of 200 milligrams per litre under the test conditions. It does not mean 200 milligrams of bacteria, or that the water currently contains that much dissolved oxygen. DO, dissolved oxygen, is a different measurement.

Check the test method as well as the number. Some methods suppress nitrification, the biological oxidation of ammonia. For example, ISO 5815-1:2019 covers dilution and seeding with nitrification suppressed, using five- or seven-day incubation. Ask the laboratory whether your result is BOD₅, carbonaceous BOD₅ (CBOD₅), or another reported basis before comparing it with a design target.

COD: a chemical measure, not a list of chemicals

COD means chemical oxygen demand. It uses chemical oxidation rather than waiting for microorganisms to consume oxygen over several days. The result represents oxygen equivalents, not the mass of an identified chemical.

COD can include organic material that does not readily biodegrade, as well as some inorganic reducing substances. It is often higher than BOD₅ for the same wastewater, but there is no universal conversion factor. The ISO 15705 sealed-tube method also has concentration and chloride limitations. If a process uses salt, tell the laboratory so it can select suitable preparation and testing.

For an owner reviewing a dyeing-factory report, the question is not simply “how high is COD?” Ask which production and cleaning streams were present when the sample was collected. COD alone cannot identify the pollutant or select a treatment process.

TSS: suspended solids, not dissolved salts

TSS means total suspended solids. A known sample volume is filtered, and the retained residue is dried and weighed under the laboratory method. The result can include organic particles and mineral material. ISO 11923 describes suspended-solids determination using glass-fibre filters and notes interference from floating oil and other immiscible organic liquids.

TSS is not TDS, total dissolved solids. Dissolved salts can pass through the filter used for a suspended-solids test. A low TSS result therefore does not establish low salinity. Nor does TSS tell you how readily every particle will settle in a clarifier.

Read the sample details before the results

A precise laboratory result can still answer the wrong question if the sample came from the wrong place or time. Before discussing treatment, check:

• Sampling point: raw inlet, equalization tank, biological tank, final outlet or a drain carrying mixed streams?
• Timing and operating conditions: collection date and time, production shift, cleaning cycle, rainfall and any shutdown.
• Sample type: a grab is a snapshot; a composite combines samples over a stated period. Ask how a composite was collected and whether it suits the parameters and purpose.
• Handling: collector, delivery time, preservation and laboratory receipt condition. Agree on bottles and transport with the laboratory before sampling.
• Reporting basis: method, units, dilution, filtered or unfiltered sample, reporting limit and any qualification or warning.

A reported “less than 5 mg/L” is not zero. It says the result is below the stated reporting threshold. Also, an inlet sample is not a final-discharge compliance sample. Label both clearly when sending them to a consultant.

For local testing enquiries, PCRWR’s Lahore office page lists water-quality sampling and analysis services. Confirm the wastewater parameters, laboratory scope, sampling arrangement and required report acceptance directly before commissioning tests.

A worked example: what does this report tell an owner?

Hypothetical example. Assume a facility treats 100 m³/day. Representative inlet and outlet results, collected through an appropriate monitoring plan under stable operation, are:

Concentration reduction (%) = (inlet − outlet) ÷ inlet × 100.

For BOD₅: (250 − 40) ÷ 250 × 100 = 84%. That describes a reduction in the measured concentration. It does not, by itself, prove compliance, confirm the plant’s rated capacity, or establish that the water is suitable for reuse.

The remaining COD deserves review alongside other results and operating records. These numbers alone cannot tell you whether the cause is difficult-to-biodegrade material, operating conditions or sampling differences. A finding should lead to investigation, not an immediate equipment purchase.

Taking inlet and outlet grabs at exactly the same time does not necessarily sample the same parcel of water. Wastewater takes time to travel through the plant. Account for this and changing production when comparing results.

Concentration is not the same as daily load

A treatment quotation needs flow as well as concentration. Convert the hypothetical inlet BOD₅ to a daily oxygen-demand load:

Load (kg/day) = flow (m³/day) × concentration (mg/L) ÷ 1,000.

100 × 250 ÷ 1,000 = 25 kg BOD₅/day. The same calculation gives 60 kg COD/day and 30 kg suspended solids/day at the inlet. These are separate indicators, not quantities to add together. The BOD₅ load is not a blower-sizing calculation.

For a second hypothetical case, 200 m³/day at 125 mg/L BOD₅ also equals 25 kg/day. The concentration has halved, but the daily BOD₅ load has not changed. This is why dilution must not be mistaken for pollutant removal. Compare mass loads using measured flows when assessing treatment performance, accounting for side streams and changing conditions.

Likewise, incoming TSS load is not the same as sludge cake production. Biological growth, chemical additions and the water remaining in sludge affect how much material must be handled.

Which standards should a facility in Punjab compare against?

NEQS means National Environmental Quality Standards. PEQS means Punjab Environmental Quality Standards. Do not assume that a table labelled “NEQS” in an old report settles the applicable requirement for every province and discharge arrangement.

For Lahore and nearby Punjab facilities, start with the provincial authority’s official PEQS page, which lists standards for municipal and liquid industrial effluents. Have the project’s environmental adviser confirm the current notification, discharge category, relevant footnotes and project-specific approval conditions. Outside Punjab, check the responsible jurisdiction instead of carrying Punjab’s requirements across automatically.

The outlet figures in this article are examples, not a pass/fail table. Compare each required parameter on the correct test basis. A good percentage reduction does not excuse an outlet exceedance, and meeting three parameters does not establish compliance with requirements for other pollutants.

Can the BOD/COD ratio choose your ETP?

In the hypothetical inlet report, BOD₅/COD = 250 ÷ 600 = approximately 0.42. This ratio can prompt questions about biological treatability, but it is not a treatment guarantee or the exact fraction a full-scale plant will remove.

Before relying on it, check that both results represent the same stream and sampling period, and ask whether the BOD method and sample conditions make the comparison meaningful. For industrial wastewater, request a treatability assessment where uncertainty could materially affect the design. Avoid buying an aeration system solely because a ratio crosses a rule-of-thumb threshold.

If you are still defining the wastewater source, our STP versus ETP guide for Pakistan explains why domestic sewage and production wastewater may need different treatment arrangements.

What should go with a report when requesting a quotation?

For a housing STP, pair the report with the development’s occupancy assumptions, measured or estimated flow, peak periods and intended discharge or reuse. For an industrial ETP, add the production schedule, process chemicals, batch releases and whether domestic sewage joins the process drains.

Ask bidders to state the following against the same design basis:

• The incoming flow and pollutant range the proposed system is designed to handle.
• Which additional tests are needed, such as pH, salinity, nutrients, oil and grease or process-specific contaminants.
• The promised outlet targets, sampling method and performance-testing conditions.
• Who operates the plant, maintains equipment, arranges testing and manages sludge.
• What is included in civil works, installation, commissioning, consumables and operator training.

Aquenvia’s industrial effluent treatment page provides further context for planning an ETP. If you need help organizing an initial technical review, share the report and facility details with Aquenvia.

Common questions

Does BOD of 80 mean 80 grams per litre?

No. If the unit is mg/L, 80 means 80 milligrams of oxygen demand per litre, equivalent to 0.08 grams per litre. Always read the units printed beside the result.

Can clear water still have high BOD or COD?

Yes. Appearance does not reveal all dissolved oxygen-demanding substances. A clear sample is not a substitute for analysis.

Can I calculate BOD from COD alone?

Not reliably using a universal multiplier. Any site-specific correlation needs paired measurements and checking as wastewater changes. It should not replace a required laboratory test.

Does low TSS mean the water is safe to reuse?

No. TSS does not measure dissolved salts, individual toxic substances or pathogens. Define the intended reuse and its quality requirements before selecting treatment or using the water.

What is the most useful first action?

Put the laboratory report beside the sampling record, flow log and operating log. Establish what water was tested, under what conditions, and which decision the result needs to support. That is a stronger starting point than choosing equipment from three numbers.

Sources checked 9 September 2026. Linked ISO pages describe test-method scopes, not discharge limits. All worked examples are hypothetical.