CHAPTER 6: Aeration pre-treatment
6.4 DISCUSSION
Figure 6-5: Comparison of the simulation with all processes on (- - -) and the effect of neglecting individual processes (—), where CO2 dissolution is turned off (A), CaCO3 precipitation is turned off (B), ammonia stripping is turned off (C), and urea hydrolysis of 0.2 mol d-1 (D).
The rate of CO2 dissolution (Equation 6) is directly linked to three variables: the concentration of CO2
in the air, the air flow rate (for a given reactor size), and the mass transfer coefficient. For each flow rate, there will be a limiting value for the mass transfer coefficient where the exponent term tends to zero. The concentration of carbonate ions is directly linked to the pH of a solution, as the pH decreases below the salinity-adjusted carbonate dissociation constant (9.35 (Millero et al., 2006)), the percentage of inorganic carbon in the form of CO3-2 will decrease, thus reducing the driving force for the formation of CaCO3 as per Equation 5. This explains the two distinct regions observed in Figure 6-1B. If CO2
dissolves too quickly, the pH will decrease below the carbonate dissociation constant before any CaCO3
can precipitate, as observed with 100% CO2 experimental data. In the work of (Altiner, 2018; Bang et al., 2011), bubbling 100% CO2 through a Ca(OH)2 slurry was investigated. In this case, the precipitation of CaCO3 was observed and the calcium concentration was reduced from 20 000 to 160 mg L-1 (Bang et al., 2011). The dissolution of excess Ca(OH)2 in the slurry acted as a buffer to maintain a high pH
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and thus, ensured that the inorganic carbon remained in the form of carbonate thereby providing a driving force for the precipitation reaction. In this current work, no excess Ca(OH)2 was present to counter the pH decrease. Therefore, if the pH decrease occurs faster than the precipitation reaction, the extent of calcium removal would be limited.
Figure 6-6 compares how different air flow rates affect the pH and calcium concentration, for a given reactor size, and assuming that the mass transfer coefficient is not limiting. The mass transfer can be improved via mixing (Lisitsin et al., 2008) or decreasing bubble size (Altiner, 2018; Bang et al., 2011) to maximize surface area and bubble retention time. At low flow rates and CO2 concentrations, the rate of CO2 dissolution is the rate-limiting step for CaCO3 precipitation. The precipitation rate is faster than the addition of CO3-2 ions to the system. Once the maximum CO2 dissolution rate is achieved, the formation of CaCO3 becomes the rate-limiting step. This is evident in Figure 6-6B, where, at flowrates greater than 5 L min-1, there is no significant increase in the rate at which calcium is removed. The rate of CaCO3 formation could be increased by increasing the operating temperature, which would decrease the 𝐾=>,3839$ (Stumm and Morgan, 1996) or by adding seeds, which would increase the 𝑘3839$ (El Fil and Manzola, 2003; Song et al., 2006). The results ultimately show that it is important to have a rate of CO32- formation greater than the precipitation rate of CaCO3, but the CO2 dissolution rate should be small enough to allow for substantial calcium removal before the buffer capacity of the solution is depleted.
Figure 6-6: Effect of varying air flow rate from1 to 10 L min-1 on the pH (A), and calcium (B) assuming maximum gas mass transfer.
Urea stability during air bubbling
After air bubbling, the pH of the treated urine was 8.5 and thus below the threshold value of 11 (Randall et al., 2016), which inhibits enzymatic urea hydrolysis. However, no significant hydrolysis occurred.
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This is likely because of the following reasons. There was a low concentration of urease or urease-producing bacteria present. At pH values below 3.5 and above 8.6 the metabolism of urease- producing Helicobacter pylori bacterium is irreversibly inhibited (Rektorschek et al., 1998), this phenomenon may potentially be inhibiting the activity of the urease-producing bacteria present in urine, even after a pH decrease is observed. In addition, all experiments were conducted at a temperature of
~20°C, which is too low for chemical hydrolysis to occur (Callahan et al., 2005). As the air/CO2
bubbling would occur continuously, and directly before RO as a pre-treatment step, it is unlikely that hydrolysis would occur and affect the recovery of urea.
Implications for a reverse osmosis process
Simulation of water removal via RO after treatment with air/CO2 bubbling showed that although the aerated urine is saturated with respect to CaCO3, scaling caused by CaCO3 might be delayed.
Rahardianto et al. (2008) observed that up to a SI of 1.2 CaCO3 membrane scaling was undetected. This occurs at a water removal of approximately 85%. Figure C-7 in Appendix C, further confirms the importance of the pre-treatment step by displaying the difference in the amounts of solids formed during water removal. For the treated urine, the simulation shows that only a small mass of solids would precipitate (< 60 mg) up to a water removal of 90%. This is significantly less than stabilized urine. For 1 L of urine, at a water removal of 50%, 1.07 g of solids would precipitate. This would cause significant membrane scaling. The potential risk of scaling could also be further reduced by adding an antiscalant, but this might reduce the quality of the product produced. This means that stabilizing fresh urine to first produce calcium phosphate and then bubbling CO2/air through the treated urine to reduce the calcium concentration in the solution could provide an effective pre-treatment method for RO. In addition, the bubbling of air into stabilized urine could also provide an innovative method to sequester CO2 (Aguilar, 2012). For example, approximately 1.32 kg CO2 could be sequestered per m3 of stabilized urine (this will vary based on the initial calcium concentration though).
Design and economic considerations Air bubbling cost vs. operating time
The power required for the air blower to treat 1 m3 of urine is a function of the air bubbling time and blower power (kW). The operating time was calculated from Figure 6-6B and was determined as the time required for 95% calcium removal for each air flow rate. The blower power requirements were calculated according to Sierra (Sierra et al., 2008) and increased with increasing air flow rate, with detailed calculations provided in Appendix C. Figure 6-7A compares the total cost and operating times required for each air flow rate. As the air flow rate increases, the marginal decrease in operating time
also decreases. Therefore, the optimum air flow rate is approximately 4 L min-1 L-1 urine, after which the marginal increase in cost outweighs the marginal decrease in operating time. At an air flow rate of 4 L min-1 L-1 urine, the air bubbling would cost $0.65 m-3 and would require an operating time of approximately 7.6 hours.
Air vs. CO2 enriched gas
Figure 6-7B compares the cost and operating time for different CO2 concentrations at optimum operating conditions. Whilst the required operating time is significantly decreased by increasing the CO2 concentration, the cost is 2.35 times more when compared to air bubbling. As CO2 costs $ 0.97 per kg (Air Liquide, Cape Town), even if CO2 is 100% efficient, approximately 1.3 kg of CO2 is required per m3 of urine resulting in a minimum cost of $1.27 m-3 urine, which is double the cost of air bubbling.
Further information regarding how flow rate and operating time impact the costing for CO2 can be found in Figure C-8. An additional advantage of air bubbling to remove excess calcium is that it can be used to sequester CO2 directly from the atmosphere (Aguilar, 2012). The cost of the power requirements for air bubbling would require the break-even sales price ($ 1.57 L-1) of the niche fertilizer product to be increased by 0.2%, based on the economic analysis conducted by Chipako and Randall (2020a).
Figure 6-7: Cost and operating time as a function of air flow rate varying from 1 to 10 L min-1 L-1 (A), cost and operating time as a function of CO2 concentration for the most cost-efficient flow rate (B).