STRUVITE CRYSTALLISATION IN SLUDGE DEWATERING SUPERNATANT
USING AIR STRIPPING: THE NEW-FULL SCALE PLANT AT TREVISO (ITALY)
SEWAGE WORKS.
P. Battistoni*, R. Boccadoro*, P. Pavan**, and F. Cecchi***
(*)Engineering Faculty, Institute of Hydraulic, University of Ancona, Via Brecce Bianche, 60123 Ancona, Italy
idrotre@popcsi.unian.i
(**)Department of Environmental Sciences, University of Venice, Calle Larga S.Marta 2137, 30123 Venice, Italy
(***)Scientific and Technological Department, University of Verona, Strada Le Grazie, 37134 Verona, Italy
ABSTRACT
The paper presents the full-scale struvite crystallization plant of Treviso (Italy) to remove phosphate from
anaerobic supernatants of sludge line. The plant was designed according to the mathematical model of the
process, previously tested in a half scale pilot plant working with real anaerobic supernatant produced by a
A2O process. Two main sections are provided to treat up to 2 m3 h-1 of supernatant flowrate; the stripping
section assures the operative pH (8.3-8.7) through carbon dioxide stripping with air, to precipitate or
crystallize struvite or hydroxyapatite on sand grains on a fluidised bed reactor. No filtration section for fines
particles is required, while a pre-precipitation of suspended solids lost by belt press station is performed.
Preliminary results on stripping tests, natural ageing of supernatant and runs at different pH are also
discussed.
KEYWORDS
Anaerobic fermentation (AF), calcium phosphate, organic fraction of municipal solid waste (OF-MSW),
struvite crystallisation process (SCP), struvite (MAP), hydroxyapatite (HAP).
INTRODUCTION
Following the indications given by the EC 271/91 Directive, for nitrogen (N) and phosphorus (P) standards
in treated wastewater discharges, the planning and upgrading of wastewater treatment plants in European
countries must be more and more oriented towards biological nutrient removal (BNR) approaches. A global
approach to wastewater and OFMSW treatment was proposed after a long time pilot scale experimentation
(Cecchi et al. (1994), Pavan et al., 1998). According to this hypothesis the liquid phase, rich in VFA, from
the anaerobic fermentation (AF) process of OFMSW is added to BNR plants inlet in order to promote the
denitrification and the biological phosphorus removal processes. This requires to fix phosphorus released in
sludge line considering that the production of a recyclable material in the form of phosphate pellets which
can be used either as slow release fertilizer or other industrial products, should be desirable. In this way the
costs of operating effective and appropriate wastewater treatment to remove phosphorus could be balanced
by the gain of a recyclable phosphate product and the reduction of sludge disposal costs, due to decreased
sludge amount and volume. However it is necessary to bring under control the formation of very small
particles, the so called fines, as these cannot be recovered and/or generate a non-manageable sludge and the
plant configuration becomes more complex. In an AF-BNR plant both the supernatants from the
gravitational thickening and anaerobic co-digestion of sludge and the solid residue of the fermented effluent
are treated considering the struvite crystallization process (SCP) (Pavan et al.; 2000).The paper presents the
new full scale plant for struvite crystallization process from anaerobic supernatants, its design and the
preliminary results obtained in the start up phase.
The full scale plant of Treviso City (Italy).
The necessity of increasing the wastewater treatment capacity from 7.000 to about 20.000 m3/d, and to
reach the target of nutrient removal imposed by the EC 271/91 Directive, are the basis that lead to the new
wastewater treatment plant of Treviso. The plant (Fig. 1) was designed considering the tests regarding
biological phosphorus removal and struvite crystallisation process from anaerobic supernatants (Cecchi et
al., 1994; Pavan et al., 1998, Battistoni et al.; 1998a). This approach allows to consider both the need of a
readily biodegradable carbon (RBCOD) source to improve the BNR process yields and the OFMSW
treatments as an alternative solution to the conventional disposal (Lötter and Pitman, 1992; Kristensen et
al., 1992b; Isaacs & Henze, 1995). The process scheme is reported in Figure 1.
Figure 1. Configuration of Treviso plant.
Wastewater
inlet
Pretreatment
Biological Treatment
(N-P removal)
Secondary
Settler
Final Treatment:
chlorination
outlet
WAS
OFMSW
Preparation
Anaerobic sludge
co-digestion
Dewatering and phosphorus
precipitation
Sludge
Fermentation
demonstrative section
Struvite
Struvite
demonstrative section
Wastewater
headworks
Wastewater treatment (BNR). The upgrading of the old conventional wastewater treatment plant (30.000
PE) was carried out adopting a modified Johannesburg configuration, avoiding the primary sedimentation to
preserve more COD for the N and P removal steps. The relative volumes of the new line, which has a
50.000 PE capacity, can be changed since the whole plant is planned with modular characteristics, such that
the anaerobic and anoxic tank volume can be varied depending on the requirements (Table 1). Furthermore,
during the whole process, it is possible to vary both the single flows and their final destination. Table 2
reports plant specifications. The plant, whose biological compartment has a total volume of 9000 m3, is
composed by:
 Pre-anoxic zone (400-1200 m3) to remove nitrates from the return activated sludge (up to 23760 m3/d);
 Anaerobic (700–1200 m3) and anoxic (1600–2200 m3) zones where the fermenter effluent is added. The
mixed liquor recycle can be performed up to 37440 m3/d (about 2.5 Qin) in the anoxic zone;
 Oxidation/nitrification tank (5500 m3), divided into two basins;
 Two secondary settlers (1300 m2), 28 m diameters each;
 Contact tank for disinfection (250 m3);
 A first pre-thickener for waste sludge prior to the anaerobic digestion process and a second thickener (210
m3) fed with residual solid from anaerobic fermentation of OFMSW and waste activated sludge;
 Single phase anaerobic digester (2200 m3) in mesophilic conditions working at an HRT of 23 d and with
an organic loading rate (OLR) of 1.75 KgTS/m3.
Anaerobic fermentation process (AF). The OFMSW anaerobic fermentation takes place in a controlled 50
m3 CSTR fermenter. The substrate, after a pre-treatment to eliminate bulky and packaging wastes, is sent to
a feedstock tank, closed (in order to reduce odour impacts) and mixed. Two on-line shredder pumps are
used also for size reduction and mixing. The sludge is sent to the fermenter by a screw pump. The fermenter
operates with an HRT within 3 and 6 days, depending from the substrate availability and the yields to be
obtained. The fermenter effluent is treated in a screw-press, which extracts the liquid fraction to employ as
RBCOD and the solid residue, sent after mixing with waste activated sludge before to the thickener and
after to anaerobic digester.
Struvite Crystallisation Process (SCP). The anaerobic supernatants produced thickening the blend between
activated sludge and AF residual solid or belt pressing digested sludges can be used to feed the plant. They
has been choose forecasting that two main points of phosphorus release will be provided according to
anaerobic conditions and volatile fatty acids availability. The SCP plant, as sketched in Figure 2, is
constituted of a pre-treatment and two operative sections: a stripping tank and a fluidised bed reactor
(FBR). The pre-treatment section is composed by a suspended solid apparatus and a reservoir tank to
manage the FBR in continuous mode notwithstanding the dewatering section runs only two or three days
per week. A stripper and a connected deareation column compose the stripping section.
Table 1. Volumes and operative conditions
Old line New line
Wastewater line
Screening and degritting, m3
181
Pre-treatments, m3
360
628
3
Pre-anoxic zone, m
400-1200
Anaerobic zone, m3
700-1200
3
Recycled settled sludge, m /d 12000
23760
3
Anoxic zone, m
1600-2200
RAS, m3/d
37440
3
Aerobic zone, m
3080
5500
MLSS, kg/ m3
3.08
3.47
F/M, kg BOD/kg MLSS d
0.130
0.125
Total reactor volume, m3
3080
9000
2
Secondary settler surface, m
530
1300
Settler surface loading rate,
0.52
0.45
3
2
m /m h
Disinfection, m3
115
250
Sludge line
Dewatering belt volume, m3
160
210
Anaerobic digester volume,
2,200
OLR, kg TS/ m3d
HRT, d
Table 2. Base planning data for Treviso
wastewater treatment plant
Total
New line
Pop. Equiv. (PE) 70000
50000
3
Q, m /d
19600
14000
Q max, m3/h
1225
BOD, kgO2/d
4770
3570
SS, kgTS/d
4900
3500
Ntot, kgN/d
843
602
Ptot, kgP/d
118
84
Old line
20000
5600
1200
1400
241
34
Q = flow rate; BOD = biological oxygen demand; SS =
suspended solids.
1.75
23
RAS = return activated sludge.
Fig. 2 SCP plant
Decanter
Stripper
Deareation column
Mixer
Stocking tank
FBR
P1
P2
The anaerobic supernatant after pre-treatment, as supplied from the reservoir tank, is sent to the stripper,
together with the recycle flowrate from FBR. The air flowrate needed for CO2 stripping is pumped from
bottom using ceramic aerators, while the effluent exits from the deareation column together with the recycle
flowrate. The system stripper+deareation column can work at three different levels (H= 1.7, 2.2, 2.7 m),
allowing different hydraulic heads and hydraulic retention times. In Table 3 the volumetric characteristics
of FBR system are reported. The reactor is a steel column (in =0.6 m, height 3 m) filled with 700 kg of
virgin silica sand (0.21-0.35 mm, m=0.265 mm) to obtain a compress bed 1.5 m height. At the bottom of
the column a steel cylinder (in =0.6 m, height 1.0 m), filled with gravel with decreasing size distribution,
work as a filter, avoiding sand return to the pump, and allowing an homogeneous distribution of the stream
to the reactor. At the top of the column an expansion tank is provided in order to prevent the loss of sand
from the reactor.
Table 3. Volumetric characteristics of FBR.
Apparatus
Characteristics and material
Volume, geometry and flowrate
Pre-treatment
Mixer tank
Volume 1.1 m3
Cylindrical (i 0.9 m) steel tank
Decanter
Volume 4.1 m3 , Max.flowrate 9 m3 h-1
Cylindrical (i 1.6 m) steel tank
Feeding tank
Concrete tank
Variable volume up to 48 m3
Stripping section
Inner diameter 1.0 m
Hydraulic head 1.7, volume 1.33 m3
Stripper (V1)
Height 3.0 m
Hydraulic head 2.2, volume 1.73 m3
Steel column
Hydraulic head 2.7, volume 2.12 m3
Inner diameter 0.5 m
Hydraulic head 1.7, volume 0.33 m3
Deareation column
Height 3.0 m
Hydraulic head 2.2, volume 0.43 m3
(V2)
Steel column
Hydraulic head 2.7, volume 0.53 m3
FBR reactor
FBR reactor
Expanded bed height: 3.0 m
i 0.6 m, Height 3 m; Steel column
free volume 0.85 m3
Expansion tank
Steel
Volume 0.28 m3
Gravel filter i 0.6 m, Height 1.0 m; Steel column
Volume 0.28 m3
Recycle pumps (1+1)
P1 Type: Progressive cavity
Flowrate 3.2-15 m3/h
Power max. 3.0 kW
Feeding pumps (1+1)
P2
Type: Progressive cavity
Flowrate 0.8-4.9 m3/h
Power max. 1.5 kW
Tair, T stripper, pH and ORP stripper
Signal to PC
Qair, Qfeeding, Qrecycle,
Process parameters and design of SCP
On the basis of average values given from the analysis of SCP influent and effluent, the following
quantities can be calculated:
Nucleation Efficiency (%)
Phosphate Conversion (X%)
(PO 4 totin PO 4 totout)
η% 
100
(1)
PO 4 totin PO 4 solout
PO 4 totin
X %  η%  L% 
100
(3)
PO 4 totin
Precipitation Efficiency (L %)
Crystallization of PO4-MAP on total removed PO4
(PO 4 totout PO 4 solout)
L% 
100
(2)
(Mg in  Mg out ) mol
PO 4 totin
PO 4  MAP% 
100 (4)
(PO 4totin  PO 4totout ) mol
Crystallization of PO4-HAP on total removed PO4
PO 4  HAP % 
PO 4 totin  PO 4 totout mol  (Mg in  Mg out ) mol
(PO 4 totin  PO 4 totout ) mol
100
(5)
In the FBR plant four hydraulic retention times can be identified, referring to Figure 2 and Tab. 3 (eq. 6-9),
where Qi is the feed flowrate, QRIC the recycle flowrate, V1 the stripping section volume, V2 the volume of
the quiet zone, V3 the FBR free volume (including tube, expansion tank and filter volumes), VEXP the volume
of the fluidised bed and  the bed porosity. In the upward movement of the liquid inside the FBR, the
number of passages (n) is defined as the ratio between the mean hydraulic retention time in the plant
(HRTT) and the time spent in the FBR reactor, while the contact time (tc) on sand grains depends on the
number of cycles and the time needed for one single passage (eq. 11); tc can be also defined as a function of
bed porosity and  if Qi and Vtot are constant. Where  is a dimensionless constant used to express the
column free volume (V3) as a function of expanded bed volume (Vexp).
:
HRT T 
(V1  V2  V3)
Qi
(6)
HRT EXP 
(V1  V2)
Qi
(7)
n
HRT stripp 
HRT FBR 
V3
Q RIC
(8)
V EXP
Q RIC
ε
(9)
HRTT
HRT FBR
tc  n HRT EXP  
(10)
Vtot
Qi
(11)
Bed porosity is defined as the vacuum volume of the expanded bed. Since FBR pilot plant operates at a
fixed VEXP, the porosity decreases while the operating time of pilot increases, due to the growth of sand
grains. To calculate the bed porosity, a weighted average sample of bed, bottom, top and medium, is
collected after every run and porosity measured according to Battistoni et al.; 2001b. Efficiency is strongly
related to the operative pH, both for experiments carried out on laboratory scale (Battistoni et al., 1998a)
and for experiments on a half scale (Battistoni et al., 2001a-b) but, as already showed, a saturational model
is not sufficiently adequate to describe the dependence of nucleation efficiency on process parameters, since
a strong dependence of , not only on pH, but also contact time (tc), does exist. Therefore a double
saturational model, taking into account both pH and tc, was devised (eq. 12, 44 runs on bench and half
scale FBR pilot plant, Battistoni et al., 1998a + Battistoni et al., 2001a-b). The expression is the following:
η %  100
 pH  7.325  t c
 pH  7.325  0.371 t c  0.0196
(12)
The values of R2 and of S.E. using this approach raise up to 0.99 and 4.9 respectively. As regards the
precipitation efficiency, L, as defined in Eq. 3, it is given by the difference between X and . The
precipitation process is obviously undesired, since it enhances particulate phosphorus escapes with fines
and a complex plant is necessary, because a filtration step is needed, thus decreasing the value of . Using
all experimental runs the following equation is obtained, that describes the behaviour of X as a function of
pH, by using a saturational model:
X %  100
 pH  7.21
 pH  7.21  0.38
(13)
for which R2=0.99, S.E.=5.2 and n= 44 (Battistoni et al.; 2001b). The physical meaning of semi-saturational
constant (0.38, Eq. 13) is given by the fact that at pH 7.59 (7.21+0.38), a 50% of conversion can be
obtained, while a pH value of 8.73 is needed for a conversion of 80%. Furthermore, the conversion and the
nucleation must be considered as concomitant phenomena, thus the phosphorus removal cannot be
optimized considering only the nucleation process on the basis of double saturational model (Eq. 12); in
fact if the conversion is sensibly higher than the nucleation efficiency, a meaningful loss of fines is
observed. This aspect is highlighted in Fig. 3, where the dependence of X on pH is shown as a continuous
line. Moreover the same figure illustrates the dependence of  on pH for different contact times; it can be
observed that to obtain a loss of fines less than 5%, the nucleation must be performed with a contact time
higher than 0.4 h and a pH higher than 8.0 (Battistoni et al.;2001b). This information underlines the fact
that greatly different results are obtained using only Eq. 12. As a matter of fact, taking into account only the
mathematical model for nucleation efficiency, a contact time equal to 0.1 h is necessary (not considering the
effect of pH), to gain a nucleation efficiency of 80%.
100
8,30
90
140
8,20
80
120
8,10
70
8,00
40
X
 tc=1.0
30
pH Test1
pH Test2
7,70
20
10
PO4-P Test3
PO4_P Test2
7,50
tc=0.1
7,40
tc=0.05
7,30
60
PO4-P Test1
7,60
tc=0.8
tc=0.4
80
pH Test3
7,80
40
20
7,20
8.0
9.0
pH
Figure 3. The dependence of % on
pH for different tc.
PO4-P (mg l-1)
50
0
7.0
100
7,90
pH

60
0
0
1
2
3
4
5
6
7
8
9
10
Time (d)
Fig. 4 Natural ageing tests
Preliminary results in SCP plant
The chemical-physical characteristics of anaerobic supernatant of Treviso plant were periodically
performed in stocking tank of SCP plant (see Fig.2). The supernatant is used without the mixing of the belt
washing waters. They reveal (Tab.4) that not enough phosphorus was present mainly because the anaerobic
digester is now managed at environmental temperature for technical problems. It is confirmed by the
contents of ammonia (averagely 118 mgN/l), alkalinity (1028 mg CaCO3/l) lower than those normally
found.
However, Mg and Ca ions concentration is enough to guarantee the stoichiometric demand to precipitate up
to 110 mg l-1 of PO4-P as MAP (magnesium ammonium phosphate) or HAP (hydroxyapatite). Waiting to
manage the digester in mesophilic conditions for obtaining medium high phosphate concentration, the
supernatant was used as matrix enriched using (NH4)2 HPO4 to increase the phosphate content and simulate
the real contents of a thickener supernatant with the aim to investigate the SCP optimum positioning. In
fact in the future plan managing the use of AF-OFMSW section will generate a residual solids stream to add
to waste activated sludge to thick. This scenario surely will provide fermentation and P release during
thickening and the need to treat, in SCP plant, this supernatant together that from belt pressing.
Table 4. Chemical-physical characteristics of anaerobic supernatant, statistical parameters
pH
CODs TSS
Alkalinity
Ptot
PO4-P NH3-N K
Mg
mg/l mg/l F mg/l
M mg/l mg/l mg/l
mg/l
mg/l mg/l
Average 7.5
123
66
0
1028
11.3
8.5
118
28
40
Min
7.4
48
60
0
960
9.2
6.6
105
26
37
Max
7.8
224
76
0
1090
16.6
10.3
136
30
42
s.d.
0.2
79
8
0
43
3
1
16
2
2
Ca
mg/l
129
120
137
6.3
Natural ageing of the supernatant at different phosphate contents was investigated (Fig.4). As previously
reported (Battistoni et al.; 1998b) the test is able to provide the behaviour of the supernatant during its slow
air equilibrium; the results show that a consistent P precipitation happens only in four six days,
contemporary to a pH increase. The slope of phosphate curve is function of initial P concentration and a P
pH
conversion up to 83% was obtained. On the basis of P, Mg and Ca molar losses it is possible to suggest the
formation of MAP and HAP both.
Preliminary Tests to evaluate
the effect of hydraulic head on
8,80
stripper-deareation system at
H3, Qair 40 m3 h-1)
different air flowrates and
8,60
H2 Qair 40 m3 h-1
constant feeding flowrate (Qi
H3 Qair 10 m3 h= 1 m3 h-1) were performed
8,40
1
(Fig. 5). The methodology
adopted was firstly to use only
8,20
H2, Qair 10 m3 h-1
FBR ON
the stripping section fed
8,00
continuously, than to start also
STRIPPER ON
the FBR reactor. In this way
7,80
the hydraulic time changes
from HRT strip (eq. 7) to
7,60
HRTt (eq. 6). Results show as
0
200
400
600
800
1000
1200
to obtain an operative pH 
Time (min)
8.3 a higher head level (h3)
must be used at a minimum air
Fig. 5 pH increment in different operative conditions of FBR
flowrate (10 m3 h-1) or lower
head one ( h1 or h2) at higher
air flowrate (40 m3 h-1).
Tab. 5 Preliminary performances results
Ptot
PO4-P
NH3-N
Ca
Mg
K
HCO3
CO3
Operative Qr
parametersQin
Qair
T
pH
h (head)
HRTt
effluent Ptot
PO4-P
Ca
Mg
K
AlkM
AlkF
Removal PO4-P
Ca
Mg
Influent
X
n
L
tc
-1
mg l
-1
mg l
-1
mg l
-1
mg l
-1
mg l
-1
mg l
-1
mg l
3 -1
m h
3 -1
m h
3 -1
m h
°C
m
h
-1
mg l
-1
mg l
-1
mg l
-1
mg l
-1
mg l
-1
mg l
-1
mmol l
-1
mmol l
-1
mmol l
1
23,8
23,1
132
137
41
29
1035
0,0
4
1
40
9,7
8,7
2,7
4,1
8,5
5,8
93,8
31,5
30,7
585
60
0,6
1,1
0,4
Test
2
24,7
21,0
100
133
42
31
1130
0,0
4
1
20
8,1
8,4
2,2
0,0
11,1
10,3
93,1
32,5
29,8
800
3
28,6
23,3
105
131
42
29
1048
0,0
4
1
20
8,5
8,1
1,7
0,0
12,9
10,8
93,8
32,5
30,7
940
0,5
1,0
0,4
0,6
0,9
0,4
Performances of experimental results
Test1
Test2
Test3
%
75,7
58,4
62,1
%
64,3
55,0
54,8
%
11,4
3,4
7,2
h
1,6
0,0
0,0
The supernatant shows 8.7 as maximum pH,
lower than that in the stripping of a classical
anaerobic supernatant but, in this case, explained
with the low alkalinity content ( quite 1000 mg l1
) . Three tests were performed with the strategy
to observe the behaviour of SCP plant using a low
P content (25 mg l-1) and different operative
conditions (Tab. 5). The pH in the FBR plant
ranges from 8.1 to 8.7 according to Qair and
hydraulic head employed, respectively from 20 to
40 m3 h-1 and from 1.7 to 2.2m. It determines a
maximum P conversion of 76% and good P
removal on grains (% from 55 to 64 %, see Tab.
5) also if it must be improved. The type of salt
precipitated can be calculated, assuming that
magnesium can only precipitate as MAP, while
calcium ions saturate firstly the phosphate
removal, then the calcite formation. In this way
the main formation of MAP is provided (67-80%
mol) but a consistent amount of calcite is also
precipitated. The comparison with previous
results (Battistoni et al.;2001b) suggests to
conclude that the calcite formation is always
coupled with a consistent loss of fines;
furthermore, the calcite formation is characteristic
in supernatant at low phosphate concentration
independently by the alkalinity concentration
(1000-2500 mg l-1).
CONCLUSIONS
The struvite crystallization process is investigated in a full scale plant in Treviso (Italy). The plat was
designed according to a process modelling previously tested indicating the air stripping of carbon dioxide
as the only way to obtain the operative pH ( pH 8.3-8.7) to precipitate or crystallize struvite or
hydroxyapatite, according to anaerobic supernatant composition, without any addition of chemicals. The
preliminary results obtained can be resumed as follows:
 Air stripping is a feasible way for the process also in supernatant at low alkalinity, like those produced
in the gravitational thickening where fermentation can take place;
 A maximum hydraulic head allows energy saving for a lower air flowrate;
 Calcite is coupled with MAP and HAP formation at low phosphate concentration;
 A loss of fines higher of 5% is obtained if calcite is formed.
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