COD and DO | Civil Engineering Optional Notes for UPSC PDF Download

The chemical oxygen requirement (COD) is a metric for measuring the consistency of water and wastewater. The COD test is often used to track the quality of water treatment plants. This test is based on the fact that, under acidic conditions, a good oxidising agent can fully oxidise almost any organic compound to carbon dioxide. The COD is the quantity of oxygen used to convert organic water pollutants to inorganic end products by chemical oxidation.
Under acidic conditions, COD is often tested using a heavy oxidant (e.g. potassium dichromate, potassium iodate, potassium permanganate). The oxidant is applied to the sample in a quantity that is considered to be excessive. After the oxidation process is complete, the amount of oxidant left in the solution is measured to determine the concentration of organics in the sample. Titration of an indicator solution is commonly used to do this. The mass of oxygen absorbed per litre of solution is measured in milligram per litre of COD.

The COD test takes just a few hours, while the Biochemical (or Biological) Oxygen Demand (BOD) test takes five days. It detects all organic toxins, including non-biodegradable ones. For each particular study, there is a relationship between BOD and COD, but it must be defined empirically. The findings of a COD evaluation will then be used to estimate a sample's BOD. Toxic compounds (such as heavy metals and cyanides) in the samples to be tested have little effect on the oxidants used in the COD test, unlike the BOD test. As a result, the COD test will be used to determine the intensity of wastes that are too harmful to be measured using the BOD method.

Note :

• The chemical oxygen requirement is a measure of the amount of oxygen that can be absorbed by reactions in a measured solution of environmental chemistry.
• It's usually measured in mass of oxygen absorbed per amount of solution, which is measured in mg / litre in SI units.

Important Points

• COD is a metric to measure organic matter content in water and wastewater.
• The COD test tracks water treatment plant quality based on oxidizing organic compounds to CO2 under acidic conditions.
• COD measures oxygen required to chemically oxidize organic pollutants to inorganic products.
• Common oxidants are potassium dichromate, iodate, permanganate.
• Excess oxidant is added, remaining amount measured (titration) to determine organic concentration.
• COD is in mg oxygen absorbed per liter (mg/L).
• COD takes a few hours, BOD takes 5 days.
• COD detects all organics including non-biodegradable, unlike BOD.
• BOD vs COD relationship must be empirically defined per study.
• COD can estimate BOD.
• Toxics like metals/cyanides don't affect COD oxidants like they do BOD.
• COD preferred for toxic wastes unsuitable for BOD method.
• COD measures oxygen-absorbing capacity in mg oxygen/L.

What is dissolved oxygen (DO)?

Dissolved oxygen (DO) is the amount of oxygen that is present in water. Water bodies receive oxygen from the atmosphere and from aquatic plants. Running water, such as that of a swift moving stream, dissolves more oxygen than the still water of a pond or lake.

All aquatic animals need DO to breathe. Low levels of oxygen (hypoxia) or no oxygen levels (anoxia) can occur when excess organic materials, such as large algal blooms, are decomposed by microorganisms. During this decomposition process, DO in the water is consumed. Low oxygen levels often occur in the bottom of the water column and affect organisms that live in the sediments. In some water bodies, DO levels fluctuate periodically, seasonally and even as part of the natural daily ecology of the aquatic resource. As DO levels drop, some sensitive animals may move away, decline in health or even die.

DO is considered an important measure of water quality as it is a direct indicator of an aquatic resource’s ability to support aquatic life. For the National Aquatic Resource Surveys (NARS), levels of DO are measured with a calibrated water quality probe meter, usually in conjunction with measurements for temperature and pH. While each organism has its own DO tolerance range, generally, DO levels less than 5mg/L are considered stressful for fish and levels  less than 3mg/L are too low to support fish. DO levels below 1mg/L are considered hypoxic and usually devoid of life.