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Journal of Environmental Quality 31:1388-1398 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Waste Management

Particulate and Dissolved Phosphorus Chemical Separation and Phosphorus Release from Treated Dairy Manure

Thanh H. Dao*,a and Tommy C. Danielb

a Henry A. Wallace Beltsville Agric. Res. Center, Animal Manure and By-Products Lab., Beltsville, MD 20705
b Crop, Soil, and Environmental Science Dep., 115 Plant Sci., Univ. of Arkansas, Fayetteville, AR 72701

* Corresponding author (thdao{at}anri.barc.usda.gov)

Received for publication November 15, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 NOTES
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
In confined animal feeding operations, liquid manure systems present special handling and storage challenges because of the large volume of diluted wastes. Water treatment polymers and mineral phosphorus (P) immobilizing chemicals [Al2(SO4)3·18H2O, FeCl3·6H2O, and Class C fly ash] were used to determine particulate and dissolved reactive phosphorus (DRP) reduction mechanisms in high total suspended solid (TSS) dairy manure and the P release from treated manure and amended soils. Co-application exceeded the aggregation level achieved with individual manure amendments and resulted in 80 and 90% reduction in metal salt and polymer rates, respectively. At marginally effective polymer rates between 0.01 and 0.25 g L-1, maximal aggregation was attained in combination with 1 and 10 g L-1 of aluminum sulfate (3 and 30 mmol Al3+ L-1) and iron chloride (3.7 and 37 mmol Fe3+ L-1) in 30 g L-1 (TSS30) and 100 g L-1 TSS (TSS100) suspensions, respectively. Fly ash induced particulate destabilization at rates >=50 g L-1 and reduced solution-phase DRP at all rates >=1 g L-1 by 52 and 71% in TSS30 and TSS100 suspensions, respectively. Aluminum and Fe salts also lowered DRP at rates <=10 g L-1 and higher concentrations redispersed particulates and increased DRP due to increased suspension acidity and electrical conductivity. The DRP release from treated manure solids and a Typic Paleudult amended with treated manure was reduced, although the amendments increased Mehlich 3–extractable P. Therefore, the synergism of flocculant types allowed input reduction in aggregation aid chemicals, enhancing particulate and dissolved P separation and immobilization in high TSS liquid manure.

Abbreviations: DRP, dissolved reactive phosphorus • PAM, polyacrylamide or polyamine polymers • TSS, total suspended solids • TSS30, total suspended solids in a 30 g L-1 manure suspension • TSS100, total suspended solids in a 100 g L-1 manure suspension


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
NOTES
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
ANIMAL FEEDING OPERATIONS generate large volumes of feces, urine, bedding, spilled feed, wash-flush water, and other processing wastes that are potentially recyclable into sources of plant nutrients, soil amendments and conditioners, and energy-producing raw materials (Edwards and Daniel, 1992; Council for Agricultural Science and Technology, 1996; Schwartz and Dao, 2000; Porteous, 2001; Magbanua et al., 2001). However, the greatest potential for accelerated surface water eutrophication usually occurs in watersheds with intensive manure production in concentrated animal feeding operations (Council for Agricultural Science and Technology, 1996; USDA and USEPA, 1998). Runoff from livestock pens, poorly designed or maintained manure storage areas, and fields where manure was repeatedly applied presents potential environmental hazards to the overall watershed. A correlation between eutrophication and P concentration in runoff and soil exists (Sharpley, 1995). Receiving-water overenrichment with mineral nutrients often leads to excessive autotroph growth, especially algae and cyanobacteria (Burkholder et al., 1992). The increased microflora biomass decay uses dissolved oxygen, which results in hypoxia or anoxia. The low oxygen level causes loss of aquatic animals and release of many materials normally bound to bottom sediments including various P forms. Phosphorus is most often the element limiting eutrophication, since many algae are able to obtain nitrogen (N) from the atmosphere.

There is increasing interest on postexcretion treatments to chemically bind or remove DRP in manure before it is applied to fields, as many soils in the USA contain excessive levels of nutrients (especially P) due to repeated heavy applications of animal manure (Moore and Miller, 1994; Barrow et al., 1997; Dao, 1999; Codling et al., 2000; Dao et al., 2001; Zhang et al., 2002). A promising technology to sequester manure P and other organic nutrients is the separation of liquid manure into particulate and liquid fractions. However, the inefficiency of mechanical liquid–solid separators used in dairy or swine production has led to the rapid loss of capacity in waste storage facilities. Particulates fill up retention ponds or lagoons rapidly, requiring frequent maintenance and cleaning. Typical mechanical separation efficiencies have ranged from 5 to 30% of particulate removal. Improving the solid–liquid phase separation process by chemical coagulants used in the drinking water treatment industry may help to remove organic and mineral matter and nutrients from the suspension and dispose of a relatively small volume of solids.

The solid–liquid separation process is a complex function of component processes of coagulation, flocculation, flotation, sedimentation, or filtration. Coagulation or aggregation increases the tendency of dispersed particulates in a wastewater suspension to interact and attach to one another, forming larger aggregates. The process is also used to remove certain dissolved components of the suspension that attach or sorb onto solid surfaces and settle out of solution as insoluble precipitates. For efficient aggregation to occur, the stability of a wastewater suspension must be first disturbed to increase the tendency of suspended particles to attach to one another. Particle destabilization mechanisms include surface charge neutralization, interparticle bridging, and particle diffuse-layer compression. Double-layer compression is an important mechanism to effect aggregation of colloidal suspension in wastewater and natural systems (O'Melia, 1998). Increasing the quantity of an electrolyte in the suspension increases the counterion concentration in the diffuse layer, which in effect reduces the thickness of the particle double layer. Surface charge neutralization involves a net reduction in the surface charge of the particles in the suspension, which also reduces the thickness of the diffuse layer of suspended particles. Interparticle bridging by a long-chained organic polymer would adsorb one or more particles and cross-link the particles together. High molecular weight organic polymers have been used to treat and clarify municipal wastewater to remove suspended solids and promote sedimentation of aggregated particles (Letterman and Pero, 1990). Long-chained, water-soluble polyacrylamides destabilize suspended charged particles by adsorbing onto them and building bridges between suspended particles. This results in larger aggregates that float or settle out of the liquid phase. Polyacrylamides (PAM) have been shown effective in reducing suspended particulates in flushed swine and dairy manure. Vanotti and Hunt (1999) used an anionic, neutral, and cationic PAM to reduce TSS in a swine manure suspension containing 18 g L-1 of solids. The cationic polymers reduced solution phase TSS by 33%, chemical oxygen demand (COD) by 38%, and organic N by 80%.

Metal salts (i.e., aluminum sulfate, aluminum chloride, ferric chloride) have been used with the organic polymers to improve the removal of suspended particulates and potentially reduce the amounts of polymers required for coagulation and flocculation. Zhang and Lei (1998) showed that polymer requirements increased linearly with TSS of the manure suspensions, at about 1.2 and 7.4% of the amount of solids in swine and dairy manure wastewater containing from 4 to 20 g L-1 TSS, respectively. Combining metal salts with the polymers lowered the polymer concentration required for flocculation and also resulted in the removal of 95% of the manure wastewater P, as compared with 53% with polymers alone.

Metal salts have been used alone as manure flocculation aids. Barrow et al. (1997) reported that FeCl3 was more effective than CaO, CaSO4, or CaCO3 in increasing particulate sedimentation in dairy manure suspensions containing 5 to 15 g TSS L-1. Using 0.3 g L-1 of FeCl3 in combination with CaO resulted in a 94% increase in sedimentation, a 56% manure N removal, and a 92% manure P removal from the suspensions. Chemical amendments containing Al, Fe, and other chelating chemicals also have been used to reduce P solubility in poultry manure (Salingar et al., 1994; Codling et al., 2000; Dao et al., 2001). Aluminum water treatment residual (WTR) and an iron-rich by-product of the titanium oxide extraction process (Dupont Chemicals, Wilmington, DE1) reduced dissolved manure P by about 40% at application rates between 25 and 50 g kg-1 (Codling et al., 2000). Dao et al. (2001) observed that another Al-WTR and the same Fe-rich by-product reduced DRP in poultry manure, at the 2:1 metal to P molar ratio, by an average of 39 and 48% in the Al- and the Fe-treated manure, respectively. Changes in P forms that occurred upon co-blending included chemical shifts from DRP and NH4F-extractable P fractions to citrate–bicarbonate–dithionite-extractable P and NaOH-extractable P. In beef cattle (Bos taurus) manure, Ca-, Al-, and Fe-rich by-products such as caliche, alum, and Class C coal combustion ash also reduced water-extractable P by 20, 60, and 85%, respectively, and plant-extractable P by 75% at 0.1 kg kg-1 rates and higher (Dao, 1999).

Co-blending animal manure and P-immobilizing mineral by-products reduced DRP concentrations but has raised concerns about the availability of P when the amended manure solids are reused in agronomic production. For example, the mixtures of cattle manure and alum, caliche, or fly ash remained stable in an Aridic Paleustalf and a Torrertic Paleustoll (Dao, 1999). Extracts of soils amended with fly ash– and caliche-treated manure at the rate of 22 Mg ha-1 had significantly lower Mehlich 3–extractable P than those of soils amended with untreated manure. Co-blending WTR with biosolids increased P sorption and decreased P availability to sorghum sudangrass [Sorghum bicolor (L.) Moench] (Heil and Barbarick, 1989). Plant tissue P concentrations were below adequate levels for bermudagrass [Cynodon dactylon (L.) Pers.] grown on three WTRs (Basta et al., 2000). Fertilizer P additions of 50 to 200 g Mg-1 did not increase tissue P of bermudagrass or yield to remedy the deficiency. Codling et al. (2000) reported that Al-WTR mixed with poultry litter at 25, 50, and 100 g kg-1 reduced Bray 1–extractable P by 38, 60, and 82%, respectively, after 7 wk of incubation. Given the complexity of P reactions, information is needed on the efficacy of the alternate metal sources and by-products in particulate and nutrient removal and manure DRP immobilization in particular from manure suspensions. The objectives for this study were to (i) develop improved understanding of key manure characteristics that control particulate aggregation and reduction in DRP from the solution phase of high TSS dairy manure (30 and 100 g L-1), and (ii) determine whether organic water treatment polymers and mineral P immobilization chemicals alter N and P release from the treated manure solids and amended soil.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 NOTES
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Reconstituted Manure and Representative Total Solid Suspensions
Dairy manure used in this study was reconstituted from separate collection of freshly excreted urine and feces from Holstein cows housed in free stalls on the USDA-ARS Beltsville Dairy Research Facility. The ratio of feces to urine was estimated to be 1.6 to 1 (Morse et al., 1993). Fecal material was mixed with acidified urine (pH 2 to conserve N by preventing the volatile loss of ammonia). Bulk batches of the reconstituted manure were kept at 4°C prior to the initiation of the study. Tap water was added to achieve two TSS concentrations of 30 and 100 g L-1, simulating manure from flushed and scraped manure collection systems.

Particulate Aggregation by Individual Polymers and Mineral Amendments
Three cationic polymers, which included C-1596, SD-2085 (polyacrylamide), and C-581 (polyamine), were evaluated for their effects on the aggregation of dairy manure suspensions in the TSS range described above. The polymers are currently marketed as generally regarded as safe (GRAS) products for use as flocculants and dewatering agents in wastewater treatment (Cytec Industries, West Paterson, NJ). Selected characteristics are presented in Table 1 . The mineral amendments included reagent-grade aluminum sulfate [Al2(SO4)3·18H2O], ferric chloride (FeCl3·6H2O), and a coal-combustion ash, Class C. The coal-combustion ash is the same material as presented in previous studies (Dao, 1999). Aliquots (40 mL) of TSS30 or TSS100 manure suspensions were weighed into tared 30- x 120-mm centrifuge tubes (50-mL capacity). Appropriate amounts of polymers or mineral P-immobilizing amendments were added to achieve concentrations in the range of 0 to 3.3 g L-1 and 0 to 100 g L-1 for the polymers and mineral amendments, respectively. The concentrations of organic polymers and fly ash were expressed on a weight basis because molecular weights of these materials were unknown to us. The metal salt molar concentrations ranged from 0 to 300 mmol Al3+ L-1 for the Al salt and 0 to 370 mmol Fe3+ L-1 for the Fe salt. In some measure, the extent of aggregation or redistribution of solid and liquid phases was determined using the actual weight of the equilibrium phases, polymers, and mineral amendments to calculate net changes to the manure liquid and solid phases. The tubes were vigorously agitated by hand for 1 min. Although the reaction and particulate separation were visible within the first 10 to 15 min, the mixtures were left standing overnight at room temperature. After 16 h, the liquid and hydrated aggregated phases were separated and weighed. Corrections for the net weight of each fraction were based on determinations of TSS of untreated TSS30 and TSS100 manure suspensions. A 5-mL aliquot of the equilibrium solution was centrifuged at 7000 x g for 20 min to determine the solution TSS. The hydrated aggregated fraction also was centrifuged at 7000 x g for 20 min. The liquid phase was removed and the residual dewatered pellet was reweighed and dried at 65°C for final TSS determination and N and P analysis. Both liquid fractions were acidified with 6 M HCl and stored frozen for N and P analysis. Upon centrifugation and sediment removal, the equilibrium solution specific gravity was near unity, thus TSS and nutrient concentrations were expressed on a volume of manure suspension basis of g L-1.


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  		Table 1. Selected properties of organic water treatment polymers.

 
Polymer and Mineral Amendment Interactions
Polymers of two chemical classes, C-1596 and C-581, were measured into 50-mL centrifuge tubes to have polymer rates of 0, 0.01, and 0.1 g L-1 of C-581 and 0, 0.1, and 0.25 g L-1 of C-1596, based the coagulation efficiency measured in the previous experiment. Mineral amendments were then weighed into duplicate centrifuge tubes to attain 0, 1, 10, and 100 g L-1 in a 2 x 2 x 3 x 4 factorial arrangement. For the metal salts, the amendment rates were equivalent to adding 3, 30, 300 mmol Al3+ L-1 and 3.7, 37, and 370 mmol Fe3+ L-1. The 30 and 100 g L-1 TSS manure slurries were added; the sample tubes were vigorously agitated and left standing overnight at room temperature. After 16 h, the liquid and solid were separated as described previously to determine the aggregated fraction and the N and P concentrations of the equilibrium solution and solid phases.

Phosphorus Extractability in Treated Manure Solids and from Soil Amended with the Treated Solids
Phosphorus extractability as affected by polymer and mineral amendments was determined in Christiana silt loam (fine, kaolinitic, mesic Typic Paleudult) amended with untreated and treated dewatered residues of TSS100 manure suspensions. A series of manure suspensions were prepared, to which the three mineral amendments were added at a single rate of 1 g L-1. For the metal salts, the amendment rate was equivalent to adding 3.0 and 3.7 mmol L-1 of Al3+ and Fe3+, respectively. Following mixing and liquid–solid separation by centrifugation, the untreated and polymer-mineral amended manure solids were added to 100-g soil samples at the rate of 10 g kg-1 soil. The final water content of the mixture was brought to 60% of the water-filled pore space with deionized water. The mixtures were incubated for 1 wk at 20°C.

At the end of equilibration period, a sample set of manure and amended soils were agitated with 25 mL of deionized water on a reciprocal shaker at 250 rpm for 20 min to determine DRP concentrations. The suspensions were centrifuged at 7000 x g and the supernatant solution was filtered (0.45 µm). The filtrate was acidified with 6 M HCl and stored frozen for later N and P analysis. A second set of manure and amended soil samples were agitated with 25 mL of Mehlich-3 extracting solution on a reciprocal shaker for 20 min (Mehlich, 1984; Soil Plant Analysis Council, 1992). The suspensions were centrifuged and the supernatant solution was filtered, acidified, and stored frozen for N and P analysis.

Phosphorus analyses were determined by the molybdenum blue method (American Public Health Association, 1998). Total N and P contents of a concentrated H2SO4 digest were determined spectrometrically in 1.5-g samples of manure or manure-amended soil with an autoanalyzer (Bran + Luebbe, Buffalo Grove, IL). Electrical conductivity (EC) measurements were also made using a digital conductivity meter (Fisher Scientific, Pittsburgh, PA). For liquid manure suspensions, 1-mL aliquots of the equilibrium solution samples were measured into 80-mL plastic vials. Appropriate volumes of deionized water were added to be within the instrument linear dynamic range. In solid manure and manure-amended soil samples, EC and pH of the untreated and treated manure samples were measured in 1:2 solid to water mixtures (Rhoades et al., 1989; Thomas, 1996).

Manure TSS, polymer, and mineral amendment treatments were replicated three times and arranged in a randomized complete block design. Differences in treatment main effects and interactions were detected following analysis of variance and the Duncan multiple range test at the 0.05 level of probability using the Statistical Analysis System (SAS Institute, 1999).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 NOTES
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Individual Polymer and Mineral Amendment Effect
Individually, the cationic organic polymers increased particulate aggregation in both TSS30 and TSS100 manure suspensions (Table 2) . Manure particulates formed an aggregated layer that floated to the surface of the manure suspension. To a large extent, the TSS concentration of the manure suspension controlled the amount of polymer required to initiate visible particulate aggregation and clarification of the equilibrium solution. In TSS30 suspensions, equilibrium solution TSS decreased at addition rates >=1 g L-1 when organic polymers were used alone. In TSS100 suspensions, we also needed polymer concentrations >=1 g L-1 to reduce TSS content of the manure liquid phase. Larger reductions, ranging from 35 to 45 g L-1 or averaging 60% less TSS, were attained at addition rates of 3.3 g L-1 of organic polymers, when compared with reductions attained in TSS30 suspensions. Reductions in TSS by all polymers appeared to converge toward a maximum of 20 g L-1. Although adjustments were made for the chemical amendment or polymer weight to calculate net particulate weights, solution-phase TSS concentrations might be overestimated when particulate concentrations were low at high polymer and amendment rates of addition. The least significant difference (LSD) at the 0.05 level of probability averaged 13 and 15 g L-1 for TSS30 and TSS100, respectively.


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  		Table 2. Effects of organic water treatment polymers on particulate removal in reconstituted dairy manure suspensions containing initial total suspended solid concentrations of 30 and 100 g L-1.

 
The hydrated aggregated fraction increased with increasing rates of polymer addition, corresponding to the decrease in equilibrium solution phase TSS (Table 2). The congealed or thickened aggregates entrapped large amounts of wastewater. About 2 to 19% and 12 to 30% of the suspension, by weight, were entrapped in the aggregated fraction in TSS30 and TSS100 manure suspensions, respectively. The aggregation, absorption, and ensuing increase in physical size produced large flocs, potentially facilitating the screening and particle filtration phases of the solid–liquid separation process. Aggregation treatment differences were more readily detected with the hydrated aggregated fractions than measurements of the equilibrium solution TSS. In the TSS30 suspension, the polymers began to aggregate particulates at application rates of 0.1 g L-1 for C-1596 and SD-2085 and 0.25 g L-1 for C-581, compared with polymer concentrations >=1 g L-1 as determined by solution TSS measurements. The polymers appeared even more efficient in the TSS100 suspension, attaining a two- to threefold increase in hydrated aggregated fractions over and above the initial TSS. These results suggested that reduced interparticle distances, compared with those in the TSS30 suspension, facilitated particle bridging by the long-chain polymers and manure particulate sorption and enmeshment onto the large organic flocs.

The absorption of wastewater by the hydrated flocs was more apparent as the dewatered aggregated fraction did not show any change in mass in the TSS30 manure suspension for all three polymers. In the TSS100 suspension, the dewatered residues were essentially not different from the untreated controls, except at rates exceeding 0.1 g polymer L-1 for C-1596 or SD-2085 and 1 g L-1 for C-581 (Table 2). Distinct phase separation boundaries were visible and increasingly clear liquid phases were obtained when polymer concentrations were increased between 0.25 to 3.3 g L-1 of suspension. The polymers turned the suspension into a clear rigid nonflowable gel at the rate of 15 and 30 g L-1 for C-1596 and SD-2085 in TSS30 and TSS100, respectively (data not shown).

Mineral amendments also reduced the equilibrium solution phase TSS at amendment rates >=50 g L-1 in TSS30 as the result of increased aggregation and sorption onto amorphous hydroxide species such as Al(OH)3 and the distinctive brown Fe(OH)3 precipitates or onto porous spheroid fly ash particles (Table 3) . Increased TSS of the manure suspension required less mineral amendment to initiate TSS reduction in the equilibrium-solution phase. In the TSS30 suspension, both Al and Fe salts decreased the equilibrium solution TSS at about 50 g L-1 whereas the TSS100 required about 10 g L-1. However, the hydrated aggregated fraction increased upon the addition of the lowest rate of metal used (1 g L-1) as wastewater entrapment and increase in floc size occurred without immediate clarification of the equilibrium solution. Maximal aggregation of suspended particulates reached 0.47 and 0.6 kg kg-1 or 60 to 70% of the suspension solids with Al and Fe salts, respectively. Ferric chloride was the more effective coagulant, reaching higher maximum aggregation of hydrated sediment, 0.6 kg kg-1 at the rate of 10 g Fe salt L-1 (37 mmol Fe3+ L-1), compared with an average of 0.4 kg kg-1 with rates >=10 g Al salt L-1 (or >=30 mmol Al3+ L-1).


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  		Table 3. Effect of mineral amendments on manure particulate aggregation in reconstituted dairy manure suspensions containing initial total suspended solid concentrations of 30 and 100 g L-1.

 
The insoluble coal-combustion by-product also exhibited some particle destabilization capacity at rates >=50 g L-1 and appeared to act as an in situ screen. Mechanistically, fly ash and manure particles flowed through the solution phase and settled out discretely under the influence of gravitational forces when total particle concentrations were low (<=10 g L-1 in TSS30). As fly ash particle concentration increased, the frequency of interparticle collisions increased between fly ash particles, manure particles, and fly ash and manure particles. Settling occurred with coalescence or aggregation because of physical contact, electrostatic interactions, and increased particle mass. Fly ash is high in silica, aluminum, iron, and calcium oxides and most of the oxides are in glassy amorphous forms (Adriano et al., 1978; Zhang et al., 2002). The finely divided fly ash has a large surface area and variable charge density that arose from silanol (~SiOH) and metal hydroxyl functional groups on the ash particle surface. At the basic pH of fly ash–amended suspensions, attractive forces between fly ash and manure particles may include hydrogen bonding and induced dipoles. Fibrous manure particulates aggregated and were dragged down by the settling of denser ash particles, resulting in the reduction in solution-phase TSS in both TSS30 and TSS100 (Table 3). Decolorization of the equilibrium solution was also visible and suggested that sorption and partitioning mechanisms may have contributed to the entrainment and co-sedimentation of fly ash and manure particulates.

Combinations of Flocculant Polymers and Mineral Amendments
The effects of manure suspension TSS on particulate aggregation remained evident in polymer and mineral amendment combinations. Particulate aggregation was only slightly affected by organic polymers in the 0.01 to 0.25 g L-1 concentration range (Fig. 1 and 2) . The synergistic effect of combinations of mineral amendments and polyacrylamide polymers occurred at Al and Fe salt concentrations <=10 g L-1. For example, alone, C-1596 showed a slight increase in aggregating manure particulates and yielded an average hydrated aggregated fraction of 0.12 kg kg-1 at rates between 0.1 to 0.25 g L-1 in the TSS30 suspension (Table 2). Alone, Al or Fe salts yielded an average hydrated aggregated fraction of 0.215 and 0.30 kg kg-1 at the amendment rate of 1 to 10 g L-1, respectively. Together, the polymers and metal salts increased aggregation by another 0.05 to 0.10 kg kg-1 (Fig. 1). Similar behavior was observed in the suspension with the polyamine C-581 (Fig. 2). In the TSS100 suspension, a gradual increase in the hydrated aggregated fraction was observed with increase in metal salt concentrations between 1 and 10 g L-1. Aggregated fractions peaked at about 50 to 60%, by weight, at polymer concentrations much less than polymers by themselves (Table 2) or mineral salts (Table 3) by themselves. Polymer rate of addition would have to reach the 1 to 3.3 g L-1 levels (instead of concentrations of 0.1 to 0.25 g L-1) while mineral amendment rates would have to exceed the 50 g L-1 level. The synergistic effect of the chemical combination suggested that increasing concentrations of electrolytes in the manure suspensions that had only organic polymers increased ionic concentration in the double layer of manure particulates, compressing its thickness (O'Melia, 1998). The water treatment polymers destabilized manure suspensions, enhancing particulates' tendency to aggregate and particle interbridging. In addition, combining the two types of manure amendments enhanced particulate aggregation while using about 75 to 90% less polymer or 10 and up to 80% less metal salt required for visible increased aggregation induced by themselves. Input reduction also meant less concern for the metal content of the residual solids upon land application.



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  		Fig. 1. Effect of total suspended solids (TSS) on the aggregation of manure particulates by polyacrylamide polymer C-1596, in combination with Al2(SO4)3·18H2O, FeCl3·6H2O, or Class C fly ash.

 


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  		Fig. 2. Effect of total suspended solids (TSS) on the aggregation of manure particulates by polyamine polymer C-581, in combination with Al2(SO4)3·18H2O, FeCl3·6H2O, or Class C fly ash.

 
The suspensions restabilized as aggregates disintegrated at an amendment rate of 100 g L-1 of metal salts. The observation suggested that surface charge inversion could have caused particle dispersion at high solution concentrations of Al3+6 or Fe3+6 aquo-metal ions (Letterman and Vanderbrook, 1983). The pH of minimum solubility of metal hydroxide precipitates is about 6.3 and 8 for freshly precipitated Al- and Fe-hydroxide, respectively (Van Benschoten and Edzwald, 1990; Letterman et al., 1999). Equilibrium solution acidity increased with increasing quantity of added metal salts and exceeded the pH of metal hydroxide minimum solubility with rates >=1 g L-1 (Table 4) . The dissolution of metal hydroxide precipitates produced positively charged hydrolysis products and soluble aquo-metal ions. These soluble species have a strong affinity for negatively charged manure particulates, inducing charge neutralization, charge reversal, and the restabilization of the manure suspension. The suspension electrical conductivity (EC) has increased rapidly with addition rates >=1 g L-1, by about an order of magnitude at the 10 and again at the 100 g L-1 amendment rates for Al and Fe salts (Table 4). At high concentrations, aquo-metal ions surrounded and complexed individual manure organic particles, resulting in less interparticle bridging. The increased electrical conductivity suggested a more positively charged solution.


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  		Table 4. Electrical conductivity (EC) and pH of the dairy manure liquid phase as a function of mineral amendment concentrations.

 
Liquid Phase Nutrient Removal
The combinations of polymers and mineral amendment in TSS30 suspensions did not affect ammonium N concentrations of the equilibrium solution, except possibly for Fe salt added at rates of 100 g L-1 (Fig. 3 and 4) . In TSS100 suspensions, Al and Fe salts appeared to lower the equilibrium solution N concentrations and no plausible explanation was found except for the potential formation of insoluble precipitate involving NH4–N similar to struvite, a magnesium-ammonium phosphate (Greaves at al., 1999).



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  		Fig. 3. Effects of co-application of cationic water treatment polymer C-1596 (0.1 g L-1) and mineral amendments on the equilibrium solution NH4–N and dissolved reactive P concentrations in two dairy manure suspensions containing initial total suspended solid concentrations of 30 and 100 g L-1.

 


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  		Fig. 4. Effects of co-application of cationic water treatment polymer C-581 (0.1 g L-1) and mineral amendments on the equilibrium solution NH4–N and dissolved reactive P concentrations in two dairy manure suspensions containing initial total suspended solid concentrations of 30 and 100 g L-1.

 
The suspension TSS, polymers, and mineral amendments had a marked effect on manure DRP. Although fly ash did not reduce suspension TSS until addition rates equaled or exceeded 50 g L-1, fly ash consistently reduced solution-phase DRP at all rates by 52 and 71% in TSS30 and TSS100 suspensions, respectively. Soluble P was removed from the liquid phase when the fly ash was used alone or in combination with the organic polymers. Sorption isotherms for PO4–P showed that fly ash sorption capacity and retention mechanisms included chemisorption and surface complexation (Dao, 1999; Dao, unpublished data, 2002). In this study, the results suggested that, similar to previous results involving beef cattle manure, fly ash reacted with phosphate anions to reduce solution DRP and suppressed P solubility in the particulate fraction. Surface functional groups such as ~SiOH reacted with the solution-phase solutes as follows:

Typical composition of Class C fly ash also showed 15 to 20% Al2O3, 5 to 15% Fe2O3, and 8 to 32% CaO (Zhang et al., 2002). Upon hydration, the fly ash silanol and metal hydroxyl groups can react with solution H2PO2-4 to reduce DRP from manure suspensions.

Aluminum and Fe salts also reduced solution-phase DRP at 1 g L-1 when applied with either C-581 or C-1596 polymers. Freshly precipitated amorphous Al(OH)3 or Fe(OH)3 can either react with dissolved phosphate P or sorbed and entrapped AlPO4 or FePO4. The sorption and precipitation of PO4–P was shown to be controlled by the integrated particles rather than the Al- or Fe(OH)3 or Al- or FePO4 individually (Hsu, 1975). Solution DRP did not change at the 10 g L-1 level, compared with the unamended manure suspensions, and then DRP substantially increased when Al and Fe salts were added at rates of 100 g L-1. Therefore, caution should be exercised in manure treatment focused on P immobilization and to guard against a tendency toward overapplication.

In TSS100 suspensions, the higher TSS concentration required the higher metal salt rate of 10 g L-1 to reduce DRP by 30 to 90% of the initial levels (Fig. 3 and 4). Again, the equilibrium solution DRP increased when Al or Fe salts were added at the rate of 100 g L-1 of manure suspension. It was not surprising to observe the elevated DRP levels when considering the hydrolyzing potential of Al and Fe salts and the fact that the manure suspensions restabilized. The low pH induced by these metal salts increased soluble aquo metal ions and soluble P species, inducing the dissolution of mineral phosphates in manure, such as calcium or iron phosphates, whether formed in situ or added to the feed. Enhanced acidic hydrolysis of organic phosphate ester linkages of partially digested feedstuff containing calcium salts of phytic acid may also have contributed to the increase in inorganic phosphate equilibrium solution concentrations shown in Fig. 3 and 4 (Dao and Schmidt, 2001). Therefore, these findings suggested that an adequate amount of metal salt promoted aggregation and P immobilization, but more reversed and negated the benefits of P reduction by metal salts in the presence of organic polymers. These results also illustrated the complexity of the chemistry of Al3+ and Fe3+ in manure wastewater. In such cases, the role of particulate and dissolved organic matter in metal phosphate and polyphosphate solubility needs further investigation.

In the liquid phase of untreated dairy manure suspension, NH4–N was the major nutrient component. Reduction or increase in equilibrium solution DRP changed the solution N to P ratio. These ratios ranged from 80 to 139 (data not shown) and were reflected in changes in those of the aggregated solid phase. In the solid phase of untreated dairy manure, the N to P ratio averaged 5.8 and 2.5 in TSS30 and TSS100 suspensions, respectively (Table 5) . The ratio was narrower than what is needed in crop production in spite of our effort to maximize the conservation of manure nutrients (i.e., acidification of the freshly collected urine fraction to keep N losses as NH3 to a minimum). With polymers and mineral amendment treatments, the N to P ratio decreased with increasing metal salt rates of addition between 1 and 10 g L-1 in TSS30 because of the precipitation of insoluble P forms in the aggregated solid fraction of TSS 30 solids, particularly for the fly ash treatment (Table 5). In the TSS100 manure solids, fly ash behaved exactly as in the more dilute manure suspensions, sequestering P in the aggregated fraction with increasing rates of addition. Aluminum and Fe salts could widen the ratios, however, by increasing the dissolution of particulate P and negating the effort to sequester solution DRP. Thus, it was important to keep mineral amendments within the narrow range of concentrations of 1 to 10 g L-1 to balance the need to conserve urine N and reduce DRP with the metal salts in liquid manure suspensions.


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  		Table 5. Dairy manure particulate nitrogen to phosphorus ratio as a function of mineral amendment concentrations in dairy manure suspensions amended with 0.1 and 0.25 g L-1 of water treatment polymer C-1596.{dagger}

 
Particulate Phosphorus Extractability
In manure particulates isolated from TSS100 suspensions treated with the mineral amendments alone, the release of DRP was reduced by all manure amendments at the 1 g L-1 rate (Table 6) . The synergism of the organic and mineral amendment combination on reducing P solubility was also evident as polymers further increased the reduction of DRP by fly ash and Fe salts and Al sulfate was more effective only in combination with the C-1596 polymer. One explanation for the synergistic behavior was that amorphous metal hydroxides sorbed DRP and metal phosphate precipitates were themselves enmeshed in the long-chain polymers. In addition, the high acidity of suspensions containing hydrolyzing metal salts can protonate the amino groups of the polyacrylamide and polyamine polymers to yield additional sites for sorption of phosphate anions (Table 5). Thus, from an environmental perspective, the potential release of DRP to runoff from a surface application of manure particulates was minimized. The acidic-fluoride Mehlich 3–extractable P was increased between 9 and 39%, compared with untreated manure solids or polymer-treated manure. The results suggested that organic P forms that may have bound and were concentrated in the polymer matrix were susceptible to dissolution under acidic conditions. In soil, the amendment–manure mixtures appeared to have maintained their stability. The DRP was reduced by 63 to 96% in soil amended with manure treated with mineral amendments alone, and 40 to 76% in soil amended with manure treated with a mixture of polymer-mineral amendments. The organic polymers increased the Mehlich 3–extractable P in soil treated with polymer and metal salt combinations. Acidic extracting solutions removed significant amounts of sorbed P upon dissolution of amorphous Al and Fe hydroxides in treated soil and manure. Thus, the greater extractability of P would have to be considered in calculations of land loading rates of treated manure. Fly ash–treated manure solids remained stable and sequestered DRP and did not affect Mehlich 3–extractable P at the 1 g L-1 rate of manure suspension to fly ash.


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  		Table 6. Phosphorus extractability from treated manure particulates and soil amended with treated manure at the rate of 10 g kg-1.{dagger}

 

   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 NOTES
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
In confined animal feeding operations, liquid manure presents special challenges in handling, storage, and manure nutrient beneficial reuse because of the large volume of diluted wastes. Organic polymers and mineral P immobilizing chemicals increased manure particulate aggregation to concentrate the solid fraction. As chemical amendments and methods of applications largely depended upon manure characteristics (particle size, TSS content, pH), concentrations of polymers or mineral amendments must be optimized to benefit from any interaction or synergism that exist in their separate mechanisms of action. Suspension TSS had a clear effect on aggregation and rates of polymer and amendment. Organic polymers and hydrolyzing Al and Fe salts individually increased the aggregation of particulates in TSS30 and TSS100 dairy manure suspensions. A synergistic aggregating reaction occurred at low concentrations of Al2(SO4)3 and FeCl3 with cationic polyacrylamide and polyamine polymers. Maximum aggregation was attained with 1 and 10 g L-1 in TSS30 and TSS100 manure suspensions, respectively. Co-applications of polymers and mineral amendments also reduced excessive amounts of DRP in manure-amended soils to reduce offsite transport risks. Reductions in DRP were achieved consistently at all rates of fly ash and hydrolyzing metal salts at rates <=10 g L-1 in the presence of organic water treatment polymers. The polymer-amendment treated particulates remained stable in soil. Release of DRP from the soil amended with treated manure was reduced. Therefore, the synergistic effect that exists between chemical aggregation aids should be optimized to achieve chemical input reduction and maximize manure particulates and P removal from liquid manure suspensions containing high TSS.


   ACKNOWLEDGMENTS
 
The authors sincerely acknowledge the technical assistance of J. Woodward-Greene and L. Heighton. Appreciation is expressed to Cytec Industries for supplying the organic water treatment polymers.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 NOTES
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
1 Mention of a trade or manufacturer name is made for information only and does not imply an endorsement, recommendation, or exclusion by the USDA-ARS. Back


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




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