CHAPTER 9: Eutectic freeze crystallization
9.4 CONCLUSIONS
An RO-EFC (11.5% N, 3.5% K) process and AD (10.5% N, 1.5% P, 4.2% K-average composition) produce the products with the highest nitrogen content composition. The dry fertilizer product produced by Simha et al. (2020a) has a higher K content because the drying medium contained K. Fertilizer application rates are based on kg of nutrient per area rather than fixed masses of fertilizer. Freeze concentration (3.9% N, 0.5% P, 0.9% K ) (Noe-Hays et al., 2021) and ND (4.2% N, 0.2% P, 1.5% K) (VUNA GmbH, 2020) had a product with a similar nutrients content. It was expected that the nutrient content of fertilizer made using RO (< 70% water removal) or NF-RO (low N recovery) would be too low and further concentration would be required to increase the nutrient content to that comparable with commercially available fertilizers. However, a comparable commercial fertilizer could be found for each of the fertilizers produced from the different treatment methods. The context of fertilizer use is also important, fertilizers with lower nutrient content would need to be applied in larger volumes to meet required nutrient application rates. This is further investigated in the economic analysis of Chapter 10.
Figure 9-6: Nitrogen and potassium content (weight %), and water removal (%) of the different fertilizers produced using different treatment methods compared to commercially available liquid fertilizers with comparable nitrogen content. Commercial 1 (Protek, 2022), commercial 2 (Rolfes Agri, 2022b), commercial 3 (Rolfes Agri, 2022a), commercial 4 (Bonsai Tree, 2022). Fertilizer compositions reported in literature include freeze concentration (FC) (Noe-Hays et al., 2021), nitrification and distillation (ND) (VUNA GmbH, 2020), and alkaline dehydration (Simha et al., 2020a).
reaching EFC conditions. Using EFC to further concentrate urine would remove 82.8% of the SO42-
ions and 15.7% of the Na+ ions from the solution, resulting in an overall water removal of 95% and a more concentrated liquid fertilizer product (for the generic synthetic urine composition).
A comparison of the experimental and modelled results showed that the model was able to accurately predict the ice formation temperature (< 0.5°C difference), however, it was less accurate at predicting the mass of ice formed (±10% difference) due to the presence of entrapped liquid and imperfect ice separation. The thermodynamic model was accurate in predicting the ice crystallization temperature, water removal required before salt formation, and the type of salt that will precipitate, which can then be used to design an EFC process.
To confirm that Na2SO4∙10H2O was crystallizing at eutectic conditions, the concentration of Ca2+, Cl-, Na+, and SO42- was measured as the solution temperature was incrementally decreased. The concentration of Na+ and SO42- in the liquid decreased after a Na2SO4 salt seed was added at -10.5°C, whilst the concentration of Ca2+ and Cl-1 continued to increase. This indicated that eutectic conditions had been reached for ice-Na2SO4·H2O. Whilst the model predicted that salt would crystallize at -9.82°C (generic synthetic urine composition) and -9.65°C (real urine composition), this did not occur, and Na2SO4 seeds needed to be added to induce salt crystallization. It was therefore only possible to determine a eutectic temperature range, which was between the ice crystallization temperature (- 10.15°C for synthetic and -9.19°C for real urine) and the ice seeding temperature (-10.5°C).
A theoretical mass balance of RO followed by FC and EFC for stabilized urine concentration, including ice washing and recycle streams, was conducted with a 95% water removal (by mass). The overall urea and K+ recovery was calculated to be 77% and 96%, respectively. Over 98% of the phosphorus would be recovered as calcium phosphate during the urine stabilization step. The final liquid fertilizer would have a urea concentration of 304g L-1 and 42.8 g L-1 K+ (11.5% N, 3.5% K) and 3.5 kg of Na2SO4∙10H2O could theoretically be recovered from 1000 kg of urine. The urea and K+ concentration in the liquid fertilizer product stream is comparable to commercially available all-purpose liquid fertilizers.
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