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INTRODUCTIONOne of the most important measurements in thedetermination of the health of a body of water is itsdissolved oxygen content. The quantity of dissolvedoxygen in water is normally expressed in parts per million (ppm) by weight and is due to the solubility of oxygen from the atmosphere around us. The atmos-phere consists primarily of nitrogen and oxygen andsince both of these gases are soluble in water, theyare present in varying amounts in all natural or manmade bodies of water. These amounts areproportional to the solubility and partial pressure of the two gases (see Table 1).

Oxygen is the gas that attracts the greatest amount of interest, simply because of its role in the cycle of allliving organisms and, specifically for our interest,those bacteria that feed on organic waste. For thisreason, it is the gas that is most commonly measuredin aquatic environments. It is important to realize

that the term "dissolved oxygen" refers to gaseousoxygen dissolved in water, and it should not beconfused with combined oxygen as found in the water molecule, H 2O.

Dissolved oxygen is very important in the treatment of domestic wastewater, as well as industrial waste fromsuch sources as the food, pulp and paper, chemical,and metals industries. Most water pollutants fromthese sources fall into one of two categories:1. Those that cannot be further broken down but

persist in or out of solution.

2. Those that are biologically degradable. There areboth organic and inorganic degradable substances,of which the organic tends to represent the largemajority.

The primary function of dissolved oxygen in a wastestream is to enhance the oxidation process by providingoxygen to aerobic bacteria so they will be able tosuccessfully perform their function of turning organicwastes into their inorganic byproducts, specifically,carbon dioxide, water, and sludge. This oxidationprocess, known as the activated sludge process, isprobably the most popular and widely used method of secondary waste treatment today and is normallyemployed downstream of a primary settling tank. Theprocess takes place in an aeration basin and isaccomplished by aeration (the bubbling of air or pureoxygen through the wastewater at this point in thetreatment process). In this manner the oxygen, whichis depleted by the bacteria, is replenished to allow theprocess to continue. For more detailed information onthis process, see the application data sheet number 4950-01, Dissolved Oxygen Measurement inMunicipal Waste Treatment.

In order to keep this waste treatment process func-tioning properly, a certain amount of care must betaken to hold the dissolved oxygen level within an

acceptable range and to avoid conditions detrimentalto the process. It is also important to make the mea-surement at a representative location on a continuousbasis to have a truly instantaneous measurement of the biological activity taking place in the aerationbasin.

TABLE 1. Solubility of Oxygen (mg/l) at VariousTemperatures and Elevations (Based on Sea

Level Barometric Pressure of 760 mm Hg)

Dissolved Oxygen Measurement

TheoryApplication Data Sheet

ADS 43-003/rev.BJanuary 2009

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Methods of DeterminationOver the years a number of methods for dissolvedoxygen determination have been developed. Thesemethods have been found to vary widely in their sensitivity, susceptibility to electrode poisoning by thepresence of undesirable compounds, and, in someinstances, sensitivity to a slow flowing or static aqueousmedia. Some methods require a very clean sample,devoid of extraneous chemical compounds, but only afew are adaptable to continuous on-line measurement.The various methods can be divided into three principlecategories:1. Laboratory methods (Winkler Method).

2. Electrochemical analysis (conductimetric, volta-metric, and galvanic).

3. Membrane electrode methods (galvanicmembrane electrodes and amperometricmembrane electrodes).

For this discussion we will confine ourselves to themembrane electrode methods.

The success of membrane electrodes in the determi-

nation of dissolved oxygen stems from the isolation of electrodes and electrolyte from the sample by meansof a semipermeable membrane. This membrane actsto protect the electrode from contamination by restrictingthe flow to gases only, and, in particular, oxygen.

The amperometric membrane technique used byRosemount Analytical is the most popular method usedindustry-wide in continuous waste water treatmentsystems. In this technique the electrodes are driven byan externally applied polarizing voltage. The currentflow between the electrodes can be directly correlatedto the amount of oxygen present in the stream.

In a typical dissolved oxygen sensor, two electrodes,

a gold cathode and a silver anode, are immersed in aspecially prepared electrolyte solution and separatedfrom the sample to be measured by a gas permeablemembrane (see Figure 1). The transfer of oxygenacross the membrane is proportional to the partialpressure of oxygen in the fluid. The chemical reactionsthat accompany this process are as follows:

Gold cathode: O 2 + 2H 2O + 4e - 4OH -

Silver anode: Ag + HCI AgCI + e - + H +

The reaction that takes place at the anode is the oxi-dation of silver to form silver chloride. This reaction isoffset at the gold cathode by the reduction of oxygen

molecules to hydroxide ions. The resulting currentflow is directly proportional to the dissolved oxygencontent of the stream.

ApplicationThe typical point for dissolved oxygen measurementin a waste treatment facility is in the aeration basin.The measurement should be made approximately twoto three feet below the surface and located near thepond exit since this point is felt by many to be thelocation at which bacterial activity in the pond canmost clearly be determined. The typical range of oper-ation should be somewhere between .5 and 2.0 ppmdissolved oxygen. If the oxygen level falls below .5ppm there is not sufficient oxygen to keep the bacteriaalive. The widespread death of bacteria and thesubsequent anaerobic condition may result in biologicalupset.

If the dissolved oxygen level rises much above 2.0ppm, there is the danger of over-population of nitrifyingbacteria over those micro-organisms that normallyperform the biodegrading function. This will increasethe oxygen demand to an even greater extent to keepup with this new demand and will ultimately cost theplant an excess in power expenses.

FIGURE 1. Typical Dissolved Oxygen Sensor

Emerson Process Management

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