Chemical Oxygen Demand (COD) Vials

There are two common reasons to measure COD in a wastewater facility:

• To protect the environment and prevent getting fined.

Environmental regulatory agencies establish COD limits for wastewater facilities that discharge treated waste to receiving bodies of water. Exceeding these limits can lead to harsh fines or other penalties. If wastewater containing high COD levels is discharged into a natural body of water, the organic contaminants may consume or react with dissolved oxygen leading to dangerously low levels of oxygen, also known as hypoxia. This condition can stress or outright kill aquatic life.

• To maximize the efficiency of the plant and lower costs.

A COD test can be used to assess the incoming influent quality so operators can react to shifting waste load conditions. This ensures the most efficient treatment processes are being used to lower costs and to ensure that the effluent is within the permit limits prior to discharge.

How do Chemical Oxygen Demand Tests Work?

There are a few standardized methods for measuring COD, but the most common is the dichromate reactor digestion method which involves the reaction of sample with potassium dichromate in a sulfuric acid solution.

The dichromate COD reagent is commonly sold in a sealed vial. Sample is added to the vial, and the reaction occurs during a digestion where the sample is heated for two hours at 150oC. As dichromate ion oxidizes organic matter in the sample, it is reduced to chromic ion, which is reflected by a change in color from orange or yellow to green. The sample is then read in a photometer or spectrophotometer to measure the change in color.

A COD test kit commonly contains sealed vials that contain COD test reagent. Sample is dosed into the vials, resealed, and then digested. To perform a COD test, a digestor (sometimes called a digestor block) is required to heat the vials. Digestors are sold separately from the test kits. A photometer or spectrophotometer is also required and sold separately unless specifically stated.