Phosphorus removal from wastewater by mineral apatite

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Phosphorus removal from wastewater by mineral apatite Nathalie Belliera, Florent Chazarenca,b,, Yves Comeaua a

Department of Civil, Geological and Mining Engineering, Ecole Polytechnique, 2900 Edouard-Montpetit, Montreal, Que., Canada H3T 1J4 Institut de recherche en biologie ve´ge´tale, Universite´ de Montre´al, 4101 rue Sherbrooke Est, Montreal, Que., Canada H1X 2B2

b

art i cle info

A B S T R A C T

Article history:

Natural apatite has emerged as potentially effective for phosphorus (P) removal from

Received 4 April 2006

wastewater. The retention capacity of apatite is attributed to a lower activation energy

Received in revised form

barrier required to form hydroxyapatite (HAP) by crystallization. The aim of our study was

12 May 2006

to test the P removal potential of four apatites found in North America. Minerals were

Accepted 18 May 2006

collected from two geologically different formations: sedimentary apatites from Florida

Available online 10 July 2006

and igneous apatites from Quebec. A granular size ranging from 2.5 to 10 mm to prevent

Keywords:

clogging in wastewater applications was used. Isotherms (24 and 96 h) were drawn after

Apatite

batch tests using the Langmuir model which indicated that sedimentary apatites presented

Hydroxyapatite crystallization

a higher P-affinity (KL ¼ 0.009 L/g) than igneous apatites (KLE0.004 L/g). The higher density

Phosphorus removal

of igneous material probably explained this difference. P-retention capacities were

Wastewater treatment

determined to be around 0.3 mg P/g apatite (24 h). A 30 mg P/L synthetic effluent was fed during 39 days to four lab-scale columns. A mixture of sedimentary material (apatite and limestone 50–50%, w/w) showed a complete P-retention during 15 days which then declined to 65% until the end of the 39 days lab scale test period. A limitation in calcium may have limited nucleation processes. The same mixture used in a field scale test showed 60% P-retention from a secondary effluent (30 mg COD/L, 10 mg Pt/L) during 65 days without clogging. & 2006 Elsevier Ltd. All rights reserved.

1.

Introduction

Adverse effects of eutrophication due to the presence of anthropogenic phosphorus (P) in surface waters are wellestablished. Conventional technologies for P-removal from wastewater are physical processes (settling, filtration), chemical precipitation (with aluminium, iron and calcium salts), and biological processes that rely on biomass growth (bacteria, algae, plants) or intracellular bacterial polyphosphates accumulation (De-Bashan and Bashan, 2004). Low cost, low maintenance extensive treatment processes based on P-retention in filters containing reactive media have been developed and showed promising results (De-Bashan and Bashan, 2004). The aim of this project was to favour P-

crystallization in a filter located at the downstream end of the sludge treatment line to reduce clogging by suspended solids. Natural apatite, a mineral found in igneous, sedimentary and metamorphic rocks (Nriagu and Moore, 1984), and in teeths and bones, was shown to favour P-removal from wastewater (Joko, 1984; Jang and Kang, 2002; Molle et al., 2005). The scope of this paper was to test and evaluate the P-removal potential of several apatites found in North America. The driving mechanisms in the process of P-crystallization consist essentially in nucleation (precipitation of hydroxyapatite (HAP): Ca10(PO4)6OH2), followed by crystal growth. Nucleation is believed to be initiated by the formation of a metastable precursor, characterised by an amorphous phase consisting in the combination of calcium phosphates at

Corresponding author. Institut de recherche en biologie ve´ge´tale, Universite´ de Montre´al, 4101 rue Sherbrooke Est, Montreal, Que., Canada H1X 2B2. Tel.: +1 514 872 3942; fax: +1 514 872 9406. E-mail address: [email protected] (F. Chazarenc). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.05.016

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a Ca/P molar ratio of between 1 and 1.5 (Zoltek, 1974). The presence of a crystal acts like a catalyst which lowers the activation energy barrier between the crystal and HAP. Seeded precipitation of calcium phosphate is favoured by a high pH, an increased contact time and a high Ca/P molar ratio. Several media favouring the seeded precipitation of phosphate from wastewater were tested, such as magnetite minerals (Karapinar et al., 2004), steel slags (Shilton et al., 2005; Kostura et al., 2005; Kim et al., 2006; Korkusuz et al., 2005; Naylor et al., 2003), calcite limestone and concrete (Song et al., 2006; Comeau et al., 2001; Molle et al., 2003). P-retention capacity of mineral apatites for wastewater treatment varied between 2.7 and 4.8 mg P/g apatite in batch experiments which were followed by successful column tests (Joko, 1984; Molle et al., 2005). For P-removal using natural apatites, adsorption mechanisms are also believed to play a role and the distinction between adsorption and crystallization is not clear (Molle et al., 2005). Considering that their concomitant action is expected to occur, adsorption is believed to enhance nucleation/crystallization (Stumm and Morgan, 1996). In this paper, the P-retention potential of three igneous apatites from Quebec, one sedimentary apatite from Florida and one sedimentary limestone was studied. Batch tests were first performed which enabled to draw isotherms and rank the samples according to their P-removal potential. Then, flow-through lab and field scale column tests were conducted which showed the better potential of sedimentary apatites mixed with limestone over igneous apatites.

2.

Material and methods

2.1.

Media tested and sample preparation

In view of full-scale filters where clogging should be avoided, a granular size of 2.5–10 mm for batch and column experiments was used. Particle-size distribution was determined (ASTM D 421-422) to estimate the specific surface area of the 2.5–10 mm material assuming a spherical shape. The density of each material was determined according to ASTM standard test methods (ASTM C 127-01 for 5–10 mm fraction, and ASTM D 854-02 for 2.5–5 mm fraction).

2.2.

Batch tests were conducted with synthetic solutions (KH2PO4 in distiled water) with a gyratory shaker (160 rpm) at 2272 1C. A mass of 35 g of material was placed in a 1 L glass flask filled with 700 mL of solution. The pH (8.070.1) and conductivity (10007100 mS/cm) were adjusted using NaOH and NaCl to be similar to a secondary treated effluent. Isotherm experiments were conducted at P solutions ranging from 5 to 150 mg P/L and lasting 24 and 96 h. All samples were analysed for pH, conductivity, turbidity, suspended solids and orthophosphates (o-PO4). P adsorption capacity was modelled using Langmuir isotherms, linearized as Ce 1 aL ¼ þ Ce , qe KL KL

(1)

where Ce is the equilibrium concentration of P in solution (mg P/L) and qe is the amount of P adsorbed (mg P/g material). The Langmuir constants are graphically defined by KL and aL. KL describes the affinity between P and the material, and is represented by the initial slope. The constant KL/aL determines the P adsorption maximum.

2.3. Four apatite-containing materials and a limestone, identified by their suppliers, were tested (Table 1). Samples were prepared as follows: crushing, rinsing, air-drying and sieving. During extraction, the associated gangue of the Cargill apatite was separated by the supplier. This may have reduced the amount of calcium and hydroxide available in the sample.

Batch tests

Column tests

Lab scale column tests were carried out using four 100 mm diameter columns filled with 2.5 L of material (Niobec, Cargill, Graymont and Cargill/Graymont mixture 50–50%w/w). A synthetic P solution (P–PO4 30 mg/L, tap water, pH 7.570.1) was continuously pumped into the bottom of each column at

Table 1 – Apatite-containing media tested Media supplier

Cargill (USA)

Location

Geographic coordinatesa

Mine or prospect deposit

Identification

Geological formation

Florida

271410 4900 N 811500 4300 W 501150 2800 N 661290 4400 W 491540 0500 N 701450 3000 W 481320 0800 N 711090 2800 W 461000 4100 N 731270 2600 W

Phosphate rock mine

Francolitehydroxyapatite Fluoroapatite

Sedimentary

Apatite

Igneous

Fluoroapatite

Igneous

Limestone

Sedimentary

Soquem (Quebec)

Sept-Iles

Arianne (Quebec)

Lac St-Jean

Niobec (Quebec)

St-H de Chicoutimi Joliette

Graymont (Quebec) a b

Geographic Reference System: WGS 84. Niobec is exploiting niobium oxide (Nb2O5).

Prospect apatite/ magnetite Prospect apatite/ ilmenite Niobium mineb Limestone quarry

Igneous

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Table 2 – Chemical and physical characteristics of the five tested media Material

Cargill Soquem Arianne Niobec Graymont a b

Density (g/cm3)

Chemical composition (%w/w) Si

Al

Ca

Mg

Fe

P

5.5 10.3 15.0 2.7 n.a.a

0.8 4.0 4.7 0.7 n.a.

31.4 8.6 7.0 25.7 n.a.

0.7 4.0 4.7 6.3 n.a.

1.1 22.0 19.0 3.6 n.a.

11.3 2.4 2.0 5.2 n.a.

Specif. area (m2/kg)

2.85 3.51 3.37 3.00 2.72

0.72 0.48 0.53 0.58 0.57

Particle sizeb d10 (mm)

d60 (mm)

2.8 3.0 3.0 3.0 3.5

4.5 6.0 6.0 6.0 7.0

n.a: not available. d10, d60: grain size of 10% and 60% material by weight.

a flow rate of 0.7 L/d for 39 days (hydraulic retention time of 1.5 d considering void space [HRTv]). Samples were taken at the inlet and outlet of the columns, every day during the first 15 days and every three days until day 39. Field scale tests were carried out using a 6.5 L total volume column filled with a mix of Cargill apatite and Graymont limestone (50–50%w/w). The columns were installed at a fish farm plant and were fed with a constructed wetland effluent (30 mg COD/L, 10 mg Pt/L) such that the HRTv was initially set at 0.33 d and operated during 65 days. The concentration was fixed at of 30 mg P/L in lab scale tests, based on annual average concentrations measured in a wetland effluent from a fish farm plant. Since the field scale tests were conducted during summer, P retention was more important in the constructed wetland resulting in a lower P concentration of the wetland effluent (10 mg P/L).

2.4.

Analytical methods

Orthophosphates were measured using the Quickchem flow injection analysis method # 10-115-01-1-Q derived from the automated ascorbic acid reduction method (Standard Methods, 1998). Ionic measurements conducted on samples collected from column tests, as well as chemical composition of tested minerals, were analysed by the atomic absorption spectroscopy method (Standard Methods, 1998).

3.

Results and discussion

3.1.

Chemical and physical media characteristics

Table 3 – Influent and effluent characteristics of lab scale columns test

Synthetic influent Niobec effluent Cargill effluent Graymont effluent Cargill/ Graymont effluent

pH

Ca2+ (mg/L)

531726

7.970.1

31.870.7

575763 491739 571749

7.670.3 7.170.4 7.970.2

31.170.9 2.872.5 32.372.6

560751

7.670.3

29.171.7

2005), giving a lower density of 2.48 g/cm3 compared to that from Cargill (Table 2), resulted in an increase of the internal porosity (Zapata and Roy, 2004) and in a larger surface of reaction (Table 3). Igneous rocks are characterized by a higher density (Table 2), suggesting a lower internal porosity due to their mode of crystallization in magma (Zapata and Roy, 2004). Apatites from Soquem, Arianne and Niobec showed concentrations varying between 2% and 5% P (Table 2) which is in the range of natural igneous apatite (Nriagu and Moore, 1984). Aluminium, magnesium and iron were also present in a relatively high proportion, notably in the Soquem and Arianne apatites (Nriagu and Moore, 1984; Al 0.05%).

3.2. The Ca and P content of the Cargill apatite (Table 2) was in the same range as those tested by Molle et al. (2005) which containing 37.3% Ca and 16.8% P, and materials tested by Joko (1984) containing 37.6% Ca and 15.2% P. The Ca/P molar ratio of 2.78 of the Cargill apatite, compared to the ratio of 1.67 of pure hydroxyapatite, suggests that some P was substituted by which is generally the case in carbonate-substituted CO2
3 (low-grade) phosphate rock like francolite (Nriagu and Moore, 1984). The presence of calcite or dolomite in the gangue associated to the apatite may also have contributed to increase this ratio (Slansky, 1986). The occurrence of isomorphic substitutions in apatite from Morocco (Molle et al.,

Conductivity (mS/cm)

Batch tests

A solubilization batch test (in distiled water) showed no electrical conductivity and pH variation with apatite samples, while dissolution of limestone from Graymont led to an increase in pH (from 8.0 to 9.5) and in conductivity. Langmuir isotherms determined after 24 h, showed a limited affinity between apatites and P, as indicated by the smooth initial slopes (Fig. 1). Graymont limestone 24 h isotherms showed a constant affinity with P either because adsorption took place regardless of the material, or due to the increase of adsorbate which proportionally augmented adsorbing sites (Sposito, 1989).

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Cargill Graymont

0.3

Langmuir Graymont

0.2

0.1

0.0 20

40

60

80

100

120

0.2

0.1

140

Ce (mgP/L)

(A)

0

Graymont

Langmuir Graymont qe (mg P/g material)

0.8

Langmuir Cargill

40

60

0.6

0.4

0.2

80

100

120

140

100

120

140

Ce (mgP/L) 1.0

Cargill

20

(B)

1.0

qe (mg P/g material)

Langmuir Soquem Langmuir Arianne Langmuir Niobec

0.0 0

Soquem Arianne Niobec

0.8

Langmuir Soquem Langmuir Arianne Langmuir Niobec

0.6

0.4

0.2

0.0

0.0 0

(C)

Soquem Arianne Niobec

Langmuir Cargill qe (mg P/g material)

qe (mg P/g material)

0.3

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20

40

60

80

100

120

140

Ce (mg P/L)

0 (D)

20

40

60

80

Ce (mg P/L)

Fig. 1 – Equilibrium isotherms of phosphate (22 1C) (Cargill apatite and Graymont limestone: (A) 24 h, (C) 96 h; Soquem, Arianne and Niobec apatites: (B) 24 h, (D) 96 h).

P-retention capacities calculated from the 24 h Langmuir isotherms (in mg P/g material: Graymont: 1.09; Arianne: 0.41; Soquem: 0.37; Cargill: 0.31; Niobec: 0.28) were low compared to those found in the literature (2.7–4.8 mg P/g apatite; Molle et al., 2005; Joko, 1984). According to 96 h isotherms (Fig. 1), a greater affinity was found between P and sedimentary materials. Igneous apatites were found to be less reactive with P, possibly due to their lower internal porosity. Considering the importance of calcium in P-retention mechanisms, the limited dissolution observed for the Cargill apatite and the absence of calcium in the synthetic P-solution possibly led to a limitation in available Ca. The low specific surface area of the material used in this study, due to the relatively coarse material, influenced the P-removal efficiency. Furthermore, apatite crystal sizes were relatively small (0.1–0.5 mm in Niobec apatite), compared to the granular size selected (2.5–10 mm), reducing the rates of P adsorption. Using 24 h Langmuir isotherms, the tested materials ordered by decreasing affinity based on KL coefficients were as follows: Cargill4Arianne4Niobec4Soquem4Graymont. Some indications in batch experiments suggest that adsorption occurred almost instantaneously and was rapidly followed by precipitation. Adsorption being a surface phenomenon, it is considered that the high densities associated with igneous materials was not favourable to these types of apatites as adsorbents, in which poor internal porosities were expected. Since the Cargill apatite presented a lower density compared to igneous apatites, this could explain the high affinity found for the sedimentary apatite. No improvement for the igneous apatite was apparent after 96 h.

Fluoroapatite presents a pH zero point of charge (pHZPC) varying between 4 and 6, a lower value than for hydroxyapatite (7.6–8.6), which may discredit compounds rich in fluorapatite (Arianne, and Soquem) as favourable substrates for phosphate adsorption. Apatites are, however, largely associated with other minerals in their natural state, and adsorption may occur in non-apatitic mineral support (especially in Arianne and Soquem media). Another linearization of Langmuir isotherm (Kd ¼ qe/ Ce ¼ f(qe)) allowed the identification of two potential P-retention mechanisms for Cargill apatite (results not shown). This was illustrated in others studies (Søvik and Kløve, 2005; Molle et al., 2005) and could correspond to an adsorption mechanism at low P content followed by precipitation mechanism at great P content. But the maximum initial P concentration in our batch experiment (150 mg P/L) was too low to clearly identify these two potential P-retention mechanisms and to go further in the discussion.

3.3.

Lab scale column experiments

Considering batch test results, only one of the igneous apatite (Niobec) was tested in lab scale column experiments. For increased availability of calcium the Cargill apatite was mixed with Graymont limestone. P-retention by the Niobec apatite was poor and the column seemed to become saturated after 15 days (Fig. 2), confirming the relatively poor efficiency of this type of material. During the first 15 days the retention by Cargill and Cargill/Graymont mixture was comparable and close to 100%. Then a slight decrease appears for Cargill’s. The removal of the associated

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P removed (mgP cumulated)

600 Niobec Cargill Graymont Cargill/Graymont

500

100% retention

400 300

50% retention

200 100 0 0

100

200

300

400

500

600

700

800

900

Padded (mgP cumulated) Fig. 2 – P-removal as a function of P added (lab scale experiment).

Table 4 – Influent and effluent characteristics of field scale column Sampling point Inlet Outlet

pH

Cond (mS/cm)

COD (mg/L)

Total P (mg P/L)

7.170.3 7.870.4

241768 286738

32711 2877

7.672.3 1.971.1

gangue of the Cargill apatite may have reduced the amount of calcium and hydroxide availability of the sample thus reducing its nucleation performance. In column tests using untreated natural apatites, Molle et al. (2005) showed a significant dissolution of calcium carbonate (5 mg/L Ca) leading to a pH increase from 7.0 to 8.0. Graymont limestone showed a relatively stable retention over time that remained near 50%. Effluent analysis at the outlet of each column showed for Cargill’s a significant consumption of Ca2+ over time and a decrease in pH (Table 4) which suggested HAP formation and crystallization. Results showed that Graymont limestone favoured P precipitation as it represented a source of Ca2+ and OH
. The P retention curve for the Cargill/Graymont mixture appeared to be the addition of the curves for the two distinct materials, avoiding the initial poor removal observed with Graymont’s alone. This suggested that Cargill’s probably provided a better seed precipitant than Graymont’s. At the outlet of the Cargill/Graymont mixture, the pH and Ca concentration were higher than in Cargill’s and appeared to favour P-retention (which equals about 200 g of P retained per m3 of column after 39 days), but they were probably not high enough to obtain more than 60% P-retention. In case of P levels of about 15–20 mg/L, the minimum inlet Ca level recommended for Ca–P precipitation, according to Jang and Kang (2002), should be 40–60 mg/L. For the field scale column test, it was chosen to use a single column of a limestone/apatite mixture as it represented the most promising media tested. The Cargill/Graymont mixture seemed to be the best compromise to increase the pH, dissolve some Ca and favour HAP precipitation.

3.4.

Field scale column experiments

The column was installed downstream of a constructed wetland of a fresh water fish farm wastewater treatment plant. Despite the variable composition of the influent (Table 4), the column worked with a total P retention efficiency of 60% without showing signs of either clogging or salting out (Fig. 3). During the field scale test, there was a conflicting effect of pH. On one hand a higher pH at the column inlet favore Ca dissolution, but on the other hand it led to a reduction of HAP formation by reducing the available OH
in solution. Having a too great pH variability at the inlet could become a problem to ensure a long term P retention, the best pH being around neutrality for HAP. The field scale column efficiency was enough to reduce P-level to reach 2 mg P/L but the long-term stability of the process remains to be established. The next step would be to test mesocosm columns under a prolonged period (e.g. 1 year) under varying influent and operational conditions such as Ca availability, retention time and pH.

4.

Conclusion

Phosphorus affinity was found to be higher with Cargill sedimentary apatite as substrate, compared to igneous apatites in batch experiments. Sedimentary apatites are believed to favour crystallization of HAP due to intrinsic characteristics, notably their internal porosity. The presence of secondary minerals associated to the gangue would be

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Removed P (mgP cumulated)

1200 Inlet cumulated P Retained cumulated P

1000

Inlet ≈ 15.5 mg P/d

800 600 400

Retention ≈ 9.5 mg P/d

200 0

0

10

20

30

40

50

60

70

Elapsed time (d) Fig. 3 – P-retention efficiency during the field scale experiment.

beneficial by contributing to a source of Ca, which is essential for Ca–P precipitation. Relatively low P-retention capacities were obtained, probably because the available Ca from dissolution rapidly became depleted. Testing pure Cargill material including the gangue would be of interest. With igneous apatites, neither HAP crystallization nor Ca–P precipitation were believed to have occurred considering the higher affinity of P with other metals like Fe, Mg, Al as pointed out in other studies. Crystallography tests could confirm this interpretation. Best pH conditions are conflicting as a low pH would favour Ca dissolution and a high pH would favour HAP crystallization. In small scale wastewater treatment plants, pH fluctuations may be important thus reducing P-retention availability. A source of calcium and hydroxide should be provided to avoid such conflicting operation conditions. Sedimentary and igneous North American apatites showed low and poor retention capacity for P-removal, respectively. Nevertheless, the potential of apatites for crystallization and P-recycling merits attention. Further studies should be conducted with regards to the critical conditions associated to pH, calcium, and orthophosphates concentrations.

Acknowledgements The authors thank Denis Bouchard for technical support and Dwight Houweling for reviewing an earlier draft of the manuscript. This research was financed by the Natural Sciences and Engineering Research Council of Canada. R E F E R E N C E S

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