One of the important objectives of waste water treatment is to remove organic matter prior to being released into a body of water. Organic substances refer to carbon-based molecules that enter the wastewater stream in the form of fecal matter, soaps and detergents, fats, greases, oils, and food particles. If organic matter is not sufficiently removed from wastewater before it is released back into the environment, human and animal populations would be subject to harmful pathogens and disease.
Bacteria that are naturally present in water will consume organic contaminants, but consumption is dependent on the water’s dissolved oxygen concentration. To ensure the organic matter is not only sufficiently, but also efficiently broken down, wastewater treatment plants utilize oxygen pumps in their aeration tanks to provide an adequate amount of dissolved oxygen for decomposition.
Effluent water with a high BOD can negatively impact the natural body of water
Biochemical oxygen demand, commonly referred to as BOD, is an empirical measurement of the oxygen required by bacteria to decompose organic matter. Thus, the efficiency of a wastewater treatment plant can be determined by measuring the BOD concentration. By comparing the dissolved oxygen concentrations in sewage influent versus effluent, treatment plants can accurately gauge how much organic matter is being removed during the treatment process. Additionally, effluent water with a high BOD can negatively impact the natural body of water where it is discharged because dissolved oxygen is limiting in aquatic environments. An input of effluent water rich in organic matter will result in consumption of this limited DO to break down that organic matter, This will result in less available DO for aquatic organisms and lead to degradation of the aquatic ecosystem.
Importance of BOD measurments
Even as one of the most commonly measured constituents of wastewater, BOD tests, commonly referred to as BOD5 tests due to the required 5 day incubation in the testing procedure, still require a fair amount of time and effort for accurate determination. In the measurement of BOD5, a sample of wastewater is pipetted into a specially made glass bottle that contains aerated diluted water. If there is not an adequate amount of bacteria in the wastewater sample, a seed population can be added to ensure the organic matter will be oxidized. The dissolved oxygen concentration is initially determined and recorded, and the bottle is then incubated in the dark for five days at 20°C. At the conclusion of five days, the dissolved oxygen concentration is again determined and recorded. The difference between the initial dissolved oxygen and the final dissolved oxygen is calculated, along with the dilution factor and seed factor, if applicable. The results of the BOD5 test are expressed as milligrams of oxygen per liter of wastewater (mg/L). Acceptable BOD values for treated wastewater are less than 30 mg/L.
