Industry Solutions
BOD vs COD vs Soluble COD: What the Numbers Mean for Industrial Wastewater Process Selection
Use BOD, COD, soluble COD and TSS together to distinguish biodegradable, particulate and dissolved load before choosing DAF or biological treatment.
BOD and COD are not interchangeable measurements.
EPA guidance defines BOD as an empirical biological oxygen-demand test and COD as a chemical oxidation measurement that can include substances not biologically stabilized.
For process selection, the most useful question is:
How much of the total load is suspended/separable, how much is dissolved, and how much of the dissolved load is biologically treatable?
1. What BOD Tells You
BOD5 measures oxygen consumed by microorganisms over a defined test period, commonly five days under standardized conditions.
It is useful as an indicator of biodegradable organic load.
It is not a complete measure of every oxidizable compound in industrial wastewater.
2. What COD Tells You
COD measures oxygen equivalent consumed by chemical oxidation under the test conditions.
It can include:
- biodegradable organics;
- slowly biodegradable organics;
- non-biodegradable organics;
- some oxidizable inorganic material.
EPA explicitly notes that COD does not necessarily correlate directly with BOD.
3. What Soluble COD Adds
Soluble COD is measured after an appropriate solids-separation step defined by the analytical method or project protocol.
It helps answer:
How much of total COD remains after suspended material is removed?
The difference between total COD and soluble COD is a useful project indicator of particulate/colloidal contribution, provided the sampling and filtration method is consistent.
4. Why TSS Must Be Read With COD
High TSS plus a large total-COD / soluble-COD gap suggests that a meaningful fraction of the load may be associated with particles.
That creates a stronger case for evaluating:
- screening;
- coagulation/flocculation;
- DAF;
- clarification.
Low TSS with total COD close to soluble COD suggests the organic load is mostly dissolved and therefore less amenable to simple physical separation.
Worked Example A - Pretreatment Has a Large Opportunity
Assume:
- flow = 500 m3/day;
- total COD = 1,200 mg/L;
- soluble COD = 500 mg/L;
- TSS = 650 mg/L;
- BOD5 = 650 mg/L.
Total COD load:
500 m3/day x 1.2 kg/m3 = 600 kg COD/day
Soluble COD load:
500 m3/day x 0.5 kg/m3 = 250 kg COD/day
The difference is:
600 - 250 = 350 kg COD/day
This does not mean a DAF will automatically remove 350 kg/day.
It means there is a large non-soluble COD fraction that deserves treatability testing for physical/chemical pretreatment.
The post-pretreatment soluble COD remains part of the biological or downstream duty.
Worked Example B - DAF Opportunity Is Smaller
For a second illustrative case, assume:
- flow = 500 m3/day;
- total COD = 1,200 mg/L;
- soluble COD = 1,050 mg/L;
- TSS = 90 mg/L;
- BOD5 = 700 mg/L.
Total COD load is again 600 kg/day.
Soluble COD load is:
500 x 1.05 = 525 kg/day
Only 75 kg/day of the measured COD difference is associated with the non-soluble fraction in this simplified interpretation.
A larger DAF is unlikely to solve the majority of a 525 kg/day dissolved COD load.
5. How to Read the BOD/COD Relationship
A BOD/COD ratio can be useful as a screening indicator, but it should not be treated as a universal design criterion.
Why?
Because the ratio depends on:
- wastewater composition;
- sampling;
- BOD inhibition;
- analytical method;
- industrial chemicals;
- biodegradation kinetics.
EPA has historically allowed COD to substitute for BOD in some industrial programs only when a reliable plant-specific relationship has been demonstrated.
Analytical Credibility Checks Before Interpreting the Numbers
BOD5
EPA Method 405.1 is a five-day biological test at 20 C. Results depend on dilution, seed organisms, pH and oxygen depletion. For nitrogen-rich samples, nitrification can contribute to oxygen demand unless carbonaceous BOD methodology is used appropriately.
Reference: https://semspub.epa.gov/work/03/130972.pdf
COD
EPA COD methods use chemical oxidation. COD therefore has no fixed relationship to BOD. High chloride can interfere with COD measurements and may require a method designed for saline samples or a chloride correction.
Soluble COD
The article should only compare total and soluble COD when the laboratory's filtration protocol is documented and used consistently. EPA process-control guidance describes soluble BOD/COD as the same analysis performed after membrane filtration.
Evidence Artifact - Minimum Lab Data Record
For every sample used in a process decision, record:
- sample location;
- grab vs composite sample;
- production condition;
- total COD method;
- soluble COD filtration method;
- BOD5 or cBOD5 method;
- TSS;
- chloride/conductivity where high salinity is possible;
- date/time and laboratory.
Without this record, ratio comparisons can look precise while mixing incompatible analytical conditions.
Mobile-Friendly Interpretation Patterns
High TSS and total COD much higher than soluble COD
A large separable fraction may be present. Next step: jar testing, DAF or settling evaluation.
Low TSS and total COD close to soluble COD
The load is mainly dissolved. Next step: biodegradability and biological-treatment review.
COD much higher than BOD
A refractory or slowly biodegradable fraction may be present, but first verify analytical interference such as high chloride.
BOD and COD both swing by production batch
The plant has variable organic loading. Next step: build an equalization profile.
High ammonia with only moderate BOD
Nitrification may drive reactor sizing. Next step: ammonia, alkalinity, DO and temperature review.
Method Precision Matters
For comparison to be meaningful:
- sample the same process condition;
- use consistent laboratory methods;
- define how soluble COD is filtered;
- avoid comparing grab and composite samples without context;
- record temperature and unusual production events.
A "soluble COD" number without a documented filtration method is weak evidence.
Technical Basis and References
- U.S. EPA, Wastewater User's Guide, defines BOD as oxygen taken up during decomposition and COD as oxygen consumed during oxidation of organic and oxidizable inorganic matter.
- U.S. EPA, Methods for Chemical Analysis of Water and Wastes, COD method: COD is an independent chemical oxidation measurement and has no fixed relationship to BOD.
- U.S. EPA BOD Method 405.1 describes the five-day test as an empirical bioassay used for municipal and industrial wastewater and engineering design criteria.
Need Help Interpreting Wastewater Analysis?
- flow;
- BOD5;
- total COD;
- soluble COD and filtration method;
- TSS;
- FOG;
- ammonia;
- pH;
- discharge target.
HydroNovus can provide a preliminary process-selection interpretation and identify the next treatability test.