Nitrogen and Phosphorus Harvesting from Human Urine Using a

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Nitrogen and phosphorus harvesting from human urine using a stripping, absorption, and precipitation process Surendra K Pradhan, Anna Mikola, and Riku Vahala Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05402 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 14, 2017

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Environmental Science & Technology

Ammonium sulfate

Human urine

Phosphate compound

ACS Paragon Plus Environment


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Nitrogen and phosphorus harvesting from human urine using a stripping,

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absorption, and precipitation process

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*Surendra K Pradhan, Anna Mikola, Riku Vahala

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Department of Built Environment, School of Engineering, Aalto University, P.O. Box 14100

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FI-00076 AALTO, Finland

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*corresponding author: - Surendra K Pradhan, [email protected], +358400973372

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ABSTRACT

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Human urine contains significant amounts of N (nitrogen) and P (phosphorus); therefore it has

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been successfully used as fertilizer in different crops. But the use of urine as fertilizer has several

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constraints, such as, the high cost of transportation, an unpleasant smell, the risk of pathogens,

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and pharmaceutical residue. A combined and improved N stripping and P precipitation technique

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is used in this study. In this technique, Ca(OH)2 is used to increase the pH of urine which

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converts ammonium into ammonia gas and precipitate P as Ca-P compound. The ammonia gas is

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stripped and passed into the sulfuric acid where ammonium sulfate and hydrogen triammonium

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disulfate is formed. The experiment was performed using 700 ml of urine and the pH of the urine

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was increased above 12. Our results showed that 85-99% of N and 99% of P (w/w) can be

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harvested from urine in 28 h at 40 oC and in 32 h at 30 oC. The harvested N (13% N w/w) and P

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(1.5% P w/w) can be used as mineral fertilizer. The economic assessment of the technique

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showed that the extraction of N and P from 1 m3 of pure urine can make a profit of €2.25.

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Keywords: - mineral fertilizer, nitrogen, phosphorus, sanitation, urine 1 ACS Paragon Plus Environment

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1. INTRODUCTION

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The urine-separating toilet is an established concept to collect urine separately from feces for

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further use as a fertilizer. One person excretes about 550 L urine/year, which is equal to 4 kg of

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nitrogen (N) 0.4 kg of phosphorus (P) and 0.9 kg of potassium (K) per year.1 Recovery and reuse

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of these nutrients improve the sustainable environment. The recycling of P is even more

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important because today most of P fertilizers are produced by mining and which may be depleted

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in 50 – 100 years.2 Therefore, urine as a fertilizer is receiving increasing research attention and

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has already been successfully used in agriculture.3,4 Although urine can be collected separately in

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big buildings or public places like airports and universities, the use of urine itself as a fertilizer is

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not feasible on a large scale. There are several constraints that make urine unattractive. For

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example: (1) it is expensive to transport large volumes of urine to farms,5 (2) urine has an

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unpleasant smell,6 (3) urine is not acceptable in many societies7 and (4) urine can contain

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pharmaceutical residues and pathogens8. This study aimed to address these issues by harvesting

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nutrients from urine.

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One of the common techniques of nutrient recovery from urine is the formation of struvite

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(MgNH4PO4·6H2O) using different Mg2+ sources including wood ash,9 MgO and NaOH,10

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MgO11 and brine12. Struvite precipitation is principally based on the chemical equilibrium of

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constituent ions in the solution and it needs the correct ratios of Mg2+:NH4+:PO43- (close to 1:1:1

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and up to 2.5:1:1).11 Ostara in Virginia (USA) and Multiform Harvest in Yakima, Washington

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(USA) are successfully producing struvite at full-scale operation with 80-90% P recovery.13 In

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general, the struvite precipitation process recovers mainly P with about 10% of N and it is a P2 ACS Paragon Plus Environment


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based slow nutrient releasing compound.14 In the struvite formation technique N recovery can be

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enhanced by adding more Mg2+ and PO43− to achieve the appropriate ratio of Mg, P and N.15

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Another technique of nutrient recovery from urine is ammonia stripping.10,16-18 Residual

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ammonium from urine was stripped and recovered in H2SO4 after the recovery of struvite.10,16

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Furthermore, N was stripped and recovered from diluted urine17 and manually hydrolyzed (using

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urease) fresh urine18. Both of these studies17,18 recovered only N. Some other techniques to

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recover nutrients from liquid waste are: nutrient recovery using zeolite,19 recovery of

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concentrated nutrients using RO (reverse osmosis)20.

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Although there are several existing techniques to recover N and P from urine, new approaches

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are needed to make the nutrients recovery process more economical and feasible. We realized

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that the recovery of N and P separately can be better, so an N or P fertilizer can be used

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separately. The earlier stripping study was conducted either combined with the struvite process

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or they only recovering N using NaOH as alkali. Here, we are developing a new approach by

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using Ca(OH)2 instead of NaOH in the stripping process as Ca(OH)2 increase the urine pH as

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well as precipitates the P. In this study, we have actually improved the stripping technique by

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combining N stripping with the P precipitation technique. The process of our technique is based

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on increasing the pH of urine and the precipitation of P (Equation 1) using Ca(OH)221 followed

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by the stripping and capturing of the resulting ammonia as ammonium sulfate and hydrogen

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triammonium disulfate using H2SO4 (Equation 2). Equations 1 and 2 describe the main reactions,

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but additional products can also form in these reactions.22 Our approach is the first technique

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where the extraction of N and P from human urine takes place simultaneously, i.e., the stripping

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of ammonia and P sedimentation in the same reactor and production of the ammonium

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compound in another reactor (reactor containing H2SO4) but all at the same time. Dry and wet

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urinals are used for collecting urine; therefore the influence of the urine concentration was

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studied.

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10Ca2+ + 6PO43- + 2OH- → Ca10(PO4)6(OH)2 ↓

Equation 1

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2NH3 + H2SO4 → (NH4)2SO4

Equation 2

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The main aim of this study is to recover nutrients from human urine and produce a mineral

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fertilizer. The objectives of the study are; (1) to determine the effect of temperature and the

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concentration of urine on the performance of the N and P harvesting process, (2) to determine the

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quality and quantity of the produced NH4 compound, and (3) to determine the quality and

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quantity of the produced Calcium + P compound fertilizer. According to our study hypothesis;

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(1) N and P can be harvested from urine using our technique and (2) the temperature and pH will

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be positively correlated with the N harvesting process.

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2. MATERIALS AND METHODS

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2.1. Urine handling and analytical methods. Urine was collected from a waterless urinal

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placed at the Helsinki Festival in the summer of 2014 and stored for about a year by Dodo (an

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NGO). About 90 L of urine in 30 L jerry cans were transported to the water lab at the

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Department of Built Environment, Aalto University, and kept in a cold room (4 oC) until used for

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the experiment. A physiochemical analysis was done before and after N and P harvesting (Table

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1). Total-N was analyzed using the Finnish standard corresponding to the ISO standard (SFS-EN

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ISO 11905-1), the automatic analyzer was equipped with an autoclave and an Ultraviolet

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Spectrophotometric Screening Method Ganimede N (Lange), Germany. NH4-N (Ammonium-N)

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was analyzed using the ISO 11732 method. The analysis was done using a Tecator 5042

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Detector/5012 Analyzer from Foss Höganäs, Sweden. NH4-N was also determined using the NH3 4 ACS Paragon Plus Environment


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gas sensing electrode Orion 900/200 (Thermo Electron Corporation, Beverly, MA, USA) after

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adjusting the sample to pH > 11. Total P was analyzed by FIA (flow injection analyzer), using a

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FiaStar 5000 analyzer from Foss, Höganäs, Sweden, following the standard method SFS-EN ISO

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6878. PO4-P was determined using the Finnish standard SFS-EN ISO 15681-1 by the FIA

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method using a FiaStar 5000 analyzer from Foss, Höganäs, Sweden. For sediment, total P and K

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was analyzed using FAAS (flame atomic absorption spectroscopy). Suspended solid (SS) was

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determined using a Whatman membrane (pore size 0.4µm) with a drying method at 105 oC for 2

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hours (SFS-EN 872, year 2005). The harvested ammonium sulfate and phosphorus compounds

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(sediment) were determined or confirmed by XRD (X-ray diffraction) using an X-ray powder

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diffractometer (PANalytical X'Pert Pro MPD α1). Quantification was based on semi-quantitative

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results performed using the “Match!” (crystal impact) program using the ICDD-PDF-4 +2014

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RDB database. XRD and FAAS analysis was done in the Department of Chemistry, and the rest

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of the analyses were done in the water laboratory at the Department of Built Environment, Aalto

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University.

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2.2. Process optimization. Increase the air flow rate17,18 and temperature18 increase the NH3

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stripping, but these parameters need to be optimized according to the experimental setup. In our

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experiment, the airflow rate was optimized at 1.1 ± 0.1 L/min because higher than 1.1 ± 0.1 L

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airflow caused a foaming issue. The amount of Ca(OH)2 was determined based on the potential

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to increase the pH (here, pH was determined by measurement) of urine, and the amount of 1M

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H2SO4 was determined by calculating the amount of ammonium-N and its molecular weight in

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urine. The dilution factor of urine was selected based on the commonly used urine separating

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flush toilet which uses about 1:4 portion of water. In order to study the effect of temperature, our 5 ACS Paragon Plus Environment

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technique was first tested at room temperature (i.e., 21 ± 1 oC), but the NH3 stripping process

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was slower (i.e. 0.05) when the pH of the pure urine was

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maintained than when the pH was not maintained (Table 2). When the pH was maintained above

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11, NH4-N harvesting was more than 99%, which is similar to the results presented by

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Basakçilardan-Kabakci et al.17 In contrast, Liu et al.18 showed that ammonia stripping was not

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increased, while the urine pH increased above 10. The temperature did not greatly affect the N

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harvesting percentage when the pH was maintained above 11 (Table 2) but the urine pH and

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ammonia reduction were positively correlated (p = 0.0001, r = 0.59). This result showed that if

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the urine pH is kept above 11, the N harvesting percentage will be similar at 30 oC and at 40 oC.

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This means that a 10 oC increase in temperature is not required after increasing the urine pH

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which results in a significant amount of saved energy. The results in efficiency of our technique

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at 30 oC will be very useful and energy-efficient in tropical countries, where the ambient

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temperature is about 30 oC all year around. The use of our technique at 30 oC seems reliable as

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Quan et al.31 and Siegrist et al.25 also reported that NH3 stripping needs to be performed at > 25

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o

C. On the other hand, the harvesting process was 8 hours faster at 40 oC when the pH was



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maintained (Figure 2). Furthermore, Ca(OH)2 is a commonly available chemical and this is re-

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useable as lime + phosphorus in agricultural soil even after used in our technique compared to

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NaOH used in previous studies.17,18 However, it is clear that the temperature, pH, and

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experimental duration need to be optimized to make this technique as economic as possible. A

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similar conclusion is presented by Liu et al.18

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3.3. Ammonium concentration and nutrient harvesting process. Our technique harvested 95-

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99% of NH4-N from pure and diluted urine at 40 oC when the pH was maintained >11 for the

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entire harvesting period (Figure 2 (D,F)). The NH4-N harvesting rate from the high NH4-N

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content urine or pure urine was slightly higher compared to the low NH4-N content urine or

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diluted urine. Pearson’s correlation also showed a weak but positive correlation between NH4-N

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harvesting and NH4-N concentration in urine (p = 0.047, r = 0.583). Liu et al.18 also reported that

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dilution of urine does not have a significant effect on NH4-N harvesting but it has some

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influence. These results suggested that it would be more economical (saving energy and time) to

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apply this technique to harvest N and P from pure urine than from diluted urine. However, the

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cost of chemicals will not be very different as the consumption of H2SO4 and Ca(OH)2 mainly

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depends on the ammonium concentration in the liquid waste. For example; in an estimated

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calculation assuming a linear scale up of our work, only 43 L of 1M H2SO4 can be used to

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harvest NH4-N (i.e. about 0.85 g of NH4-N) from 1m3 of diluted urine (1:4 times dilution) while

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214 L of 1M H2SO4 is needed to harvest NH4-N (i.e., about 4.4 g of NH4-N) from 1m3 of pure

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urine. Similarly, a smaller amount of Ca(OH)2 was used to increase the pH of diluted urine

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compared to the amount used for pure urine. Variations in Ca(OH)2 consumption and phosphorus

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removal efficiency are mainly dependent upon the presence of NH4+ and CO32- in the solution.32


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Although the stored urine contained 4.5 g/L of NH4-N (Table 1), some of the ammonia

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volatilized during the preparation of the experiment or before the stripping process. We noticed

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that there is a risk of ammonia evaporation while urine is mixed with Ca(OH)2 and the pH is

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increased to >12. There was an NH4-N loss of 0.5-6% during the N and P harvesting process

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(Table 2). This might be due to evaporation during sampling, air flow and pH adjustment. N loss

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was comparatively lower in the pH-maintained pure urine compared to the others (Table 2). The

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experiment with pH-maintained pure urine had lower residual NH4-N, so there was less ammonia

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to evaporate compared to urine from other treatments. However, the NH3 loss was not due to

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nitrification, as a high pH inhibits the nitrification process33 and a high temperature of 40 oC is

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not a favorable condition for nitrification. Furthermore, there was about 10% of NH4-N loss

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during the drying/crystallization of the acid solution which is similar to the results presented by

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Basakçilardan-Kabakci et al.17 This might be because some ammonium ions remain in the acid

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solution and they evaporate during the drying/crystallization process. In fact, if (NH4)2SO4 is

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already formed, N will not evaporate in this temperature.34 The loss of ammonia might have

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influenced the result of the total N content being only 13%. The ammonia loss during the

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experiment can be controlled in a pilot plant where the possible evaporation points can be

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mitigated.

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3.4. Characteristics of the products. The ammonium sulfate production was similar in both

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temperatures when urine pH was maintained above 11 whereas Liu et al.18 found that recovery

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increases when the temperature increases. This might be because our experimental duration was

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28 hours and pH was >11 while Liu et al.18 conducted an experiment for 24 hours and pH was

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99% P,

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as reported by Fernandes et al.21 As pH is the main factor for P precipitation, the experimental

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temperature did not affect the P harvesting rate. But the concentration of P was positively

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correlated to the P harvesting percentage (p < 0.0001, r = 0.912). Most of the P in urine was in

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the form of PO4-P (Table 1); similarly, most of the P in the sediment was also PO4-P. This

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sediment containing about 1.5 % P containing sediment can be used as fertilizer + lime in forest

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soil37 and agricultural soil37 especially with low pH soil to increase productivity. Although we

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have not done any specific treatment to remove pharmaceutical residues and heavy metals from

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our sediment, the previous study showed a low risk of using sediment from urine. For example,

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Ronteltap et al39 showed that the heavy metals contained in urine sediment are lower than even

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the European standard, and only 9 reduces a significant amount of pathogens40 and



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inactivates Ascaris eggs.

E coli and Salmonella spp can be reduced 8 logs at pH 12 for 15

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seconds of contact time.42

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3.6. Economic assessment. In general, 1 m3 of urine contains 4.5 kg of NH4-N and 0.35 kg of

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total P.1 Among them, 4.2 kg of NH4-N and 0.35 kg of total P can be harvested using 20.9 kg of

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H2SO4 (98%) and 22 kg of Ca(OH)2. Based on the calculation the harvesting of N and P from 1

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m3 of urine can make a profit of €2.25 (Table 3). The economic assessment was based on a

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theoretical calculation of the upscale 1 m3 controlled pilot plant assuming a linear increase in the

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chemical dose from a 1 L reactor. However, additional studies are needed to convert the

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hydrogen triammonium disulfate mixture into ammonium sulfate and to produce pure CaCO3

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and phosphate precipitates. The revenue gain assumes that pure compounds were produced

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without the additional cost.

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ACKNOWLEDGEMENTS

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We would like to thank the Kone Foundation for funding the study upon the decision of 2014.

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We would also like to thank Mr. Taneli Tiittanen for the XRD analysis and Mr. Kirmo Kivela

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from Dodo.org for providing urine for the experiment.

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Perry, R.H.; Green, D.W. Perry's chemical engineers' handbook. McGraw-Hill: New

Quan, X.; Wang, F.; Zhao, Q.; Zhao, T.; Xiang, J. Air stripping of ammonia in a wateraerocyclone

reactor.

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2009,

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983-988;

Szogi, A.A.; Vanotti, M.B. Removal of Phosphorus from Livestock Effluents. J. Environ.

Skadsen, J. Effectiveness of high pH in controlling nitrification. J. American Water


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inherent Ca2+ in phosphorus removal from wastewater system. Water Sci. Technol. 2016, 73 (7),

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Page 22 of 27

Page 23 of 27

450


Table 1. Physico-chemical properties of urine, effluent and sediment. Parameters

Before N and P

Effluent after N and P harvesting

harvesting pH not maintained pure

pH maintained pure urine

pH maintained diluted urine

urine At 30 oC

At 40 oC

At 30 oC

At 40 oC

At 30 oC

At 40 oC

pH

9.3 ± 0.1

8.8 ± 0.2

8.5 ± 0.2

11.9 ± 0.3

11.8 ± 0.2

10.5 ± 0.8*

10.7 ± 3*

Conductivity

27.3 ± 4.5

13.5

13.5 ± 1

16.1 ± 4

15.5 ± 3.4

2.9 ± 0.4

3.2 ± 0.5

NH4-N (g/L)

4.5± 0.2 (0.9)**

0.6 ± 0.2

0.8 ± 0.4

0.3 ± 0.01

0.1 ± 0.02

0.02± 0.001

0.01± 0.001

Total-N (g/L)

5.0 ± 0.2 (1)**

0.6±0.06a

0.4±0.04b

0.2±0.07c

0.2±0.07c

0.06±0.01d

0.03±0.01d

Total-P (mg/L)

328±1.1(66)**

1.5 ± 6

1.03 ± 0.6

1.8 ± 0.8

1.2 ± 0.1

1.6 ± 0.6

0.9 ± 0.3

PO4-P (mg/L)

309± 10 (62)**

0.1

0.1

0.1

0.1

0.02

0.02

Total-K (g/L)

1.7 (0.2)**

1.33 ± 0.04

1.38 ± 0.1

1.38 ± 0.04

1.37 ± 0.05

0.29 ± 0.01

0.3 ± 0.01

SS (mg/L)

180 (36)**

44 ± 34

30 ± 24

5±2

1±3

2±1

4±1

Cl (g/L)

4.5

NA

NA

NA

NA

NA

NA

Total-N (%)

-

0.23 ± 0.03

0.1

0.34 ± 0.01

0.22 ± 0.01

0.15 ± 0.04

0.2 ± 0.02

Total-P (%)

-

1.6±0.05a

1.6±0.04a

1.6±0.03a

1.4±0.07a

1±0.05b

0.9±0.05b

PO4-P (%)

-

1.6±0.12

1.6±0.1

1.3±0.06

1.4±0.1

0.60.5

0.9±0.04

Total-K (%)

-

0.4 ± 0.07

0.42±0.15

0.42± 0.07

0.42±0.13

0.06 ± 0.01

0.09 ± 0.01

(mS/cm)

In sediment

451

Mean ± Stdev (N = 6). *Although the pH of the urine was maintained at >11 it was a little lower

452

at the end of the experiment. ** The value in the parenthesis is for diluted urine. NA= not

453

analyzed. Different letters in the same row mean significantly different results (P > 0.05) but the

454

analysis did not include the row “Before N and P harvesting”. The result presented as a

455

percentage was calculated based on weight (W/W).

456



Page 24 of 27

457

Table 2. N and P harvesting efficiency, production of (NH4)2SO4 compound, and P sediment in

458

different experiments. Experimental conditions

NH4-N

NH4-N

P

NH4-N

(NH4)2SO4

Sediment

removed %

harvested

harvested

loss %

compound (DW

(DW

%

%

kg/m3 urine)

urine)

kg/m3

At 30 oC (pure urine)

87 ± 1a

85 ± 2a

99

2 ± 1a

24 ± 0.5a

26.9 ± 0.4a

At 40 oC (pure urine)

92 ± 4 ab

86 ± 4a

99

6 ± 3b

25.5 ± 1.1ab

25.6 ± 1.1a

At 30 oC (pH maintained

94 ± 2b

96 ± 3bc

99

0.5 ± 1c

25.8 ± 0.8ab

32.4 ± 0.3b

98 ± 2cd

99 ± 1c

99

1 ± 1c

25.6 ± 0.4b

29.5 ± 0.9c

96 ± 1c

92 ± 5ab

99

5 ± 5abc

4.5 ± 0.4c

9.1 ± 0.1d

99 ± 1d

95 ± 4bc

99

4 ± 3abc

5 ± 0.2c

8.9 ± 0.2d

pure urine) At 40 oC (pH maintained pure urine) At 30 oC (pH maintained diluted urine) At 40 oC (pH maintained diluted urine)

459

Mean ± SD, (N = 6). Different letters in the same column mean significantly different results (P

460

> 0.05), DW = dry weight. The % result was calculated based on W/W.

461 462 463 464 465 466 467 468 469


Page 25 of 27

470


Table 3. Theoretical economic assessment of N and P harvesting using our technique. Treatment

Used amount

Cost/price (€)

References

H2SO4

20.9 kg

6

(€290/ton)43

Ca(OH)2

22 kg

2

(€93/ton)44

Energy for aeration

0.025 kWh/nm3

0.10

(calculated

as

€0.072/kWh

in

as

€0.072/kWh

in

Finland) Energy for drying the acid

1

(calculated

and sediment H2SO4

for

Finland) effluent

1.47 kg

(€290/ton) 43

0.4

neutralization Total treatment cost

9.50

Revenue Revenue from (NH4)2(SO4)

25.2 kg of

(€461/ton of (NH4)2SO4 of 21% N)

7.3

+ H(NH4)3(SO4)2 mixture

(Cemagro Finland)

(13% N) Revenue from CaCO3 + P

4.45 (4.05+0.4)

(€1363/ton of phosphate, USDA 2013),

(€135/ton

of

36%

Ca,

Nordkalk) Total revenue

11.75

Total profit

2.25

471

Note:- The price of produced (NH4)2(SO4) + H(NH4)3(SO4)2 mixture is calculated as the price of the N

472

content in it and the price of Ca(OH)2 is calculated as the price of calcium. Investment and labor

473

costs are not included in the calculation.

474



475

45.

476

Experimental design.

477 478

Figure 1. Experimental setup. (Volume of the urine bottle was 1L, volume of the acid bottle

479

1 was 250 mL and the volume of acid bottle 2 was 100 mL).

480

481


Page 26 of 27

Page 27 of 27


482 483

484 485

Figure 2. The relation between NH4-N removal from urine, NH4-N harvested in sulfuric acid,

486

and the pH of urine. (A,B) pH not maintained pure urine, (C,D) pH maintained pure urine, (E,F)

487

pH maintained diluted urine. Standard deviation is shown as an error bar. The nutrient harvesting

488

experiment at 30 oC was conducted for 32 h (A,C,E) and at 40 oC for 28 h (B,D,F). The NH4-N

489

results are shown on the primary axis and the pH result is shown on the secondary axis.

490


Nitrogen and Phosphorus Harvesting from Human Urine Using a

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