The biochemical oxygen demand (BOD) test is a measurement of the quantity of oxygen required by bacteria to biologically oxidize organic material under aerobic conditions. Biochemical oxygen demand (BOD) is usually expressed in mg/L but can also be expressed in lbs/day. The organic matter serves as food for the bacteria and the cell receives energy from the organic matter during its oxidation. By measuring the amount of oxygen consumed by the bacteria, the amount of BOD, or food for the bacteria, can be calculated.
Biochemical oxygen demand testing can be separated into three measurable categories: total biochemical oxygen demand (tBOD), soluble biochemical oxygen demand (sBOD), and carbonaceous biochemical oxygen demand (CBOD). Total biochemical oxygen demand measures all biodegradable material in the sample. Soluble biochemical oxygen demand measures the dissolved biodegradable material in the sample. Carbonaceous biochemical oxygen demand measures the amount of oxygen required by bacteria to biologically oxide the carbonaceous fraction of organics and removes interference from nitrification.
To test the biochemical oxygen demand (BOD), the water samples of interest are diluted with nutrient rich water (containing a phosphate buffer solution, a magnesium sulfate solution, a calcium chloride solution, and a ferric chloride solution) and seeded with a known amount of bacteria. Dilutions are determined by estimating the amount of biochemical oxygen demand (BOD) in the sample using the measured chemical oxygen demand (COD) or another suitable surrogate. The dissolved oxygen (DO) of the sample is measured initially after mixing the solution and again after five days for the BOD5‚ or after one day for the BOD1 test. The seed (bacteria) added to each solution will degrade the biodegradable material in each sample relative to the amount of oxygen removed from the water. The sample dilution and amount of oxygen removed can be used to calculate the amount of biodegradable material in the sample in mg/L. Each sample must produce a residual of 1.0 mg/L of dissolved oxygen and have had a dissolved oxygen uptake of at least 2.0 mg/L.
Quality control for this procedure is sometimes misinterpreted from the Standard Methods procedure and care should be taken to ensure compliance is met. This is almost always an NPDES permitted test that requires reporting to state or federal environmental compliance departments.