Environmental Technology, Vol. 22. pp 1279-1286
© Selper Ltd, 2001
ASSESING THE POTENTIAL FOR STRUVITE RECOVERY
AT SEWAGE TREATMENT WORKS
S. A. PARSONS1*, F. WALL1, J. DOYLE1, K. OLDRING2 AND J. CHURCHLEY2
1School
of Water Sciences, Cranfield University, Cranfield, Bedfordshire, U.K
Trent Water Ltd, Avon House, St Martins Road, Coventry, U.K.
2Severn
(Received 12 February 2001; Accepted 30 April 2001)
ABSTRACT
Struvite in wastewater treatment plants was identified as early as 1939. Problems with struvite formation date back to the
1960s when it was noticed at the Hyperion treatment plant, Los Angeles. Operators at the plant noticed crystalline deposits
on the underside of post digestion screens. The digested sludge stream was diluted and it was thought the problem was
solved, until five years later when the normal gravity flow of digested sludge had decreased to such a stage that pumping
was required. This paper reports the findings of a series of experiments undertaken to identify the potential of recovering
struvite from sludge liquors. Seven sludge treatment works (STW) have been investigated including one detail. A number
of the works has the potential to form over 100 mg l-1 of struvite.
Keywords:
Phosphorus removal, phosphorus recovery, struvite, scaling, sludge liquors.
INTRODUCTION
Since the implementation of the EC Urban Waste Water
Treatment Directive, (UWWTD) 97/271/EC (21st May 1991) a
number of fundamental changes in wastewater treatment
have occurred [1]. Two of the changes directly impact upon
water companies’ treatment of sludge produced from
wastewater treatment facilities

Dumping of sewage sludge at sea is now prohibited.

Nitrogen and phosphorus limits have been imposed to
reduce the potential of eutrophication of sensitive inland
and coastal waters.
The removal of compounds containing nitrogen and
phosphorus is a key element of the UWWTD and is intended
to reduce and prevent eutrophication of sensitive inland and
coastal waters. Eutrophication can be defined in two ways:
either the result of over fertilisation of aquatic environments,
with anthropogenic inputs being the cause [2]; alternatively a
natural phenomenon which increases the organic load in a
lake due to increased nutrients.
With more stringent standards imposed regarding
nutrient removal, processes have been developed to remove
compounds containing nitrogen and phosphorus. The result
of removing greater concentrations of phosphorus from the
wastewater is that the wasted sludge has a greater
concentration of phosphorus, nitrogen and magnesium (Table
1). This combination of ions found in sludges produced from
nutrient removal, specifically biological nutrient removal
(BNR) processes, can result in the formation of a mineral
called struvite.
Struvite is a white crystalline substance consisting of
magnesium, ammonium and phosphorus in equal molar
concentrations (MgNH4PO4.6H2O). Struvite forms according
to the reaction shown below.
2
Mg

3
 NH4  PO4  6 H2O MgNH4 PO4 (H2O)6
Struvite has a distinctive orthorhombic crystal structure and
can be identified via X-ray diffraction (XRD) by matching the
1279
Table 1.
Comparison of BNR and activated sludges.
Parameters
Units
BNR
sludge
Activated
sludge
Total-COD
Total Solids
Total Nitrogen
Total Phosphorus
Soluble Phosphorus
mg l-1
mg l-1
mg l-1
mg l-1
mg l-1
15320
12620
486
335
55
13640
11080
399
143
34
Total Calcium
Total Magnesium
pH
mg l-1
mg l-1
mg l-1
686
108
7.2
247
39
7.4
intensity and position of the peaks produced to a database for
the crystal structure. The crystal habit and polarity are
discussed in detail elsewhere [3].
Struvite precipitation can be separated into two stages:
nucleation and growth. Nucleation occurs when constituent
ions combine to form crystal embryos. Crystal growth
continues until equilibrium is reached and in systems
continuously replenished with struvite constituents; e.g.
wastewater treatment plants, crystal growth continues
indefinitely.
Struvite
precipitation
is
controlled
by
pH,
supersaturation, temperature and the presence of impurities
such as calcium [4] and can occur when the concentrations of
Mg2+, NH4+ and phosphate ions PO43- exceed the solubility
product (Ksp ) for struvite. The Ksp is given by the following
expression:
Ksp = [Mg 2+] [NH4+] [PO43-]
The relationship between Ksp and pH indicates that
struvite solubility decreases with increasing pH, which in
turn leads to an increase in the struvite precipitation potential
(SPP) of a water.
The recovery of struvite has recently become an issue at
wastewater treatment works. This is because of two reasons
(i) it is a possible resource which has potential use as a
fertiliser and (ii) recovering struvite can lead to reduced
problem with struvite fouling which is a major problem in
wastewater treatment plants.
Natural sources of struvite
include guano deposits and cow manure and it has been
shown to be a highly effective source of nitrogen, magnesium
and phosphorus for plant foliage and soil applications.
Struvite can be recovered from wastewater and sludges etc. if
the chemical constituents of the sludge are such that the K sp
value is exceeded. A range of processes has been used to
recover struvite including fluidised bed reactors and pellet
reactors, Table 2. In all of these processes the precipitation is
Table 2. Examples of P recovery at bench, pilot and full scale.
Reference
Scale
Source
Method
P removal
[16]
[17]
Waki et al., 1987
Somiya et al., 1989
bench
aeration
aeration /pH
88% after 120 mins
>90% at pH 9.5
[13]
Ohlinger et al., 2000
bench FBR
digester liquor
synthetic
digester liquors
digester liquor
>80%
[18]
Kabdasli et al., 2000
bench
textile waste
aeration + seed
crystals
aeration
[19]
Lind et al., 2000
bench
human urine
[20]
[26]
Webb and Ho 1992
Munch and Barr,
2001
Matsumiya et al.,
2000
Battistoni et al., 1998
bench
pilot reactor
piggery waste
dewatering liquors
Pilot scale reactor
belt liquors
pilot FBR
belt press
liquors
digester liquor
aeration
80%
NaOH/MgCl2
>90%
digester liquor
NaOH/MgCl2
aeration
~90% removal at
1.5 Mg:P
61.7–89.6 % removal
of PO43>90%
[21]
[12]
[22]
[23]
Yamamoto et al., pilot reactor
1988
Suigimori et al., 1995 pilot reactor
[27]
Battistoni et al., 2001
full scale FBR
[14]
Taruya et al.,
full scale FBR
belt press
liquors
dewatering liquors
[24]
Susckha et al., 2000
bench/full scale
dewatering liquors
1280
adsorption on to
zeolite
MgSO4/NaOH
aeration /pH/
MgOH2
seawater
MgCl2/MgOH2
1-2 Mg:P
aeration
~90% removal of
NH3
65-80% removal of
NH3
94%
>70%
-
[15]
Jaffer et al., 2001
bench FBR/full
centrifuge
liquors
achieved by either (i) pH changes or (ii) concentration changes
or both. The pH is changed either by dosing base in the form
of NaOH or MgOH2 or by aeration of the liquors to degas the
solution. The second method to initiate precipitation is to
increase the concentration of one of the constituent ions,
usually Mg, so that the driving force for precipitation in
promoted.
Struvite scale problems were first identified in
wastewater treatment plants as early as 1939 [5]. Whilst
studying digestion Rawn (1939) found crystalline material
identified as struvite, in the digested sludge supernatant lines.
Problems with struvite formation date back to the 1960s when
it was noticed at the Hyperion treatment plant, Los Angeles
[6]. Operators at the plant noticed crystalline deposits on the
underside of post digestion screens. The digested sludge
stream was diluted and it was thought the problem was
solved, until five years later when the normal gravity flow of
digested sludge had decreased to such a stage that pumping
was required. The pipeline had diminished in size from
twelve inches in diameter to six [6]. Benisch et al. identified
annual costs related to struvite scales typically range between
$2,000 and $10,000 per MGD of secondary dry weather
capacity for the majority of treatment plants with anaerobic
digesters [7]. It is therefore essential to identify sites where
scale may become or already is a problem and identify
preventative methods.
This paper assesses the potential for forming struvite at
a number of sewage treatment works and reviews the
processes available to recover pure reusable material.
MATERIALS AND METHODS
Mass Balance
Six sewage works in the Severn Trent Water region
were selected to assess the impact of sludge treatment, water
hardness and treatment process on scale formation. Weekly
samples of digester feed sludge, digested sludge and centrate
liquors were collected over a six-week period.
Computer Simulations
A commercially available computer model (Struvite
version 3.1) developed by R. E. Loewenthal and I. Morrison
of the Department of Civil Engineering, Cape Town, South
Africa, was used to predict the SPP based upon the chemistry
of the water tested. This model was based upon research
incorporating a number of equations involving ionic
equilibria and ion activity coefficients. The computer software
model was used to assess the formation of struvite at seven
sewage works and why these may be particularly prone to
struvite precipitation . The computer model required the total
NaOH/MgCl2
97%
concentrations of magnesium, ammonia species and inorganic
phosphorus as well as alkalinity, pH, temperature and partial
pressure of carbon dioxide. From these data the SPP was
calculated.
Formation and Dissolution Experiments
A jar tester (Phipps and Bird PB-900) was used in a
series of experiments to investigate the formation of struvite
in liquors taken from Coleshill sludge distruction plant (SDP).
X-ray diffraction (XRD) analysis was used to identify the
precipitates formed with inductively coupled plasma (ICP)
analysis applied in a series of dissolution experiments to
compare and determine the purity of magnesium and
phosphorus in the precipitates formed. Again the computer
model was applied to predict the SPP based upon the
chemistry of the water tested.
Impact of Turbulence upon pH in real Liquors.
A 1-litre sample of centrate liquor was placed in a jar
and mixed at 300 rpm in an attempt to produce turbulent
conditions. A pH probe and stopwatch were used to measure
the change in pH over time. The pH meter was first calibrated
and fixed in the solution to prevent movement and damage.
RESULTS AND DISCUSSION
Struvite Formation
The SPP for each sampling point was calculated using a
computer model called Struvite (Version 3.1). This model was
developed for the Water Research Commission, South Africa
[8]. A series of experiments were undertaken with real sludge
liquors to test whether the computer software model could
predict struvite formation. The masses of precipitate formed
were calculated from the ICP data gained from the remaining
magnesium and phosphorus concentrations in the real liquors
(Figure 1). The formation of struvite based upon the data
input into the computer model initially occurs at a pH of 7.1.
In comparison struvite formation was not observed in both
real and synthetic liquors until a pH of 7.5 was reached. The
mass of precipitates formed appears to display a more linear
relationship with respect to pH than the model predicts. The
synthetic data generated from the previous jar tests regarding
the mass of struvite formed displays a closer relationship to
the data generated from the real liquors. The trendline
plotted from the synthetic data had an R2 value of 0.93.
Trendlines plotted for the masses of struvite formed from real
liquors based upon magnesium and phosphorus
concentrations remaining in solution produced R2 values of
0.75 and 0.82 respectively.
The model whilst useful does tend to under predict the
1281
formation of struvite formation and this must be considered
when looking at the potential for recovery.
The model was used to assess the potential for
formation and hence recovery has been investigated at seven
Sewage Works Assessment
Figure 1. Comparison of the Struvite model with actual sludge liquors.
Severn Trent Water sites. Six sewage treatment works and
one sludge destruction plant have been assessed for their
potential to form calcite and struvite scales. Three of the
works (Coleshill, Wanlip and Mansfield) are known to suffer
from scale formation although only, Coleshill had previously
been identified as having a struvite scale problem [9]. The
chemistry of the sludge liquors is compared in Table 3. The
works selected cover a range of pH, hardness and phosphate
concentrations. The Struvite model was used to compare the
precipitation potential of both calcite and struvite for all seven
liquors (Table 4).
The impact of chemical phosphorus removal on struvite
Table 3. Average levels of selected determinants in settled sludge liquor.
Determinant
(mg l-1)
Barston
Coleshill
Mansfield
Milcote
Oswestry
Spernal
Wanlip
pH
Total dissolved solids
Alkalinity
Ammonia
Calcium
Magnesium
Orthophosphate
7.6
1425
2226
523
257
46
2
7.9
1200
3382
888
94
19
96
7.9
902
3700
872
123
52
65
7.5
1360
1883
426
165
34
9
7.2
622
2795
658
185
31
80
7.6
2323
3267
660
321
53
8
7.7
918
3267
957
188
41
99
Table 4.
Comparison of average struvite and calcite precipitation potential.
Site
Barston
Coleshill
Mansfield
Milcote
Struvite formation
potential (mg l-1)
Calcite formation
potential (mg l-1)
-102
99
64
-56
183
-117
100
79
1282
Oswestry
Spernal
Wanlip
72
-80
177
formation can be seen by comparing the precipitation
potential of Spernal and Barston which have iron dosing and
Milcote that has aluminium dosing. Unlike BNR sludges
which release phosphorus during digestion in chemical
sludge the phosphorus is bound and stays within the sludge.
This is clear when comparing the phosphorus levels in the
sludge liquors (<10 mg l-1) with that of a BNR sludge, 282 mg
l-1 [10]. Therefore for all of these works the potential for
forming struvite is significantly reduced and the model
predictions were always negative for struvite precipitation.
Whilst the chemical phosphorus removal had a significant
effect on struvite formation it had little effect on the potential
to form calcite. Spernal has the greatest propensity of all
works sampled to form calcite, as high as 500 mg l -1, and
calcite scale has been identified (by XRD) in the liquor pumps
leading to a 20% loss in efficiency.
Table 5.
Site
Barston
Coleshill
Mansfield
Milcote
Oswestry
Spernal
Wanlip
Potential value of sludge liquor streams.
Struvite formation
potential (mg l-1) @
pH 8.5
Value*p/m3
liquor
2
105
237
13
162
16
222
0.14
7
16
0.9
11
0.17
15
28
230
128
Effect of pH
pH is the main driving force behind the formation of
both calcite and struvite. As the pH is increased then so does
the potential to form both minerals (Table 5). The liquor
samples collected are only snapshots of what is happening at
a sewage treatment works so it is important to consider what
happens through the processes. Pumping or aerating sludge
liquors has the effect of degassing the solution and removing
CO2. This degassing can lead to localised increases in pH by
as high as 1 pH unit. Ohlinger et al. showed how increasing
turbulence leads to CO2 liberation, an increase in pH and
hence an increase in struvite precipitation [11]. The time
taken for struvite crystals to nucleate is termed the induction
time and this is affected by turbulence where a doubling in
mixing speed leads to a halving of the induction time.
Samples of liquors from Wanlip and Barston were aerated to
show what effect degassing could have on liquor pH. As the
liquor is aerated the pH raises from ~ 7.5 to 8.5. Liquors from
Coleshill were stirred to degas them. Figure 2 shows the
effect of stirring on Coleshill liquors at a range of different
alkalinities. This pH change has a significant effect on the
potential to form both calcite and struvite scales. For example
increasing the pH of Barston liquor to 8.5 raised the
precipitation potential to 222 mg l-1 of struvite. The same pH
increase for Wanlip led to an increase in the calcite
precipitation potential to 616 mg l-1. A number of researchers
have used CO2 stripping to preferentially precipitate
struvite as a means to recover phosphorus (see Table 2)
including Battistoni et al. who aerated BNR centrate liquors,
*Value – potential value of struvite £670/tonne
1283
Figure 2. Effect of stirring on the pH of different alkalinity Coleshill centrifuge liquors.
Table 6. Comparison of sludge and liquor streams at Slough STW, UK [15].
Stream
P-PO4
mg l-1
NH4
mg l-1
Mg
mg l-1
Ca
Alk
pH
mg l-1 mg l-1
Temp
SPP
mg l-1
Thickened SAS
Belt
Liquor
Digested sludge
Centrifuge
Liquor
Centrifuge Cake
Crude sewage
Settled sewage
Effluent C
Effluent A
14.2
14.2
389
1.6
482
11
NA
121
50.7 6.9
285.8 7.3
18.3
17.2
-21
-342
154
94.9
1166
615
153
44
NA
56
98.8
2580
7.3
7.6
26.8
24.1
198
140
94.9
5.7
5.7
5.6
0.6
4477
16.1
23.9
0.8
1.4
1049
8.9
8
8.1
6.7
NA
118
111
109
112
482
360
356
199
268
7.0
7.9
7.7
8.2
7.6
NA
14
14
13.4
13
420
-341
-352
-276
-440
precipitating struvite and recovering 80% of phosphorus from
the liquor [10,12].
Recovery of Struvite
Recovery experiments have typically been undertaken
with sludge liquors. Jaffer et al., compared liquid and sludge
streams from Slough sludge treatment works (STW), UK [15].
They identified those streams that had the greatest potential
for struvite formation (Table 6). Whilst the digested sludge
and centrifuge cake had the highest potential they contain
mainly solids making recovery very difficult. The digested
sludge liquors were chosen to be the best option for recovery.
The evaluation of digested sludge liquors from the seven STW
reported here showed that 4 out of the seven works
consistently had the potential to form over 100 mg l -1 of
struvite.
A range of processes have been investigated to recovery
struvite [10-27], Table 2. Experiments have been undertaken
on bench, pilot and full scale and precipitation is usually
encouraged by changing the pH by either dosing base in the
form of NaOH or MgOH2 or by aeration of the liquors to
degas the solution. The second method to initiate
precipitation is to increase the concentration of one of the
constituent ions, usually Mg, so that the driving force for
precipitation in promoted.
Some authors have used magnesium hydroxide as a
source of magnesium ions and to raise the pH. Salutsky et al.,
1284
achieved 90% phosphorus recovery with magnesium
hydroxide addition, but at a temperature of 25oC [25]. Munch
and Barr also used magnesium hydroxide as a dual function
chemical and obtained an average of 94% phosphorus
removal as struvite [26]. However, using magnesium
hydroxide to serve both functions means that the magnesium
dose or the pH can not be optimised independently of each
other. The Phosnix process uses magnesium hydroxide, but
also has sodium hydroxide addition to control the pH. The
sodium hydroxide requirement is less with magnesium
hydroxide, than with magnesium chloride [14].
An
advantage of using magnesium chloride over magnesium
hydroxide is that magnesium chloride disossociates faster
than magnesium hydroxide, resulting in shorter reaction
times. A shorter reaction time means a smaller full-scale
reactor can be constructed as the hydraulic retention time can
be reduced.
Some authors have managed to crystallise phosphorus
from wastewater without the addition of any chemicals.
Battistoni et al., (1997) removed 80% of phosphorus from belt
press liquors, from a treatment plant that had nitrification,
denitrification and anaerobic digestion [10]. The liquors were
aged and air stripped to remove carbon dioxide. The pH was
raised from 7.9 to 8.3-8.6 and the phosphorus was removed as
struvite. The liquor had a very high Mg:P ratio of 3.7 : 1, so
did not require magnesium addition. However, when the
work was repeated with centrate liquor from a biological
nutrient removal plant, phosphorus removal was achieved as
a mixture of struvite and hydroxyapatite. The Mg:P ratio had
decreased to 0.22:1 and was no longer sufficient to exclusively
form struvite [12].
Recently 10% seawater (containing
1250 mg l-1) has been used as the magnesium source at a pilotplant [21].
Typical processes used are fluidised bed reactors (FBR)
or pellet reactors, where the material is collected as small solid
pellets. There are a number of full scale plants operating
across the world, the largest of which are in Japan. The three
main plants operating are situated at Shimane Prefecture
(500 m3 day-1), Fukuoka Prefecture (170 m3 day-1) and at Osaka
South Ace Centre (266 m3 day-1). They each use magnesium
addition (magnesium hydroxide or chloride) and pH
adjustment (to pH 8.1 – 8.9) to cause struvite precipitation.
The fine crystals of struvite formed are grown to 0.5-1 mm
granules by air stirring or recirculation stirring over an
average 10 day residence time. The extracted granules are
separated (fine crystals are returned to the reactor) and either
left to stand or air-dried down to 10% water content. The
Shimane Prefecture reactor removes 90% of soluble
phosphates from the treated liquors. The recovered struvite is
sold as a quality fertiliser for 1 – 200,000 Euros tonne-1.
In Trevisio, Italy, a full scale FBR is being used to treat
dewatering liquor from the belt press treating anaerobic
digester sludge from an 85,000 pe sewage works (Figure 3).
The plant has recently been commissioned. P removals as
high as 86% have been reported using just aeration [27].
It is clear that processes exist to recover struvite from
sludge liquors. The assessment of the seven sludge liquors
reported here shows that those works without chemical
phosphorus removal all have the potential to form struvite in
concentrations up to 237 mg l-1. If left untreated this will lead
to fouling problems.
CONCLUSIONS





Many STW in the UK have the potential to form struvite
at reasonable concentrations (>100 mg l-1).
Digester sludge liquors typically have the highest
potential to form struvite although this is usually limited
by the magnesium concentration in the liquor.
Degassing solutions by mixing or aeration significantly
increases the potential to form struvite.
The use of chemical phosphorus removal processes
removes the potential of liquors to form struvite.
Recovery methods exist and are usually based on
chemical addition and can recover over 90% of the
struvite from the liquor.
ACKNOWLEDGEMENTS
The authors would like to thank Severn Trent and the
UK Engineering Physical Sciences Research Council for their
sponsorship.
1285
Figure 3. Struvite crystallisation in sludge dewatering supernatant using air stripping: the new-full scale plant at Treviso (Italy)
sewage works [27].
REFERENCES
1.
Council of the European Communities, Directive concerning the collection, treatment and discharge of urban wastewater
and the discharge of wastewater from certain industrial sectors. (91/271/EEC). Official Journal L 135/40 (1991).
2. Harremoes, P. The challenge of managing water and material balances in relation to eutrophication . Water Sci. Technol. 37,
9-17. (1998).
3. Abbona, F., Calleri, M. and Ivaldi, G. Synthetic struvite, MgNH4PO4.6H20: correct polarity and surface features of some
complementary forms. ACTA Crystallographica Section b-Structural Science, 40, 223-227. (1984)
4. Bouropoulos N. and Koutsoukos P.G. Spontaneous precipitation of struvite from aqueous solutions. J. Crystal Growth 213,
381-388 (2000).
5. Rawn, A.M., A.Perry Banta. and Pomeroy, R. Multiple stage sewage digestion. Trans. ASCE, 105, 93-132. (1939).
6. Borgerding, J. Phosphate deposits in digestion systems. J. Water Pollut. Control Fed., 44, 813-819. (1972).
7. Benisch M., Clark C., Sprick R. G. and Baur R. Struvite deposits: a common and costly nuisance. WEF Operation Forum,
December. (2000).
8. Loewenthal R.E., Kornmuller U.R C. and van Heerden E.P., Modelling struvite precipitation in anaerobic treatment
systems. Water Sci. Technol., 30, 107-116 (1994).
9. Doyle J.D., Philp R., Churchley J., and Parsons S.A. Analysis of struvite precipitation in real and synthetic liquors. Trans.
Inst. Chem. Eng., 78, 480-488. (2000).
10. Battistoni, P., Fava, G., Pavan, P., Musacco, A. and Cecchi, F. Phosphate removal in anaerobic liquors by struvite
crystallisation without addition of chemicals: preliminary results. Water Res., 31, 2925-2929. (1997).
11. Ohlinger K.N., Young T.M and Schroeder E.D. Kinetics effects on preferential struvite accumulation in wastewater. J.
Environ. Eng. 125, 730-737. (1999).
12. Battistoni, P., Pavan, P., Cecchi, F. and MataAlvarez, J. Phosphate removal in real anaerobic supernatatnts: modelling and
performance of a fluidised bed reactor. Water Sci. Technol., 38, 275-283. (1998).
1286
13. Ohlinger K.N., Young T.M. and Schroeder E.D. Post digestion struvite precipitation using a fluidized bed reactor. J.
Environ. Eng., 126, 361-368 (2000).
14. Taruya T., Ueno Y. and Fujii M. Development of phosphorus resource recycling process from sewage. IWA 2000 July, Paris,
(2000).
15. Jaffer Y., Clark T.A., Pearce P. and Parsons S. A. assessing the potential of full scale phosphorus recovery by struvite
formation. Water Res., accepted (2001).
16. Waki N, Kondo H and Nishida M. Study of phosphorus removal from digester supernatant by aeration. J. Water
Wastewater, 29, 10-14. (1987).
17. Somiya I., Tsuno H. and Yoshio M. Study of phosphorus and ammonia removal by struvite recovery. In: Proc. 26th Ann.
Conf. on Sewerage Research. Fulouka City, 400. (1989). (in Japanese)
18. Kabdasli I., Tunay O., Ozturk I., Yilmaz S. and Arikan O. Ammonia removal from young landfill leachate by magnesium
ammonium phosphate precipitation and air stripping. Water Sci. Technol., 41, 237-240. (2000).
19. Lind B.B, Ban Z., and Byden S. Nutrient recovery from human urine by struvite crystallization with ammonia adsorption
on zeolite and wollastonit. Bioresource Technol. 73, 169-174. (2000).
20. Webb, K.M. and Ho, G.E. Struvite (MgNH4PO4.6H2O) solubility and its application to a piggery effluent problem. Water Sci.
Technol., 26, 2229-2232. (1992).
21. Matsumiya Y., Yamasita T., and Nawamura Y. Phosphorus removal from sidestreams by crystallisation of magnesiumammonium-phosphate using seawater. Water Environ. Manage. 14, 291-296. (2000).
22. Yamamoto K., Mastsui R., Fujii M. and Tanaka Y. Phosphorus removal by MAP crystalisation. J. PPM, Apr., 18. (1998). (in
Japanese)
23. Sugimori N., Ito T. and Nakamura T. Phosphorus removal from dewatering filtrate. In: Proc. 31st Ann. Conf. on Sewerage
Research. Kobe City, 505. (1994).
24. Suschka J. and Poplawski S. Phosphorus recovery - laboratory scale experiments. Polish-Swedish seminar, May, 8pp.
(2000).
25. Salutsky M.L., Dunseth M.G., Ries K.M. and Shapiro J.J. Ultimate disposal of phosphate from wastewater by recovery as
fertiliser. Effluent Water Treat. J., 22, 509-519. (1972).
26. Munch E.V. and Barr K. Controlled struvite crystallisation for removing phosphorus from anaerobic digester sidestreams.
Water Res. 35,151-159. (2001).
27. Battistoni P., Boccadoro R., Pavan P. and Cecchi F. Struvite crystallisation in sludge dewatering supernatant using air
stripping: the new-full scale plant at Treviso (Italy) sewage works. Second International Conference on Recovery of Phosphates
from Sewage and Animal Wastes, 12-13 March CEEP, Holland (2001).
1287