Ind. Eng. Chem. Res. 2005, 44, 6701-6707
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PROCESS DESIGN AND CONTROL Phosphorus Removal from Supernatants at Low Concentration Using Packed and Fluidized-Bed Reactors Paolo Battistoni,*,† Barbara Paci,† Francesco Fatone,‡ and Paolo Pavan§ Institute of Hydraulics and Transportation Infrastructures, Engineering Faculty, Marche Polytechnical University, Via Brecce Bianche, 60131 Ancona, Italy, Department of Science and Technology, University of Verona, Strada Le Grazie 15, 37134 Verona, Italy, and Department of Environmental Sciences, University of Venice, Dorsoduro, 2137-30123 Venice, Italy
The paper deals with a treatment for obtaining phosphorus recovery from anaerobic supernatants with low phosphate concentration (30-50 mg PO4-P/L). The study was carried out at demonstrative scale (inflow up to 2 m3/d), and operative conditions as fluidized or packed bed were investigated. To determine the best operative conditions in the fluidized bed reactor, two upward velocities of 42 and of 64 m/h were performed. Very high phosphorus removal (up to 75%) was observed, and the real performances were similar to those predicted by the mathematical model. Regarding the fluidized bed operative condition into the reactor, the average value of V′10 was maintained at 110 mL/L throughout the treatment of 234 m3 of anaerobic supernatants. In the second part of the experiment, the polyelectrolyte escape from the dewatering station caused the packing of a great part of the bed, and only a minimum content of the grains remained fluidized (20 000 mg/L). Good performances in phosphorus removal were observed (45-55%), but they were lower than those expected in the fluidized bed (64-69%). Big crystal dimensions in the bottom of the reactor were shown in the packed bed (φ50 ) 0.5-1.4 mm), whereas they maintained a dimension up to 0.5 mm in the fluidized bed. Suspended grains showed a dimension strictly related to the recycle flow rate (φ50 ) 0.21 mm at 12 m3/h and -φ50 ) 0.35 mm at 18 m3/h). The phosphorus mass balance showed a production of ∼60 kg of grains in the fluidized bed and 111 kg in the packed bed during the experimental periods. Both were mainly constituted by struvite (97%) with smaller amounts of calcite and hydroxyapatite. A low content of organic substances in phosphorus salts extracted from the bottom of the reactor was quantified by a COD to a TS ratio of 2-3%. Introduction The sludge line supernatants in biological nutrient removal (BNR) wastewater treatment plants (WWTP) undergo a gradual enrichment of nitrogen and phosphorus content. This is due to the hydrolytic and fermentative processes that come about in the succession of physical and biological treatments which the sludge undergoes. In BNR plants, the high phosphorus concentration in supernatants is strictly related to phosphorus released from P-accumulating organisms (PAO) under anaerobic conditions and in the presence of volatile fatty acids (VFA).1 To keep high P removal performances in the main stream, this phenomenon imposes an adequate treatment of the sludge or the only supernatants before their recirculation. Among the different available technologies, the recovery of phosphorus as a readily reusable salt, such as struvite (MAP) or hydroxipatite (HAP), is * To whom correspondence should be addressed. E-mail:
[email protected]. † Marche Polytechnical University. ‡ University of Verona. § University of Venice.
certainly to be preferred.2 The main phosphorus fertilizers in use today are TSP (triple super phosphate) with 47% of P2O5; DAP (diammonium phosphate) with 18% of N, 46% of P2O5; and MAP (magnesium ammonium phosphate) with 12% of N and 52% of P2O5. In these three fertilizers, the greater part of phosphorus is watersoluble, and they are often combined with other types, so as to supply nourishment like K and S. Phosphate salt recovery avoids disposing of sludge produced by phosphorus chemical precipitation; furthermore, a recyclable phosphate product balances the costs of the phosphorus removal treatment. Other advantages of a plant that is able to produce pellets are linked to its easier design and less costly construction. Estimating the feasibility of the phosphorus recovery, considering also the economic aspect, it has been observed that the minimal potentiality of the plant should probably be 30 000 EI.3 The most used technologies for phosphorus removal from wastewater by crystallization are DHV Crystallactor and the struvite crystallization processes (SCP). The DHV Crystallactor system consists of a degassing device, a fluidized bed reactor partially filled with seed material, and two pressure filters.4 According to this process, the dosage of concentrated sulfuric acid is
10.1021/ie050186g CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
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necessary for carbon dioxide degassing process, whereas lime is requested to gain the stoichiometric ratio for precipitation of calcium phosphates (Ca3(PO4)2). Otherwise, no reagent dosages are requested by the auto nucleation process in fluidized bed reactors, the SCP process. Here, only the chemical-physical properties of the anaerobic supernatants are used, since the operative pH conditions are reached by stripping CO2 with air. To crystallize phosphorus as MAP or HAP, it is necessary to perform the process in the zone between thermodynamic and kinetic equilibrium conditions defined as metastable condition. Such a condition can be controlled with an appropriate operative pH.5 The methodology is simple, and it is based on the use of an up-flow reactor in which a number of feeding recycles are performed. In this case, the generated turbulence by the fall of the recirculation current is enough to maintain the operative pH with a consequent energy saving.6 A methodology to evaluate reactor performances is based on the calculation of three parameters: nucleation efficiency, that is, the quantity of phosphate removed and recovered as grains (η%, eq 1); global phosphate conversion (X%, eq 2); and fines precipitation efficiency (L%, eq 3), that is, the phosphate present as small particles in the effluent.
η% ) 100
(PO4,totin - PO4,totout) PO4,totin
X% ) η% + L% ) 100 L% ) 100
(PO4,totin - PO4,solout) PO4,totin
(PO4,totout - PO4,solout) PO4,totin
(1)
(2)
(3)
where Ptot is the phosphate measured in unfiltered acidified samples and Psol is the phosphate measured in 0.45-µm filtrate, both in the influent (in) and in the effluent (out) of the plant. The fines precipitation is obviously an undesired phenomenon because it increases the quantity of particulate phosphates that escape from the reactor. It has been verified that for values of L > 5%, a substantial escape can be observed; therefore, a filtration unit is necessary at the end of the plant.6 In Europe, the supernatants that come from the dewatering unit have very high phosphate concentrations (250-300 mg Ptot/L), with a main content of orthophosphate (84%) and a marginal one of polyphosphate (16%). In Italy, in consequence of the law concerning reformulation of detergents with a minimal content of phosphates, the BNR WWTP’s supernatants present low PO4-P concentration (40-80 mg PO4-P/L).7 A previous research8 has shown that in the metastable state, the nucleation efficiency (η%) is strictly related to the operative pH and the contact time (tc). These experiments have been carried out on laboratory scale and in semiscale pilot plants, using silica sand as seed material. Their results were applied to confirm the validity of a mathematical model, even in short contact time conditions in a recent study.6 The introduction of the double saturational model determines the nucleation efficiency according to the contact time and operative pH (eq 4), whereas pH is the only factor involved in conversion (eq 5).
tc (pH - 7.325) η% ) 100 × (pH - 7.325) + 0.371 tc + 0.0196 pH - 7.21 X% ) 100 (pH - 7.21) + 0.38
(4) (5)
At the same time, conversion and nucleation must be seen as two connected phenomena. Thus, once fixed at an operative pH, an adequate contact time must be chosen to perform high nucleation efficiency; otherwise, a meaningful loss of fines can occur. An experience concerning the removal of phosphorus as MAP under autonucleation conditions in a demonstrative plant9 was proposed. This work has turned out to be very promising, but it showed some problems on bed packing in the case of polyelectrolyte escape from the dewatering station. In fact, this can cause the reversal of the eletrokinetic potential of sludge flocks. Therefore, the performances of an autonucleation system to remove phosphorus from anaerobic supernatants were studied in this work. The reactor was fed with real supernatants supplied from a, A2O process with Johannesburg modification, and the performances under conditions of fluidized and packed bed were investigated. Methodology Demonstrative Plant. The demonstrative plant (Figure 1) was fed with anaerobic supernatant produced by belt pressing of anaerobically digested sludge. The supernatant was collected separately from belt washing waters to avoid its dilution. The pretreatments consisted of mixer and decanter sections (1.3 and 4.7 m3) to remove suspended solid; then the flow was accumulated in an equalization tank (42 m3). The feeding flow rate was first stripped with air (stripping column 1.7 m3) if required, then it flowed into the deaeration column (0.4 m3) and was pumped into the FBR reactor (1 m3). By this way, the SCP plant treated up to 2.0 m3 h-1 of anaerobic supernatant in continuous mode. A Dortmund apparatus (0.8 m3) at the top of FBR avoided wash-out of fines (linear upflow velocity of 6 m/h). The flow rate from FBR was recycled to the stripping column. The final effluent was obtained from the deaeration column discharge. The different operative conditions (feeding, air and recycle flow rate, operative pH, oxidation reduction potential) were logged using an apparatus for on-line measurement. Averaged daily samples were taken from
Figure 1. The SCP demonstrative plant.
Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6703 Table 1. Characteristics of Supernatants
av SD
pH
Alktot mg CaCO3/L
CODs mg O2/L
NH4 N mg N/L
TSS mg/L
PO4sol mg P/L
PO4tot mg P/L
Ca mg/L
Mg mg/L
K mg/L
Ca/Mg
8.1 0.3
941.1 204.9
79.2 6.2
373.5 88.2
32.9 24.5
42.1 9.3
48.8 19.6
88.1 21.0
45.2 19.3
106.2 26.4
2.0 1.1
the supernatant and from the inlet and the outlet of the demonstrative plant. They were analyzed and characterized in terms of pH, Al kpH4, Al kpH6, Ptot, PO4-P, NH4N, Mg, Ca, and TSS according to the Standard Methods methodology.10 In the FBR system, four hydraulic retention times can be identified (eqs 6 vs 9).
HRTT ) HRTstripp ) HRTFBR )
(V1 + V2 + V3) Qi
(6)
(V1 + V2) Qi
(7)
V3 QRIC
(8)
VEXP HRTEXP ) QRIC
(9)
HRTT HRTFBR
tc ) nHRTEXP
% pi* )
grams material sievei × 100 grams total material
(13)
i
where Qi is the feeding flowrate, QRIC is the recycle flowrate, V1 is the stripping section volume, V2 is the volume of the quiet zone, V3 is the FBR free volume, VEXP is the volume of the fluidized bed, and is the bed porosity. In the upward movement, the number of cycles in the fluidized bed (n) is defined as the ratio between hydraulic retention time in the whole plant and that spent in the fluidized bed (eq 10). The contact time (tc) on the fluid bed (eq 11) is defined by the product of the number of cycles and the individual contact time in the fluidized bed (HRTEXP).
n)
where M is the mass of grains and VEXP is the volume of the fluidized bed. The reactor management was performed by discharging periodically the accumulated material in the bottom of the reactor (sample C) to avoid the reduction of the expanded zone and the excessive increase of their sizes. Sieve Analysis of B and C Samples. The basic principle of this technique is the following: A sample of known weight is passed through a set of sieves of known mesh sizes; the sieves are arranged in downward decreasing mesh diameters and mechanically vibrated for a fixed period of time (about 15 min); the weight of the sediment retained on each sieve is measured and converted into a percentage of the total sediment sample (eq 13 vs eq 15),
(10) (11)
% ki** )
∑0 % kept by sievei
(14)
where a single asterisk (*) indicates the partial kept by sieve i, the double asterisk (**) indicates the cumulative kept by sieve i, and ∑i0 % kept by sievei is the total material kept by the mesh sieves wider than sieve i.
% undersizei ) 100 - % ki
(15)
When the total material passed through every sieve is known, it is possible to design the sieve distribution curve that permitts one to define clearly the sample size. The grain growth is estimated considering Φ50 as the reference value and is defined as the sample diameter corresponding to 50% of cumulative undersize. Results And Discussion
FBR and PBR Reactor Control. The reactor was checked by performing daily a V′10 test5 to determine the fluidized bed consistency. The procedure consisted of placing 1 L of sample in an Imhoff cone to settle for 10 min; the settled solids amount, expressed as milliliters per liter, was recorded. The V′10 was executed on samples taken at four different heights of the reactor, and the average value was considered (sample B); it indicated the amount of solids present in a particular section of the reactor. A sample was used to measure the bed porosity (), defined as the vacuum volume of the expanded bed.6 To calculate the bed porosity, a weighted average expanded bed sample was collected. The sand was air-dried for 48 h, and 20 g was loaded into a beaker containing a known volume of water. The water volume expansion was related to the dried sand and used to calculate its specific volume (Vsp ) vol/g sand). The porosity of the expanded bed was calculated according to eq 12.
)
VEXP - (VSPM) VEXP
(12)
Anaerobic Supernatants. The chemical-physical characteristics of the supernatant (Table 1) demonstrated the good performances of the decanter section in the removal of suspended solids lost from the dewatering station. The low concentration of TSS (av 32.9 mg L-1) avoided the suspended solids accumulation in the bed. Furthermore, Ca and Mg ion concentrations (88.1 and 45.2 mg L-1, respectively; Table 1) were enough to satisfy the precipitation of phosphorus as MAP or HAP, at the same time the Ptot/P-PO4 ) 1.15 ratio indicated the presence of nearly totally soluble phosphorus. The supernatants have been compared with those used in the previous works (Table 2). This allows one to consider the following: while in the works611 the stoichiometric requirement for the precipitation of MAP or HAP was met by all the parameters except for the Mg; in this study, all the chemical-physical parameter values were lower than the others except for the Mg ion, which was higher. This last evidence suggested a partial dilution due to the belt washing waters, as compared to the supernatants produced from centrifuges.
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the B and C zones was not very evident; however, a more round shape can be observed in the C zone crystals. Using a density of 1.56 g/mL, by the V′10 of the FBR, it was possible to calculate (eq 16) the weight of the grains making the fluid layer (B zone). This value was equal to 139 kg, and it must be compared with that extracted from the bottom of the reactor (layer C), equal to 60 kg.
Table 2. Comparison between Supernatants ref
PO4 tot mg/L
NH4 N mg/L
HCO3 mgCaCO3/L
Ca mg/L
Mg mg/L
6 11 this work
85 139 49
713 914 373
2900 3550 941
178 153 88
36 24 45
Fluidized Bed. Two periods at steady-state conditions were studied using a feeding flow rate of 1 m3 h-1 (period A, Table 3). The condition of metastable equilibrium was defined from the operative pH of 7.8-8.2; also in this case, the pH was reached without air blowing. The obtained performances were compared considering the change of the upward velocity (42 m/h, determined by a recycle flowrate of 12 m3/h, run A1; 64 m/h with 18 m3/h, run A2). The state of fluidized bed was proved by an average V′10 of 110 mL/L (104 mL/L, A1; 118 mL/L, A2; Table 3) during the treatment of 234 m3 of anaerobic supernatants. The phosphorus mass balance showed a high crystallization performance (η % ) 71.6% in A1 run, η % ) 74.6% in A2 run). In both cases, conversion (X%) increased lightly in terms of η %, with a consequent limited loss of fines (5.8% A1, 4.9% A2, Table 3). The real performances in the case of the fluidized bed were very close to those expected from the mathematical model (Table 4). In the A1 run, in which an optimal pH of 8.2 was used, the nucleation efficiency (η ) 72%) perfectly agreed with the value shown from the model (η ) 70%). The A2 run performances were higher than A1, probably because of the higher V′10. The sieve analyses were carried out on samples taken from zones B and C of the FBR. During the A1 run, V′10 and φ50 for zones B and C remained constant (Figure 2a). After the change of the recycle flowrate (Qr from 12 m3/h to 18 m3/h, Figure 2a), the increased upward velocity determined an immediate increase of V′10 from 104 mL/L to V′10 ) 118 mL/L (Figure 2b). At the end of run A2, the resulting φ50 for zones B and C was higher than those observed in run A1. The φ50 for zone B changed from 0.21 to 0.35 mm, and even φ50 for zone C increased from 0.35 to 0.5 mm. In other words, the B zone grain dimensions during run A2 increased up to the C zone grain dimensions during run A1. All these results are consistent with the fact that the size of the suspended grains (B zone) and those of the settled grains (C zone) are strictly related to the upflow velocity or the hydrodynamic conditions inside the reactor. The FBR management or the time between two extractions from the C zone can influence only this last group of grains, which growth is not actually demonstrated. The characterization of the grains was completed through the macroscope photos (Figure 3a,b), which showed an evident crystalline structure for both the B and C zones. The difference between the dimensions of
V′10
[mlL]γ[kgL]V
FBR[L]
) amount of solid
(16)
Supposing only MAP formation, the mass balance on the basis of phosphorus removed allows one to calculate the formation of 60 kg of P salts treating 234 m3 of supernatant, quite similar to the amount extracted from the C zone. Packed Bed. This second phase of the research allowed us to verify the behavior of a bed that was originally fluidized and was subsequently packed. The origin of this phenomenon was the polyelectrolyte loss in the dewatering section caused by its excessive dosage for sludge conditioning. The packed bed was studied under two conditions of feeding flow rate: 1 and 2 m3 h-1 (B1 and B2 runs, respectively; Table 5). In both runs, a constant recycle flow rate of 18 m3/h was used, leading to an upflow velocity of 64 m/h, similar to that normally adopted to fluidize the bed. To reach the metastable operative pH of 8.0-8.2, the only turbulence generated from the fall of the recycling stream was enough to strip the right amount of carbon dioxide. The complete saving of air blowing was possible, also, thanks to the influent anaerobic supernatant pH equal to 8 ( 0.27. The state of the reactor as a packed bed was demonstrated from the V′10 tests that did not exceed 13 mL/ L. Its physical meaning is that using a specific weight of 1.54 g/mL, the fluidized bed overhanging the packed one has never worked with a concentration higher than 20 000 mg/L of fluidized grains. This concentration did not change meaningfully (8 mL/L for B1 run, 13 mL/L for B2; Table 5) during the treatment of 592 m3 of supernatants. In previous research9 the steady-state condition of a fluidized bed was characterized by a V′10 of 100 mL/L, corresponding to 154 000 mg/L of suspended grains. The phosphorus mass balance showed a minimum nucleation efficiency equal to 45.3% in run B2 and a maximum performance (nucleation efficiency 55.3%) in run B1. The conversion (X%) was slightly higher than nucleation, and as a consequence, the loss of the fines was limited (max 7.5% run B2, Table 5). The comparison between experimental and theoretical performances, using the mathematical model forecast-
Table 3. Performances and Operative Conditions of FBR Plant in Steady State Periods to Fluidized Bed period
run
cumulative treated volume, m3
external P addition
feeding m3/h
Qr m3/h
Qair m3/h
bed porosity
specific weight, g/mL
V′10 ml/L
pH
X%
η%
L%
A
A1 A2
55 179
no no
1 1
12 18
0 0
0.86 0.85
1.56 1.56
104 118
8.2 7.8
77.5 79.6
71.6 74.6
5.8 4.9
Table 4. Comparison of the Performances from Calculation and Model in Fluidized Bed Feeding
inlet flow
outlet flow
from model
from calculation
periods
m3/d
P Ptot mg/L
P PO4 mg/L
pH
P Ptot mg/L
P PO4 mg/L
X%
η%
L%
X%
η%
L%
A1 A2
1 1
40 48
33 40
8.2 7.8
15 17
12 15
72.7 61.2
70 56
2.7 5.2
77.5 79.6
71.6 74.6
5.8 4.9
Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6705
Figure 2. (a) Sieve analysis, Ø50 diameter of B and C zones of FBR. (b) Sieve analysis, conditions of work of the bed.
Figure 3. (a) Macroscope photos, B zone sample. (b) Macroscope photos, C zone sample.
Figure 4. (a) Sieve analysis, Ø50 diameter of B and C zone material of reactor in packed bed condition. (b) Sieve analysis, bed working conditions. Table 5. Performances and Operating Conditions of System FBR in the Stationary Periods to Packed Bed period
run
cumulative treated volume, m3
external P add
feeding flowrate, m3/h
Qr m3/h
Qair m3/h
bed porosity
specific weight, g/mL
V′10 ml/L
pH
X%
η%
L%
B
B1 B2
251 341
no no
1 2
18 18
0 0
0.99 0.98
1.54 1.54
8 13
8.2 8.0
57.3 52.8
55.3 45.3
1.95 7.5
Table 6. Comparison of the Performances from Mass Balance and from FBR Model in Packed Bed inlet flow
outlet flow
from model
from mass balance
period
feeding, m3/d
P Ptot, mg/L
P PO4, mg/L
pH
P Ptot, mg/L
P PO4,,mg/L
X%
η%
L%
X%
η%
L%
B1 B2
1 2
47.8 31.7
43.8 25.8
8.2 8.0
20 15.6
18 13.5
72 68
69.4 64
2.6 4.0
57.3 52.8
55.3 45.3
1.95 7.5
ing the FBR reactor,6 showed how the expected performances are greater regarding the real ones (Table 6). This aspect could still confirm a reactor where fluidization is not the main phenomenon. Sieve analyses were carried out on settled grains, and the results showed a substantial increase in crystal dimensions in C zone (Φ50C ) 0.5-1.4 mm) with the supernatant treated volume. Conversely, the fluidized grains (B zone) increased their dimension slightly (Φ50B ) 0.2-0.4 mm). This was explained in part by the
constant recycle flow rate of 18 m3/h (Figure 4a). All these variations were linked to a B zone at low concentration of suspended grains (V′10 ) 8-13 mL/L, Figure 4b), confirming the main work of the reactor under packed bed conditions. The comparison, the B zone in fluidized and packed beds showed how, even in the presence of different amounts of grains in B zone (139 kg A period, fluidized bed; 16 kg B period, packed bed), their dimension was almost unvaried (φ50 ) 0.2-0.3 mm). On the contrary,
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Figure 5. (a) Macroscope photos, B zone material. (b) Macroscope photos, C zone material. Table 7. Phosphorus Salts Formed by Analysis of Supernatantsa period
zone
P as MAP %
P as HAP %
XMAP % ww
XHAP % ww
Xcalcite % ww
A
A1 A2 B1 B2
74 67 98 76
26 33 2 24
80 71 99 85
12 24 1 12
8 5 0 3
B a
Weight-on-weight results.
the C zone result was completely different: φ50 ) 0.61.4 mm in the packed bed; φ50 ) 0.35-0.5 mm in the fluidized bed. Through the macroscope photos (Figure 5a,b), the crystalline structure was shown, together with the different dimensions previously measured for samples of the B and C zones. The phosphorus mass balance showed a production of ∼111 kg of MAP after the treatment of 592 m3 of supernatants. This amount approximately corresponds to the increase in the packed bed, confirming its main role in phosphorus crystallization. All these results are consistent with a grain distribution mainly located at the C zone (Figure 6). Chemical Analysis. The formed phosphorus salts were determined through chemical analyses of the supernatant influent and effluent of the reactor. The removal of phosphates happened in a meaningful way in all the stationary periods with the formation of MAP (Table 7). Even the chemical analysis carried out on periodically extracted crystals shown as the medium composition in weight of grains was mainly MAP. In Table 8, the averaged values in terms of percentage for different runs are illustrated. The content of the organic substances, quantified by COD on TS % ratio, was analyzed. The results (Table
Figure 6. Grain distribution in packed bed.
Table 8. B and C Zone Analysis av of period period
sample
MAP %
HAP %
CALCITE %
A
fluidized bed (B) bottom of reactor(C) fluidized bed (B) bottom of reactor (C)
94.9 100
0.9 0
4.2 0
97
0.5
2.5
B
Table 9. Organic Content in Produced Crystals av of period (COD/TS), % ref 9 this work
run
sample B
sample C
A F A1 A2 B1 B2
1.7 1.8 1.7 2.6 1.8 2.1
1.2 1.4 2.2 4.9 2.3 2.5
Table 10. Input Parameters in Continuous Feeding Reactor parameter NH4 N (mg/L) PO4 tot P (mg/L) Mg (mg/L) Ca (mg/L) pH log Ksp struvite log Ksp hydroxiapatite ionic strength (M) temp (°C)
value 225 18 40 73 8.0 -12.6 -44.3 0.012 25
9) showed an average concentration of 3.6% in period A and 2.4% in B period. These values can be considered insignificant for an agricultural use of MAP produced and are very similar to those previously found.9 This was confirmed as the upflow velocity adopted both in packed and in fluidized bed avoided the accumulation of the sludge lost during the dewatering process. Chemical Equilibrium Modeling. To establish the optimal pH for struvite and hydroxyapatite precipitation while minimizing the coprecipitation of other crystalline phases, Visual MINTEQ (ver 2.15, EPA 1991)12 simulations were carried out. This model, based on the thermodynamic equilibrium between the species, allowed the creation of a personal database in which the
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possible solid phases are inserted. In this study, a custom thermodynamic database was created for struvite and hydroxyapatite with their characteristic values of solubility product (Ksp), enthalpy values, and stoichiometric coefficients. With this database, the program can calculate the equilibrium concentrations of ion species and precipitates on the basis of the input of component ion concentrations, pH, temperature, and ionic strength. The model was tested under conditions of a continuously fed reactor with data of the effluent plant (Table 10). It revealed conditions of metastable equilibrium with possible precipitation of HAP in concentration of 0.0630 mmol/L. The discrepancies between real and model-calculated results are attributable to the following observations: 1. The formation of the MAP is thermodynamically disadvantaged regarding HAP; in fact, the solubility product of MAP is higher than HAP (log Ksp ) -12.6 vs log Ksp ) -44.3, respectively). 2. To ascertain the nature of the solid phase obtained under kinetic conditions, a comparison among the supersaturation curves of synthetic solutions and that of a real supernatant has been carried out in a previous work.13 It has been verified that at low values of pH (7.8-8.2), there is contemporary formation of HAP and MAP. A hypothesis of MAP nucleation on the HAP crystal was advanced. This theory is confirmed also by the experimental results obtained in the present work. 3. The inhibition on HAP formation exerted by alkalinity 14 must have an important role on the main kinetic MAP formation. It was showed that a significant role can be exerted at 2000-3000 mg CaCO3/L.13 The present work demonstrates that even 1000 mg of CaCO3/L is able to inhibit HAP formation. Conclusions The paper has examined the treatment of lowstrength phosphate supernatants typical of BNR Italian WWTPs. The main results are (1) FBR is particularly efficient, its performances can be forecasted by mathematical model fitted using high phosphate concentration supernatant. (2) PBR can be formed starting from a previous FBR condition, later packed by polyelectrolyte lost from the dewatering section. In this case, the interruption of treatment is not required because a packed bed can remove phosphorus, even if at lower performance rates. (3) In FBR and PBR reactors, negligible inclusions of organic suspended solids are found in material extracted from the reactor. (4) High crystalline material is obtained; it is extracted from the bottom of reactor; its dimension is up to 1.4 mm in PBR
and no higher than 0.5 mm in FBR. In both cases, the management of material extracted is very easy and inexpensive. Literature Cited (1) McGrath J. W.; Quinn J. P. Biological phosphorus removal. Phosphorus in Environmental Technology: Principles and Applications 2004, 272-290. (2) Evans, T. D.; Johnstan, A. E. Phosphorus and crop nutrition: principles and practice. Phosphorus in Environmental Technology: Principles and Applications 2004, 93-116. (3) Jeanmaire, N.; Evans, T. Technical-economic feasibility of P-recovery from municipal wastewaters. Environ. Technol. 2001, 22, 1355-1360. (4) Piekema, P. G. The case study of a phosphorus recovery sewage treatment plant at Geestmerambacht, Holland-design and operation. Phosphorus in Environmental Technology: Principles and Applications 2004, 470-495. (5) Battistoni, P.; Pavan, P.; Cecchi, F.; Mata Halvarez, J. Effect of composition of anaerobic supernatants from an anaerobic, anoxic and oxic (A2O) process on struvite and hydroxyapatite formation. Annal. Chim. 1998, 88, 761-772. (6) Battistoni, P.; De Angelis, A.; Prisciandaro, M.; Boccadoro, R.; Bolzonella, D. P Removal from anaerobic supernatants by struvite crystallization: long-term validation and process modelling. Water Res. 2002, 36 (8), 1927-1938. (7) Metcalf & Eddy Wastewater Engineering. Treatment, Disposal and Reuse, 3rd ed.; McGraw-Hill, Inc.: New York, 1991. (8) Battistoni, P.; De Angelis, A.; Pavan, P.; Prisciandaro, M.; Cecchi, F. Phosphorus removal from a real anaerobic supernatant. Water Res. 2001, 35 (9), 2167-2178. (9) Battistoni, P.; Boccadoro, R.; Fatone, F.; Pavan, P. Autonucleation and crystal growth of struvite in a demonstrative fluidized bed reactor (FBR). Proc. Int. Conf. Struvite; Cranfield University, June 17-18, 2004; Cranfield-Bedfordshire: Cranfield. (10) APHA. Standards Methods for the Examination of Water and Wastewater. 16th ed.; American Public Health Association: Washington, DC, 1985. (11) Battistoni, P.; Pavan, P.; Cecchi, F.; Mata-Halvarez, J. Phosphate removal in real anaerobic supernatants: modelling and performance of a fluidized bed reactor. Water Sci. Technol. 1998, 31, 275-283. (12) Environmental Protection Agency. A Geochemical Assessment Model for Environmental Systems, Version 2.02; EPA/600/ 3-91/021; U.S. Government Printing Office: Washington, DC, 1991. Visual Minteq was developed by KTH (Swedish Royal Institute of Technology). (13) Battistoni, P.; Pavan, P.; Prisciandaro, M.; Cecchi, F. Struvite crystallization: a feasible and reliable way to fix phosphorus i anaerobic supernatants. Water Res. 2000, 34 (11), 30333041. (14) Jenkins, D.; Ferguson, J. F.; Menar, A. B. Chemical processes for phosphate removal. Water Res. 1971, 5, 369-389.
Received for review February 16, 2005 Revised manuscript received June 7, 2005 Accepted June 13, 2005 IE050186G
