CHAPTER 6: Aeration pre-treatment
6.3 RESULTS
Effect of air flowrate and CO2 concentration on the pH and calcium concentration The rate at which the pH and calcium concentration change with time for different flow rates of air is shown in Figure 6-1. The faster the air flow rate, the quicker the pH and calcium concentration decreased. However, the marginal decrease in reaction time from a flow rate of 1.5 to 3 L min-1 was
much greater than from 6 to 9 L min-1.Figure 6-1B shows two distinct regions where the calcium concentration decreases and then remains constant. The 9 L min-1 flow rate resulted in a pH of 8.5 and a concentration of 78 mg Ca2+ L-1 after 6 hours. The final calcium concentration ranged from 18 to 179 mg L-1. The deviation in the 1.5 L min-1 pH curve was due to a build-up of calcite on the pH probe overnight, the probe was cleaned and re-calibrated at 20 hours.
The experimentally determined pH and calcium concentration, as a function of time for varying CO2
concentration between 0.04 to 100%, are shown in Figure 6-1C and Figure 6-1D, with the time in Log hours. Increasing the CO2 concentration decreased the time required to reach the system's equilibrium pH from over 24 hours with air to 1 hour with 1% CO2 and 5 minutes for 100% CO2. The concentration of CO2 that will dissolve in a liquid is governed by Henry’s Law. Therefore, an increase in CO2
concentration in the air also defined the equilibrium pH. The time required to reach a minimum calcium concentration was reduced to 1 hour for 1% CO2. However, for 100% CO2 there no significant calcium removal was observed. The reason for this is explained in section 4.2.
Figure 6-1: Experimentally determined pH (A) and remaining calcium concentration (B) vs time for air flowrates of 1.5, 3, 6, and 9 L min-1. Experimentally determined pH (C), and remaining calcium concentration (D) for a flow rate of 1.5 L min-1 and CO2 concentration of 0.04%, 1%, and 100%. The lighter shading for the
0 200 400 600 800 1000 1200 1400
0.001 0.01 0.1 1 10 100
Calcium remaining (mg L-1)
Time Log(hours) 6
7 8 9 10 11 12 13
0.001 0.01 0.1 1 10 100
pH (-)
Time Log(hours) 8
9 10 11 12 13
0 3 6 9 12 15 18 21 24 27
pH (-)
Time (hours)
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0 3 6 9 12 15 18 21 24 27
Calcium remaining (mg L-1)
Time (hours)
A B
1.5 L min-1 3 L min-1 6 L min-1 9 L min-1
1.5 L min-1 3 L min-1 6 L min-1 9 L min-1
C D
100 % CO2
1 % CO2
0.04 % CO2
Urea stability
Figure 6-2A shows that the concentration of urea remained constant throughout the 30 hours of air bubbling, of which for more than 24 hours the pH was below the minimum threshold pH of 11, thus indicating that no significant urea loss had occurred during the experiment. The concentration of free and saline ammonia decreased at the start of the experiment as it was stripped from the solution (Figure 6-2B). Stabilized urine will have an initial concentration of ammonia. However, the concentration is typically less than 400 mg L-1 (Figure 6-2) and is significantly less than the concentration present in hydrolyzed urine. The concentration of ammonia after the initial stripping process remained constant, further indicating that urea hydrolysis did not occur.
Figure 6-2: Urea concentration (A), ammonia concentration (B), and pH over 30 hours with an air flow rate of 3 L min-1.
Analysis of precipitates
The precipitate was analyzed via SEM and XRD, both of which confirmed the sample to be calcium carbonate. The XRD analysis indicated that no other crystalline phase was present. No calcium hydroxide was present in the precipitant. The crystal morphology can be determined from SEM imagery which can be found in Appendix C. SEM showed that the particles exist in a cube-like formation with a size range of approximately 2-5 µm (Figure C-3). The regular cubic formation indicates that the calcium carbonate is in the form of calcite (Siva et al., 2017), which is in agreement with the XRD analysis.
8 9 10 11 12 13
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0 3 6 9 12 15 18 21 24 27 30
pH (-)
Urea (mg-N L-1)
Time (hours)
Urea pH
8 9 10 11 12 13
0 50 100 150 200 250 300 350 400
0 3 6 9 12 15 18 21 24 27 30
pH (-)
Ammonia (mg-N L-1)
Time (hours)
Ammonia pH
A B
Model fit
A comparison of the experimental and simulated pH and calcium data for 0.04% CO2 (A and B), 1%
CO2 (C and D), and 100% CO2 (E and F) is shown in Figure 6-3. Comparison of the experimental and simulated data and model calibration for different air flow rates can be found in Figure C-4 in Appendix C.
Figure 6-3: Comparison of simulated (- - -) and experimental (—) results for a flowrate of 1.5 L min-1 and a 8
9 10 11 12 13
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pH (-)
Time (hours) Series1
Series2
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0 3 6 9 12 15 18 21 24 27
Calcium (mg L-1)
Time (hours)
7 8 9 10 11 12 13
0 0.5 1 1.5 2 2.5 3
pH (-)
Time (hours)
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0 0.5 1 1.5 2 2.5 3
Calcium (mg L-1)
Time (hours)
6 7 8 9 10 11 12 13
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
pH (-)
Time (min)
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0 1 2 3 4 5 6 7 8 9
Calcium (mg L-1)
Time (hours)
C D
E F
NSE = 0.985 NSE = 0.990
NSE = 0.985 NSE = -14.1
A B
NSE = 0.979 NSE = 0.975
Simulated Experimental
In all cases, the model captured the pH curve well with a Nash-Sutcliff model efficiency (NSE) coefficient greater than 0.96. The model also captured the calcium concentration well with an NSE coefficient greater than 0.95 for all cases, except for the 100% CO2 curve where the NSE coefficient was -14. In this case, the model captures the trend where the calcium concentration decreases slightly in the first five minutes and then remains relatively constant but not the absolute values (Figure 6-3D).
Scaling potential after treatment
The treated stabilized urine will be saturated with respect to CaCO3 after aerating it with air/CO2. As stated earlier, CaCO3 is a common scaling component. Therefore, it was important to determine how the CaCO3 present after air/CO2 bubbling would affect the operation of an RO process. A simulation of how the scaling index (SI) of CaCO3 increases as water is removed (concentrated by RO) for different urine compositions was therefore conducted, shown in Figure 6-4A. Approximately 85% water removal could be achieved before a CaCO3 SI of 1.2 is reached. Urine composition does not have a significant impact on the scaling index. Figure 6-4B shows the relationship between temperature and SI as water is removed. Water removal was simulated at 20, 25, and 30°C, assuming air bubbling took place at 20°C followed by water removal at 25°C (20/25) and 15°C (20/15). Water removal at temperatures between 20 and 30°C does not result in a significant variation in the scaling index. If water removal occurs at a higher temperature than the air bubbling step, the solution is initially under-saturated with respect to CaCO3 and over-saturated if water removal occurs at a lower temperature than air bubbling.
However, as water is removed, the difference in scaling index converges to the same value regardless of composition and water removal temperature.
Figure 6-4: (A) CaCO3 scaling index as a function of water removal for five different urine compositions, (B) the effect of temperature on the CaCO3 scaling index as a function of water removal, where the water is removed at 20°C, 25°C, and 30°C, the solution is aerated at 20°C and the water is removed at 25°C, the solution is aerated at 20°C and the water is removed at 15°C.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0 10 20 30 40 50 60 70 80 90 CaCO3Scaling Index
Water removed (%) 20 degC
25 degC 30 degC 20 then 25 20 then 15
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0 10 20 30 40 50 60 70 80 90 CaCO3Scaling index
Water removed (%) U1
U2 U3 U4 U5
A B 20°C
25°C 30°C 20/25°C 20/15°C