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Department of Plant and Soil Sci., Univ. of Delaware, Newark, DE 19717-1303
* Corresponding authors (chpenn{at}vt.edu, jtsims{at}udel.edu)
Received for publication September 4, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: Alox, Feox, and Pox, oxalate-extractable aluminum, iron, and phosphorus, respectively • Al-P, Ca-P, and Fe-P, aluminum-, calcium-, and iron-related phosphorus, respectively • BNR, biological nutrient removal • DPS, degree of phosphorus saturation • DRP, dissolved reactive phosphorus • EPA3050 P, total phosphorus analyzed by the USEPA 3050 acid–peroxide digestion method • FeO-P, iron-oxide-coated filter paper strip–extractable phosphorus • ICP–AES, inductively coupled plasma atomic emission spectroscopy • LS-P, loosely soluble phosphorus • M1-P, Mehlich-1 phosphorus • M3-P, Mehlich-3 phosphorus • PL, poultry litter • RS-P, reductant-soluble phosphorus • STP, soil test phosphorus • WSP, water-soluble phosphorus • WWTP, wastewater treatment plant
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Nutrient management laws and regulations focused on reducing P inputs to surface waters have recently been passed in Delaware (1999), Maryland (1998), and Virginia (1999) (Sims, 2000). All of these laws will use "high" soil test P levels in some manner to identify sites where P applications must be restricted or discontinued. For example, in Maryland, the Water Quality Improvement Act of 1998 dictated that application of P in fertilizers, manures, and biosolids will be based upon soil test P levels, site conditions (topography, drainage, proximity to surface waters), and P source management factors (application timing and method, relative availability of P in the organic P source) (Coale, 2001). The Delaware Nutrient Management Act of 1999 allows a maximum P application rate to "high" P soils equivalent to the "three year crop removal rate" (Sims, 1999); thus, "high" P soils in Delaware will usually only receive P applications once every three years. Annual P applications equal to crop P removal would also be permitted but are unlikely due to equipment limitations that preclude the application of very low rates of manure and/or biosolids P. The Virginia Poultry Waste Management Bill of 1999 targets the state's poultry industry and mandates that P application rates shall not exceed the greater of crop nutrient needs or crop nutrient removal. The actions in Maryland are of considerable significance to municipalities in that state, and throughout the region, that rely upon land application for the beneficial use of biosolids. In most U.S. states biosolids application rates are based on the N requirement of the crop and the concentrations and loading rates of several trace elements, as defined in the USEPA 503 rule (USEPA, 1994). If some form of P-based management was to be required, such as limiting biosolids application based on a soil test P concentration considered to be "high" or "excessive," the land available to use biosolids as agricultural soil amendment would be significantly limited because of the percentage of soils in these states that are already considered to be high in P (Sims et al., 2000). Further, past research has shown that the current nutrient management practice used for biosolids (continuous N-based applications) will cause soil P to accumulate to levels above those needed for optimum crop production (Kelling et al., 1977; Pierzynski, 1994; Peterson et al., 1994; Maguire et al., 2000a, b).
As P-based guidelines and regulations are developed for these states and others, it is important to consider that past research has shown that P in biosolids may be less mobile with respect to leaching and runoff than other P sources (e.g., fertilizers, manures). Biosolids P is often less soluble and plant available due to the addition of chemicals (e.g., metal salts and/or lime) at the WWTP (Kirkham, 1982; McCoy et al., 1986; Frossard et al., 1996a). Withers et al. (2001) measured runoff P from field plots that had previously received either triple superphosphate, liquid cattle manure, liquid anaerobically digested biosolids, or dewatered biosolids cake. Runoff P was related to the amounts of P extracted from the different sources by either water or NaHCO3. The authors concluded that, "the results suggest there is a lower risk of P transfer in land runoff following applications of biosolids compared with other agricultural P amendments at similar P rates."
In addition to the differences in P availability between biosolids and animal manures, P availability and P loss from biosolids-amended soils may vary depending upon the WWTP process that was used to produce the biosolids (Kyle and McClintock, 1995; Maguire et al., 2001; Jokinen, 1990; Soon et al., 1978). For example, Rydin and Otabbong (1997) leached 35 mm of water through soils amended with either Fe or Al biosolids and found that less P was released from Fe biosolids compared with Al biosolids. This variation in P availability and P loss from different biosolids is most likely due to differences in solubility in the forms of inorganic P that result from different WWTPs.
Some past research has investigated the effects of wastewater treatment process on soil P and P leached through soil pots and columns, but little information is available on the effect of WWTP process on P losses in runoff. Thus, our objective was to determine, using a rainfall simulation study, the effects of WWTP process on the forms of P in biosolids and biosolids-amended soils and P losses in runoff.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soils were characterized for soil pH (1:1 soil to water ratio), buffer pH (Adams–Evans buffer), organic matter (OM; by loss on ignition at 360°C), cation exchange capacity (CEC; at pH 7 by the ammonium saturation method), and sand, silt, and clay (hydrometer method) following standard protocols of the University of Delaware (Sims and Heckendorn, 1991). Soil test P was analyzed by Mehlich-1 (M1-P, 1:4 ratio of soil to 0.05 M HCl + 0.0125 M H2SO4, 5-min reaction time, filtration with Whatman [Maidstone, UK] #2 paper), and analysis by inductively coupled plasma atomic emission spectroscopy (ICP–AES).
Biosolids and Poultry Litter Collection and Characterization
Biosolids from eight different wastewater treatment plants (WWTP) were chosen to represent the range in treatment processes used in the Mid-Atlantic USA, with respect to digestion and the addition of lime and metal salts [e.g., FeCl3, Al2(SO4)3; Table 1]
. Three of the biosolids (Biosolids 4 and 8) received only Fe salt additions, one received both Fe salt and Al2(SO4)3 (Biosolids 7), three received both lime and Fe salts (Biosolids 3, 5, and 6), and two received neither lime or Fe salts (Biosolids 1 and 2). Three of the biosolids were anaerobically digested (Biosolids 2, 7, and 8), one was aerobically digested (Biosolids 4), three were not digested (Biosolids 3, 5, and 6), and one (Biosolids 1) was produced by the biological nutrient removal (BNR) process. The BNR process removes P by treating wastewater in an anaerobic zone, followed by treatment in an aerobic zone. In this process microorganisms exhibit P uptake above normal levels, using the P for cell maintenance, synthesis, and energy transport, thus converting wastewater P to microbial biomass P (Furrer and Bollinger, 1981; Metcalf and Eddy, 1991).
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Rainfall Simulation Experiments
Three experiments were conducted using eight biosolids (Table 1) and the two soils described above (Table 2)
. Each experiment consisted of six treatments, replicated three times: (i) four biosolids types, (ii) one PL, and (iii) one control (unamended soil). The first two experiments used Biosolids 1, 2, 3, and 4 and the Elsinboro (Experiment 1) and Woodstown (Experiment 2) soils; the third experiment used Biosolids 5, 6, 7, and 8 and the Woodstown soil. The Elsinsboro soil was limed with CaCO3 prior to use to achieve the minimum pH required for biosolids application. In all experiments, the biosolids and PL were added to each soil at a rate of 200 kg EPA3050 P ha-1. This P rate was chosen because it approximated the amount of EPA3050 P added when biosolids are land-applied at the average plant-available nitrogen (PAN) rate of the eight biosolids tested (Table 1) (Maguire et al., 2001). Actual biosolids application rates (dry weight basis) ranged from 8.2 to 29.4 Mg ha-1 and averaged 15.2 Mg ha-1. Soils and amendments were mixed together in a cement mixer for 5 min and then poured into wooden runoff boxes approximately 100 cm x 20 cm x 5 cm in size, leveled, and presaturated 24 h before being placed under a rainfall simulator to ensure that runoff would occur during the rainfall event. Soils were presaturated by adding 5.0 and 3.5 L of water, respectively, to the Elsinboro silt loam and Woodstown sandy loam. Soil moisture measurements (gravimetric, at 105°C) on soil samples collected before the rainfall event showed that the average water contents of the soils were 0.22 and 0.15 kg kg-1 for the Elsinboro silt loam and Woodstown sandy loam, respectively. Preliminary studies found that presaturated soils in runoff boxes did not become reduced after a 24-h time period, as measured with a redox probe.
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is an empirical constant used to relate soil P sorption capacity to Alox and Feox and typically ranges from 0.4 to 0.6; we used a value of 0.5 (Schoumans, 2000):
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The rainfall simulator consisted of a single Tee Jet HH-SS-50WSQ nozzle (Spraying Systems, Wheaton, IL) attached to a 3- x 3- x 3-m metal frame, and calibrated to achieve an intensity of 75 mm h-1 at 90% uniformity. The runoff boxes were placed randomly under the rainfall simulator on steel racks adjusted to a 5% slope. Rainfall events were 15 min long and all runoff was collected in 9-L plastic containers. Experiment 1 (Elsinboro soil) consisted of three separate rainfall events: (i) Event 1, conducted 24 h after soil presaturation; (ii) Event 2, conducted 7 d after Event 1; and (iii) Event 3, conducted 30 d after Event 1. However, because there were no statistical differences in treatment effects on any form of runoff P after Event 2 in this experiment, only two rainfall events were conducted for each of the second two experiments.
Runoff subsamples were pipetted in 10-mL aliquots from bulk runoff samples that were being mixed on a stir plate to keep all sediment in suspension. These subsamples were analyzed as follows: (i) DRP (40 mL runoff filtered through 0.45-µm Millipore filter papers); (ii) FeO-P (40 mL runoff + iron-oxide-coated filter paper strip, 16-h reaction time, followed by dissolving Fe and P from the filter paper strip for 1 h in 1 M H2SO4); (iii) total P (100 mL of runoff digested by the EPA3050 method); and (iv) sediment concentration (40 mL of runoff evaporated at 120°C in glass beakers of known mass, then weighed again after all water had evaporated). Runoff DRP and FeO-P were analyzed by the Murphy and Riley colorimetric method (Murphy and Riley, 1962) and runoff EPA3050 P extracts were measured by ICP–AES.
Statistical Analyses
Data were tested for normality by the Shapiro–Wilkes statistic conducted by the PROC UNIVARIATE procedure of the Statistical Analysis System (Version 8.0) and found to be normally distributed (SAS Institute, 1998). All correlation, regression, and analysis of variance procedures were conducted by standard procedures of SAS.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Biosolids and Poultry Litter Properties
As expected, the pH of lime-treated biosolids was higher (pH > 11) than biosolids not treated with lime, where pH values ranged from pH 5.6 to 6.8; the PL had a pH of 7.5. Total C ranged from 240 to 440 g kg-1, with BNR biosolids having the highest total C concentrations (Table 1). The C to N ratios of the biosolids and PL ranged from 9 to 13.
Total (EPA3050) P in the eight biosolids and the PL ranged from 6.8 to 24.5 g kg-1; the median value reported for U.S. biosolids by Linden et al. (1995) was 23 g kg-1. We also found, as reported by Maguire et al. (2001), that Pox exceeded EPA3050 P for all biosolids except those that had received lime (Biosolids 3, 5, and 6), suggesting that most P in the unlimed biosolids was associated with amorphous oxides of Fe and Al. The fact that Pox was less than EPA3050 P in lime-treated biosolids suggests that liming at the WWTP results in more biosolids P associated with Ca than with Fe and Al, or that the neutralizing potential of lime-treated biosolids affects the pH of the ammonium oxalate solution (pH 3), and reduces its efficiency at P extraction.
Extractable (M1-P and M3-P) phosphorus and WSP concentrations were higher in biosolids that had not been treated with Fe (Biosolids 1 and 2) and in PL (Biosolids 9), than in the Fe or Al biosolids (Biosolids 3–8) (Table 3) . Biosolids Pox to (Alox + Feox) molar ratios followed the same trend. When expressed as percentages of EPA3050 P, we found that M3-P, M1-P, and WSP in the biosolids without Fe and the PL ranged from 29 to 68%, 17 to 52%, and 10 to 45%, respectively, compared with 0.4 to 13%, 0.3 to 6%, and 0.4 to 5% in Fe or Al biosolids.
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Wastewater treatment process also affected the distribution of inorganic forms of P in the biosolids, as measured by sequential chemical extraction (Table 4) . From 0.2 to 44% of total (sum of fractions) P was in the loosely soluble fraction (LS-P). As with extractable and WSP, concentrations of LS-P and percentages of total P found as LS-P were higher in the non-Fe biosolids and PL (Biosolids 1, 2, and 9) than Fe + Al biosolids. Among Fe biosolids, those produced using Fe and lime (Biosolids 3, 5, and 6) had greater concentrations and percentages of LS-P than Fe + no lime biosolids (Biosolids 4, 7, and 8). Biosolids LS-P concentrations were also significantly correlated with M3-P, M1-P, and WSP (r = 0.94, 0.88, and 0.98, respectively, each significant at the 0.001 probability level).
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Biosolids and Poultry Litter Effects on Soil Phosphorus, Aluminum, and Iron
Biosolids application consistently and significantly increased all forms of soil P in the Elsinboro and Woodstown soils (Table 5) . Since all biosolids and the PL added the same amount of total P (200 kg P ha-1, equivalent to 11–29 Mg biosolids ha-1), the increase in each form of soil P, relative to the control, is a measure of the effect of biosolids type on the relative plant availability and potential mobility of added P. A comparison of the interaction between soil type and biosolids type on P availability can also be made because the Elsinboro and Woodstown soils were both amended with Biosolids 1 through 4 and PL (Experiments 1 and 2, Table 5).
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Of particular interest, because of the extra cost involved in adding a lime stabilization process to the WWTP, were the differences we noted in soil P increases when soils were amended with Fe + no lime biosolids (Biosolids 4, 7, and 8) compared with Fe + lime biosolids (Biosolids 3, 5, and 6). For example, amending the Elsinboro soil with Biosolids 4 (Fe + no lime) resulted in smaller increases in soil FeO-P, Pox, M3-P, and WSP than Biosolids 3 (Fe + lime). The same patterns were evident in the Woodstown soils when comparing Fe + no lime biosolids–amended soils with Fe + lime biosolids–amended soils (Table 5). Soils amended with Fe + lime biosolids also had a greater percentage change in soil FeO-P per unit of EPA3050 P added compared with Fe + no lime biosolids (Fig. 1) . Thus, P in Fe + lime biosolids became more available than P in Fe + no lime–treated biosolids after being added to the soil. As noted earlier, the higher P availability in soils amended with Fe + lime biosolids is probably related to the greater solubility of biosolids Ca-P relative to biosolids Fe-P, Al-P, and RS-P in biosolids. Maguire et al. (2001) also showed that biosolids amended with only metal salts consistently caused the smallest increases in soil P (WSP and FeO-P), followed by those produced using metal salts and lime, and then those that received no metal salts or lime.
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Our results also showed that amending soils with biosolids not only affected the amount and solubility of soil P, but also the potential for soils to sorb P because biosolids added constituents are known to be important in soil P retention (Al, Ca, Fe). Past research has shown that the Al and Fe content of biosolids can be important to P mobility and availability in biosolids-amended soils (Chang et al., 1983; De Haan, 1980; Frossard et al., 1996b; Jenkins et al., 2000; Rydin and Otabbong, 1997; Soon et al., 1978). Depending upon the WWTP, the biosolids added as much or more of Al, Ca, and Fe to soils as P and, particularly for the Woodstown soil, the amount added was appreciable relative to that in the unamended soil (Table 3). Amending these soils with biosolids at the rate of 200 kg EPA3050 P ha-1 also added an average of 3 and 9% of native Alox, and 8 and 30% of native Feox in the Elsinboro and Woodstown soils, respectively (Table 2). In general, soils amended with biosolids produced using Fe (Biosolids 3, 4, 5, 6, and 8) or Fe + Al (Biosolids 7) had higher Feox and/or Alox concentrations, and thus lower DPS values, than soils amended with non-Fe biosolids and PL (Biosolids 1, 2, and 9) (Table 5). This has implications for the availability and mobility of P in soils because added biosolids Fe + Al may mitigate the potential for P losses in runoff or leaching, particularly if applied on a regular basis to soils with high P saturation, such as the Woodstown (Maguire et al., 2000a). Added biosolids Fe (or Al) may provide the soil with additional sites for P adsorption in highly-P saturated, Al-P dominated soils such as the Woodstown soil used in this study. This observation is supported by the significant, negative correlation between added biosolids Feox and soil WSP (r = -0.60, significant at the 0.05 probability level) in the Woodstown soil. This correlation, however, was not significant for the Elsinboro soil (r = -0.24), which had a lower DPS value than the Woodstown (Table 2). Neither of the two soils showed any significant relationship between added biosolids Alox and soil WSP (r = 0.19 and 0.12 for the Woodstown and Elsinboro soils, respectively). Differences in the relative percentages of Feox added to Elsinboro and Woodstown soils may explain why added biosolids Feox did not have as much of an effect on soil WSP in the Elsinboro as the Woodstown. With respect to the Elsinboro soil, on average, biosolids amendments added about 8% of Feox relative to that already present in the soil, compared with an average of 30% for the Woodstown. Further evidence supporting the benefit of biosolids Fe on reducing "available" P in highly P-saturated soils such as the Woodstown is provided by the fact that FeO-P concentrations in Woodstown soils amended with Fe + no lime biosolids actually decreased relative to the control soil (Table 5, Fig. 1). Other authors have found that the addition of waste products produced using Fe can reduce the solubility of soil P. Maguire et al. (2001) reported that when biosolids applications increased soil Alox and Feox, soil WSP or FeO-P changed very little or decreased. Kyle and McClintock (1995) found that the addition of Fe and Al from biosolids decreased P solubility in soils as indicated by reductions in P leaching. Soon and Bates (1982) showed that the application of biosolids to soils caused an increase in soil Fe- and Al-oxide content, which resulted in increased P retention. In the present study, however, only the addition of Feox (compared with Alox) appeared to have strongly affected the availability of soil P.
Phosphorus in Runoff from Biosolids- and Poultry Litter–Amended Soils
In general, total P (EPA 3050 P) concentrations in runoff were higher for the Elsinboro soil mainly because sediment concentrations in runoff from this fine-textured soil were also greater (Table 6) . Further evidence for this is provided by the significant correlation between sediment and total P concentrations in runoff for the first two rainfall events (r = 0.66, significant at the 0.01 probability level). This phenomenon (an increase in runoff total P with increases in total solids in runoff) was also found by Yli-Halla et al. (1995) and others (Sharpley, 1997; Withers et al., 2001). We also observed, however, that the percentage of total P in runoff that was bioavailable (FeO-P) and the DRP concentrations in runoff were consistently higher for the Woodstown soil (mean over all treatments and events = 25% and 0.39 mg L-1) than the Elsinboro soil (mean = 6% and 0.03 mg L-1) (Table 6). This reflects the higher DPS and STP values in the Woodstown relative to the Elsinboro (Table 2) and is similar to the results of Sharpley (1997), who found that soil "P sorption saturation" was significantly and positively correlated with DRP concentrations in runoff from Oklahoma soils.
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Runoff P concentrations were also affected by biosolids type. For Event 1 of Experiments 1 and 2, runoff DRP, FeO-P, and EPA3050 P concentrations were consistently highest from soils amended with BNR biosolids, probably due to the fact that these biosolids also contained the highest concentrations and percentages of WSP and extractable P (Tables 3 and 6). Following BNR biosolids-amended soils, runoff P losses tended to be highest from Fe + lime biosolids, no Fe + no lime biosolids, PL, and Fe + no lime biosolids (Table 6). Similar results were noted in a leaching study by Kyle and McClintock (1995), who found that more soluble P was leached from soils receiving BNR biosolids compared with soils amended with Fe or Al biosolids. The similarity in P runoff losses from PL and non-Fe or Al biosolids–amended soils is most likely a result of the fact that no FeCl3 or Al2(SO4)3 was added to or used in the production of either material.
For Event 1 of all three experiments, amending soils with Fe + lime biosolids (Biosolids 3, 5, and 6) generally resulted in high concentrations of runoff DRP, FeO-P, and EPA3050 P, while soils amended with Fe + no lime biosolids (Biosolids 4, 7, and 8) consistently had the lowest concentrations of runoff P (Table 6). This can be attributed to the inorganic P forms of the biosolids; the majority of the P in Fe + no lime biosolids was Fe-P and Al-P. As noted above, adding a lime treatment to biosolids produced with Fe (Fe + no lime to Fe + lime) caused a shift in P distribution from Al-P and Fe-P to LS-P and Ca-P forms (Table 4). As the amounts of biosolids and PL LS-P + Ca-P (more soluble P) increased relative to Fe-P + Al-P + RS-P (less soluble P), the DRP concentrations in runoff also increased (Fig. 2) .
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Predicting Phosphorus Concentrations in Runoff from Biosolids-Amended Soils from Biosolids Properties and Soil Phosphorus
As with other P sources, the ability to predict, from simple, rapid tests, how biosolids and soil P forms and concentrations affect P runoff is important to biosolids management in P-sensitive watersheds (Maguire et al., 2001; Sibbesen and Sharpley, 1997; Sims et al., 2000). In terms of the inorganic P fractions in the biosolids used in this study, we found that LS-P and Ca-P were linearly correlated with Event 1 DRP concentrations from both the Elsinboro (r = 0.84, significant at the 0.05 probability level, and r = 0.90 significant at the 0.01 probability level, respectively) and Woodstown soils (r = 0.81 and 0.80, both significant at the 0.01 probability level, respectively) (Table 7)
. Note that for the relationship between runoff DRP and Ca-P added (Table 7), BNR biosolids were not included due to the fact that this biosolids had very little Ca-P; its source of runoff DRP was LS-P (Table 4). With the exception of biosolids Al-P in the Elsinboro soil (r = 0.71, significant at the 0.05 probability level) other inorganic P fractions of biosolids P were not significantly correlated with runoff DRP concentrations.
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It seems clear from our results that, due to differences in soil and biosolids properties, the effect of P application on soil P and runoff P will vary with soil type and that rapid tests, while useful, may not be the most effective means to characterize the risk of P loss. For example, we found that the ratio of runoff P to desorbable soil P (FeO-P) was helpful in determining how soil type and P source affected P losses. The ratio of runoff DRP to soil FeO-P (ratios multiplied by 100 for presentational clarity) for the Elsinboro soil ranged from 0 to 0.25, with an average of 0.11, compared with the Woodstown soil, which ranged from 0.71 to 1.44, with an average of 1.22. This indicates that FeO-P in the Woodstown soils was more susceptible to P loss than that in the Elsinboro soil. As a result, the Elsinboro soil could potentially have higher concentrations of FeO-P than the Woodstown and still have lower DRP losses in runoff. This difference between the ability of the two soils to retain P during rainfall events is highly correlated with soil DPS (Fig. 4) . The DPS values of biosolids-amended Woodstown soils were three to five times greater than Elsinboro DPS values (Fig. 4), resulting in a greater ratio of runoff DRP to soil FeO-P.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Our results support this and also emphasize the importance of the interaction between soil physico–chemical properties and biosolids type on the forms and mobility of P. Specifically, we found that, in the near term, WWTP process affects extractable P concentrations in biosolids and biosolids-amended soils and runoff P concentrations from biosolids-amended soils. In the longer term, as biosolids equilibrate with soils, the release of P to runoff will be affected more by soil properties (e.g., DPS) than biosolids characteristics. Biosolids produced by the BNR process had the highest concentrations of "available" P (WSP, M1-P, M3-P) and thus caused the greatest increases in soil and runoff P. After BNR biosolids, available P concentrations in biosolids and biosolids-amended soils, as well as P concentrations in runoff, were greatest from no Fe + no lime biosolids, followed by Fe + lime biosolids, and then Fe + no lime biosolids. Phosphorus runoff concentrations from PL-amended soils were most similar to soils amended with no Fe + no lime biosolids. The Fe + no lime biosolids actually caused certain forms of extractable soil P to decrease (relative to control) when amended to highly P saturated soils such as the Woodstown sandy loam. Both the decrease in soil P for Fe + no lime biosolids–amended soils and the low P concentrations in runoff from these soils were due to the fact that this biosolids type had the lowest molar ratio of Pox to (Alox + Feox) and available P concentrations and also added high amounts of Fe to the soil. The addition of Fe to soils through biosolids application may be beneficial in the long term, from the perspective of preventing P losses through runoff by increasing soil P sorption capacity. We also found that runoff P concentrations could be predicted with reasonable accuracy by several "quick-tests" of biosolids and biosolids-amended soils (e.g., WSP, FeO-P, M3-P). These tests could potentially be useful for rapid, inexpensive assessments of the potential risk of nonpoint P loss from biosolids-amended soils.
Based on these results some general recommendations can be made. If preventing P loss from soil to water is a major goal, biosolids should be applied to fine-textured soils with low STP and high amounts of Fe and Al. However, for WWTP in areas such as the Delmarva Peninsula, where P losses from agricultural lands to surface waters are a serious concern and many of the soils are sandy, high in STP, and have low P sorption capacities, a treatment process that uses FeCl3 and no lime would be the best choice. Alternatively, if P loss to water is not a major concern and biosolids are primarily applied to provide P to crops, the BNR process or a process that involves the addition of both FeCl3 and lime would be most effective. Biosolids produced by WWTP processes that use FeCl3 and lime would be best for soils with low STP but that also may become high in STP with multiple applications. In these settings, biosolids provide available P to crops and may also increase the P sorption capacity of the soil by adding significant amounts of Fe. In all cases it is also critical to take active measures to control soil erosion and the loss of particulate P.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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