CHAPTER 11: Conclusions and future work
11.1 CONCLUSIONS
Human urine can be used as a more sustainable alternative to synthetic fertilizers. Source-separation and nutrient recovery from urine also reduces the nutrient loads and treatment requirements at traditional wastewater treatment plants. Despite this, the widespread use of urine as fertilizer is not yet commonplace. Whilst methods for separate urine collection and stabilization (to prevent urea degradation and the resulting malodor) are well established, urine is 97% water and the cost of large- scale collection and transportation from urban to agricultural areas is not economically feasible. This has led to the investigation of a variety of methods (of varying complexity) to remove the water and concentrate urine. However, many of these technologies are still being tested at the laboratory scale or are being designed for treatment at the individual toilet/household scale. To be truly revolutionizing, a urine concentration method that can operate at a significant scale and which is energy efficient is required.
Reverse osmosis (RO) was identified as a promising, yet understudied, concentration technique because it is energy efficient when compared to other urine concentration methods. Full-scale RO has been proven to be commercially viable as it is widely used to desalinate seawater. In typical RO processes, the brine produced is a waste stream that requires disposal. When concentrating urine with RO there is zero waste as the brine is the product fertilizer and the permeate can be used as a non-potable water source. Limited research into urine concentration using RO was attributed to the potential for membrane scaling, low urea recovery, and unknown concentration limits. In addition, if urine is not stabilized the urea breaks down into ammonia, which is lost to the atmosphere through volatilization. The use of an RO process therefore also had to consider how stabilization affects the treatment process.
Chapter 5 and Chapter 6 focused on pre-treatment methods for Ca(OH)2 stabilized urine. Bicarbonate salt addition reduced the Ca2+ concentration to less than 10 mg L-1. Air bubbling removed between 85%
and 98% of the Ca2+ (depending on operating time, air flow rate, and initial concentration). Although air bubbling reduced the urine pH to 8.5 (which is below the threshold for urea hydrolysis prevention), no urea hydrolysis was observed for at least 18 hours after the pH reached 8.5.
In Chapter 7, urine pre-treated with air bubbling and bicarbonate salt addition was concentrated using RO and the effectiveness of the pre-treatment methods were compared to a process with no pre-treatment. Both pre-treatment methods were equally effective. Air bubbling was chosen as the preferred pre-treatment method as it does not require salt addition and the associated greenhouse gas emissions associated with the salt’s production. In addition, air bubbling sequesters CO2 from the atmosphere. Air bubbling resulted in a pH (8.0-8.5) which was within the membrane design operating parameters unlike with NaHCO3addition (pH-11.0), and it did not add additional Na+ ions.
Stabilization of urine with an acid (citric acid) resulted in the formation of uric acid dihydrate crystals and an unidentified organic compound during concentration. Neither could be removed with a pre-treatment step, and it was concluded that concentration of acidified urine should be achieved using evaporation or freeze concentration instead. Real urine, stabilized with Ca(OH)2 and pre-treated with air bubbling, was concentrated using a seawater RO membrane operating at 55 bar. Water removal of 60% was achieved and 85.5% of the urea and 98.5% of the potassium was recovered in the brine, and more than 99% of the phosphorus was recovered as a separate solid phosphate-based fertilizer. The final liquid had a concentration of 11.2 g-N L-1 and 3.66 g-K L-1, however, the final composition will depend on the feed urine composition.
Pharmaceuticals and salts present in urine are concentrated with the nutrients using RO. In Chapter 8, a hybrid NF-RO process was tested to purify the urea-rich stream further. Both loose and tight NF membranes could be used to remove pharmaceuticals (loose > 70%, and tight > 99% removal) at a 75%
water removal (as permeate). The permeate from the tight NF membrane (75% water removal) contained 48% of the urea and the permeate from the loose NF membrane contained 78% of the urea.
Urea purity was increased from 37% to 89%, removing 90% of the organics and 96% of the ions present in the urine using the tight NF membrane. Using the loose NF membrane, the purity increased to 56%, and 78% of the organics and 44% of the ions were removed.
The permeate from a tight NF membrane was further treated with a high-pressure SWRO membrane.
An overall water removal of 80% resulted in a urea recovery of 32.7%. Further concentration of the permeate using loose NF membranes (to 80% overall water removal) would increase the overall urea recovery to 52%. Based on this analysis, a decision tree was developed that can be used to determine the optimal treatment train based on the desired product. In general, the increased purity comes at the cost of urea recovery. Water removal in the RO step was limited to an overall water removal of 80% as a significant decrease in urea recovery was observed as the water removal increased beyond this percentage.
Water removal achieved using RO will have a limit as the osmotic pressure of the feed increases to the point that it is equal to the pump’s operating pressure. As the urine becomes more concentrated there is also the potential for fouling and scaling as solubility limits of different aqueous species are reached.
In Chapter 9, EFC was investigated to further concentrate the urine as freezing processes are not affected by membrane scaling and the technology has the potential to simultaneously remove undesirable salts.
A thermodynamic model was first used to predict crystallization temperatures, water removal (as ice) before salt crystallization began, and the types of salts that would crystallize. The experimental analysis determined that the model accurately predicted ice crystallization temperatures (< 0.5°C difference).
The model also predicted the mass of ice formed at a fixed temperature with a ±10% accuracy since the model did not account for impurities trapped in the ice.
Overall, this research was the first to show experimentally that Na2SO4 ∙10H2O crystallizes from urine at eutectic conditions. A theoretical mass balance (including ice-washing and recycle streams) was used to show that the liquid fertilizer composition after RO-EFC (95% water removal) would be 304 g urea L-1 and 42.8 g K L-1 (11.5% N, 3.5% K), and 3.5 kg of Na2SO4∙10H2O could theoretically be recovered from 1000 kg of urine. The system would have an overall urea and potassium recovery of 77.1% and 96%, respectively.
An economic analysis comparing the different treatment methods was conducted in Chapter 10. The analysis showed that treatment processes using RO for the bulk of the water removal have the lowest energy consumption (16 kWh m-3, 70% water recovery), followed by freeze concentration (119-162 kWh m-3, 70-95% water removal), and evaporative processes (154 -198 kWh m-3, >95% water removal) was the most energy intensive. The value of each product produced was estimated based only on the N- content of the final product. Using a basis of 7.5 m3 input urine per week and a niche fertilizer market size of 140 L per week for the Cape Town region only, RO-EFC produced the product with the highest gross value. FC had the highest value if the market size increased to 0.81 m3 per week, and RO-only had the highest value for a market size > 0.2 m3 per week. However, this analysis did not include capital and operating expenses, therefore the net value as a function of market size remains uncertain. It is, however, evident that an accurate market size will influence the preferred treatment method. The use of urine-based fertilizers (UBF) for commercial use may require transport up to 200 km from the treatment facility to the end-user. The volume of UBF transported increases as N-content decreases. Transport costs as a percentage of the product's gross value could account for between 0.8% (RO-EFC – 11.5%
N) and 3.2% (RO – 1.9% N) of the fertilizer’s gross value (75 km) depending on the product composition.
In conclusion, membrane and hybrid freezing processes can be used to produce different liquid fertilizers with comparable compositions to commercially available fertilizers. Different process configurations can be used to remove pharmaceuticals and salts as well as to further concentrate the product. The choice of treatment configuration will be influenced by the size of the urine collection system since the capital costs for different treatment methods will vary based on the feed volume. In addition, the treatment method will also depend on the niche fertilizer market size, the fertilizer use (ornamental plants versus edible crops), and the associated fertilizer regulations for each type of product. This novel work ultimately offers new urine concentration methods for fertilizer production.