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Chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water.Most applications of COD determine the amount of organic pollutants found in surface water.
It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older references may express the units as parts per million (ppm).
COD is often measured as a rapid indicator of organic pollutant in water. It is normally measured in both municipal and industrial wastewater treatment plants and gives an indication of the efficiency of the treatment process. COD is measured on both influent and effluent water. The efficiency of the treatment process is normally expressed as COD Removal, measured as a percentage of the organic matter purified during the cycle.
Biochemical oxygen demand or B.O.D. is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.
Biochemical Oxygen Demand (BOD) is an empirical test that determines the relative oxygen requirements of wastewater, effluent and polluted waters. BOD tests measure the molecular oxygen utilized during a specified incubation duration for the biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidize inorganic material such as ferrous iron and sulfides. The most common BOD test consists of a 5 day period in which a sample is placed in an airtight bottle under controlled conditions temperature (20ºC ± 1ºC), keeping any light from penetrating the sample to prevent photosynthesis. The Dissolved Oxygen (DO) in the sample is measured before and after the 5 day incubation period, and BOD is then calculated as the difference between initial and final DO measurements. BOD can be considered a more "natural" test in determining the oxygen required to oxidize organic matter, however it does not account for rapid changes in conditions. COD is often preferred for daily analysis since it is inherently more reproducible, accounts for changing conditions and takes a short time to complete
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