Dissolved Oxygen (DO) in Wastewater Information: 

Properties:

Dissolved Oxygen (DO) is a water quality parameter that is very important in secondary treatment processes. DO levels represent the amount of oxygen that is dissolved and dispersed throughout a water sample. Bacteria and microorganisms use dissolved oxygen to break down organic material, therefore reducing concentrations of DO. In wastewater treatment, microorganisms are added in flocs to aerobically digest and remove organic matter.1 As a result, efficient treatment relies on microorganism health and dissolved oxygen concentration.

Sources:

In nature, DO levels are higher in streams with choppy, turbulent water, at colder temperatures, and during the day when aquatic plants release oxygen. Common sinks for DO in the environment include microbiological degradation of organic compounds and algae blooms.

In wastewater treatment, DO levels decrease as Biological Oxygen Demand (BOD) and microorganism activity increase. If DO concentrations are too low, microorganisms will die off and treatment efficiency will therefore decrease. Aeration and bubbler systems are needed to keep DO levels around 2 mg/L and to evenly distribute DO throughout the flocs that contain microorganisms.1

Health/Environmental Concerns:

As mentioned, DO is important for environmental health. Low concentrations of DO can indicate that there are high amounts of organics in the water and that water quality is poor.2

In regard to wastewater treatment, DO is an important parameter to monitor and control in secondary treatment processes. If DO levels drop too much, microorganisms used to remove organic material will die off, rendering treatment inefficient. This will increase the concentration of organic material that reaches receiving waterways upon discharge. Additionally, organic material can react unfavourably with chlorine-containing disinfectants in the final stages of treatment and form carcinogenic compounds like THM’s.

Action:

During wastewater treatment, DO levels should be kept around 2 mg/L when microorganisms are used for organic material removal.1 One common error made in water treatment plants is the addition of excess oxygen; this is a waste of energy and puts unnecessary stress on machinery and equipment.1

1 Water and Wastewater Industry Application Data Sheet: Dissolved Oxygen Measurement in Wastewater Treatment. Emerson Process Management, Rosemount Analytical Inc: Irvine, CA. 2019. 

2 Science: Dissolved Oxygen. National Institute of Water and Atmospheric Research: New Zealand. (Accessed 13 Mar. 2020).