Source-separated urine contains most of the excreted nutrients, which can be recovered by using nitrification to stabilize the urine before concentrating the nutrient solution with distillation. The aim of this study was to test this process combination at pilot scale. The nitrification process was efficient in a moving bed biofilm reactor with maximal rates of 930 mg N L−1 d−1. Rates decreased to 120 mg N L−1 d−1 after switching to more concentrated urine. At high nitrification rates (640 mg N L−1 d−1) and low total ammonia concentrations (1,790 mg NH4-N L−1 in influent) distillation caused the main primary energy demand of 71 W cap−1 (nitrification: 13 W cap−1) assuming a nitrogen production of 8.8 g N cap−1 d−1. Possible process failures include the accumulation of the nitrification intermediate nitrite and the selection of acid-tolerant ammonia-oxidizing bacteria. Especially during reactor start-up, the process must therefore be carefully supervised. The concentrate produced by the nitrification/distillation process is low in heavy metals, but high in nutrients, suggesting a good suitability as an integral fertilizer.

Separating urine at the source is an effective approach to recover nutrients from wastewater, given that urine contains most nutrients, which humans excrete (Larsen & Gujer 1996). Researchers have targeted specific nutrients (e.g. nitrogen, phosphorus) and developed technologies to reclaim them from the liquid (Larsen et al. 2013). Alternatively, our research group opted for another procedure: water is separated from urine, leaving behind a concentrate, which contains all nutrients. After extensive laboratory work (Udert et al. 2003; Udert & Wächter 2012), a pilot plant was started up at Eawag's main building, Forum Chriesbach. The plant operates in two stages: first, half of the total ammonia ( and NH3) in urine is biologically converted into nitrate , which is then concentrated with a distiller.

The aim of the nitrification step is to prevent ammonia (NH3) losses and malodor. During nitrification bacteria convert half of the total ammonia into non-volatile nitrate and, as the pH drops from pH 9 to values around 6, the other half is stabilized as non-volatile ammonium . About 90% of the organic substances are mineralized including compounds which are responsible for the malodor (Troccaz et al. 2013). The resulting stabilized solution can be concentrated without any substantial nitrogen losses in order to simplify transportation, storage or direct application as fertilizer.

Four main factors determine the suitability of the pilot-scale reactor for nutrient recovery: first, nitrification rates determining the reactor size; second, the energy demand; third, stability of the biological processes; and fourth, the quality of the concentrate produced.

High urine nitrification rates of 380 mg N L−1 d−1 were observed in a laboratory-scale moving bed biofilm reactor (MBBR) at temperatures of 25 ± 0.3 °C and over a period of 80 days (Udert et al. 2003). However, stronger changes in temperature and urine composition have to be expected for the pilot-scale reactor, which may influence long-term performance.

The primary energy demand for the overall process (nitrification and distillation) has been calculated as 30 W cap−1 based on laboratory results and literature data (Udert & Wächter 2012). This is two and a half times higher than for the treatment of municipal wastewater in a conventional wastewater treatment (Maurer et al. 2003). The energy consumption was estimated for the urine composition given in Udert et al. (2006). The concentrations from urine collected in urine-diverting toilets may, however, not reach equally high values (Goosse et al. 2009), which will cause a higher energy demand for water removal by distillation.

The stability of the biological process is particularly challenging, as it requires the well-tuned interplay of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). A simple pH control was proposed to achieve high process stability in a urine nitrification reactor (Udert & Wächter 2012). The pH value is kept in a narrow range by intermittent dosage of stored urine based on the following principle: when no urine is pumped, the pH decreases due to nitrification; during urine dosage, pH increases due to the high pH value and alkalinity of stored urine. Udert & Wächter (2012) tested this strategy for the steady-state operation of a laboratory reactor, but not for reactor start-up, which is the most critical phase in reactor operation.

The nitrification process converts 50% of the total ammonia to nitrate, and heterotrophic bacteria degrade 90% of the organic substances (measured as chemical oxygen demand) (Udert & Wächter 2012). Most other compounds though are not degraded or removed during nitrification and become concentrated during distillation. If the concentrate is supposed to be used as a fertilizer, its composition needs to satisfy legal standards with respect to various compounds (e.g. heavy metals; Council Regulation (EEC) 1991).

This paper summarizes our previous experience with the nitrification/distillation pilot plant at Eawag. The results provide important information for further research and pilot studies. We address the nitrification rates, energy demand and the quality of the concentrate as fertilizer based on the results from the pilot-scale installations. The factors influencing process stability are discussed based on laboratory nitrification experiments.

Urine collection and composition

Urine was collected from urine-diverting flush toilets (NoMix toilets, Roediger Vacuum, Hanau, Germany) and waterless urinals through a separate piping system at Eawag's main building (Dübendorf, Switzerland). In the building, an average of 110 L of urine is collected daily (only working days). In two 1,000 L tanks, women's urine and men's urine are stored separately before they are treated in the nitrification/distillation process. Urine from the women's storage tank is less concentrated by a factor of approximately two with respect to all compounds compared with men's urine (Table 1). The main reason for the lower concentration of women's urine, is the collection with NoMix toilets only: leaky valves allow some flushing water to enter the collection tank.

Table 1

Composition of stored urine as well as composition of concentrate (produced from women's urine) compared to threshold values for organic farming from European legislation (Council Regulation (EEC) 1991)

  Women's urineMen's urineConcentrateCouncil Regulation (EEC)
  Average ± Std. Dev.1Average ± Std. Dev.1Average ± Std. Dev.2Threshold value
pH [-] 8.9 ± 0.1 9.0 ± 0.1 4.1  
Total ammonia-N [mg L−11,990 ± 420 4,140 ± 870 23,100 ± 4,700  
Nitrate-N [mg L−1<10 <10 24,400 ± 2,200  
Dissolved COD [mg L−12,010 ± 540 3,860 ± 870 4,650 ± 1,030  
Total inorganic C [mg L−11,020 ± 250 2,080 ± 260 <5  
Total phosphate-P [mg L−1106 ± 17 242 ± 23 2,130 ± 180  
Potassium [mg L−1854 ± 143 1,470 ± 130 17,400 ± 2,800  
Sodium [mg L−1881 ± 239 1,760 ± 90 19,400 ± 2,700  
Sulfate [mg L−1308 ± 87 708 ± 109 8,620 ± 810  
Chloride [mg L−11,630 ± 400 2,980 ± 440 35,300 ± 2,900  
Calcium [mg L−113.5 ± 11.0 n/a 428 ± 37  
Magnesium [mg L−1<4 n/a <4  
Iron [mg L−1n/a n/a 0.6 ± 0.1  
Manganese [mg L−1n/a n/a 0.4 ± 0.5  
Boron [mg L−1n/a n/a 17.2 ± 0.8  
Cobalt [mg L−1n/a n/a 0.1 ± 0.1  
Copper [mg L−1n/a n/a 0.4 ± 0.3 70 
Chromium [mg L−1n/a n/a 0.2 ± 0.1 70 
Zinc [mg L−1n/a n/a 14.2 ± 0.9 200 
Cadmium [mg L−1n/a n/a <0.05 0.7 
Nickel [mg L−1n/a n/a <0.1 25 
Lead [mg L−1n/a n/a 0.27 45 
  Women's urineMen's urineConcentrateCouncil Regulation (EEC)
  Average ± Std. Dev.1Average ± Std. Dev.1Average ± Std. Dev.2Threshold value
pH [-] 8.9 ± 0.1 9.0 ± 0.1 4.1  
Total ammonia-N [mg L−11,990 ± 420 4,140 ± 870 23,100 ± 4,700  
Nitrate-N [mg L−1<10 <10 24,400 ± 2,200  
Dissolved COD [mg L−12,010 ± 540 3,860 ± 870 4,650 ± 1,030  
Total inorganic C [mg L−11,020 ± 250 2,080 ± 260 <5  
Total phosphate-P [mg L−1106 ± 17 242 ± 23 2,130 ± 180  
Potassium [mg L−1854 ± 143 1,470 ± 130 17,400 ± 2,800  
Sodium [mg L−1881 ± 239 1,760 ± 90 19,400 ± 2,700  
Sulfate [mg L−1308 ± 87 708 ± 109 8,620 ± 810  
Chloride [mg L−11,630 ± 400 2,980 ± 440 35,300 ± 2,900  
Calcium [mg L−113.5 ± 11.0 n/a 428 ± 37  
Magnesium [mg L−1<4 n/a <4  
Iron [mg L−1n/a n/a 0.6 ± 0.1  
Manganese [mg L−1n/a n/a 0.4 ± 0.5  
Boron [mg L−1n/a n/a 17.2 ± 0.8  
Cobalt [mg L−1n/a n/a 0.1 ± 0.1  
Copper [mg L−1n/a n/a 0.4 ± 0.3 70 
Chromium [mg L−1n/a n/a 0.2 ± 0.1 70 
Zinc [mg L−1n/a n/a 14.2 ± 0.9 200 
Cadmium [mg L−1n/a n/a <0.05 0.7 
Nickel [mg L−1n/a n/a <0.1 25 
Lead [mg L−1n/a n/a 0.27 45 

1Sample number >8.

2Three samples.

Pilot-scale nitrification reactor and vacuum distiller

The urine treatment facilities are located in the basement of the building (Figure 1). The nitrification reactor consists of two columns with a diameter of 32 cm and a liquid volume of 120 L. The reactors are continuously fed with stored urine and contain high-density polyethylene biomass carriers (Kaldnes® K1) with a bulk volume of 60% of the total reactor volume and a specific surface of 500 m2 m−3 (Rusten et al. 2006). The reactor's aeration acts as the driving force for the mixing of the carriers and liquid. To achieve complete mixing, the aeration rate had to be set to a value of 2 m3 h−1 (per column). Due to the strong aeration, the dissolved oxygen concentration in the reactor is constantly above 7 mg L−1. The urine is pumped from the storage tanks into the reactor with a membrane dosing pump (Delta 730, Prominent, Heidelberg, Germany); the flow rate can be adjusted gradually from 0 to 30 L h−1. Initially only one reactor column was in use.
Figure 1

The nitrification reactor (left column) and distiller (right) at Eawag (Photograph: B. Etter).

Figure 1

The nitrification reactor (left column) and distiller (right) at Eawag (Photograph: B. Etter).

Close modal

In order to reduce water loss during aeration of the biological reactor, the air from Eawag's pressurized air supply is humidified in a column containing distilled water prior to injection into the reactor. In the nitrification reactor, the instrumentation comprises pH, temperature, and dissolved oxygen probes (Tophit CPS491D, Oxymax COS61D, Liquiline CM448, Endress & Hauser AG, Reinach, Switzerland). Between the nitrification reactor and the distiller, an intermediate storage tank is installed, which holds 600 L nitrified urine corresponding to the volume of one distillation batch. The average concentration factor during distillation is between 20 and 25.

The distiller is a commercially available industrial vapor compression vacuum distiller (KMU-Loft Cleanwater, Hausen, Germany). It is set to operate at approximately 500 mbar working pressure. The vapor compression occurring in the vacuum pump is the sole source of heating, as the heated vapor is recycled into a heat exchanger located in the distiller's sump. The distiller is operated in semi-batch mode, i.e. the liquid volume in the sump is kept constant at 20 L by compensating for evaporated liquid with new influent. Thus, over a distillation batch, the concentration in the sump continuously augments up to a given level. Once a set volume of liquid has been distilled, the process is halted and the remaining concentrated liquid is drained from the distiller. In the case of our set-up, the distilled water is recycled into the toilet flush system. In the distiller, temperature, pressure, electric conductivity (as a proxy for concentration), and electricity consumption are measured and recorded. In both, the urine supply tanks and the intermediate storage tank holding the nitrified urine, differential pressure sensors (Vegaflex 61, VEGA Grieshaber KG, Schiltach, Germany) record the liquid level.

Start-up experiments in laboratory reactors

Laboratory experiments were conducted to test the start-up procedure. Each of the two reactors had a volume of 7 L. The pH and oxygen concentrations were measured continuously. Data were stored in a data logger (Memograph M, RSG40, Endress & Hauser, Reinach, Switzerland), which was also used to control the influent pumps (SCi-Q 400, Watson Marlow, Falmouth, UK). The airflow was maintained at 1 L min−1 using a flow controller (EL-FLOW, Bronkhorst, Reinach, Switzerland), resulting in dissolved oxygen concentrations above 7 mg L−1. The reactors were stirred magnetically at 500 rpm. The temperature was maintained at 25 °C with a thermostat (F32, Julabo Labortechnik GmbH, Seelbach, Germany).

For start-up, the reactors were filled with 0.7 L of activated sludge from the nitrification tank of Eawag's municipal wastewater treatment plant (WWTP), 140 mL of men's urine, and tap water. The reactors were operated with suspended biomass but no biofilm carriers. Men's urine was added automatically according to the pH control mechanism described above (Udert & Wächter 2012). The two reactors were operated in two different pH intervals: reactor 1 (R1) between 5.80 and 5.85 and reactor 2 (R2) between 6.2 and 6.25. As the low pH setpoints were initially not reached, four and two times 50 mL of urine were added manually within the first 15 days to R1 and R2, respectively.

Analytical methods

Chloride, sulfate, phosphate, nitrate, potassium and sodium were analyzed with ion chromatography (Metrohm, Herisau, Switzerland). Magnesium, calcium, iron, copper, zinc, manganese, cobalt, copper, chromium, cadmium, nickel, lead and boron were determined with inductively coupled plasma optical emission spectrometry (Ciros, Spectro Analytical Instruments, Kleve, Germany). The total ammonia, total nitrite (nitrite and nitrous acid) concentration as well as the chemical oxygen demand (COD) were measured photometrically with cuvette tests (LCK 303, LCK 342, LCK 614 Hach-Lange, Berlin, Germany). Total inorganic and organic carbon (TIC and TOC) were measured with a TIC/TOC analyzer (IL550 OmniTOC, Hach-Lange, Berlin, Germany).

Biomass analysis

To characterize the distribution between suspended and attached nitrifying bacteria, biomass samples of the suspended biomass and of the carrier material were taken from the pilot-scale reactor. Samples were taken on 1 day in month 15 and on a second day, in month 29. The nitrification rates were substantially different in both cases (see Table 2). Furthermore, biomass samples were removed from each of the two laboratory reactors after 142 days of reactor operation. The biomass samples were stored at −20 °C prior to analysis. DNA was extracted using the FastDNA SPIN Kit for Soil (MP Biomedicals, Santa Ana, CA, USA) and analyzed by Research and Testing Laboratory (Lubbock, TX, USA) with bacterial tag-encoded FLX amplicon pyrosequencing using the primer pair 341F (5′-CCTACGGGNGGCWGCAG-3′)/785R (5′-GACTACHVGGGTATCTAATCC-3′) targeting the bacterial 16S rRNA gene pool (Herlemann et al. 2011).

Table 2

Performance data of the pilot-scale nitrification reactor at different time points

TimeNitrification rateInfluent ammoniaDischargeCOD to N ratiopHTemperatureParticulate COD
[month][mg N L−1 d−1][g m−2 d−1][mg N L−1][L d−1][-][-][°C][mg L−1]
12–16 310 ± 50 1.0 ± 0.2 1,800 ± 140 42 ± 5 1.3 ± 0.6 5.9 ± 0.2 23.7 ± 0.9 2,260 ± 470 
29–30 640 ± 160 2.1 ± 0.5 1,790 ± 50 84 ± 17 1.2 ± 0.1 5.8 ± 0.1 26.3 ± 1.0 3,600 
36–40 120 ± 50 0.4 ± 0.1 4,100 ± 450 7 ± 3 1.0 ± 0.2 6.0 ± 0.1 22.5 ± 0.6 220 ± 150 
TimeNitrification rateInfluent ammoniaDischargeCOD to N ratiopHTemperatureParticulate COD
[month][mg N L−1 d−1][g m−2 d−1][mg N L−1][L d−1][-][-][°C][mg L−1]
12–16 310 ± 50 1.0 ± 0.2 1,800 ± 140 42 ± 5 1.3 ± 0.6 5.9 ± 0.2 23.7 ± 0.9 2,260 ± 470 
29–30 640 ± 160 2.1 ± 0.5 1,790 ± 50 84 ± 17 1.2 ± 0.1 5.8 ± 0.1 26.3 ± 1.0 3,600 
36–40 120 ± 50 0.4 ± 0.1 4,100 ± 450 7 ± 3 1.0 ± 0.2 6.0 ± 0.1 22.5 ± 0.6 220 ± 150 

During the first two operational phases (month 12 to 16, and 29 to 30) the reactor was fed with women's urine, and in the third phase (month 36 to 40) with men's urine.

Nitrification rates in the pilot-scale reactor

The nitrification reactor was fed for 3 years with women's urine, after which men's urine was dosed for another half a year of reactor operation. Nitrification rates in women's urine reached 310 mg N L−1 d−1 during the months 12 to 16 (Table 2). The COD of the particulate organic matter, a measure for suspended biomass, was 2,260 mg COD L−1. The biomass fraction of the nitrifiers in the suspended biomass was very low, while AOB and NOB were predominant in the biofilm attached to the carriers (Figure 2).
Figure 2

Relative abundance of Nitrosomonas sp. and sequences from the family of Bradyrhizobiaceae in suspension and attached to the carrier during reactor operation with women's urine. The Bradyrhizobiaceae sequences showed 100% identity to Nitrobacter, but could not be uniquely attributed to this genus. The relative abundance was determined for 1 day in month 15 and 29, respectively, by 16S rRNA amplicon sequencing.

Figure 2

Relative abundance of Nitrosomonas sp. and sequences from the family of Bradyrhizobiaceae in suspension and attached to the carrier during reactor operation with women's urine. The Bradyrhizobiaceae sequences showed 100% identity to Nitrobacter, but could not be uniquely attributed to this genus. The relative abundance was determined for 1 day in month 15 and 29, respectively, by 16S rRNA amplicon sequencing.

Close modal

AOB sequences affiliated with the Nitrosomonas europaea lineage and NOB sequences with the genus of Nitrobacter (Figure 2). The predominance of these nitrifiers was not surprising, as AOB from the Nitrosomonas europaea lineage are often selected in environments with high ammonia concentrations (Koops et al. 2006), while NOB of the genus Nitrobacter are adapted to higher nitrite concentrations than Nitrospira (Nowka et al. 2015). The relative abundance of all other AOB or NOB was below 0.1%.

The nitrification rate increased to 630 mg N L−1 d−1 at higher temperatures of 26.3 °C (months 29 to 30). At temperatures of 27.0 °C, the maximal rate of 930 mg N L−1 d−1 (3.1 g N m−2 d−1) was reached, corresponding to a discharge of 120 L d−1 of women's urine. The high rates were maintained for only 10 days, because not sufficient urine was available from the women's urine tank. In this phase, both the particulate COD and the relative abundance of nitrifying bacteria in suspension increased (Table 2, Figure 2). Hence, the nitrification rate increased at higher temperatures, because both suspended and attached nitrifiers contributed to the overall conversion rate.

Following the switch from women's to men's urine (months 36 to 40), the nitrification rate dropped to average values of 120 mg N L−1 d−1. The particulate COD concentrations decreased drastically to 220 mg L−1. In the biofilm of the carrier material, nitrifying biomass competes with heterotrophic bacteria for oxygen and space (Hem et al. 1994). The increased attachment of bacteria to the carrier material (visual observation) after the switch to men's urine further boosted this competition, resulting in lower nitrification rates.

Energy efficiency of the nitrification/distillation pilot plant

The average electric energy demand for distillation was 107 ± 31 Wh L−1 of urine for 33 distiller runs. The energy required for distillation with respect to nitrogen depends on the initial degree of dilution of the urine solution, represented by the total ammonia concentration (Figure 3). Hence, for the less concentrated women's urine in this study, more energy was required for distillation than for the more concentrated men's urine.
Figure 3

Energy demand for the nitrification/distillation process as a function of the total ammonia in the urine influent. The solid line represents the energy demand for distillation; the dotted lines are the sum of the nitrification/distillation process at different nitrification rates. The three large dots represent the energy demand for the evaluated operation periods as specified in Table 2.

Figure 3

Energy demand for the nitrification/distillation process as a function of the total ammonia in the urine influent. The solid line represents the energy demand for distillation; the dotted lines are the sum of the nitrification/distillation process at different nitrification rates. The three large dots represent the energy demand for the evaluated operation periods as specified in Table 2.

Close modal

The energy demand for nitrification is mainly caused by aeration. Due to the fact that the airflow was used for mixing, the airflow was kept constant independently of the nitrification rate. The electric energy demand for nitrification is relatively low (11 Wh gN−1) at high nitrification rates (640 mg N L−1 d−1), but it increases to values of 59 Wh gN−1 for the low rates observed in men's urine (120 mg N L−1 d−1, Figure 3).

The electric energy demand for the overall process amounted to 71 Wh gN−1 in the best case (640 mg N L−1 d−1, 1,790 mg NH4-N L−1 in influent). By assuming a conversion efficiency of 31% for electricity production (average European electricity mix; UCPTE 1994) and a nitrogen production of 8.8 g N cap−1 d−1 (Maurer et al. 2003), the primary energy demand amounted to 84 W cap−1 (71 W cap−1 for distillation and 13 W cap−1 for nitrification), which corresponds to 1.9% of the overall primary energy demand in the European Union (4,430 W cap−1 in 2012; The World Bank 2015).

The energy demand is clearly higher than the estimated primary energy demand of 10 W cap−1 for the combination of conventional treatment of municipal wastewater and industrial fertilizer production (Maurer et al. 2003) (nitrification/pre-denitrification in WWTP, average N- and P-fertilizer production in Europe, P-precipitation in WWTP including energy for sludge transport and incineration). However, the overall energy demand can still be reduced significantly, and additional energy demands in urban water management, e.g. for flushing water provision, have not been considered so far. In fact, the lower flushing water requirement for urine-diverting toilets has been shown to be an important energetic advantage of urine separation (Ishii & Boyer 2015).

Collecting urine that is as concentrated as possible is an important requirement to save energy. If urine is collected in a more concentrated form, however, the aeration rate should be reduced due to the lower nitrification rate. Stirrers instead of the aeration could be used to provide sufficient mixing under such conditions. Alternatively, different reactor types, e.g. membrane bioreactors, could achieve lower energy demands.

Process stability during reactor start-up

The start-up of the urine nitrification reactor was the main challenge during the operation of the pilot-scale reactor, mainly due to the accumulation of nitrite and the growth of acid-tolerant AOB (see below). The urine addition to the pilot-plant was increased manually, and several trials were required for successful reactor start-up. To simplify the start-up procedure, a simple pH control was tested in two laboratory reactors.

The strict pH control allowed for the automated start-up of the laboratory reactors: as the nitrification rate increased due to biomass growth, more urine was dosed automatically (Figure 4). Due to the limited alkalinity in urine, only 50% of the total ammonia in the influent was converted to nitrate. Thus, the total ammonia and nitrate concentrations increased concomitantly as more and more of the reactor content was replaced by nitrified urine.
Figure 4

Concentrations of ammonium (NH4-N, ♦), nitrate (NO3-N, ▪) and nitrite (NO2-N, ●) as well as the influent rate during the automated start-up of two laboratory reactors with pH control. The pH is controlled between 5.80 and 5.85 in R1 and 6.20 and 6.25 in R2 by switching on and off the influent pump.

Figure 4

Concentrations of ammonium (NH4-N, ♦), nitrate (NO3-N, ▪) and nitrite (NO2-N, ●) as well as the influent rate during the automated start-up of two laboratory reactors with pH control. The pH is controlled between 5.80 and 5.85 in R1 and 6.20 and 6.25 in R2 by switching on and off the influent pump.

Close modal

The pH setpoints had an influence on nitrite production: nitrite increased to a maximal value of 109 mg N L−1 in R2, while the nitrite concentrations remained below 1 mg N L−1 at all times in R1 (Figure 4). NOB are inhibited by nitrous acid (HNO2), but despite this inhibition NOB were still capable of removing the excess nitrite in R2 (Anthonisen et al. 1976), which was not always the case during the start-up of the pilot-scale application.

Nitrite accumulated in R2 but not in R1, as the influent rate increased faster in R2 compared to R1 (Figure 4). The faster increase in the urine dosage can be explained by the faster growth of AOB at higher pH values (Fumasoli et al. 2015), which boosted the automated urine addition. AOB and not NOB control the influent regime, because protons are released during ammonia oxidation to nitrite.

The inflow reached a maximal rate of 240 mL d−1 in R1 (pH 5.8) and 370 mL d−1 in R2 (pH 6.2) corresponding to a hydraulic residence time of 28.8 d in R1 and 18.9 d in R2, respectively (Figure 4). In the course of the experiment the inflow rates decreased, which may be explained with the increasing salt concentrations; high salt concentrations are known to reduce AOB and NOB activity (Moussa et al. 2006).

By switching off the influent pump, the pH in R1 dropped to 4.6, whereas the pH in R2 decreased to a minimal value of 5.7 only (Figure 5). This strong difference in the final pH minimum indicates that different AOB populations were selected in the two reactors with different pH setpoints. Indeed, the main AOB with an abundance of 3.1% in reactor R2 (pH 6.2) affiliated with the Nitrosomonas europaea lineage, which is similar to the AOB population during long-term operation of the pilot-scale reactor (Figure 2). In R1 (pH 5.8), however, Nitrosospira spp. were selected at pH 5.8 with relative abundance of 0.8%, while hardly any Nitrosomonas spp. were found.
Figure 5

After switching off the influent pump, the pH decreases to the low pH limit of the particular AOB population present in the laboratory reactors.

Figure 5

After switching off the influent pump, the pH decreases to the low pH limit of the particular AOB population present in the laboratory reactors.

Close modal

Acid-tolerant AOB are a potential risk in urine nitrification reactors, because some acid-tolerant strains can decrease the pH to values as low as 2.2 (Fumasoli et al. in preparation), in the case where no or only very low amounts of urine are added (e.g. during holidays), which could buffer the pH value. Low pH values can inhibit NOB and lead to chemical decomposition of nitrite (Udert et al. 2005), during which significant amounts of harmful gases, such as nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O) and HNO2 are released (Fumasoli et al. in preparation).

An appropriate selection of the pH setpoint is therefore important for urine nitrification. A too high pH setpoint can lead to the accumulation of nitrite and subsequent inhibition of NOB, whereas a too low setpoint can foster the selection of acid-tolerant AOB and a consequent process destabilization. Online nitrite monitoring would be a possibility to explore the maximum pH setpoint. Unfortunately, online sensors for the high concentrations to be expected in urine treatment are currently not available, but recent tests with an ultraviolet spectral probe are promising (Mašić et al. 2015).

Quality of the concentrate for fertilizer purposes

The concentrate has a high nitrogen content, but it also contains other important nutrients, such as phosphorus and potassium (Table 1). Most of the compounds from the initial influent are recovered in the final concentrate. Exceptions are the organic substances, ammonia and TIC: organic substances are oxidized during the biological treatment, about half of the ammonia is nitrified to nitrate and nearly all TIC is lost due to CO2 volatilization, mainly during the biological treatment.

Studies on pharmaceuticals removal showed that some antiviral and antibiotic compounds were substantially degraded in the nitrification reactor. Remaining pharmaceuticals can be removed after the nitrification process by adsorption onto activated carbon (Oezel Duygan et al. in preparation).

The heavy metal content in the concentrated nutrient solution is far below the general limits for fertilizers in organic agriculture (Council Regulation (EEC) 1991; Table 1). The heavy metal concentrations in the concentrate are close to the expected values taking into account the heavy metal concentrations in urine (Ronteltap et al. 2007) and a concentration factor of 20 to 25. The heavy metal content in urine is generally low, because heavy metals are mostly excreted with feces (Jönsson & Vinnerås 2013).

The greenhouse trials with synthetic solutions representing concentrated nitrified urine showed that the fertilizer performed equally or better than commercial fertilizers (Bonvin et al. 2015). The trials were conducted on Italian ryegrass (Lolium multiflorum) for a growth period of 11 weeks in sandy, slightly acidic (pH = 6.5) soil with moderate phosphorus levels.

The nitrification/distillation process produces a concentrated nutrient solution, which could be well suited as an integral fertilizer. Current challenges of the system are the high energy demand and the possible process destabilization due to nitrite accumulation or low pH values. The inhibition by nitrite as well as the growth of acid-tolerant bacteria could be prevented by online monitoring and controlling the nitrite concentration. This should be the main focus of further research. The energy demand can be decreased by collecting urine as concentrated as possible and by energetically optimizing the nitrification reactor.

We thank the Bill and Melinda Gates Foundation for the funding received for the VUNA project (www.vuna.ch; Grant No. OPP1011603) and Eawag for infrastructure funds to construct the reactors. We also thank Karin Rottermann and Claudia Bänninger-Werffeli for their dependable laboratory analyses, and Mathias Mosberger and Corine Uhlmann for reactor operation and sampling. Furthermore, we greatly appreciated the frequent knowledge and experience exchange with our project partners in South Africa, Christopher Buckley, Maximilian Grau, Sara Rhoton, and Lungiswa Zuma, who have been operating a similar reactor combination in Durban.

Anthonisen
A. C.
Loehr
R. C.
Prakasam
T. B.
Srinath
E. G.
1976
Inhibition of nitrification by ammonia and nitrous acid
.
Journal of the Water Pollution Control Federation
48
(
5
),
835
852
.
Bonvin
C.
Etter
B.
Udert
K. M.
Frossard
E.
Nanzer
S.
Tamburini
F.
Oberson
A.
2015
Plant uptake of phosphorus and nitrogen recycled from synthetic source-separated urine
.
AMBIO
44
(
2
),
217
227
.
Council Regulation (EEC)
1991
No. 2092/91 on organic production of agricultural products and indications referring thereto on agricultural products and foodstuffs. OJ L198, 22 July 1991, 1–15
.
Fumasoli
A.
Bürgmann
H.
Weissbrodt
D. G.
Wells
G. F.
Beck
K.
Mohn
J.
Morgenroth
E.
Udert
K. M.
(in preparation) The growth of γ-proteobacterial ammonia oxidizing bacteria causes strong acidification in high strength nitrogen wastewater
.
Goosse
P.
Steiner
M.
Udert
K. M.
Neuenschwander
W.
2009
NoMix-Anlage. Erste Monitoringergebnisse im Forum Chriesbach (NoMix Toilet System. First monitoring results in Forum Chriesbach)
.
Gas Wasser Abwasser
7
,
567
574
(in German, English summary).
Hem
L. J.
Rusten
B.
Ødegaard
H.
1994
Nitrification in a moving bed biofilm reactor
.
Water Research
28
(
6
),
1425
1433
.
Herlemann
D. P. R.
Labrenz
M.
Jürgens
K.
Bertilsson
S.
Waniek
J. J.
Andersson
A. F.
2011
Transitions in bacterial communities along the 2000 km salinity gradient of the Baltic Sea
.
The ISME Journal
5
(
10
),
1571
1579
.
Jönsson
H.
Vinnerås
B.
2013
Closing the loop: recycling nutrients to agriculture
. In:
Source Separation and Decentralization for Wastewater Management
(
Larsen
T. A.
Udert
K. M.
Lienert
J.
, eds).
IWA Publishing
,
London
.
Koops
H.-P.
Purkhold
U.
Pommerening-Röser
A.
Timmermann
G.
Wagner
M.
2006
The lithoautotrophic ammonia-oxidizing bacteria
. In:
The Prokaryotes
. (
Dworkin
M.
Falkow
S.
Rosenberg
E.
Schleifer
K.-H.
Stackebrandt
E.
, eds).
Springer
,
New York
, pp.
778
811
.
Larsen
T. A.
Gujer
W.
1996
Separate management of anthropogenic nutrient solutions (human urine)
.
Water Science and Technology
34
(
3–4
),
87
94
.
Larsen
T. A.
Udert
K. M.
Lienert
J.
2013
Source Separation and Decentralization for Wastewater Management
.
IWA Publishing
,
London
.
Mašić
A.
Santos
A. T. L.
Etter
B.
Udert
K. M.
Villez
K.
2015
Estimation of nitrite in source-separated nitrified urine with UV spectrophotometry
.
Water Research
85
,
244
254
.
Maurer
M.
Schwegler
P.
Larsen
T. A.
2003
Nutrients in urine: energetic aspects of removal and recovery
.
Water Science and Technology
48
(
1
),
37
46
.
Moussa
M. S.
Sumanasekera
D. U.
Ibrahim
S. H.
Lubberding
H. J.
Hooijmans
C. M.
Gijzen
H. J.
van Loosdrecht
M. C. M.
2006
Long term effects of salt on activity, population structure and floc characteristics in enriched bacterial cultures of nitrifiers
.
Water Research
40
(
7
),
1377
1388
.
Oezel Duygan
B. D.
Udert
K. M.
Remmele
A.
McArdell
C. S.
(in preparation) Fate of pharmaceuticals in source-separated urine during storage, biological treatment and powdered activated carbon adsorption
.
Rusten
B.
Eikebrokk
B.
Ulgenes
Y.
Lygren
E.
2006
Design and operations of the Kaldnes moving bed biofilm reactors
.
Aquacultural Engineering
34
(
3
),
322
331
.
The World Bank
2015
World DataBank, World Development Indicators. http://databank.worldbank.org
(accessed 1 April 2015)
.
Troccaz
M.
Niclass
Y.
Anziani
P.
Starkenmann
C.
2013
The influence of thermal reaction and microbial transformation on the odour of human urine
.
Flavour and Fragrance Journal
28
(
4
),
200
211
.
UCPTE
1994
Yearly Report 1993
.
UCPTE (Union pour la coordination de la production et du transport de l’électricité)
,
Vienna
,
Austria
.
Udert
K. M.
Fux
C.
Münster
M.
Larsen
T. A.
Siegrist
H.
Gujer
W.
2003
Nitrification and autotrophic denitrification of source-separated urine
.
Water Science and Technology
48
(
1
),
119
130
.
Udert
K. M.
Larsen
T. A.
Gujer
W.
2005
Chemical nitrite oxidation in acid solutions as a consequence of microbial ammonium oxidation
.
Environmental Science and Technology
39
(
11
),
4066
4075
.
Udert
K. M.
Larsen
T. A.
Gujer
W.
2006
Fate of major compounds in source separated urine
.
Water Science and Technology: Water Supply
54
(
11–12
),
413
420
.