Dissolved Oxygen, an Intro

Oxygen is one of the most important compounds in supporting life. In terms of water and wastewater quality and treatment processes, oxygen concentration can be represented by the value of dissolved oxygen (DO), which tells how much oxygen is dissolved in the water. Dissolved oxygen is one of the key parameters for determining the quality of a certain aquatic environment like lakes, rivers, seawater, etc. and can help determine if an aquatic environment is free from eutrophication process caused by nutrients like nitrogen and phosphorus. Additionally, it is important for oxidation-reduction reactions. Having an environment with enough DO is necessary to support aquatic organisms like fish, plankton, and aquatic plants.

In wastewater treatment, Dissolved oxygen (DO) concentration is an important parameter in the operation of biological treatment, both for aerobic and anaerobic processes. In aerobic processes, such as activated sludge process, DO is of interest optimize system performance. On the other hand, it can be a crucial parameter for anaerobic process too, because the presence of oxygen is not expected, and therefore DO should be zero.

Dissolved oxygen concentrations can be used determine biochemical oxygen demand (BOD). BOD₅ is measured by finding the difference in DO concentration between the first day and after an incubation period of 5 days at 20°C of a closed bottle sample. The removed oxygen in the sample was consumed by the organisms. DO is also a parameter to determine maximum specific growth rate constant for nitrification processes, especially for the Nitrosomonas bacteria.

Typically, DO is measured in milligrams per liter (mg/L), which means the mass of oxygen (mg) in a liter of water. For class A waters, such as the South Fork Palouse River, the minimum standard DO concentration that should be maintained is about 8.0 mg/L[1]. In further application, dissolved oxygen measurement is also performed to measure oxygen concentration in seawater that is also important in environmental and marine science.

There are several ways to measure DO level in the water. There are colorimetric, titrimetric, and polarographic methods[2]. Currently, the polarographic method is most commonly used, which uses a digital DO meter as its main device. Every method has its own advantages and drawbacks, but using DO meter is very practical and easy to use.

The common method for titrimetric DO measurement is the Winkler Method. Although the Winkler Method is less convenient than the digital measurements, this method is still relied on for several purposes, such as to calibrate the accuracy of digital DO meters. Compared to digital DO meters, this method is reasonably cheaper, which can make it attractive in some settings. Also, in some cases where the oxygen is in very low concentration, the Winkler Method is more reliable than a DO meter.[3]

The Winkler Method is based on oxidation-reduction reactions occurring between oxygen in the water and reagents that are purposely added to react with the oxygen. This method basically is an iodometric titration. The end point of this titration process then is converted to dissolved oxygen concentration as mg/L.

The reactions occurring in the process of Winkler Method are described as follows.

 

DO meters, on the other hand, use signals from a probe that is inserted to the water. The DO meter works based on polarographic principle that translates the movement of the membrane of the stirrer (the electrodes) around the water measured containing oxygen, into the signal that can be read directly as DO concentration in mg/L. Therefore, measuring DO by this device will be much faster than using Winkler Method, because there is no need to perform the wet chemistry actions and make unit conversions to determine the concentration. However, to maintain the accuracy of a DO meter, regular calibration is needed.

[1] Jim Carroll. (2007). “South Fork Palouse River Dissolved Oxygen and pH Total Maximum Daily Load Study” e-report, accessed September 13^th 2014

[2] Revital Katznelson. (2004). “Dissolved Oxygen Measurement Principles and Methods” http://www.swrcb.ca.gov/water_issues/programs/swamp/docs/cwt/guidance/311.pdf, accessed October 6 2014

[3] ibid