The over-arching aim of this research was to develop an integrated treatment process to concentrate urine whilst maximizing the recovery of key nutrients (N, P, K). This chapter covers the general and recurring methods that were used throughout this research, including the method for urine collection, make-up methodology and composition for synthetic urine, thermodynamic modeling, and analytical methods. Additional experimental conditions and equipment are described in more detail in the relevant chapters.
4.1 URINE COLLECTION AND STABILIZATION
Real urine
All real urine used throughout this work was collected anonymously from men and women working and studying in the New Engineering Building at the University of Cape Town. The majority of urine used throughout this research was stabilized with 10 g L-1 Ca(OH)2 (Randall et al., 2016). The urine was collected in novel fertilizer-producing urinals, developed by Flanagan and Randall (2018), which were pre-dosed with the appropriate amount of Ca(OH)2 (98%, Kimix, South Africa). The urinals were manually mixed twice daily during the collection phase by gently swirling the container contents. This ensured a homogenous solution pH above 12. After collection, the urine was stored in sealed 25 L containers and used within one month of collection. All urine was filtered (1,2µm, Ahlstrom-Munksjö, Helsinki, Finland) to remove excess Ca(OH)2 and other precipitates before experiments were conducted. Due to COVID-19, the limited presence of students and staff on campus made it difficult to collect sufficient volumes of urine with the same composition, hence many experiments using real urine had different compositions. Detailed urine compositions from each experiment are available in the relevant Appendix for each chapter ((Table B-1, Table C-1, Table D-1, and Table E-1).
Synthetic urine
The composition of real urine can vary significantly. Synthetic urine is therefore useful because the composition can be kept constant throughout experiments. In addition, the modeling software (described in detail below) used throughout this research does not include many of the organics present in urine (other than urea) in its database. Therefore, a synthetic urine recipe was developed without organics such that those experimental results could be directly compared to the thermodynamic model.
Table 6 details the synthetic urine recipe. The synthetic recipe was developed based on average ion concentrations in urine observed in literature and was composed of salts recommended by (Pronk et al., 2006b). However, some salts were replaced by acetate salts (ammonium acetate, and sodium acetate) to mimic the organic content and pH of real fresh urine more closely. The synthetic urine recipe was based on “fresh” urine to which the relevant stabilizer was added and then treated as per the real urine.
Table 6: Recipe for fresh synthetic urine in g L-1, pH = 5.7.
Compound Concentration
(g L-1)
Purity (%)
Supplier
Urea CO(NH2)2 13.0 99.5 Sigma Aldrich
Sodium chloride NaCl 3.65 99.0 Sigma Aldrich
Potassium chloride KCl 3.87 99.0 Sigma Aldrich
Sodium dihydrogen phosphate NaH2PO4 1.38 99.0 Sigma Aldrich
Sodium sulfate Na2SO4 1.43 97.0 Sigma Aldrich
Ammonium acetate CH3OONH4 2.24 98.0 Sigma Aldrich
Calcium chloride dihydrate CaCl2∙2H2O 0.56 100 Kimix
Magnesium chloride MgCl2 0.24 98.0 Sigma Aldrich
Sodium Acetate NaC2H3O 1.00 98.0 Kimic
4.2 THERMODYNAMIC MODELING
All thermodynamic modeling was conducted using OLI Stream Analyzer (OLI Systems Inc, 2022). The Mixed Solvent Electrolyte (MSE) database was used as this is not limited by the ionic strength of the solution. Thermodynamic properties of aqueous species are calculated based on the revised Helgeson- Kirkham-Flowers (HKF) framework and the activity coefficients for complex high ionic strength solutions (like urine) are based on the combined work of Bromley, Zemaitis, Pitzer, Debye–Huckel (Lewis et al., 2010). Analysis of an aqueous stream should have a balance of anions and cations. Simply inputting the concentrations of the ions measured in urine does not always result in the cations and anions balancing as it is not analytically possible to measure every species in urine, and accurately.
Therefore, directly inputting the urine recipe based on the salts used ensured the cations and anions were balanced.
4.3 ANALYTICAL METHODS
Liquid Analysis
Colorimetric methods were used to determine the concentration of urea, total ammoniacal nitrogen (TAN), PO4-P, Mg2+, Ca2+, SO42-, Cl-, and K+, where required. This process was automated with a GalleryTM Discrete Analyzer (ThermoFisher Scientific, Massachusetts) using standard methods of the equipment. The samples were diluted, and the pH was adjusted (with dilute HCl, 0.01M) to between 7 and 8 to prevent ammonia volatilization where necessary. The concentration of sodium ions was determined using inductively coupled plasma optical emission spectrometry (5900 SVDV, Agilent,
vials (HI94754B-25, Hanna, Johannesburg) and a multiparameter photometer (H183399, Hanna, Johannesburg). The COD was used as a proxy for the concentration of organics (Li and Liu, 2018).
Solids could also be analyzed for specific ions by redissolving a known mass in 100 mL of de-ionized water. All liquid samples were analyzed within 30 minutes of sampling.
Scanning electron microscope
Scanning electron microscopy (SEM) (FEI Nova SEM 230, FEI, USA) was used to analyze the CaCO3
crystal morphology in Chapter 5 and the RO membrane surface in Chapter 7. The SEM was equipped with an Oxford X-Max dispersive X-ray spectroscopy (EDS) detector (Oxford Instruments, England) which was used to characterize precipitates on the membrane surface using elemental analysis. The analysis was carried out on the Oxford INCA software.
X-ray diffraction
X-ray diffraction (XRD) analysis was conducted in Chapter 5 and Chapter 6, to confirm the precipitation of CaCO3, using a D8 advanced diffractometer (Bruker, Germany) which was outfitted with a position-sensitive detector (LYNXEYE) and Bragg Brentano geometry was used for the analysis.
Power to the Co anode was set at 35 kV and 40 mA. A range of 20° to 120° (d-1 = 0.19 to 0.97 Å-1) with a 0.017° step size (0.84 seconds per step) was used to acquire the diffraction patterns. The ICDD database (PDF4+, released 2020) was used to compare the diffraction patterns to the reference data files.
REFERENCES
Flanagan, C.P., and Randall, D.G. 2018. Development of a novel nutrient recovery urinal for on-site fertilizer production. Journal of Environmental Chemical Engineering 6(5), 6344-6350.
Lewis, A.E., Nathoo, J., Thomsen, K., Kramer, H., Witkamp, G., Reddy, S. and Randall, D.G. 2010.
Design of a Eutectic Freeze Crystallization process for multicomponent wastewater stream.
Chemical Engineering Research and Design 88(9), 1290-1296.
Li, D. and Liu, S. 2018. Water quality monitoring and management: basis, technology, and case studies, Academic Press, Massachusetts, USA.
OLI Systems Inc. 2022. OLI Stream Analyzer, version 11.0, OLI Systems Inc, New Jersey, USA.
Pronk, W., Palmquist, H., Biebow, M. and Boller, M. 2006. Nanofiltration for the separation of pharmaceuticals from nutrients in source-separated urine. Water Research 40(7), 1405-1412.
Randall, D.G., Krähenbühl, M., Köpping, I., Larsen, T.A. and Udert, K.M. 2016. A novel approach for stabilizing fresh urine by calcium hydroxide addition. Water Research 95, 361-369.