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a Lethbridge Research Centre, Agriculture and Agri-Food Canada, 5403 - 1 Avenue S., Lethbridge, AB, Canada T1J 4B1
b Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, Room 439, St. Paul, MN 55108-6028
* Corresponding author (crosen{at}umn.edu)
Received for publication May 8, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BTC, breakthrough curve • DOP, dissolved organic phosphorus • ICP–AES, inductively coupled plasma–atomic emission spectroscopy • PP, particulate phosphorus • SBL, sugarbeet lime • SRP, soluble reactive phosphorus • STP, soil test phosphorus • TDP, total dissolved phosphorus • TP, total phosphorus • TSP, total soluble phosphorus • WEP, water-extractable phosphorus • WTR, water treatment residual
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Phosphorus sorption by soils plays a pivotal role in reducing the potential for P loss through leaching. Repeated land application of manures, biosolids, and wastewater typically elevates soil P levels, thus reducing its P sorption capacity (Elliott et al., 2002). While considerable effort has been devoted to reducing P loss in surface runoff through erosion control measures, the potential for P loss through leaching has traditionally received little attention (Eghball et al., 1996; Hesketh and Brookes, 2000). Recent studies, however, indicate that P leaching can be a significant problem in poorly drained soils high in organic matter (Sharpley et al., 1994) and in soils with a long history of manure (Breeuwsma et al., 1995; Heckrath et al., 1995) and waste application (Zvomuya et al., 2005, 2006). The potential risk to surface waters supplied by shallow aquifers underlying such soils can be considerable.
The food-processing industry, which generates large volumes of wastewater, has for years faced challenges from regulations governing wastewater discharge into surface waters and from ever-increasing wastewater treatment costs (Mullan et al., 2002). As an alternative, food processors can use cropland irrigation as a means of wastewater disposal (Robbins and Smith, 1977; Geber, 2000). However, high wastewater application rates or prolonged application periods on a limited land area can result in P buildup and saturate the P sorption capacity of the soil. Westerman et al. (1995) reported that soil solution P increased from 3 mg L–1 1 yr after the start of swine lagoon effluent (56 mg L–1 P) application to >30 mg L–1 after 3 yr of application. Once soil P levels are elevated, continued wastewater application can lead to P leaching and thus pollution of unconfined shallow aquifers and, indirectly, the surface waters.
Regulations aimed at minimizing eutrophication of surface waters have recently come into effect in parts of the United States (Sims, 2000). The laws specifically target P and use soil test phosphorus (STP) levels to determine whether P applications at given sites should be continued, restricted, or discontinued. Because land availability is limited, continued operation of food-processing plants will necessitate remediation of the available high P soils. Chemicals, especially aluminum sulfate [alum; Al2(SO4)3·14H2O] and ferric chloride (FeCl3), are extensively used by wastewater treatment plants to remove P from the wastewater (Galarneau and Gehr, 1997). The successful use of these chemical amendments and Al- and Fe-based water-treatment residuals (WTRs, that is, the solid or semisolid residue generated during the treatment of water or wastewater with, inter alia, Al and Fe compounds to remove suspended solids and dissolved chemicals, e.g., P) to tie-up P in animal manure is well documented (Dao et al., 2001; Dou et al., 2003). Moore and Miller (1994) reported significant reduction in soluble P levels in poultry litter following application of Al, Ca, and Fe compounds.
The efficacy of Al-based WTRs in sorbing and precipitating soluble manure P has also been demonstrated (Peters and Basta, 1996). Aluminum WTR and an iron-rich titanium-processing by-product reduced soluble reactive phosphorus (SRP) by 39 and 48%, respectively (Dao et al., 2001). Results from a recent study by Rhoton and Bigham (2005) demonstrated the feasibility of using ferrihydrite as an amendment for adsorbing soil solution P. However, because of the pH-dependent charge of ferrihydrite, its effectiveness will be limited to acid soils.
The mechanism of P sequestration by alum and FeCl3 is thought to involve the reaction of ortho-P with these compounds to form precipitates (e.g., variscite and strengite), or adsorption on Al or Fe hydroxide formed by hydrolysis. Moore and Miller (1994) proposed the following reaction between alum and phosphate following application of alum to poultry litter:
 | [1] |
Moore et al. (2000) suggested that the amorphous phosphate compound/complex formed in Eq. [1] would likely eventually transform into a crystalline mineral such as variscite (AlPO4·2H2O) or wavellite.
Recent advances in spectroscopic techniques have permitted closer scrutiny of the mechanisms involved in P fixation, and there are indications that metal-P precipitation may not be as important as previously thought. Using X-ray absorption near edge structure (XANES) spectroscopy, Peak et al. (2002) found no evidence of Al phosphate precipitation in alum-amended poultry litter. Instead, their results suggested the formation of amorphous Al(OH)3, which then reacted with phosphate via adsorption mechanisms (Eq. [2]):
 | [2] |
Hunger et al. (2004) reported that phosphate associated with Al in alum-amended poultry litter was present either as a poorly ordered wavellite [Al3(PO4)2(OH)3· 5H2O] or as phosphate surface complexes on Al hydroxide formed by the hydrolysis of alum.
Regulations in Minnesota require that sprayfields that are irrigated with wastewater are bermed to prevent runoff into nearby water bodies. Most of the P loss from these sprayfields, therefore, occurs via leaching, particularly from coarse-textured soils (Zvomuya et al., 2005). Leaching of P is a cause for concern in parts of northern Minnesota where ground water is a significant source of water to some trout streams (Winter et al., 1998). Zvomuya et al. (2005) demonstrated that P leaching past the 1.5-m depth in a wastewater-irrigated sandy loam in northern Minnesota originated from the soil rather than from the low-P tertiary-treated wastewater applied in winter to the sprayfields. Sequestration of soil P using suitable amendments could help reduce the risk of P leaching from these soils.
The objectives of this study were to (i) test the efficacy of Al-, Fe-, and Ca-based chemical and by-product amendments for immobilizing soil P through laboratory incubation studies, and (ii) evaluate the field performance of the most promising amendment in a seasonally frozen, high P outwash soil that has received repeated applications of potato-processing wastewater for several years.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil Analysis
Soil pH and EC were measured potentiometrically in a 1:1 soil to water suspension using an Orion 920A ISE meter (Thermo Electron Corp., Beverly, MA). Total P concentration was determined in air-dried soil (<2 mm) by the ascorbic acid method (Murphy and Riley, 1962) after perchloric (HClO4)–nitric (HNO3) acid digestion (Kuo, 1996) on a block digester. Water-extractable P was determined after shaking 2 g soil with 20 mL of deionized water for 1 h, followed by filtration of the suspensions through 0.45-µm membrane filters (Self-Davis et al., 2000). For Bray-1 P determination, 20 mL Bray solution (0.03 M NH4F + 0.025 M HCl) were added to 2 g soil and shaken for 5 min on a reciprocating shaker (Bray and Kurtz, 1945). The suspensions were then filtered through a Fisher Q2 quantitative filter paper (1-µm pore size; Fisher Scientific, Hampton, NH). Water- and Bray 1-extractable P concentrations were determined in the supernatant by the ascorbic acid method (Murphy and Riley, 1962) at 880 nm.
Statistical Analysis
Data were analyzed using the SAS statistical package (SAS Institute, 1999) based on the split-split-plot arrangement with three replications, with date as the main plot, amendment as the subplot, and relative rate as the sub-subplot. All data were tested for normality of distribution using the UNIVARIATE procedure of SAS before statistical analysis. Data conforming to the normal distribution were analyzed using the MIXED procedure of SAS (Littell et al., 1996), with replication by sampling date and replication by sampling date by amendment as the random effects. Subsequently, orthogonal polynomial contrasts were also tested and when significant they were followed by regression analysis to determine relationships between the dependent variables and the rates. Any data not conforming to the normal distribution were normalized by log-transformation before parametric analysis. Effects were considered significant at the 5% probability level.
Field Breakthrough Study
Site Description
The field experiment was conducted in sprayfields adjacent to a potato processing plant near Park Rapids, MN (46°98' N, 95°09' W). The soil at the site was a Verndale sandy loam, with an average bulk density of 1.63 Mg m–3. Chemical properties measured in composite samples (0- to 0.3-m depth) taken from the sprayfields before the start of the experiment were pH: 7.7; Bray-1 P (Sims, 2000): 108 mg kg–1; organic matter (Nelson and Sommers, 1996): 1.9%; and WEP (Self-Davis et al., 2000): 72 mg kg–1.
The 120-ha sprayfields received manure and potato waste applications before 1996 and have received wastewater applications since 1996 when the Minnesota Pollution Control Agency (MPCA) issued a permit allowing disposal of wastewater from the plant. The sprayfields were bermed to ensure wastewater did not run off from the fields. The sprayfields were seeded with reed canarygrass (Phalaris arundinacea L.) in May 2001 and interseeded with the same crop in May 2003 using a no-till drill. Reed canarygrass was chosen because of its ability to survive the ice buildup during winter wastewater application. The reed canarygrass was harvested two times during the growing season.
The MPCA's requirements were that nutrient concentrations in wastewater applied in the sprayfields during the winter (October through March) must not exceed 6 mg TP L–1, 10 mg NO3–N L–1, and 20 mg total Kjeldahl nitrogen (TKN) L–1. These criteria were met by precipitation (using alum) and biological removal of P and by denitrification of N using a sequencing batch reactor during the wastewater treatment process. Nutrient concentrations in wastewater applied during the growing season were not regulated, except that N addition from all sources, including the wastewater, must not exceed 336 kg ha–1, which was the agronomic N requirement of the reed canarygrass grown in the sprayfields.
Lysimeter Construction and Installation
Two drainage lysimeters (1.5 m deep x 28.9 cm internal diameter) installed in the sprayfields in 1996 were used in the study. The lysimeters were constructed using minimally disturbed soil cores that were collected from areas outside the sprayfields by pushing a PVC tube into the soil with a backhoe. Soil in the 0- to 0.3-m layer, which would have been affected by past cultivation, was removed before collection of the 1.2-m-long minimally disturbed core and replaced at installation with surface soil from the installation site. A 23-L collection vessel fitted with a sample access tube was attached to the bottom of the PVC containing the soil core. The lysimeter was gently lowered into a 2-m-deep hole previously dug using a 0.6-m-diameter power auger. The sample access tube, protected using an electrical conduit, was buried in the ground away from the lysimeter. The hole was backfilled with subsoil and bentonite added along the lysimeter wall to prevent preferential flow. The lysimeter was then covered with soil from the 0- to 0.3-m layer retained during augering. Further details on lysimeter construction and installation are given in Zvomuya et al. (2005).
Amendment
Alum was chosen for field evaluation based on results from the laboratory incubation study (see Results and Discussion, below). Alum presents greater opportunity for field application because it forms the least soluble P compounds compared with Fe and Ca within the pH range 2 to 9 (Stumm and Morgan, 1981). Furthermore, solubility of Al-P compounds is governed purely by pH and these compounds are less sensitive to changes in redox potential (Eh), compared to ferric phosphate (Stumm and Morgan, 1981). At our sprayfield site, winter wastewater application on seasonally frozen soils results in extended ponding during the winter and early spring, leading to anaerobic soil conditions, which may solubilize Fe-P reaction products.
Microplot Establishment and Breakthrough Test Setup
Microplots (2 x 2 m) were established in July 2003 at two high STP sprayfield sites where drainage lysimeters had previously been installed, with each lysimeter in the center of the microplot. Soils in the microplots averaged 119 mg kg–1 WEP and 585 mg kg–1 Bray-1 P in the 0- to 0.15-m depth. Technical grade alum (1.3 kg m–2 or 0.32x) was uniformly applied in the microplots and thoroughly incorporated into the 0- to 0.2-m soil layer using a tractor-mounted rotary tiller. Wastewater (3.2 mg L–1 TP) was immediately applied using a center pivot system to moisten the soil and facilitate reaction of the alum with soil P. Subsequent wastewater irrigations throughout the experiment were scheduled as for the rest of the sprayfields in which the microplots were located.
A 12-gauge galvanized steel cylinder (2.44-m i.d. by 0.60-m height) was pounded into each microplot (with the lysimeter in the center) to a depth of 0.1 m using a 7-kg sledgehammer. The first breakthrough test was run 60 d before alum application and the second test 41 d after alum application. Composite soil samples were taken from the 0- to 0.15-m layer just before alum application (Day 0) and again on Days 17, 27, and 41 for determination of WEP concentration, pH, and EC. Water-extractable P in the 0- to 0.15-m depth averaged 118 mg kg–1 before the first breakthrough test and 119 mg kg–1 just before alum application.
Wastewater Application
Wastewater was hauled to each microplot using a 12.9-m3 tanker truck and applied to the cylinder using a perforated hose according to procedures outlined by Zvomuya et al. (2005). Wastewater used in the first breakthrough test (before alum application) contained 3.6 mg L–1 TP and 259 mg L–1 Br tracer (added as KBr), whereas that for the second test (41 d after alum application) contained 3.2 mg L–1 TP and was not spiked with the Br tracer. A hydrostatic head of approximately 0.3 m was maintained in the cylinder, with a maximum allowable depletion of 0.03 m. Approximately 6 pore volumes of wastewater were leached through each lysimeter in both tests. Leachates in the lysimeters were extracted every 15 to 60 min using a generator powered vacuum pump. The samples were collected in 500-mL plastic bottles and immediately placed in a cooler. Bromide concentration in leachates from the first test was generally measured within 3 h of sampling. The samples were then stored frozen until P analysis.
Wastewater and Leachate Analysis
Bromide concentration in the wastewater and leachate samples was measured using the Orion 920A ISE meter in conjunction with a Br ion selective electrode (ISE) (Orion 94-35; Thermo Electron Corp.). Total P and total soluble phosphorus (TSP) concentrations were determined in unfiltered and filtered samples, respectively, by the ascorbic acid method (Murphy and Riley, 1962) following digestion with HClO4–HNO3 on a block digester. Soluble reactive phosphorus (SRP, i.e., dissolved molybdate reactive P) was similarly determined, but without digestion, after filtration of the leachates through 0.45-µm membrane filters. Dissolved organic phosphorus (DOP) was estimated as the difference between TSP and SRP, and particulate phosphorus (PP) as the difference between TP and TSP.
Statistical Analysis
Data were analyzed with each lysimeter as a replicate using the MIXED procedure with LSMEANS (SAS Institute, 1999). Leachate TP and SRP concentrations before and after alum application were compared for leachate pore volumes of 
1. Statistical significance was assessed at the 5% probability level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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20 mg kg–1 are unlikely to result in significant leaching below the 1.5-m depth (Zvomuya et al., 2005). Our results are consistent with those obtained by Ann et al. (2000), who reported that P immobilization effectiveness in soils from a constructed wetland decreased in the order FeCl3 > alum > Ca(OH)2 > calcite > dolomite.
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0.5x, CaCO3 had the least effect on WEP. Sugarbeet lime addition significantly decreased WEP concentration at rates above 1.5x (12.8 g SBL kg–1). At the highest SBL rate (5.0x or 42.7 g kg–1 soil; data not shown in the graph), WEP decreased by 53% (compared to the untreated control) after 63 d of incubation. Despite the effectiveness of alum and FeCl3 in immobilizing P, cost remains a major obstacle to their widespread application on P-enriched soils. Current pricing information indicates that one metric ton of alum costs between $235 (0.07 mol–1 Al) and $468 ($0.14 mol–1 Al), depending on form (liquid vs. dry) and iron content (iron-free vs. not iron-free), while one metric ton of technical grade FeCl3 costs between $300 ($0.05 mol–1 Fe) and $351 ($0.06 mol–1 Fe) (Chemical Market Reporter, 2005). However, in situations such as the winter sprayfields from which our soil was collected, site-specific application of the amendments could reduce the costs. During ice- and snow-melt in these winter sprayfields, melt-water usually accumulates in topographic depressions, which have much higher WEP levels than the upland areas (Zvomuya et al., 2005). Chemical amendment application can, therefore, be confined to these high-P depressions rather than applying uniformly over the entire sprayfield.
In these soils, the risk of ground water pollution is greatest during snowmelt and after heavy rainfall. The presence of topographic depressions further aggravates the potential pollution problem because snowmelt runs off into these depressions, which in turn act as areas of focused recharge with higher hydraulic loading (increased depth of water) than intended. Because P solubility increases under reducing environments (Sah and Mikkelsen, 1986), prolonged saturation of the soil under the depressional ponds further decreases P adsorption, thereby increasing the potential for P leaching.
Since ponding in the topographic depressions during spring ice- and snow-melt results in prolonged soil saturation and reducing conditions, Fe-based compounds and complexes may be less effective because of their sensitivity to oxidation-reduction (redox) conditions. For example, Ann et al. (2000) showed that ferric phosphates, such as strengite (FePO4·2H2O), formed under oxidizing conditions following soil application of FeCl3 will dissolve and release P to the soil solution. Also, P adsorbed on Fe-oxides will be mobilized. On the other hand, Al-P reaction products, such as variscite (AlPO4·2H2O), are less likely to be influenced by redox conditions (Moore and Reddy, 1994). Alum should therefore be the preferred amendment on high P soils subjected to anaerobic conditions.
Where alum or FeCl3 costs are prohibitive, WTRs from water treatment plants that utilize Al and Fe compounds for coagulation could be used as low cost Al- and Fe-based amendments. Sugarbeet lime, which was the most effective of the Ca-based amendments tested in this study, may also be a less expensive alternative to alum and FeCl3 if it can be procured nearby. It has been suggested that Ca-based amendments sequester P via formation of insoluble Ca-P compounds, such as hydroxyapatite [Ca10(PO4)6(OH)2] and fluorapatite [Ca10(PO4)6F2] (Parfitt, 1978). However, recent XANES spectroscopy results suggest that high molecular weight organic compounds in some organic materials may retard the formation of the more stable hydroxyapatite phases from the initially formed dicalcium phosphates (Peak et al., 2002). Therefore, SBL may have limited effectiveness in soils, such as the one investigated in this study, irrigated with potato processing wastewater high in organic content.
Bray-1 Phosphorus
Only FeCl3 and (at rates of 2.0x) SBL had a significant effect on Bray-1 P concentration. Bray-1 P concentration decreased linearly from an initial 600 mg kg–1 soil to 97 mg kg–1 (84% decrease) when the rate of FeCl3 was increased from 0 to 1x (6.8 g kg–1) (Fig. 1b). Further increase in FeCl3 rate resulted in minimal reduction in Bray-1 P concentration. It is noteworthy that even at the higher FeCl3 rates, which nearly eliminated WEP, Bray-1 P remained above 60 mg L–1. Bray-1 P tests of 30 mg kg–1 are considered in the high to very high category for most crops (Rehm et al., 2001; Sawyer et al., 2002).
At rates of >2.0x (17 g kg–1; data for higher rates not shown), SBL significantly reduced Bray-1 P concentration in the soil, with a 38% (230 mg kg–1) reduction at the highest rate investigated (5.0x or 42.7 g kg–1). The other Ca-based amendments, CaCO3 and WTR, had no significant effect on Bray-1 P concentration in the soil and the Bray-1 P remained above 500 mg kg–1 even after 63 d of incubation.
While alum was about as effective as FeCl3 in reducing WEP in the soil, it did not significantly reduce Bray-1 P after 63 d of incubation (Fig. 1b). This was because the Bray-1 extractant promotes P release by decreasing Al activity in the soil solution through the formation of more stable AlF4– complexes. In this regard, Bray-1 P may not be a good indicator of plant available P in alum-treated soils. Nevertheless, alum is still a good choice since it reduces water soluble P, which is of greater environmental importance with regard to leaching and water quality.
It should be noted that the Bray-1 P method used in this study is not recommended for alkaline soils. Nevertheless, it was the method of choice because of the anticipated low pH values following soil application of alum and FeCl3 and the need to use the same method to facilitate comparison among treatments.
Soil Conductivity and pH
Ferric chloride had the greatest effect on soil EC, followed by alum (Fig. 2a
). The EC increased from 0.5 dS m–1 in the untreated soil to 9.0 and 3.6 dS m–1, respectively, at the highest rates (2.0x) of FeCl3 and alum. At the rates (3.9 g alum kg–1 soil and 1.5 g FeCl3 kg–1 soil) required to reduce WEP to the environmentally safe level of 20 mg kg–1, however, EC was below 2 dS m–1 and would not be expected to adversely affect plants growing on the soil. Calcium carbonate, WTR, and SBL application had no significant effect on EC.
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Field Breakthrough Study
Water-Extractable Phosphorus
Water-extractable P concentration in the 0- to 0.15-m soil layer decreased significantly (P < 0.05) from a mean of 119 mg kg–1 before alum application to 17.7 mg kg–1 17 d after alum application (Table 3). There was no significant change in WEP concentration after Day 17. It is clear from the data that immobilization of P by alum in this soil was a rapid reaction. Once formed, Al-P reaction products are known to remain stable at pH levels similar to those (6.2–7.6 units) measured in this study (Stumm and Morgan, 1981).
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Electrical conductivity increased significantly from 0.47 dS m–1 before alum application to 2.19 dS m–1 17 d after application (Table 3). The EC decreased slightly, but nonsignificantly on Days 27 and 41 compared to Day 17. The EC levels measured in this study are in the low range and unlikely to have any adverse effect on most crops (United States Salinity Laboratory Staff, 1954). Also, the EC increase is likely a temporary effect, which would decrease with subsequent irrigation.
Bromide Breakthrough
During the breakthrough test before alum application, Br concentration in the wastewater averaged 259 mg L–1. Bromide was detected in the leachates after 0.2 pore volumes of wastewater had percolated through the lysimeters (Fig. 3
). Leachate Br concentration reached 50% of wastewater Br concentration (C/C0 = 0.5, where C is leachate Br concentration at time t and C0 is Br concentration in the wastewater) after 0.5 pore volumes. Relative Br concentration (C/C0) peaked at 0.93 after one pore volume. Transport parameters for the sites have previously been reported by Zvomuya et al. (2005). Bromide breakthrough curves (BTCs) show that there were no preferential flow pathways and the chemical transport followed the convective-dispersive transport model.
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3 pore volumes of wastewater. It is not clear whether this was related to a possible change in the stability of Al-P reaction products with residence time. However, there was no corresponding increase in TP concentration, reflecting a concomitant decrease in PP plus DOP. Also, despite the indication by the Br BTCs that there was no preferential flow, the seemingly higher TP (Fig. 4b) and SRP (Fig. 5b) at zero pore volume after alum application compared to the values at the end of the tests before alum application (Fig. 4a and 5a) could not be easily explained.
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Earlier breakthrough tests using the same wastewater and lysimeters installed at high STP and low STP sites indicated that leachate P originated from the soil rather than the tertiary-treated wastewater (Zvomuya et al., 2005). This was corroborated by P simulations using the CXTFIT model, which indicated that P applied in the wastewater at the soil surface under ponded conditions would not be expected in the leachate at the 1.5-m depth until 20 pore volumes had percolated through the profile (Zvomuya et al., 2005). Results from the present study further support these earlier findings, since relative P concentrations in the leachates were consistently greater than one (C/C0 > 1) during the breakthrough tests. Our results are also consistent with those of Mamo et al. (2005), who demonstrated in a laboratory column study using soils from the same sprayfields that the soil acted as a source or sink of P in the soil solution, depending on soil and wastewater P levels.
Phosphorus fractionation was similar for leachates collected before and after alum application. The leachates contained, on average, 80% SRP, 15% PP, and 5% DOP. The corresponding speciation of wastewater P was 65% SRP, 31% PP, and 3% DOP. The difference in P speciation, particularly SRP and PP, between the leachate and the wastewater, further supports the conclusion that leachate P originated from the soil rather than the wastewater applied during the experiment.
Results from this study have important implications on the continued use of land application as a cost-effective wastewater disposal method for the food-processing industry. Since repeated waste applications eventually lead to saturation of the soil's P sorption capacity, alum application may be an effective means of reclaiming this sorption capacity and thus extending the treatment capacity of a field. The widespread use of alum to ameliorate P-impacted soils will ultimately depend on the value of benefits in using this practice relative to the cost of the amendment. However, in situations such as the present study where WEP in the sprayfields is highest in depressions, focal points for ice- and snow-melt accumulation during spring thaw, costs can be minimized by site-specific application of the alum in depressional areas only, as discussed above.
There is need for longer duration field experiments to assess the stability of Al-P reaction products formed after application of alum, and determine how frequently the amendment should be applied in sprayfields receiving year-round applications of food-processing wastewater. Also, future studies should evaluate the effectiveness of adding alum directly to the wastewater before it is land-applied. This may ensure intimate contact between the Al and P and circumvent the need for deep incorporation of the solid amendment.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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  J. W. Leader, E. J. Dunne, and K. R. Reddy Phosphorus Sorbing Materials: Sorption Dynamics and Physicochemical Characteristics J. Environ. Qual., January 4, 2008; 37(1): 174 - 181. [Abstract] [Full Text] [PDF]
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