Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) consumed by microorganisms for the biochemical oxidation of organic and inorganic matter.
The BOD measurement is both an important indicator on the functional effectiveness of the plant, as well as a measure to assess the impact of plant effluent on receiving waters.
It is an empirical test to determine the relative oxygen requirements of wastewater effluents for the biochemical degradation of organic material and oxidation of inorganic material such as sulphides and ferrous iron.
The values obtained for BOD5 are always lower than those for COD, for 2 reasons:
• Activated sludge bacteria cannot degrade some of the compounds oxidized chemically in the COD test.
• Some of the carbon removed during the BOD test is not oxidised, but ends up in new bacterial biomass. So the BOD is only measuring the biodegradable carbon that is actually oxidised by the bacteria.
Carbonaceous BOD (CBOD) - the amount of oxygen used by a mixed population of heterotrophic microorganisms to oxidize organic compounds in the dark at 20 C over a period of 5 days. BOD:N:P ratio of 100:5:1 is thought to be the optimum ratio of nutrients needed by bacteria.
Since BOD is composed primarily of readily biodegradable organic carbon, it is easily removed by a healthy activated sludge microbial population. In the event that the BOD removal efficiency is low then that implies a problem with plant functioning or toxicity in the influent.
This measurement protocol is based on the amount of oxygen required to microbiologically degrade a sample under arbitrary standard conditions of a five day incubation at 20°C. This represent the total oxygen consumed from several physiological process including utilization of organic compounds during chemoorganoheterotrophic bacterial respiration, respiration by protozoa, subsequent death and lysis of the biomass and the respiration of the products.
It is very important that the distilled water used for dilution water be of high grade and free from contaminants (such as copper and chlorine) which could inhibit the growth of bacteria. Add 1 mg/L each of phosphate buffer, magnesium sulfate solution, calcium chloride solution and ferric chloride solution. Distilled water should be allowed to equilibrate in the incubator or with outside air for at least 24 hours at 20 C before use. To avoid dust or dirt contamination while allowing oxygenation, use a paper towel, cotton plug, or sponge to cover the bottle opening.
The BOD test relies on a measurable depletion of DO over a specified period of time. Because most samples of wastewater will have a BOD higher that the amount of oxygen available in the BOD bottle during the incubation period, the samples must be diluted. This dilution is done by adding dilution water to the sample in the BOD bottle. If the sample is not diluted, the biological activity of the microorganisms will use up the DO in the BOD bottle before the five day incubation time is up. If the final DO is too low, the BOD cannot be determined. There is no way of knowing at what point during the five days the DO reached zero.
One of the most difficult steps in the BOD procedure is deciding how much sample to place in the BOD bottles for incubation. Some plants have influent and effluent BOD’s that do not vary greatly over time, while others fluctuate greatly from day to day. In all cases, several different dilutions of each sample should be prepared to obtain the desired DO depletions.
Once a general range for the BOD of a sample has been determined, the dilutions can be established which will ensure that at least one dilution will meet the criteria for valid BOD results.