pharmaceuticals were representative of the eleven pharmaceuticals (five OTCs and six ARVs) chosen for the current work since two OTCs and three ARVs were used.
The optimization of the HC system was considered for this work since it also conserved more than 90% of urea (refer to section 4.6). Besides being a new pharmaceutical removal technology, it was less invasive than the optimization of the hydrogen peroxide method which would require more equipment and a new setup for the combined hydrogen peroxide/ultraviolet system. Furthermore, the optimization of GAC was not considered because it already had a high pharmaceutical removal efficiency (˃94%). Additionally, changing the dosage of calcium hydroxide would not be beneficial since a dosage of 10 g L-1 was an overestimate for the stabilization of urine (Randall et al., 2016).
4.5 Optimized hydrodynamic cavitation system
Figure 24: Optimized hydrodynamic cavitation pharmaceutical degradation results:
Optimized hydrodynamic cavitation sample 1 (A), Optimized hydrodynamic cavitation sample 2 (B). The respective abbreviations of the pharmaceuticals labelled under each graph are as follows: paracetamol (PARA), chlorpheniramine maleate (CHL), stavudine (STA), lamivudine (LAM) and zidovudine (ZID).
The chromatographs of the samples for the optimized HC system are given in Figure 25. The chromatograms of the samples at three different times (time 0, 15 and 30 minutes) are superimposed to illustrate the extent of degradation due to the optimized HC system. The asterisks indicate the formation of degradation products. Although the degradation products were not identified, they confirmed the degradation of the primary pharmaceuticals.
Figure 25: Optimized hydrodynamic cavitation pharmaceutical degradation chromatograms:
sample 1 (A), sample 2 (B). The colour coded chromatograms correspond to the time during the HC system as follows: 0 minutes (red); 15 minutes (green) and 30 minutes (blue). The respective abbreviations of the pharmaceuticals represent the following pharmaceuticals:
paracetamol (Para), chlorpheniramine maleate (Chloro), stavudine (Sta), lamivudine (Lami) and zidovudine (Zido).
4.5.2 The effect of temperature and pH
The temperature of the optimized HC system increased linearly over time. Figure 26 shows
the change of temperature and pH over time for the optimized cavitation system. The R-squared values for sample 1 (A) and sample 2 (B) were 0.95 and 0.98 respectively, which
confirm the linearity of temperature over time. The full details are given in Annexure C4.
Figure 26: Optimized hydrodynamic cavitation system temperature and pH analysis:
Optimized hydrodynamic cavitation sample 1 (A), Optimized hydrodynamic cavitation sample 2 (B).
Furthermore, the pH was consistent throughout the experiments. The sample solutions were adjusted to pH 2 following the results shown in section 4.2.4. This implied that the urine treated using HC would be stabilized by acidification instead of alkalinization. Rather than dosing with calcium hydroxide then adjusting the solution pH to 2, the urine solution will be adjusted to pH 2. Both the urea and the ammonium would be conserved (refer to the discussed in section 2.5.1), since the ammonium ions form at a low pH.
4.5.3 Cavitation device
Although it was said that a circular venturi is more effective than an orifice (Saharan et al., 2013), the cavitation device was changed from a circular venturi to a 3 mm orifice. This was to verify whether the statement made by Saharan and co-workers (2013) was valid for all sample solutions. The results from this work showed that a 3 mm orifice was more effective in the removal of pharmaceuticals than a circular venturi. This may be attributed to the throat diameter of the venturi. It is known that hydroxyl radicals are produced when cavitation bubbles (generated in the throat of the venturi) collapse due to the pressure difference created from a low-pressure zone to a high pressure (Warade et al., 2016). However, a big throat diameter of a venturi limits the pressure drop, which results in less hydroxyl radicals being generated, consequently less pharmaceutical degradation.
4.5.4 Inlet pressure
The optimum inlet pressure of the HC system was found by varying the inlet pressure of the HC system. Since a 3 mm orifice was chosen as the cavitation device for the optimized system, the optimum pressure was determined for a 3 mm orifice. Three pressures were investigated:
200 kPa, 300 kPa and 400 kPa. The results from this work showed that the optimum inlet pressure was 400 kPa (given in Annexure C1). The degradation at of paracetamol at 200 kPa, 300 kPa and 400 kPa was 19.5%, 18.8% and 29.6% respectively. This suggests that the cavitation bubbles formed at 200 kPa and 300 kPa were not big enough, since a significant pressure drop is required to produce and grow the cavitation bubbles (Kuldeep et al., 2014).
Therefore, less hydroxyl radicals were produced at 200 kPa and 300 kPa when compared to 400 kPa because they are derived from the collapse of the cavitation bubbles which enables the dissociation of water to form hydroxyl radicals (Saharan et al., 2012).
4.5.5 Effect of temperature
There is a concern over the rise in temperature of the urine sample solution when using the HC system. This is because urea degrades at a temperature of more than 40˚C (Randall et al., 2016). The optimized system reached a maximum temperature of almost 50˚C (shown in Annexure C4). The temperature matches the temperature at which the maximum cavitation aggressiveness occurs (Šarc et al., 2017). This means that at a temperature of 50˚C, the cavitation is most effective in the degradation of micropollutants such as pharmaceuticals.
The urea was analyzed to find out how much of it had degraded. Contrary to what Randall and co-workers (2016) found, more than 95% of the urea in the sample solution treated by the optimized system was conserved. However, the system was run for 30 minutes, and the temperature of the sample solution was above 40˚C for the last 10 minutes. Additionally, the loss of urea is not immediate once the temperature of a solution is above 40˚C.
Since the degradation of the urea was determined in relative terms and not absolute terms, the samples may have both lost urea. The samples were kept at 4 ˚C since it is known that the rate of urea hydrolysis increases with an increase in storage temperature (Hellstrom et al., 1999). This was to make sure that there was no hydrolysis that occurred which would result in the loss of ammonium.
4.5.6 Practical application of the hydrodynamic cavitation system
The optimized hydrodynamic cavitation (HC) system would require 1.84 kW m-3 (refer to Annexure F). The energy required is realistic for industry application considering that Yen (2016) used 1.73 kW m-3 to decolourize textile water using the UV/H2O2 process. The energy from the treatment process by Yen (2016) is comparable to the energy which is required to operate the HC system at the optimized operating conditions used for this work (Yen, 2016).
Furthermore, the HC system would be scalable since it requires a simple reactor design for larger-scale operation capacity (Yuequn et al., 2016). A study by Garuti and co-workers (2018) investigated a full-scale HC pre-treatment process in an agricultural biogas plant. The system was equipped with three digesters (each with a capacity of 1.4 ML) which were installed in series. The findings from the study revealed that the system was efficient and could be operated at a low energy. Furthermore, it was easy to implement the cavitation system (Garuti et al., 2018). With that being said, it would be possible to implement the HC system in a urine treatment plant.
Chipako and Randall (2020a) proposed a decentralized system for the collection of urine (used for fertilizer production) for the City of Cape Town. An estimated amount of urine that could be collected from malls within the City of Cape Town was 392 m3 yr-1, which translates to approximately 1080 L d-1 (Chipako and Randall, 2020a). The findings from the study suggest that the 80 L cavitation system would need to be scaled by a factor of at least 20, which can accommodate for a flow of up to 1600 L d-1.
Chipako and Randall (2020b) made a further recommendation for fresh urine treatment.
However, the inclusion of a HC system to degrade the pharmaceuticals found in urine would alter the proposed urine treatment train. Instead of base stabilization with calcium hydroxide, the fresh urine would be stabilized by acidification (pH 2). Citric acid is recommended for use to acidify the urine since citric acid is the main organic acid found in citrus fruits (Grewal and Kalra, 1995). Therefore, traces of citric acid in urine-derived fertilizer will not be harmful for human handling and consumption. The stabilized urine would not be filtered since the solubility of citric acid is higher than that of calcium hydroxide. Furthermore, precipitation does not occur at low pH.The last stage will be an evaporation process to concentrate the urine solution instead of reverse osmosis. This is because acidified urine results in brown
fouling on the reverse osmosis membrane surface which reduces the permeate flux of the reverse osmosis process (Courtney and Randall, 2022).However, the cost and energy required for evaporation is up to three times more than what is required for reverse osmosis (Ek et al., 2006).