struvite formation from the supernatants of an anaerobic digestion pilot plant
TRANSCRIPT
Bioresource Technology 101 (2010) 118–125
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Bioresource Technology
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Struvite formation from the supernatants of an anaerobic digestion pilot plant
L. Pastor a, D. Mangin b, J. Ferrer a, A. Seco c,*
a Departamento de Ingeniería Hidráulica y Medio Ambiente, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spainb Laboratoire d’Automatique et de Génie des Procédés (LAGEP), Université Claude Bernard Lyon 1, bât 308G ESCPE-Lyon, 2ième étage, 69622 Villeurbanne Cedex, Francec Departamento de Ingeniería Química, Universidad de Valencia, Doctor Moliner, 50, 46100 Burjassot, Valencia, Spain
a r t i c l e i n f o
Article history:Received 9 June 2009Received in revised form 3 August 2009Accepted 4 August 2009Available online 3 September 2009
Keywords:Phosphorus recoveryStirred crystallizerStruviteAnaerobic digestion
0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.08.002
* Corresponding author. Tel.: +34 96 354 43 26; faxE-mail address: [email protected] (A. Seco).
a b s t r a c t
This work studied the influence of the characteristics of the supernatants on the struvite precipitationprocess. Eighteen experiments with the supernatants generated in an anaerobic digestion pilot plantwere performed in a stirred reactor. In order to obtain the pH control during the crystallization process,a Fuzzy Logic based controller was used. High phosphorus precipitation and recovery efficiencies wereobtained. The composition of the supernatants was analyzed in order to study its influence on the solidsformed from those solutions. The presence of calcium reduced the percentage of phosphorus precipitatedas struvite leading to the formation of amorphous calcium phosphate (ACP), which tended to be lost withthe effluent of the reactor. Calcite was also formed when supernatants with high magnesium:phosphorus(Mg/P) and calcium:phosphorus (Ca/P) molar ratios were employed. Some ammonium volatilization byconversion to NH3 occurred in all the experiments. The use of air to increase the pH to an adequate valueshowed to be feasible. Aeration cleaned struvite crystals from suspended solids, which makes aerationinteresting for struvite separation. However, aeration slightly increased the loss of phosphorus withthe effluent of the reactor and promoted ammonium volatilization.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Legislation and numerous policies are being implemented toincreasingly obligate wastewater treatment plants (WWTP) to re-move phosphate from effluent streams. Biological phosphorus re-moval treatment followed by anaerobic sludge digestion is one ofthe most suitable processes for phosphorus recovery. After sludgeseparation, subsequent liquors contain high concentrations of sol-uble phosphate and ammonium, and lower concentrations of solu-ble calcium and magnesium. Significant proportions of this solublephosphorus can be recovered by adding a crystallization processafter sludge digestion. Crystallization yields white orthorhombiccrystals known as magnesium ammonium phosphate hexahydrate(MAP), which is commonly known as struvite.
At present, phosphorus is obtained by exploitation of geologicalsources, and this is gradually diminishing natural supplies. Phos-phorus precipitation in the form of struvite from the centrates ofan anaerobically digested sludge would contribute to diminish thisphosphate rocks exploitation. Therefore, the benefits of applicationof a struvite precipitation process in a WWTP would be benefittwofold: to reduce the consumption of phosphate rocks, and to ob-tain a recovered product that can be used as a fertilizer (Ahmed
ll rights reserved.
: +34 96 354 48 98.
et al., 2006) or used as raw material in the phosphorus industry(Steén, 2004).
Apart from these benefits, being able to control struvite crystal-lization prevents scenarios such as spontaneous crystallization inwastewater treatment elements, preventing uncontrolled deposi-tion of struvite within the plant infrastructure. These depositionscan cause significant operational problems. These kinds of prob-lems are widely reported in the literature (Kummel et al., 2005;Barat et al., 2007).
Different studies about struvite formation from different liquorscan be found in the literature what show the importance of recov-ering phosphorus and the value of the struvite product. Betweenthese liquors urine (Ganrot et al., 2007), dairy manures (Uludag-Demirer et al., 2005), landfill leachates (Kim et al., 2007), andagro-industry wastes (Moerman et al., 2009) have been studied.
Currently, only few full-scale struvite crystallization plants ex-ist. This is due to the lack of information about the benefits of stru-vite as a fertilizer, its value on the market, and problems related tothe crystallization process such as the need to control the pH andthe cost of reagents.
Crystallization occurs in two stages: nucleation and crystalgrowth. Predicting or controlling these mechanisms is complexsince they are controlled by a combination of physical–chemicalparameters such as: pH of the solution from which struvite precip-itates, supersaturation, mixing energy, temperature, and presenceof foreign ions such as calcium. Due to the difficulty of modelling
L. Pastor et al. / Bioresource Technology 101 (2010) 118–125 119
all the processes involved in struvite precipitation (supersaturationdistribution inside the reactor, the dominating nucleation mecha-nisms, the phosphate thermodynamics, and chemistry), someexperiments to verify the feasibility of working under certain con-ditions must be carried out.
A previous work by this research group has focused on thestudy of struvite precipitation for phosphorus recovery as a routefor avoiding problems in sludge treatment processes (Marti et al.,2007, 2008). Furthermore, crystallization studies using syntheticliquors and a pilot scale stirred struvite reactor in order to investi-gate the conditions at which struvite precipitation can occur havebeen done (Pastor et al., 2008).
In this work, the supernatants generated in an anaerobic diges-tion pilot plant were used to carry out three types of experimentsin the same stirred reactor in order to study the influence of thesupernatant composition and the presence of high calcium concen-trations on the struvite precipitation. The possibility of reachingthe operational pH by aeration of the supernatants has been alsostudied. These parameters have been considered of great impor-tance from the point of view of full-scale operation of a struvitecrystallization plant. In this sense, it is important to reduce the costof the operation avoiding the use of reactants (air instead of chem-ical reagents) and it is also needed to determine the supernatantscharacteristics that optimize the struvite production in comparisonto other precipitates (calcium phosphates).
The aim of this work has been the employment of real liquorsfrom an anaerobic digestion pilot plant to precipitate struvite in or-der to validate the previous results obtained with synthetic liquors.Working with real supernatants allows studying uncontrollablefactors and the scale up of the precipitation process.
2. Methods
2.1. Crystallization pilot plant
The lay out of the crystallization pilot plant is shown in Fig. 1. Itwas composed of the crystallization reactor, three stainless steelinjection tubes for the influent and reactants, two peristalticpumps, one membrane pump, and two balances. The reactor wasa stirred tank reactor that was composed of two parts: a reactionzone, and a settling zone to prevent fine particles from being dri-ven out with the effluent. The reaction zone was designed accord-ing to the typical dimensions of a perfectly mixed reactor (Manginand Klein, 2004). This zone was equipped with four baffles to pre-vent the formation of vortexes and to favour mixing. The space be-tween the baffles and the tank wall prevents the accumulation of
pH + T
Conductivity
Peristaltic pump
Influent storage tank
Settling zone
Reaction zone
Fig. 1. Crystallizati
solid particles on the baffles. The settling zone was locatedabove the reaction zone and is cone-shaped with an angle of 45�between the two zones. This part was also equipped with a baffleto guide the flow. The effluent flowed out at the top of the settlingzone over a weir. The crystallization pilot plant and the reactor aredescribed in detail in Pastor et al. (2008).
To study the influence of aeration in the process a crown wasplaced at the bottom of the reactor to allow air to enter. The diam-eter of the crown was 75 mm (lower than the agitator diameter)and it was made of a silicone pipe with an internal diameter of6.4 mm. Holes with a diameter of 0.4 mm were equally distributedon its surface. The crown was placed under the agitator with theholes directed towards the top of the vessel. This placement al-lowed the air bubbles to be broken into smaller ones when theypassed through the agitator.
2.2. Substrates
Centrates obtained after centrifuging an anaerobically digestedsludge and supernatants from the thickener of an anaerobic diges-tion pilot plant were used as a feed stream to the crystallizer.
The anaerobic digestion pilot plant consisted of a gravity thick-ener, an anaerobic digester, and a secondary digester, all of whichwere built with polypropylene. The anaerobic digester was a com-pletely mixed type reactor of 160 l in effective volume. The contentof the digester was completely mixed by the recirculation of thetank content using alternative pumped cycles. The reactor wasequipped with redox, pH, and temperature electrodes. Digestedsludge was collected in the secondary digester for its laterdewatering.
The anaerobic digester and the crystallization reactor describedabove are part of a group of pilot plants that are operated to studybiological nutrient removal (BNR) and phosphorus recovery bystruvite precipitation. These pilot plants are located in theCarraixet WWTP in Valencia (Spain) and are operated for fermen-tation of primary sludge (Bouzas et al., 2007), biological nitrogenand phosphorus removal (Garcia-Usach et al., 2006), and anaerobicdigestion of waste sludge (Marti et al., 2007). The pilot plants arefed with 40 l/h of raw municipal wastewater from the degritterof the full-scale WWTP.
2.3. Experimental procedure
Eighteen crystallization experiments were carried out using thesupernatant obtained after centrifuging the anaerobic digestionsludge (centrate). In some experiments, the supernatant from the
Peristaltic pump
Membrane pump
Balance
Balance
Computer
MgCl2⋅6H2O
NaOH
on pilot plant.
120 L. Pastor et al. / Bioresource Technology 101 (2010) 118–125
thickener that is placed before the anaerobic digester was also em-ployed. In the experiments where both of these supernatants wereused, the supernatants were mixed in the same proportions atwhich they were produced.
The experiments were carried out to study the feasibility ofrecovering phosphorus as struvite. The characteristics of the li-quors employed as a feed stream, the high calcium concentrationin the influent and the use of aeration to increase the pH werethe parameters studied.
The influent characteristics in the experiments were mainlydetermined by the concentrations of the supernatants employed.The Mg/P molar ratio in the inlet stream was the only parameteradjusted by the addition of MgCl2�6H2O to achieve the desired va-lue. The Mg/P, N/P, and Ca/P molar ratios studied varied between0.5–1.8, 5.0–23.1, and 0.2–2.3, respectively.
The crystallization reactor was operated in continuous mode forthe liquid phase and in batch mode for the solid phase. This way ofoperating provided enough solids for growth and for obtaininglarge crystals. The hydraulic retention time in the reaction zonewas maintained around 2.5 h in all the experiments carried out.
Samples of the supernatants used as a feed solution were takenevery day to measure total phosphorus, orthophosphate, ammo-nium, dissolved magnesium, calcium, and potassium. Partial alka-linity (ALKP), and total (TSS) and volatile (VSS) suspended solids inthe feed solutions were also analyzed (data not shown). The efflu-ent of the crystallization reactor was analyzed for total phospho-rus, orthophosphate, ammonium, total and dissolved magnesium,calcium, and potassium. Effluent total concentration samples wereprepared by adding nitric acid (HNO3) with agitation to dissolve allexisting solid particles that could had been carried out. Solublesamples of the effluent were prepared by filtering the samples with0.45 lm filter paper and testing the liquors. All the analyses wereperformed in accordance with Standard Methods (APHA, 2005).Phosphorus and ammonium analyses used a standard spectropho-tometer technique. Magnesium, calcium, and potassium analyseswere carried out by atomic absorption. At the end of the experi-ments, the precipitated solids inside the reactor were recoveredfrom the reaction zone and were air dried. These solids were ana-lyzed by X-ray diffraction (XRD) in order to check whether or notstruvite crystals were formed. The XRD measurements were per-formed on a Bruker AXS D5005 powder diffractometer.
2.4. Fuzzy Logic for pH control
To achieve the desired operation pH in the reactor, a NaOH0.4 M solution was added. The pH was controlled by employingthe Fuzzy Logic algorithm control described in Chanona et al.(2006).
In four of the eighteen experiments, the required pH wasachieved by CO2 stripping with air. The above-mentioned algo-rithm was modified in order to achieve a pH control by aeration in-stead of by NaOH addition. The modification consisted of a changein the manipulated variable. In this case, the manipulated variablewas the opening percentage of the air valve that allows air to enterthe reactor. In those cases where aeration was not enough toachieve the desired pH, the algorithm control kept the air valvecompletely opened (100%) and started to act over the NaOH pumpas described in Chanona et al. (2006).
3. Results and discussion
3.1. Supernatant characteristics. Process efficiencies
The composition of the feed solutions (supernatants) deter-mined the composition of the influent stream to the crystallizer.
The parameter a indicates the fraction of raw sludge liquid to totalsludge liquid in the resulting solutions. It is defined by
a ¼ raw sludge supernatantdigested sludge supernatantþ raw sludge supernatant
ð1Þ
The PO4–P and NH4–N concentrations in the feed solutions var-ied between 50 and 170 mg/l and between 100 and 700 mg/l,respectively. The Mg2+ concentration in the supernatants did notshow high variability. Nevertheless, in some cases the Mg/P molarratio in the supernatants was lower than 1, this meant a deficit inmagnesium for struvite precipitation, making addition of extramagnesium necessary. A MgCl2�6H2O solution (720 mg Mg2+/l)was used as a source of extra magnesium in those experiments.
The calcium concentration in the supernatants varied between35 and 160 mg/l. The relatively high calcium concentration wasdue to the hardness of the surface water in the Mediterranean areaof Spain.
Table 1 shows the final composition of the solutions enteringthe reactor after mixing the three flow rates (supernatants, theNaOH solution and the MgCl2�6H2O solution when added) as wellas the operational conditions of each experiment. This table alsoindicates if the pH was adjusted by the addition of NaOH or by aer-ation. The working pH was fixed at 8.7 except for one experiment,which was carried out at pH 8.5. A previous study working withsynthetic supernatants (Pastor et al., 2008) has shown good resultsworking at this pH (8.7) with regard to phosphorus precipitationand recovery efficiencies and reagent consumption.
To study the extension of phosphorus precipitation and phos-phorus recovery, two types of efficiencies were calculated for eachexperiment: recovery and precipitation. The precipitation effi-ciency represents the maximum phosphorus precipitation from athermodynamic point of view. It was calculated as the ration be-tween the difference of the influent and effluent orthophosphateconcentration and the influent orthophosphate concentration.Conversely, the recovery efficiency takes into account both precip-itation and crystal growth efficiency and it was calculated on thebasis of total phosphorus concentrations in the influent and efflu-ent streams. Similar recovery and precipitation efficiencies in eachexperiment indicate that large struvite crystals have precipitatedwith minimal loss of fine crystals with the effluent. The differencebetween the two efficiencies (Defficiencies) is related to the presenceof fines in the effluent stream and with the quantity of phosphoruslost with the effluent.
The efficiencies obtained for each experiment are shown in thelast columns of Table 1. Precipitation and recovery efficiencies val-ues between 78% and 95% and between 46% and 86% were ob-tained, respectively.
These results show the feasibility of recovering phosphorusfrom the supernatants obtained in an anaerobic digestion pilotplant employing an agitated crystallizer. Nevertheless it is neces-sary to carry out an overall mass balance for P in the whole pro-cess (anaerobic digestion plus crystallization process) to evaluatethe significance of the recycling process. It must be taken into ac-count that not all the phosphorus in the sludge is released to thesupernatants. This P mass balance study is beyond the scope ofthis study. However, the work carried out by Marti et al. (2008)has demonstrated that with a correct sludge line managementup to 68% of the soluble phosphorus in the system could be avail-able for the crystallization process after the anaerobic digestionprocess.
3.2. Solids precipitated
In order to identify the solids precipitated inside the reactor, thefollowing analyses were carried out:
Tabl
e1
Char
acte
rist
ics
ofth
ein
fluen
tst
ream
,ope
rati
onal
cond
itio
ns,a
ndef
fici
enci
esac
hiev
ed.
Exp
Infl
uen
tst
ream
Infl
uen
tm
olar
rati
ospH
Ope
rati
onal
con
diti
ons
Effi
cien
cies
P t(m
g/l)
PO4–P
(mg/
l)N
H4–N
(mg/
l)M
g2+
(mg/
l)C
a2+
(mg/
l)K
+(m
g/l)
aM
gM
g/P
N/P
Ca/
PC
a/M
gpH
pHad
just
HR
Th
T(�
C)
Rec
Prec
Def
fici
enci
es
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
adde
dm
olar
mol
arm
olar
mol
ar
Infl
uen
tco
mpo
siti
onR
115
4.7
±2.7
150.
6±4
.154
1.7
±17.
293
.2±2
.569
.0±1
.621
3.1
±5.5
0.0
Yes
0.8
8.0
0.3
0.5
8.7
NaO
H10
.721
.080
.289
.69.
4R
213
0.8
±1.8
127.
4±3
.160
4.0
±21.
111
0.4
±4.4
92.0
±4.8
231.
3±7
.00.
0Y
es1.
110
.50.
60.
58.
7N
aOH
10.5
23.7
83.1
95.4
12.4
R3
146.
4±3
.014
0.6
±5.4
474.
6±1
1.2
113.
4±2
.767
.4±1
.417
9.4
±3.4
0.0
Yes
1.0
7.5
0.4
0.4
8.7
NaO
H10
.524
.386
.793
.87.
2R
415
5.3
±2.5
145.
8±3
.535
5.2
±7.8
104.
5±2
.565
.8±0
.218
5.7
±4.2
0.4
Yes
0.9
5.4
0.4
0.4
8.7
NaO
H10
.828
.485
.189
.64.
5R
514
8.5
±3.9
141.
1±7
.952
6.4
±6.9
86.9
±5.9
81.1
±4.5
202.
2±5
.00.
0Y
es0.
88.
30.
40.
68.
5N
aOH
10.5
26.3
69.2
78.6
9.4
R6
168.
7±1
.215
5.4
±3.5
605.
6±2
2.5
60.9
±1.8
34.4
±2.4
256.
1±2
.20.
0Y
es0.
58.
60.
20.
38.
7N
aOH
10.5
24.8
46.1
60.9
14.8
R7
85.7
±1.8
73.4
±1.4
463.
8±1
2.2
53.2
±3.2
44.7
±5.3
240.
3±4
.40.
2Y
es0.
914
.00.
50.
68.
7N
aOH
11.1
24.9
72.4
82.7
10.4
Aer
atio
nR
879
.2±2
.765
.2±2
.846
5.4
±31.
050
.7±2
.865
.9±2
.616
3.6
±3.5
0.2
No
1.0
15.4
0.8
0.8
8.7
Air
12.5
21.4
54.7
79.5
24.8
R9
69.9
±2.2
57.9
±2.5
483.
4±2
4.9
36.1
±1.3
73.1
±6.2
193.
3±4
.90.
2N
o0.
818
.51.
01.
38.
7A
ir12
.422
.145
.383
.237
.9R
1094
.1±3
.188
.2±2
.353
4.3
±16.
566
.5±5
.067
.2±4
.614
7.7
±4.9
0.1
Yes
1.0
13.5
0.6
0.6
8.7
NaO
H11
.226
.172
.882
.09.
2R
1110
8.0
±4.3
96.0
±2.9
581.
4±1
9.1
60.8
±2.2
78.6
±2.4
150.
4±3
.80.
1Y
es0.
813
.20.
70.
88.
7A
ir+
NaO
H11
.622
.475
.487
.812
.3R
1271
.7±2
.065
.9±2
.040
3.3
±12.
450
.2±1
.674
.7±4
.016
3.8
±3.4
0.1
No
1.0
13.4
0.9
0.9
8.7
NaO
H11
.029
.281
.287
.46.
2R
1379
.2±1
.969
.7±2
.841
1.9
±8.3
49.7
±4.5
74.0
±5.1
174.
4±3
.60.
1N
o0.
913
.10.
80.
98.
7A
ir11
.221
.773
.188
.915
.9H
igh
Ca
con
cen
trat
ion
R14
72.4
±3.2
66.3
±4.9
688.
6±1
9.0
61.3
±1.7
156.
1±4
.022
4.5
±6.4
0.0
Yes
1.2
23.1
1.8
1.5
8.7
NaO
H11
.726
.166
.193
.927
.8R
1561
.3±1
.250
.1±3
.040
0.4
±7.3
69.7
±1.6
91.9
±2.5
102.
2±1
.70.
4N
o1.
817
.81.
40.
88.
7N
aOH
11.9
24.5
79.5
84.9
5.4
R16
63.9
±2.7
51.8
±4.4
436.
4±1
2.5
53.0
±1.7
73.6
±4.7
126.
7±1
.40.
2N
o1.
319
.41.
10.
88.
7N
aOH
12.0
23.5
72.3
87.0
14.7
R17
46.4
±2.9
43.0
±2.5
129.
0±2
.149
.8±0
.912
9.9
±3.2
35.3
±0.8
0.9
No
1.5
6.7
2.3
1.6
8.7
NaO
H11
.821
.969
.983
.013
.1R
1854
.7±3
.344
.3±1
.199
.6±0
.958
.7±0
.611
3.3
±2.8
66.7
±2.2
0.9
No
1.7
5.0
2.0
1.2
8.7
NaO
H11
.727
.061
.778
.817
.1
av.:
aver
age;
sd.:
stan
dard
devi
atio
n.
L. Pastor et al. / Bioresource Technology 101 (2010) 118–125 121
– Chemical analyses of the solids collected.– Mass balances in the reactor taking into account the average
concentrations of the influent and effluent streams.– X-ray analyses of the solids collected.
3.2.1. Chemical analysesThe chemical analyses of the solids precipitated and collected in
the experiments were carried out dissolving the solids with nitricacid and analysing the resulting solutions for PO4–P, NH4–N,Mg2+, Ca2+, and K+. Equal molar concentrations of PO4–P, NH4–N,and Mg2+ were obtained in those solutions, which indicates thatthe solids collected were struvite according to struvite compositionP:N:Mg = 1:1:1. Neither calcium nor potassium was found in thesolutions. This can not confirm that calcium phosphates or potas-sium compounds did not precipitate as they could have been lostwith the effluent.
3.2.2. Mass balancesIn order to check which precipitates were formed during the
experiments mass balances to the reactor were carried out. Table2 shows the mmol/l of PO4–P, NH4–N, Mg2+, Ca2+, and K+ precipi-tated that were calculated by the difference of soluble concentra-tions in the influent and the effluent streams (Tables 1 and 2).The effluent concentrations shown in Table 2 are the average con-centrations once the stationary state was achieved at the end of theexperiments. Phosphorus, ammonium, magnesium, and calciumprecipitated in all the experiments. Potassium was not removedin any of the experiments, indicating that potassium struvite(MgKPO4�6H2O) had not been formed, probably due to the highammonium concentration in the experiments. Different authors(Schuiling and Andrade, 1999; Wilsenach et al., 2007) have ob-served that potassium struvite (KMgPO4�6H2O) could precipitateinstead of MAP only in the case of low ammonium concentrations,which it is not the case in any of the experiments carried out.
Moreover, in order to check if potassium struvite precipitatedthe Saturation Index (SI) for potassium struvite in the solution in-side the reactor were calculated. The SI is used to describe the sat-uration state of the aqueous phase composition versus differentsolids (Eq. (2)). When SI = 0, the solution is in equilibrium; whenSI < 0, the solution is undersaturated and no precipitation occurs;when SI > 0, the solution is supersaturated and precipitation occursspontaneously. Therefore, the SI values can be used to evaluate theeffect of the solution composition on the tendency and extent ofthe precipitation, according to the following equation:
SI ¼ logIAPKsp
ð2Þ
where IAP represents the ion activity product and Ksp represents thethermodynamic solubility product.
The SI for potassium struvite in the solution inside the reactorwere calculated using the equilibrium speciation model MINTEQA2(Allison et al., 1991), assuming complete mixing of the three feedflows. The activity coefficients were obtained from the Daviesequation with a Davies B parameter of 0.3. This equation is quitesatisfactory for values of ionic strength up to about 0.2 mol/l (Da-vies, 1962). The ionic strength of the feed solutions used in allthe experiments was between 0.03 and 0.08 mol/l. The potassiumstruvite pKsp used was 10.6 (Taylor et al., 1963). The negative val-ues for the potassium struvite pKsp obtained in all the experimentsshowed that the solution was undersaturated at the conditionsachieved in the reactor, which proved that no potassium struviteprecipitated.
The possible precipitates that can appear when working withsolutions containing Mg2+, PO3�
4 , NHþ4 , and CO2�3 are struvite,
Tabl
e2
Efflu
ent
char
acte
rist
ics
and
mm
ol/l
prec
ipit
ated
ofth
edi
ffer
ent
ions
invo
lved
.
Exp
Effl
uen
tst
ream
mm
ol/l
prec
ipit
ated
P mea
/Pca
l
P t(m
g/l)
PO4–P
(mg/
l)N
H4–N
(mg/
l)M
g t(m
g/l)
Mg2
+(m
g/l)
Ca t
(mg/
l)C
a2+
(mg/
l)K
t(m
g/l)
K+
(mg/
l)PO
4–P
NH
4–N
Mg2
+C
a2+
K+
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
av.
sd.
mm
ol/l
mm
ol/l
mm
ol/l
mm
ol/l
mm
ol/l
Infl
uen
tco
mpo
siti
onR
130
.6±9
.115
.7±2
.146
6.0
±11.
514
.4±3
.47.
6±0
.578
.9±1
3.1
51.7
±0.3
217.
1±2
.021
3.1
±4.1
4.4
5.4
3.5
0.4
0.0
1.1
R2
22.2
±1.0
5.8
±1.1
510.
2±1
2.1
23.8
±1.3
20.4
±1.0
106.
5±2
.577
.4±0
.823
0.9
±3.9
230.
8±3
.93.
96.
73.
70.
40.
01.
0R
319
.5±2
.88.
7±1
.839
8.9
±11.
321
.6±1
.219
.8±0
.381
.2±3
.665
.0±0
.817
7.6
±2.6
177.
4±2
.74.
35.
43.
90.
10.
01.
1R
423
.1±0
.515
.2±1
.228
9.5
±0.9
18.9
±0.6
13.8
±0.1
66.7
±3.7
55.1
±0.1
185.
1±3
.518
4.5
±2.4
4.2
4.7
3.7
0.3
0.0
1.1
R5
45.8
±0.4
30.2
±0.1
442.
7±5
.38.
7±1
.35.
5±0
.277
.4±4
.354
.8±1
.520
1.3
±4.3
202.
0±2
.33.
66.
03.
40.
70.
00.
9R
690
.9±5
.660
.8±2
.052
1.5
±8.2
4.6
±0.2
1.6
±0.4
60.7
±2.5
20.5
±0.3
254.
4±2
.925
2.0
±5.8
3.1
6.0
2.4
0.3
0.1
1.1
R7
23.7
±1.0
12.7
±0.2
383.
5±8
.815
.5±0
.49.
7±0
.058
.8±2
.030
.3±3
.124
1.9
±4.3
241.
1±1
.52.
05.
71.
80.
40.
01.
0A
erat
ion
R8
35.9
±1.4
13.4
±0.3
417.
9±0
.024
.8±1
0.2
11.4
±1.2
62.8
±1.3
57.0
±0.0
163.
9±3
.316
3.3
±2.5
1.7
3.4
1.6
0.2
0.0
1.0
R9
38.2
±1.1
9.7
±1.0
344.
0±0
.017
.9±2
.28.
2±0
.193
.3±4
.446
.6±0
.019
2.9
±1.6
193.
8±0
.61.
610
.01.
10.
70.
01.
0R
1025
.6±1
.115
.9±0
.848
7.5
±4.1
18.3
±1.9
14.2
±0.7
77.1
±4.6
63.5
±2.2
147.
2±1
.514
6.4
±1.1
2.3
3.3
2.2
0.1
0.0
1.1
R11
26.5
±0.2
11.7
±1.1
449.
3±3
.611
.7±0
.96.
7±1
.080
.5±7
.263
.9±0
.715
0.3
±2.6
147.
0±1
.62.
79.
42.
20.
40.
11.
1R
1213
.5±0
.28.
3±0
.138
8.8
±7.9
18.2
±0.6
15.4
±0.2
67.3
±2.2
42.1
±0.7
164.
6±5
.416
0.8
±4.9
1.9
1.0
1.4
0.8
0.1
0.9
R13
21.3
±2.8
7.7
±0.5
316.
5±1
5.3
19.1
±2.3
9.5
±0.3
90.7
±2.2
55.6
±4.4
171.
1±4
.317
0.3
±5.2
2.0
6.8
1.7
0.5
0.1
1.0
Hig
hC
aco
nce
ntr
atio
nR
1424
.5±5
.34.
1±0
.460
5.0
±7.0
30.3
±0.4
27.8
±0.5
124.
2±1
.595
.2±7
.723
0.2
±2.8
225.
8±3
.42.
06.
01.
41.
50.
00.
8R
1512
.6±0
.37.
6±0
.037
1.3
±4.5
39.2
±0.2
36.8
±0.1
73.1
±0.0
52.1
±3.5
98.3
±2.9
98.9
±3.8
1.4
2.1
1.4
1.0
0.1
0.7
R16
17.7
±0.7
6.7
±0.0
351.
1±8
.827
.4±0
.020
.7±0
.787
.3±0
.046
.4±3
.412
5.0
±0.0
125.
4±0
.91.
56.
11.
30.
70.
00.
8R
1714
.0±0
.57.
3±0
.312
3.7
±3.3
40.4
±0.5
39.4
±0.8
81.8
±1.8
71.6
±2.4
36.1
±0.5
35.9
±0.4
1.2
0.4
0.4
1.5
0.0
0.8
R18
20.9
±0.8
9.4
±0.4
86.2
±2.3
57.7
±2.1
51.0
±0.8
110.
9±1
5.5
51.2
±1.3
67.5
±1.8
65.4
±2.1
1.1
1.0
0.3
1.6
0.0
0.8
av.:
aver
age;
sd.:
stan
dard
devi
atio
n.
122 L. Pastor et al. / Bioresource Technology 101 (2010) 118–125
magnesite (MgCO3) and newberyite (MgHPO4�3H2O). According toMusvoto et al. (2000) struvite precipitates at neutral and higher pHand at Mg/Ca molar ratios >0.6; newberyite precipitates signifi-cantly only at lower pH (<6.0) and at high concentrations of mag-nesium and phosphorus. Therefore, due to the Mg/Ca molar ratiosin the experiments (>0.6) and the working pH (>6) it was not con-sidered newberyite as a possible precipitate. Moreover, the nega-tive value of the SI for newberyite in the solution calculated withMINTEQA2 confirmed that newberyite not precipitated. Normally,magnesite precipitates in negligible quantities in the supernatantscoming from an anaerobic digestion process (van Rensburg et al.,2003).
Taking into account the above statements and the results ob-tained from the chemical analyses of the solids collected, the onlymagnesium compound considered to be formed was struvite.Therefore, the quantity of phosphorus precipitated as struvite(PMAP) can be calculated from the magnesium precipitated. Table2 shows that, in all the experiments, equal or higher moles of phos-phorus than magnesium precipitated. The phosphorus that did notprecipitate as struvite was considered to precipitate as some cal-cium phosphate (Pprec Ca).
Calcium can precipitate as calcium phosphate, calcite (CaCO3)and dolomite (CaMg(CO3)2). Nevertheless, Mamais et al. (1994)mentioned that dolomite has very low precipitation rates. The pre-cipitation rate of calcite is influenced by the presence of magne-sium, iron, phosphates, and dissolved organic compounds, whichdiminish its precipitation rate and increase its solubility (Plantand House, 2002). Due to the low precipitation rate of dolomiteand the relative high concentrations of magnesium in the reactorcalcite and dolomite were not considered to easily precipitate inthe experiments carried out.
Even though hydroxylapatatite (HAP) is the most stable ther-modynamically form of calcium phosphate, the presence of mag-nesium ions in the solutions can reduce the solubility of the ACPand increase the time needed for the ACP to transform into HAP.There are many parameters that can affect this transformationsuch as impurities, ageing time, pH, ACP particle size, agitationspeed, and solution composition (Li et al., 2007). A previous studyby Pastor et al. (2008) that used the reactor described above andsolutions prepared in the laboratory showed that the transforma-tion of ACP to HAP does not take place within the length of timethat the experiment last. Moreover, samples of the crystals takenin the experiments with the highest calcium concentrations inthe influent did not show any other crystals than the ones withthe typical struvite shape. Based on these results, the calciumphosphate precipitated is considered to be amorphous calciumphosphate (ACP).
The last column of Table 2 shows the ratio between the moles ofprecipitated phosphorus that were obtained from the analyses atthe influent and effluent streams of the reactor (Pmea) and the mo-les of precipitated phosphorus that were calculated (Pcal) from themoles of magnesium and calcium that were precipitated in thereactor (Pcal = PMAP + Pprec Ca), assuming that the solids formed werestruvite and ACP. If this ratio (Pmea/Pcal) had a value around 1, theassumption about the solids formed would be correct. As Table 2shows, in most of the cases the ratio Pmea/Pcal is close to 1. Never-theless, the ratio obtained in experiment R15 has a value of 0.7,indicating that not all the calcium precipitated was precipitatedas ACP. Therefore, calcite could also have been formed. In this sameexperiment the Mg/P and Ca/P molar ratios were high (1.8 and 1.4,respectively), so the excess of calcium that did not precipitate withphosphorus could have precipitated as calcite. In other experi-ments carried out under similar conditions, the ratio Pmea/Pcal tooka value of 0.8, indicating that calcite could also have been formed.
Assuming that struvite was formed, the precipitation of phos-phorus, ammonium, and magnesium should have been equimolar.
L. Pastor et al. / Bioresource Technology 101 (2010) 118–125 123
Table 2 shows that in all the experiments higher moles of ammo-nium than magnesium were removed. This was due to the ammo-nium volatilization by conversion to NH3. It can also be observedthat aeration (experiments R9, R11, and R13) promoted the volatil-ization of ammonium. Aeration did not promote the volatilizationin experiment R8 due to the low air flow rate used in this experi-ment. This nitrogen volatilization must be taken into account whenworking on the industrial scale since the NH3 is a greenhouse gas.
3.2.3. X-ray analysesThe XRD analyses resulted in the presence of struvite in all the
experiments carried out. The good correlation between the peaksof the diffractograms obtained for the solids collected in the reac-tor and the peaks of the struvite pattern confirmed that the solidsobtained were struvite.
3.3. Aeration
3.3.1. pH control by aerationFour experiments (R8, R9, R11, and R13) were carried out
adjusting the pH by aeration of the solution inside the reactor. Aer-ation promotes CO2 stripping from the supernatant and the conse-quent increase in pH.
Fig. 2 shows the profiles of the pH and the percentage of the airvalve opening in experiment R9, which was carried out by aeration.It can be observed that the Fuzzy Logic algorithm control was able
0
10
20
30
40
50
60
70
80
90
100
0 10 20 3Time (h)
%
Fig. 2. pH and percentage of the air v
0
10
20
30
40
50
60
70
80
90
100
0.02 0.12 0.22 0.32 0.42SIMAP
% P
MAP
Fig. 3. Percentage of phosphorus precipitated that has fo
to maintain the pH at a stable value when the experiments wereconducted with aeration. Moreover, the algorithm was able toachieve an adequate pH (8.7) in the reactor by aeration only, whatwould reduce the cost in reagents. Battistoni et al. (1997) and Su-zuki et al. (2002) studies have already shown the feasibility toreach pH values around 8.5–8.6 only by aeration.
It was in only one experiment (R11) where aeration was not en-ough to increase the pH to a value of 8.7. In this case, the controlalgorithm started to act over the NaOH pump leaving the air valvecompletely opened. The need for extra NaOH in this experimentwas due to the low alkalinity as compared to the other experi-ments that were carried out with aeration. Moreover, the quantityof phosphorus and magnesium precipitated in this experiment washigher than in the others (Table 2), which produced a higher de-crease in the pH according to struvite precipitation reaction.
3.3.2. Influence of aeration on the process efficiencies and crystalsformed
Experiments R8 and R9 were carried out to check the possibilityof increasing the pH by aeration and to test the Fuzzy Logic algo-rithm control. Then, two pairs of experiments, R10–R11 andR12–R13 were carried out to study the influence of aeration onphosphorus precipitation and recovery efficiencies and on the crys-tals formed. Each pair of experiments was carried out using thesame feed solution. The only difference between them was theway the pH was increased: by the addition of NaOH or by aeration.
0 40 507.0
7.5
8.0
8.5
9.0
9.5
10.0
pH
% Air valvepHset point
alve opening in experiment R9.
0.52 0.62 0.72 0.82 0.92/SIACP
rmed struvite as a function of the ratio SIMAP/SIACP.
124 L. Pastor et al. / Bioresource Technology 101 (2010) 118–125
Table 1 shows the influent concentrations, the operational con-ditions, and the efficiencies achieved in these experiments. Theparameter Defficiencies (Table 1) indicates the quantity of phospho-rus lost with the effluent in a precipitated form. The difference be-tween the experiments with and without aeration is that aerationslightly increased the loss of phosphorus in the form of crystalswith the effluent. The upwards air flow was enough to maintainthe smallest crystals in suspension in the upper part of the reactorwhat allowed these crystals to go out with the effluent of thereactor.
Pictures (data not shown) of crystal samples from the upperpart of the reactor and from the reaction zone in experimentsR12 (NaOH) and R13 (air) were taken from the agitated solution in-side the reactor during the course of the experiments. It was ob-served that the air slightly pulled the crystals upward indicatingthat aeration slightly increased the loss of phosphorus in the formof crystals with the effluent. Moreover it was also observed that theair cleaned the reaction zone from suspended solids. Thus, aerationwould be suitable for a subsequent cleaning and separation of thestruvite crystals formed.
3.4. Influence of calcium concentration
Since all the magnesium precipitated formed struvite, the per-centage of precipitated phosphorus that precipitated as struvite(%PMAP) can be calculated by
% PMAP ¼Mg2þ
prec
PO4 � Pprec� 100 ð3Þ
This %PMAP has been represented in Fig. 3 as a function of the ra-tio of the saturation index for struvite and the saturation index forACP (SIMAP/SIACP) since the solids that were formed were mainlystruvite and ACP. The struvite pKSMAP used for the SIMAP calculationwas 13.31 (Pastor et al., 2008), and the ACP pKSACP used was 26.52,which was proposed by Seckler et al. (1996).
Fig. 3 shows that the ratio of the saturation indexes controls theformation of MAP or ACP. It can be observed that the presence ofcalcium, which increases the ACP saturation index and reducesthe SIMAP/SIACP ratio, diminished the percentage of phosphorus pre-cipitated as struvite.
Table 1 shows that the experiments with high calcium concen-trations in the influent lost a high quantity of precipitated phos-phorus with the effluent of the reactor (higher values of theparameter Defficiencies). As stated above, the presence of calciumin the influent diminishes the percentage of phosphorus precipi-tated as struvite and gives rise to the formation of ACP. Le Correet al. (2005) showed in their work that an increase in the calciumconcentration diminished the struvite crystal size and inhibited itsgrowth. Therefore, the influence of calcium on the loss of phospho-rus with the effluent can be due to:
– The formation of ACP, which tends to be lost with the effluent(Pastor et al., 2008).
– The influence of calcium on the struvite particle size, whichfavours struvite loss with the effluent.
4. Conclusion
The agitated crystallizer has shown to obtain satisfactory re-sults for the struvite precipitation (78–95% of precipitation effi-ciency and 46–86% of recovery efficiency). The solids formedwere mainly struvite, followed by amorphous calcium phosphate,and then by calcite. Potassium struvite did not precipitate in anyexperiment.
A pH stable value was achieved both with NaOH and aerationusing a Fuzzy Logic based controller.
The presence of calcium diminished the percentage of phospho-rus precipitated as struvite and gave rise to ACP formation. More-over, high calcium concentration in the influent increased thephosphorus lost with the effluent.
Acknowledgements
This research work has been supported by the Spanish ResearchFoundation (MCYT, Project PPQ2002-04043-C02-01), which isgratefully acknowledged. CEEP is also gratefully acknowledged.
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