CHAPTER 6: Aeration pre-treatment
6.5 CONCLUSIONS
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).
It was determined that the rate of CO2 dissolution is the key process controlling the pH of the solution, and ultimately the calcium is removed as solid CaCO3. At low flow rates, CO2 dissolution was the rate- limiting step whilst at high flow rates and/or high CO2 concentrations, the fast pH decrease limited the precipitation of CaCO3.
It was also shown that even though the pH of the solution had dropped below the threshold pH for enzymatic urea hydrolysis to occur (<11), no urea hydrolysis was detected, most likely because the urease-producing bacteria had been inhibited at the high initial pH values (>12.5) and the reaction time was too fast.
Through simulations, it was shown that after pre-treatment with air bubbling, 85% of the water could be removed from the urine before CaCO3 scaling would be detected. However, the exact nature of membrane scaling for the proposed process needs to be investigated further experimentally and in a full-scale RO system.
The cost of using air enriched with CO2 was more than double compared to using an air blower. Whilst using higher concentrations of CO2 resulted in a significant decrease in operating time, it did not justify the overall operating cost. The optimum operating condition was determined to be an air flow rate of 4 L min-1 L-1 urine and an operating time of 7.6 hours. At these conditions, 95% of the calcium could be removed for a cost of $0.65 m-3 (R11 m-3). This represents only 0.2% of the break-even sales price ($1.57/ R27 L-1) of a liquid fertilizer product that could be produced in a RO system.
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