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TECHNICAL REPORTS

Waste Management

Precipitation of Liquid Swine Manure Phosphates Using Magnesium Smelting By-Products

^a Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd., Quebec, QC, Canada G1V 2J3
^b Norsk Hydro Canada Inc., 7000 Raoul-Duchesne Blvd., Bécancour, QC, Canada G9H 2V3

^* Corresponding author (parentg{at}agr.gc.ca)

Received for publication May 2, 2006.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Swine manure contains considerable amounts of total (P) and soluble phosphorus (PO₄–P) which may increase the soil P content when applied in excess to crop requirements and, consequently, risk water eutrophication. The feasibility of using magnesium (Mg) from the by-product of electrolysis and foundries (BPEF) for the removal of P from liquid swine manure was studied by adding up to 3 g of Mg as BPEF per liter of nursery (NU) and grower-finisher (GF) swine manure in 25-L plastic buckets. Changes in P and other elements were monitored for up to 360 h. Small amounts of Mg as BPEF (0.5 and 1.0 g Mg L^–1 manure) reduced the total P concentration of the liquid fraction by 70 to 95% of both manure types with respect to the control treatment of mixed raw manure. A settling period of 8 h or more was necessary to significantly reduce the liquid fraction's total P concentration for both manure types. Reduction of PO₄–P varied from 96 to 100% in the liquid fractions for both manure types, which along with natural settling, explains most of the total P reduction in that fraction. The addition of BPEF did not influence the N content of manure. The low P liquid fraction can be safely applied to saturated P soils whereas the high P solid fraction offers the opportunity of transporting manure to agricultural soils deficient in P. Since N is conserved, both liquid and solid fractions could be valuable fertilizer manure by-products.

Abbreviations: BPEF, by-product of electrolysis and foundries • DM, dry matter • GF, grower-finisher swine manure • LSD, least significant difference • NU, nursery swine manure • PO₄–P, water-soluble phosphorus

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
SWINE manure has one of the highest contents of total P and soluble inorganic P among farmyard manures (Peperzak et al., 1959; Barnett, 1994). Repeated applications of manure beyond crop requirements increase the soil concentration of P (Simard et al., 1995) and the degree of P saturation (Nelson et al., 2005). As a result, P can be lost through runoff or leached into the ground water (Daniel et al., 1998), causing eutrophication of streams and lakes (Sharpley et al., 1999). To prevent excessive algae growth in fresh water, it is more effective to control P than N (Daniel et al., 1994). Within North America, regional regulations establish limits for manure applications on soils with a high P content. Consequently, several swine operators have insufficient land to spread all of their manure within the regulatory limits.

Removing P from manure and converting it into a valuable product for use outside the farm is a potential solution (Greaves et al., 1999). Physical separation, mechanically or by gravity-based sedimentation, effectively separates solids and liquids (Zhang and Westerman, 1997) but without chemical coagulants it has a limited effect on the dissolved P fraction of liquid manures (Westerman and Bicudo, 2000).

Precipitation of the inorganic dissolved P in manure is possible with coagulants, such as metal salts (Moore and Miller, 1994; Zhang and Lei, 1998), alkaline earth metal-containing compounds (Vanotti et al., 2003), or with industrial by-products (Stout et al., 1998; Dao, 1999; Barrington et al., 2004). Using Al or Fe metal salts in manure separation, however, may lead to increased soil content of those elements. Ippolito et al. (2003) report an increase in desorbed Al concentration in soils after applying residuals from water treatments. Aluminum-based agents may be linked to brain disease and their use should be avoided (World Health Organization, 1998).

Magnesium-based agents are used to remove nutrients from wastewater (Wu and Bishop, 2004) and struvite (MgNH₄PO₄.6H₂O), considered to be a slow release fertilizer (Durrant et al., 1999), is produced. Soluble P was reduced by 76 to 91% in swine slurries when treated with Mg as magnesium chloride (MgCl₂) (Burns et al., 2001). Although using Mg for P removal is technically feasible, the expense of high-grade Mg compounds makes this practice uneconomical (Chimeros et al., 2003). When magnesium hydroxide, for example, is used to remove soluble P from wastewater, the costs are eight to ten times more than for calcium hydroxide. However, the feasibility of using a more economical source of Mg, such as low-grade magnesium oxide, was confirmed for treating wastewater (Chimeros et al., 2003).

The Norsk Hydro Canada Inc. (NHCI) plant, located in Bécancour (QC, Canada), produces pure Mg. The process involves the dissolution of magnesite ore (MgCO₃) in hydrochloric acid (HCl) and the formation of anhydrous MgCl₂ granules that are then decomposed by electrolysis to produce metallic Mg. Because the by-product of electrolysis and foundries (BPEF), generated during the electrolysis, cannot be recycled in the production of metallic Mg, it could be a low-cost source of Mg for precipitating P in swine manure. Its effectiveness to precipitate P in swine manure, however, has not been reported. The objective of this study was to establish the feasibility of using BPEF as a Mg source to remove P from liquid swine manure. Specifically, the study sought to determine the amount of Mg as BPEF needed to be added to the manure and its required settling period.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Sources and Transfer of Liquid Swine Manure
Two types of liquid swine manure (nursery, NU and grower-finisher, GF) were collected from two farms located in St-Germain de Grantham, QC, Canada. The manure from each farm was transferred from the concrete pit to a plastic tote tank (1500 L) on 1 May 2003. Before its transfer, the liquid swine manure was stirred for at least 4 h with a lagoon pump and brought to the Jean-Charles-Chapais Research Farm in Lévis (QC, Canada). On 2 May 2003, the liquid swine manure was transferred to 25-L plastic buckets using a tube connected to the bottom drain valve of the tote tank while vigorously stirring with an electric tote tank mixer; each bucket received 20.1 L of liquid manure. Four samples per manure type were collected during transfer and analyzed; selected chemical characteristics are presented in Table 1. A total of 28 buckets, representing the experimental units, were prepared for each manure type and placed on the concrete floor of a barn. The temperature in the barn during the settling period was maintained above 13°C; the maximum temperature reached was 28°C.

Manure Sampling and Analysis
The liquid fraction of the manure treated with BPEF and that from the mixed control treatment were sampled with a plastic large-tip opening transfer pipette fitted with a rubber bulb at 2, 8, 24, 48, 168, and 360 h after the BPEF application. The transfer pipette allowed samples to be drawn without disturbing the liquid fraction. The transfer pipette was washed with distilled water before each collection. Manure in each bucket reached a height of 33 cm. Two depths were sampled which represented 30 and 60% of the depth of the manure from the bucket top at 23 and 13 cm from the bottom of the buckets. A 50-mL sample was collected for each sampling depth, at each sampling time.

The solid fraction at the bottom of the buckets was sampled only at the end of the experiment (360 h). The mixed control treatment was not considered for this sampling because it had no solid fraction. The liquid fraction was carefully and slowly removed using a small electric vacuum pump connected to a Pyrex aspirator bottle to prevent any mixing with the solid fraction. After the removal of the liquid fraction, the solid fraction was collected into 50-mL plastic bottles using a plastic spatula. The spatula was washed with distilled water before collecting each sample. All collected samples were frozen at –20°C until analysis.

Unfiltered samples were thawed at 4°C for 96 h and then digested using a modified wet acid (H₂SO₄ and H₂SeO₃) ashing digestion method to determine total N, P, Ca, and Mg (Chapman and Pratt, 1961). The digestions were processed on aliquots of 7 mL from the liquid fraction, 5 mL from the mixed control treatment, and 1 mL from the solid fraction. Samples were heated first at 100°C for 30 min on a digestion system to evaporate most of the moisture in the samples and to prevent any boiling over. Subsequently, the temperature was raised gradually over 1 h from 100 to 400°C. Samples were then digested for 45 min at 400°C and the supernatant was analyzed.

Phosphate (PO₄–P) was extracted by shaking unfiltered solid and liquid manure samples with distilled water in 35-mL Nalgene Oak Rigde polypropylene tubes with a manure/water ratio of 1:4 (Keeney and Nelson, 1971). Samples were agitated on a reciprocal shaker at 250 rpm for 30 min, centrifuged 10 min at 23 000 x g (15 000 rpm, Sorvall RC-5C plus, rotor SA-600, Sorvall, Newton, CT), and filtrated through VWR brand (West Chester, PA) #410 filter paper.

Concentrations of total Mg and Ca were determined by atomic absorption spectroscopy (PerkinElmer 3300, Überlingen, Germany). Concentrations of total N and P, and PO₄–P were determined on a Lachat QuikChem 8000 flow injection analysis system (Zellweger Analytic Inc., Lachat Instrument Division, Milwaukee, WI, methods No. 15-501-3 and 12-115-01-1-A). Concentrations were expressed in mg L^–1 manure by assuming a specific gravity of manures near unity. The pH of samples was determined using glass bulb electrodes connected to a dual-channel Accumet pH meter.

Statistical Analysis
The two sampling depths were averaged to represent the chemical composition of the liquid fraction of each manure type; there was no significant interaction between sampling depth, sampling time, and the amount of BPEF added (data not shown), except for the PO₄–P concentration of the liquid fraction. The experiment followed a split-plot design with four replicates. Manure type was assigned to the main plots and applications of Mg as BPEF to sub-plots. Due to significant interactions between sampling time and amount of Mg as BPEF or manure type, the analysis of variance for the liquid fraction was performed for each sampling time using the General Linear Models procedure (SAS Institute, 1990). A protected least significant difference (LSD) at the 0.05 level of probability was used to compare treatments. For the solid fraction, the effect of BPEF amounts was partitioned into linear and quadratic contrasts.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Liquid Fraction
Total Phosphorus
The difference between the concentration of total P in the mixed control treatment and that of the check with no BPEF (0 g Mg L^–1 manure) indicates the effect of natural settling. The greater settling of total P in the GF manure, compared with the NU manure (Fig. 1), can be explained by its higher DM content (Table 1). The observed maximum reduction in total P concentration noted with the natural settling of the manure was 52% for the GF manure and 23% for the NU manure. The total P concentration of both manure types with no BPEF applied remained relatively stable over the experimental period after the natural settling had occurred during the first 2 h of the settling period.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Total P concentration in the liquid fraction of two types of liquid swine manure (nursery and grower-finisher) treated with different amounts of Mg in the form of by-product of electrolysis and foundries (BPEF) as a function of time (2–360 h) with x-axis on a log scale; the mixed control treatment received no BPEF but was stirred before each sampling. Vertical bars represent the LSD values (P 0.05) at each sampling time.

Total P concentration in the liquid fraction of the GF manure increased slightly at 2 h after the BPEF application when 2.0 g Mg L^–1 manure or more was used (Fig. 1). For a settling period of 8 h or more, additions as low as 0.5 g Mg L^–1 manure were sufficient to reduce total P concentration of the liquid fraction for both manure types. Higher BPEF additions did not significantly reduce further the total P concentration of the liquid fraction after 8 h. Most of the reduction in total P concentration of the liquid fraction occurred within the first 24 h. The total P concentration continued to decrease until the end of the sampling period, but at a much slower rate.

Water-Soluble Phosphorus
Natural settling, without BPEF, reduced the liquid fraction PO₄–P concentration of both manure types (23% with NU manure and 36% with GF manure, when averaged across all sampling times) compared with the mixed control treatment (Fig. 2). This natural settling had therefore less influence on the concentration of PO₄–P of the liquid fraction of GF manure than on the concentration of total P.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Soluble phosphorus (PO4–P) concentration in the liquid fraction of two types of liquid swine manure (nursery and grower-finisher) treated with different amounts of Mg in the form of by-product of electrolysis and foundries (BPEF) as a function of time (2–360 h) with x-axis on a log scale; the mixed control treatment received no BPEF but was stirred before sampling. Vertical bars represent the LSD values (P 0.05) at each sampling time.

Total Nitrogen, Total Magnesium, pH, and Total Calcium
Additions of BPEF had no effect on the concentration of total N (Fig. 3) in the liquid fraction of both manure types after a settling period of up to 168 h. Greater BPEF additions significantly decreased total N concentration in the liquid fraction of GF manure after a settling period of 360 h.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Total N concentration in the liquid fraction of two types of liquid swine manure (nursery and grower-finisher) treated with different amounts of Mg in the form of by-product of electrolysis and foundries (BPEF) as a function of time (2–360 h) with x-axis on a log scale; the mixed control treatment received no BPEF but was stirred before sampling. Vertical bars represent the LSD values (P 0.05) at each sampling time.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Total Mg concentration in the liquid fraction of two types of liquid swine manure (nursery and grower-finisher) treated with different amounts of Mg in the form of by-product of electrolysis and foundries (BPEF) as a function of time (2–360 h) with x-axis on a log scale; the mixed control treatment received no BPEF but was stirred before sampling. Vertical bars represent the LSD values (P 0.05) at each sampling time.

The addition of BPEF increased the pH of the liquid fraction for both manure types (Fig. 5). This increase was explained by the calcium carbonate equivalent content of BPEF (854 g kg^–1; Table 2). Averaged across all sampling times, the pH of the liquid fraction of both the mixed control treatment and the check without BPEF were 7.1 for the GF manure and 7.4 for the NU manure. Reduction of the liquid fraction's PO₄–P concentration of both manure types is associated with the increase in pH of the liquid fraction (Fig. 5). As the pH of the liquid fraction of both manure types increased, the concentration of PO₄–P decreased, suggesting that Mg-P compounds formed or that P precipitation is affected by the pH of the liquid fraction.

View larger version (21K): [in this window] [in a new window]   Fig. 5. pH in the liquid fraction of two types of liquid swine manure (nursery and grower-finisher) treated with different amounts of Mg in the form of by-product of electrolysis and foundries (BPEF) as a function of time (2–360 h) with x-axis on a log scale; the mixed control treatment received no BPEF but was stirred before sampling. Vertical bars represent the LSD values (P 0.05) at each sampling time.

View larger version (21K): [in this window] [in a new window]   Fig. 6. Total Ca concentration in the liquid fraction of two types of liquid swine manure (nursery and grower-finisher) treated with different amounts of Mg in the form of by-product of electrolysis and foundries (BPEF) as a function of time (2–360 h) with x-axis on a log scale; the mixed control treatment received no BPEF but was stirred before sampling. Vertical bars represent the LSD values (P 0.05) at each sampling time.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

By-Product of Electrolysis and Foundries Addition and Settling Period Requirements for Phosphorus Removal
When comparing the BPEF-treated manure with the naturally settled manure (0 g Mg L^–1 manure) for sampling times of 8 to 360 h, the PO₄–P reduction represented 76 to 79% of the total P reduction in the liquid fraction for the NU manure and 94 to 100% for the GF manure. Although most of the total P reduction in the BPEF-treated manure was attributed to PO₄–P reduction, the addition of BPEF may also have led to the precipitation of some solid particles containing P.

Total P concentration of the GF manure at 2 h increased slightly when 2.0 or 3.0 g Mg L^–1 manure of BPEF was added. This minor and short-lasting increase may be explained by the presence of magnesium hydride (MgH₂) in BPEF, which produces hydrogen when it is in contact with water (Huot et al., 2003). This release of hydrogen gas induced turbulence that suspended P from the solid fraction. This turbulence was not observed in the PO₄–P concentration for both manure types, hence indicating that the solid particles of swine manure are comprised primarily of organic or insoluble particulate P.

The PO₄–P reduction occurred rapidly after the addition of BPEF. According to Wu and Bishop (2004), the reduction of PO₄–P occurs as quickly as 20 min after increasing the pH and adding Mg-containing chemicals to wastewater. The PO₄–P concentration continued to decrease up to 168 h after BPEF addition. A settling period duration of BPEF with swine manure of 2 h or more is therefore required to observe a significant PO₄–P reduction compared with the mixed raw manure and the naturally settled manure (0 g Mg L^–1 manure). A settling period duration of 168 h or more would maximize the PO₄–P reduction in the liquid fraction.

Nitrogen in Manure
Adding BPEF to swine manure increased the pH of the liquid fraction for both manure types. Increasing the pH of swine manure usually leads to ammonia volatilization (Panetta et al., 2005). The absence of significant changes in the concentration of total N in the liquid fraction for a settling period duration of 168 h or less suggests that volatilization of ammonia did not significantly occur. Ammonia volatilization is affected by numerous factors such as pH, temperature, wind speed, surface cover, and concentration of ammonium in manure. Having the manure unstirred during the settling period probably minimized ammonia volatilization (Panetta et al., 2005). The relatively low air temperature in the building and the absence of wind may also explain the low ammonia volatilization in the first 7 d of the settling period (Arogo et al., 1999; Sommer and Olesen, 2000). For a settling period of 360 h, however, the pH values were the highest and the slight reduction in the concentration of total N with additions of BPEF of 1.5 g Mg L^–1 manure, or more, may be attributed to the volatilization of ammonia with the increased pH of the liquid fraction.

Minerals Formed in Manure
Optimal pH conditions (8.5–9.0; Stratful et al., 2001) for forming struvite were potentially met for the solid fraction after BPEF was added. High Mg concentrations enhance struvite formation in wastewaters (Schulze-Rettmer, 1991). The formation of struvite in wastewaters and slurries, following the addition of Mg, usually results in the reduction of both NH₄–N and PO₄–P concentrations in the liquid fraction. There is, however, no evidence that struvite was produced in this experiment. No significant reduction of total N concentration occurred in the liquid nor solid fractions of both manure types within the first 168 h of the settling period.

Newberyite is a magnesium phosphate that precipitates with struvite (Abbona et al., 1986). Struvite precipitates only when the pH is sufficiently high; however, newberyite is formed when the concentration of dissolved Mg is very high and the pH is relatively low (Abbona and Boistelle, 1988). The addition of Mg to a water solution may involve the formation of whitlockite and brushite (Abbona and Franchini-Angela, 1990). Burns et al. (2003) also obtained different forms of Mg-P compounds and concluded that brushite and other phosphate-containing compounds were synthesized using MgCl₂ with manure. Analyses of manure samples by X-ray diffraction would be required to determine the nature of the mineral compounds formed after the addition of BPEF to manure.

Calcium and Magnesium Composition of By-Product of Electrolysis and Foundries-Treated Manure
Additions of BPEF to swine manure increased the Ca concentration of the liquid fraction for both manure types at 2 h. This result can be explained by the presence of Ca in the BPEF (Table 2). Total Ca concentration of the liquid fraction, however, decreased gradually during the settling period. This reduction could be attributed to the chemical speciation of PO₄–P with Ca. The decrease of the total Ca concentration of the liquid fraction, however, was greater than that of total P and PO₄–P concentrations between 168 and 360 h. The increased pH of the liquid fraction for both manures types may also have decreased the solubility of Ca and led to it being precipitated to the bottom of buckets, hence contributing to the decrease of the liquid fraction total Ca concentration.

The liquid fraction total Mg concentration was greater at 168 and 360 h than at 2 to 48 h. This increase during the settling period was attributed to the dissolution of slowly soluble Mg forms contained in BPEF, which represented around 50% of the total Mg (Norsk Hydro Canada Inc., 2000).

Total Phosphorus Content of the Solid Fraction of By-Product of Electrolysis and Foundry-Treated Manure
Even if the addition of Mg as BPEF greatly reduced the total P and PO₄–P concentrations of the liquid fraction, no significant increase of the total P concentration was observed in the solid fraction. This might be explained by the presence in the solid fraction of the remaining slowly soluble or insoluble fraction of the BPEF (50% of the BPEF; Norsk Hydro Canada Inc., 2000), which caused a dilution of the total P and PO₄–P. Increasing the total Ca and Mg concentrations of the solid fraction with increasing amounts of Mg as BPEF at 168 and 360 h provides evidence for the presence of BPEF in the solid fraction (Table 3). This lack of increase in the solid fraction total P concentration could also be related to our sampling procedure. It is possible that part of the Mg-P compounds, primarily those formed later during the settling period, might be located at the top of the solid fraction and could have been removed with the liquid fraction.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The results of this study demonstrated the feasibility of using BPEF as a Mg source to remove P from liquid swine manure. The addition of 0.5 to 1.0 g Mg L^–1 manure as BPEF and a settling period of 8 h or more almost eliminated all the PO₄–P in the liquid fraction of swine manure and reduced the total P content, compared with the mixed raw manure. The utilization of this low P liquid fraction on high P soils would comply with environmental regulations and supply N to crops. In the long term, the use of this low P liquid fraction would reduce soil P levels and, therefore, the risk of water eutrophication. The solid fraction also had a reduced PO₄–P concentration for a 0.5 g Mg L^–1 manure addition and its utilization would be environmentally advantageous because of the reduced P solubility. The utilization of this Mg- and Ca-enriched solid fraction would also increase the Mg and Ca content of soils.

	ACKNOWLEDGMENTS

 
The authors would like to acknowledge the technical assistance of Catherine Pinsonneault-Bélanger, Linda Gaulin, Caroline Morin, and Marie Bélanger; also the assistance of C.A. McRae, EditWorks, for structural editing of the manuscript. This research was supported in part by Norsk Hydro Canada Inc. and by the Matching Investment Initiative Program of Agriculture and Agri-Food Canada. An application for a U.S. (No. 10/536,896) and a Canadian (No. 2507388) patent protection on the concept has been submitted by Agriculture and Agri-Food Canada.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Contribution no. 814 from Agriculture and Agri-Food Canada.

	REFERENCES

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