What is the Difference Between BOD and COD Testing

When oxygen is taken out of receiving streams by any demand leaving in the effluent of a wastewater treatment plant, serious problems can develop with aquatic plant life and fish & that is why either BOD or COD testing, and sometimes both of these parameter tests is required all around the Country. What is the difference between BOD and COD testing?

The main difference between BOD and COD is that BOD is the amount of oxygen that is consumed by bacteria while decomposing organic matter under aerobic conditions whereas COD is the amount of oxygen required for the chemical oxidation of total organic & inorganic matter in H2O.

The main difference between BOD and COD is that BOD is the amount of oxygen that is consumed by bacteria while decomposing organic matter under aerobic conditions whereas COD is the amount of oxygen required for the chemical oxidation of total organic matter in water.

Difference Between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) Test


The biochemical oxygen demand (BOD) represents the amount of dissolved oxygen (DO) consumed by biological organisms when they decompose organic matter in water. COD is the only method to measure the amount of industrial waste in water, which cannot be measured under BOD. The plants that treat wastewater from commercial operations measure COD.

The chemical oxygen demand (COD) is the amount of oxygen consumed when the water sample is chemically oxidized. BOD and COD can reduce the DO of lakes and rivers, and low concentrations can cause eutrophication and harm aquatic life.

Oxygen demand measurements are used to estimate water pollution levels entering and leaving wastewater treatment and industrial facilities. Particularly close attention is paid to effluents as high oxygen demand levels indicate a danger to aquatic life.

Since treatment facilities are considered point sources, they are regulated under the Clean Water Act (CWA) which in turn sets facility-specific pollutant release targets as part of the National Permit Discharge Elimination System (NPDES) permitting process.

NPDES permits usually specify limits related to oxygen demand based on either the Chemical Oxygen Demand test (COD) test or Biochemical Oxygen Demand (BOD) testing methods.

Of the two tests, COD test analysis is more commonly performed as it takes less time and is more replicable. Furthermore, treatment operators can quickly react to changes in oxygen demand and modify treatment processes appropriately.

COD test measurements will always be higher than BOD measurements because COD measures everything that can be oxidized in the sample, whereas BOD measures only what can be oxidized through biological processes.

Plant engineers may establish a statistically validated COD/BOD test ratio to demonstrate their ability to reliably predict a BOD test value based solely on a COD measurement to their permitting authority. The COD test value may be used as justification for BOD removal from a plant permit. These test methods are designed to indicate the amount of contamination in a water sample.

Almost all wastewater treatment plants are required to measure one of these three items as a measure of the pollution value in the water. COD should always measure higher than TOC and then BOD.

COD or Chemical Oxygen Demand is the total measurement of all chemicals in the water that can be oxidized. TOC or Total Organic Carbon is the measurement of organic carbons. BOD- Biochemical Oxygen Demand is supposed to measure the amount of food (or organic carbons) that bacteria can oxidize.


Municipal wastewater discharge and industrial processes can create water high in COD/BOD, requiring careful treatment before discharge to preserve the health of waterways. Many people tend to confuse these two metrics since they are used for the same purpose to find out the degree of water contamination. The permissible limit of BOD biochemical oxygen demand is 30 mg/L. 

The permissible limit of COD is 250 to 500 ppm. These limits are measured in the Discharge Parameter test samples in the effluent of the plant and reported for State compliance.

However, unlike the BOD test, the COD test measures all oxidizable matter in a sample of water, and not just organic compounds. Thus, BOD is typically lower than COD for the same sample. Moreover, a standard COD test can be done in a few hours, unlike a BOD test which requires a standard 5-day incubation period.

BOD and COD test measurements are taken to determine the pollution level of wastewater. COD value is always greater than the BOD value of a particular water body. BOD measures the oxygen demand for the decomposition of organic material by the microbes in the wastewater. COD measures the oxygen demand for the decomposition of both organic and inorganic waste in wastewater. This is the difference between bod and cod.



Biochemical Oxygen Demand BOD Test


COD vs. BOD Testing Methods for Testing Water Quality
COD vs. BOD Testing Methods for Testing Water Quality

Coming out of the ’70s from wastewater treatment training and eventually running my own wastewater plant the most important training I received and passed on to new Operators was learning to run the BOD test twice a week for a sewage treatment plant. The more stringent you ran the tests the better results and control you had over operating a treatment plant.

BOD refers to the biological oxygen demand, which measures the amount of dissolved oxygen (DO) required by aerobic organisms (oxygen demand) to break down organic waste material present in a given water sample at a given temperature and specified time.

Since the BOD test is a biochemical process, it is not a precise quantitative test. However, a BOD water sample is a widely used test method, indicating the organic quality of water. BOD is determined by incubating a sealed water sample for five days and measuring the loss of oxygen from the beginning of the test to show the amount of organic matter present in concentration.

Two measurements are to be taken for the calculation of the BOD test of a water sample. One is the initial DO and the second is the final DO after five days. BOD is expressed in milligrams of oxygen consumed per liter of the sample for five days (BOD) of incubation at 20 °C  water sample. BOD directly affects the amount of DO of water in rivers and streams.



Difference Between BOD and COD | Compare the Difference Between Similar Terms



What is a Trickling Filter in Wastewater Treatment?

Trickling filters are used to remove organic matter from wastewater an aerobic treatment system that uses microbes attached to a filter medium rock, gravel, or plastic that removes organic matter from wastewater sprayed on it, followed by a recirculation tank It can handle peak shock loads to loads. ..…………………………………………………………………………… Read more


Chemical Oxygen Demand COD Wastewater Test


COD (Chemical Oxygen Demand) Kalibrierstandards für AQUAfast KolorimeterWhen treated wastewater is discharged into the environment through effluent, it can introduce pollution in the form of organic content to receiving waters. High levels of wastewater COD indicate concentrations of organics that can deplete dissolved oxygen in the water, leading to negative environmental and regulatory consequences. To help determine the impact and ultimately limit the amount of organic pollution in water, oxygen demand is an essential measurement.

Chemical oxygen demand (COD) is the amount of dissolved oxygen that must be present in water to oxidize chemical organic materials, like petroleum. COD is used to gauge the short-term impact wastewater effluents will have on the oxygen levels of receiving waters.

BOD: BOD is a biological oxidation process. COD: COD is a chemical oxidation process and COD describes the amount of oxygen required to chemically break down pollutants, while BOD indicates the amount of oxygen required to break down organic pollutants biologically with microorganisms.

There is a correlation between COD and BOD, however, it must be experimentally established before using one parameter to express another. If you run both tests you will come up with a ratio by dividing the BOD results by the COD results.  Make sure to watch the video above. Usually, COD analysis (which is a much faster and more accurate method) is used to estimate BOD using the established correlation.


What are the 3 Stages of Wastewater Treatment?

  1. Primary Stage-separates settable organic solids & inorganic material that won’t degrade
  2. Secondary Stage-removes suspended & soluble solids converting them to settable solids using Biological Oxidation
  3. The tertiary stage uses chemical & physical treatment to create H2O closer to potable quality ..………………………………………………………………………………. Read more


JimGalloway Author/Editor

*How many times have you driven past the Wastewater Treatment Plant and held your breath and said to yourself how can anybody work there, well, I did. Wastewater treatment has come a long way from where I started back in the ’70s. 

Jim has been a Consulting/Operator and Superintendent in Pennsylvania-held a Class A type 1 for Biological Treatment

  • Conventional Activated Sludge with Anaerobic Digestors-4 million gals. per/day,
  • Tertiary Treatment with Phosphorus and Denitrification Removal
  • Numerous Extended Aeration Treatment Plants
  • Contact Stabilization Treatment Plants


References: University of Georgia Wastewater Lab Testing



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