The Effects of Stir Speed, Airflow Rate, Diffusers, and Volume on. Water Aeration

Transcription

1 The Effects of Stir Speed, Airflow Rate, Diffusers, and Volume on Water Aeration Cycle 3 - Group B1 Group Leader: David Nguyen Katarina Van Wonterghem Joseph Jimenez Justin Chung CBEMS 140A University of California, Irvine Irvine, California November 15, 2016

2 Abstract Water aeration is an important concept and process used in applications such as pond destratification to support fish growth and water waste treatment to remove foul taste, odor from hydrogen sulfide, and other chemicals. An effective aeration process is vital for conserving energy and keeping processing costs low. This experiment explores the oxygenation effects of three stirring speeds, airflow rates, diffusers, and water level heights. Given these various conditions, the overall absorption coefficient (K) and oxygenation capacity (R) was calculated to determine the most efficient settings for each scenario. In each experiment, the highest settings (316 rpm, 9 L/min, and 20 L) offered the highest K and R values and changes in air flow rate had the biggest impact on aeration. Comparatively, K and R values for a volume of 20 liters were 8.59 hr 1 and 1.13 g/hr while a flow rate of 9 L/min gave values of 13.1 hr 1 and 1.75 g/hr. In the case of the three various diffusers, the donut airstone produced the highest K and R at 11.6 hr 1 and 1.51 g/hr, a result of the numerous fine bubbles less than 3-5 mm in diameter that created a larger surface area for oxygen diffusion and the large bubble column that created a more turbulent mixing regime. Objective The main objective of this experiment was to explore the effects of stirring speed, air flow rate, diffusers, and water height on the aeration process for water. Specifically, oxygen concentration in water will be collected to calculate the overall absorption coefficient (the value of concentration of gas in liquid when the atmosphere is composed entirely of the gas under consideration) and oxygenation capacity (the rate of absorption of oxygen during aeration of completely deoxygenated water at 10 C) in each subsequent experiment. Conclusively, this data

3 will be analyzed to confirm oxygen coefficient and oxygen capacity trends, and aim to confirm if bubble size, bubble column and bubble residence time affect the gas-liquid phase diffusion rate in each subsequent experiment. Background Aeration of wastewater began in England as early as 1882 and major advances in aeration technology were developed soon after by Arden and Lockett in One of the earliest patents for a diffuser was in 1904 in Great Britain for a perforated metal plate diffuser [1]. In the early stages of aeration, porous tubes, perforated pipes, double perforated tubes with fibrous material in the annular space and nozzles were used. When issues rose from inefficient diffusivity, an investigation of finer bubbles ensued. Development of sandstone, firebrick, mixtures of sand and glass, and pumice diffusers were tested as potential materials but proved to be too dense which also created high head losses. Thus, Jones and Atwood, Ltd created a concrete cast porous plate that proved to be reliable and were used for many years by Great Britain (Eckenfelder). Around the same time in the United States, bonded silica sand porous plates were produced by Filtros and used in sludge plants. Eventually, aluminum oxide and silica were bonded to these plates raising the permeability from 14.1 m 3 /h to 188 m 3 /h [1]. Clogging of fine bubble diffusers was a problem for porous diffusers and perforated pipes. A study at the Sanitary District of Chicago in 1924 suggested using coarse diffusers to avoid fouling. Soon after, new methods of cleaning the installed diffusers were adopted. By the 1950s, however, many plants decided to use larger orifice type diffusers instead. Many of the newer diffuser designs were built for easy maintenance and accessibility that produced a coarser bubble, which in turn sacrificed oxygen transfer efficiency [1].

4 Ethics Wastewater treatment in order to remove foul taste, odor from hydrogen sulfide, and other chemicals is one of the main applications of aeration. Aeration promotes bacterial growth in wastewater by providing oxygen to the microbes that biodegrade the waste [2]. The supplied oxygen is used by the microbes to break down organic matter into carbon dioxide and water. If an insufficient amount of dissolved oxygen is present, the microbes will not have enough time to biodegrade properly in the septic tanks. This will lead to hydrogen sulphide and methane formation causing foul odor and corrosion in plumbing systems. Organic acids can also form creating low ph conditions that leach metal ions such as iron, manganese, copper, lead, and zinc from the plumbing systems as well. These undesirable conditions make wastewater treatment more difficult and less cost effective. Another area where aeration is used is to oxygenate stagnant bodies of water. Ponds will naturally undergo stratification conditions where multiple layers of different oxygen concentrations are created [3]. The bottom layer will be oxygen-deprived and may cause a decline in ecosystem efficiency and prevent fish from obtaining enough oxygen to survive. Theory Determining the oxygen capacity of a system can be done using both steady and unsteady state analysis. While steady state analysis may be more accurate, unsteady state still provides a reasonable approximation and is much simpler to perform. When a gas liquid interface occurs between soluble fluids, the gas will diffuse into the liquid until the gas concentration in the liquid is in equilibrium with the gas concentration in the atmosphere, forming a steady state. Steady state equilibrium depends on the steady state concentration of the particular gas in the fluid when

5 the atmosphere is exclusively composed of the pure gas. This concentration is referred to as the coefficient of absorption, K. Empirically this K value can be derived using Equation 1 where D 1 [Eq. 1] K = lnd 1 lnd 2 t 1 t 2 = logd 1 logd 2 t 1 t 2 and D 2 are oxygen concentrations in mg/l and t 1 and t 2 are the respective times of those readings. D t is calculated by taking the saturation concentration C s at the measured temperature and subtracting the measured oxygen concentration C t. Saturation concentrations were found in provided tables such as Table A10 in the appendix. The oxygen capacity of a system, or the rate of absorption of oxygen during the aeration of completely deoxygenated water at 10 C, is also calculated from collected data using Equation 2. In this equation K is the coefficient of [Eq. 2] R (10) = KFC (10) V grams/hour absorption found using Equation 1, F is a temperature correct factor also found in Table A10 in the appendix, C s is the saturation concentration from Table A10, and V is the volume of the fluid. Materials and Methods Aeration of water was conducted using an aeration tube to bubble air through a tank at different air flow rates, water stirring rates, water volumes, and air diffusers. The equipment shown in Figure 1 included a Perspex water tank with a capacity of 24.5 liters mounted on a base and backboard, a stirrer, a temperature and oxygen probe, and an aeration tube. Air was supplied to the tank through a pump attached to a flow meter measuring flow rates from 0 L/min to 12 L/min, with an observed maximum output to be 9 L/min when fully opened. Control dials on the backboard allowed the air flow rate and stirring rate to be adjusted. Calibration of the

6 oxygen probe was tested by taking readings in a 5% sodium sulfate solution and while the probe was exposed to air to ensure the device read 0% oxygen and 100% oxygen respectively. In each experimental testing, measurements were taken in both percentage oxygen and milligrams per liter of dissolved oxygen. To deoxygenate the water, a 10% solution of sodium sulphite and a 1% solution of the catalyst cobaltous chloride were created using provided chemicals weighed out on a scale and dissolved in DI water. Before proceeding with each experimental Figure 1: Aeration Apparatus test, the water in the tank was deoxygenated by adding ml of 10% sodium sulphite and 4-7 ml of 1% cobaltous chloride in a three to one ratio until the oxygen concentration was below 10%. Once the experimental trial was set up for each parameter being evaluated, temperature ( C) and oxygen concentration data were recorded every minute from the time the air flow was turned on until the oxygen concentration reached steady state. Steady state was determined by the group to be no more than a 0.05 mg/l change per minute.

7 Oxygen Transfer Under Non-Steady State conditions (Experiment A) The tank was filled to a volume of 20.0 liters by inputting water until the height on the depth gauge read 3l.1 centimeters and was deoxygenated using the method discussed above. To establish a graph with a general oxygen concentration over time, the flow rate was adjusted to 5 L/min, the plastic sparger was attached to the air flow outlet, and the stirring rate was set to a quarter turn resulting in 160 revolutions per minute (rpm), as this was the lowest speed at which the contents of the tank were adequately stirred. Effect of Stirring Speed On Coefficient of Absorption and Oxygen Capacity (Experiment B) For this subsequent experiment, the volume of water and air flow rate were left at 20.0 liters and 5 liters/min respectively and the sparger was attached to the air flow outlet. Water was again deoxygenated using the same method as before. Stirring speeds were set to 130 rpm (¼ turn), 212 rpm (½ turn), and 316 rpm (full turn) for each test run with water being deoxygenated between runs. Effect of Air Flowrate On Coefficient of Absorption and Oxygen Capacity (Experiment C) To examine the effect air flow rate had on water aeration, the volume of water in the tank was kept constant at 20.0 liters, the stirring speed was kept constant at 130 revolutions per minute, and the sparger was attached to the air flow outlet. Water was deoxygenated using the same method as before. Air flow rates to the tank were set to a low value of 5 L/min, a medium value of 8 L/min, and a maximum flow rate with the valve fully open of 9 l/min. The water in the tank was deoxygenated between runs. Effect of Diffuser Type On Coefficient of Absorption and Oxygen Capacity (Experiment E) For this subsequent experiment, the volume of water, stirring rate, and air flow rate were left at 20.0 liters, 130 rpm, and 5 liters/min respectively. Water was again deoxygenated using

8 the same method as before. Three different diffusion attachments were tested by attaching, a plastic sparger, a treble airstone, and a donut airstone to the air flow outlet in separate tests. The water was deoxygenated between each trial. The results of this experiment were compared to the single air stone used by other groups. Effect of Water Volume On Coefficient of Absorption and Oxygen Capacity (Experiment F) To examine the effect water volume had on water aeration, air flow rate was kept constant at 5 L/min, the stirring speed was kept constant at 130 revolutions per minute, and the sparger was attached to the air flow outlet. Water was deoxygenated using the same method as before. The volume of water in the tank was adjusted to a low value of 7.7 liters, medium value of 14.3 liters, and maximum value of 20.0 liters for each trial. Water in the tank was deoxygenated between runs. Results and Discussion Oxygen Transfer Under Non-Steady State Conditions (Experiment A) An oxygenation graph under non-steady state conditions at a fixed temperature, stirring rate, and air flow rate was produced. Figure 2 shows the relationship of the concentration differential in logarithmic scale with time. This trial is used to create a baseline for comparing the data obtained from stirring speed, airflow rate, diffuser type, and water volume experiments. It is later experiments showed that increasing stirring speed, increasing airflow rate, and choosing a diffuser that produces many small bubbles increases oxygenation rate, whereas water volume had no clear relationship with oxygenation rate.

9 Figure 2. Standard Oxygenation Graph under Non-Steady State Conditions The graph consists of a linear region up until t = 15 min where concentration reaches an asymptote, signifying the concentration approaching the saturation value. The linear fit generated a high R 2 value of 0.985, meaning the aeration rate was constant. Using the equations discussed in the theory section, the coefficient of absorption (K) was calculated to be hr 1 and the oxygenation capacity (R) was calculated to be g/hr. The factors that affected K were all factors that affected the slope of the oxygenation rate, which were stir speed, airflow rate, diffuser type, and water volume. The effect of these different factors was tested in later trials. Because K is proportional to R, these factors also affected R. Additionally, temperature of the water affected R because temperature determined the correction factor and saturation concentration of the water. Group A3 had less linear data with a R 2 of 0.94, shown in Figure 3. A linear trend line added to their data points gave a slope of and generated a K value of hr 1. Their

10 volume of water was also 20.0 L, but the temperature of their water on that given day was 23.9 C, giving a Cs of 8.5 mg/l and a correction factor of Using equation 2 the R value found using their data was g/hr. The flat region at the beginning of Figure 3 signifies no observable increase in oxygen concentration in the water. This could have occurred because group A3 added too much sodium sulphite causing the water to be de-oxygenated before the probe could read the oxygen supplied by the air flow. Enough oxygen needed to be supplied to react with all of the sodium sulphite before the oxygen concentration in the water could increase. Figure 3. Group A3 Standard Oxygenation Graph

11 Effect of Stirring Speed On Coefficient of Absorption and Oxygen Capacity (Experiment B) The effect of stirring speed on oxygen absorption had a positive correlation, when stirring speed increased, oxygen absorption rate increased. Figure 4 shows a plot of the logarithm of the concentration differential with respect to time. The graphs have a linear region and an exponential region where the concentration asymptotically approaches the saturation value. Plotting all three different stirring speeds 130 rpm, 212 rpm, 316 rpm, provides a trend of increasing absorption rate is reflected in the Figure 4: Increasing Stirring Speed, Increases Oxygen Absorption Rate increasing magnitude of the slope. The slope of the trend lines at varying speeds with their respective R 2 values are presented in Table 1. A linear trend line fit yielded high R 2 values shown in Table 1.

12 Table 1: Slope Magnitude and R 2 values for Variable Stirring Speed Magnitude of Slopes R 2 values Low (130 rpm) Medium (212 rpm) High (316 rpm) The same trend is reflected in the increasing coefficients of absorption and oxygenation capacity at higher speeds as shown in Table 2. Table 2: Increasing Stirring Speed Increases Coefficient of Absorption (K) and Oxygenation Capacity (R) Low(130 rpm) Medium(212 rpm) High(316 rpm) Coefficient of Absorption (K) 8.59 hr hr hr 1 Oxygenation Capacity (R) 1.13g/hr 1.19g/hr 1.33g/hr From the data, a simple mathematical model can be derived to ascertain the relationship between stirring speed and oxygenation capacity. Figure 5 shows a plot of oxygenation capacity versus stirring speed. The trend line shown in Figure 5 is a linear model for the relationship between oxygenation capacity and stirring speed. The absorption coefficient increased with increased stirring speed rate because of two reasons: water deformation increased the surface area available for oxygen transfer by increasing

13 Figure 5: Increasing Stirring Speed Increases Oxygenation Capacity the surface area of the bubbles, and higher levels of turbulence increased the availability of unsaturated liquid in contact with the bubble. The stirrer induced vortices in the water and at higher stirring speeds the vortices would become deeper exposing more water to the air. Fick s equation for mass transfer implies that with increased surface area there is an increase in flux of mass. Fick s equation also states that the concentration gradient is the driving force of mass transfer. With faster stirring speeds, the saturated fluid in contact with the air was replaced with non-saturated fluid more rapidly. This increased the concentration gradient at the boundary layers causing increased mass transfer. To further investigate the relationship of stirring speed and rate of absorption more speeds need to be tested. According to the Equation 2, increasing the stirring speed will increase the rate of mixing indefinitely, however realistically there must be a limit to this value. To find the

14 upper and lower limits speeds ranging from 5 rpm to over 1000 rpm should be tested. This predicts an exponential curve that plateaus after a certain speed is reached. Group B5 s data was plotted, shown in Figure 6, and shows the same increasing slope magnitude with increased stirring speed. Their data was irregular but still the general trend of increasing absorption rate was seen. Figure 6: Group B5, Increasing Stirring Speed Increased slope magnitude. Effect of Air Flowrate On Coefficient of Absorption and Oxygen Capacity (Experiment C) A positive correlation between air flow rate, absorption coefficient and oxygen capacity was observed in the data. Increasing air flow rate decreased the time it took the liquid to reach the saturation concentration level. This is observed in Figure 7, the plot of concentration over time for flow rates of air at 5L/min, 8L/min, 9L/min.

15 Figure 7: Increasing Airflow Increases Oxygen Absorption Rate The absolute magnitude of the slopes increased with increasing air flow, as seen in Table 3. A higher value slope means the liquid reached saturation concentration faster than a lower slope. With a trend of increasing slope magnitude when increasing airflow, it can be concluded that liquid absorption rate increases with airflow. Table 3: Increasing slope Magnitude and R2 values for Variable Airflow Slopes R2 values low(5l/min) medium(8l/min) high(9l/min)

16 The oxygenation capacity and absorption coefficients also increased with airflow rate as seen in Table 4. Also included in table is the time it took the liquid to reach steady state concentration at each flow rate. Final steady state concentration values are listed in Table 4. The time to reach steady state can provide a relative picture of the time to reach saturation concentration. Table 4: Increasing Airflow Increases Coefficient of Absorption and Oxygenation Capacity and Decreases the Time to Steady State Airflow Rates Low(5L/min) Medium(8L/min) High(9L/min) Coefficient of Absorption (K) 8.67hr hr hr 1 Oxygenation Capacity (R) 1.14g/hr 1.51g/hr 1.75g/hr Time took to Steady-State Concentration 18.0mins 15.0mins 14.0mins Steady-State Concentration 7.92mg/L 8.08mg/L 8.11mg/L Because the concentration of oxygen will asymptotically approach saturation point, to save time the experiment was conducted until a defined steady state. This captures the relative magnitudes of the time at different airflows but does not reveal the actual time it takes to reach saturation point. The saturation concentration for oxygen in water under the experimental conditions of 1 atm and 23.4 C was 8.60 mg/l. Decreasing the size of the bubbles will increase absorption coefficient and oxygenation capacity. This is because mass transfer of oxygen depends on exposed surface areas. Smaller bubbles allowed more air to be in contact with the liquid at any given moment. This is because the ratio of surface area to volume increases as volume decreases. Decreasing bubble velocity will increase the absorption coefficient and oxygenation capacity. Bubble velocity refers to the time the bubble spends under water before reaching the surface. The

17 longer the bubbles spend under water more mass transfer can take place. The saturated liquid and gas boundary layers cause resistances to mass transfer. The limiting variable in the liquid boundary layer is the diffusion of oxygen in water. Oxygen is carried away from the boundary layer by diffusion. Since no more oxygen can dissolve in a saturated solution the oxygen in the boundary layer must be carried away to continue mass transfer. The limiting variable in the gas boundary layer is the diffusion rate of oxygen through the air. Oxygen is supplied to the boundary layer by gas diffusion. To maximize the mass transfer rate the gas boundary layer needs to remain saturated to keep up the supply of oxygen that is available to dissolve in the liquid boundary layer. Therefore, decreasing bubble velocity increases the time of mass transfer increasing the likelihood of oxygen overcoming diffusion resistances Figure 8 shows oxygenation capacity plotted against airflow rates fitted with a linear trend line. The trend line equation is a simple mathematical model of the relationship between air flow rate and oxygenation capacity. Figure 8: Increasing Airflow Increases Oxygenation Capacity

18 Group B5 s data was plotted, shown in Figure 9, and analyzed and found the same trend of increasing magnitude of the slope with increased airflow. Figure 9: Group B5, Increasing Airflow, Increased Magnitude of Slope Effect of Diffuser Type On Coefficient of Absorption and Oxygen Capacity (Experiment E) Diffusers that created a larger bubble column and smaller bubbles led to an increase in oxygen absorption. Figure 10 shows the relationship of logarithm of the concentration differential with time using the sparger, treble airstone, and donut airstone. The slope of the trend lines across the 3 diffuser types and their R 2 values are presented in Figure 10. All three graphs have the linear region where oxygen transfer is constant as well as the asymptotic zone where the oxygen concentration is reaching the saturation concentration.

19 Figure 10: Oxygenation Graphs Across Different Diffusers The sparger had larger air holes, producing more coarse bubbles than the treble and donut airstones. The two airstones produced same size bubbles, but the donut airstone produced the larger bubble column. The steepness of slope increased when bubbles were finer and bubble column size was larger as shown in Figure 10. The steeper the slope, the faster the liquid reaches saturation concentration. Thus, as the surface area of oxygen transfer increased and bubble size decreased, the rate of oxygenation also increased. Oxygenation capacity and coefficient of absorption also increased as size of bubble column increased and bubble size decreased. Table 5 shows K and R values as well as the time to reach saturation concentration with each diffuser type. Because of time constraints, the trials were stopped when the defined steady state was reached and not when the trial reached saturation concentration.

20 Table 5: Decreasing Bubble Size and Increasing Number of Bubbles Increases Coefficient of Absorption and Oxygenation Capacity and Decreases Time to Steady State Bubbler Type Sparger Treble Airstone Donut Airstone Coefficient of Absorption (K) 8.67 hr hr hr 1 Oxygenation Capacity (R) 1.14g/hr 1.34g/hr 1.52g/hr Time took to Steady-State Concentration 18.0mins 16.0mins 15.0mins Steady-State Concentration 7.92mg/L 8.00mg/L 8.02mg/L The trials using the donut and treble airstones reached steady-state concentration faster than the trial using the sparger. This is because the airstones produced finer bubbles than the sparger. According to literature, finer bubbles increased oxygen transfer. The smaller the bubble, the more bubble surface area per unit volume and thus the greater oxygen transfer exchange (mooer). Between the donut and treble airstones, the donut reached steady-state concentration faster. This is because the donut airstone created a larger air bubble column than the treble airstone. The larger the air bubble column, the more bubbles, thus the more oxygen being transferred. Additionally, a larger bubble column created a more turbulent mixing regime, which caused more mixing in the system. An increase in mixing increased rate of oxygen transfer as observed in the stirring rate experiment. Although both bubble size and bubble column size affect the rate of oxygen transfer, it is observed that bubble size has a greater impact. This is shown in the greater difference in steepness of slope between the three different diffusers in Figure x3. The difference in steepness

21 between the airstones is much less than the difference in steepness between the airstones and the sparger, meaning the effect of bubble size was greater than the effect of bubble column size. Effect of Water Volume On Coefficient of Absorption and Oxygen Capacity (Experiment F) There was no observable trend when changing the water height. It was hypothesized that as water level decreased, oxygen absorption rate would increase. However, experimental data does not support this trend as shown in Figure 11. Figure 11: Oxygenation Graphs at Different Water Heights The lowest water height level had the greatest coefficient of absorption coefficient and lowest oxygenation capacity as expected. Additionally, oxygenation capacity increased with water height as hypothesized. Time to reach steady-state concentration was expected to decrease as water height decreased, however, it took 19 minutes to reach steady-state concentration at a

22 water height of 220mm but only 18 minutes at 311mm as shown in table 6. This means the intermediate water height had the lowest oxygen transfer rate, which was not expected. This means that human error was involved or there must have been a different driving force of aeration rate at different water heights. Table 6: Water Height s Effect On Oxygenation Values Water Column Height 120 mm 220 mm 311 mm Coefficient of Absorption (K) 9.67hr hr hr 1 Oxygenation Capacity (R) 0.496g/hr 0.705g/hr 1.13g/hr Time took to Steady-State Concentration 15.0mins 19.0mins 18.0mins Steady-State Concentration 7.95mg/L 7.97mg/L 7.92mg/L One reason the data did not match what was expected was because the 120mm height trial started at an initial concentration of 0.85 mg/l, the 220mm trial started at 0.80 mg/l, and the 311mm trial started at 0.75 mg/l. Because each trial was run at a different initial concentration, the effect of water height on aeration rate cannot be accurately observed. Another reason the data did not follow the expected trend was because the effect of residence time may have been more profound at higher water heights, and the effect of stir speed on aeration rate may have had a more profound effect at lower water heights. At higher water heights, the air bubbles spend more time in the water and therefore transfer more oxygen to the water. Additionally, the effect of stirring is less profound because the stirrer is only mixing a fraction of the water. At lower water heights, the air bubbles transfer less oxygen to the water, however, the effect of stirring is much greater because the stirrer is mixing a large portion of the water when the height is low. Additionally, because the stirrer was so close to the top of the water head, the stirring could have drawn in oxygen from the atmosphere. When conducting this

23 experiment for the 120 mm height it was observed that the oxygen concentration increased even before airflow was turned on, increasing the validity of this reason. When water height is in between high and low water heights, both the effect of stirring is less impactful and the residence time of the bubbles is decreased, leading to an overall lower oxygen transfer rate. Group B5 s data was plotted, shown in Figure 12. Again it is observed that the intermediate water height had a lower oxygen transfer rate than both the low and high water height trials. This supports the hypothesis that at high water heights, longer residence time increases the rate of oxygen transfer whereas at low water heights, stirring rate increases the rate of oxygen transfer. Figure 12: Group B5 Oxygenation Graph across Different Water Heights Because there were no observable trends between the three water heights, it was impossible to see the effect of water height on aeration rate. In order to make a more accurate observation, trials must be run at least every 25mm starting from 100mm to 325mm. With this many trials, the hypothesis that different parameters affect aeration rates more profoundly at different water

24 heights may be tested. The trials would allow the most efficient water height to be observed and allow a theory to be developed about how aeration rate at different water heights is driven by different parameters such as stir speed and air bubble residence time. Conclusion Aeration is a process seen in nature in waterfalls, streams, floods, and water plant photosynthesis. Fine bubble and coarse bubble aeration are utilized to accelerate the naturally occurring process through the use of mechanical stirring, sufficient air flow rate, specific air diffusers, and water height. In the case of wastewater treatment, the aeration process will promote bacterial growth to allow biodegradation to occur. The oxygen concentration and temperature was easily measured using the oxygenation probe, and calculations of the overall absorption coefficient and oxygenation capacity were performed to find relations and trends. Overall, rotation per minute (rpm) and airflow rate increased linearly with overall absorption coefficient and oxygenation capacity. Increasing the rpm by one will have an increase of overall absorption coefficient by 0.116% and oxygenation capacity by 0.126%. Increasing the airflow rate by 1 L/min will increase K by 9.43% and R by 9.74%. The donut airstone was the favored diffuser over the plastic sparger with an increase in K and R values by 25.4%. Height level of the tank seem to appear to have parabolic relation with the K values while the R values are linear. Knowing the effects of all the variables listed would help one choose the best settings needed for any aeration network design in consideration to have the most stable and efficient system.

25 References [1] Eckenfelder, W.W Aeration: Principles and Practice, Volume 11. Water Quality Management Library [2] Oxymen Smarter Aeration [Online]. Available: Accessed: Nov. 12, [3] Why is Aeration Important [Online]. Available: Accessed: Nov. 12, [4] Bubble Diffuser Differences, [Online]. Available: Accessed: Nov. 12, [5] Disc Diffusers, Airston and Silica Diffusers [Online]. Available: Accessed: Nov. 12, 2016

26 Appendix Table A1: Baseline. Stir speed = 130 rpm, airflow rate = 5 L/min, plastic sparger, water level height = 311 mm Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct) Table A2: 212 rpm stirring speed, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct)

27 Table A3: 316 rpm stirring speed, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct)

28 Table A4: 8 L/min airflow rate, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct) Table A5: 9 L/min airflow rate, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct)

29 Table A6: Treble airstone diffuser, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct) Table A7: Donut airstone diffuser, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct)

30 Table A8: 120 mm water height, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct) Table A9: 220 mm water height, all other variables the same to baseline Saturation Conc, Cs Temp. Correction Factor, F Measured Oxygen Conc, Ct Coeff. Of Abs, K (1/hr) Oxygenation Capacity, R (g/hr) Time (min) Temp ( C) (Cs - Ct)

31 Table A10: Saturation concentration and temperature correction factor at given temperature Saturation Conc, Cs Temp. Correction Factor, F Temp ( C)