Chemical oxygen demand is a measure of the amount of organic matter oxidized by a strong chemical oxidant. COD is used to measure organic matter in commercial, industrial and municipal wastes that contain compounds toxic to biological life where the BOD₅ test would not work. COD, similar to BOD, provides an index to assess the effect the discharged wastewater will have on the receiving environment.

Higher COD levels mean a greater amount of oxidizable organic material in the sample, which will require more oxygen in the downstream components. Higher oxygen demands can lead to anaerobic conditions. The COD test is often used as an alternate to BOD due to shorter testing time. The COD levels in a wastewater sample are always greater than BOD₅ levels because more compounds can be chemically oxidized in the COD test than can be biologically oxidized in the BOD test. In most cases, once the COD/BOD₅ relationship is known for a particular facility, the COD concentration of a sample can be used to approximate the BOD₅ concentration, but it varies from site to site. The COD test can generally be done within several hours, whereas a BOD₅ test takes five days. A COD test is performed when a quick determination of oxygen demand is needed.

Analysis
The most common method to measure COD is Method 410.4. This method involves using a strong oxidizing chemical, potassium dichromate (Cr₂O₇^2-), to oxidize the organic matter in the solution to carbon dioxide and water under acidic conditions. Often, the test also involves a silver compound to encourage oxidation of certain organic compounds and mercury to reduce the interference from oxidation of chloride ions. The sample is then digested for approximately two hours at 150 degrees C. The amount of oxygen required is calculated from the quantity of chemical oxidant consumed. When choosing a lab to perform analysis of wastewater characteristics, a certified lab is always the best choice. These labs use standard procedures, and through most states, you can obtain a list of certified labs.

Impact of COD on onsite systems
A high COD value may indicate either high amounts of organic material or cleaners and chemical use. In residential wastewaters, the relationship between the COD and BOD is about 2-to-1, but this ratio increases as the toxicity of the waste is increased and it becomes more challenging to treat.

---

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.

This article is part of the series: An Installer's Guide to Wastewater Characteristics

• The Installer’s Guide to BOD5
• An Installer’s Guide to Total Suspended Solids
• An Installer’s Guide to COD
• An Installer’s Guide to FOG
• An Installer’s Guide to Phosphorus
• An Installer’s Guide to Wastewater Pathogens
• An Installer’s Guide to Nitrogen