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Introduction
Dissolved oxygen (DO) is an essential measurement parameter in aerobic bioreactors. The growth of all cells is heavily dependent on DO because it acts as a terminal electron acceptor in aerobic respiration. However, if excessive amount of DO is added to the process, it may limit the growth of the culture and promote undesirable organisms. Consequently, the measurement of DO is critical to effective operation of systems. Today, a variety of sensors are available in the market, each with its own advantages and disadvantages.
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Dissolved Oxygen
Dissolved oxygen is a physical distribution of oxygen molecules in water. Oxygen does not react with water, but mixes with it. There are two main sources of DO in water: atmosphere and photosynthesis. [2]
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Also dissolved oxygen data or BOD (biological oxygen demand) is needed to determine effluent water quality. It is a common environmental procedure to determine the amount of microorganisms in a sample. This measurement is used in wastewater treatment, food manufacturing and filtration facilities where this quantity is important for the process and final product. “High concentrations of DO predict that oxygen uptake by microorganisms is low along with the required break down of nutrient sources in the medium” [1].
Types of DO sensors
There are two main types of dissolved oxygen sensors: optical (luminescent) and Clark electrochemical (membrane covered electrode or amperometric). These main types have subtypes, slightly differing from each other, see figure 1.
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A reduction reaction will occur when a suitable potential is applied to the electrode: ox + ne- → red
Where ox is the oxidized species, red is reduced species, n is the number of electrons transferred and e- is an electron.
A concentration gradient of ox caused by its depletion at the electrode surface leads to mass transport by diffusion. This leads to a flux of ox, Jox (mol/m2s) that related to the reduction current, ired, through the electrode with an area A according to Faraday’s law [5]:
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Measuring dissolved oxygen with either sensor type
For both Neither electrochemical and nor optical dissolved oxygen sensor, they sensors do not measure the concentration of dissolved oxygen in mg/L or ppm (parts per million which is equivalent to mg/L). Instead, they measure the pressure of oxygen that is dissolved in the sample is being measured. To interpret the readings from the measurement, the pressure of the dissolved oxygen is expressed as DO %-saturation. To explain this in detail, the The instrument converts the dissolved oxygen pressure value from the sensor to % Saturation -saturation by dividing the sensor output in mmHg by 160*** (the pressure of oxygen in air at 760 mmHg) and then multiplying 38 39by by 100. For example, a measured oxygen pressure of 150 mmHg would be displayed by a sensor as 93.8 % Saturation -saturation (150/160 * 100). Source: [6]
***The pressure of oxygen at sea level is 160 mmHg because oxygen is about 21% of the earth’s atmosphere and 21% of 760 (average sea level barometric pressure) is about 160 mmHg.
The fact that the sensor measures the pressure instead of the concentration for dissolved oxygen is known to be true because a sample of fresh water can dissolve more can be illustrated by two water samples: one of fresh water and the other of sea water. The sample of fresh water can dissolve more oxygen than a sample of sea water at the same temperature and at the same altitude (or under the same barometric pressure). However, the sensor’s output signal is identical in both samples since the oxygen pressure is identical in both media. See the following figure Figure 9 for an example of this concept.
Figure 9. DO sensors measure %-saturation. Source: [6]
Variables that affect DO measurements
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Temperature is the most significant variable for the measurement accuracy. Therefore, it should be ensured that the temperature sensor on the probe is working correctly. Temperature can influence the DO measurement in two ways [6]:
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Three main methods for calibrating DO sensor are
- Winkler titration
- Air-saturated water
- (See, for example http://web.colby.edu/colbyatsea/2011/02/11/winkler-titrations-measuring-dissolved-oxygen/ or http://water.usgs.gov/owq/FieldManual/Chapter6/Archive/Section6.2.pdf)
- Air-saturated water (See, for example http://water.usgs.gov/owq/FieldManual/Chapter6/Archive/Section6.2.pdf)
- Water-saturated air (Hanna Instruments's video of calibration: http://www.youtube.com/watch?v=sxI_PS7b8XI) Water-saturated air [6]
Cleaning and Maintenance
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