Chapter 14  COD Chemical Oxygen Demand  return to Table of Contents

                               

 

                 The COD test will give a good estimate of the first stage oxygen demand for most wastewaters.  An advantage of the COD test over the biochemical oxygen demand (BOD) test is 2 to 3 hours versus 5 days.  The COD test also is used to measure the strength of wastes that are too toxic for the BOD test.  The COD test should be considered an independent measurement and not a quick substitute for the BOD test.  The COD is usually higher than the BOD, but the amount will vary from waste to waste.  The COD test should be considered an independent measurement of organic matter in a sample rather than a substitute for the BOD test.

                 Chemical Oxygen Demand measures the ability of hot chromic acid solution to oxidize organic matter.  This analyzes both biodegradable and non-biodegradable (refractory) organic matter.  Expressed as O₂.

The results of the COD (chemical oxygen demand) tests are usually higher that the corresponding BOD test for several reasons.  Many organic compounds which are dichromate oxidizable are not biochemically oxidizable;  Certain inorganic substances, such as sulfides, sulfites, thiosulfates, nitrites  and ferrous iron are oxidized by dichromate, creating an inorganic COD,  which is misleading when estimating the organic content of the wastewater. If this is true, a BOD value for a glucose/glutamic acid standard should be 60-70% of the COD value for the same sample. Check a few old Water Pollution Studies ... you will find that BOD is ~64% of COD for the last umpteen studies. The BOD to COD ratio is nothing more than the BOD concentration divided by the COD concentration for the same sample (e.g., if BOD is 60 mg/L, and COD is 100 mg/L for a sample, the ratio is 60/100, or 0.60). There are three fairly reliable "rules of thumb" correlations between COD/BOD. 1. Ratios for COD to BOD of 0.5 to 2 are usually found in potable water or exceptionally clean surface or ground water.  2. Ratios of COD to BOD of 2 to 4 are usually seen in routine domestic/municipal sewage wastes. 3. Ratios of COD to BOD of 4 to 6 are usually indicative of industrial type wastes. Of course, each specific treatment system may be checked for its own particular ratios.

In some industrial effluents (pretreatment program), BODs can be higher than CODs (for example, some effluents which are high in sugars , as can be found in the bakery industry, or soda bottling. Some industrial effluents will have higher demand because of the higher quantities of chemicals that demand oxygen. Which chemicals can be quite a wide variety. It depends... As previously suggested, you must first establish the COD/BOD correlation for YOUR wastewater. The benefit to using COD is that you have your results in less than 3 hours.

   

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