Ind. Eng. Chem. Res. 2006, 45, 663-669
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PROCESS DESIGN AND CONTROL Phosphorus Removal from Anaerobic Supernatants: Start-Up and Steady-State Conditions of a Fluidized Bed Reactor Full-Scale Plant 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 the full-scale application of the struvite crystallization process (SCP) used for phosphorus recovery. In this study this kind of technology and its core, a fluidized bed reactor (FBR), have been used for the treatment of anaerobic supernatants with low phosphate concentrations. Covering four different runs, the experimental results are referred to both start-up and steady-state periods, while the discussion is focused both on the optimal operating conditions and on the performances obtained. All four start-up periods were performed using the same methodology that can be considered suitable also for starting up every industrial SCP application. In fact, in working with these operating conditions, the time needed to obtain the right amount of well fluidized bed was 6-8 days. During the steady-state conditions, 1-8 m3 h-1 supernatants were treated and high phosphorus removal by crystallization (58-72%) was obtained; moreover, these values were very close to the mathematical model previsions (60-69%). The recovered material was mainly composed of struvite (average 95%). Finally, the used operating conditions can be considered suitable for managing industrial plants. Introduction High phosphorus concentrations in biological nutrients removal (BNR) wastewater treatment plants (WWTPs) are strictly linked to phosphorus released from phosphorus-accumulating organisms (PAOs). This phenomenon is particularly remarkable in anaerobic conditions in the presence of adequate concentrations of volatile fatty acids (VFA).1 Moreover, a peculiarity of this type of plant is that the sludge line supernatants undergo a gradual enrichment of phosphorus content. The consequent phosphorus mass feedback to the plant head tanks can determine a decrease of performance and a serious risk of overcoming the phosphorus limit on the main plant effluent. This requires an adequate treatment of the sludge or supernatants in order to remove phosphorus before their recirculation into the main influent stream. Among the different technologies for phosphorus removal, chemical precipitation presents several disadvantages, and the worst, which can cause drawbacks from both technical and economical points of view, probably consists of the impossibility of achieving a correct recycling of the formed phosphorus salts. On the contrary, the removal of phosphorus by its recovery, for instance as a salt and in a form ready for its reuse,2 must be preferred. According to this principle, the crystallization of hydroxyapatite (Ca5(PO4)3OH, HAP) or struvite (NH4MgPO4‚ 6H2O, MAP) represents a very favorable technique. In fact, the recovery of MAP in pellet form could balance the costs of * To whom correspondence should be addressed. E-mail:
[email protected]. † Marche Polytechnical University. ‡ University of Verona. § University of Venice.
phosphorus biological removal treatment thanks to two main reasons: the gains linked to a recyclable phosphate product and the reduction of the sludge disposal amount. Therefore, the authors propose the study of an autonucleation system able to remove phosphorus from anaerobic supernatants. The discussion pays attention both to the start-up phase (in supersaturation conditions with phosphorus salt addition) and to the best steadystate operating conditions. The aim of the experimental work was the definition of feasible managing procedures for proposal to real industrial plants. Thus, in accordance with this aim, the fluidized bed reactor (FBR) was fed with real supernatants supplied from a full-scale A2O process adopting the Johannesburg modification (the Treviso WWTP). State of the Art. The struvite crystallization process (SCP) uses the chemical-physical properties of the anaerobic supernatants without any addition of external reagents, since the operative pH conditions are reached by stripping the CO2 with air.3 Precipitation in an amorphous or crystalline form of MAP and/or HAP can be realized under supersaturation and metastable conditions, respectively. In both cases the technology is simple, and it is based on the use of an up-flow reactor where the feed is recycled a number of times. In the FBR system four hydraulic retention times (HRTs) can be defined (eqs 1-4).
HRTT )
V1 + V2 + V3 Qi
HRTstripp )
V1 + V2 Qi
(2)
V3 QRIC
(3)
HRTFBR )
10.1021/ie050796g CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2005
(1)
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HRTEXP )
VEXP QRIC
(4)
In eqs 1-4, Qi is the feed flow rate, QRIC is the recycle flow rate, V1 is the stripping section volume, V2 is the volume of the deaeration column, 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 a ratio between the hydraulic retention time in the whole plant (HRTT) and the one spent in the fluidized bed reactor (HRTFBR, eq 5). The contact time (tc) on the fluidized bed (eq 6) is defined by the product of the number of cycles and the individual contact time in the expanded bed (HRTEXP). In some cases the turbulence generated by the fall of the recirculation flow is enough to maintain the operating pH with consequent energy savings.4
n)
HRTT HRTFBR
tc ) n(HRTEXP)
(5) (6)
The methodology for evaluating the process performances is based on the calculation of three parameters: nucleation efficiency (η%, eq 7), phosphate conversion (X%, eq 8), and precipitation efficiency (L%, eq 9):
η% ) 100
Ptot,in - Ptot,out Ptot,in
X% ) η% + L% ) 100 L% ) 100
Ptot,in - Psol,out Ptot,in
Ptot,out - Psol,out Ptot,in
(7)
(8)
(9)
where Ptot is the phosphate measured in unfiltered acidified samples and Psol is the phosphate measured in 0.45 µm filtrate, in both the influent and effluent of the plant. Supersaturation condition is obviously an undesired phenomenon because it increases the quantity of fines escape from the top of the reactor. It has been verified that for L values >5% a substantial escape of fines can be observed; such a phenomenon involves the installation of a filtration unit at the end of the plant.4 Previous research3 showed that the nucleation efficiency (η%) is strongly related to the operative pH and the contact time (tc) when the plant operates in metastable conditions. These experiments were first carried out in the laboratory and on a middlescale pilot plant, using silica sand as seed material. The introduction of a double saturation model determined the nucleation efficiency according to the contact time and operative pH (eq 10), while pH is the only factor determining the conversion (eq 11).
tc pH - 7.325 η% ) 100 (pH - 7.325) + 0.371 tc + 0.0196
(10)
pH - 7.21 X% ) 100 (pH - 7.21) + 0.38
(11)
In eqs 10 and 11 the physical meaning is that conversion and nucleation must be seen as two connected phenomena. This is why, once an operative pH has been fixed, an adequate contact
Figure 1. The SCP demonstrative plant.
time must be chosen to perform high nucleation efficiency; otherwise a meaningful loss of fines can take place.4 Regarding the phosphorus contents, in European countries the supernatants coming from dewatering sludge stations have very high phosphate concentrations (250-300 mg of Ptot/L), composed of mainly orthophosphate (84%) and in a minor fraction polyphosphate (16%). In Italy, by the issuing of a law that fixed very low phosphate contents for detergent composition, the BNR WWTP’s supernatant presents low phosphorus concentrations (40-80 mg of PO4-P/L). As for the management of the start-up phase, previous research5 evidenced some problems regarding the packing of the bed in FBR reactor in the case of polyelectrolyte escape from the dewatering station. This fact shows that the use of already formed grains as seed material in consecutive start-up phases is not always possible because of the inadequacy of the bed. For this reason, before a steady-state condition was performed, it was necessary to define a simple and precise startup methodology. The operative pH was always reached by stripping of the CO2 with air. Start-up experience in a bench-scale pilot plant6 showed rapid struvite formation working with magnesium and total phosphorus concentrations of 100 and 110-120 mg/L, respectively (molar ratio 1:1.1-1.05) and a pH value of about 9.0. Previous experimentations7 carried out on a demonstrative scale have shown that the best operating pH ranged from 7.8 to 8.2. Moreover, in start-up periods a metastable equilibrium (pH 8.3-8.4) allowed obtaining an adequate fluidized bed and was preferred to the supersaturation conditions in order to avoid pipe scaling. The magnesium used to reach the correct molar ratio has been obtained from magnesium chloride; this choice was suggested by its easy handling and good solubility. Therefore, most of the magnesium was readily available. Methodology Demonstrative Plant. The demonstrative plant (Figure 1) was fed with anaerobic supernatant produced by a belt press dewatering station treating anaerobically digested sludge. The supernatant was collected separately from belt washing waters to avoid dilution. The pretreatments consisted of a mixer and decanter section (1.3 and 4.7 m3, respectively) to remove suspended solids; then supernatant was accumulated in an equalization tank (42 m3). The feed flow rate was first stripped with air (stripping column 1.7 m3); then it flowed to the deaeration column (0.4 m3) and was pumped into the FBR reactor (1 m3). In this way the SCP plant treated up to 8.0 m3
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 665 Table 1. Supernatant Characteristics period A B C D
cumul treated vol, m3
pH
Alktot, mg/L
CODs, mg/L
NH4-N, mg/L
TSS, mg/L
P-PO4,sol, mg/L
Ptot,mg/L
Ca, mg/L
Mg, mg/L
K, mg/L
Ca/Mg
K/Mg
234 186 194 169
8.1 8.2 8.1 8.0
941.1 1339 1036.3 1093.3
79.2 79.9 70.2 83.4
264.5 274.0 283.0 268.0
32.9 33.5 28.3 39.6
42.1 37.3 43.6 44.6
48.8 43.8 54.3 52.9
88.1 113.6 110.5 125.8
43.0 44.1 46.2 56.7
87.0 86.0 64.7 62.7
2.0 2.6 2.4 2.2
2.0 2.0 1.4 1.1
8.1 0.08
1102.4 169.75
78.2 5.63
272.4 8.10
33.6 4.64
41.9 3.23
50.0 4.72
109.5 15.72
47.5 6.27
75.1 13.20
2.3 0.26
1.6 0.45
average SD
h-1 anaerobic supernatant in continuous mode. A Dortmund apparatus (0.8 m3) at the top of the FBR avoided fines washout (linear velocity of 6 m/h). The FBR effluent was recycled to the stripping column. The final effluent went out from the deaeration column discharge. The different operative parameters (feed, air and recycle flow rates, operative pH, oxidation reduction potential) were measured online and logged. Averaged daily samples were taken from the supernatant and from the inlet and the outlet of the demonstrative plant. They were analyzed and characterized in terms of pH, alkalinity content (AlkpH 4, AlkpH 6), Ptot, PO4-P, NH4-N, Mg, Ca, and total suspended solids (TSS) according to Standard Methods methodology.8 FBR Start-Up. Using (NH4)2HPO4 and MgCl2, 4 m3 of mother solution with phosphorus concentration of 2000 mg/L and magnesium concentration of 2300 mg/L was prepared and stored in a tank. A stocking pH of about 4.7 was gained by adding 19 L of H2SO4 diluted at 33%. The dosage from the tank into the stripping column was made, only during start-up study, with a flow rate of 17 L/h. FBR Reactor Control. The reactor was daily checked by performance of a V′10 test9 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 amount of settled solids, 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 (sample B) or the value at the bottom of the reactor (sample C) was considered. Chemical and sieve analyses were periodically performed together with the bed porosity () measurement, which is defined as the vacuum volume referred to the unit volume of the expanded bed.4 The method used to calculate the bed porosity is as follows: a weighed average expanded bed sample was collected after each run, the grains were air-dried for 48 h, and 20 g was loaded in a beaker containing a known water volume. The water volume expansion can be related to the dried grains and used to calculate its specific volume (Vsp ) volume per grams of sand). The porosity of expanded bed can be calculated according to eq 12.
)
VEXP - (VSPM) VEXP
by each sieve was weighted, and then it was converted into a percentage of the total sediment sample (eqs 13-15):
% pi* )
(g of material sieve)i × 100 g of total material
(13)
where pi* is the partial kept by sieve i i
% ki* )
∑0 %(kept by sieve)i
(14)
ki* is the cumulative kept by sieve i; ∑i0 %(kept by sieve)i is the total material kept by the sieves with mesh wider than sieve i
% undersizei ) 100 - % ki
(15)
Once the total amount of material passed through every sieve was known, the plot of the sieve distribution was possible. By this curve the sample sizes were clearly defined. The reference value to estimate the grain growth was called Φ50, defined as the sample diameter corresponding to 50% of the cumulative undersize (eq 15). Results and Discussion Anaerobic Supernatants. The supernatant chemical-physical characteristics (Table 1) showed some relevant aspects: low phosphate and suspended solids concentrations. The first aspect is typical of plants located in northern Italy, where the low phosphorus content in wastewater inlet causes also low concentrations in the secondary streams of the removal process.10 The second aspect showed the good performances of the decanter section in removing the suspended solids lost from the dewatering station. As a matter of fact, averaged TSS concentrations of 33.6 mg L-1 in the feed can be considered values that avoid any suspended solids accumulation in the FBR. Furthermore, on the basis of the phosphate concentration, the calcium stoichiometric request for the precipitation of HAP was satisfied (average Ca ion concentration ) 109.5 mg L-1; Table 1); by contrast, the concentration of Mg ions was not enough
(12)
where M is the mass of grains and VEXP is the volume of the fluidized bed. Furthermore, microscopic analyses were performed both on material periodically wasted from the bottom and also at the four different heights of the FBR. The material distribution was measured by sieve analysis. The basic principle of this technique is as follows: a sample of known weight was passed through a set of sieves of known mesh sizes. These sieves were arranged in downward-decreasing mesh diameters and mechanically vibrated for a fixed time (about 15 min). The sediment retained
Figure 2. Trend of Mg and K ions with phosphate in anaerobic supernatant.
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Table 2. Supernatant Characteristics in Start-Up Periods period 1 2
cumul treated vol, m3
pH
Alktot, mg/L
CODs, mg/L
NH4-N, mg/L
TSS, mg/L
P-PO4,sol, mg/L
Ptot, mg/L
Ca, mg/L
Mg, mg/L
K, mg/L
139 197
8.4 8.3
1179.5 917.3
69.3 72.3
309.7 299.5
25.8 31.6
82.6 99.2
105.1 104.3
77.6 123.7
68.0 118.7
83.1 82.6
8.4 0.1
1048.4 185.4
70.8 2.1
304.6 7.2
28.7 4.1
90.9 11.7
104.7 0.6
105.1 39.0
93.4 35.9
82.9 0.4
average SD
Table 3. Operative Conditions of FBR Plant in Start-Up Periods period
cumul treated vol, m3
external P addn?
feed flow rate m3 h-1
Qr, m3 h-1
Qair, m3 h-1
bed porosity
sp wt, g/mL
V′10 average, mL/L
total Alk, mg/L
pH
1 2
139 197
yes yes
1 1
10.4 12.1
38.8 41.6
0.87 0.91
1.5 1.6
64 36
1180 917
8.4 8.3
for a complete formation of MAP (average Mg ion concentration ) 47.5 mg L-1; Table 1). The supernatant presented a constant Ca/Mg ratio (2.3; standard deviation (SD) ) 0.3) and near totally soluble phosphorus (Ptot/P-PO4 ) 1.19). Figure 2 shows the K ions ranging from 62.7 to 85.7 mg/L and they were strictly related to phosphate content. An analogous trend was observed for Mg ions. Really, the observed decreasing trend of K with Mg ion concentrations was unexpected. In fact during phosphorus accumulating organism (PAO) hydrolysis, phosphate, magnesium, and potassium are normally released in the same mode of polyphosphate synthesis. These last were reported to be 0.335 M Mg M-1 P and 0.258 M K M-1 P, respectively.11-13 This means that the expected K/Mg molarto-molar ratio was 0.77, while the weight-to-weight ratio was 1.24. The results obtained can be only explained with a partial struvite precipitation inside the digester. As a result, low P-PO4 concentrations show a K/Mg ratio equal to 2.4, higher than the one expected; by contrast, high P-PO4 concentrations coincided with a K/Mg weight-to-weight ratio ranging from 1.2 to 1.0, similar to the theoretical one. Start-Up. Before operating in steady-state conditions, logically the SCP plant needed a fluidized bed. The grains discharged previously from the FBR were not suitable because of their fast packing during air-drying. Thus, it was necessary to use a simple and reliable methodology to start-up the FBR. Two start-up phases were studied. The metastable conditions for struvite formation were reached operating at a phosphate concentration of about 100 mg L-1 (90.9 average value; Table 2) and at an operative pH of 8.3-8.4. The stoichiometric ratio Mg:N:P ) 1:1:1 was obtained by adding (NH4)2PO4 and MgCl2 solution to the supernatant, while the operative pH conditions were reached by stripping of the CO2 with different air flow rates according to the alkalinity content: 38.8 m3 h-1 for period 1 and 41.6 m3 h-1 for period 2 (Table 3). The main different operating conditions were the recycle flow rate (10.4 m3 h-1 for period 1 and 12.1 m3 h-1 for period 2)
Figure 3. Bed growth in start-up periods 1 and 2.
Table 4. Performances of FBR Plant in Start-Up Periods inlet flow
outlet flow
period
P-Ptot, mg/L
P-PO4, mg/L
Ptot, mg/L
P-PO4, mg/L
X%
η%
L%
1 2
105.1 122.7
82.6 99.2
24.3 22.3
19.3 18.6
71.7 82.1
67.4 78.6
4.3 3.5
Table 5. Phosphorus Salts Formed by Supernatant Analysis (Weight on Weight Results) period
P as MAP, %
P as HAP, %
XMAP, % w/w
XHAP, % w/w
Xcalcite, % w/w
1 2
63 96
37 4
80 87
20 1
0 12
and the characteristic ratios P-PO4/Ca, P-PO4/Mg (1.06 and 1.21 respectively for period 1 and 0.80 and 0.84 respectively for period 2; see Table 2). The different recycle flow rates determined an upward linear velocity respectively of 36 and 43 m/h. Following bed growth by the V′10 control, the time needed to obtain a well fluidized bed (respectively 64 mL/L, period 1, and 36 mL/L, period 2; Table 3) ranged from 6 to 8 days (Figure 3). At least two completely different conditions were gained. In both cases a good parabolic growth of the fluidized bed was observed (see R2 of the interpolated equations in Figure 3). The phosphorus mass balance showed good nucleation efficiency in periods 1 and 2, respectively: η ) 67.4% and 78.6% (see Table 4). The conversion (X%) was slightly higher than nucleation in both cases; as a consequence, the loss of the fines was limited (maximum 4.3% in period 1; Table 4). The higher values of η% and X% during period 2 are probably due to the higher concentrations of Ca and Mg (see Table 2). The formed phosphorus salts were determined on the basis of chemical analysis of influent and effluent of the reactor. The results showed that the removal of phosphates was very satisfactory and the grains were formed mainly of MAP (Table 5). The results of sieve analysis, carried out on meaningful samples withdrawn at four different heights, evidenced a constant value of grain dimension in period 1 (Φ50 ) 0.180.21 mm; Figure 4a). By contrast, the grains of period 2 increased their dimension slightly, from Φ50 ) 0.18 mm to Φ50 ) 0.35 mm (Figure 4b). Coupling these results with V′10 values (64 mL/L in period 1 and 36 mL/L during period 2), it can be observed that the lower are V′10 values and the higher are the Φ50 values. This was justified from the recycle flow rate adopted in period 1, which was lower than that in period 2 (10 vs 12 m3 h-1). In other words, the grains settle to the bottom of the bed when they overcome the maximum size fit for the fluidized state and this value is related to the upflow velocity; thus an equilibrium between upflow velocity and grain size is reached. Furthermore,
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 667
Figure 4. (a) Sieve analysis: Φ50 of fluidized bed and bed working conditions for period 1. (b) Sieve analysis: Φ50 of fluidized bed and bed working conditions for period 2. Table 6. Sieve Distribution period
wt of grains in fluid bed, kg
MAP from mass bal, kg
1 2
83 58
88 127
higher upflow velocities allow reaching bigger grain dimensions, which increases the bed porosity. The V′10 test and the specific weight can be used to estimate the amount of solids in the FBR through eq 16.
V′10
[mLL]γ[kgL]V
FBR[L]
) amount‚of‚solid
(16)
where γ is the specific weight of the grains. To verify the grain formation mechanism, a comparison between the weight of grains in the fluidized bed and those formed, calculated on a mass balance basis, was performed. The results (Table 6) confirm the conclusions previously suggested. In particular, in period 1 the total amount of grains formed constitutes the fluidized bed (see Table 6, where 88 kg is formed and 83 kg constitutes the fluidized bed); in period 2 about half the struvite is in fluidized form and half is fallen in the bottom part of the reactor. Steady State. To define the optimal control parameters, four periods at steady-state conditions were studied using increasing feed flow rate: 1 m3 h-1 for period A; 2 m3 h-1 for period B; 4 m3 h-1 for period C; 8 m3 h-1 for period D. See Table 7. The condition of metastable equilibrium was defined from the operative pH of 8.0-8.2; in this case the pH was gained without blowing air. An upward velocity ranging from 42 m/h (Qr ) 12 m3 h-1) to 64 m/h (Qr ) 18 m3 h-1) was adopted. The phosphorus mass balance showed a crystallization performance strictly related to the feed flow rate. From period A to period D an exponential growth of feed flow rate was adopted and a decrease of conversion and crystallization
Figure 5. Start-up and steady-state V′10.
performances was observed (Table 7). The operative pH was quite constant at 8.0-8.2 and no air blowing was needed, since the turbulence of recycle was enough to strip carbon dioxide. Fines loss was small, even if a gradual increase from 5-5.8% to 7.6-7.9% was observed increasing the feed flow rate. The main effect of feed flow rate variation was evidenced on the contact time: this gradually decreases at higher feed flow rate according to eq 6 (Table 8). The explanation of this result is in agreement with the mathematical model (eqs 10 and 11) where the lowering of the nucleation efficiency (η%) is due to the lowering of the contact time (tc) (Table 8). The global consideration is that, on changing the feed from 1 to 8 m3 h-1, phosphorus removal lowers from 72 to 58%. This performance is considered good and reveals the great elasticity of the FBR. Even operating at higher feed flow rates, the observed loss of performance can be considered acceptable.
Table 7. Performances and Operative Conditions of FBR Plant in Steady-State Periods to Fluid Bed period
cumul treated vol, m3
external P addn?
feed flow rate, m3 h-1
Qr, m3 h-1
Qair, m3 h-1
bed porosity
sp wt, g/mL
V′10, mL/L
pH
X%
η%
L%
A B C D
234 186 194 169
no no no no
1 2 4 8
18 12 18 18
0 0 0 0
0.86 0.94 0.96 0.92
1.50 1.60 1.50 1.50
110 61 45 50
8.2 8.2 8.1 8.0
77.5 69.6 66.0 65.2
71.8 61.9 58.1 57.7
5.8 5.0 7.9 7.6
Table 8. Comparison of Performances from Calculation and Model period
feed flow rate, m3 h-1
A B C D
1 2 4 8
inlet flow P-Ptot, mg/L P-PO4, mg/L 40.3 45.6 54.0 53.9
35.3 38.9 43.8 45.2
outlet flow P-Ptot, mg/L P-PO4, mg/L 15.3 17.1 22.5 22.5
12.3 13.6 18.4 18.7
from model η% L%
pH
t c, h
X%
8.2 8.2 8.1 8.0
2.13 1.16 0.59 0.27
70.1 72.3 70.1 67.5
67.0 69.1 65.5 60.4
3.1 3.2 4.6 7.1
from mass balance X% η% L% 77.5 69.6 66.0 65.2
71.8 61.9 58.1 57.7
5.8 5.0 7.9 7.6
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Figure 6. (a) Sieve analysis: bed working conditions for period A. (b) Sieve analysis: Φ50 diameters of B and C layers of FBR for period B. (c) Sieve analysis: conditions of work of the bed for period C. (d) Sieve analysis: bed working conditions for period D. Table 9. Sieve Distribution period A B C D
top (fluidized bed), kg 82.5 37.6 13.5 7.5
bottom (wasted), kg 12 10 27 31.5
total, kg 94.5 47.6 40.5 39
P from mass bal, kg 46 44 48 42
After the start-up period, the state of the fluidized bed is maintained constant during the steady-state period (Figure 5) by periodic extractions of grains. The sieve analyses were carried out on samples extracted both from the fluidized bed (TOP) and from the bottom (BOTTOM). In all steady-state periods top and bottom grain dimensions are related to recycle flow rate. As a matter of fact, top grains reach up to 0.30 mm if the recycle flow rate is 12 m3 h-1 (period B; Figure 6b) and a further increase is observed (0.35 mm) if the recycle flow rate adopted is 18 m3 h-1 (periods A, C, D; Figure 6 a,c,d). The same situation can be observed for bottom layer, where Φ50 grows up to 0.5 mm for a recycle flow rate of 18 m3 h-1 and to only 0.35 mm for a recycle flow rate of 12 m3 h-1. A constant grain dimension of the top layer suggests the following behavior: the fluidized grains reach the maximum dimension fluidized at the operative upward velocity, then settle to the bottom with the consequent formation of the bottom zone, which maintains more or less the same volume if frequent extraction is organized. The phosphorus mass balance shows a production of approximately 45 kg of MAP in periods B, C, and D (Table 9); a complete agreement between phosphorus salts from mass balance and material extracted from the FBR can be observed with the exclusion of period A. This fact can be explained by looking at the V′10 of the expanded bed before starting steadystate period A (100 mL; Figure 5). This was the highest value between those measured before starting periods B, C, and D; it can be due to an anomalous start-up phase. Adopting a V′10 of 52 mL, equal to the mean value between those observed at the end of the second and third start-ups (Figure 5), the top of the
Table 10. Phosphorus Salts Formed by Supernatant Analysis (Weight on Weight Results) period A B C D
P as MAP, %
P as HAP, %
XMAP, % w/w
XHAP, % w/w
Xcalcite, % w/w
71 81 53 71
29 19 47 29
76 89 62 83
17 8 27 14
7 3 11 3
Table 11. Phosphorus Salts Formed under Steady-State Conditions average of period period
samples
MAP, %
HAP, %
calcite, %
A
top bed bottom of reactor top bed bottom of reactor top bed bottom of reactor top bed bottom of reactor
94.9 100
0.9 0
4.2 0
100 96.5
0 0.9
0 2.6
94.7 89.6
0.9 0.8
4.4 9.6
B C D
fluidized bed quantity can be recalculated to 39 kg. If the bottom waste is added (12 kg), a total amount of 51 kg is obtained; this value is comparable to the 46 kg carried out from the phosphorus mass balance (Table 9). Chemical Analysis. The influent and effluent supernatant mass balances show the main formation of MAP in every steadystate period (Table 10); some calcite formation is also observed. The chemical analysis carried out on periodically extracted material from the FBR (top and bottom) confirmed mainly struvite (MAP) formation as is shown in Table 11. Conclusions The paper has investigated the treatment of low strength phosphate supernatants typical of BNR Italian WWTPs. The start-up phases and different operative steady-state conditions were studied. The main conclusions of the study were the following:
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 669
(i) A start-up procedure to initialize new plants or to restart a FBR after maintenance has been individuated as feasible: a solution at high phosphate concentration (2000 mg/L) using (NH4)2HPO4 and MgCl2 can be added to perform supersaturation conditions and to obtain in 5-6 days a reactor ready to be operative. Its fluidized bed contains 45-100 mL/L of grains according to the upflow velocity used. (ii) Using anaerobic supernatant at low phosphate concentration (maximum 50 mg of P-PO4/L), a flow rate ranging from 1 m3 h-1 to a maximum of 8 m3 h-1 can be used to feed the same FBR. Performances in phosphorus removal are quite similar and also a higher loss of grains can be tolerated thanks to the high elasticity of the reactor. This means that periodic or seasonal overflow rates can be easily treated without any plant modification. (iii) A continuous or frequent extraction of crystallized phosphorus salts ensures constant operative conditions of the fluidized bed; their dimension is linked to the upflow velocity adopted. Literature Cited (1) McGrath, J. W.; Quinn, J. P. Biological phosphorus removal. Phosphorus in EnVironmental Technology: Principles and Applications; 2004; pp 272-290. (2) Evans, T. D.; Johnstan, A. E. Phosphorus and crop nutrition: principles and practice. Phosphorus in EnVironmental Technology: Principles and Applications; 2004; pp 93-116. (3) Battistoni, P.; De Angelis, A.; Pavan, P.; Prisciandaro, M.; Cecchi, F. Phosphorus removal rom a real anaerobic supernatant. Water Res. 2001, 35 (9), 2167-2178. (4) 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 (5) Battistoni, P.; Boccadoro, R.; Fatone, F.; Pavan, P. Auto-nucleation and crystal growth of struvite in a demonstrative fluidized bed reactor (FBR). In Proceedings of International Conference On StruVite: its role in recoVery and reuse; Cranfield, June 17-18, 2004; University of Cranfield, Cranfield, Bedfordshire, UK. (6) Jaffer, Y.; Pearce, P. Phosphorus recovery via struvite production at Slough sewage treatment works, UKsa case study. Phosphorus in EnVironmental Technologies: Principles and Applications; 2004; pp 402427. (7) Battistoni, P.; Paci, B.; Fatone, F.; Pavan, P. Phosphorus removal from Supernatants at Low Concentration Using Packed and Fluidized-Bed Reactors. Ind. Eng. Chem. Res. 2005, 44, 6701-6707. (8) APHA. Standards methods for the examination of water and wastewater, 16th ed.; American Public Health Association: Washington, DC, 1985. (9) Battistoni, P.; Pavan, P.; Cecchi, F.; Mata Alkvarez, J. Effect of composition of anaerobic supernatants from an anaerobic, anoxic and oxic (A2O) process on struvite and hydroxyapatite formation. Ann. Chim. 1998, 88, 761-772. (10) Metcalf & Eddy. Wastewater Engineering. Treatment, Disposal and Reuse, 3rd ed.; McGraw-Hill, Inc.: New York, 1991. (11) Arvin, E.; Kristensen, G. H. Exchange of organics, phosphate and cations between sludge and water in biological phosphorus and nitrogen removal process. Water Sci. Technol. 1995, 17, 147-162. (12) Wentzel, M. C.; Ekama, G. A.; Marais, G. v. R. Processes and modelling of nitrification denitrification biological excess phosphorus removal systems. Water Sci. Technol. 1992, 25, 59-82. (13) Jardin, N.; Po¨pel, H. J. Refixation of phosphates released during bio-P sludge handling as struvite or aluminium phosphate. Water Sci. Technol. 1993, 28 (1), 263-271.
ReceiVed for reView July 6, 2005 ReVised manuscript receiVed October 14, 2005 Accepted October 18, 2005 IE050796G
