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Rinnakkaistallenteet Luonnontieteiden ja metsätieteiden tiedekunta
2019
Recovery of nitrogen and phosphorus from human urine using membrane and precipitation process
Pradhan, Surendra K
Elsevier BV
Tieteelliset aikakauslehtiartikkelit
© Elsevier Ltd
CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/
http://dx.doi.org/10.1016/j.jenvman.2019.06.046
https://erepo.uef.fi/handle/123456789/7684
Downloaded from University of Eastern Finland's eRepository
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Recovery of nitrogen and phosphorus from human urine using
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membrane and precipitation process
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*Surendra K Pradhan1, Anna Mikola1, Helvi Heinonen-Tanski2, Riku Vahala1 3
1Department of Built Environment, School of Engineering, Aalto University, P.O. Box 4
15200 FI-00076 Aalto, Finland 5
2Department of Environmental and Biological Sciences, University of Eastern Finland, 6
POB 1627, 70211, Kuopio, Finland 7
*corresponding author: Surendra K Pradhan, Surendra.pradhan@aalto.fi, +358 400973372 8
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2 Abstract
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The nitrogen (N) and phosphorus (P) contents in human urine have been recovered using 12
struvite precipitation and N-stripping techniques. Struvite precipitation technique recovers 13
mainly phosphorus whereas N-stripping technique only recovers nitrogen. In this study, we 14
developed an NPharvest technique which recovered both nitrogen and phosphorus separately 15
in the same process, enabling their use independently. The technique used Ca(OH)2 to 16
increase the pH of urine converting ammonium into NH3 gas and simultaneously 17
precipitating P with Ca. The NH3 gas is passed through a gas permeable hydrophobic 18
membrane (GPHM) and reacts with H2SO4 forming ammonium sulfate. Our result showed 19
that more than 98% (w/w) of N and P can be harvested from urine in 8 hours at 30 oC. The 20
harvested ammonium sulfate contained 19% (w/w) N, and the sediment contained 1–2%
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(w/w) P. The extraction of N and P from 1 m3 of urine could give a profit of 1.5 €.
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Keywords: - Ammonium sulfate; gas permeable hydrophobic membrane; nitrogen; nutrients 24
recycle; phosphorus; urine 25
26
3 1. Introduction
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Human urine is rich in nitrogen (N), phosphorus (P), and potassium (K) (Jonsson et al., 2004).
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Eco-toilet (collecting separately urine and feces) is an established technique to collect human 29
urine for further use. N and P from urine can be recovered as struvite (MgNH4PO4·6H2O) 30
(Lind et al., 2000; Etter et al., 2011; Morales et al., 2013). Several full-scale struvite 31
production plants (mainly for P recovery from wastewater or reject water) have been 32
established by several companies (Mikosana, 2015). The struvite precipitation process 33
recovers about 99% of P and only about 10% of N. The stripping technique is used to recover 34
N from urine (Basakcilardan-Kabakci et al., 2007; Liu et al., 2015) and reject water (Wu et 35
al., 2013) but it recovers only nitrogen. Therefore, it would be useful to develop a new 36
technique to extract N and P separately and enable their independent use.
37 38
Ammonia from different liquid wastes has been recovered using gas permeable hydrophobic 39
membrane (GPHM) (Garcia-Gonzalez and Vanotti, 2015; Vanotti et al., 2017; Kuntke et al., 40
2016). GPHM used in bio-electrochemical system recovered 95% ammonia from urine in 41
11-15 days (Kuntke et al., 2016) and about 90% of ammonia from swine wastewater (Vanotti 42
et al., 2017) and from digested manure (Garcia-Gonzalez et al., 2016). The energy 43
consumption of the membrane process can be 0.18 kWh/kg NH3 (Zarebska et al., 2015)while 44
the aeration technique may need 21 kWh/kg NH3 (Maurer et al., 2003). Increasing the liquid 45
pH is the major costs of both membrane process and stripping (Zarebska et al., 2015). On the 46
other hand, Ca(OH)2 was used to precipitate P from swine manure (Fernandes et al., 2012) 47
and urine (Pradhan et al., 2017). Ca(OH)2 was also used to increase the pH of fecal slurry for 48
its hygienization (Ogunyoku et al., 2016).
49
4 50
In this study, we have improved our previous “NPharvest technique” so that the purity of 51
ammonium sulfate will be increased and the need for energy and Ca will be lowered. The 52
process was improved by combining membrane and precipitation techniques to recover N 53
and P from urine. The mechanism of this technique is based on increasing the pH of urine 54
using Ca(OH)2 and precipitating P (Equation 1) and simultaneously converting NH4+-N into 55
ammonia gas. This gas is passed through GPHM and reacting with H2SO4 and forming 56
ammonium sulfate (Garcia-Gonzalez and Vanotti, 2015) (Equation 2). Both N and P will be 57
recovered simultaneously in the same run. The Ca(OH)2 is easily available globally, and the 58
Ca-based end-product has better value for agriculture, especially in acidic soils (Goulding, 59
2016) as it combines the benefits of P and calcium for liming the soil.
60 61
5Ca2+ + 3PO43- + OH- → Ca5(PO4)3OH ↓ Equation 1 62
2NH3 + H2SO4 → (NH4)2SO4 Equation 2
63 64
The N and P harvesting process can be influenced by the temperature (Huang and Shang, 65
2006), pH (Garcia-Gonzalez and Vanotti, 2015) and concentration of nutrients. We used 66
both, pure and diluted urine because dry and wet flush urinals (1:5 v/v) are used for collecting 67
urine. This study was conducted at 30, 20 and 8 oC presenting ambient temperatures in tropics 68
to temperate. The main objectives of this study are: (1) to test the NPharvest technique to 69
recover N and P, and produce fertilizers from urine at different temperatures and in different 70
dilutions, (2) to determine the quality of the NPharvest recovered products, i.e. ammonium 71
sulfate and P in sediment, and (3) economic assessment.
72
5 73
2. Materials and methods 74
2.1. Urine handling, membrane unit preparation and physiochemical analysis 75
This study is the continuation of our earlier study about N and P recovery from urine (Pradhan 76
et al., 2017) but in this study, we used GPHM for N recovery at low temperature. Urine was 77
collected from a waterless urinal placed at the Helsinki Festival in the summer of 2014 and 78
stored at 4 oC until this experiment. The GPHM tube of 60 cm length and 5.6 mm outer 79
diameter (contact area 106 cm2) and wall thickness 0.22 mm made from ePTFE (extended 80
polytetrafluoroethylene, Zeus International) was used. An acid resistant metal coil and net 81
spacer were used as an inner supporting material for the membrane. Both ends of the GPHM 82
tubes were connected to the PVC pipes. Total-N, NH4+-N, Total-P, PO4-P, suspended solids 83
(SS) for water, and total-P and K in sediment were analyzed (Table 1) using the technique 84
and standards as explained (Pradhan et al., 2017). The ammonia concentration of urine was 85
determined before and after the experiment using the NH3 gas sensing electrode Orion 86
900/200 (Thermo Electron Corporation, Beverly, MA) after adjusting the sample to pH > 11.
87
This sample was analyzed for NH3 using the ISO 11732 method using a Tecator 5042 88
detector/5012 analyzer from Foss Höganäs, Sweden. Although we used NH3 gas 89
measurement, eventually it is measured as NH4-N and the end value is presented as NH4-N.
90
Alkalinity was determined with an automatic titrator (Metrohm, Schott Instruments) by 91
measuring the amount of 100 mmol hydrochloric acids required to reach an end-point pH of 92
4.5 and a pH of 7 and reported as CaCO3 mg/L (SFS-EN-ISO 9963-1). The pH was monitored 93
using a pH meter (Finnolab, Inolab pH 720 from WTW). The purity of the harvested solid 94
ammonium sulfate and sediment (calcium and phosphorus compounds) were confirmed by 95
6
X-ray diffraction (XRD) Quantification of the XRD result was based on semi-quantitative 96
results performed using the “Match!” software for phase identification from powder 97
diffraction data.
98 99
2.2. Experimental design 100
101
Figure 1. Experimental setup.
102
2.2.1. Experiment 1 (pure urine). Pure urine (700 mL) was taken into a 1 L glass bottles 103
(Duran Germany) and its pH was adjusted to pH 12 with 13 g of Ca(OH)2 (purity 96%). The 104
bottles were closed to avoid NH3 emissions before the experiment. Then 130 mL of 1M 105
H2SO4 (prepared from 98% concentrated H2SO4) was taken into a 250 mL bottles. The coiled 106
GPHM were submerged in urine, and the connected PVC pipes were passed through a pump 107
and the pipe ends were submerged in 1M H2SO4 (Figure 1). The acid was circulated at the 108
rate of 33 ± 3 mL/min, and the urine liquid was stirred at 150 rpm. During the circulation 109
process, the ammonia gas passed through the GPMH and contacted the circulating acid 110
forming ammonium sulfate.
111 112
2.2.2. Experiment 2 (diluted urine). Amount of 700 mL diluted urine (diluted as 1 portion 113
pure urine and 4 portions deionized water) was taken into a 1 L glass bottles where 3 g of 114
7
Ca(OH)2 was added to achieve pH 12 to maintain similarities to the pH of pure urine in 115
experiment 1. Now, 28 mL of 1M H2SO4 was taken into a 100 mL bottles. The experimental 116
setup was done as in Experiment 1.
117
Both experiments were conducted at 8, 20 and 30 oC until NH4-N concentration of urine 118
became very low. The acid and urine samples were analyzed every 4 hours, and the 119
experiments were conducted with three experimental setups with 2 replicates. After 120
completing the experiments, the sediments were carefully separated from the supernatant.
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The sediments and the ammonia-adsorbed H2SO4 solutions were dehydrated at 60 oC for 24- 122
48 hours. The recovered crystallized/solids (NH4)2SO4 and sediments were ground and 123
analyzed for nitrogen, phosphorus, and potassium (NPK). The crystalline structure of 124
(NH4)2SO4 and P in sediments was also analyzed using XRD. The supernatants or effluents 125
were analyzed for residual NPK and SS.
126 127
2.3. Process optimization 128
The total NH3/NH4 content in urine was measured and the free ammonia concentration was 129
estimated using the table of shifting ammonium and ammonia gas based on the urine pH 130
(Thurston et al., 1979). The experiment was pre-tested to optimize different variables, 131
conditions, and setup (data not shown). We noticed that the high flow rate of acid inside 132
GPHM increased the efficiency of ammonia harvesting. A high flow rate in the peristaltic 133
pump increased air bubbles inside the tube and may disrupt the acid flow. Similarly, the high 134
flow rate of acid increased the pressure on the membrane (in this case, it was negative 135
pressure). Therefore, the acid flow rate was optimized based on our experimental setup. The 136
8
membrane was prepared and tested in a different model. The coiled model (Figure 1) was 137
more promising, so it was used for the entire experiment.
138 139
The pH of stored urine was about 9 but our target pH was 12. The amount of Ca(OH)2 needed 140
to increase the urine pH was determined based on the potential to increase the pH of urine 141
(here, pH was determined by measurement). The amount of 1M H2SO4 needed for each test 142
was determined based on the molar weight of ammonium in the urine. However, we used 143
about 6% more acid than the theoretical calculation to make sure there was enough acid to 144
react the harvested ammonia. Furthermore, a higher concentration of acid reduces the risk of 145
ammonium sulfate crystallization as it avoids the solution becomes supersaturated (Mullin et 146
al., 1970).
147 148
2.4. Membrane and ammonia harvesting process 149
GPHM is made of Teflon and has been used for ammonia removal by several studies (Vanotti 150
et al., 2017; Garcia-Gonzalez et al., 2016; Kuntke et al., 2016). The basic principle of this 151
membrane is that only gas can pass through this hydrophobic membrane. The surface area, 152
membrane thickness, contact time, temperature, etc., influence the ammonia gas transfer rate 153
(Kaljunen, 2018). The ammonia concentration is higher in the urine side forcing ammonia to 154
diffuse to the acid side. This gaseous ammonia will be in contact with water in the acid side 155
and turn into an ammonium ion, which reacts with the sulfate ion forming ammonium sulfate.
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In this process, the pH of the acid side must be maintained <2 pH to guarantee enough SO4-
157
ions to react with NH4+. 158
159
9 2.5. Data analysis
160
Although the experiment was conducted in a small volume (700 mL), the experimental result 161
and economic assessment are calculated for 1m3 of urine assuming a linear result. The basic 162
data calculation and the graph plots were performed using Microsoft Office Excel 2016.
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These data were transferred into an IBM SPSS 23 for statistical analysis. The data were tested 164
for its distribution before performing the appropriate statistical analysis. The data from Table 165
1 and Table 2 were analyzed using One Way ANOVA combined with Tukey.
166
167
3. Results and discussion 168
169
3.1. The effect of alkali and pH 170
Fresh urine contains most of the N as urea and its pH is about 7. The urea will hydrolyze into 171
ammonia/ammonium and >99% of the ammonium will convert into ammonia gas at pH 11 172
at 20 oC (Thirston et al., 1979). In this study, urine pH above 12 was used because the pH 173
can slightly be decreased during the nutrient harvesting process (Figure 2) so that there can 174
be a risk that ammonia gas will convert back into ammonium if the pH decreases below 11.
175
The pH reduction during the NPharvest process was higher at a higher temperature (i.e. 30 176
oC) compared to low temperature (i.e. 8 oC). The reduction of pH was higher during the first 177
four hours of the experiment time at 20 and 30 oC. This is due to the high NH4+ reduction 178
during the first four hours of the experiment (Figure 2). The reduction of pH during the 179
extraction of ammonia was also reported earlier (Garcia-González and Vanotti, 2015).
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However, the pH reduction was much lower in the GPHM technique compared to the aeration 181
N-stripping technique (Pradhan et al., 2017).
182
10 183
Ca(OH)2 was used in this study due to its feasibility for the NPharvest process as found by 184
Ogunyoku et al. (2016) and Pradhan et al. (2017). The amount of alkali needed to increase 185
the pH of urine depends on the concentration of ammonium-N and dissolved CO2 (i.e. HCO3) 186
in the urine. The aeration in urine (1.1 L/min) at 20 oC for 10 minutes slightly decreased the 187
alkalinity i.e. from 3.4 g/L CaCO3 to 3.2 g/L CaCO3. Since aeration increases risks of NH3
188
losses, we did not aerate the urine before the process. Randall et al. (2016) recommended 189
using about 10 kg Ca(OH)2/m3 to achieve pH 12 of fresh, un-hydrolyzed urine. According to 190
our calculation the need of Ca(OH)2 would be 19 kg/m3 for one-year-old undiluted urine and 191
3 kg/m3 for diluted urine (1:4) to achieve pH 12.3 assuming a linear scale-up. Consumption 192
of Ca(OH)2 in this study was thus a little lower compared to the aeration stripping technique 193
(Pradhan et al., 2017). The aeration with ambient air reduces the pH because of CO2 in 194
ambient air reacts with Ca(OH)2 producing less soluble CaCO3 (Taylor, 1964). Therefore, 195
the aeration process needs additional alkali to maintain the pH (Pradhan et al., 2017). The 196
need for alkali varies, as the quality of urine also varies in each batch.
197 198
11 199
200
Figure 2. NH4+-N harvesting from urine at different temperatures and different dilutions of 201
urine. The pH of urine is shown in the secondary axis. The standard deviation is shown in 202
the error bar.
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204
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Table 1. Physico-chemical properties of urine before and after the experiment in supernatant 205
and sediment (mean ± standard deviation).
206
Urine before experiment
Effluent after N and P harvest at different temperatures
Pure urine Diluted urine
8 oC 20 oC 30 oC 8 oC 20 oC 30 oC
*In liquid
pH 9.4 ± 0.2 12.7 11.9 11.5 12.5 12.3 11.8
NH4+-N (kg/m3) 3.9 ± 0.2 0.3 ± 0.05 0.05 ± 0.02 0.09 ± 0.02 0.02 ± 0.01 0.03 ± 0.01 0.01 Total-N (kg/m3) 4.4 ± 0.2 0.2 ± 0.04 0.03 ± 0.01 0.11 ± 0.03 0.04 ± 0.01 0.04 ± 0.01 0.03 ±
0.01 Total-P (g/m3) 227 ± 16 0.8 ± 0.2a 0.2 ± 0.1ab 1.2 ± 0.1b 0.5 ± 0.1 0.2 ± 0.1 1 ± 0.5
PO4-P (g/m3) 225 ± 15 NA NA NA NA NA NA
Tot-K (kg/m3) 1.74 1.4 ± 0.09a 1.5 ± 0.04a 8.2 ± 0.04b 0.4 0.4 0.4 In sediment
N (g/kg) - 0.03 0.04 0.04 0.03 0.03 0.03
P (g/kg) - 10.3 ± 0.2a 9.4 ± 0.5ac 8.7 ± 1.9c 4.9 ± 0.7b 10.7 ± 0.6a 8.5 ± 0.5c K (g/kg) - 14.8 ± 3.8a 8.3 ± 1.6c 7.5 ± 1.6c 1.2 ± 0.3b 2.6 ± 0.4b 2.6 ± 0.4b 207 The different letters (a, b, c, and d) in the same row (i.e., for P and K) are significantly different (P<0.05). *For pure urine data was compared
208
with pure urine, and diluted urine data was compared with diluted urine. NA = not analyzed.
209 210
3.2. Membrane and ammonia harvesting process 211
Our study achieved more than 98% ammonia harvesting within 24 hours, even at 8 oC (Figure 212
2). Our process is thus faster with higher ammonia recovery than that 81% recovery in one 213
month (García-González and Vanotti, 2015; García-González et al., 2016) or in several days 214
(Vanotti et al., 2017). One reason for our good result can be the thickness of the membrane 215
13
wall. We used a 0.22 mm thick membrane, whereas García-González and Vanotti (2015) and 216
García-González et al. (2016) used a 0.77 mm thick membrane. The second reason for our 217
high N harvesting result might be the high circulating rate of acid (31 mL/min per 100 cm2 218
of the membrane) whereas the acid circulation rate of 2.6 mL/min per 100 cm2 of the 219
membrane was used by García-González and Vanotti (2015) and García-González et al.
220
(2016). The third reason might be that our process converted 99% of ammonium into 221
ammonia, whereas all the ammonium was maybe not converted into ammonia gas in the 222
studies of García-González et al. (2016) and Vanotti et al. (2017). The fourth reason might 223
be that the urine used in our study contains less SS, (180 mg/L) than swine manure (Vanotti 224
et al., 2017)or digested manure (García-González et al., 2016), which might affect the 225
efficiency of the membrane. Fouling in the membrane was not noticed in this experiment, 226
but the experimental time was too short to make any conclusions about this.
227
228
3.3. Effect of temperature and operational time 229
At 30 oC, 90–100% of NH4+-N was harvested within a harvesting time of 8 hours, whereas 230
16 and 24 hours were needed at 20 oC and 8 oC to harvest 98–100% of the NH4+-N. This 231
might be because of the high ammonia gas availability at a higher temperature (Thurston et 232
al., 1979) as seen by the decreasing pKa value of NH4+-N with increasing temperature. Thus, 233
the pKa values of NH4+-N are 9.893, 9.493 and 9.184 at 8, 20 and at 30 °C (calculated using 234
the Equation 3 (Emerson et al., 1975) and Equation 4 (Gulyas et al., 2014)). The second 235
reason might be faster diffusion with a lower viscosity at high temperature. The third reason 236
can be the higher reaction rate at a higher temperature. Nutrient harvesting at 30 oC would 237
need only 25% of the experimental time compared to the experiment at 8 oC (Table 2 and 238
14
Figure 2). The temperature of 30 oC is favorable to tropical countries where the ambient 239
temperature is about 30 oC year-round. Furthermore, the result at 8 oC showed that this 240
technique can operate even at low temperatures, although the experimental period will be 241
longer.
242 243
𝑝𝑝𝑝𝑝𝑎𝑎𝑤𝑤 = 0.09108 +2729.92𝐾𝐾𝑇𝑇 Equation 3 244
𝑝𝑝𝑝𝑝𝑎𝑎 =𝑝𝑝𝑝𝑝𝑎𝑎𝑤𝑤+ [0.1552𝐿𝐿𝑚𝑚𝑚𝑚𝑚𝑚−1−0. 0003142𝐿𝐿𝑚𝑚𝑚𝑚𝑚𝑚−1𝑝𝑝−1× (𝑇𝑇 −273,2𝑝𝑝)] ×𝐼𝐼 Equation 4 245
Equation 3 is the pKa value for NH3 in water, and it was recalculated for urine in Equation 246
4. T is the absolute temperature, I is an ionic strength for urine, and pKaw is the pKa value for 247
ammonia water.
248 249
Although the experiment was conducted for 8, 16 or 24 hours to harvest 99% of ammonia, it 250
is possible that shorter harvesting times could be more feasible. For example, at 20 oC the 251
NH4+-N harvesting for 8 hours was 88 ± 7% for pure urine and 85 ± 8% for diluted urine 252
(Figure 2). Similarly, at 8 oC, the NH4+-Nharvesting in 16 hours was 90 ± 7% for pure urine 253
and 93 ± 8% for diluted urine. Therefore, about half of the operation time might be 254
economical compared to operating a long time to achieve > 99% harvesting. A detailed 255
economic assessment is discussed in section 3.7. Anyhow, removal of N needs to be 256
considered according to environmental permissions to limit the emissions of NH3 to the 257
environment.
258 259
3.4. Effect of Ammonium Concentration 260
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The NH4+ harvesting efficiencies for pure and diluted urine were not significantly different 261
(Table 2). This is very important in practice since the amount of flushing water can vary as 262
well as the chemical composition of urine varies according to diet, physical activity and 263
climate etc. (Schouw et al., 2002). In the first four hours of the total 24 hours and 16 hours 264
of experiments, the NH4+-N harvesting rate was slightly higher for pure urine compared to 265
diluted urine. Pearson’s correlation showed a positive correlation between NH4+-N 266
harvesting and NH4+-N concentration in urine (p < 0.005, r = 0.859). Pradhan et al. (2017) 267
also showed a positive correlation between N-concentration and NH4+-N harvesting. In fact, 268
the overall result showed that the first four hours of experimental time is the most important 269
time for NH4+-N harvesting. After that, the harvesting rate decreased. This might be because, 270
in the beginning, there is a high concentration of ammonia in urine and more SO4- ion in acid 271
to react. With time, the concentration of ammonia and sulfate ion will decrease, and the 272
reductions of these ions in the process will reduce the chance of contact between free 273
ammonium and free sulfate inside the GPHM tube.
274 275
Table 2. Production of (NH4)2SO4 and P-precipitate in sediment after different experimental 276
conditions.
277
Types of urine Pure urine Diluted urine
Process temperature and time 8 oC/24 h 20 oC/16 h 30 oC/8 h 8 oC/24 h 20 oC/16 h 30 oC/8 h (NH4)2SO4 (DW kg/m3 urine) 22.8 ± 1.8a 25 ± 1b 22 ± 0.7a 4.2 ± 0.1 5 ± 0.1 4.4 ± 0.2 NH4+-N in harvested crystals % 16.8 ± 2.2 18.4 ± 0.5 20.4 ± 0.5 18.1 ± 2.1 18 ± 2 20 ± 1.2 Sediment (DW kg/m3 urine) 23 ± 0.7a 24 ± 0.2a 25 ± 0.4b 4.5 ± 0.1y 4.5 ± 0.1y 5 ± 0.1x NH4+-N harvested % 98 ±3 100 ± 4 99 ± 3 100 ± 2 100 ± 1 100 ± 4
P harvested % 99.6 99.9 99.5 99.8 99.9 99.6
16
The significance of the data is presented using the letters a, b, x, and y. The different letters (a, and b for pure urine and x and y for diluted
278
urine) in the same row are significantly different (P<0.05).
279
The ammonia loss in this experiment was <1% possible due to evaporation during sampling 280
or analyses. It was noticed that the evaporation of ammonia is higher at high pH, so that there 281
is a risk of ammonia loss during sampling and analysis (mass transfer coefficient for urine 282
exposed to ambient air for 10 seconds during sampling; i.e., a total of 40 seconds for the 283
whole experiment is 0.003 m/s, calculated assuming 99% of N as NH3 using Equation 5 284
(Arogo et al., 1999)). Therefore, ammonia exposure time in the experimental process needs 285
to be reduced to reduce the risk of ammonia loss. If there is free ammonia in the ammonium 286
sulfate solution, it can evaporate during drying. About 10% of the ammonia loss during the 287
drying process has been reported (Basakcilardan-Kabakci. et al., 2007; Pradhan et al., 2017).
288
In this study, the ratio of ammonia and sulfuric acid was closely maintained and mixed for 289
one hour before drying so all the ammonia can react with sulfate. The ammonia loss during 290
drying was negligible so the dried ammonium sulfate contained above 19% N. Supplying of 291
more ammonia gas in the ammonium sulfate solution (i.e. up to the solution pH 6-7) might 292
increase the N% in dried ammonium sulfate.
293 294
𝑑𝑑𝑑𝑑
𝑑𝑑𝑑𝑑 =𝑝𝑝𝐾𝐾0(𝐶𝐶𝐿𝐿− 𝐶𝐶𝑎𝑎𝑎𝑎𝑎𝑎) Equation 5
295
Here, M is the mass of the volatile compound (kg), t is time in seconds, K is the overall mass 296
transfer coefficient of the ammonia (m/s), AO is the interfacial surface area (m2), CL is the 297
concentration of ammonia dissolved in the liquid (g/L), and Cair is the concentration of the 298
ammonia in the air (g/L).
299 300
17
3.5. Characteristics of Produced Ammonium Sulfate and Phosphorus Sediment 301
White and light brown crystals of ammonium sulfate were recovered after drying the 302
ammonia adsorbed acid solution. The ammonium sulfate crystals contained 17–21% of N, 303
which is similar to the N in commercial ammonium sulfate (i.e., 21% NH4+-N). This result 304
is better than our previous result where the produced ammonium sulfate contained only 13%
305
of N (Pradhan et al., 2017). It was noticed that the ammonium sulfate recovered from 306
different experiments can vary as the harvesting rate also varies. The XRD result showed that 307
40-90% of the product was pure ammonium sulfate ((NH4)2SO4) compound and the rest was 308
hydrogen triammonium disulfate (H(NH4)3(SO4)2). The percentage of pure (NH4)2SO4 can 309
be increased by increasing the ammonia to the sulfuric acid ratio (Fukami et al., 1997).
310 311
Our NPharvest technique harvested 99% of P in sediment with Ca(OH)2, which is important 312
since globally the availability of phosphorus will be limited. As most of the P in urine is in 313
the form of phosphate, the harvested P is assumed to be Ca3(PO4)2 which can be used as 314
phosphorus fertilizer. The P-% (about 1%) in sediment was comparatively lower since it 315
contained a significant amount of Ca. The P recovery in sediment was higher at 8 oC 316
compared to higher temperatures (Table 1), this finding is similar to the result presented by 317
Song et al. (2002). The XRD result showed that the sediment contained 98% CaCO3 and 318
about 1% potassium chloride (KCl) since urine contains also K and Cl (Pradhan et al., 2017).
319
320
3.6. Effluent quality 321
The effluent from an experiment at high temperature had a lower pH than effluent from the 322
experiment at a low temperature, which is similar to our earlier results (Pradhan et al., 2017).
323
18
The pH in the effluent was high and should be neutralized before disposing into the 324
environment. The effluent pH can be neutralized using about 1.5 g of H2SO4/m3. Urine may 325
contain few pathogens but the pH 12 will hygienize urine or decrease significant numbers of 326
pathogens (Ogunyoku et al., 2016) including Escherichia coli and Salmonella spp 327
(Mendonca et al., 1994).
328 329
3.7. Economic and energy assessment 330
The economic assessment showed that the energy needed to produce 1 kg of N fertilizer (i.e., 331
≈ 4.75 kg of ammonium sulfate) from pure urine is 8 kWh at 8 and 20 oC, and 11 kWh at 30 332
oC. In fact, our NPharvest technique needed 0.3 kWh energy for pumping to harvest 1 kg of 333
N at 20 oC which is much less than energy needed to produce 1 kg of N by Haber-Bosch, 334
stripping and struvite precipitation process (supplementary material, Table 1). However, a 335
feasibility study for a full-scale NPharvest plant is needed to support this statement as there 336
are several other costs (salary, transportation, interest, etc.), which need to be considered for 337
a full-scale plant.
338 339
The NPharvest experiment at 20 oC would be economically feasible already at 8 hours of 340
experimental time, but the maximum profit is achieved at 16 hours of experimental time. If 341
the ambient temperature would be 20 oC, the experiment at 20 oC instead of 30 oC would 342
save about 1 €/m3 for heating 10 oC (calculated as specific heat capacity for water and 343
assuming no heat loss in the system). In fact, the NPharvest process is more efficient at 30 344
oC, and therefore this process would be cheaper in tropical countries (if the heat energy is 345
free of charge).
346
19 347
Nutrients recovery from diluted urine was more expensive than the recovering from pure 348
urine since the energy consumption to operate 1 m3 of both types of urine is similar, but the 349
recovery of N and P will be lower from diluted urine than from pure urine (Table 2). It was 350
clear that nutrient harvesting from diluted urine at 30 oC is not feasible because of the high 351
heat energy cost and low revenue generation. The process will be economically beneficial 352
after 12 hours and 16 hours of process time at 20 oC and at 8 oC, respectively (data not 353
shown).
354 355
The economic assessment showed that this technique can be a profitable business, as shown 356
in Table 3, and thus the harvest of N and P from 1 m3 of urine using this technique can make 357
a profit of about 1.5 € (investment and labor costs are not included in the calculation). The 358
revenue from P is calculated from the price of current rock phosphate, as P% in our sediment 359
is similar to the rock phosphate (i.e. about 2%) (Wiklund, 2015). The revenue from CaCO3
360
was calculated using half the price of the commercial product due to its possible impurities 361
(Table 3). However, the profit of the NPharvest technique to treat urine can be affected by 362
other factors, such as urine transportation, ambient temperature, electricity, treatment plant 363
size, etc. The most expensive part of this cost analysis is the energy needed for drying the 364
end products. Maybe solar drying could be used in some areas. Additional studies are needed 365
to produce ammonium sulfate containing 21% N or other N-based fertilizer that might have 366
a high market demand and better market price (Pradhan et al., 2018). Furthermore, the 367
additional revenue can be generated by collecting the fee to dispose of urine in a recycle 368
20
company/center for urine treatment. The fee to dispose of human waste is about 5 €/m3 in a 369
Finnish wastewater treatment plant (Pradhan et al., 2018).
370
Table 3. Economic calculation of cost and benefits of 1 m3 pure urine process at 20 oC for 371
the batch test. Price of the chemicals fluctuates due to several factors.
372
Variables Used amount Cost/price
(€/m3 treated urine)
References/sources
Chemicals
H2SO4 (98%) 18 kg 5.2 €290/ton (Rothrock et al., 2013)
Ca(OH)2 18.6 kg 1.7 €93/ton (Miller, 2012)
Pumping energy 1.2 kWh 0.08 Calculated as €0.07/kWh in Finland (www.sahkonhinta.fi) Drying energy (≈23 kg) 59 kWh
(2.5 kWh/kg of N fertilizer)
4.1 calculated as €0.07/kWh in Finland (www.sahkonhinta.fi)
H2SO4 for effluent neutralization
1.47 kg 0.4 €290/ton (Miller, 2012)
Total treatment cost 11.48
Revenue
Revenue from (NH4)2(SO4) (19% N)
23.3 kg 9.72 €461/ton of (NH4)2SO4 of 21%
N (Cemagro, 2018)
21
Revenue from PO4-P 0.35 kg 0.03 €82/ton for rock phosphate was used for the calculation (Index Mundi, 2018)
Revenue from CaCO3 24 kg 3.24 €68/ton of Ca was used for the calculation (actual market price is €135/ton of 36% Ca)
Nordkalk
Total revenue 12.99
Total profit 1.5
Note: After sedimentation and dewatering, the sediment contains a very small portion of moisture, so the drying energy was not calculated
373
for the sediment.
374
4. Conclusions 375
It is concluded that our technique (NPharvest) can produce ammonium sulfate fertilizer by 376
harvesting N from urine. Our technique also harvests P in sediment with calcium carbonate, 377
the percentage of P in urine and its sediment is low. This sediment can be used in agriculture 378
as a P-added lime or used as raw material to extract P. The economic analysis showed that 379
this technique can make a profit by harvesting N and P from urine. In broad thinking, this 380
idea will improve human life by reducing environmental contamination and increasing 381
agricultural products.
382
22 383
Acknowledgments: We would like to thank the Kone Foundation 2014 and the Maj and 384
Nessling Foundation 2015 and the Ministry of Environment, Finland (RAKI2)-2017 for their 385
financial support. We would also like to thank Mr. Taneli Tiittanen for the XRD analysis and 386
Mr. Kirmo Kivela from Dodo.org for providing urine for the experiment.
387
388
Declaration of interest: None 389
390
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