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.
Background
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.
http://www.eutechinst.com/techtips/tech-tips15.htm
Ambient air contains about 20% oxygen and is essential for breathing, also for fish and other aquatic organism. Dissolved oxygen is the amount of free oxygen in water suitable for the breathing purpose. If there is not enough oxygen, it is letal to fish: the amount of 2 mg/l is deadly and the amount between 2 and 5 mg/l affects fish health.
is this relevant to DO? 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” (http://www.eidusa.com/Theory_DO.htm).
Types
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.
Figure 1. Diagram of sensor types. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Different sensor types suit some applications better that the others. These properties will be discussed later on the page, meanwhile the applications can be found from Figure 2.
Figure 2. Best applications for different types of sensors. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Optical Sensors
Optical sensing of oxygen is based on the measurement of the red fluorescence of a dye/indicator illuminated with a modulated blue light as shown in Figure 3.
Figure 3. Principal of oxygen detection using fluorescent dye. Source: A comparison of amperometric and optical dissolved oxygen sensors in power and industrial water applications
The probe emits a blue light of the proper wavelength that causes the dye in the sensing element to luminesce or glow red. Oxygen constantly diffuses through the paint layer, affecting the luminescence of the sensing layer. The amount of oxygen passing through to the sensing layer is inversely proportional to the lifetime of the luminescence in the sensing layer.
The sensor measures the lifetime of the dye’s (sensing layer’s) luminescence, caused by the presence of oxygen, with a photodiode (light detector) in the probe. To increase the accuracy and stability of the measurement the reading is compared to a reference. The lifetime of the luminescence from excitation by the red light acts as the reference (“the sensor emits a red light that is reflected by the dye layer back to the photodiode in the sensor” https://www.fondriest.com/pdf/ysi_do_handbook.pdf)), so the lifetime of luminescence of the blue light is compared to it, and the stable oxygen concentration is calculated by the probe.
The oxygen concentration is determined with the Stern-Volmer equation, that sets the relationship between luminescence lifetime (intensity) and oxygen concentration see Figure 4.
Figure 4. Stern-Volmer equation. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
The most significant advantage of an optical dissolved oxygen sensor is low maintenance cost and the possibility of less frequent calibration. Other advantages and disadvantages can be found from Figure 5.
Figure 5. Advantages and disadvantages of optical sensors. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Electrochemical Sensors
Electrochemical DO electrodes are divided into two separate types: polarographic and galvanic. These electrodes are constructed with an anode and a cathode submerged in an electrolyte solution. An oxygen-permeable membrane is used to confine the cathode. When the cathode is polarized with a constant voltage, dissolved oxygen molecules diffusing through the membrane is reduced at the cathode. Then, an electrical signal produced by the cathode travels to the anode and then to the instrument. The oxygen tension versus the electrode current can be calibrated since the diffusive flux is a function of the partial pressure of oxygen in the flow.
The oxygen-reduction reaction at the cathode can be presented as:
O2 + 2H2O + 2e- → H2O2 + 2OH-
H2O2 + 2e- → 2OH-
As in the case for the polarographic electrodes, a voltage is applied externally while an internal potential is generated as in the galvanic electrodes.
Figure 6. An illustration of an electrochemical sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Amperometry
Amperometry is a technique used to detect ions in a solution based on electrical current produced by electrochemical reaction of an electro-active species.
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:
ired = - n×F×A×Jox ↔ Jox = - ired / n×F×A
Where:
ired = reducing current (A)
Jox = flux of dissolved oxygen (mol/m2s)
n = number of moles (mol)
F = Faraday's constant
A = surface area of the electrode (m2)
The electrical current is now proportional to the amount of DO transported by the electrode. The driving force for Jox is the concentration gradient (∂C/∂x) of ox near the electrode. Fick's first law of diffusion is used to relate the original concentration of ox to the measured current:
ired = - n×Jox×A×F = n×F×A×D×∂C/∂x
Where:
D = diffusion coefficient (m2/s)
∂C/∂x = concentration gradient ( x = 0 => C = 0, means the concentration of ox at the electrode drops to 0 )
A few assumptions are made in order to obtain a simple expression for the gradient.
- The slope of the gradient is linear,
- The thickness of the layer (s) is fixed.
- The concentration of ox drops to zero. x = 0, C = 0.
The equation above becomes:
ired = n×F×A×D×C / s
Polarographic Electrode
A typical polarographic electrode consists of a silver anode, a gold or platinum cathode and an electrolyte solution (KCl or AgCl). In order to create a sensor, a constant voltage of 0.8 volts is applied to the probe, and a digital meter is installed to read the DO response measured by the sensor.
Figure 6. A simplified diagram of a polarographic sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Limitations
- Response time is described as the time required for the electrode to reach >90% of the output. Typical response time for polarographic sensors are is 30 sec, which makes them not compatible to be used for dynamic measurements.
- Warm-up time for this type is approximately 10 minutes. Wrong readings will occur if measurements are made when the required amount of time has not been attained.
- Chloride ions in the electrolyte will be eventually consumed resulting in gradual drift in the electrode signal. The electrolyte must be replaced.
- Since the electrode consumes oxygen, readings are affected by flow across the sensor tip. Thus enough flow rate at the membrane (or sample renewal rate) must be ensured for accurate results.
Galvanic Electrode
ADD YOUR PART HERE
Measuring dissolved oxygen with either sensor type
Variables that affect DO measurements
There are several parameters that affect the DO measurement accuracy and reliability, they are temperature, salinity, atmospheric (barometric) pressure and flow (stirring).
----- are we gonna discuss about stirring?
Temperature
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:
- Diffusion of oxygen through the membrane (electrochemical) or sensing element (optical) on the probe increases/decreases with higher/lower temperature due to change in molecular activity (up to 4% difference per °C).
With digital sensors, the effect of temperature can be compensated with software, as the temperature is known; with analog sensors, compensation is done by adding a thermistor (a temperature-sensitive resistor) into the circuit. - Ability of water to dissolve oxygen is directly proportional to temperature. Warmer water dissolves less oxygen than colder water. Consequently, with same saturation rate, warmer water contains less oxygen in absolute terms. The absolute (mg/L) concentration must be therefore compensated according to the temperature of the sample.
Salinity
Similarly with temperature, increasing water salinity decreases its ability to dissolve oxygen. Some of the DO sensors measure also conductivity, and the value is used for calculating salinity and, based on that, oxygen concentration. If built-in conductivity sensor is available, it is important to ensure that it is calibrated and working correctly. If the conductivity is measured with separate sensor, the salinity value must be entered by the user.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Pressure
As mentioned earlier, DO sensors measure the dissolved oxygen pressure in the water (or air), not the absolute concentration. This pressure depends not only on the oxygen concentration, but also on the atmospheric (barometric) pressure, which varies according to elevation and weather. The atmospheric pressure is not, however, needed to be known to obtain correct concentration values. Proper calibration of the sensor is enough to ensure proper measurements.
When the sensor is calibrated, known atmospheric pressure is used. After calibration, the measurements are correct, even though the pressure would change.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Some of the DO sensors do the pressure compensations automatically; see for example Hanna Instruments's model HI 98186.
http://www.hannainst.com/usa/prods2.cfm?ProdCode=HI%2098186&id=004002
Calibration
Electrochemical sensors are more prone to drift and require more frequent calibrations than optical sensors. In principle, teady-state galvanic and polarographic sensors need calibration daily when in use. If the measurements, however, are reliable also with less frequent calibrations, calibration frequency can be reduced.
Optical sensors are more stable than traditional electrochemical sensors. It has been shown that optical sensors can hold their calibration for months. It is still recommended to calibrate the sensor regularly to obtain most correct measurements. The calibration is done by measuring known sample and comparing the measured value to the known real value.
Three main methods for calibrating DO sensor are
- Winkler titration
- Air-saturated water
- Water-saturated air
Cleaning and Maintenance
[need to be paraphrased]
The component of the sensor that is sensitive to contamination is the membrane. Contamination results in lower readings when measuring or lesser slopes when calibrating because a portion of the membrane surface is not available for the diffusion of oxygen. The attempt to compensate for the contamination by adjusting the instrument does not agree with the water principle. It is preferable to clean the membrane. Acetic or citric acid with a concentration of 5--10% (percent in weight!) is used for calcium and iron oxide deposits and warm (<50C) household detergent is used for fats and oils.
Avoid strong mechanical treatment of the membrane during all cleaning activities because its thickness is on the order of m and it is easily destroyed. It is best to use a soft paper towel. Dissolved oxygen not clean the sensor in an ultrasound bath as this may cause the coating of the anodes to peel off.
Regeneration of the sensor becomes necessary when the function responds or when the slope (S) < 0.6 has decreased markedly when calibrating.
Basically, regeneration is required when the electrolyte solution is depleted, when the gold cathode has become coated with lead or silver, when the reference electrode is to xified or when the membrane is damaged or contaminated.
It consists of exchanging the electrolyte solution, cleaning the electrodes and exchanging the membrane head.
It is important to follow the operating manual exactly! Mistakes are then easily avoided.
The following points should be emphasized:
The sensor must be disconnected from the meter. When the sensor is connected and submersed in the cleaning solution, no chemical reaction takes place between the solution and the oxidized reference electrode surface; instead, the cleaning solution may become electrolyzed!
Use the cleaning or electrolyte solution suitable for the particular sensor as stated in the operating manual! A solution that is suitable for silver electrodes cannot regenerate lead electrodes!
Only the gold cathode should be polished; the counter electrode is merely wiped clean with a soft cloth to wipe away easily removable salt deposits! A spotty coating after regeneration of the lead or silver electrodes does not impair measurements!
When polishing the gold electrode, only use the moistened EID abrasive film that has a special grain that polishes and do not scratch!
It is also recommended to use a new membrane head since the used membrane cannot necessarily guarantee that the membrane fits correctly against the gold cathode which is ensured by a spacing lattice on the inside of the membrane. Baggy clothing don't fit either!
Please note: The spacing lattice is clearly visible when the membrane head is held up against the light.
The result of an oxygen measurement can be documented in several ways:
Display of the concentration: The instrument requires the appropriate data of the calibration curve and uses them to calculate the concentration in mg/L (ppm is identical in this case), allowing for the temperature dependency of the individual parameters
Display of the percentage of oxygen saturation: The instrument measures the sensor current and calculates the partial pressure of oxygen according to the calibration. The current air pressure is measured for the calculation of the saturation partial pressure. The display corresponds to the quotient, converted into a percentage.
Applications
A. Foods and Beverages
Many foodstuffs are packed in a Modified Atmosphere Packaging where a low or controlled oxygen level is necessary. Dissolved oxygen levels in some drinks, such as beer, should be kept in specific range. Practice of adding oxygen under pressure to bottled water to make oxygenated water has become more common. These dissolved oxygen measurements required dissolved oxygen probes that can be cleaned at elevated temperatures without being removed from the application.
B. Environmental monitoring
EID's dissolved oxygen data loggers can be left to record dissolved oxygen fluctuations in lakes, rivers etc. Deep sea oxygen probes are used in oceans and deep lakes. EId's dissolved oxygen electrodes with fast response are used to map the dissolved oxygen content of lakes and fishing waters. EId's dissolved oxygen probes are not only raised and lowered in the water, but also towed through the water at different depths to give a total picture of the state of the area concerned.
C. Fish Farming - Aquaculture
Fish farmers needed multi-channel dissolved oxygen meters. Additionally, they need dissolved oxygen monitoring and dissolved oxygen control equipment. EID's dissolved oxygen monitoring and logger are encompass alert units with both high dissolved oxygen alarm and low dissolved oxygen alarm. Equipment introduced by EID in 1977 for controlling dissolved oxygen they are been used all over the world.
D. Water treatment (Re-circulating)
The water is cleaned and filtered through mechanical and biological filters. Ozone can be added to "burn off" pollutants, either by direct ozone injection or by UV ozone activation. This process can be controlled using a redox or ORP measurement. The pH of the water is measured and controlled using a pH meter and pH controller. The dissolved oxygen content is measured and pure oxygen is injected. This oxygen injection can also be used to strip off carbon dioxide. Often only a small proportion of the water is oxygenated at high pressure. The resulting super-saturated water is mixed with the main flow to give healthy dissolved oxygen levels in the growth tanks. EID's In-line dissolved oxygen electrodes, Twist and lock mount dissolved oxygen electrodes or flow cell dissolved oxygen electrodes can be used in such high pressure oxygenation systems.
E. Hatchery and growth tanks
Water level as well as dissolved oxygen should be measured in each tank - the water supply to one tank could be cut off. Oxygen level alarms are set on the dissolved oxygen measurements. Aeration or oxygen injection to each tank is not often practiced in smaller indoor tanks, oxygen is added to the inlet or re-circulated water. Aeration or oxygen injection is, however, seen in larger tanks, requiring a separate dissolved oxygen meter with dissolved oxygen controller system for each tank. This is easily done with EID's MultiProbe TechnologyTM dissolved oxygen metering, logging and control equipment.
F. Sea cages
Since it is difficult to control the dissolved oxygen content of the sea. Dissolved oxygen measurement is very important because feed uptake and dissolved oxygen levels are interconnected. Intensive feeding after fish have experienced low dissolved oxygen levels can not only be a waste of food, but can actually harm the fish. The measurement of dissolved oxygen levels enables feed to be dosed optimally and, if relayed to the shore can warn that the cage should be moved if extremely low dissolved oxygen levels should occur.
G. Transport tanks
Dissolved oxygen measurement should also be performed during transport to the processing plant. A healthy fish gives a better finished product when contain the right level of oxygen. Another situation requiring dissolved oxygen measurement is the transport of juvenile fish to tanks or cages for growing.
H. Oxygen generator control
Pure oxygen meters and oxygen controllers equipment are also used in aquaculture. The purchase of liquid oxygen in bulk is often the most economic solution, but there are many cases where oxygen generators are installed locally. Two of the many advantages of using pure oxygen are that 1) it is possible to super-saturate the water with oxygen and 2) you save pump energy since pumping air means pumping 79% nitrogen and "only" 20.9% oxygen.
I. Waste Water Treatment
It is no longer enough just to filter the water and dump the detritus in the sea. The larger part of the waste is mainly organic, and this must be broken down in sludge tanks and the effluent water controlled and treated as necessary.
Sludge tank dissolved oxygen measurement and control is kept at approximately 2 mg/l.
Flow measurement, suspended solids measurement, sludge blanket detection, conductivity measurement, nitrate measurement and phosphate measurement utilizing EID's Industrial electrodes are also all used to enable the efficient and effective cleaning of waste water.
J. Safety Monitoring
Both oxygen detection in flammable gas and oxygen monitoring in ambient air are examples of this. Blanket gas is often used where flammable substances occur. Blanket gas is gas that cannot burn or sustain fire, i.e. it does not contain oxygen. Volumetric oxygen measurement is carried out both on the blanket gas and the surrounding air, the latter for worker safety. Special versions of the EID's dissolved oxygen electrodes are approved for use in potentially dangerous atmospheres, i.e. in classified areas.
K. Measuring biochemical oxygen demand
The BOD test requires a commitment of five (5) days from initial sample collection to the end of the analysis. During this time, samples are initially seeded with microorganisms and supplied with a carbon nutrient source of glucose-glutamic acid. The sample is then introduced to an environment suitable for bacterial growth at reproducible temperatures, nutrient sources and light within a 20C incubator such that oxygen will be consumed. Quality controls, standards and dilutions are also run for accuracy and precision. Determination of the dissolved oxygen within the samples can be determined through titration. The difference in initial DO readings (prior to incubation) and final DO readings (after a five (5) day incubation period) predicts the BOD of the sample. A suitable detection limit as per environmental quality control is 1 mg/l.
BOD calculations
Steps to calculate Biochemical Oxygen Demand (BOD). They and are based on the addition of a nutrient source (carbon - glucose - glutamic acid) and no nutrient source.
1. The BOD of the blanks (no nutrient source) = DOFinal - DOInitial
2. The BOD of the nutrient added samples = (DOFinal - DOInitial) time dilution factor per 300ml
* 300 ml is based on the volume contained in BOD bottles
The BOD of the sample and standards are calculated by subtracting the final DO from the initial DO and multiplying this factor by the dilution factor. The final value is determined by subtracting out the BOD for the blank from the BOD that has been nutrient enriched.
References
Erlich Industrial Development. D. O. theory. http://www.eidusa.com/Theory_DO.htm
Eutech Instruments, 1997. Introduction to Dissolved Oxygen. http://www.eutechinst.com/techtips/tech-tips15.htm
Fraden J. Handbook of Modern Sensors. New York: AIP Press/Springer; 2004. p. 508-510.
Hanna instruments. Dissolved Oxygen and BOD meter, HI98186. http://www.hannainst.com/usa/prods2.cfm?ProdCode=HI%2098186&id=004002
Regtien P. Measurement Science For Engineers. London: Kogan Page Science; 2004. p. 257-263.
YSI, 2009. The Dissolved Oxygen Handbook. https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Contributors:
Eila Myllylä, Chi-En Shan, Olga Gerasimenko, Judit Nyári & Nhat Nguyen