Neha
Jagtap
*ab and
Treavor H.
Boyer
b
aDepartment of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure & Environment (ESSIE), University of Florida, P.O. Box 116450, Gainesville, Florida 32611-6450, USA. E-mail: njagtap@ufl.edu; Tel: +1 352 346 0057
bSchool of Sustainable Engineering and the Built Environment (SSEBE), Arizona State University, P.O. Box 873005, Tempe, Arizona 85287-3005, USA
First published on 28th August 2018
This study investigated an integrated, multi-process approach of using struvite precipitation, ammonia stripping–acid absorption, and evaporation to recover phosphorus (P), nitrogen (N), and potassium (K), respectively, from stored urine. The process produces separate nutrient products that can then be recombined to produce customized fertilizers of any NPK ratio. Bench-scale experiments were conducted using three stored urine solutions: synthetic urine, synthetic urine with six endogenous metabolites, and real urine. For struvite precipitation, MgCl2·6H2O, MgCO3, and MgO were tested and dosed at a molar ratio of 1.1:1 Mg:P. There was a statistically significant difference between total phosphate (TP) recovered by each magnesium (Mg) source and urine solution; MgCl2·6H2O (91–94% TP recovered) > MgCO3 (55–77%) > MgO (52–66%) and real urine > synthetic urine with six endogenous metabolites > synthetic urine. For ammonia stripping–acid absorption, there was a statistically significant difference between TAN recovery and experimental stripping conditions where increasing both the pH and temperature recovered a higher percent of TAN compared to solely increasing the pH or temperature of the solution. In real urine, consumed cost for stripping increased as follows: control condition of pH 9.2, 22 °C < elevated pH condition of pH 10.5, 22 °C < elevated temperature condition of pH 9.2, 70 °C. There was no statistically significant difference between the Mg source and TAN recovery in real urine and synthetic urine with metabolites but there was in synthetic urine. Furthermore, the amount of TAN recovered in real urine and synthetic urine with metabolites was consistently greater than or approximately equal to synthetic urine. This suggests that using synthetic urine as a proxy for real urine is not suitable for N recovery. For evaporation, there was a statistically significant difference between the urine solution and conditions for N recovery (i.e., temperature and/or pH) on K recovery and product purity. As the pH was increased, the purity of the final K product, potash, decreased due to sodium from NaOH. Results from this study show that an integrated, multi-process approach to urine treatment can achieve approximately 99% N, 91% P, and 80% K recovery as fertilizer products.
Water impactUrine is a unique waste stream because it contains nutrients that are valuable in agriculture yet problematic in excess in aquatic environments. Unlike traditional fertilizer production that requires finite resources, urine is widely available. Separation and treatment of urine can recover nitrogen, phosphorus, and potassium as separate products of value, thereby reducing the negative impacts of nutrients on the environment. |
The chemical composition of human urine varies with diet, physiological factors, and further changes upon storage.12 When excreted from the human body, urine contains a high concentration of N in the form of urea, inorganic cations (Na+, Ca2+, Mg2+, K+), inorganic anions (Cl−, SO42−, PO43−), organic compounds (e.g., endogenous metabolites such as creatinine, taurine, hippurate) and possibly trace contaminants.13–15 This is referred to as fresh urine in the literature. When urine is stored, bacteria containing the urease enzyme hydrolyze urea to produce ammonia and bicarbonate.16 Urea hydrolysis raises the pH of urine from pH 6 to pH 9. Other impacts of storage and hydrolysis include the presence of a high concentration of ammonia, which can act as a biocide to reduce the number of pathogens, and create supersaturated conditions that favor the precipitation of magnesium phosphate and calcium phosphate minerals.17,18 This is referred to as stored urine in the literature. Although previous studies show low or no presence of citrate and hippurate in stored urine,19,20 there is limited published data on the extent of endogenous metabolite degradation during storage.6,20 Therefore, one important assumption made for this study was that the concentration of metabolites in fresh urine and stored urine were equal and no degradation of endogenous metabolites occurred.
Although urine contains numerous nutrients, direct application of urine as a liquid fertilizer has disadvantages. These include a predetermined NPK ratio that may be suitable for some plants but not others as well as labor and storage costs of collecting and transporting large volumes of urine.21–24 These disadvantages can be mitigated by producing separate N, P, and K products which will result in two main benefits: 1) the ability to customize the ratio of NPK in the fertilizer by recombining each individual product to fit the nutrient needs of any crop and 2) to concentrate nutrients in solid and liquid fertilizers that will have higher nutrient density than liquid urine.5,7,24
Struvite (MgNH4PO4·6H2O) precipitation has gained attention because it is an effective treatment process for P recovery. With the addition of a magnesium source (Mg source) 95% of P can be recovered within an hour in the form of an odorless, slow release fertilizer with little heavy metal contamination when compared with commercial fertilizers.21,24–26 Although struvite contains multiple key nutrients, N and P, the elemental composition of struvite and the composition of urine limits the amount of N that can be recovered from urine. Considering the composition of synthetic urine used in this study (Table 1) and the concentration of Mg2+ added for struvite precipitation, 15.4 mmol L−1, approximately 6.8 g L−1 N or 97% of N would remain in solution if all the P precipitated as struvite. The processes of struvite precipitation has also been investigated to recover potassium as K-struvite, KMgPO4·6H2O. However, the precipitation of K-struvite is not likely until N is depleted to concentrations below K from urine.27–30 Consequently, a significant portion of N and K remain in the solution after the struvite precipitation process.
Chemicals | Synthetic urinea | Synthetic urine with metabolitesa | Real urinea | |||
---|---|---|---|---|---|---|
mmol L−1 | mg L−1 | mmol L−1 | mg L−1 | mmol L−1 | mg L−1 | |
a Concentrations are measured except endogenous metabolites, which were calculated in synthetic urine based on recipe and not measured (nm) in real urine. b TAN = total ammonia nitrogen = NH3 (g) + NH4+ (aq) (mg L−1 N). c TP = total phosphate = H2PO4− + HPO42− (mg L−1 P). | ||||||
SO42− | 15.14 | 1453 | 14.73 | 1414 | 12.74 | 1223 |
Na+ | 98.26 | 2259 | 112.4 | 2585 | 57.66 | 1326 |
K+ | 37.72 | 1471 | 43.21 | 1685 | 27.73 | 1081 |
TANb | 453.8 | 6350 | 459.3 | 6430 | 359.8 | 5037 |
Cl− | 96.40 | 3413 | 94.82 | 3356 | 73.15 | 2590 |
TPc | 13.3 | 411 | 15.2 | 470 | 11.1 | 344 |
Conductivity (μS cm−1) | 38.45 | 39.20 | 45.24 | |||
Citrate | 2.486 | 2.486 | nm | — | ||
Creatinine | 0.5633 | 0.5633 | nm | — | ||
Glycine | 1.237 | 1.237 | nm | — | ||
Hippurate | 2.804 | 2.804 | nm | — | ||
L-Cysteine | 0.8058 | 0.8058 | nm | — | ||
Taurine | 0.9919 | 0.9919 | nm | — | ||
pH | 9.2 | 9.2 | 9.0 |
Due to the high concentration of total ammonia nitrogen (TAN), NH3(g) + NH4+(aq), in stored urine, many technologies have been investigated for the recovery of N, including air stripping–acid absorption (hereafter referred to as ammonia stripping–acid absorption) which has shown high nutrient recovery (>80% N recovery).31–34 The ammonia stripping–acid absorption process targets N recovery by removing ammonia from solution and concentrating it in a sulfuric acid solution. This is done by shifting the solution ammonia acid/base equilibrium towards NH3 (unionized ammonia) via a temperature and/or pH increase. Using a sufficient air to liquid flow ratio, NH3 is transferred from the liquid to gas phase and is subsequently ionized via absorption by sulfuric acid to produce (NH4)2SO4, ammonium sulfate, a liquid N fertilizer. Since all of the K is present in the soluble form, K should remain in solution and not be affected by the ammonia stripping–acid absorption process.
Precipitation in the form of K-struvite, KMgPO4·6H2O, has been researched as a treatment for the recovery of K from N-depleted urine. However, due to the elemental composition of K-struvite additional chemical input of Mg2+ and P is required for effective K recovery.28,30
TAN recovery by ammonia stripping–acid absorption32,34,35 and TP recovery by struvite precipitation21,25,27,36–50 have been studied extensively to recover a significant portion of N and P in urine. For example, major conclusions from previous research include >95% TP recovery using MgCl2 and >90% TAN recovery by increasing the pH and temperature of the solution. The gap in knowledge lies in the few studies that combine N and P treatment in series to produce separate N and P products,30,31,51–54 and minimal research on treatment processes to recover N, P, and K at significant concentrations.30,55,56 For example, major conclusions on treatment processes to recover N, P, and K include a lack of significant N and K recovery via precipitation without the equimolar addition of P and Mg2+ to N or K+ in solution.
NPK are all essential for plant nutrition yet the majority of studies done on nutrient recovery from urine focuses on N and P recovery. This is mainly because of the high concentration of N in urine and the limited availability of raw P sources which are location specific. Based on the urine composition shown in Table 1, the concentration of nutrients NPK increase from P < K < N where the concentration of N is approximately 4.5 times greater than K, which is approximately 3.5 times greater than P. Considering the approximate concentration of NPK in urine and the market value of each NPK fertilizer, the economic value of each nutrient ($ nutrient per 10000 L urine) was calculated for N, P, and K as shown in Table S1 in the ESI.† The economic value of NPK in urine decreases from N > K > P where the value of N is approximately 1.7 times greater than K, which is approximately 14 times greater than P. Results from this table show that K has comparable economic value to N and greater economic value compared to P, and should therefore be a focus for studies investigating nutrient recovery from urine. Due to the agricultural and economic value of separate N, P, and K products, identifying effective nutrient recovery treatment processes are necessary. Furthermore, understanding the effect of treatment processes on subsequent treatment operations is required for maximum nutrient recovery from urine. As a result, this study aims to fill this gap in knowledge by providing an integrated, multi-process approach of combining nutrient recovery processes into a single sequence to understand the recovery of N, P, and K as separate products of economic and agricultural value.
The goal of this study was to investigate the downstream impacts of each treatment process on NPK recovery and identify conditions that would yield high NPK recovery as agricultural products. The specific objectives of this study were to (i) compare the effect of urine solution chemistry on NPK recovery; (ii) compare the process of struvite precipitation using three Mg source inputs (MgCl2·6H2O, MgO, and MgCO3) in terms of TP recovery (%), struvite product purity, and urine supernatant solution chemistry (pH and ion concentration); (iii) compare TAN recovery (%), liquid ammonium sulfate product purity, and urine solution chemistry (ion concentration) via ammonia stripping–acid absorption, using urine supernatant with pH (chemical) and temperature (physical) adjustments, (iv) evaluate the composition of the final product, potash, compare K recovery (%), and compare the mass of product via evaporation, and (v) evaluate the economic value of each urine derived fertilizer product.
The University of Florida Institutional Review Board approved real urine collection as exempt. Informed consent was obtained for any experimentation with human subjects. Undiluted real human urine was collected from healthy males and females ages 16–50 and stored for four months at 20–30 °C. The extent of urea hydrolysis was 80% as determined by total nitrogen (TN) and TAN concentrations. Initial nutrient concentrations were established by samples taken after the storage period (pre struvite precipitation).
Based on the solubility (g/100 mL at 25 °C) of Mg sources in water (MgCl2·6H2O = 52.9, MgO = 0.0086, MgCO3 = 0.0139), preliminary Mg2+ solubility tests were conducted to determine the solubility of each Mg source in solutions of increasing ionic strength and the expected TP recovery for struvite precipitation experiments. Experiments were done in triplicate using the same parameters detailed for struvite precipitation experiments (Mg:P molar ratio, mixing speed and time, and settling time), 250 mL of deionized (DI) water, and a modified synthetic urine made without the addition of phosphate (referred to as synthetic urine with no P). Solubility was determined based on the concentration of Mg2+ in solution.
Solid samples were taken at two points: post struvite precipitation and post evaporation. Samples were ground to a powder using a mortar and pestle then examined by X-ray diffraction (XRD). XRD analysis was performed using a Siemens D5000 X-ray diffractometer with monochromic Cu Kα radiation. The scanning rate used was 2.0° min−1 with a 2θ range of 10–70. The quality of precipitate obtained was compared with standard XRD patterns from the International Centre for Diffraction Data (ICDD).
The percent of TP recovered by each Mg source, percent of TP in the product, and the crystal structure identified using XRD are shown in Table 2. The amount of TP recovered by each Mg source was MgCl2·6H2O > MgCO3 > MgO, where only MgCl2·6H2O followed trends reported in literature25,48 whereas MgO and MgCO3 did not. For example, MgO recovered <66% TP, unlike the study conducted by Lind et al. in which >99% TP recovery was observed using a ratio less than 1.1:1 Mg:P (experimental dose).51 Etter et al. observed up to 91% TP recovery via filtration when MgO was dosed at 1.1:1 Mg:P molar ratio and less than 50% via sedimentation.25 Several struvite precipitation studies were conducted with MgO, however, the dose was greater than 1.1:1 Mg:P molar ratio and therefore could not be used for direct comparison.25,31,64 Due to the low solubility of MgCO3 in water, MgCO3 has not been investigated for struvite precipitation, but rather it has been treated to produce soluble MgO.42,65 Krahenuhl et al. saw only 1% of the added Mg2+ had dissolved in solution after six hours of calcination at 400 °C, which is less than the >55% TP recovered by MgCO3 in synthetic urine in this study.
Urine solution | Magnesium source | Ce,a mean (mmol L−1) | Ce,a std (mmol L−1) | TP recovered (%) | P in solidb (%) |
---|---|---|---|---|---|
Initial TP concentration of urine solution is as follows: synthetic urine with metabolites (15.2 mmol L−1), synthetic urine (13.3 mmol L−1), and real urine (11.1 mmol L−1). Reported values are the averages of triplicates. a Ce = effluent concentration. b XRD analysis identified struvite as the dominant mineral phase for all solids. | |||||
Synthetic urine | MgCl2·6H2O | N/A | N/A | 92.7 | 12.0 |
MgO | 6.93 | 0.544 | 47.8 | 10.1 | |
MgCO3 | 5.99 | 0.478 | 54.9 | 10.1 | |
Synthetic urine with metabolites | MgCl2·6H2O | N/A | N/A | 93.6 | 12.5 |
MgO | 5.20 | 0.868 | 65.7 | 10.8 | |
MgCO3 | 5.23 | 1.59 | 65.5 | 11.4 | |
Real urine | MgCl2·6H2O | N/A | N/A | 91.3 | 12.6 |
MgO | 3.74 | 0.535 | 66.4 | 10.7 | |
MgCO3 | 2.58 | 0.520 | 76.8 | 10.5 |
Based on preliminary Mg2+ solubility experiments (Table S4 in ESI†), the solubility of MgO and MgCO3 increased with ionic strength (i.e., synthetic urine with no P > DI water) and were expected to recover approximately 100% and 30% TP, respectively, in synthetic urine since all of the dissolved Mg2+ would precipitate with P and N species. However, MgO and MgCO3 did not follow this trend and recovered approximately 52% and 55% of TP during struvite precipitation experiments, respectively, in synthetic urine (Table 2). Conductivity measurements were taken as a surrogate of ionic strength (Table 1) and the increasing trend was synthetic urine < synthetic urine with metabolites < real urine. For struvite precipitation experiments, TP recovered when MgCl2·6H2O was used was approximately equal (91–94%) in all urine solutions. The specific results are further explained in the ESI,† section 1.2. However, TP recovery when MgO and MgCO3 was used as the Mg source for struvite precipitation was highest in real urine followed by synthetic urine with metabolites and finally synthetic urine. The TP results supported the idea that the solubility of MgO and MgCO3 increased with increasing ionic strength.
Regardless of the Mg source and urine solution, all solid materials obtained were crystalline and agreed well with the peak positions of the standard XRD pattern for struvite (Fig. S1 in ESI†). Based on the stoichiometry of MgNH4PO4·6H2O, the theoretical percent of P in struvite is 12.6%, which is approximately four times greater than the percent of P in liquid urine. Samples that contained MgCl2·6H2O consistently had values of approximately 12.6% (12.6% in real urine, 12.5% in synthetic urine with metabolites, and 12.0% in synthetic urine), while MgO and MgCO3 consistently had values below 12.6%, indicating that an approximately pure struvite was produced albeit with some difference. Previous struvite precipitation work reported MgO as a more effective source for struvite precipitation than MgCO3 because of lower costs, greater percent of dissolved Mg2+ in solution, higher TP recovery, and higher P content in the product.25,42,43 A simple economic cost analysis for the production of struvite from 10000 L of real urine was calculated as shown in Table S5 in the ESI.† Although MgO had the greatest net profit, MgCO3 had the highest cost per mol of P recovered. Furthermore, MgCO3 recovered a greater percent of TP compared with MgO and had comparable struvite purity (approximately 11% in real urine, 11% in synthetic urine with metabolites, and 10% in synthetic urine), demonstrating that it is an equally effective Mg source for struvite precipitation. To determine which Mg source would be the most effective (high nutrient recovery and low cost) the location of the treatment process must be considered. For example, the abundance of MgCO3 in Nepal could reduce consumed costs by using a locally available resource and make MgCO3 more cost effective than pretreating MgCO3 to produce MgO and/or buying MgO from the market.25
The purpose of using three urine solutions was to compare the effect of urine solution chemistry under identical experimental conditions on NPK recovery, e.g., the effect of endogenous metabolites on nutrient recovery. The amount of P in the struvite product and TP recovered from solution increased as the conductivity of the solution increased, suggesting that metabolites had a positive effect on TP recovery and struvite purity. When MgCO3 was used as the Mg source, the percent of TP recovered from solution was 52%, 62%, and 66% in synthetic urine, synthetic urine with metabolites, and real urine, respectively (Table 2). Previous literature observed the opposite effect where the endogenous metabolites either had no effect on the precipitation potential of struvite and could be neglected or acted as organic complexing agents and reduced the amount of free cations in solution and thus the extent of struvite precipitation. For example, Udert et al. 2003 investigated precipitation effects in a urine collecting system and concluded that endogenous metabolites (organic compounds), specifically citrate and oxalate, had a negative effect on the struvite precipitation process because the metabolites would adsorb to the crystals during the nucleation step and inhibit the rate of precipitation by a factor of 4.17 However, the difference in precipitation potential was mainly noticed during the hydrolysis process. Once at least 16% of the urea was hydrolyzed, the precipitation potential in solutions with organic complexing agents and without organic complexing agents only differed slightly until complete urea hydrolysis was reached.17,19 For this study, approximately 80% of urea was hydrolyzed in real urine and 100% of urea was hydrolyzed in the synthetic urine solutions. Therefore, the difference in urea hydrolysis from previous literature and the current study could explain for the variation observed in this study. However, since TP recovery in the presence of metabolites has not been observed in pervious literature, future research detailing the mechanisms of prevalent metabolites in urine is required to fully understand and compare this result from this study.
Since stored urine is buffered by ammonia and carbonate, there was no significant increase or decrease in pH due to the Mg source (as shown in Table S6 in the ESI†).
Results from the ANOVA two-way test with replication for TP recovery (Table S3.a in ESI†) show that p-values for factor (i) and (ii) are <0.05. Therefore, there is a statistically significant different between TP recovery and varying Mg sources, where MgCl2·6H2O recovered the greatest TP, and between TP recovery and varying urine solutions, where the greatest TP was recovered in real urine.
Urine solution | Mg source | Ammonia stripping–acid absorption conditions | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
pH 9.6, 55 °C | pH 10, 40 °C | pH 10.5, 22 °C | pH 9.2, 70 °C | pH 9.2, 22 °C | ||||||
% Na | % Ka | % Na | % Ka | % Na | % Ka | % Na | % Na | % Ka | ||
a Percent nutrient in the product calculated as w/w. | ||||||||||
Real urine | MgCl2·6H2O | 1.5 | 4.4 | 1.4 | 5.1 | 1.2 | 2.6 | 2.4 | 0.83 | 6.6 |
MgO | 1.4 | 4 | 1.3 | 5.1 | 1.3 | 3.4 | 2.1 | 0.96 | 6.6 | |
MgCO3 | 1.6 | 3.7 | 1.4 | 4.9 | 1.3 | 3.5 | 2.2 | 0.62 | 6.6 | |
Synthetic with metabolites | MgCl2·6H2O | 1.7 | 6.6 | 1.6 | 7.1 | 1.4 | 5.3 | 2.1 | 0.98 | 6.7 |
MgO | 1.8 | 6.7 | 1.7 | 6.7 | 1.5 | 5.5 | 1.9 | 0.91 | 6.5 | |
MgCO3 | 2.0 | 5.9 | 1.6 | 6.1 | 1.9 | 5.5 | 2.3 | 1.1 | 9.2 | |
Synthetic | MgCl2·6H2O | 1.7 | 6.7 | 1.6 | 7.4 | 1.3 | 5.8 | 2.2 | 0.93 | 9.3 |
MgO | 1.9 | 5.5 | 1.6 | 6.2 | 1.4 | 5.2 | 2.3 | 0.96 | 9.2 | |
MgCO3 | 2.0 | 5.2 | 1.8 | 6.6 | 1.6 | 5.2 | 1.7 | 0.98 | 9.1 |
Increasing both the pH and temperature of the solution recovered the greatest percent of TAN in real urine and synthetic urine with metabolites, regardless of the Mg source. The trend of TAN recovery in real urine for this study follows trends in literature where solely increasing the temperature, rather than solely increasing the pH, of the solution resulted in a higher percentage of TAN recovery, pH 9.2, 70 °C > pH 10.5, 22 °C > pH 9.2, 22 °C (Fig. 1).32,33,66 Although solely increasing the temperature resulted in a higher percent of TAN recovered, operational issues such as excess foaming, precipitation, and evaporation were observed. Specific details are further explained in the ESI,† section 2.2. Experimental conditions were designed to yield TAN recovery of approximately 90%, however, only temperature adjusted samples recovered >90% TAN while samples adjusted for only pH (condition pH 10.5, 22 °C) recovered <75% TAN in all urine solutions. During ammonia stripping–acid absorption experiments, temperature was maintained using a water bath while pH was not monitored throughout. For pH measurements, samples were adjusted (to the condition pH) and measured before and after the treatment process (Table S9 in the ESI†). Due to the slight variation in pH, it is difficult to conclude why samples only adjusted for pH recovered <75% TAN instead of >90% TAN.
Results from the ANOVA two-way test with replication (Table S3.b and c in ESI†) for TAN recovery in samples containing MgCl2·6H2O, MgO, and MgCO3 indicate that there was a statistically significant difference between TAN recovery by varying ammonia stripping–acid absorption condition (p-value <0.05) and between TAN recovery in varying urine solutions. This suggests that the percent of TAN recovered by MgO, for example, in all urine solutions was significantly different, where a greater percent was recovered in real urine for all conditions aside from the baseline and pH 9.2, 70 °C. Furthermore, samples containing MgO and MgCO3 consistently had higher TAN recovery compared to samples containing MgCl2·6H2O for all conditions aside from the baseline and pH 9.2, 70 °C as well as in all urine solutions. For experiments done with real urine and synthetic urine with metabolites, there was no statically significant difference (p-value >0.05) between TAN recovery and varying Mg source but there was a statistically significant difference for experiments done with synthetic urine. This suggests that using synthetic urine as a proxy for real urine is not suitable for N recovery studies.
In post ammonia stripping–acid absorption, the (NH4)2SO4 product was kept in liquid form to minimize N loss and the percent N (w/w) in the product was calculated (Table 3).33,35 Calculation details for percent N (w/w) in the product are explained in the ESI,† eqn (S1)–(S6). The H2SO4 solution used in this study had the capability of producing a 2.67% N (w/w) product (eqn (S3) and (S4) in the ESI†). Condition pH 9.2, 70 °C produced the highest percent N (w/w) in the product, however due to operational issues detailed in the ESI,† this condition would not be ideal for treatment. The ammonium sulfate product derived from treating 100 mL of real urine contained approximately 1.18–1.47% N (w/w). Although this is less than the 8% N (w/w) in commercial liquid ammonium sulfate products on the market,67 it is 2–3 times more concentrated that the percent N (w/w) in urine. Furthermore, the concentration and volume of sulfuric acid (H2SO4) could be increased and reused to treat multiple batches of urine and produce a product with a higher N content (eqn (S5) and (S6) in the ESI†). Assuming the urination volume for each person is 1.4 L d−1, a single person can supply enough N to produce an 8% N ammonium sulfate liquid fertilizer in less than a month.
The mass of K present in the stripping column post ammonia stripping–acid absorption treatment, as shown in Table S10 in the ESI,† and the percent K in the potash product, as shown in Table S11 in the ESI,† followed the same trend where K decreased from pH 9.2, 22 °C > pH 10, 40 °C > pH 9.6, 55 °C > pH 10.5, 22 °C in all urine samples. Due to the high solubility of K+ in solution, similar masses of K was expected before and after the ammonia stripping–acid absorption treatment process. However, this was not the case. Furthermore, all ammonia stripping–acid absorption conditions aside from the baseline condition, pH 9.2, 22 °C had over 15% of the initial mass of K that was not present in either the stripping or absorption column. Since XRD results were insufficient to draw conclusions and did not identify any insoluble K minerals, Visual MINTEQ 3.1 was used to predict possible K precipitates that could explain for the unaccounted K. None were identified. Although XRD and Visual MINTEQ 3.1 did not identify K precipitates or insoluble K minerals, both tools have limitations, and it may be possible that the elevated pH conditions resulted in precipitation of K. Since samples were filtered prior to potassium analysis, it is possible that the precipitates were excluded by the filter and therefore unaccounted for during the measurement.
Potash is a common term for nutrient forms of the element K such as KCl and K2SO4.68 Therefore, potash purity was determined for all samples by comparing potash nutrients, K, SO42−, and Cl− (% w/w) to Na, which is undesired by soils and plants because it inhibits the uptake of desired nutrients such as N and K which are essential to plant growth (Table S11 in the ESI†). Specific details are discussed in the ESI,† section 2.3. The percent of Na+ in solution was consistently greater than the percent of potash in all samples, and increased with pH, where condition pH 10.5, 22 °C had the greatest percent of Na by mass in the product followed by pH 10, 40 °C > pH 9.6, 55 °C > pH 9.2, 22 °C. Samples adjusted for pH contained approximately 30–50% by mass Na indicating that the use of NaOH as a pH adjustment would not be suitable for effective nutrient recovery. Marketable potash fertilizers range from 17–52% K,69 which is over 4 times greater than what was obtained in this work. One potential solution to reduce Na+ in solution and increase K+ in solution would be to substitute KOH ($480 per metric ton) in lieu of NaOH ($400 per metric ton) for pH adjustment during ammonia stripping–acid absorption (specific cost details located in section 2.5 in ESI†). However, the volume of KOH must be considered to draw concrete conclusions on the effect on potash purity. Furthermore, it is important to note that the presence of organic compounds such as endogenous metabolites and pharmaceuticals, could render the potash product unusable, and further treatment, such as the addition of acid and the use of biochar to reduce the odor and pharmaceutical concentration in urine should be explored.70,71
Results from the ANOVA two-way test with replication (Table S3.d and e in ESI†) indicate that there was a statistically significant difference between K recoveries for varying ammonia stripping–acid absorption conditions (p-value <0.05) as well as between K recoveries for varying urine solutions. Following the baseline condition (pH 9.2, 22 °C) which recovered the greatest percent K in all urine solutions, i.e., the mass of K present in the stripping column post ammonia stripping–acid absorption, increasing both pH and temperature (pH 10, 40 °C) recovered the second greatest percent K in all urine solutions. The trend for increasing K recovery under condition pH 10, 40 °C, for samples containing MgCl2·6H2O and MgO was real urine > synthetic urine with metabolites > synthetic urine, whereas the trend for samples containing MgCO3 was real urine > synthetic urine > synthetic urine with metabolites. These findings, which have not been previously reported in the literature, indicate that the urine solution and conditions for N recovery (i.e., temperature and/or pH) dictate the purity of K in the final product whereas conditions for P recovery (i.e., Mg source) have no significant effect on the purity of the K product.
P fertilizer ($) | N fertilizer ($) | P and N fertilizer ($) | K fertilizer ($) | N and K fertilizer ($) | NPK fertilizer ($) | |
---|---|---|---|---|---|---|
a Total nutrient recovery involving K fertilizer could not be calculated due to cross contamination of potassium during the ammonia stripping–acid absorption process. | ||||||
pH 9.6, 55 °C | 4.99 | −75.89 | −70.89 | 13.12 | −62.77 | −57.77 |
pH 10, 40 °C | 4.99 | −75.50 | −70.50 | 13.44 | −62.06 | −57.06 |
pH 10.5, 22 °C | 4.99 | −75.88 | −70.88 | 12.09 | −63.79 | −58.79 |
pH 9.2 70 °Ca | 4.99 | −73.78 | −68.78 | — | — | — |
pH 9.2, 22 °C | 4.99 | −4.34 | 0.65 | 14.06 | 9.72 | 14.72 |
• To maximize N, P, and K recovery and purity of the corresponding products from real urine, MgCl2·6H2O is recommended as the input for struvite precipitation followed by an increase in pH and temperature to pH 10 and 40 °C. Under these conditions, over 91% of P, 99% of N, and 80% of K can be recovered.
• The Mg source used for P recovery did not impact subsequent N or K recovery in real urine or synthetic urine with metabolites.
• Conditions for N recovery affected subsequent K recovery where increasing both the pH and temperature of urine (pH 10, 40 °C) maximized K recovery and only increasing the pH decreased product purity.
• Replacing NaOH with KOH, another strong base to adjust the pH of urine, would increase the concentration of K+ (desired by soils and plants) in solution rather than Na+ (undesired by soils and plants) and create a K product of higher purity.
• TP, TAN, and K recovery in real urine was consistently higher than or equivalent to synthetic urine. Therefore, using synthetic solutions as a proxy is not suitable for robust conclusions regarding complete NPK recovery from real urine.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ew00004b |
This journal is © The Royal Society of Chemistry 2018 |