Phosphate and Potassium Recovery From Source Separated
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Transcript of Phosphate and Potassium Recovery From Source Separated
ARTICLE IN PRESS
Available at www.sciencedirect.com
WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 5 8 – 4 6 6
0043-1354/$ - see frodoi:10.1016/j.watres
�Corresponding autE-mail address: J
journal homepage: www.elsevier.com/locate/watres
Phosphate and potassium recovery from source separatedurine through struvite precipitation
J.A. Wilsenach�, C.A.H. Schuurbiers, M.C.M. van Loosdrecht
Delft University of Technology, Department of Biotechnology, Julianalaan 67, 2628BC, Delft, The Netherlands
a r t i c l e i n f o
Article history:
Received 10 July 2006
Received in revised form
9 October 2006
Accepted 11 October 2006
Available online 28 November 2006
Keywords:
Crystallisation
Precipitation
Phosphate
Potassium
Recovery
Struvite
Urine
nt matter & 2006 Elsevie.2006.10.014
hor. Fax: +27 21 888 [email protected] (J.A
A B S T R A C T
Phosphate can be recovered as struvite or apatite in fluidised bed reactors. Urine has a
much higher phosphate concentration than sludge reject water, allowing simpler (and less
expensive) process for precipitation of phosphates. A stirred tank reactor with a special
compartment for liquid solid separation was used to precipitate struvite from urine.
Magnesium ammonium phosphate as well as potassium magnesium phosphate are two
forms of struvite that were successfully precipitated. Liquid/solid separation was very
effective, but the compaction of struvite was rather poor in the case of potassium struvite.
Crystals did not form clusters and maintained the typical orthorhombic structure.
Ammonium struvite had slightly lower effluent phosphate concentrations, but an
average of 95% of influent phosphate was removed regardless of ammonium or potassium
struvite precipitation. Fluid mechanics is believed to be important and should inform
further work.
& 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Phosphate in wastewater has to be removed to prevent
eutrophication of surface waters. Chemical phosphate re-
moval is expensive and the produced inorganic solids
complicate the sludge treatment, which further adds to costs
(Paul et al., 2001). Metal phosphates, such as FePO4, are
generally considered to be unavailable as plant fertilizer. For
other reasons too (e.g. heavy metal content) most wastewater
sludge containing phosphate eventually ends in landfills.
Since phosphate rock is a finite resource, phosphate should
be recovered from liquid wastes in more sustainable systems.
Nowadays, biological excess phosphate removal is well
understood and has been implemented in advanced biologi-
cal nutrient removal (BNR) plants in many countries. Organ-
isms that can take up phosphate in excess of their nutrient
requirement, release this excess phosphate in anaerobic
r Ltd. All rights reserved.. Wilsenach).
conditions, such as sludge digesters or the anaerobic com-
partments of advanced BNR plants. The phosphate concen-
tration in sludge reject water can be quite high, i.e.
85–95 g P=m3 (e.g. von Munch and Barr, 2001; Battistoni et
al., 2002; Jaffer et al., 2002). This may lead to precipitation and
scaling in pipes that increase operating and maintenance
costs (Neethling and Benisch, 2004). Controlled precipitation,
however, allows phosphate recovery. Phosphate can also be
recovered directly via a phosphate pump from the anaerobic
compartment of advanced BNR pants, although at lower
concentrations than sludge reject water (Brandse et al., 2001).
Fluidised bed reactors have been developed to crystallise
Ca3(PO4)2, which could be a secondary ore in industrial
phosphorus production (Eggers et al., 1995; Giesen, 1999).
Boundary conditions include degassing, additional seeding
material and flow control, which all add to the complexity
and costs. Struvite, MgNH4PO4 � 6H2O, on the other hand, can
ARTICLE IN PRESS
Table 1 – Composition of synthetic urine
Salt g/l mM
CaCl2 � 2H2O 0.65 4.4
MgCl2 � 6H2O 0.65 3.2
NaCl 4.60 78.7
Na2SO4 2.30 16.2
Na3citrate � 2H2O 0.65 2.6
Na2–(COO)2 0.02 0.15
KH2PO4 4.2 30.9
KCl 1.60 21.5
NH4Cl 1.00 18.7
NH2CONH2 (urea) 25.0 417
C4H7N3O (creatinine) 1.10 9.7
WAT ER R ES E A R C H 41 (2007) 458– 466 459
be used directly as a slow-release fertilizer and has a
potentially higher market value (von Munch and Barr, 2001;
Ueno and Fujii, 2001).
Urine contains around 80% of the total nitrogen (N),
70% of the potassium (K) and up to 50% of the total phos-
phate (P) loads in municipal wastewater, but adds less
than 1% of the volume (Larsen and Gujer, 1996). Modern
no-mix toilets and waterless urinals have been developed
to collect urine separately and largely undiluted (Larsen
et al., 2001). The immediate benefits of urine separation
would be an increased capacity and better effluent quality
of existing treatment plants, with a lower overall re-
source consumption (Wilsenach and van Loosdrecht, 2003).
Source separated urine could be treated at a central plant,
but this raises the problem of transport. One solution could
be de-central treatment (e.g. in office blocks or hos-
pitals), after which the treated liquid would be discharged
via existing sewers. Fluidised bed reactors are relatively
complicated processes even at centralised treatment plants
(e.g. Abe, 1995; Ueno and Fujii, 2001; von Munch and
Barr, 2001; Adnan et al., 2004) and would hardly be viable
on a smaller scale. The economies of scale dictate that
de-central processes should therefore be simpler in
general and relatively maintenance free. The aim of this
study was to design and investigate a low-tech system for
struvite recovery from source separated urine. The issue of
de-central or central treatment is left open for discussion.
Simple and efficient low-tech processes are obviously bene-
ficial to all.
The hydrolysis of urea releases ammonium and bicarbo-
nate in urine, which increases the pH and determines the
concentration of ions involved in equilibria of chemical
speciation. The phosphate concentration ðPO3�4 Þ therefore
increases in urealysed urine at the same total inorganic
phosphate concentration of around 800 g P=m3 (Ciba Geigy,
1977). This leads to supersaturation of struvite, which has
been found to precipitate naturally in urine collection
systems with all the available Mg2þ (Udert et al., 2003c, b).
Biological nitrification of urine removes all the available
bicarbonate (Udert et al., 2003a), but since the molar ratio of
NHþ4 :K:P:Mg in urine is roughly 260:13:6:1 (e.g. Ciba Geigy,
1977; Griffith et al., 1976), ammonium in nitrified urine would
still be sufficient for struvite precipitation with alkalinity
dosing. In the case of complete ammonium oxidation, or
nitrogen removal, potassium struvite ðKMgPO4 � 6H2OÞ could
be precipitated instead.
A continuous stirred tank reactor has been recovering
potassium struvite for more than five years from animal
manure in the Netherlands (Schuiling and Anrade, 1999). This
reactor is however not optimised for good settling character-
istics. The outflow of struvite particles from such reactors to
liquid/solids separation devices could lead to scaling in
downstream conduits. We therefore designed and tested a
lab-scale precipitator that incorporated a special compart-
ment for liquid/solid separation. The effects of operating
parameters (e.g. hydraulic retention time (HRT), mixing
intensity and pH) on the performance of the precipitator are
discussed. Differences between ammonium struvite and
potassium struvite precipitation and recovery were also
examined.
2. Materials and method
2.1. Synthetic urine mixture
Thermodynamics and kinetics of precipitation in synthetic
urine do not differ from real urine (Ronteltap et al., 2003).
Synthetic urine according to Griffith et al. (1976) was therefore
used in all experiments. The phosphate concentration was
increased by 50% above this recipe in order to be more
representative of most recent studies. The composition of the
urine mixture is shown in Table 1. Small amounts of urease
were added to hydrolyse urea, which increased the pH to
around 9.4. Some natural precipitation occurred in the
influent (with Mg and Ca in urine) leaving a P concentration
of around 750 mg P/l.
2.2. Batch tests
The use of different magnesium additives to recover
MgNH4PO4 � 6H2O (MAP) as well as KMgPO4 � 6H2O (KMP) were
investigated in 250 ml stirred and unstirred flasks, adding
MgO and MgCl2 at different Mg:P ratios. KMP precipitation
was investigated with a different synthetic urine solution,
containing no urea and only a small amount of NH4Cl, which
is closer to the chemical composition after nitrification–deni-
trification of urine. The initial P concentration was around
460 mg P/l and NHþ4 –N was around 40 mg N/l. The lower P
concentration simulates the conditions in biological treat-
ment of urine, in which some dilution prevents microbial
inhibition. In KMP batch experiments with MgCl2, the pH was
initially increased from 7.4 to 9.4 through NaOH addition. No
extra base was added for experiments with MgO addition.
Batches were run for 48 h.
2.3. Continuous stirred tank reactor (CSTR) for struviteprecipitation and settling
Fig. 1 illustrates two alternatives for liquid/solid separation in
the experimental CSTR. The initial set-up was used in
experiments for continuous MAP precipitation, in which
the treated liquid flows inwards after liquid/solid separation
(Fig. 1a). A second set-up, believed to be an improvement, was
ARTICLE IN PRESS
Effluent
Mg2+Influent
Struvite
Acid/BaseInfluent
and Mg2+
Struvite
Effluent
a b
Fig. 1 – Schematic drawing of struvite precipitator with cross sections through alternative liquid/solid separation devices. (a)
Inward flow device, with effluent flowing from reactor wall upwards in single effluent line. Used for MAP precipitation with
influent pH49 and Mg2þ dosed in separate line. (b) Outward flow device, with effluent flowing upwards towards the reactor
wall and out through three effluent lines (only two shown). Used for KMP precipitation, with influent pHo7 and Mg2þ mixed
with influent.
WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 5 8 – 4 6 6460
used (Fig. 1b), in which liquid flows outwards after liquid/solid
separation. The reactor was made of acrylic glass and had
a height of 300 mm and a diameter of 100 mm. The volumes
of the precipitation and settling sections were both 0.77 l.
An axial flow impeller (propeller-type) was submerged half-
way into the precipitation zone and connected to a variable
speed motor. In the case of MAP precipitation, the influent
had a pH of 9.4 and MgCl2 (1 M) was added continuously with
a separate dose pump. Influent for KMP precipitation had a
pH of around 6 and the required MgCl2 was mixed directly
into the influent. A pH probe was placed directly into the
reactor just below the liquid surface. For experiments with
KMP, the pH was monitored on-line and controlled by
addition of 1.0 M NaOH or 0.1 M HCl solutions. Set points for
minimum and maximum pH values defined a narrow band of
0.2 pH units. Samples were taken directly from the precipita-
tion zone. All experiments were done at room temperature,
i.e. 23–24�C.
2.4. X-ray diffraction (XRD) and analysis
Samples of both MAP and KMP precipitants were mixed
with ethanol and ground in a mortar. A small amount was
added to a substrate consisting of a silicon single crystal
wafer. After evaporation of the ethanol, a thin layer of sample
is obtained with a weight of about 10 mg. The XRD measure-
ments were performed on a Bruker-AXS D5005 diffractometer
equipped with an incident beam CuKa1 monochromator and
a Braun position sensitive detector (PSD). The 2y range
was 5–70� with a stepsize of 0:04� and a counting time per
step of 1 s.
All ammonium and phosphate concentrations were mea-
sured using standard commercial Dr. Lange spectrophot-
ometer equipment.
3. Results
3.1. Batch tests
The most important results from batch tests are shown in Fig.
2. In the case of MAP precipitation, an excess ammonium
concentration and bicarbonate buffer ensured a near con-
stant pH of 9.4. With addition of MgCl2, the phosphate
removal is linear with an increasing Mg:P ratio, and 99%
removal was achieved with MgCl2:P ¼ 1. Practically, the same
result was obtained with MgO addition (not shown in Fig. 2).
In the case of phosphate removal through KMP, results are
somewhat different regarding the pH. With MgCl2 as magne-
sium source, the pH decreased with increasing phosphate
removal, presumably according to
Kþ þMg2þ þHPO2�4 ! KMgPO4 þHþ. (1)
The decrease in the pH to 8.2 resulted in a relatively low
phosphate removal, even with an overdose of magnesium (P
removal was only 75% with Mg:P ¼ 2). However, with further
base addition to pH ¼ 9.1, the P removal efficiency increased
to 95% with Mg:P ¼ 1. With MgO as magnesium source, no
additional base was required. Although the reaction is
presumably the same as Eq. (1), the oxide neutralises acid
and results in a slight net increase in pH. At Mg:P ¼ 1, the pH
exceeded 9, which was sufficient for complete phosphate
removal, similar to MAP precipitation. With Mg:P ¼ 1, the
ammonium concentration decreased from 41 mgNHþ4 –N/l
initially to 22 mg NHþ4 –N/l at equilibrium. This indicates that
less than 1% of the phosphate was removed as MAP, the
greatest percentage presumably being KMP. With further MgO
additions (Mg:P ¼ 1.5 and 2) the pH increased further, but
without further effect on the P removal efficiency. Although
Miles and Ellis (2001) reported lower phosphate removal
ARTICLE IN PRESS
WAT ER R ES E A R C H 41 (2007) 458– 466 461
efficiencies (through struvite) at pH 10, due to the NHþ4 –NH3
equilibrium moving towards NH3, this is not important in
untreated urine with excess ammonium concentration, and
irrelevant regarding potassium struvite.
Summing up, almost all phosphate could be removed at
Mg:P ¼ 1, regardless of the magnesium source or whether
ammonium or potassium struvite is precipitated. Confirming
previous research, pH 9 or higher is crucial for complete
struvite precipitation from urine (e.g. Ronteltap et al., 2003).
3.2. Precipitation efficiency in continuous operation: MAPand KMP
In the baseline experiment, the struvite precipitator was filled
with untreated urine and the influent pumps for synthetic
urine and dissolved MgCl2 were started. The resulting HRT
was 2 h, based on the volume of the precipitation chamber.
The Mg:P ratio was 4.2. The soluble P effluent concentration
0
25
50
75
100
0
0.5 1.0 1.5 2.0
Mg2+:Ptot
MAP (MgCl2)KMP (MgCl2)
KMP (MgO)pH (MgCl2)
pH (MgO)
pH
P -
rem
oval eff
icie
ncy (
%)
7
8
9
10
Fig. 2 – Phosphate removal efficiencies from batch
experiments for potassium struvite precipitation (KMP)
with different Mg2þ additives and ammonium struvite
(MAP) with constant pH ¼ 9:4 and MgCl2.
Table 2 – Summary of operational parameters and results of s
Exp. Struvite pH Mg:P HRT (h)number type (feed)
1 MAP 9.4 2.6 2.0
2 MAP 9.4 2.6 2.0
3 MAP 9.4 1.6 0.5
4 MAP 9.5 1.1 2.0
5 MAP 9.4 1.1 0.57
6 MAP 9.4 1.1 0.92
7a MAP 9.4 1.3 1.55
8a KMP 9.0 1.3 1.63
9a KMP 9.0 1.3 1.63
10a KMP 8.7 1.3 1.56
a Outward flow device (Fig. 1b).
dropped to 18 mg P/l at steady state (precipitation efficiency of
97.5%), which was reached within 3 h of continuous opera-
tion. This is in line with findings from Ronteltap et al. (2003)
who determined that struvite is more soluble in urine than in
other liquids and that effluent concentrations of around
16 g P=m3 could be expected with an initial Mg:P ratio of 1.
The XRD analysis showed that precipitant was predomi-
nantly struvite, with some chloride salts. Table 2 shows the
most important results of all further experiments, which
were all started with the effluent from previous experiments
in order to reach steady state quicker. With a Mg:P ratio of 2.6,
the effluent P concentration was quite low, i.e. 10 mg P/l.
However, the decrease in removal efficiency with a Mg:P ratio
of 1.1 was almost insignificant compared to the influent
concentration.
Whether the precipitator was stirred (mixed) or not, had
virtually no effect on the soluble P effluent concentration
(experiments 1 and 2). The precipitator was operated at
different HRTs, i.e. 0.5, 0.9 and 2.0 h, for continuous periods of
up to one day. No difference could be found in precipitation
efficiency for different HRTs. Experiment 5 was in fact a series
of seven experiments where the effects of different mixing
speeds—from 50 to 600 rpm—were investigated. Mixing speed
had practically no effect on the precipitation efficiency.
In experiments 8–10, small amounts of ammonium were
added to the influent, representative of conditions after
nitrification–denitrification of urine. Remaining ammonium
was partly removed with precipitation and final effluent
concentrations with an average 18 mg NHþ4 –N were measured
(influent ammonium was 52 mg NHþ4 –N). This means that
with effluent phosphate concentrations around 38 mg P/l
(influent phosphate was 320 mg P/l for experiments 8–10),
only 17% of phosphate was precipitated as MAP, with the rest
presumably KMP. The XRD analysis confirmed that the
precipitant was predominantly struvite. The MAP samples
from the experiments 1–7 all had the same line pattern,
which correlated perfectly with struvite. KMP samples all
gave the same line pattern, which was slightly different from
MAP samples, but still had a very strong correlation with
struvite. The XRD analysis of KMP samples revealed no other
truvite precipitation
Mixing Effluent Volume Scalingspeed conc. index on wall(rpm) (mg P/l) (ml/g P) (% of influent P)
100 9 – –
0 11 – –
100 25 – –
100 24 – –
50–600 33� 2:0 44� 5 (Fig. 3)
100 26 – –
100 25 – –
100 35 500 1.2
300 35 310 5.2
100 62 370 –
ARTICLE IN PRESS
WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 5 8 – 4 6 6462
crystal formations. Precipitant scraped off the impeller blades
was also predominantly struvite.
3.3. Liquid/solid separation in continuous operation
3.3.1. Scaling of struviteExcessive scaling occurred in all MAP experiments. At least
50% of the total precipitant (by volume) had to be removed
mechanically from the impeller blades, reactor wall and the
outside of the internal effluent pipe. Fig. 3 shows the effect of
mixing speed on the collection of MAP precipitant in
experiment 5, with total duration of 135 min (i.e. 4 times the
HRT). Higher mixing speeds in general led to a higher
percentage of scaling on the reactor wall, etc. At best, 50%
of precipitant collected directly in the settler (50 rpm) while at
worst only around 30% collected directly in the settler (400,
500 and 600 rpm). By contrast, very little scaling occurred
during experiments with KMP precipitation. Still, a thin layer
of precipitant was observed on the reactor wall. After
identical KMP experiments, with equal influent flows and
equal total running times (8 h), but different mixing speeds of
100 and 300 rpm (i.e. experiments 8 and 9, respectively), the
reactor was flushed twice with tap water. The reactor was
filled with distilled water and the pH was reduced to 5.8. The
reactor was mixed for 24 h and then drained into a clean
vessel where the phosphate concentration was measured.
Based on observation of the acrylic glass reactor walls,
lowering the pH was effective to completely dissolve this
layer of precipitant from the reactor wall. The P concentration
after lowering the pH in experiment 8 (100 rpm) was 16 mg P/l,
while after experiment 9 (300 rpm) the P concentration was
74 mg P/l. This correlates to, respectively, 1.2% and 5.2% of the
influent load that precipitated on the reactor wall, indicating
the effect of higher mixing speed on wall scaling.
3.3.2. Differences between precipitation and removalefficiencyAlthough good precipitation efficiencies were observed for all
experiments involving MAP precipitation, fines were always
Mixing speed (rpm)
0 200 600400
0.2
0.4
0
0.6
Vo
lum
e p
recip
itan
t sett
ler/
tota
l p
recip
itan
t vo
lum
e
Fig. 3 – Ratio of MAP precipitant volume in the settler to the
total MAP precipitant volume after mechanical removal
from impeller, internal effluent pipe and reactor walls, for
different mixing speeds (refer Fig. 1a for liquid/solid
separation device).
present in the effluent. The total phosphate effluent concen-
tration was around 51 mg P/l at 50–300 rpm, and 78 mg P/l at
600 rpm. When these concentrations are compared to that in
Table 2 (experiment 5), it seems that around 6% of influent
phosphate was present in particulate form in the effluent. At
higher mixing speeds (500 and 600 rpm), transfer of mixing
power from the precipitation chamber to the settler was
evident and resulted in bigger losses of fines in the effluent.
This was improved with the outward flow device (Fig. 4a).
Precipitant can be seen in the precipitation compartment
with a hazy liquid. The impeller and connecting shaft are
barely visible through the liquid/solid mixture. A white
powdery substance (struvite) can be seen in the compaction
compartment, with the settling zone almost completely
transparent. Settling particles, leaving the precipitation
compartment, can be seen in the detail photograph (Fig. 4b).
Practically, no precipitant was lost and no blockages occurred
during any of the experiments with the outward flow device
(experiments 7–10).
3.3.3. Struvite volume index of MAP and KMP precipitantThe ‘struvite volume index’ of the combined MAP precipitant
(collected in settler plus the scaling removed) was determined
for the different mixing speeds. This was determined by
calculating the volume occupied by the total precipitant and
then filtering and weighing the mass of precipitant. We
assumed all precipitant to be struvite, based on the XRD-
analysis. The average volume occupied by MAP after a load of
2.5 g P was 103� 12 ml. The volume index for various mixing
speeds was 44 ml/g P on average for the seven runs in
experiment 5 (50, 100, 200, etc. up to 600 rpm), with small
differences between runs. These are still only indicative
figures for comparative purposes. The volume index of KMP
precipitant was determined based on the volume occupied by
the precipitant, and the load of P removed. The volume
occupied by KMP, after a load of 1.4 g P was already around
450 ml. The volume index for KMP was between 310 and
500 ml/g P, for different operating parameters shown in Table
2, i.e. roughly ten times higher than the MAP volume index. In
experiment 10, the pH was lowered to 8.7. It was believed that
this would decrease the supersaturation to favour secondary
crystal growth instead of primary nucleation, thereby in-
creasing the crystal sizes and improving settling. The volume
index was however not much improved, compared to that of
MAP precipitant from experiment 5. Although practically no
precipitant was lost during experiments 7–10, removal of
precipitant from the compaction zone was problematic.
When precipitant was released and flow became turbulent,
the precipitant was re-suspended and mixed uniformly
through the settling zone.
4. Discussion
4.1. Precipitation efficiency
Struvite precipitation is in general a very efficient way of
phosphate removal from urine. Even when stoichiometric
and kinetic parameters were critical at the same time (i.e.
Mg:P ¼ 1.1 and HRT ¼ 0.6 h), the effluent P concentration
ARTICLE IN PRESS
Fig. 4 – Struvite precipitator: (a) general view of experimental set-up during precipitation, settling and compaction, with
outward flow device for liquid/solid separation; (b) detail showing a plume of KMP crystals being discharged from the
precipitation chamber into the settling zone; (c) detail showing the inward flow device for liquid/solid separation, with
scaling on reactor wall and axial flow impeller clearly visible (MAP).
WAT ER R ES E A R C H 41 (2007) 458– 466 463
was still low, and only slightly higher than with oversupply of
magnesium, or with longer contact time (compare experi-
ment 5 to experiments 3 and 4). However, this is of academic
interest only, because the precipitation efficiency was still
96%. Potassium struvite precipitation ðKMgPO4 � 6H2OÞ was
shown to be almost as efficient in phosphate removal as the
more familiar MgNH4PO4 � 6H2O. Although the effluent P
concentration was a little higher for KMP (38 vs. 25 mg P/l),
this is still not significant compared to the influent concen-
tration. Batch tests have also shown that complete KMP
removal (99%) is possible. Although not investigated in this
study, precipitation kinetics could play a more important role
with potassium struvite. The practical insignificance of HRT
on precipitation efficiency, found in this study, is similar to
findings of others. Battistoni et al. (2002) used an empirical
double saturation model to show that beyond a certain
minimum contact time (around 30 min) only pH plays a role
in nucleation efficiency. In untreated and hydrolysed urine,
however, the pH is already between 9.4 and 9.5. This means
that magnesium addition would always lead to struvite
precipitation in untreated urine, where the excess ammo-
nium concentration apparently drives struvite precipitation
to ensure high removal efficiencies (Stratful et al., 2001). In
untreated urine, the NHþ4 :P ratio is 43, while the K:P ratio is
only 2. The work by Ronteltap et al. (2002) suggests that the
thermodynamics and kinetics of potassium struvite precipi-
tation in urine would be different, not only from potassium
struvite precipitation in animal manure, but also from
ammonium struvite in urine. The different ion activity
products in different liquids would then explain the slightly
better precipitation efficiency of MAP relative to KMP. A more
fundamental and quantitative understanding of potassium
struvite precipitation in urine is still lacking.
4.2. Crystallisation and recovery efficiency
Crystallisation efficiency is just as important as good
precipitation efficiency. Crystal retention time is therefore
an important parameter. This was pointed out by Battistoni et
al. (2002), who also clearly differentiated between precipita-
tion and nucleation, i.e. secondary crystal growth sufficiently
large to prevent outflow of fines (precipitant) from a fluidised
bed reactor. In this study, increasing the mixing speed to
maintain more solids in suspension for a larger effective
precipitation area only resulted in transfer of mixing power to
the settling zone of the inward flow (Fig. 1a). Since the
opening between the two compartments was at the peri-
meter, the liquid shear stress would have been highest there,
i.e. at the outside of the vortex. This problem was eliminated
by the outward flow device with the opening at the centre (Fig.
1b). Although the transfer of mixing power to the sedimenta-
tion zone was eliminated, an increase in mixing speed still
only led to more precipitation on walls. Struvite has a specific
density of about 1.7 and would therefore be transported
outwards under influence of centrifugal forces.
Fig. 5a shows the typical needle-like orthorhombic struc-
ture of struvite from a MAP sample produced in experiment 7
(outward flow device). Increasing the crystal size through
lower supersaturation (i.e. lower pH) was only partly success-
ful. Fig. 5c shows a rare example of a crystal formed at pH 8.7,
ARTICLE IN PRESS
Fig. 5 – Microscope images of struvite crystals from precipitator with outward flow device: (a) experiment 7; typical needle-
like MAP crystals; (b) experiment 9; typical KMP crystals, pH 9.0; (c) experiment 10; example of one big KMP crystal, pH 8.7; (d)
experiment 8; general view of KMP crystals, pH 9.0 and (e) experiment 10; general view of KMP crystals, pH 8.7. Images (a)–(c)
are 400 times magnified. Images (d) and (e) are 50 times magnified. Samples were dried and salt can be seen in all images.
WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 5 8 – 4 6 6464
which is substantially bigger than the crystals shown in Fig.
5b (formed at pH 9.0, experiment 9). The comparison of the
average thickness of crystals in Fig. 5d (KMP, pH 9.0,
experiment 8) and Fig. 5e (KMP, pH 8.7) is more representative
and shows that limiting the supersaturation could play some
role in struvite crystal growth. Crystals formed at pH 8.7 are
not much longer but noticeably thicker. No obvious differ-
ences were observed between the sizes and shapes of crystals
produced in experiment 9, at 300 rpm, and experiment 10.
Better mixing and the limiting of supersaturation might
therefore be equally important in order to increase crystal
size. Even though the crystals settled well (refer Fig. 4a and b)
the compaction of KMP crystals, which still had a very high
struvite volume index, could not be improved in these
experiments. These figures are believed to be representative
of the experiments, but they still give mostly qualitative
information.
The crystal in Fig. 5c has dimensions very similar to that of
von Munch and Barr (2001). The shorter and more bulky
crystals would lead to better compaction compared to the
longer and thinner crystals. Crystals produced by von Munch
and Barr (2001) were all still individual struvite crystals (i.e. no
clusters like Ueno and Fujii, 2001; Adnan et al., 2004), with a
median size of 110mm and length-to-thickness ratios of 3–4.
The images in Fig. 5 show crystals with length-to-thickness
ratios of 10 or more.
The large difference between the volume index of experi-
ment 5 (44 ml/g P) and that of experiments 8–10 (around
400 ml/g P) is also illustrated by comparison of Fig. 4a with Fig.
4c. Whereas the impeller and shaft are barely visible in Fig. 4a
due to crystals in suspension, almost no crystals in suspen-
sion can be seen in Fig. 4c. Since no flow baffles were installed
in the precipitation compartment, the vertical flow compo-
nent of mixing is believed to have been small in comparison
to the rotational movement of the liquid, which approached
that of solid body rotation. Although the crystals were kept in
suspension, there could have been little movement of
particles relative to each other. The effectiveness of mixing
could therefore have been poor and an increase of mixing
speed could have had less effect to improve the mixing. The
main difference between the precipitation chambers in the
two alternative configurations is the presence of the effluent
pipe in the inward flow device. We speculate that better
mixing, caused by eddies around the effluent pipe, would
increase the number of particle collisions. Scaling around the
effluent pipe can also be seen in Fig. 4c. A further difference in
physical operation between the inward and outward flow
devices was the dosing of magnesium, as illustrated in Fig. 1.
In the outward flow device, magnesium was already mixed
with the influent, which had a relatively low pH. This
situation would evidently have resulted in localised areas
with a high degree of struvite supersaturation in the
precipitation chamber, especially with poor mixing, which
could have favoured primary nucleation. The struvite volume
index was not measured in experiment 7 (MAP in outward
flow device). Fig. 5a and b give no evidence to suggest that
these MAP crystals have different particle shapes and sizes
than KMP. This would suggest that differences in the struvite
volume index is rather due to mechanical differences
discussed above, than differences between MAP and KMP.
4.3. Future application and further research
The potassium struvite produced at the calf manure treat-
ment plant in Putten, The Netherlands is still separated as
slurry, rather than solids (Verhoek, 2005). Struvite crystals
were also shown to cluster together, forming composite
crystals with typical diameters of 20–25mm (Schuiling and
Anrade, 1999). These crystals are produced from two CSTRs in
series without any baffles and have a typical volume index of
ARTICLE IN PRESS
WAT ER R ES E A R C H 41 (2007) 458– 466 465
60–70 ml/g P (based on KMP) after thickening in a storage tank
(Verhoek, 2005). One obvious difference between the potas-
sium struvite from calf manure and urine (this study) is the
presence of many fines in the calf manure influent, such as
pieces of animal hair and organic matter, which could
possibly improve crystal growth. Virtually all precipitation
takes place in the first of this series of reactors, but the
downstream reactor and storage tank could improve the
coagulation or clustering of precipitant. Crystals could also
grow further through ageing.
Recycling of precipitated crystals to the precipitation zone
was not investigated in this study. This would possibly
improve secondary crystal growth. Mechanical abrasion
would eventually break down long thin crystals, promoting
growth of thicker crystals. Addition of a MgO:P ratio of around
0.3–0.4 to biologically treated urine, would not only leave
more than 50% of phosphate in treated urine in solution, but
would also increase the pH only to around 8.5. Although this
was not investigated, it could prevent supersaturation, which
would lead to better secondary growth. A 50:50 mixture of
MgO and MgCl2 could be the ideal magnesium additive for
recovering phosphate for treated urine. This would have to be
investigated in more detail.
This study indicates that downstream dewatering is still
required after simple struvite precipitation. Good mixing
seems to be crucial to ensure secondary crystal growth
instead of primary nucleation. Further research should focus
on quantifying the effects of different mixing scenarios on
secondary crystal growth. This should include the issues of
baffle size and distribution, impeller type, reactor dimensions
and the possibility of partial crystal recirculation. The
operational problem of scaling could be turned into an
advantage, if for instance a plastic film could be devised to
cover baffles, which could then easily be removed.
If say 90% of the ammonium in urine is removed (or
converted to nitrite/nitrate) in a biological step, the remaining
ammonium could be removed as struvite. Even if too little
ammonium remains for complete phosphate removal
through MAP precipitation, potassium is sufficient for com-
plete phosphate recovery through KMP precipitation. At this
stage, implementation of struvite recovery on a de-central
scale seems to be inopportune. In a transitional period, the
mixing of some urine with supernatant would be the most
logical step in improving phosphate recovery.
5. Conclusions
Struvite precipitation from urine (or urine mixed with sludge
reject water) should be further developed as a process
downstream of biological N-removal. This allows recovery of
some potassium and at the same time act as a polishing
process for improved ammonium removal. Addition of MgO
provides sufficient alkalinity for struvite precipitation in
nitrified urine.
The precipitation of struvite is less of a problem than
engineering the fate of the precipitant itself. The particle size
of precipitant appears to be increased by the separate and
combined effects of limiting supersaturation and good
mixing. These should be examined further. The effectiveness
of mixing and suspension of particles should be increased,
while the mixing speed should be decreased to limit
centrifugal force and wall scaling. Localised supersaturation
could be prevented by diffuse dosing of magnesium, e.g. in a
fractal manifold, rather than point dosing. An approach using
computational fluid dynamics would benefit further studies.
Acknowledgements
This study was financially supported by the STOWA (Dutch
Foundation for Applied Water Research). The XRD analysis
was done by Niek van der Pers, Material Sciences (TU Delft).
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