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Published in J. Environ. Qual. 33:1895-1903 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Waste Management

Water-Soluble and Solid-State Speciation of Phosphorus in Stabilized Sewage Sludge

Xiao-Lan Huang and Moshe Shenker*

Department of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76-100, Israel

* Corresponding author (Shenker{at}agri.huji.ac.il).

Received for publication December 8, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Three chemicals, ferrous sulfate (Fe-sul), calcium oxide (CaO), and aluminum sulfate (alum), were used to stabilize phosphorus (P) in fresh, anaerobically digested sewage sludge (FSS). The chemically stabilized sludge materials and biosolids compost (BSC) were compared with the FSS with respect to water-soluble phosphorus (WSP) content in its inorganic (WSPi) and organic (WSPo) forms as well as water-soluble organic carbon (DOC). Solid-state P speciation was further probed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray elemental spectrometry (EDXS). Water-soluble P was effectively controlled by a wide range of Fe-sul or CaO additions to the sludge (Ca to P ratio = 3.47–17.72, Fe to P ratio = 1.01–16.53), but by only a narrow range (Al to P ratio = 1.04–2.87) of alum addition. The WSP content in the BSC was also depressed, but to a lesser extent. The pH in the treated sludge ranged from 3.0 to 12.5 and served as a key factor to control P chemistry. No correlation was observed between DOC and WSPo. No crystallized Ca-P minerals were detected in the CaO-stabilized sludge, but brushite crystallization seemed to be obtained by low addition of Fe-sul and alum. Variscite and strengite crystallization was obtained following high addition of Fe-sul or alum, as detected by XRD and SEM–EDXS. Adsorption of P by newly formed Fe-hydroxide seems to play an important role in the Fe-sul-stabilized sludge. We concluded that administration of the tested chemicals at the proper rate can effectively reduce the hazard of P release and leaching from sludge.

Abbreviations: Alum, aluminum sulfate • BSC, biosolids compost • CaO, calcium oxide • DOC, water-soluble organic carbon • EDXS, energy-dispersive X-ray elemental spectrometry • Fe-sul, ferrous sulfate • FSS, fresh, anaerobically digested sewage sludge • SEM, scanning electron microscopy • WSP, water-soluble phosphorus • WSPi, inorganic water-soluble phosphorus • WSPo, organic water-soluble phosphorus • XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
EUTROPHICATION OF FRESH WATERS is a growing environmental problem worldwide, and excess P is well documented as its most common cause in many aquatic systems (Correll, 1998). In general, P emissions from point sources have declined, whereas agricultural land has become a dominant P source (Department for Environment, Food and Rural Affairs, 2002). A significant fraction of P pollution originates from recycled organic waste, including biosolids, used as soil amendments (Haygarth and Jarvis, 1999; Sharpley and Rekolainen, 1997; Sharpley et al., 1994; Sims et al., 2000; Smil, 2000). Sewage sludge is an inevitable byproduct of wastewater treatment and its handling and disposal are the most costly phases of sewage treatment. Agricultural application provides a cost-effective alternative to sludge disposal, but it is essential that the sludge be stabilized before its application on agricultural land to minimize potential environmental problems (Switzenbaum et al., 1997). Only environmentally safe practices in the agronomic use of sewage sludge will ensure this approach as a long-term outlet for this troublesome waste stream.

Sewage sludge generally contains appreciable amounts of N and P and has significant inorganic-fertilizer replacement value for these major plant nutrients (Hue, 1995; Kirkham, 1982; Smith, 1996; Sommers, 1977). Phosphorus content in sewage sludge usually ranges from 8 to 62 g kg–1 on a dry weight basis and most of it (70–95%) is in inorganic forms (Brobst, 1999; Commission on Geosciences, Environment, and Resources, 1996; Fine and Mingelgrin, 1996; Frossard et al., 1994). While much of the sewage N may escape as gaseous compounds during water and sludge treatment, the removed P is entirely retained in the sludge. Thus, the ratio of P to N in the sludge is significantly higher than that required by plants (Brobst, 1999; Hue, 1995; Smith, 1996; Sommers, 1977). Hence, the current practice of applying biosolids in agriculture at rates based on the N requirements of crops results in an excessive P supply relative to crop needs. Many studies have shown that prolonged addition of sludge to soil results in increased total and available soil P and may increase P losses (Chang et al., 1983; Clapp et al., 1994; Otabbong, 1997; Peterson et al., 1994), especially in cases where the applications rates were based on N requirements (Bossche et al., 2000; Withers et al., 2001). Water-soluble phosphorus (WSP) reflects the most available and prone-to-leaching P fraction in sludge, and great variation in its content has been reported (Elliott et al., 2002).

Calcium stabilization of sewage sludge has been well developed for the production of commercial products (Logan and Burnham, 1995; Logan and Harrison, 1995; Wright et al., 1998). It has been confirmed that using chemical amendments such as ferrous sulfate (Fe-sul) and aluminum sulfate (alum) is an efficient method of controlling P pollution from animal waste applied on acidic sandy soils (Dao, 1999; Moore and Miller, 1994; Shreve et al., 1995).

Phosphorus mineralogy has been explored in different types of sludge and different minerals have been reported (Frossard et al., 1994, 1997; Hinedi et al., 1989). However, data on mineralogical characteristics and transformations in chemically stabilized sludge are scarce. The objectives of this paper were to examine the effects of Fe-sul, CaO, or alum stabilization on WSP and solid P speciation in the treated sewage sludge in comparison with fresh, anaerobically digested sewage sludge (FSS) and biosolids compost (BSC). This information is essential to our understanding of P chemistry after these sludge materials are applied as amendments to soils. Our aim is to elaborate these processes and provide some groundwork studies that we believe are essential for sewage sludge pretreatment methods for environmentally safe, long-term agronomic application.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sludge and Stabilization Treatments
The FSS was obtained from the sewage treatment plant in Netanya, Israel. The raw material had a pH of 7.54 (1:60 solid to water) and contained 73% water, 630 g kg–1 total organic matter, and 29.8 g kg–1 total P. Total metal content was (g kg–1): 72.1 Ca, 7.9 Fe, 7.2 Al, 6.1 Mg, 3.7 K, 3.1 Zn, 0.37 Cu, 0.105 Mn, 0.061 Cr, and 0.046 Ni. These levels are expressed on a sludge dry weight basis (105°C).

Three chemicals, FeSO4·7H2O, CaO, and Al2(SO4)3·18H2O (designated Fe-sul, CaO, and alum, respectively), each at six chemical to FSS ratios, were used to stabilize the sludge. All chemicals were of chemically pure (CP) grade. After thorough mixing with the designated amount of the chemical, the treated sludge was incubated at room temperature in loosely covered containers to reduce drying but allow aeration. After 1 mo, covers were removed to allow air-drying of the stabilized sludge. The entire process took 4 mo. The air-dried, chemically stabilized sludge was milled and stored for analysis. The final products are designated according to the molar ratio of metal to P, as listed in Table 1.


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  		Table 1. Molar ratios of metal (Fe, Ca, or Al) to P in treated and untreated sludge.

 
The biologically stabilized product (BSC) was prepared in a 2-m-wide, 2-m-high windrow consisting of the same FSS, mixed with chopped yard waste in a volumetric ratio of 2:1 sludge to yard waste. The pile was turned over once a week for the first two months, twice a month for the following two months, and once a month until the end of compost preparation, using a front loader for mechanical aeration. After 50, 156, and 214 d, samples were freeze-died, milled, and stored for analysis.

Sludge Analysis
Water-soluble P was extracted with deionized water. Sludge samples of 0.5 g were shaken end-to-end for 16 h with 30 mL of deionized water in a 50-mL centrifuge tube at 25°C; the tubes were then centrifuged at 12000 x g for 10 min and the supernatant was passed through a 0.45-µm filter (Schleicher & Schuell, Dassel, Germany). Inorganic water-soluble phosphorus (WSPi) was determined directly in the filtrates and total WSP was determined after digesting aliquots of the filtrates in an autoclave at 103.5 kPa with acidified (NH4)2S2O8 (Clesceri et al., 1998). The WSPi in the initial filtrates and WSP in the digested samples were determined colorimetrically using the Murphy and Riley (1962) method. The difference between the total WSP and WSPi in the extracts was designated organic water-soluble phosphorus (WSPo). Concentrations of Fe, Ca, and Al in the water extracts were measured using an inductively coupled plasma–atomic emission spectrometer (ICP–AES) (Spectro, Kleve, Germany). The pH of the water extracts was measured by a combined glass electrode and the water-soluble organic carbon (DOC) concentration was determined with a Formacs Combustion TOC Analyzer (Skalar Analytical, Breda, the Netherlands) following acidification of the tested water extracts.

All sludge materials were digested with HNO3–HClO4 (Clesceri et al., 1998) to determine total content of Fe, Ca, Al, and P. The metals were analyzed with ICP–AES, and total P with the Murphy and Riley (1962) method.

Mineralogical characterization of selected chemically stabilized sludges was conducted on powdered samples by X-ray diffraction (XRD) using a Philips (Eindhoven, the Netherlands) diffractometer with Co K{alpha} radiation (PW 1720). Specimens were scanned from 4 to 54°2{theta}. The various minerals were identified based on Joint Committee on Powder Diffraction Standards (1974). Selected chemically stabilized sludge samples were subjected to scanning electron microscopy (SEM) (JSM-5410LV scanning microscope; JEOL, Peabody, MA), equipped with energy-dispersive X-ray elemental spectrometry (EDXS) (Oxford Instruments, Witney, UK). At least 50 particles in each sample were analyzed for element content as calculated by the ZAF method (seven iterations).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The total WSP content in the FSS was 3.52 g kg–1 P (Table 2), 11.5% of the total P in the sludge; 96% of the WSP was in inorganic forms (WSPi). The effect of the chemicals and biological treatments on the P status of the sewage was significant. The P content and its organic–inorganic composition in water extracts of stabilized sludge were considerably affected by the stabilization processes, the time of composting, the chemical used, and its dose in the chemically treated sludge. The WSP was effectively suppressed to less than 5% of that in the untreated sewage sludge by all chemicals at their most efficient chemical to sewage sludge ratio (Table 2), but each chemical displayed a unique response pattern to the dose used, as detailed below.


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  		Table 2. Water-soluble phosphorus (WSP) in stabilized sewage sludge and untreated sewage sludge (FSS).{dagger}

 
The main minerals in the FSS were silicates (quartz and cristabalite) and carbonates (calcite, vaterite, and dolomite), but no phosphate minerals were detected by XRD (Fig. 1). The mineralogy of P in the stabilized sludge changed significantly, depending on the chemical used and its rate.



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  		Fig. 1. X-ray diffraction (XRD) patterns of the ferrous sulfate (Fe-sul)-stabilized and untreated (the lowest Fe to P molar ratio) sludge. Mineral symbols are as follows: BR, brushite; G, gypsum; VA, ferrian variscite; LI, lipscombite; RO, rozenite; SD, siderotil; HJ, hydronium jarosite; MG, magnetite; CR, cristabalite; Q, quartz; C, calcite; V, vaterite; and DO, dolomite.

 
Biosolids Compost
Composting of the sewage sludge resulted in P stabilization as indicated by the significant decrease in the total WSP, WSPi, and WSPo concentrations compared with the original sludge (Table 2). The lowest values of WSP and WSPi were found in the 156-d BSC (559 and 508 mg kg–1, respectively). The lowest DOC content was 3.34 g kg–1 in the 156-d BSC, compared with 7.50 g kg–1 in the untreated sludge, 7.30 g kg–1 in the 50-d BSC, and 4.32 g kg–1 in the 214-d BSC. The pH of the BSC also varied with time, from its original value of 7.54 ± 0.12 in the untreated sludge to 7.14 ± 0.02 at 50 d, 7.37 ± 0.08 at 156 d, and 7.30 ± 0.03 at 214 d. In general, the decrease in WSPo was enhanced with time (Table 2), but no relationship was found with the corresponding DOC. Soluble Fe, Ca, and Al changed slightly during composting, reaching the highest Fe and Al and lowest Ca concentrations at 156 d (Table 3).


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  		Table 3. Water-soluble metals in stabilized and untreated (FSS) sewage sludge.{dagger}

 
Ferrous Sulfate–Stabilized Sludge
The Fe-sul was the most effective at depressing the total WSP concentrations (Table 2). At the low Fe-sul addition, resulting in an Fe to P molar ratio increase from the original 0.14 to 0.58, the WSP decreased from its original level of 3520 to 922 mg kg–1; at higher Fe to P ratios (Fe to P ratio ≥ 1.90), it decreased sharply to much lower levels (<80 mg kg–1). The lowest WSP (38.7 mg kg–1, 1.1% of the control) was found at the Fe to P ratio of 3.04. Most of the P stabilization was assigned to the inorganic fraction, WSPi, which decreased to less than 0.5% of its value in the control. The lowest WSPi (9 mg kg–1) was also obtained in the same sample (3.04 Fe to P sludge ratio). On the other hand, the WSPo decrease was much smaller (down to 13.7% of the control at the Fe to P ratio of 3.04), and WSPo became the dominant P fraction at Fe to P ratios of 1.01 or higher (Table 2).

The DOC also decreased from 7.5 to 3.7 g kg–1 with the increase in Fe to P ratio (Fig. 2), but there was no significant relationship between the WSPo and DOC concentrations. The pH of the water extract decreased significantly, from 7.5 in the FSS to 3.0 in the stabilized sludge (Fig. 3). This decrease in pH was attributed mainly to the hydrolysis of Fe3+ and precipitation of Fe-hydroxides. As expected, the low pH resulted in the dissolution of indigenous carbonates, which could be observed using XRD analyses (Fig. 1). Carbonate peaks seen in the FSS curves (calcite, vaterite, and dolomite) decreased in the sludge with the Fe to P ratio of 0.58 and completely disappeared at higher Fe to P ratios, while water-soluble Ca increased (Table 3). At the highest Fe to P ratio, Ca concentrations decreased due to dilution of the sludge by the added chemical.



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  		Fig. 2. Effect of chemical addition on water-soluble organic carbon (DOC) concentration in the treated and untreated (the lowest metal to P molar ratio for each treatment) sludge. Data are the averages and standard deviations of three replicates.

 


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  		Fig. 3. Effect of chemical addition on the pH of water extracts (1:60, 16 h) from treated and untreated (the lowest metal to P molar ratio for each treatment) sludge. Data are the averages of three replicates; standard deviations were lower than 0.12 in all cases.

 
The change in mineral composition was quite pronounced (Fig. 1). Phosphorus originating from the sludge, and SO2–4 originating from the added Fe-sul, precipitated with the Ca originating from the dissolved Ca-carbonates to form brushite (CaHPO4·2H2O) and/or gypsum (CaSO4·2H2O), as evidenced by the diffraction lines at 7.57, 4.24, 3.05, 2.93, and 2.6 Å, which are characteristic of both minerals. Rinaudo et al. (1996) and Hina et al. (2001) pointed out that there are almost no differences in the crystallographic and structural analogies between gypsum and brushite. Both crystal structures are made of tetrahedral HPO2–4 and SO2–4 linked by Ca and water molecules, hence the XRD cannot distinguish between them. However, element dot-mappings of the low-ratio (0.58) Fe to P sludge provided solid information: there were at least two different P species, one combined with Ca (Ca-P) and the other with Fe (Fe-P) (Fig. 4). The Fe to P molar ratio in the Fe-P particles was in the range of 0.96 to 11.04 (4.18 ± 3.16, n = 25), indicating the possible presence of Fe-P minerals or P adsorbed to Fe-hydroxides. The Ca to P molar ratio in the Ca-P particles was in the range of 1.12 to 12.83 (4.05 ± 3.54, n = 24), indicating that the P may reside in Ca-P minerals with a Ca to P ratio of 1, such as brushite, or it may be adsorbed to, or coprecipitate with, calcite. The dot-mappings also showed evidence of gypsum formation, with a Ca to S molar ratio in the range of 0.98 to 6.45 (1.93 ± 1.76, n = 29). Variscite with ferrian [(Al, Fe)PO4·2H2O] (5.42, 4.29, 3.07, 4.86, 3.94, and 2.88 Å) and lipscombite [Fe3(PO4)2(OH)2] (3.33, 3.20, and 2.06 Å) were detected by the XRD at a high (6.15) ratio of Fe to P sludge, along with siderotil (FeSO4·5H2O) (4.89, 3.73, 5.76, and 5.73 Å), rozenite (FeSO4·4H2O) (4.47, 5.46, and 3.97 Å), and hydronium jarosite [Fe3(SO4)2(OH)5·2H2O] (3.09, 5.11, and 3.13 Å). No peaks of vivianite [Fe3(PO4)2·8H2O] were found in any of the Fe-sul-stabilized sludge, probably because of rapid oxidation of the ferrous ion during the sludge treatment. Magnetite (Fe3O4), with diffraction lines at 2.53, 3.7, and 2.97 Å, occurred in the Fe-sul-stabilized sludge, as also reported by Barrado et al. (1998)(2002). It seems that most of the oxidized Fe had precipitated as amorphous Fe(OH)3, which was not detected by the XRD analysis. These Fe-hydroxides have a large surface area and a high affinity to phosphate, and thus they may effectively adsorb P (De Haas et al., 2000; He et al., 1996). Based on these results, we can assume that the main chemical reactions leading to WSP suppression in the Fe-sul-stabilized sludge were ferrous oxidation and Fe-hydroxide precipitation accompanied by acidification and calcite dissolution, and resulting in Ca-P precipitation at low Fe to P ratios. The Ca-P precipitate was transformed to or coexisted with Fe-P minerals at higher Fe to P ratios. Adsorption of P by newly formed Fe-hydroxide seems to play an important role in the P stabilization.



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  		Fig. 4. Electron micrographs of ferrous sulfate (Fe-sul)-stabilized sludge (Fe to P ratio = 0.58) showing location of Ca, Fe, S, and P, respectively. The particles of (a) Fe-P, (b) Ca-P, and (c) gypsum are shown on the scanning electron microscopy (SEM) image (left).

 
Calcium Oxide–Stabilized Sludge
As did the Fe-sul treatment, the CaO treatment also resulted in decreased WSP with increasing Ca to P molar ratio in the stabilized sludge (Table 2). At a Ca to P ratio of 3.47, the total WSP was 181 mg kg–1, which amount to 5.1% of its level in the untreated sludge. Most of the WSP decrease was assigned to WSPi, while WSPo did not decrease much and became the dominant fraction at Ca to P ratios of 3.47 and above. The lowest WSP and WSPi values (66 and 1 mg kg–1, respectively) were obtained at the highest ratio of Ca to P (Table 2). As opposed to the Fe-sul treatment, the decrease in WSP as the Ca to P ratio increased was accompanied by a pH increase (up to 12.5) rather than a decrease (Fig. 3), and a DOC increase to 20.1 g kg–1 (Fig. 2) in the sludge with the highest Ca to P ratio (17.72). The pH increase is attributed to CaO hydrolysis, while the buffer capacity at low (up to Ca to P ratio of 3.47) rates of CaO additions may be partially attributed to Ca-P precipitation accompanied by proton release, as is indicated by the sharp decrease of WSPi in this Ca to P ratio range (Table 2). Similar to the other chemical treatments, the DOC was positively related to the pH and increased as pH increased, probably due to pH-dependent increase of the negative charge of its macromolecules. As for Fe-sul, no correlation was found between WSPo and DOC.

The presence of portlandite [Ca(OH)2)], with diffraction lines at 2.63 and 4.9 Å (Fig. 5) in the highest Ca to P ratio sludge, indicates that at this high level of CaO addition, some of the added chemical is still active and may react as the treated sludge is incorporated into the soil. The absence of any detectable Ca-P minerals in any of the CaO-stabilized sludge could have been the result of a high concentration of DOC in this treatment (Fig. 3), which coats the seed Ca-P particles and inhibits crystal growth. A similar mechanism was reported previously by Inskeep and Silvertooth (1988) and Grossl and Inskeep (1991)(1992).



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  		Fig. 5. X-ray diffraction (XRD) patterns of the CaO-stabilized and untreated (the lowest Ca to P molar ratio) sludge. Mineral symbols are as follows: PO, portlandite; CR, cristabalite; Q, quartz; C, calcite; V, vaterite; and DO, dolomite.

 
Based on these results, we assume that the main chemical reactions in the CaO-stabilized sludge are CaO hydrolysis and Ca-P precipitation as noncrystalline nanoparticles of Ca-P phases coated with adsorbed organic matter.

Alum-Stabilized Sludge
The alum treatment significantly differed from the Fe-sul and CaO treatments in that it had a narrow effective molar ratio range (Al to P ratio= 1.04–2.87) of P-solubility reduction (Table 2). The total WSP decreased sharply to 77 mg kg–1, 2.2% of that in the FSS, at the 1.65 molar ratio of Al to P; but at higher alum rates, the WSP increased sharply to 522 mg kg–1 at an Al to P ratio of 2.87, 2.16 g kg–1 at an Al to P ratio of 5.07, and up to 5.30 g kg–1 at an Al to P ratio of 9.55. The three highest Al to P ratios (5.07, 9.55, and 22.17) resulted in P dissolution from fractions that were not soluble in the untreated sludge, and up to 56% of the total P became water soluble at the highest alum level. Most of the WSP increase originated from an increase in the inorganic fraction. Similar to the Fe-sul treatment, the pH dropped significantly, to 3.6 (Fig. 3), and the DOC to 2.6 g kg–1 (Fig. 2). As for the Fe-sul and CaO systems, no correlation was found between DOC and WSPo. Most of the WSPo decrease, from 199 mg kg–1 in the untreated sludge to undetectable levels, occurred in the Al to P ratio range of 0.32 to 5.07 (Table 2), while the DOC concentrations did not change much (from 7.5 to 6.2 g kg–1; Fig. 2) in this Al to P ratio range. At the highest Al to P ratio (22.17), the DOC decreased further, to 2.6 g kg–1, while the WSPo increased from undetectable levels to 78 mg kg–1 (Table 2). Similar to the Fe-sul system, the pH drop was assigned to metal (Al3+ in this case) hydrolysis and the precipitation of Al-hydroxides. Water-soluble Fe and Al increased sharply as the pH decreased due to following increasing alum addition rates (Table 3). At the lower range of alum addition rates (Al to P ratio rates of 1.04 and 1.65), the pH appeared to be buffered by the dissolution of calcite, as indicated by an increase in water-soluble Ca (Table 3). At the Al to P ratio of 2.87, all of the calcite was dissolved, as shown by the pH drop and by the XRD analysis. Higher alum rates diluted the sludge and resulted in decreasing water-soluble Ca.

Similar to the case with Fe-sul-stabilized sludge, the sludge-originated dissolved Ca precipitated with SO2–4 to form gypsum, and/or with sludge-originated P to form brushite (Fig. 6). At a low rate of Al to P ratio (1.04), the diffraction lines of these minerals (7.57, 4.24, 3.05, 2.93, and 2.6 Å) were detectable by XRD. The EDXS of typical particles from this sludge (Fig. 7) further supported the notion that both brushite and gypsum reside in these sludge samples. One type of particle (Fig. 7A) exhibited a Ca to P molar ratio of 1.06 (concentration of 9.14% P and 12.57% Ca), while another type (Fig. 7B) exhibited a Ca to S molar ratio of 0.97 (6.59% S and 7.97% Ca). These particles may consist of brushite and gypsum, respectively, with approximately 50 to 60% impurities. Higher alum rates, resulting in a 9.55 Al to P ratio, led to gypsum–brushite XRD lines of lower intensity. Since brushite solubility increases with decreasing pH while gypsum solubility is irrespective of pH, we assume that much of the intensity of the gypsum–brushite XRD lines is indeed attributable to brushite. At these high alum treatments (9.55 Al to P ratio), the sludge P was also transformed to a myriad of crystalline Al-P and Fe-P minerals, such as variscite (AlPO4·2H2O) (5.37, 4.26, and 3.04 Å) and strengite with aluminum substitution [(Fe,Al)PO4·2H2O] (5.46, 4.33, and 3.08 Å) (Fig. 6). Alunogen [Al2(SO4)3·17H2O], with diffraction lines at 13.4, 4.48, and 3.36 Å, also occurred at this high alum treatment, indicating, similar to the case with the high CaO treatment, that some of the added chemical is still active and may react as the treated sludge is incorporated into the soil. No crystalline Al-hydroxides (gibbsite, bayerite, nordstrandite, boehmite, or diaspore) were detected in any of the alum-treated sludge materials. The possibility of brushite formation at low Al to P ratios is supported by a recent study of Peak et al. (2002) who, using XANES spectroscopy, found brushite formation in low-rate alum-treated poultry litter (<0.85 Al to P ratio). In contrast to our case, no brushite was detected by these authors at higher alum rates (1.13 Al to P ratio). Peak et al. (2002) also did not observe any Al-P precipitation, perhaps due to the lower concentration of inorganic P in the poultry litter compared with the sludge.



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  		Fig. 6. X-ray diffraction (XRD) patterns of the alum-stabilized and untreated (the lowest Al to P molar ratio) sludge. Mineral symbols are as follows: BR, brushite; G, gypsum; VA, variscite; AL, alunogen; ST, aluminaum strengite; CR, cristabalite; Q, quartz; C, calcite; V, vaterite; and DO, dolomite.

 


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  		Fig. 7. Typical X-ray spectra (energy-dispersive X-ray elemental spectrometry, EDXS) of particles of alum-stabilized sludge (Al to P ratio = 1.04). The element content is given in g kg–1 as calculated by the ZAF method: (A) P = 91.4, Ca = 125.7, Al = 17.1, and S = 24.1 for the Ca-P particle; (B) P = 8.2, Ca = 136.7, Al = 11.5, and S = 116.2 for the Ca-S particle.

 
As opposed to the Fe-sul treatment, no evidence for precipitation of hydroxides was detected by XRD, and no adsorption of phosphate was detected by SEM–EDXS. This may be a result of the lower stability of Al hydroxides compared with ferric hydroxides. Indeed, geochemical calculations based on metal concentrations in the water extracts and relevant pH had indicated oversaturation even for the least stable amorphous Fe(OH)3 in the Fe-sul-treated sludge (all rates), while undersaturation was found for amorphous Al(OH)3 in the alum-treated sludge as pH dropped [Al to P ratios of 2.87 and above; detailed calculations, carried according to stability constants of Lindsay (1979), are not shown]. Thus, it is assumed that similar to the Fe-sul treatment, at high alum treatments' pH drop was combined with dissolution of Ca-P and probably also amorphous Al-P minerals, but unlike Fe-sul treatments this process was not accompanied by a massive formation of new adsorption sites of fresh hydroxides, and hence WSPi sharply increased at higher application rates of alum.

The main chemical reactions of alum in the sludge involve (i) Al3+ hydrolysis and Al-hydroxide precipitation, resulting in a decrease in pH; (ii) acid dissolution of calcite, which supplies soluble Ca; (iii) Ca-P and/or Ca-S precipitation as brushite and gypsum, respectively, at low Al to P ratios; and (iv) crystalline Al-P mineral formation at higher Al to P ratios as SO2–4 increases and effectively competes for the released Ca while brushite solubility increases due to the lower pH.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
All of the tested chemicals—Fe-sul, CaO, and alum—significantly changed the WSP content and speciation. Both properties depend on the type of chemical used and its molar ratio in the sludge. Water-soluble P was effectively controlled by a wide range of chemical to sludge ratios using Fe-sul and CaO (Fe to P ratio = 1.01–16.53, Ca to P ratio = 3.47–17.72), but by only a narrow range (Al to P ratio = 1.04–2.87) with alum. Biosolids compost, compost of FSS, also suppressed the WSP content, but less effectively than the chemicals. There was no correlation between WSPo and DOC in any of the stabilized sludges. Phosphorus mineralogy of the sludge was also notably altered by the chemicals and their application rates. No crystallized Ca-P was detected in the CaO-stabilized sludge. Brushite was formed by low rates of alum and Fe-sul, and was transformed to and coexisted with other alum- or Fe-P minerals at higher ratios of both chemicals. As far as P solubility is concerned, it was concluded that low rates of each of the tested chemicals could effectively reduce the hazard of P pollution from sludge. However, the retention mechanisms are pH-dependent and thus Fe-sul- or alum-stabilized sludge, in which much of the sludge P is retained by adsorption to Fe- or Al-hydroxides at the low pH of the treated sludge, may release much of the adsorbed P if incorporated into a soil of higher pH. Similarly, P retained by Ca-P precipitation in high-pH CaO-stabilized sludge may be released when the sludge is incorporated into a soil of lower pH. In such instances, residual activity of the used chemicals may be of importance to the process in the sludge-amended soil. Further studies are needed aiming to characterize these sludge–soil interactions and to assign the proper pretreatment for each sludge material before its application to a given soil. This study is of high relevance and importance to those trying to achieve beneficial use of sludge while reducing environmental risks, thereby providing a long-term outlet for this waste.


   ACKNOWLEDGMENTS
 
This research was supported by the Israeli Ministry of the Environment.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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