Human urine: A novel source of phosphorus for vivianite production.

Human urine contributes up to 50 % of the phosphorus load in domestic wastewater. Decentralized sanitation systems that separately collect urine provide an opportunity to recover this phosphorus. In this study, we leveraged the unique and complex chemistry of urine in favor of recovering phosphorus as vivianite. We found that the type of urine affected the yield and purity of vivianite, but the kind of iron salt used, and reaction temperature, did not affect the yield and purity. Ultimately, it was the urine pH that affected the solubility of vivianite and other co-precipitates, with the highest yield (93 ± 2 %) and purity (79 ± 3 %) of vivianite obtained at pH 6.0. Yield and purity of vivianite were both maximized when Fe:P molar ratio was >1.5:1, but <2.2:1. This molar ratio provided sufficient iron to react with all available phosphorus, while exerting a competitive effect that suppressed the precipitation of other precipitates. Vivianite produced from fresh urine was less pure than vivianite produced from synthetic urine, because of the presence of organics in real urine, but washing the solids with deionized water improved the purity by 15.5 % at pH 6.0. Overall, this novel work adds to the growing body of literature on phosphorus recovery as vivianite from wastewater.

Concentrating stabilized human urine using eutectic freeze crystallization for liquid fertilizer production.

Resource recovery from source-separated urine can be used to produce fertilizers and provide a more sustainable alternative to mineral fertilizers. Reverse osmosis can be used to remove up to 70% of the water in urine that has been stabilized with Ca(OH)2 and pre-treated with air bubbling. However, further water removal is limited because of membrane scaling and equipment operating pressure limitations. A novel hybrid eutectic freeze crystallization (EFC) and RO system was investigated as a method to concentrate human urine, whilst simultaneously crystallizing salt and ice under EFC conditions. A thermodynamic model was used to predict the type of salts that would crystallize, their associated eutectic temperatures, and how much additional water removal was required (using freeze crystallization) to reach eutectic conditions. This innovative work showed that at eutectic conditions, Na2SO4∙10H2O crystallizes simultaneously with ice in both real and synthetic urine, thus providing a new method to concentrate human urine for liquid fertilizer production. A theoretical mass balance of a hybrid RO-EFC process, including ice washing and recycle streams, showed that 77% of the urea and 96% of the potassium could be recovered with a 95% water removal. The final liquid fertilizer would have a composition of 11.5% N and 3.5% K, and 3.5 kg of Na2SO4∙10H2O could be recovered from 1000 kg of urine. Over 98% of the phosphorus would be recovered as calcium phosphate during the urine stabilization step. A hybrid RO-EFC process would require 60 kWh m-3 of energy, which is substantially less than other concentration methods.

A hybrid nanofiltration and reverse osmosis process for urine treatment: Effect on urea recovery and purity.

Human urine can be treated and concentrated using reverse osmosis (RO), but this process also concentrates pharmaceuticals and undesirable salts along with valuable nutrients such as urea. The final fertilizer product, therefore, has limited use on salt-sensitive crops or edible crops as the effects of pharmaceuticals remain a concern. Tight and loose nanofiltration (NF) pre-treatment was investigated as a method to recover urea in the permeate (which could then be further concentrated using RO), whilst pharmaceuticals and undesirable salts would be removed in the brine. Both NF options removed pharmaceuticals (NF90 - 99%, NF270 - 70%). Using a loose NF membrane, 78% of the urea was recovered in the permeate (80% water removal), however, the salt removal was poor (44%), and the urea purity only increased from 37 to 56%. Tight NF membranes provided better rejection of salts and organics (96% and 90% removed respectively). The urea purity in the permeate (75% water removal) from the tight NF process was 89%, however, the urea recovered was lower (48%). The tight NF permeate was then further concentrated with RO. At an overall water removal of 80%, 32.7% of the urea could be recovered with a purity of 85%. A decision tree was also developed to determine the optimum treatment process based on the desired final product. This decision tree could be used to determine the economic feasibility of each treatment process based on the final product choice and product value.

Precipitation to remove calcium ions from stabilized human urine as a pre-treatment for reverse osmosis.

Concentration of Ca(OH)2 stabilized urine by reverse osmosis (RO) has the potential to cause CaCO3 scaling on the membranes. The aim of this research was to determine whether the addition of carbonate salts could be used to precipitate CaCO3 prior to RO concentration and how to accurately dose the salts. Dosing of NaHCO3 or Na2CO3 reduced the calcium concentration to <0.18 mmol L-1, whilst maintaining a pH > 11. This is the pH threshold for enzymatic urea hydrolysis in urine, but above the operating pH range of most membranes. However, the pH could be decreased by adding an acid. Measuring conductivity as a proxy for the calcium concentration was found to be an effective method to determine the dose of salt required. Simulations with other carbonate-producing salts (KHCO3, MgCO3, and NH4HCO3) were also shown to be effective. However, NH4HCO3 ($0.53 m-3 urine) was the only other salt comparable in cost to NaHCO3 ($0.49 m-3 urine) and resulted in a final pH within the normal operating range of membranes. The addition of NH4HCO3 would add extra N to the urine rather than sodium ions when dosing NaHCO3. The choice of salt will ultimately depend on what liquid fertilizer composition is desired.

Concentrating stabilized urine with reverse osmosis: How does stabilization method and pre-treatment affect nutrient recovery, flux, and scaling?

Human urine can be used as a fertilizer, however, due to the high water content (97%), concentration is required to make transportation economically feasible. Reverse osmosis (RO) has been identified as an energy efficient concentration method. Furthermore, to maximize nitrogen recovery from source-separated urine it should be stabilized with an acid or base to prevent urea hydrolysis. However, the method of stabilization will have an impact on the downstream RO process. Calcium hydroxide is often used as a base stabilization method for human urine but would require pre-treatment to remove excess calcium and subsequent membrane scaling. Three pre-treatment methods such as air bubbling, NaHCO3 addition, and NH4HCO3 addition, were investigated in this study. Each method successfully reduced the scaling potential and air bubbling was determined to be the most effective method as it resulted in the highest nutrient recovery during concentration and did not require the addition of any chemicals. Base stabilization with air bubbling pre-treatment was then compared to urine stabilized with citric acid. Acid stabilized urine had a higher nitrogen recovery (7.6% higher). However, solids formed in the real acidified urine and during concentration a brown organic compound formed on the membrane surface. The crystals were identified as uric acid dihydrate and the brown organic fouling resulted in a significant decrease in permeate flux as compared to the base stabilized urine with air bubbling pre-treatment. At a 60% water recovery, 85.5% of the urea and 99.2% of the potassium was recovered in the brine stream and more than 99.9% of the phosphorus was recovered as a separate solid calcium phosphate fertilizer. Whilst nutrient recovery was higher with acid stabilization, the membrane fouling that occurred with this stabilization method meant that base stabilization with air bubbling pre-treatment was the preferred treatment option. It is recommended that acid stabilized urine be concentrated using evaporation processes instead.

Calcium removal from stabilized human urine by air and CO2 bubbling.

Stabilization of urine with calcium hydroxide prevents enzymatic urea hydrolysis, thus allowing for maximum nitrogen recovery. The process also produces a calcium phosphate bi-product which has value as a fertilizer. However, the treated solution is saturated with calcium that could ultimately result in calcium carbonate scaling of reverse osmosis membranes during urine concentration. This would result in a decrease in maximum water removal and an increase in operational costs. This study therefore investigated if bubbling air and carbon dioxide through stabilized urine could remove calcium ions as calcium carbonate. The process was modelled to better understand the mechanisms controlling the reactions in the process. The model was then used to determine the most cost and time efficient operating conditions. Calcium removal of between 85-98% was achieved at air flow rates of 1.5 to 9 L min-1. Increasing the CO2 concentration from 0.04% (air) to 1% decreased the reaction time from 20.5 h to 2.5 h but the cost of CO2 outweighed the shorter operating time. Air bubbling was the more cost-efficient option. It was estimated that 95% of the calcium could be removed in 7.6 h at an air flow rate of 4 L min-1 L-1 of urine and at a cost of $0.65 m-3. It was also determined that even if the pH decreased to below 11, the urine remained stabilized and no enzymatic urea hydrolysis occurred.