Phosphorus forms and extractability in dairy manure: A case study for Wisconsin on-farm anaerobic digesters

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Bioresource Technology 99 (2008) 425–436

Phosphorus forms and extractability in dairy manure: A case study for Wisconsin on-farm anaerobic digesters Kerem Gu¨ngo¨r, K.G. Karthikeyan

*

Biological Systems Engineering Department, University of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706, United States Received 12 January 2006; received in revised form 20 October 2006; accepted 22 November 2006 Available online 7 February 2007

Abstract The effect of anaerobic digestion on phosphorus (P) forms and water P extractability was investigated using dairy manure samples from six full-scale on-farm anaerobic digesters in Wisconsin, USA. On an average, total dissolved P (TDP) constituted 12 ± 4% of total P (TP) in the influent to the anaerobic digesters. Only 7 ± 2% of the effluent was in a dissolved form. Dissolved unreactive P (DUP), comprising polyphosphates and organic P, dominated the dissolved P component in both the influent and effluent. In most cases, it appeared that the fraction of DUP mineralized during anaerobic digestion became subsequently associated with particulate-bound solids. Geochemical equilibrium modeling with Mineql+ indicated that dicalcium phosphate dihydrate, dicalcium phosphate anhydrous, octacalcium phosphate, newberyite, and struvite were the probable solid phases in both the digester influent and effluent samples. The water-extractable P (WEP) fraction in undigested manure ranged from 45% to 70% of TP, which reduced substantially after anaerobic digestion to 25% to 45% of TP. Anaerobic digestion of dairy manure appears capable of reducing the fraction of P that is immediately available by increasing the stability of the solid phases controlling P solubility. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Calcium and magnesium phosphates; Phosphorus extractability; Geochemical equilibrium modeling; Mineql+; Dissolved and particulate phosphorus

1. Introduction During the past few decades, the livestock industry in the United States (US) has been influenced by several economical and technological factors geared towards fewer, densely populated, and more efficient animal feeding operations (AFOs) (USDA and USEPA, 1999; FAO, 2001). For instance, the number of dairy operations decreased from 334,180 to 81,440 between 1980 and 2004 (USDANASS, 2004). In the same period, average cow density per operation tripled and average annual milk production per cow increased from 5400 kg to 8600 kg (USDA-NASS, 2004). Due to livestock concentration in fewer AFOs, average animal manure production per AFO also has increased

*

Corresponding author. Tel.: +1 608 262 9367; fax: +1 608 262 1228. E-mail address: [email protected] (K.G. Karthikeyan).

0960-8524/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.11.049

significantly. More cropland is, therefore, required to efficiently absorb the increasing amount of manure nutrients in the areas with high-density livestock operations. However, most AFOs have very limited cropland where manure is being intensively applied in excess of agronomic nutrient needs. The concentrated AFOs (CAFOs) having more than 1000 animal units produce 68% of the excessive phosphorus (P) originating from animal production systems (USDA, 2003). Agricultural runoff and subsurface flow transport the excess nutrients to downstream water bodies and contribute to the eutrophication problem (Sharpley et al., 2001). Currently, agriculture (fertilizer and manure application) is the leading pollution source for the impaired streams and lakes in the USA (USEPA, 2000). Approximately 50% of the large AFOs store manure in earthen basins for a period of at least three months prior to land application (USDA-APHIS, 2004). Half of these earthen basins are designed to treat manure for partial

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solids, nitrogen, and pathogen removal and are generally termed anaerobic lagoons. Current storage facilities are unsatisfactory to provide environmentally sound solutions to the mounting problems associated with manure generation at the CAFOs. Therefore, attempts are underway to adopt high-rate and efficient waste treatment processes for animal manure. An example of these processes is anaerobic digestion and a brief review and farm-scale application of this treatment method can be found elsewhere (Gu¨ngo¨r and Karthikeyan, 2005a). The literature on anaerobic manure digestion mainly focuses on the influence of reactor configuration and manure characteristics on (i) solids destruction, (ii) biogas production, (iii) odor reduction, and (iv) pathogen removal. Very limited information is available on the effect of anaerobic digestion on animal manure P dynamics. Gerritse and Vriesema (1984) investigated P speciation (inorganic versus organic) and phase distribution (dissolved versus particulate) using four liquid cow manure samples, two of which were anaerobically digested. For the undigested samples, total organic P (Po) constituted only 10% of total P (TP) on an average. While a substantial portion (38%) of total Po was in a dissolved form, the corresponding fraction was only 1% for total inorganic P (Pi). They also reported that anaerobic digestion did not affect the phase distribution of P species with total Po being 14% of TP in the treated manure. Although there are several studies confirming that undigested dairy manure P is predominantly inorganic in nature (McAuliffe and Peech, 1949; Peperzak et al., 1959; Barnett, 1994; Dou et al., 2000; Sharpley and Moyer, 2000), only a few of them (e.g., Gerritse and Vriesema, 1984) focused on P phase distribution and found that Pi is predominantly particulate-bound. As high as 70% of TP can be released from the solid phase during water extraction depending on the water-to-manure ratio (Dou et al., 2000; Ajiboye et al., 2004; Brandt et al., 2004; He et al., 2004; Gu¨ngo¨r and Karthikeyan, 2005a). A straightforward water-extractable P (WEP) test has been recently proposed to estimate the availability of manure P to runoff (Kleinman et al., 2002). Currently, research is underway to standardize the protocol for WEP determination and incorporate it into P management tools, e.g., P indices (Studnicka et al., 2005; Sullivan et al., 2005). Elemental composition and extraction data strongly support the hypothesis that Pi in animal manure is predominantly in Ca- and/or Mg-bound forms (Dou et al., 2000; Kleinman et al., 2002; O’Connor et al., 2002; ChapuisLardy et al., 2003; Gu¨ngo¨r and Karthikeyan, 2005a). In previous studies (Gu¨ngo¨r and Karthikeyan, 2005a,b) using batch reactors, we quantified water extractability of P from anaerobically digested manure and evaluated the stability of probable Ca– and Mg–P solid phases controlling P solubility using a geochemical equilibrium model, Mineql+ (Westall et al., 1976). Depending on the inoculum-to-substrate ratio in the batch reactors, anaerobic digestion influenced water extractability of P (Gu¨ngo¨r and Karthikeyan, 2005a). For instance, while WEP decreased by as much as

28% at an inoculum-to-substrate ratio of 0.3, a high inoculum-to-substrate ratio of 2.0 increased WEP by 40%. Mineql+ simulations showed that MgNH4PO4 Æ 6H2O (struvite), CaHPO4 Æ 2H2O (dicalcium phosphate dihydrate, DCPD), CaHPO4 (dicalcium phosphate anhydrous, DCPA), Ca4H(PO4)3 Æ 3H2O (octacalcium phosphate, OCP), and Ca3(PO4)2 (beta-tricalcium phosphate, b-TCP) were the probable P solid phases in both untreated and anaerobically digested manure. Although batch reactor studies provide insights on the effect of anaerobic digestion on P speciation and extractability, the results cannot be directly translated to full-scale and continuously operating on-farm digesters. The major goal of this study was to evaluate P dynamics during anaerobic treatment at six full-scale on-farm digesters in Wisconsin, USA. Specific objectives are to: (i) determine P speciation (i.e., dissolved reactive P (DRP), total dissolved P (TDP), and TP), phase distribution (dissolved versus particulate), and water P extractability in both influent and effluent samples collected from the study farms and (ii) investigate the probable Ca– and Mg–P solid phases using Mineql+ modeling. 2. Methods 2.1. On-farm anaerobic digester systems Six full-scale, on-farm anaerobic digesters located in Wisconsin were sampled in this study. The following abbreviations will be used, henceforth, to address the digesters at the dairy operations: DS, G, S, T, W1, and W2. Four of these digesters are located at different dairies whereas W1 and W2 receive manure from the same dairy operation. Only the DS dairy operation uses a flush system for manure collection. The remaining operations utilize a scrape system. A slated screen and an equalization basin precede the digesters at the DS and T dairies, respectively. Only the manure collected from milk cow pens is conveyed to W1; on the other hand, W2 influent includes milk, prefresh, dry, and fresh cow manure (Kenn Buelow, personal communication). Solid–liquid separation of anaerobically digested manure from every digester is achieved using mechanical separators. Digestate (liquid portion) is then discharged to manure ponds and subsequently landapplied. Separated manure solids are recycled as bedding. Design and operational characteristics of the digesters are given in Table 1 (Kramer, 2002; Kramer, 2004). 2.2. Dietary information Through communication with the operators, information on the composition of diets used at DS, S, T, and W dairy operations was obtained (Personal Communications, 2004). Furthermore, the DS and W operations provided detailed composition of dry, pre-fresh, and lactating cow diets. Operators at the remaining dairies only provided information on the lactating cow diet. The P content in

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Table 1 Design and operational characteristics of the on-farm digesters Name

Head-feeding digester

Temperature range

Type

Target temperature (°C)

RAS

Target hydraulic retention time (d)

DS G S T W1 W2

1100 875 1000 2400 3000 3000

Mesophilic Mesophilic Mesophilic Mesophilic Mesophilic Mesophilic

Mixed plug-flow loop Mixed plug-flow loop Plug-flow Complete-mix Mixed plug-flow Mixed plug-flow

38 38 37–41 38 38 38

Yes Yes No No Yes Yes

20 20 20 20 20 20

RAS: Return activated sludge.

Table 2 Calcium, magnesium, and phosphorus composition of the dairy cow diets Name

Ca (%)

Mg (%)

P (%)

Ca:Pa

Mg:Pa

DS S T W1 W2

0.71–1.23 1.00 0.92 0.86 0.66–0.91

0.23–0.35 0.35 0.40 0.36 0.36–0.46

0.30–0.42 0.40 0.43 0.36 0.28–0.36

1.83–2.43 1.94 1.66 1.85 1.83–2.28

0.99–1.33 1.13 1.20 1.29 1.67–2.12

a

Molar ratio.

the diet varied from 0.28% to 0.43%, with the diet at the W1 and T dairies containing the lowest and highest levels, respectively (Table 2). It should be noted that the P content of lactating cow diets at all of the dairy operations was below the US average of 0.48% (Wu and Satter, 2000). Dry cow dietary P content at DS and W dairies was 0.30% and 0.31%, respectively, which agreed very well with the recomended value of 0.30% (Etgen et al., 1987). On the other hand, all of the lactating and dry cow diets had higher Mg content (Table 2) as compared to the minimum daily requirements of 0.25% and 0.16%, respectively (Etgen et al., 1987). Similarly, the diets used for the lactating and dry cows contained higher Ca levels (Table 2) than the recommended values of 0.80% and 0.40%, respectively (Etgen et al., 1987). There was a fairly narrow range of Ca:P and Mg:P molar ratios with the Ca:P molar ratio being higher in all diets (Table 2), with the exception of the pre-fresh diet at the W dairy farm. 2.3. Analytical methods To cover seasonal variations, four different sampling campaigns were performed within a 1-yr period: September (fall) and December (winter), 2004; March (spring) and July (summer), 2005. Samples influent to and effluent from each digester were collected on the same day using 1-L containers. For the DS digester, digestate was sampled instead of its effluent, since there was no physical access to the effluent. Influent and digestate samples from DS digester were collected using the same procedure as described above for the other digesters. Samples were transported to the laboratory in coolers packed with ice on the collection day. Total solids (TS) content was determined by pre-weighing the sub-samples (directly transferred from 1-L containers) and removing moisture in an oven at 105 °C for 24 to 48 h (Peters et al., 2003). Volatile solids (VS) determination

followed the initial steps of TS analysis; additionally, organics were removed in a muffle furnace at 550 °C for a minimum of 24 h. Filtrates of the sub-samples were obtained by initially centrifuging them at 18,000 rpm (29,749g) for 10 min followed by passing the supernatant through a 0.45 lm pore size MF-Millipore mixed cellulose ester (Millipore, Billerica, MA) membrane filter. Filtrates were analyzed for pH, electrical conductivity (EC), dissolved reactive P (DRP), total dissolved P (TDP), ammonium ðNHþ 4 Þ (except September 2004 samples), bicarbonate ðHCO 3 Þ (except September 2004 samples), and dissolved Ca and Mg. Electrical conductivity and pH were measured using an Accumet AR-50 pH/conductivity meter (Fisher Scientific, Pittsburgh, PA). The meter was calibrated using single (1.4 dS m1) and three point (pH = 4, 7, 10) calibration methods before each set of EC and pH measurements, respectively. Pre-determined amounts of sub-samples and sub-sample filtrates were digested using a persulfate and sulfuric acid digestion method (QuikChem 10-115-01-4-E) for TP and TDP analyses, respectively. A Lachat BD 46 block digester (Hach Company, Loveland, CO) was used to digest the samples using a two-step sequence: 250 °C for 30 min and 380 °C for 40 min. One or two boiling chips were added to each tube to avoid splashing during digestion. Cold fingers were placed on top of the tubes before the 380 °C digestion step to initiate H2SO4 reflux and to avoid sample loss. The digestion sequence was repeated more than once to obtain relatively colorless digestate when necessary. The digestates were then diluted with Milli-Q grade deionized (DI) water (final volume 50 mL). Polyphosphate and compost references with known P concentrations were digested with the sub-samples for quality control purposes for TDP and TP analyses, respectively. Dissolved reactive P, TDP, and TP were determined using an automated molybdate-ascorbic acid method (QuikChem 10-115-01-1-A for DRP; QuikChem 10-11501-4-E for TDP and TP) on a Lachat QuikChem Flow Injection Analyzer (FIA) 8000 (Hach Company, Loveland, CO). Ammonium was also determined on the Lachat QuikChem FIA using an automated method based on the Berthelot reaction (QuikChem 10-107-06-1-B). A slightly modified version of the titration method proposed by Jenkins et al. (1983) was used to determine HCO 3 in the subsample filtrates: 1–2 mL of filtrate were diluted to 50 mL using DI water and titrated with 0.1 or 0.01 N HCl until

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a pH of 5.75 was reached. True HCO 3 alkalinity was calculated using Eq. (1) (Jenkins et al., 1983): TBA ¼ 1:25  ALK5:75

ð1Þ

where TBA = true bicarbonate alkalinity using an endpoint pH of 5.75 (mg L1 CaCO3), ALK5.75 = alkalinity measured using endpoint pH of 5.75. Dissolved Ca and Mg concentrations in the filtrates were measured following standard methods (air–acetylene flame and lanthanum chloride added as a suppressant for phosphate) on an atomic absorption spectrophotometer (GBC Scientific Equipment, Melbourne, Australia) (APHA, 1995). Manure sub-samples were subjected to extraction using a 200-to-1 water-to-manure (dry-weight equivalent) ratio and 1 h shaking time. Dissolved reactive P measurement of the extracts was assigned as WEP (Kleinman et al., 2002). WEP was reported as the percentage of TP extracted as DRP by the above water extraction procedure. Triplicate manure samples were used for the analyses of the December 2004, March 2005, and July 2005 samples. September 2004 samples were analyzed in duplicates.

each simulation, probable P solid phases were identified using the saturation index (SI) criterion: the P solid phases with an SI value higher than 1 and lower than +1 were labeled as ‘‘probable’’ phases (Bril and Salomons, 1990; Gu¨ngo¨r and Karthikeyan, 2005b). If SI of a solid phase is below 1, manure slurry is undersaturated with respect to that particular phase, which is highly soluble and unlikely to exist under given pH, EC, and component concentrations. On the other hand, manure slurry is supersaturated with respect to P solid phases having SI values above +1 indicating that these solid phases are insoluble under the given conditions and their existence is thermodynamically favorable. However, precipitation of an insoluble P solid phase could be extremely slow and improbable (i.e., kinetically limited) due to the presence of inhibiting components in manure. This effect has been discussed in our previous publication (Gu¨ngo¨r and Karthikeyan, 2005b). Therefore, the solid phases satisfying the above SI criterion were regarded in this study as the ‘‘probable’’ phases that could control P solubility in the influent and effluent samples.

2.4. Chemical modeling Geochemical equilibrium software, Mineql+, was used for determining the probable P solid phases (Westall et al., 1976) in influent and effluent samples from the onfarm anaerobic digesters. Average values of EC, HCO 3, NHþ , dissolved Ca and Mg were used in the simulations. 4 The range in values of these analytes is given in Table 3. Ionic strength was calculated from EC using an empirical equation (Snoeyink and Jenkins, 1980): I ¼ 1:6  105  EC

ð2Þ

where I is the ionic strength (M) and EC is the electrical conductivity (lS cm1). The P solid phases selected for Mineql+ simulations in our previous study (Gu¨ngo¨r and Karthikeyan, 2005b) were also used for this work: OCP, Ca5(PO4)3OH (hydroxylapatite, OHAP), MgHPO4 Æ 3H2O (newberyite), DCPA, DCPD, Mg3(PO4)2, struvite, and b-TCP. The selected phases were transferred from the ‘‘Dissolved Solids’’ to the ‘‘Species Not Considered’’ compartment of the software to prevent the most thermodynamically stable phase from controlling P levels. For each data set collected between December 2004 and July 2005, a separate simulation was performed yielding a total number of 34 simulations (17 influent and 17 effluent samples). Following

2.5. Regular operation criteria for phosphorus speciation analysis A major objective of this study was to investigate the changes occurring in DRP and TDP concentrations of manure slurry subjected to anaerobic digestion. Limited sample-set did not facilitate application of parametric statistical methods on the influent and effluent data. Therefore, an unconventional ‘‘regular operation’’ condition criterion was applied. This criterion was basically designed to eliminate data affected by: (i) irregularities associated with digester operation and/or influent manure characteristics, and (ii) P precipitation in a digester. A common irregularity in digester operation is caused by engine breakdown that decreases reactor temperature and affects overall performance. Another irregularity could be due to dilution of raw manure slurry with milking parlor wastewater and subsequent changes in the influent characteristics. Elimination of irregular data points facilitated a robust comparison of DRP and TDP concentration between the influent and effluent. Therefore, the following criterion, based on an a priori knowledge of anaerobic digestion process was used to determine whether the digesters were operating regularly at the time of sampling:

Table 3 Range of input variables for Mineql+ simulations Sample type a

Influent Effluenta a

I (mol L1)

Ca (mmol L1)

Mg (mmol L1)

1 NHþ 4 (mmol L )

1 CO2 CaCO3) 3 alkalinity (mg L

1 PO3 4 (mmol L )

0.10–0.45 0.15–0.49

3.22–19.7 2.62–12.1

9.00–28.2 9.71–21.2

41.6–152 81.1–169

3100–9000 6900–12,100

0.184–1.34 0.297–0.606

December, 2004; March and July, 2005 samples. The minima and maxima given above are the averages of triplicates.

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IF VSinf  VSeff  SDVSðinfÞ  SDVSðeffÞ > 0 AND jTPinf  TPeff  SDTPðinfÞ  SDTPðeffÞ j=TPinf < 0:15 THEN regular operation condition exists where VSinf = average influent VS concentration (%), VSeff = average effluent VS concentration (%), TPinf = average influent TP concentration (mg P L1), TPeff = average effluent TP concentration (mg P L1) , SDVS(inf) = standard deviation of influent VS concentration (%), SDVS(eff) = standard deviation of effluent VS concentration (%), SDTP(inf) = standard deviation of influent TP concentration (mg P L1), SDTP(eff) = standard deviation of effluent TP concentration (mg P L1). The first term in the above criterion is based on the fact that organic solids are degraded and, therefore, effluent VS should be lower than influent VS in a properly operating anaerobic digester. When the 68% confidence intervals, i.e., the interval including 68% of the data, of the influent and effluent VS levels overlap or the confidence interval of effluent VS is significantly higher than that of influent VS, this situation is interpreted as a disturbance to the regular operation condition. The second term of the criterion stems from the assumption that TP concentration does not change significantly during anaerobic digestion. Precipitation of P as a new solid phase and subsequent settling within the digester may invalidate this assumption. However, with the exception of the digester at the S dairy, the other digesters sampled were of a mixed type (Table 1) and, therefore, the effect of P precipitation/settling should be minimal. When the difference between the confidence intervals of the influent and effluent TP levels was more than 15% of average influent TP concentration, the regular operation condition was rejected. When both of these logical conditions were satisfied for a particular data set, it was labeled as a regular operation data set and analyzed further to evaluate the effect of anaerobic digestion on P speciation between DRP and TDP. Phosphorus phase distribution (dissolved versus particulate) was determined using the average TDP:TP ratios. 3. Results and discussion 3.1. Phosphorus speciation and phase distribution As mentioned earlier, influent–effluent sample combinations were collected from all the digesters except the one at the DS farm. For the DS digestate, effluent samples (digestate) were collected from the mechanical separator. Therefore, the data acquired for the DS farm can be used for combined performance analysis of both the anaerobic

429

digester and mechanical separator. Since the major goal was to investigate the effect of anaerobic digestion on DRP and TDP levels, data from the DS farm was excluded for this portion of our analysis. The first step involved analyzing influent and effluent VS and TP concentrations for G, S, T, W1, and W2 digesters to identify the data sets satisfying the regular operation condition. Based on the criteria given above, it was determined that the following digesters were operating under regular condition: G digester (September 2004; March and July 2005), S digester (September 2004; March and July 2005), T digester (September and December 2004; July 2005) (Figs. 1 and 2). The digesters W1 and W2 did not meet the criterion for any of the sampling dates (data not shown). Under regular operation condition, the average VS removal ranged from 33% to 43%, 22% to 28%, 30% to 53% for G, S, and T digesters, respectively. These removal efficiencies fall within the VS removal range of 18% to 66% reported for five different on-farm anaerobic digesters treating dairy manure in the state of New York, USA (Wright et al., 2004). The dissolved P component (given by TDP) comprises DRP (orthophosphates) and dissolved unreactive P (DUP, which is equal to TDP-DRP). Organic P and polyphosphates are the main species contributing to DUP. A comparison of DRP and TDP levels (Figs. 3 and 4, respectively) indicates that DUP is the dominant fraction of the dissolved phase in the influent manure samples. The DUP:DRP ratio for influents was as high as 3.28, 5.44, and 3.94 for the G, S, and T dairies, respectively. On the other hand, the highest value of the DUP:DRP ratio observed for the anaerobic digester effluents was 1.88, 2.30, and 1.96 for the G, S, and T dairy operations. The above trend could be attributed to substantial decreases in DUP concentration after anaerobic digestion (Fig. 4). Mesophilic conditions in these digesters could have accelerated mineralization of organic P, since enzymes degrading organic matter are more active around 30 °C to 40 °C (Boerth, 2003). Although the experimental results suggest mineralization of DUP, a consistent increase in the effluent DRP concentrations was not observed (Fig. 3). Hence, it is likely that the mineralized portion of DUP must have subsequently re-precipitated as an inorganic P solid phase or became associated with particulate solids and, thereby, removed from the dissolved phase. Phosphorus partitioning between dissolved and particulate phases in dairy manure was determined using the samples that satisfied the regular operation criteria. In the anaerobically undigested (influent) samples, 11 ± 4% of TP was in the dissolved form on an average (median = 10%). After anaerobic digestion, there was a decrease in the dissolved P fraction, which was 6 ± 2% of TP (median = 6%). When data from all the samples were used to calculate the P phase distribution, without the regular operation condition separation, comparable results were obtained: influent P in the dissolved phase = 12 ± 4% of

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G

Sampling Month

Jul-05

Mar-05

Dec-04

Sep-04 0.00

2.00

4.00

6.00

8.00

10.00

VS (%)

a

Influent

Effluent

S

Sampling Month

Jul-05

Mar-05

Sep-04

0.00

2.00

4.00

6.00

8.00

10.00

VS (%)

b

Influent

Effluent

T

Sampling Month

Jul-05

Mar-05

Dec-04

Sep-04 0.00

2.00

4.00

6.00

8.00

10.00

VS (%)

c

Influent

Effluent

Fig. 1. Influent and effluent volatile solids (VS) concentration trends for the on-farm anaerobic digesters: (a) G digester, (b) S digester, and (c) T digester (G, S, and T denote dairy operations). Error bars show ±1 standard deviation of replicates.

TP (average) with a median of 12% and effluent P in the dissolved phase = 7 ± 2% of TP (average) with a median of 7%. Overall, P mineralization appears to play an important role during anaerobic digestion of dairy manure, yet it did not affect the predominance of DUP in the dissolved phase. Mineralized P presumably became particulatebound and did not contribute to the dissolved fraction in the digested manure samples.

3.2. Mineql+ simulations: probable Ca– and Mg–P solid phases Mineql+ simulations revealed that DCPD, DCPA, OCP, newberyite, and struvite are the probable P solid phases in dairy manure and anaerobic digestion did not influence their type and stability. These results agree well with those from our earlier work, using batch-scale reactors, conducted on water extracts of anaerobically digested

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G

Sampling Month

Jul-05

Mar-05

Dec-04

Sep-04 0

200

400 600 TP (mg-P/L) Influent Effluent

800

1000

S

Sampling Month

Jul-05

Mar-05

Sep-04

0

200

400 600 TP (mg-P/L) Influent Effluent

800

T

Jul-05 Sampling Month

1000

Mar-05

Dec-04

Sep-04

0

200

400 TP (mg-P/L) Influent

600

800

Effluent

Fig. 2. Influent and effluent total phosphorus (TP) concentration trends for the on-farm anaerobic digesters: (a) G digester, (b) S digester, and (c) T digester (G, S, and T denote dairy operations). Error bars show ±1 standard deviation of the replicates.

and undigested manure (Gu¨ngo¨r and Karthikeyan, 2005b). One of the phases identified previously was b-TCP, which fulfilled the probable phase criterion only at high extractant or water-to-manure ratios. In this study, the SI value for b-TCP was consistently greater than +1 indicating that the manure slurry was supersaturated with respect to this phase before and after anaerobic digestion. Therefore, bTCP is not likely to control P solubility in dairy manure. Newberyite was also identified as a probable P solid phase in the current study whereas we previously reported

(Gu¨ngo¨r and Karthikeyan, 2005b) that the water extracts were undersaturated with respect to newberyite even at low water-to-manure ratios. Hence, newberyite, being a highly soluble Mg–P solid phase, may exist in dairy manure before or after anaerobic digestion, but would dissolve completely upon contact with relatively low volumes of an extractant. Manure characteristics (i.e., pH, ionic composition and concentration) can be expected to vary among different dairy operations and even seasonally within the same

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G

Sampling Month

Jul-05

Mar-05

Sep-04

0.00

10.00

a

20.00 DRP (mg-P/L) Influent Effluent

30.00

40.00

S

Sampling Month

Jul-05

Mar-05

Sep-04

0.00

10.00

b

20.00 DRP (mg-P/L) Influent Effluent

30.00

40.00

T

Sampling Month

Jul-05

Dec-04

Sep-04

0. 00

c

10. 00

20. 00 30. 00 DRP (mg-P/L) Influent Effluent

40. 00

50. 00

Fig. 3. Influent and effluent dissolved reactive phosphorus (DRP) trends for the on-farm anaerobic digesters operating under regular operation condition: (a) G digester, (b) S digester, and (c) T digester (G, S, and T denote dairy operations). Error bars show ±1 standard deviation of the replicates.

operation. This variation would influence the SI value of the P solid phases. For example, while struvite was determined as a probable P solid phase in the DS influent sample collected in July 2005, it was not identified in Mineql+ simulations of samples collected in December 2004 and March 2005 for the same operation. Therefore, all Mineql+ simulation outputs were compiled to determine the frequency of a certain P solid phase to exist in the influent and effluent samples. Henceforth, the term frequency will

be used to report the number of simulations for which a particular P solid phase had an SI above 1 and below +1, i.e., satisfied the probable P solid phase condition. The frequency of the P solid phases decreased in the following order (Fig. 5): DCPD  DCPA > newberyite > OCP  struvite (for simulation of influent samples); DCPA > DCPD > OCP > newberyite > struvite (for effluent sample simulations). The frequency of occurrence of the two highly soluble Ca– and Mg–P solid phases, i.e.,

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433

G

Sampling Month

Jul-05

Mar-05

Sep-04

0.00

20.00

a

40.00 TDP (mg-P/L) Influent Effluent

60.00

80.00

S

Sampling Month

Jul-05

Mar-05

Sep-04 0.00

20.00

b

40.00

60.00 TDP (mg-P/L) Influent Effluent

80.00

100.00

120.00

T

Sampling Month

Jul-05

Dec-04

Sep-04

0.00

c

20.00

40.00

60.00

80.00

100.00

120.00

TDP (mg-P/L) Influent Effluent

Fig. 4. Influent and effluent total dissolved phosphorus (TDP) trends for the on-farm anaerobic digesters operating under regular operation condition: (a) G digester, (b) S digester, and (c) T digester (G, S, and T denote dairy operations). Error bars show ±1 standard deviation of the replicates.

DCPD and newberyite, was 18% and 38% lower in the effluent, respectively, as compared to that in the influent. On the other hand, frequency of a less soluble Ca–P solid phase, OCP, was 71% higher in the effluent. The most stable or insoluble Mg–P solid phase used in our simulations was struvite and its frequency of occurrence did not change following anaerobic digestion. The frequency trends observed for DCPD, newberyite, and OCP indicate that the concentration of a more stable Ca–P solid phase in dairy manure may increase after anaerobic digestion. Since

influent and effluent samples consistently yielded SI values > +1 for the highly stable Ca–P phases, namely b-TCP and OHAP, it is very unlikely that these P phases would exist in dairy manure. 3.3. Effect of anaerobic digestion on water-extractable phosphorus Influent and effluent WEP levels for the samples collected in July 2005 from all the six digesters are given in

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100.0

Influent

Frequency of Occurrence (%)

90.0

Effluent

80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 DCPD

DCPA

OCP

Newberyite

Struvite

Solid Phase Fig. 5. Frequency of occurrence (%) of the probable phosphorus solid phases in the influent and effluent samples of the on-farm anaerobic digesters (sampling events: December, 2004; March and July, 2005). [DCPD: dicalcium phosphate dihydrate, CaHPO4 Æ 2H2O; DCPA: dicalcium phosphate anhydrous, CaHPO4; OCP: octacalcium phosphate, Ca4H(PO4)3 Æ 3H2O; newberyite: MgHPO4 Æ 3H2O; struvite: MgNH4PO4 Æ 6H2O]. Frequency refers to the number of Mineql+ simulations in which a particular P solid phase had a saturation index (SI) value above 1 and below +1.

80.0 70.0

INFLUENT

60.0

WEP (% TP)

50.0 40.0 30.0 EFFLUENT

20.0 10.0 0.0 0.00

5.00

10.00

15.00

TS (%) DS

G

S

T

W1

W2

Fig. 6. Water-extractable P (WEP) for influent and effluent samples from the on-farm anaerobic digesters as a function of total solids (TS) content (sampling event: July 2005). Influent and effluent WEP values are grouped together within rectangular boxes.

Fig. 6. It should be noted that effluent WEP for the DS operation is for the samples from the mechanical separator that followed the anaerobic digester. The WEP for undi-

gested manure ranged from 45% to 70% whereas anaerobically digested manure had substantially lower WEP values (25–45%). Specifically, the WEP of the anaerobically

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digested manure was 47%, 45%, 22%, 36%, 40%, and 28% lower as compared to that for the undigested manure for the DS, G, S, T, W1, and W2 digesters, respectively. The TS content of the undigested manure ranged for all the dairies between 8.5% and 12.5% except for the W1 influent. Influent and effluent TS content for W1 digester was 2% and 4%, respectively. Apparently, irregular operation conditions prevailed at the time of sampling in July 2005 for the W1 digester. The influent TS level for W1 digester was exceptionally low in July 2005 as compared to the 5– 10% range observed for the other sampling events. It is most likely that milking parlor wastewater was discharged into the W1 influent channel, which diluted the manure solids shortly before the July 2005 sampling trip. The TS content of the digested manure for all the dairies was between 4% and 8%. The influent VS level ranged from 7% to 11%, excluding the W1 sample, and effluent VS was between 3% and 6% (data not shown in Fig. 6). As discussed earlier, phase distribution of P in the dairy manure was dominated (ca. 90%) by the particulate-bound form with anaerobic digestion increasing the particulate-P content slightly. Therefore, the formation of a more stable P solid phase from DRP alone cannot explain the substantial reductions in WEP levels of dairy manure following anaerobic digestion as shown in Fig. 6. A plausible explanation could be that anaerobic digestion process transforms the existing P solid phases and increases their stability. Anaerobic biodegradation of organic matter, which causes a reduction in VS and TS content of dairy manure (Gu¨ngo¨r and Karthikeyan, 2005a), may have enhanced the stability of P solid phases. Further research is necessary to elucidate the mechanisms responsible for the increase in stability of P solid phases during anaerobic digestion of dairy manure. 4. Conclusions Both undigested and anaerobically digested dairy manure contain less than 20% of TP in a dissolved form, which is dominated by dissolved unreactive P species. Influent and effluent DRP and TDP trends suggested the mineralization of DUP during anaerobic digestion and subsequent partitioning of orthophosphates into a particulate-bound form. In most cases, this mechanism resulted in a slight increase in the particulate-P content of digested manure. Probable P solid phases in the anaerobically undigested and digested manure were DCPD, DCPA, OCP, newberyite, and struvite. The water extractability of manure P decreased substantially (22–47%) following anaerobic digestion. The on-farm anaerobic digesters appear to have a positive impact in reducing the immediate extractability of dairy manure P, attributable to the destruction of organic constitutents that are known to inhibit the formation of stable P solid phases. Additional research is necessary to specifically examine the mechanism causing an increase in P solid phase stability after anaerobic digestion. This mechanism could be exploited for the design of on-

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farm anaerobic digestion systems to maximize P recovery from animal manures. Acknowledgements The authors sincerely thank Prof. Emeritus James C. Converse (University of Wisconsin, Madison, WI, USA) for his mentorship, Mr. Joseph M. Kramer for providing contact information for the dairy farms, and the owners of the dairy operations for their full cooperation. This research was supported by the USDA-CSREES (Wisconsin) Hatch Program, Project No. WIS04655. References Ajiboye, B., Akinremi, O.O., Racz, G.J., 2004. Laboratory characterization of phosphorus in fresh and oven-dried organic amendments. J. Environ. Qual. 33, 1062–1069. APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. APHA, AWWA, and WPCF, Washington, DC. Barnett, G.M., 1994. Phosphorus forms in animal manure. Bioresour. Technol. 49, 139–147. Boerth, T., 2003. Enzymatic hydrolysis of organic phosphorus in soils and dairy feces. University of Wisconsin-Madison Doctor of Philosophy Dissertation, USA. Brandt, R.C., Elliott, H.A., O’Connor, G.A., 2004. Water-extractable phosphorus in biosolids: implications for land-based recycling. Water Environ. Res. 76 (March/April), 121–129. Bril, J., Salomons, W., 1990. Chemical composition of animal manure: a modelling approach. Neth. J. Agric. Sci. 38 (3), 333–351. Chapuis-Lardy, L., Temminghoff, E.J.M., De Goede, R.G.M., 2003. Effects of different treatments of cattle slurry manure on waterextractable phosphorus. Neth. J. Agric. Sci. 51 (1–2), 91–102. Dou, Z., Toth, J.D., Galligan, D.T., Ramberg Jr., C.F., Ferguson, J.D., 2000. Laboratory procedures for characterizing manure phosphorus. J. Environ. Qual. 29, 508–514. Etgen, W.M., James, R.E., Reaves, P.M., 1987. Dairy Cattle Feeding and Management, seventh ed. John Wiley & Sons, New York. FAO, 2001. Global Livestock Production and Health Atlas. (accessed 7.10.2005). Gerritse, R.G., Vriesema, R., 1984. Phosphate distribution in animal waste slurries. J. Agric. Sci. 102 (February), 159–161. Gu¨ngo¨r, K., Karthikeyan, K.G., 2005a. Influence of anaerobic digestion on dairy manure phosphorus extractability. Trans. ASABE 48 (4), 1497–1507. Gu¨ngo¨r, K., Karthikeyan, K.G., 2005b. Probable phosphorus solid phases and their stability in anaerobically digested dairy manure. Trans. ASABE 48 (4), 1509–1520. He, Z., Griffin, T.S., Honeycutt, C.W., 2004. Phosphorus distribution in dairy manures. J. Environ. Qual. 33, 1528–1534. Jenkins, S.R., Morgan, J.M., Sawyer, C.L., 1983. Measuring anaerobic sludge digestion and growth by a simple alkalimetric titration. J. WPCF 55 (5), 448–453. Kleinman, P.J.A., Sharpley, A.N., Wolf, A.M., Beegle, D.B., Moore, P.A., 2002. Measuring water-extractable phosphorus in manure as an indicator of phosphorus in runoff. Soil. Sci. Soc. Am. J. 66 (6), 2009– 2015. Kramer, J.M., 2002. Agricultural Biogas Casebook. (accessed 21.09.2005). Kramer, J.M., 2004. Agricultural Biogas Casebook, 2004 Update. (accessed 20.09.2005). McAuliffe, C., Peech, M., 1949. Utilization by plants of phosphorus in farm manure: I. Soil Sci. 68, 179–184.

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