Dissolved oxygen- DO

UNICERT is the leading inspection body in the area of water quality test for Dissolved oxygen (DO) and objectives to reduce environmental emission/pollution and enhance environmental performance to the society.

Dissolved oxygen (DO):

Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other liquids. It is an important parameter in assessing water quality because of its influence on the organisms living within a body of water. In limnology (the study of lakes), dissolved oxygen is an essential factor second only to water itself.  A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality.

Non-compound oxygen, or free oxygen (O2), is oxygen that is not bonded to any other element. Dissolved oxygen is the presence of these free O2 molecules within water. The bonded oxygen molecule in water (H2O) is in a compound and does not count toward dissolved oxygen levels. One can imagine that free oxygen molecules dissolve in water much the way salt or sugar does when it is stirred


Effects of Dissolve Oxygen in Water:

Dissolved oxygen is necessary to many forms of life including fish, invertebrates, bacteria and plants. These organisms use oxygen in respiration, similar to organisms on land. Fish and crustaceans obtain oxygen for respiration through their gills, while plant life and phytoplankton require dissolved oxygen for respiration when there is no light for photosynthesis. The amount of dissolved oxygen needed varies from creature to creature. Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen (1-6 mg/L), while shallow water fish need higher levels (4-15 mg/L)⁵.

Microbes such as bacteria and fungi also require dissolved oxygen. These organisms use DO to decompose organic material at the bottom of a body of water. Microbial decomposition is an important contributor to nutrient recycling. However, if there is an excess of decaying organic material (from dying algae and other organisms), in a body of water with infrequent or no turnover (also known as stratification), the oxygen at lower water levels will get used up quicker ⁶.

A high DO level in a community water supply is good because it makes drinking water taste better. The amount of oxygen water can hold depends upon temperature (more oxygen can be dissolved in colder water), pressure (more oxygen can be dissolved in water at greater pressure), and salinity (more oxygen can be dissolved in water of lower salinity). Many lakes and ponds have anoxic (oxygen deficient) bottom layers in the summer because of decomposition processes depleting the oxygen.

The amount of dissolved oxygen often determines the number and types of organisms living in that body of water. For example, fish like trout are sensitive to low DO levels (less than eight parts per million) and cannot survive in warm, slow-moving streams or rivers. Decay of organic material in water caused by either chemical processes or microbial action on untreated sewage or dead vegetation can severely reduce dissolved oxygen concentration. This is the most common cause of fish kills, especially in summer months when warm water holds less oxygen anyway.

Dissolved oxygen (DO) refers to the volume of oxygen that is contained in water. Oxygen enters the water as rooted aquatic plants and algae undergo photosynthesis, and as oxygen are transferred across the air-water interface.


Interested Parties including Regulatory Authorities:

  1. Persons affected DO
  2. Industries, Laboratories reducing DO
  3. Personal and commercial activities regarding DO
  4. Private / Govt. Projects to control DO
  5. Local Environmental Department/ Authorities
  6. Local Government Authorities like Municipalities, City Corporation etc.
  7. Local Law Enforcing Agencies like Police, Magistrate and Regulatory Authorities etc.


Benefit of Monitoring:

By monitoring long-term contamination trends, every country establishes baseline contamination levels, making it possible for early identification of contamination events. Daily events and long term trends are captured and taken step to reduce environmental emission/ pollution and enhance environmental performance of the society.