BOD (biological oxygen demand) - The water quality indicator

6 September 2022,

3 Minute Water and Waste Water Video Tutorials by AET

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BOD – Biological oxygen demand is the most important used assessment to determine water quality.
Its a measurement unit and stands for the needed amount of oxygen by aerobic biological organisms to break down organic matter.
COD in comparison stands only for everything that can be chemically oxidized and is therefore less specific.
The biochemical process of BOD removal can be described with the equation:
Organic matter + O2 is converted into Co2 + H20
As higher the degree of water pollution as more organic matter is present and as more Oxygen is needed for the oxidization, hence, as higher is the BOD level.
The most commonly laboratory test method used is called the BOD5 test. For this testing method one liter of waste water is filled into a testing bottle and the consumed oxygen during 5 days of incubation is measured. The test method uses 5 days of incubation time as after 5 days most organic material has already been degraded.
The test sample is continuously agitated under the absence of light and a sensor measures the decline of pressure caused by oxygen consumption.
Typical values for BOD levels are 2 to 8mg/l in moderately polluted rivers. In comparison untreated sewage in the US has a BOD level of about 300mg/l whereas European sewage averages around 600mg/l. The lower BOD level in the US are caused by a greater water use per capita.
Waste water treatment plants use different technologies to increase BOD removal significantly. One common method is to increase the amount of biological organisms by providing more surface area for attached growth. Fill media plastic blocks consists of corrugated sheets and ensure a good water and air mixing to supply the biological organisms with nutrients and oxygen. Each fill media block can provide up to 250 m2 surface area for attached growth!