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WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 5 8 – 4 6 6

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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 2682.Wilsenach@csir.co.za (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 –

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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

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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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