Biochemical Oxygen Demand (BOD) &
Dissolved Oxygen (DO)

Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand, BOD, as it is commonly abbreviated, is one of the most important and useful parameters (measured characteristics) indicating the organic strength of a wastewater.  BOD measurement permits an estimate of the waste strength in terms of the amount of dissolved oxygen required to break down the wastewater.  The specifics of the analysis are discussed in detail in Standard Methods for the Examination of Water and Wastewater.  The BOD test is one of the most basic tests used in the wastewater field.  It is essentially a measure of the biological and the chemical component of the waste in terms of the dissolved oxygen needed by the natural aerobic biological systems in the wastewater to break down the waste under defined conditions.  Generally the BOD test is carried out by determining the dissolved oxygen on the wastewater or a diluted mixture at the beginning of the test period, incubating the wastewater mixture at 20°C, and determining the dissolved oxygen at the end of 5 days.  The difference in dissolved oxygen between the initial measurement and the fifth day measurement represents the biochemical oxygen demand. 

While BOD describes the biological oxidation capacity of a wastewater, it is not a measure of the total potential oxidation of the organic compounds present in the wastewater.  A number of chemical tests are used to measure this parameter, either in terms of the oxygen required for virtually complete oxidation, or in terms of the element carbon.  Probably the most common test for estimating industrial wastewater strength is the Chemical Oxygen Demand (COD) Test.  This test essentially measures the chemical oxidation of the wastewater by a strong oxidizing agent in an acid solution.  The value for the COD test is always greater than the BOD test and is not always a good indication of BOD values for the same waste. 

A test which measures carbon and which is being used to a greater extent in measuring wastewater strength is the TOC (Total Organic Carbon) test where the carbon is oxidized by catalytic combustion to carbon dioxide and the carbon dioxide is measured. 

Dissolved Oxygen (DO)

The dissolved oxygen concentrations in a wastewater before and after treatment are very important.  While dissolved oxygen concentrations are necessary to carry out the BOD determination, as described above, dissolved oxygen levels are also quite important in determining how satisfactory a biological wastewater treatment plant is operating.  For example, for satisfactory biological wastewater decomposition (i.e. treatment) some dissolved oxygen must be present.  If it is not, the system will be inefficient and is said to be anaerobic.  Septic conditions follow, accompanied by a variety of nuisance conditions such as odor and color changes. 

Normally, oxygen is not a very soluble gas and dissolved oxygen concentrations in wastewaters are very low.  For example, dissolved oxygen concentrations of a few milligrams per liter (or parts per million) are commonplace in water.  The solubility of oxygen is such that dissolved oxygen levels in clean water are affected by temperature and salt concentrations expressed as chlorides. 

When microorganisms and an available food supply are present, dissolved oxygen will be consumed.  Since many of the components present in a raw wastewater can serve as a nutrient for microorganisms, most domestic wastewaters will undergo some decomposition and usually any available dissolved oxygen supplies are consumed during travel through the sewer system.  Generally, raw wastewater will have little if any dissolved oxygen present while wastewater in the aeration tanks, final settling tanks, or in the final effluent will probably have at least measurable dissolved oxygen concentrations.