Clean Urine
A research to the removal of phosphate from separately collected urine by means of the precipitation
of struvite and the profitability and the social acceptance of separation toilets
Robert van Houten
Marijn Siemons
Jeroen Wagenaar
Bonhoeffer College Bruggertstraat,
Enschede, the Netherlands,
17 March 2010
1
Abstract
Purpose
Phosphate in wastewater is removed at a wastewater treatment plant, because even small
concentrations of phosphate in water lead to excessive growth of algae, which would extract all oxygen
from the surface water. But since the concentration of phosphate is reduced by grey water and rain in
the sewers, the phosphate-removal is difficult and costly. Furthermore, phosphate is a limited resource
that is generally used in fertilizers. A shortage of phosphate could lead to a major food crisis.
Human urine contains 45% of the phosphate in waste water. A solution to remove phosphate more
effectively from waste water would be to remove the phosphate from the urine before it reaches the
sewers. To achieve this, the urine has to be separated from the faeces and flush water. There are special
designed toilets for this purpose, called separation toilets or no-mix toilets. The separation of urine at
the source results in very concentrated phosphate, which makes removal much easier.
The phosphate can be removed from urine if magnesium-ions are added to the urine. These will
precipitate and form the crystal: magnesium ammonium phosphate (MAP), that is also known as
struvite. Struvite is a fertilizer which can be used in agriculture and in this way phosphate can be
recycled.
This essay discusses different aspects of struvite precipitation: the effects of pH-values, different Mg2+salts, the growth of struvite crystals and the settling speed of struvite. With the results of these
researches a reactor design is made for a public building. A small prototype-reactor of this design is build
and tested on functionality. This essay also discusses the profitability of separation toilets, which need
less water to flush, and the social acceptance of the toilets.
Procedure
Several experiments are executed, these procedures are followed:
- To find the ideal pH-value for the precipitation of struvite, the following procedure is
followed. Eight measuring cups are filled with a different buffer solution, rising in pH-value.
The measuring cups are then filled with equal amounts of struvite and stirred. Then visual
observations indicate how much struvite is dissolved.
- The settling speed of struvite measurements involves a measuring cup filled with water and
struvite and a video camera. The settling of struvite is monitored for 30 minutes.
- To determine the possible differences between different Mg2+ salts the procedure that is
used involves filling 8 measuring cups with urine, adding MgO to the first four, and MgCl2 to
the last four. The struvite is filtrated and then measured on a weighing scale.
- The test setup of the growth of the struvite crystals involves a test reactor. In this reactor
different currents are produced with air. The struvite crystals are examined with a
microscope.
- The prototype of the reactor is tested by measuring the struvite production, with different
quantities of MgCl2, different mixing times and with MgO as Mg2+-source.
- The production of urine is estimated in the Bonhoeffer College, by counting the usages of
toilets in one toilet section. The information that is gathered is used to make an estimation
of the total usages. With that data the urine production per day is estimated.
- A survey about comfort, scent and other subjects, is held under 55 11th grade students and
the results are compared to other surveys.
2
Results
The results are that struvite precipitates better in a basic solution, while it dissolves in acid solutions.
MgCl2 forms more struvite then MgO, but is more expensive. The settling speed of struvite is
12mm/min. The growth of the struvite crystals in hard to achieve and is not suitable for a simple batchreactor in public buildings. The prototype of the reactor works properly, mixing times between 1 and 5
minutes are sufficient.
A waterless urinal reaches its breakeven-point after 10.000 flushes and the separation toilets after 5800
flushes. For the Bonhoeffer College Enschede the breakeven-point would be reached after 3,2 years if al
toilets were replaced by separation toilets. The urine production at the Bonhoeffer College Enschede is
estimated at 70L per day.
The rectangle-shape of the separation toilets needs serious reconsideration. Something should be done
about the smell of the waterless urinals. 58% of the students would not like to have a separation toilet
at their home.
Conclusion
To remove the phosphate in a reactor the pH-value should be basic, around 8,5. MgO is considered as a
better source for magnesium-ions than MgCl2 , because it is more cost effective. The settling speed is
high enough to separate the struvite with settling in a batch reactor. The design that is made works
properly and is very suitable for public buildings.
Separation toilets are a profitable and an interesting investment for both households and public
buildings. But the toilets need much improvements to prevent smell and add more sit comfort.
In conclusion, the precipitation of struvite is an easy and cheap way to remove phosphate from urine at
the source, which can be easily applied in public buildings or schools.
3
Table of contents
Abstract
2
Table of contents
4
Preface
6
Chapter 1 - Separation at source
7
Introduction
Materials and Methods
Urine flow
Economical application
Results
Urine flow
Economical application
Discussion
Urine flow
Economical application
Conclusion
8
9
9
10
11
11
12
15
15
15
16
Chapter 2 – Struvite
16
Introduction
Materials and Methods
pH-value
MgO and MgCl
Settling speed
Crystal growth
Results
pH-value
MgO and MgCl
Settling speed
Crystal growth
Discussion
pH-value
MgO and MgCl
Settling speed
Crystal growth
Conclusion
18
19
19
20
21
22
23
23
23
24
25
26
26
26
27
27
28
4
Chapter 3 – Design of a reactor
29
Introduction
Reactor Design
Separation
Reactor type - continue or batch?
Supply
Mixing
Extraction of urine and struvite
Scaling
30
31
31
32
33
33
33
33
Chapter 4 – Test reactor
36
Introduction
Reactor
Materials and method
Blanco test
Less magnesium
Mixing time
Results
Discussion
Blanco test
Less magnesium
Mixing time
Conclusion
37
38
39
39
39
40
41
42
42
42
42
43
Chapter 5 – Social acceptance
44
Introduction
Materials and methods
Results
Discussion
Conclusion
45
46
47
48
50
Acknowledgements
51
References/Bibliography
52
Appendices
53
5
Preface
In this report for the O&O (Research & Design) we discuss the possibilities and applications of the
removal of phosphate, by means of the precipitation of struvite, from urine, before the urine
reaches the sewers. Struvite is a crystal that forms when phosphate precipitates with ammonium
and magnesium. Urine already contains these substances, but magnesium in a much smaller degree.
Therefore, only a small amount of phosphate precipitates normally. Struvite can be used as a
fertilizer.
We got involved in this subject because several separation toilets and waterless urinals were
installed in our school. These toilets separate the urine at the source from the faeces and flush
water. The urine is collected separately in the front part of the toilet, and is removed to a storage
tank. The faeces with additional flush water is dumped in the sewer. The waterless urinals are not
flushed, the urine flows away automatically and is collected in a storage tank.
During this project we got help from several companies and we would like to thank the water board
Regge & Dinkel, Norit, Saxion High School Enschede, University of Wageningen, Water Research Lab
Wetsus, Waterstromen bv Steenderen and Technasium Overijssel. These parties have provided us
with materials and data. Several companies have ongoing researches to the removal of phosphate
and the precipitation of struvite, and are therefore specifically interested in our project.
Phosphate-removal, by means of struvite-precipitation, and separation toilets definitely have a
future. It increases the efficiency of phosphate removal and the separation toilets save water.
Moreover, phosphate is a limited resource; there is not an infinite amount of it on earth. According
to the Water board Regge & Dinkel, 15% of the total phosphate usage in the Netherlands can be
provided with this method. It is also very economical. When removing phosphate at the source, it
does not has to be removed at wastewater treatment plants, which saves both space and money.
Struvite will also be profitable when it will become an acknowledged fertilizer.
Robert van Houten, Marijn Siemons and Jeroen Wagenaar
6
Chapter 1 – Separation at source
Introduction
7
Materials and Methods
Urine flow
Economical application
8
8
9
Results
Urine flow
Economical application
10
10
11
Discussion
Urine flow
Economical application
14
14
14
Conclusion
15
7
Introduction
There are two ways to separate urine at the source: with separation toilets and with waterless urinals
(figure 1.1).
The separation toilets have different looks and workings compared to regular toilets. The separation
toilets separate the urine from the faeces and collect the urine in a storage tank. The toilet is divided in
two parts, one for collecting urine and one for the faeces and toilet paper. The separation toilets only
use 500 ml of flush water(4), and a small amount of the flush water is collected in the front part where
the urine is collected. So the separately collected urine is slightly diluted. The waterless urinals do not
flush at all, the urine flows away automatically. This results in highly concentrated urine.
If separation toilets are to be installed in public buildings it is necessary to give an estimation of the
profitability of separation toilets and waterless urinals. For that, the amount of urine produced in a
public building is necessary to estimate the urine flow and then it is possible to calculate when it will
reach it’s breakeven-point.
Figure 1.1 Separation toilet and waterless urinal
8
Materials and Methods
Urine flow
Purpose
The purpose of this research is to find out how many times, on average, the toilets are used per day at
the Bonhoeffer College Bruggertstraat Enschede, the Netherlands. This information is necessary for an
estimation of the urine flow and profitability of the separation toilets and waterless urinals in schools.
Method
The number of usages of one toilet section was counted during one school day. As it is not possible to
count the amount of the 5th and the 6th class, the amount of the 5th class is assumed to be the average
of the previous 4 hours. During the 6th class half of the students have gone home, because of their
timetables. Therefore the amount of visitors of the 6th class is assumed to be the half of the average of
the first 4 hours.
It is also not possible to count how many regular toilets are used at the boys toilet, but it is possible to
count the amount of visitors. Therefore, based on own experiences, it is assumed that 10% of the boys
who visits the toilet-section make use of the regular toilets and 90% of the urinals.
The amount of usages of, the toilet section which was used for the research, is estimated to be 50% of
the total usages, because of its position. To calculate the amount of usages of the toilets per year the
results of this research is used as an average per day.
Hypothesis
Per day about 50 people make use of the toilet section and around 50 people make use of the urinals.
Data
For the research we used the following data:
-
Everyone produces on average 0,150 L(1) of urine every time they go to the toilet
9
Economical application
Purpose
The purpose of this research is to find out how economical a separation toilet is compared to a regular
toilet, and to find out after how much time the breakeven-point is reached.
Method
It is possible to determine the profit which is made by a separation toilet or waterless urinal compared
to a regular toilet or normal urinal, if the prices of the toilets and water are known. A few calculations
are made with the data that was obtained from the research to the amount of usages on the Bonhoeffer
College in Enschede, the Netherlands.
Hypothesis
A separation toilet will reach its breakeven-point after 3 years.
Data
For this research the following data was used:
Regular toilet
Separation toilet
Regular urinal
Waterless urinal
6 liter flush water(5)
0,5 liter flush water(4)
2,5 liter flush water(5)
no flush water
The water in the province Overijssel, the Netherlands, costed €0,026 per liter in 2004(5).
These are the prices of the toilets:
All-inclusive costs of separation toilets
All-inclusive costs of waterless urinal
All-inclusive costs of regular toilet
All-inclusive costs of regular urinal
€1300,- (5)
€1050,- (5)
€470,- (5)
€420,- (5)
10
Results
Urine flow
Measurements
Teachers
Total
Total
Total
urinals*
(men)
toilets*
1st Class
17
11
1
29
19,1
9,9
nd
2 Class
7
8
3
18
10,8
7,2
1st Break
16
23
2
41
20,3
20,7
3rd Class
11
15
0
26
12,5
13,5
th
4 Class
22
21
1
44
25,1
18,9
2nd Break
18
18
0
36
19,8
16,2
*It is assumed that 90% of the boys that visit the toilet section make use of the urinal and 10% of the
toilet.
Girls
Boys
Counting during the 5th and 6th class was not possible, so an estimation is made. The figures for the 5th
class are the average of the 1st, 2nd, 3rd and 4th class. The figures for the 6th class are half of that.
5th Class
6th Class
Girls
Boys
14,3
7,1
13,8
6,9
Teachers
(men)
1,3
0,6
Total
29,4
14,6
Total
toilets*
17
5,4
Total
urinals*
12,4
6,2
These estimated values are used to calculate the total urine flow per day.
Total
Girls
Boys
112,4
116,7
Teachers
(men)
8,9
Total
29,4
Total
toilets*
133
Total
urinals*
105
Toilet usage in the entire school : 133 ∙ 2 ≈ 270 usages per day
Urinal usage in the entire school : 105 ∙ 2 ≈ 210 usages per day
Urine production in the entire school : (266+210) ∙ 0,150 ≈ 70 L
A school year has 200 days
Toilet usage per year: 270 ∙ 200 = 54.000 visits
Urinal usage per year: 210 ∙ 200 = 42.000 visit
11
Economical application
Figure 1.2 Breakeven-point of a separation toilet, regular toilet, waterless urinal and urinal.
From figure 1.2 can be determined:
The break even of the separation toilets is at approximately 5.800 flushes.
The break even of the waterless urinals is at approximately 10.000 flushes.
12
Processing
1. How much money does the school save every year on water costs if all toilets were to be
replaced by separation toilets?
Research to the amount of users indicated that every school year about 54.000 people visit the toilet
when nature calls, and 42.000 men visit the urinals. The amount of money saved every year can be
determined with the water prices.
Water consumption per year:
Toilets:
Urinals:
Separation toilets:
Waterless urinals:
54.000 ∙ 6,0 liter = 320.000 = 3,2 ∙ 105 liter per year
42.000 ∙ 2,5 liter = 105.000 = 1,1 ∙ 105 liter per year
54.000 ∙ 0,5 liter = 27.000 = 2,7 ∙ 104 liter per year
42.000 ∙ 0,0 liter = 0 liter per year
Water costs per year:
Toilets:
Urinals:
Separation toilets:
Waterless urinals:
3,2 ∙ 105 ∙ €0,026 = € 8300,- per year
1,1 ∙ 105 ∙ €0,026 = € 2700,-per year
2.7 ∙ 104 ∙ € 0,026 = €700,- per year
0 ∙ € 0,026 0 = €0,- per year
Savings:
Separation toilets:
Waterless urinals:
€8300,00 - €700 = €7600,€ 2700,-
With the separation toilets the school saves €7600,- a year and with the waterless urinals €2700,-. With
those numbers the school would save approximately €10.300,- in total every year.
2. After how many years is the breakeven-point reached?
At the school there are 17 toilets and 10 urinals. If all these toilets and urinals are replaced by separation
toilets and waterless urinals, the investment would be:
17 ∙ 1300,- + 10 ∙ 1050,- = €32.600,The school would save € 10.300,- on water costs. The breakeven-point is reached in:
32.600,- / 10.300,- = 3,1 years
The breakeven-point would be reached in 3 years and 2 months.
13
3. How many people have to visit the separation toilets every day to reach the breakeven-point
after 5 years?
For public buildings it is convenient to have a minimum amount of users necessary to reach the
breakeven-point after a certain amount of time.
After 5800 flushes the break-even of a separation toilet with regards to a regular toilet is reached, for
waterless urinals 10.000 flushes. 5 year has 1825 days.
To reach breakeven-point after 5 years, the amount of people who have to use the separation toilet on
average per day must be:
5800 / 1825 = 3,2
For waterless urinals it is:
10.000 / 1825 = 5,5
This can also be put in a diagram:
Breakeven-point in:
Year
Days
1
365
5
1825
10
3650
15
5475
20
7300
Min. Amount of users per day
Toilets
Urinals
15,9
27,4
3,2
5,5
1,6
2,7
1,1
1,8
0,8
1,4
4. When is the breakeven-point in a regular household reached?
When taking into account that an average household has 3 inhabitants, which make use of the toilet
4 times per day, this makes a total of 12 usages per day, the breakeven-point can be calculated. It is
also assumed that a regular household has 2 toilets and no urinals.
Cost of the separation toilets: 2 · €1300,- = €2600,Water to be saved:
€2600 / €0,026 = 100.000 L
A separation toilet saves with every flush 5,5 liters compared to a regular toilet.
Amount of flushes:
100.000 / 5,5 = 18.181,8
Breakeven-point after:
18.181,8 / 12 = 1515.15 days = 4,2 years
The breakeven-point in a regular household is reached after 4,2 years.
14
Discussion
Urine flow
The expected amount of toilet visits (50 toilet and 50 urinals usages) proved to be quite low with regards
to the actual result (133 toilet and 103 urinal usages). As the percentage of visits to the specific toilet
section relevantly to the entire school was roughly estimated, the actual outcome of the research may
be inaccurate. There is no other research to compare it with. It is virtually impossible to calculate a
precise result, because there will always be peaks and drops.
A possible improvement to this research would be to count the use of the toilets for a longer period of
time at every toilet section. Possibly every day during one to two weeks would be optimal.
Economical application
The hypothesis for this research was that the breakeven-point for separation toilets is reached after 3
years at the Bonhoeffer College Bruggertstraat Enschede. The results showed that this point will be
reached after 4,2 years, so the estimation was pretty accurate.
As the water prices were from 6 years ago, the prices are quite outdated. But as the price of water
probably has risen, the separation toilets will only save more money. Of course the accuracy of the
research could be improved if more recent prices were used.
15
Conclusion
The toilets at the Bonhoeffer College Bruggerstraat are used in total 270 times. The urinals are used
approximately 210 times. Per school year there are 54.000 toilet users and 42.000 urinal users. An
average 70 liters urine is produced per day.
The separation toilets save relatively a lot of water, and thus money, compared to a normal toilet. A
separation toilet is profitable compared to a regular toilet after 5800 flushes and urinals after 10.000
flushes.
If on a school with 1000 students and teachers all toilets would be replaced with separation toilets and
waterless urinals, then these would be profitable after 3 years and 2 months. This is relatively fast,
because a toilet section lasts longer than 3 years.
It is calculated that on average of 3,2 people per day have to make use of the separation toilet in a
public building for a period of 5 years to be profitable. For waterless urinals 5,5 people per day on
average are needed. In a household it takes only 4,2 years when the breakeven-point is reached.
In conclusion, separation toilets save a lot of water and are an interesting investment for both public
buildings and households.
16
Chapter 2- Struvite
Introduction
18
Materials and Methods
pH-value
MgO and MgCl
Settling speed
Crystal growth
19
19
20
21
22
Results
pH-value
MgO and MgCl
Settling speed
Crystal growth
23
23
23
24
25
Discussion
pH-value
MgO and MgCl
Settling speed
Crystal growth
26
26
26
27
27
Conclusion
28
17
Introduction
Magnesium precipitates with ammonium and phosphate into a crystal, called MAP or struvite (figure
2.1), according to the following equation:
Mg2+ + NH4+ + PO43- + 6H2O  MgNH4PO4 · 6H2O
Because this is a precipitation it will react automatically. To remove all phosphate, magnesium has to be
added because there is not enough magnesium in urine to react with all the phosphate. Ammonium is in
much larger quantities present. If the struvite precipitates completely almost 10 grams is formed in
every liter of urine. For more details on the contents of urine and the calculations, see Appendix A.1 and
B.2.
If the precipitation of struvite is to be used to remove phosphate a reactor is needed. If a reactor has to
remove the phosphate as efficiently as possible, certain properties of struvite have to be known. There
are two kinds of properties of struvite which are essential: the chemical properties and the physical
properties. The chemical properties are the optimal pH-value of the precipitation and the difference in
magnesium salts. The physical properties are the settling-speed of struvite and the crystal growth. These
properties are needed if a reactor has to be built.
Figure 2.1 Struvite
18
Materials and Methods
Chemical properties
Optimal pH-value of struvite precipitation
Purpose
The purpose of this research is to find out if different pH-values make a difference in the precipitation of
struvite.
Hypothesis
Struvite precipitates at pH 8 or higher, at lower pH-values struvite does not precipitate.
Method
First struvite is made for this research. The struvite in the urine is settled, and the urine is then poured
off. After that, distilled water is added to remove any urine left and is again poured off.
Several measuring cups are filled with pH-buffers (figure 2.2). After that 4ml of struvite suspension is
added to every cup. Because of the high costs of measuring the phosphate concentration, it is decided
the effects of the pH-value are determined with visual observation. The following pH-values are used:
measuring cup 1
measuring cup 2
measuring cup 3
measuring cup 4
measuring cup 5
measuring cup 6
measuring cup 7
measuring cup 8
pH-value
2,8
4,4
5,2
6
7
8,4
9,2
10
For the buffer solutions see appendix D.
Figure 2.2 Research pH-values, from high (left) to low pH-values(right).
19
Difference in Mg2+-salts
Purpose
The Magnesium salts which are not harmful to the environment, are MgO and MgCl 2. The purpose of
this research is to determine if struvite precipitates different with MgO or MgCl2.
Hypothesis
Struvite precipitates equal with different Mg2+-salts, and thus the amount which is expected to be
measured is 2,9 g struvite for both Mg2+-salts. See for the calculations Appendix A.
Method
There are 2 measuring cups of 0,5 L, which are filled with urine, which has been stored. To determine
how much of the PO43- has precipitated, the amount of struvite that is formed is measured.
In one measuring cup 4,3g MgCl2· 5H2O is added and the solution is stirred. In the other measuring cup
0,85g MgO is added and the solution is stirred. The struvite is removed from the urine with filtration
(figure 2.3)and the weight of residue is measured. This is repeated 4 times and then the average is
calculated. For the calculations see Appendix A.2
Figure 2.3 Filtration of urine with struvite
20
Physical properties
Settling-speed
Purpose
It is important to know how fast struvite settles, because one can then calculate how tall the settling
part of the reactor has to be. One can also determine how long it takes to reach a certain concentration
of struvite.
Hypothesis
Struvite settles first at a speed of 1 mm/min, after a few minutes it settles much slower. The
concentration that can be achieved by settling is estimated at 20%.
Method
To make the struvite visible while it settles, it is put in water instead of in urine, because urine is not
transparent enough. To make this struvite suitable for this research it was first cleaned. This is done by
first making struvite in urine by adding Mg2+, let it settle and then pour off the urine. Distilled water is
added to the struvite and the water is poured off again. The struvite is now suitable to use it for this
research.
The suspension is put in a tall thin measuring cup (figure 2.4). The width of the measuring cup does not
influence the settling speed. The measuring cup is placed on a stable and flat surface. Then a bright light
is placed behind the measuring cup to make the settling of struvite clearly visible. The settling is then
captured by a camera, which records 30 minutes. In this way the settling speed can be easily
determined.
Previous observations showed that struvite settles fast and that a layer of struvite is formed on the
bottom. But after a day this layer had become smaller. To measure this phenomenon this height is
measured after 1, 2 and 5 days. This will give an indication how struvite behaves after a long period of
settling. It is assume that the maximum concentration is achieved after 5 days. This mass-percent is
measured. Because the height is proportional with the concentration, the concentrations after 30 min, 1
and 2 days are then calculated.
The results are written down in two tables, one with the information of 30 minutes and one with
information over 5 days. After that the concentrations of the different heights are calculated. These data
are processed in two diagrams.
21
Figure 2.4 Research settling speed, a measuring cup with a struvite substance
Crystal Growth
Purpose
To determine if a current increases the size of struvite crystals.
Hypothesis
The current will increase the size of struvite crystals and has influence on the shape of the crystals.
Method
A test reactor is made of plexiglas of the following dimensions: 25cm x 20cm x 2,5 cm. In the bottom of
the test reactor a hole is drilled for the air tube. 2 Triton 2000 cc aquarium pumps were used for the air
supply. Two wooden pieces were used to put in the reactor to make a flow possible. These pieces were
9cm x 1cm x 2,5cm. See figures 2.5 and 2.6.
Five different experiments are executed, each with other configurations, to test the effect of a current
The struvite crystals are examined with a microscope before the experiment and after to determine if
the crystal have grown in size.
Airflowtype
Conditions
Test 1
1 entrance
Struvite in suspension
Test 2
2 entrances
Struvite in suspension
Test 3
2 entrances
PO43- precipitates with MgCl2 and NH4+
Test 4
2 entrances
PO43= precipitates with MgO and NH4+
Test 5
2 entrances
Suspension of struvite and shell sand
Figure 2.5 Test reactor with one tube
Figure 2.6 Test reactor with two tubes
22
Results
Chemical properties
Optimal pH-value of struvite precipitation
In measuring cup 1 till 4 there was a clear solution visible. In measuring cup 5 struvite was clearly visible.
In 6, 7 and 8 there was increasingly more struvite in the measuring cups visible.
Difference in Mg-salts
Test 1
MgCl2
MgO
Amount of struvite in
grams
2,9957
0,5079*
Test 2
MgCl2
MgO
Amount of struvite in
grams
3,6597
2,414
Test 3
MgCl2
MgO
Amount of struvite in
grams
1,4098*
1,3808
Test 4
MgCl2
MgO
Amount of struvite in grams
0,8721*
2,0307
Average
MgCl2
MgO
Amount of struvite in
grams
3,3277
1,8974
*These values are inaccurate due to several inconsistencies during research. Therefore, these values are
not taken in consideration when calculating the averages.
23
Physical property
Settling-speed
Short term data
The data of the short term research are put in a diagram (figure 2.7).
Figure 2.7 Settling of struvite
According to the graph the struvite settles evenly in the first 250 seconds. The gradient of the first 250
seconds represents the settling speed of struvite. The program Coach 5 was used to determine the
gradient (figure 2.8).
Figure 2.8 Derivative of the settling of struvite of the 250 seconds.
The gradient is: - 0,00021 m/s
Then the speed is : 12 mm/min
24
Long term data
It is assumed that the maximum concentration, which can be acquired through settling, is acquired after
5 days. Of this sample the concentration of dry-struvite is measured. This is done by the Water board
Regge & Dinkel. This turned out to be 5,37%. After that the other concentration could be calculated by
using the fact that the height is proportionate with concentration. With this done, the following table is
achieved:
Time (days)
0 (30 min)
1
Height struvite (mm)
20
14
Concentration (%)
2,95*
4,22*
2
12
4,92*
11
5,37
5
*Calculated
Crystal growth
In all the experiments, with the reactor (figure 2.9) the crystals were of the same size and shape. All
crystals were between 0,2 and 0,5 mm long.
Test
Test 1
Test 2
Test 3
Test 4
Test 5
Size (mm)
0,2-0,5
0,2-0,5
0,2-0,5
0,2-0,5
0,2-0,5
Figure 2.9 The test reactor
25
Discussion
Chemical properties
Optimal pH-value of struvite precipitation
The data from this research matched the hypothesis. Above a pH-value of 8 the struvite precipitated
best (figure 2.10), at lower pH-values the struvite did not precipitate at all. Other results of already
published data also match the data. The already published data(3) was more accurate than this research
and contained at different pH-values the different phosphate-concentrations. The optimal pH-value for
the precipitation was 8,5 , according to STOWA and the water board Regge & Dinkel(3). Stored urine
already has a pH-value of 8,5(3), so no substances have to be added to precipitate all phosphate.
Unfortunately, due to budget, the phosphate-concentrations could not be measured in this research.
To improve this research, instruments that measure the phosphate concentration should be used. Also,
more different pH-values could be used for research.
Figure 2.10 Research pH-values, basic solution.
Difference in Mg2+-salts
In general the amount of struvite, which was measured, turned out to be quite the same. The expected
amount was 2,9 g struvite, however with MgCl2 an average of 3,3 g struvite was measured and with
MgO 1,9 g struvite. It is slightly more, but this can be explained with the fact that not all of the crystal
water has evaporated. According to a research of STOWA(3), struvite precipitates for 90% with MgO, and
99-100% with MgCl2. The amount of MgO that was measured was substantially lower than the amount
measured by MgCl2. This matches the research of STOWA. However, it did not matched on the
proportions. With MgO, struvite precipitated around 40% less than with MgCl 2. But as MgO is much
cheaper than MgCl2, it is better to use MgO. MgO is also used in the struvite reactor at Steenderen(9),
which removes phosphate from the waste water of a factory.
There are a few ways to clarify the fact that the amount of struvite was less than expected. The
concentration of phosphate in the urine could be not as high as assumed. Also, the filtration method
that was used, could contain some (major) flaws, but this is less likely than the first clarification.
To improve this research the phosphate concentration could be measured before and after the
precipitation. Then the amount of phosphate could be compared to each other. Again, due to budget,
this could not be measured in this research.
26
Physical properties
Settling-speed
The settling-speed that was measured turned out to be much higher than expected. The 12 mm/min
that was measured is believed to be high enough for a reactor with a height of 1,5 meter to gain a high
concentration. There is no research which measured this before, so it cannot be compared. The gradient
after 250s is getting smaller, so the struvite settles less fast. This is explained because at a certain
moment the struvite particles at the bottom slow down the other struvite particles. The concentration
that can be achieved was lower than expected. After 5 days of waiting the concentration that was
achieved was 5,37%.
A possible error in this research is the start amount of struvite. This amount did not match the amount
of struvite in urine, when all the phosphate has precipitated. More research should be done to the
effect of the start amount of struvite on the concentration that can be achieved by settling.
Crystal Growth
The results do not match the hypothesis at all. None of the crystals were larger in size or had a different
shape. The current had no effect on the crystals. Other researches(8) did succeed in creating larger
crystals with a certain current speed. Some other ions like Ca2+ were used to make larger crystals. But
this is hard to achieve and maybe not suitable for a reactor in public buildings.
To improve this research better instruments should be used and more precise measuring equipment.
More experiments should also be done in more varied conditions for more results.
27
Conclusion
Struvite precipitates best in a basic solution, it dissolves in an acid solution. The optimal pH-value for the
precipitation is 8,5. Stored urine already has this value, so no substances have to be added.
Magnesium oxide should be added to the urine for the precipitation (figure 2.11), because it is cheaper
than magnesium chloride and it is not harmful for the environment. The settling speed of struvite is
12mm/min and the concentration that was reached after five days was 5,37%. The growth of struvitecrystals is difficult to achieve and is maybe not suitable for a relatively simple reactor for public
buildings.
Figure 2.11 Filtrated urine
28
Chapter 3 – Design of a reactor
Introduction
30
Reactor Design
Separation
Reactor type - continue or batch?
Supply
Mixing
Extraction of urine and struvite
Scaling
31
31
32
33
33
33
33
29
Introduction
Now that the properties of struvite are known, it is possible to make a design for a reactor which
removes phosphate from urine by means of struvite precipitation. The reactor (figure 3.1) is designed
for a public building like a school. There are six issues which are discussed: what type the reactor should
be, the supply of urine and magnesium, the mixing of the urine, the separation of the struvite from the
urine, the extraction of struvite and scaling, a phenomenon that occurs when struvite attaches to a
surface.
Figure 3.1 A possible reactor design
30
Reactor Design
A reactor which removes the phosphate needs certain requirements such as the supply of MgO and
urine, the mixing, the separation of struvite, etc. The most important requirement is the separation
section, because this is defining for the reactor design. This is discussed first. After that the process type
is discussed (batch or continue). Then the supply, mixing and extraction are discussed. One of the things
that should also be taken in consideration is scaling, this is discussed last. Then the operation of the
reactor is explained and the properties of the reactor are given.
Separation
The struvite has to be separated from the treated urine. There are a few ways of separation which can
be used. To make a decision which method should be used in the reactor, all advantages and
disadvantages have to be known. Here they are discussed:
Settling
Struvite will settle because it has a higher density than urine. When it has settled down the clean urine
can be removed and the remaining struvite will have a higher concentration.
Advantages:
This is a very simple method. The obtained concentration of struvite is believed
to be sufficient for a fertilizer.
Disadvantages:
It is time-consuming (maximum concentration appeared to be 5,39 % after five
days). Also the struvite will always contain some urine.
Centrifuge
This method is based on the difference in density of both substances.
Advantage:
A higher concentration of struvite is possible.
Disadvantages:
Continuous processing is not possible and it is very costly.
Filtration
The filtration method is based on the difference in size of the particles.
Advantage:
This method is quite effective. Using this method a concentration of almost
100% can be obtained.
Disadvantage:
The struvite must be removed from the filter on a regular basis. This requires
complicated and expensive installations.
Evaporation
The evaporation method is based on the difference in boiling point of the components in the solution.
Evaporation of the solution also salts from the urine are left as a residue in the struvite. This can be
solved by rinsing the solution. Rinsing is done by adding water to the mixture and then drain it by using
the settling method. Repeating this process will leave a mixture of struvite and water. Evaporation of
this mixture will leave pure struvite.
Advantage:
Using this method a concentration of 100 % can be obtained.
Disadvantage:
This process requires a lot of energy to evaporate the water. Rinsing the struvite
takes a lot of time because settling is a slow process. The settling tanks require a
lot of space.
Conclusion
Settling is, in this case, compared to the other methods the best. It is a simple method that does not
require a complicated installation. Therefore it is relatively cheap. All other methods require large
and/or expensive installations.
31
Reactor type – Continue or Batch?
The best way to separate the struvite of the urine is by means of settling. This settling requires time. In a
continuous processing reactor urine is supplied and drained constantly. The struvite is also removed
continuously. All these flows effect the settling process of the struvite. This can be seen in figure 3.2.
Figure 3.2 Flows in the settling tank
So with a continuous reactor not al struvite-crystals will settle and struvite-crystals would be drained
with the urine and scaling would occur on the pipes. With a batch reactor the struvite could have
enough time to settle and the urine could be drained without any struvite-crystals and there would be
no scaling.
The urine production is not constant, as people do not go to the toilet very often at night, and a public
building is closed in the evening. This would mean that a continues reactor needs a buffer tank. A batch
reactor would not need one, as the urine that is produced that day could react in the afternoon and the
struvite could settle in the night. Then the struvite and urine can be extracted in the morning and the
reactor is emptied and ready to process new urine. The reaction and the settling could be done in the
same tank. This would make the reactor very simple. From this can be concluded that a batch process is
the best option.
32
Supply
The reactor should be supplied with magnesium oxide and urine. Magnesium oxide has to be dosed in
the right proportions. This can be done easiest when the magnesium is in solution. But magnesium oxide
does not dissolve in water, it will settle after some time. So the tank that stores the solution requires a
mixer to keep the magnesium oxide in suspension. The urine can be supplied normally with a pump.
Mixing
To let all magnesium react with the ammonium and phosphate the solution has to be mixed. This,
however, cannot be done by a mixer because then scaling will occur (described below). The urine can
also be mixed by means of a current. This current can be established by pumping air in the urine with a
flexible tube. Because scaling does not occur on flexible moving objects, the urine is mixed without any
additional scaling.
Extraction of struvite and urine
Now that all phosphate has precipitated the struvite needs to be extracted. The extraction of the
struvite and the draining of the clean urine must be separated. The struvite settles at the bottom of the
reactor, where a valve is placed. If all the struvite is settled at the bottom the urine can be pumped
away. The pumping should be slow, because it is unwanted that struvite is pumped with the urine. The
pumping should be slower then 12mm/min, because the struvite would settle faster than it is drained.
The valve which is placed at the bottom remains closed during the precipitation and settling. It opens
after the clean urine is pumped away. The struvite will drop in a reservoir through the valve and the
valve closes again.
Scaling
Scaling is an adverse effect that occurs on the surface of the reactor or tubes. Scaling means that
struvite attaches to a surface. Scaling is unwanted and needs to be minimized. There is less scaling when
there is a constant current. When there is not a constant current, struvite will stick to a surface. Tubes
will get clogged due to this scaling effect. However, on flexible moving objects scaling does not occur.
This is already used in a struvite precipitation reactor in Steenderen(9). They make use of flexible tubes to
add the magnesium oxide solution to the wastewater and scaling did not occur at all on the flexible
tubes. The previous research shows that struvite dissolves in an acid solution, so scaling in the reactor
can be removed with acid.
33
Operation reactor
The operation of the reactor (figure 3.3) is as follows: the school closes around four o’clock and the
toilets will not be used any more. Then the process will start. A suspension of magnesium oxide in water
is added and mixed with the urine. It takes about ten minutes for all phosphate to precipitate into
struvite. After that the mixing is stopped. The struvite can then settle from 4.30 pm until 7.00 am. The
bottom of the reactor will gradually be filled with struvite. This is a period of 14,5 hours. According to
previous research, the struvite concentration will be 3,74%. The clean urine will be pumped away when
all the struvite has settled. Then the valve opens and the struvite drops in the reservoir. The reactor
should be cleaned once in a month with an acid solution to remove any scaling. For more details on all
the calculations, see Appendix B.
Figure 3.3 Batch reactor functional design.
34
Reactor size
During daytime the urine is collected in the reactor. Previous research showed that 70 liters of urine is
produced per day at the Bonhoeffer College. This means that the reactor has to be about 100 liters for
peaks. The research about settling speed of struvite showed that struvite settles quite fast. So the height
of the reactor does not have much influence on the concentration of struvite that is reached by settling.
Therefore a height of 1,5 meters is suitable and the diameter should then be 0,164 meters. The
container tank should be about 370 liters if it is emptied once a month. For the calculations see
Appendix B.1 and B.4.
Reactor properties
The reactor that would be placed at the Bonhoeffer College Enschede has the following properties:
Capacity Reactor
100 L
Height reaction tank
1500 mm
Diameter reaction tank
164,4 mm
MgO usage per liter urine
1,7 g
MgO usage per day
9,8 g
Struvite production per liter urine
119 g
Struvite production per day
690 g
Capacity struvitetank
370 L
The calculations can be found in Appendix B.
35
Chapter 4 – Test-reactor
Introduction
37
Reactor
38
Materials and method
Blanco test
Less magnesium
Mixing time
39
39
39
40
Results
41
Discussion
Blanco test
Less magnesium
Mixing time
42
42
42
42
Conclusion
43
36
Introduction
In the previous chapter a design is made for a batch reactor. A small prototype of this reactor is made
and tested to see if the design works properly, this is done by filtrating (Figure 4.1) and measuring the
amount of struvite. Two tests are done as a blanco test, to see if the reactor works properly. After that,
three tests are done to test the influence of the mixing duration on the amount of struvite that is
produced.
Figure 4.1 Filtration of struvite
37
Reactor
The following prototype (figure 4.2) is used for the tests:
Figure 4.2 Reactor prototype
Capacity
De test reactor can contain 4,5L urine.
Urine supply
The urine is pumped in the reactor with a Masterflex L/S pump.
Mg-solution supply
The magnesium solution is added by hand.
Airflow
To produce the airflow, three Triton 2000 CC aquarium air-pumps are used.
Urine extraction
The extraction point of the effluent is placed above the level of the settled struvite. In this way, the
struvite is not extracted with the urine.
Struvite extraction
The struvite is extracted at the bottom where a rubber tube is placed and tightened. To extract the
struvite, the rubber tube is untightened and the struvite flows down.
38
Materials and Method
Blanco test
Purpose
The purpose of this experiment is to determine if the reactor works properly and produces sufficient
struvite. Major questions are: Does the struvite extract well through a rubber tube and is there any
struvite extracted with the urine?
Hypothesis
26,1g struvite will be produced in the reactor.
Method
4,5L of urine is pumped in the reactor. The pH of the urine is measured. De urine consists of 50% male
and 50% female urine. After that, 38,6g MgCl2∙5H20, which is dissolved in water, is added (See
Appendix C.1). The urine is mixed for 15 min, it is believed that is enough. The struvite settles for 15
hours and then the urine is extracted. After that, the rubber tube is untightened and the struvite is
extracted and collected in a measuring cub. The struvite is filtrated and measured on a balance. This is
repeated for 2 times.
Less magnesium
Purpose
In the blanco test less struvite was produced than expected. The purpose of this experiment is to
determine if the same amount of struvite is produced when less magnesium is added.
Hypothesis
Female urine is diluted with a factor of 4. This means the 50% female contains 4 times less phosphate
and only 24,1g MgCl2∙5H2O has to be added (See for calculations Appendix C.2). But in the blanco tests
around 7,9g was produced. This suggests only 11,7g MgCl2∙5H2O has to be added (See for calculations
Appendix C.3). If with 11,7 MgCl2 around 7,9g struvite is produced, then is that the correct amount.
Method
The same method as the blanco test is used, but now only 24,1g and 11,7g is added.
39
Mixing time
Purpose
To determine if a shorter mixing time, or none at all, will produce the same amount of struvite (figure
4.3).
Hypothesis
No mixing will result in less struvite, because the magnesium-ions will not be spread out evenly in the
solution. Not all the phosphate and ammonium will be able to react with the magnesium. A shorter
mixing time than 15 minutes can be sufficient.
Method
The same method is used with as the previous experiment. But now the mixing time is variable: no
mixing at all, 1 minute and 5 minutes. The results are compared to the blanco test, to determine if it had
any effect.
Figure 4.3 Filtrated struvite with 5 min mixing time
40
Results
Blanco
Test 1
Test 2
Less Magnesium
Test 1
Test 2
Mixing Test
No mixing
1 minute mixing
pH
9
9
Mixing time (min)
15
15
MgCl2··5H2O added (g)
38,53
38,53
Struvite (g)
7,19
8,4698
9
9
15
15
24,096
11,697
8,1203
8,7974
9
9
0
1
24,096
24,096
5,3716
7,9645
5 minute mixing
9
5
24,096
8,8385
41
Discussion
Blanco Test
Less struvite is produced than expected, this can be explained in two ways. The urine consists of 50%
male and 50% female urine. The female urine is diluted with water, which means that there is less
phosphate in the urine than expected.
The other possibility would be that the mixing time was too short. More tests should be executed to
make sure 15 minutes is long enough to mix the solution.
The struvite extraction works properly, but some flushing with water is needed to extract all struvite,
but the loss is not significant. There is also no struvite in the urine effluent, which is also good.
This experiment could be improved if more tests were executed to provide a more reliable result.
Less Magnesium
The hypothesis was correct, less MgCl2∙5H2O produced the same amount of struvite. Thus the dilution of
the urine must be taken in consideration to determine the amount of MgCl2∙5H2O.
This research can be improved if more tests were executed. The exact amount can also be determined if
more experiments were done with different amounts of MgCl2∙5H2O or by measuring the PO43concentration.
Mixing Time
The results match the hypothesis. No mixing results in considerably less struvite (5 grams). 1 minute
mixing produces around 8g struvite. 5 minutes produces more than 1 minute mixing, but also more as
the blanco test. This suggests that 1 minute of mixing is enough, more tests should be done to explain
the higher amount of struvite with 5 min mixing.
This experiment could be improved if more tests were executed and more different mixing times were
tested.
42
Conclusion
Overall, the reactor works properly but does not produce the amount of struvite expected. This is
explained because the female urine is diluted. The reactor produces an average of 8,2g struvite. The
struvite is extracted well through the rubber tube, but some struvite remains in the reactor. There is no
struvite in the urine. 11,7g of MgCl2 ∙ 5H2O is enough to form the maximum amount of struvite. The
urine is best mixed when it is mixed for 5 minutes. This short mixing time reduces the scaling in the
reactor.
43
Chapter 5 – Application in society
Introduction
45
Materials and methods
46
Results
47
Discussion
48
Conclusion
50
44
Introduction
It is important to make it attractive for people to install separation toilets, because at the moment
people are not aware of the existence of the separation toilets. A survey is held under 55 11th grade
students. They answered questions about comfort, scent and their ideas about separation at source. The
results are compared to other surveys about the acceptance of separation toilets. The acceptance of
separation at source is import to efficiently apply struvite as a fertilizer in agriculture (figure 5.1).
Figure 5.1 Application of struvite in agriculture
45
Materials and Methods
Survey
Purpose
The purpose of this research is to find out what the social acceptation of separation toilets are, which is
vital information for the separation toilets to succeed in society.
Hypothesis
The females are expected to be quite unsatisfied, due to the distinct smell of the separation toilets.
Males are expected to be neutral, since it wouldn’t matter much to them.
Method
A survey was held under 55 11th grade students of the Bonhoeffer College, to find the customer’s
opinion on separation toilets. These students had been using separation toilets on the school for roughly
a year at the time the survey was held.
46
Results
Survey
This survey is held under 55 11th grade VWO students. There were 24 boys and 31 girls.
47
Discussion
Survey
According to this survey, the design of the separation toilets needs serious reconsideration. The
rectangle shape of the toilet seat is uncomfortable and therefore it discourages people to use the
separation toilet. The waterless urinals tend to smell more than regular urinals, since the urine is not
flushed away with water. A solution has to be found to get rid of the foul smell of urine. These problems
have to be solved in order to apply waterless urinals and separation toilets on a large scale.
A lot of research to the social acceptance of separation toilets and waterless urinals has been done in
Switzerland by the Swiss Federal Institute of Aquatic Science and Technology (EAWAG). [6][7]
According to their first questionnaire in 2003[6] among a focus group of citizens, generally the separation
toilets and the “urine-fertilized” were very well received (figure 5.2). 80% of the participants liked the
thought behind separation toilets, and 60% were willing to purchase a toilet in their own household,
which is significantly more than in the questionnaire that was held among the Bonhoeffer College
Bruggertstraat students. Although most participants indicated that separation toilets would be more
feasible in public buildings.
The participants were very positive about urine-based fertilizers as well, 80% of the participants replied
that they would have vegetables made with urine fertilizers rather than with artificial fertilizers,
although, before full implementation, an in-depth research needs to be done to the possible human
health risks with such fertilizers.
Figure 5.2 Answers to the question (A) Could you imagine purchasing a NoMix toilet? (B) Could you
imagine moving into an apartment with a NoMix toilet? The percentages are given on the top of each
bar. Results from Eawag research in 2003. [6]
48
Another research was done by Eawag in 2006, a survey among young adults to sound out their opinion
on separation toilets and urine-based fertilizers[7]. The participants were divided in a group of long-term
users and a group who are new to the concept. Especially the long-time users were positive about the
concept, 70% of these participants find the concept of separation toilets convincing, whereas 10% gave
a negative response and 20% had no opinion. Again, a significant difference with the students at the
Bonhoeffer College. Another surprising result of this survey was that women were far more positive
than men, which, again, is a difference between the Dutch survey.
An interesting result is that, even though the willingness to pay more for such toilets, the overall
acceptance is quite high (figure 5.3), making the application of separation toilets in the society in the
future very likely.
Figure 5.3 Answers to: “How do you judge the NoMix toilet compared to a conventional one?” As asked
in an Eawag questionnaire held in 2006. [7]
49
Conclusion
In order to let people accept separation at source, more research needs to be done in the possible
health risks of using urine-based fertilizers for growing vegetables. Participants of surveys indicated
they prefer vegetables grown with urine-based fertilizers rather than those with artificial fertilizers, if
health risks can be prevented. The design and comfort of separation toilets needs to be improved
according to several surveys that were held.
A good way to stimulate people to use separation toilets and waterless urinals would be by means of a
subsidy. This subsidy could be distributed by the local government.
Subsidy for separation toilets could come in different forms. One way is to make it more attractive by
lowering the water taxes for households or organisations that have separation toilets. Not only would
the customer save on water costs, due to the low water usage of these toilets, the price of water itself
would be lowered as well.
Another possible form of stimulation could be to subsidize the purchase of separation toilets. The
government would pay a part of the purchase price, making the purchase cheaper for the customer and
thus making it more attractive.
50
Acknowledgements
During this project we got aid from several people and companies. Our special thanks goes to our
Physics teacher and project supervisor Benno Berendsen and our Chemistry teacher Gerard Kransen. We
would also like to thank Mathijs Oosterhuis of water board Regge & Dinkel, professor International
Water Technology Harry Futselaar of Norit Nederland BV, Michiel Beukers, student at the Saxion
Hogeschool Enschede and Philipp Kuntke, researcher at Water Research Lab Wetsus, for their continuing
support and material help.
51
References/Bibliography
1Waterschap
Regge & Dinkel. “Schone Urine/Clean Urine,” Technasium Project, Dec. 2008.
2Kuntke,
Philipp. “Recovery of Nutrients and Energy from Source Separated Urine,” Wetsus results
2008, Apr. 2009.
3Wilsenach,
Jac. Stowa. “Stowa-Desar; options for separate treatment of urine,” 2005.
4Mels,
Adriaan. Zeeman, Grietje. Bisschops, Iemke. Stowa. “Stowa; Brongerichte inzameling en
lokale behandeling van afvalwater,” 2005.
5School
records, Bonhoeffer College Bruggertstraat, Enschede, the Netherlands 2009
6Eawag,
“Investigating consumer attitudes towards the new technology of urine separation,” Water
Science and Technology Vol 48 No 1, 2003.
7Eawag,
“Young users accept NoMix toilets,” Water Science & Technology Vol 54, 2006.
Faculty of Chemistry, Wroclaw University of Technology, “Nucleation and Crystal Growth Rates of
Struvite in DTM Type Crystallizer with a Jet-Pump of Descending Suspension Flow in a Mixing
Chamber,” American Journal of Agricultural and Biological Sciences, 2007.
8
9Waterstromen
BV, Steenderen, Postbus 8; 7241 JD Lochem.
52
Appendices
A. Contents of urine
This is an overview of the contents of stored urine(2):
Matter
Total-N
ClNa+
K+
PO4 3Mg2+
SO42pH-value
gram per liter
7,4
4,4
3,0
2,0
4,0
0,1
3,0
8,5
A.1 The optimal ratio
Urine does not contain a perfect ratio of nitrogen, phosphate and magnesium to make al phosphate to
precipitate into struvite. There is more nitrogen in urine than phosphate and magnesium. The amount of
magnesium can be neglected. If one wants to get the maximum amount of struvite, one should add
magnesium ions to make the equation correct.
Molar mass N:
Amount of mole N:
14,01 u
7,4 / 14,01 = 5,3 ∙ 10-1 moles per liter
Molar mass P04:
Amount of moles P04:
94,97 u
4,0/ 94,97 = 4,2 ∙ 10-2 moles per liter
Because the equation says that the ratio phosphate : magnesium is 1:1, is the amount of magnesium
equal to the amount of phosphate.
Molar mass Mg:
Amount of Mg needed:
24,31 u
4,2 ∙ 10-2 moles per liter
Struvite:
Amount of moles MgNH4PO4 ∙ 6H20:
Molar mass:
Amount of gram MgNH4PO4 ∙ 6H204:
Amount of gram MgNH4PO4:
4,2 ∙ 10-2 moles
245,418 u
4,2 ∙ 10-2 ∙ 245,418 = 10,3 grams
4,2 ∙ 10-2 ∙ 137,298 = 5,77 grams
53
A.2 MgO and MgCl2 ∙ 5H2O
For 1 liter:
Molar mass MgCl2 ∙ 5H2O
Amount of grammes MgCl2 ∙ 5H2O
Molar mass MgO
Amount of grammes MgO
204,34 u
8,6 grams per liter
40,32
1,7 grams per liter
For 0,5 liter:
Amount of grams MgCl2 ∙ 5H2O for 0,5L
Amount of grams MgO
8,6 / 2 = 4,3 grams
1,7 / 2 = 0,85 grams
54
B. Reactor calculations
B.1 Calculations – Reaction tank
100 L = 100 dm3
Surface bottom = 100 / 15 = 66700 mm²
Radius bottom = √(6,67) / π = 82,2 mm
Diameter reactor = 0,822 ∙ 100 ∙ 2 = 164,4 mm
B.2 Calculations - Struvite
Values
Molecule mass struvite = 233,322 u
70 L urine produced per day
4g PO43-  4,2 ∙ 10-2 M
Calculations
1 L urine contains:
4,2 ∙ 10-2 ∙ 245,418 = 10,3 g struvite
The amount of struvite that is precipitated per day:
10,3 ∙ 70 (L) = 721 g struvite
B.3 Calculations – Struvite concentration
Settling time from 4.30 PM until 07.00 AM is 14½ hours
Settling speed of struvite is 12 mm/min
Struvite concentration after 30 min is 2,95% and after 1 day it is 4,22%
Calculations
The distance struvite settles in that time is:
14½ ∙ 12 = 10,440 m
The struvite concentration after 15 hours of settling:
15/24 ∙ (4,22-2,95) = 3,74%
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B.4 Calculations – Volume reservoir
It is assumed that the space 1 gram of struvite occupies 1 ml of space.
According to the previous calculations the following values can be noted down:
The concentration after 15 hours of settling us 3,74%
In the reactor the maximum amount of formed struvite every day is 690 gram.
Calculations
The volume of the reservoir is:
20 ∙ 721 ∙ 10-3 ∙ (100/3,74) ≈ 386L
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C. Magnesium
C.1 MgO and MgCl2 in urine
Amount of moles PO43- per liter
Amount of moles PO43- per 4,5 liter
Amount of moles Mg2+ per 4,5 liter needed
Molar mass MgCl2 ∙ 5H2O
Amount of grammes MgCl2 ∙ 5H2O needed
4,2 ∙ 10 -2 moles
1,89 ∙ 10 -1 moles
1,89 ∙ 10 -1 moles
204,34 u
1,89 ∙ 10 -1 ∙ 204,34 = 38,6 g
C. 2 Struvite in diluted urine
4g PO43- per liter urine
In 4,5L
Molar mass PO43Amount of moles in 4,5L
Molar mass struvite
Amount of grams struvite in 4,5L
4 ∙ 4,5 = 18g PO4394,97 u
18 / 94,97 = 0,190 moles PO43137,412 u
0,190 ∙ 137,412 = 137,412g MgNH4PO4
50%/50% male/female in urine
Male urine 4g PO43- per liter
Female urine 1g PO43- per liter
Total grams PO43Molar mass PO43-Total moles PO43Molar mass struvite
Total struvite
4,5 / 2 = 2,25L
2,25 ∙ 4 = 9g PO432,25 ∙ 1 = 2,25g PO439 + 2,25 = 11,25
94,97 u
11,25 / 94,97 = 0,1185
137,412 u
0,1185 ∙ 137,412 = 16,3 g MgNH4PO4
C.3 MgCl2 ∙ 5H2O in 7,9 grammes of struvite
Molar mass struvite
Amount of moles struvite in 7,9g
Molar mass MgCl2 ∙ 5H2O
Amount of grams MgCl2 ∙ 5H2O
137,412 u
7,9 / 137,412 = 0,0575 moles MgNH4PO4
204,34 u
0,0575 ∙ 204,34 = 11,69g MgCl2 ∙ 5H2O
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D. Buffer solutions
Amount in
percents
pH
0,2 M K2HPO4
Amounts in ml
0,1 M Citric acid
0,2 M K2HPO4
0,1 M Citric acid
2,8
15
85
12
68
3,6
20
70
24
56
4,4
45
55
36
44
5,2
55
45
44
36
6,0
65
35
52
28
6,8
75
25
60
20
7,6
95
5
76
4
0,2 M Boric acid
0,2 M NaOH
0,2 M Boric acid
0,2 M NaOH
8,4
85
15
68
12
9,2
65
35
52
28
10,0
50
50
40
40
58