Oxygen in Liquids (DISSOLVED OXYGEN)
Dissolved Oxygen – the amount of dissolved oxygen in a body of water as an indication of the degree of the health of water and its ability to support a balanced aquatic ecosystem. Oxygen – is a clear, colorless, odorless, and tasteless gas that dissolves in water. Small but important amounts of it are dissolved in water. OXYGEN: Aquatic Life Depends on it
Plants and Animals depend on dissolved oxygen for survival. Lack of dissolved oxygen can cause aquatic animals to leave quickly they are or face death. Factors Affecting Oxygen Levels
Rate of Photosynthesis
Degree of Light Penetration (turbidity & water depth)
Degree of Water Turbulence or Wave action
The amount of oxygen used by respiration and decay of organic matter Oxygen in the Balance
Dissolved Oxygen levels that are at 90% and 110% saturation level or higher consistently considered healthy or good. If the Dissolved Oxygen are below 90%, there may be large amounts of oxygen demanding materials. What Is Dissolved Oxygen In Water?
Dissolved oxygen in water is vital for underwater life. It is what aquatic creatures need to breathe. Why Is Dissolved Oxygen Important?
Just as we need air to breathe, aquatic organisms need dissolved oxygen to respire. It is necessary for the survival of fish, invertebrates, bacteria, and underwater plants. How Is Dissolved Oxygen Measured?
Dissolved oxygen concentration can be reported as milligrams per liter, parts per million, or as percent air saturation.
It is very similar to the galvanic cell. However, the polarographic cell has two noble-metal electrodes and requires a polarizing voltage to reduce the oxygen.
The dissolved oxygen in the sample diffuses through the membrane into the electrolyte, which usually is an aqueous KC1 solution. If there is a constant polarizing voltage (usually 0.8 V) across the electrodes, the oxygen is reduced at the cathode, and the resulting current How is proportional to the oxygen content of the electrolyte. This current flow is detected as an indication of oxygen content.
All galvanic cells consist of an electrolyte and two electrodes (Figure 8.43c). The oxygen content of the electrolyte is equalized with that of the sample. The reaction is spontaneous; no external voltage is applied. In this reaction, the cathode reduces the oxygen into hydroxide, thus releasing four electrons for each molecule of oxygen. These electrons cause a current flow through the electrolyte.. The magnitude of the current flow is in proportion to the oxygen concentration in the electrolyte.
Flow through Cells
In the flow-through cells, the process sample stream is bubbled through the electrolyte. The oxygen concentration of the electrolyte is therefore in equilibrium with the sample’s oxygen content, and the resulting ion current between the electrodes is representative of this concentration. These types of cells are usually provided with sampling consisting of (but not limited to) filtering and scrubbing components and flow, pressure, and temperature regulators.
Thallium cells are somewhat unique in their operating principle and cannot be classified into the category of either galvanic or polarographic cells. At the same time, they are of the electrochemical type. One thallium-electrode cell design is somewhat similar in appearance to the unit illustrated on Figure 8.43c except that it has no membrane or electrolyte. This cell has a thallium outer-ring electrode and an inner reference electrode. When oxygen contacts the thallium, the potential developed by the cell is a function of the thallous ion concentration at the face of the electrode, and the ion concentration is in proportion to the concentration of dissolved oxygen.
In this case, a compound containing ruthenium is immobilized in a gas-permeable matrix called a sol-gel. Sol-gels are very low-density, silica-based matrices suitable for immobilizing chemical compounds such as the ruthenium compound used in this measurement technique. Effectively, the sol-gel is equivalent to the membrane in a conventional DO sensor. Using fiber optics, light from a light-emitting diode is transferred to the backside of the sol-gel coating. The emitted fluorescence is collected from the backside of the sol-gel with another optical fiber and its intensity is detected by photodiode. A simplified sensor design is shown in Figure 8.43g.
If no oxygen is present, the intensity of the emitted light will be at its maximum value. If oxygen is present, the fluorescence will be quenched, and the emitted intensity will decrease.
The Winkler Method is a technique used to measure dissolved oxygen in freshwater systems. Dissolved oxygen is used as an indicator of the health of a water body, where higher dissolved oxygen concentrations are correlated with high productivity and little pollution.
Biochemical Oxygen Demand (BOD)
Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria
present working to decompose this waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline.
Biochemical oxygen demand (BOD) is a measure for the quantity of oxygen required for the biodegradation of organic matter (carbonaceous demand) in water.It can also indicate the amount of oxygen used to oxidise reduced forms of nitrogen (nitrogenous demand), unless their oxidation is prevented by an inhibitor. A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20 ̊̊̊̊C).
BOD is devided in two parts which is Carbonaceous Oxygen Demand and the Nitrogenous Oxygen Demand.
Carbonaceous Oxygen Demand – it is the amount of oxygen consumed by the microorganisms during decomposing carbohydrate material.
Nitrogenous Oxygen Demand – it is the amount of oxygen consumed by the microorganisms during decomposing nitrogenous materials.
Relationship of DO and BOD
If the Dissolve Oxygen (DO) of a water is high, the Biological Oxygen Demand (BOD)is low. If the BOD of the water is hight, the DO is low.Therefore DO and BOD is inversely Proportional to each other.
Why we should need to know BOD?
BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die.
Knowledge of oxygen utilization of a polluted water supply is important because:
1. It is the measure of the pollution load, relative to oxygen utilization by other life in the water; 2. It is the means for predicting progress of aerobic decomposition and the amount of self-purification taking place; 3. It is the measure of the oxygen demand load removal efficiency by different treatment process.
Factors that contributes to variations in BOD
Is the bacterial culture that affects the oxidation of materials in the sample. If the biological seed is not acclimated to the particular wastewater, erroneous results are frequently obtained.
The BOD results are also greatly affected by the pH of the sample, especially if it is lower than 6.5 or higher than 8.3. In order to achieve uniform conditions, the sample should be buffered to a pH of about 7.
Standard test condition calls for a temperature of 20 ̊C (68 ̊F). field tests often require operation at other temperatures and, consequently, the results tend to vary unless temperature corrections are applied.
The presence of toxic materials may result increase in the BOD value as a specific sample is dilluted for the BOD test.Consistent value may be obtained either by removing the toxic materials from the sample or By developing a seed that is compatible with the toxic material in the sample.
The usual standard lab test incubation time is 5 days, results may occur at a flat part or occur at a steeply rising portion.Depending on the type of seed and the type of oxidable material, divergent result can be expected.
In the usual course BOD test, the oxygen consumption rises steeply at the beginning of the test owing to attack on carbohydrate materials. Another sharp increase in oxygen utilization occurs sometime during 10th to 15th day in those samples containing nitrogenous materials.
How we determine or measure BOD?
Five-Day BOD Procedure
The BOD test takes 5 days to complete and is performed using a dissolved oxygen test kit. The BOD level is determined by comparing the DO level of a water sample taken immediately with the DO level of a water sample that has been incubated in a dark location for 5 days. The difference between the two DO levels represents the amount of oxygen required for the decomposition of any organic material in the sample and is a good approximation of the BOD level.
1. Take 2 samples of water
2. Record the DO level (ppm) of one immediately using the method described in the dissolved oxygen test. 3. Place the second water sample in an incubator in complete darkness at 20oC for 5days. If you don’t have an incubator, wrap the water sample bottle in aluminum foil or black electrical tape and store in a dark place at room temperature (20 ̊C or 68 °F). 4. After 5 days, take another dissolved oxygen reading (ppm) using the dissolved oxygen test kit. 5. Subtract the Day 5 reading from the Day 1 reading to determine the BOD level. Record your final BOD result in ppm.
Generally, when BOD levels are high, there is a decline in DO levels. This is because the demand for oxygen by the bacteria is high and they are taking that oxygen from the oxygen dissolved in the water. If there is no organic waste present in the water, there won’t be as many bacteria present to decompose it and thus the BOD will tend to be lower and the DO level will tend to be higher. At high BOD levels, organisms such as macro invertebrates that are more tolerant of lower dissolved oxygen may appear and become numerous. Organisms that need higher oxygen levels) will NOT survive.
Extended BOD Test
Continuation of BOD test beyond 5 days shows a continuing oxygen demand, with a sharp increase in BOD rate at the 10th day owing to nitrification. The latter process involves biological attack on nitrogenous organic material accompanied by an increase in BOD rate. The oxygen demand continues at a uniform rate for an extended time.
Manometric BOD Test
In the manometric procedure, the seeded sample is confined in a closed system that includes an appreciable amount of air . As the oxygen in the water is depleted, it is replenish by the gas phase. A potassium hydroxide (KOH) absorber within the system removes any gaseous carbon dioxide generated by bacterial action. The oxygen removed from the air phase results in a drop in pressure that is that is removed with a manometer. This fall is then related to the BOD of the sample.
Electrolysis System for BOD
The measuring principle for all electrolytic respirometers is quite similar. As micro-organisms respire they use oxygen converting the organic carbon in the solution to CO2 gas, which is absorbed to alkali. This causes a reduction in the gas pressure, which can be sensed with various sensors or membranes. A small current is created in electrolysis cell and this generates oxidation/reduction reactions in the electrolysis cell and oxygen is formed at the anode.
Electrolysis of water can supply oxygen to a closed system as incubation proceeds . At constant current, the time during which electrolysis generates the oxygen to keep the system pressure constant is a direct measure of the oxygen demand. The amount of oxygen produced by the electrolysis correlates with the amount of oxygen consumed by bacteria.
Chemical Oxygen Demand (COD)
Is the standard method for indirect measurements of the amount of pollution in a sample of water that cannot be oxidized biologically. Is based on the chemical decomposition of organic and inorganic contaminants, dissolved or suspended in water.
Why Measure Chemical Oxygen Demand?
It is often measured as a rapid indicator for organic pollutant in water. Normally measured in both municipal and industrial wastewater treatment plants and gives an indication of the efficiency of the treatment process. It is measured on both influent and effluent water.
Standard Dichromate COD Procedure
A sample is heated to its boiling point with known amounts of sulfuric acid and potassium dichromate. The loss of water is minimized by the reflux condenser.
After 2 h, the solution is cooled, and the amount of dichromate that reacted with oxidizable material in the water sample is determined by titrating the excess potassium dichromate with ferrous sulfate. Dichromate consumed is calculated as to oxygen equivalent for the sample and stated as milligrams of oxygen per liter of sample (ml/l).
Factors preventing the concordance of BOD values to COD values: Many organic materials are oxidizable by dichromate but not biochemically oxidizable, and vice versa. For example, pyridine, benzene, and ammonia are not attacked by the dichromate procedure. A number of inorganic substances such as sulfide, sulfites, thiosulfates, nitrites, and ferrous iron are oxidized by dichromate, creating an inorganic COD that is misleading when estimating the organic content of wastewater. Although the factor of seed acclimation will give erroneously low results on the BOD tests, COD results are not dependent on acclimation.
Chlorides interfere with the COD analysis, and their effect must be minimized in order to obtain consistent results. The standard procedure provides for only a limited amount of chlorides in the sample. This is usually accomplished by diluting the sample to achieve a lower chloride concentration and interference. This can be a problem for low COD concentration samples, as the dilution may dilute the COD concentration below the detection level or to levels at which accuracy and repeatability are poor.
The term COD usually refers to the laboratory dichromate oxidation procedure, although it has also been applied to other procedures that differ greatly from the dichromate method but which do involve chemical reaction. These methods have been embodied in instruments both for manual operation in the laboratory and for automatic operation online. They have the distinct advantage of reducing analysis time from days (5-day BOD) and hours (dichromate, respirometer) to minutes.
Automatic On-Line Designs
Takes a 5 cc sample from the flowing process stream.
Injects it into the reflux chamber after mixing it together with dilution water (if any) agents. One ozone-based scheme enriches dilution water with and with two reagents: dichromate solution and sulfuric acid. The reagents also contain an oxidation catalyst (silver sulfate) and a chemical that complexes chlorides in the solution (mercuric sulfate). The mixture is boiled at 302°F (150°C) by the heater.
Vapors are condensed by the cooling water in the reflux condenser. During which the dichromate ions are reduced to trivalent chromic ions, as the oxygen demanding organics are oxidized in the sample. The chromic ions give the solution a green color. The COD concentration is measured by detecting the amount of dichromate converted to chromic ions by measuring the intensity of the green color through a fiber-optic detector. The microprocessor-controlled package is available with automatic zeroing, calibration, and flushing features.
Sampling and Traditional Parameter
pH, Standard Units
6.0 – 9.0
Biological Oxygen Demand (BOD)
≤ 30 ppm
Chemical Oxygen Demand (COD)
≤ 200 ppm
COD has a large value than BOD because BOD measurement is based only in decomposition of organic matter while COD measures the decomposition of both organic and Inorganic compound.
Sources of Error
Cause of using nonhomogeneous sample is the largest error.
Use of volumetric flasks and volumetric pipettes with a large bore. Oxidizing agent must be precisely measured.
Make sure that the vials are clean and free of air bubbles.
Always read the bottom of the meniscus at eye level.
Total Oxygen Demand (TOD)
The quantitative measurement of the amount of oxygen used to burn the impurities in a liquid sample. Thus, it is a direct measure of the oxygen demand of the sample. Measurement is by continuous analysis of the concentration of oxygen in a combustion process gas effluent. A quantitative measurement of all oxidizable material in a sample water or wastewater as determined instrumentally by measuring the depletion of oxygen after high-temperature combustion. BOD and COD have long time cycles. COD use corrosive reagents with the inherent problem of disposal. Analysis is faster, approximately 3 min, and uses no liquid reagents in its analysis. Can be correlated to both COD and BOD.
Unaffected by the presence of inorganic carbon.
Also indicate noncarbonaceous materials that consume or contribute oxygen Since the actual measurement is oxygen consumption. Reflects the oxidation state of the chemical compound.
The oxidizable components in a liquid sample introduce into the combustion tube are converted to their stable oxides by a reaction that disturbs the oxygen equilibrium in the carrier gas steam. The momentary depletion in the oxygen concentration in the carrier gas is detected by an oxygen detector and recorded as a negative oxygen peak.
Upon a signal from a cycle timer, the air actuator temporarily moves the valve to its “sample fill” position. At the same time, an air-operated actuator moves a 20-ul sample through the valve into the combustion tube. A stream of oxygen-enriched nitrogen carrier gas moves the slug of sample into the combustion tube.
Rotary Sampling Valve
A motor continuously rotates a sampling head, which contains a built-in sampling syringe. For part of the time, the tip of the syringe is over a trough that contains the flowing sample. 2 or more cam ramps along the rotational path cause the syringe plunger to rise and fall, thus rinsing the sample chamber. Just before the syringe reaches the combustion tube, it picks up a 20-ul sample. As it rotates over the combustion tube, it discharges the sample.
Platinum-lead Fuel Cell
Fuel Cell – Generates a current in proportion to the oxygen content of the carrier gas passing through it. Before entering the cell, the gas is scrubbed in a potassium hydroxide solution, both to remove acid gases and other harmful combustion products to humidify the gas. The oxygen cell and the scrubber are located in a temperature-controlled compartment. The fuel cell output is monitored and zeroed to provide a constant baseline. The output peaks are linearly proportionate to the reduced concentration of oxygen in the carrier gas as a result of the sample’s TOD. Yttrium-doped Zirconium Oxide Ceramic Tube
Coated on both sides with a porous layer of platinum. It is maintained at an elevated temperature and also provides an output that represents the reduction in oxygen concentration in the carrier gas that is a result of the sample’s TOD. The operation of these oxygen detectors involves the ionization of oxygen in both a sample and a known reference gas stream. When the sample and reference gas streams come in contact with the electrode surfaces, oxygen ionizes into O-2 ions.
The oxygen ion concentrations in each stream is a function of the partial pressure of oxygen in the stream. The potential at each electrode will depend on the partial pressure of oxygen in the gas stream. The electrode with higher potential (higher oxygen concentration) will generate oxygen ions, whereas the electrode with lower potential (lower oxygen concentration) will convert oxygen ions to oxygen molecules. Calibration
Analysis is by comparison of peak heights or areas to a standard calibration curve. To prepare this curve, known TOD concentrations of a primary standard (KHP) are prepared in distilled and deionized water. Standard solutions are stable for several weeks at room temperature. Water solutions of other organic compounds can also be used as standards.
Several analyses can be made at each calibration concentration, and the resulting data are recorded as parts per million (ppm) TOD vs. peak height or area.
Many regulatory agencies recognize as the basis for oxygen-depleting pollution control only BOD or COD (preferably BOD) measurements of pollution load, because they are concerned with the pollution load on receiving waters, which is related to lowering the DO due to bacterial activity. If other methods described are to be used to satisfy legal requirements of pollution load in effluents or to measure BOD removal, it is important to establish a correlation between the other methods and BOD or COD (preferably BOD).
a measurement of property of the sample, i.e. the amount of oxygen required for bacterial oxidation of bacterial food in the water, the BOD dependence of the oxygen demand on the nature of the food as well as on its quantity dependence of the oxygen demand on the nature and amount of the bacteria
Another extensive study concluded the following:
(1) A reliable statistical correlation between BOD and COD of a wastewater and its corresponding TOD can frequently be achieved, particularly when the organic strength is high and the diversity in dissolved organic constituents is low. (2) The relationship is best described by a least squares regression with the degree of fit expressed by the correlation coefficient (3) The observed correspondence of COD-TOD was better than that of COD-BOD for the wastewaters. (4) The BOD-COD ratio of an untreated wastewater is indicative of the biological treatment possible with the particular wastewater. Comparison:
The oxygen required when a population of bacteria causes the oxidation reaction in a population of bacteria.
The oxygen equivalent when the oxidation is carried out with a chemical oxidizing reagent such as potassium dichromate.
The oxygen equivalent when oxidation is caused by heating the sample in a furnace in the presence of a catalyst and oxygen.
Utilize bacteria to oxidize the pollutants
Measured through chemical oxidation and catalytic combustion techniques
Oxidize the sample in a catalyzed thermal combustion process and detect both the organic and inorganic impurities in a sample
5 days – 30 mg/L
2 hours – 250-500 ppm
3 minutes – 100-100,000 mg/L
3 – 20% / $500 – $20,000
2 – 10% / $8,00 – $20,000
2 – 5% / $5,000 – $20,000