COD Limits

Question:

We are an Asian-based company investigating the construction of a metal finishing facility in the United States. What are the regulatory limits for COD? J.T.

Answer:

J.T., for our readers’ benefit, let me first define COD or chemical oxygen demand. COD is a measure of the amount of oxygen required to breakdown the organic matter in a sample under very strong oxidizing conditions. During the COD test, the sample is subjected to boiling with sulfuric acid and a strong oxidizer, such as potassium dichromate. We know that the COD test is widely used in other parts of the world for industrial dischargers.

COD is not commonly used in these United States for the government regulation of wastewater discharges. Typically, the five-day biochemical oxygen demand (BOD5) is used, so let me define BOD5. BOD5 is a measure of the amount of oxygen that is consumed by bacteria as it breaks down organic matter in a sample during a five-day period under standardized conditions. When the USEPA evaluated electroplating and metal finishing wastewaters during its development of the electroplating (40CFR413) and metal finishing (40CFR433) categorical pretreatment standards, it did not set limitations for either BOD5 or COD, leaving these pollutants’ regulation up to the agency that issues the discharge permit.

If you are looking at a direct discharge into a surface water (stream, river, lake, etc), your BOD5 limit could range from 5–50 mg/liter, depending upon your wastewater flow, receiving stream flow and the water quality standards of the receiving waters. None of the few direct metal finishing discharges that I know treat specifically for BOD5, but it is possible that your discharge may need polishing, such as using activated carbon, to consistently meet your BOD5 limit, especially if it is on the low end of the above range.

If you are looking at an indirect discharge into a sanitary sewer, there is a high probability that BOD5 and COD would not be an issue even though some sewer districts do regulate for COD. Typically, “normal” domestic sewage is defined with a BOD5 concentration of between 200–300; the exact concentration is based upon a local sewer district’s definition. Based upon our experience, the vast majority of metal finishing wastewaters have a BOD5 lower than 200 mg/liter. Even if COD is regulated, it is typically at a concentration several times higher than “normal” domestic BOD5, therefore, likely not an issue. None of our clients who discharge into sanitary sewers are required to treat for BOD5 or COD.

The BOD5/COD ratio, which can be used to estimate one parameter if the other is known, varies very widely depending how quickly the organics in the wastewater can be degraded in the BOD5 test. Just to illustrate how widely these ratios can vary, here are some examples from literature sources: machine oil»0.16, ethanolamine»0.4, kerosene»0.55, butyl cellosolve»0.7 and methanol»0.75. Since metal finishing wastewater can include low concentrations of a variety of organics, testing is the best way to determine BOD5.

There are two types of organics that are typically regulated either by USEPA, the states, and/or local sewer districts: total toxic organics (TTOs) and oil and grease. In the electroplating and metal finishing pretreatment standards referenced above, the USEPA and states regulate the TTO at a daily maximum of 2.13 mg/liter, which is a summation of all quantifiable values greater than 0.01 mg/liter. There are about 100 specified TTO chemicals, the vast majority likely not found in metal finishing wastewaters; if you have painting or degreasing operations, low levels of TTOs, such as 1,1,1-trichloroethane, trichloroethylene, and toluene, may be detected. Sometimes we have found detectable concentrations of phenol, but always under the limit. Interestingly, the most common TTO that we have detected in very low concentrations of metal finishing wastewaters is chloroform, which is a byproduct of drinking water disinfection. Lastly, if one treats cyanide wastewaters by alkaline chlorination, the reaction of chlorine with organics in the wastewater can lead to the creation of several TTOs.

Oil and grease is regulated by USEPA under the metal finishing categorical standard for direct dischargers with an average monthly limit of 26 mg/liter and daily maximum limit of 52 mg/liter, however, state water quality standards usually limit oil and grease to even lower concentrations. Furthermore, many direct discharge permits not only have a numerical limit, but also state the wastewater discharge is not to have or cause an oil sheen or film.

For indirect dischargers into sanitary sewers, the oil and grease limit can vary widely, but is usually somewhere between 50–500 mg/liter. Since you will likely treat for metals, your wastewater pretreatment system effluent will likely be able to meet these limits.