Scholarly article on topic 'Centrifuge separation effect on bacterial indicator reduction in dairy manure'

Centrifuge separation effect on bacterial indicator reduction in dairy manure Academic research paper on "Agriculture, forestry, and fisheries"

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Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — Zong Liu, Zachary S. Carroll, Sharon C. Long, Aicardo Roa-Espinosa, Troy Runge

Abstract Centrifugation is a commonly applied separation method for manure processing on large farms to separate solids and nutrients. Pathogen reduction is also an important consideration for managing manure. Appropriate treatment reduces risks from pathogen exposure when manure is used as soil amendments or the processed liquid stream is recycled to flush the barn. This study investigated the effects of centrifugation and polymer addition on bacterial indicator removal from the liquid fraction of manure slurries. Farm samples were taken from a manure centrifuge processing system. There were negligible changes of quantified pathogen indicator concentrations in the low-solids centrate compared to the influent slurry. To study if possible improvements could be made to the system, lab scale experiments were performed investigating a range of g-forces and flocculating polymer addition. The results demonstrated that polymer addition had a negligible effect on the indicator bacteria levels when centrifuged at high g forces. However, the higher g force centrifugation was capable of reducing bacterial indicator levels up to two-log10 in the liquid stream of the manure, although at speeds higher than typical centrifuge operations currently used for manure processing applications. This study suggests manure centrifuge equipment could be redesigned to provide pathogen reduction to meet emerging issues, such as zoonotic pathogen control.

Academic research paper on topic "Centrifuge separation effect on bacterial indicator reduction in dairy manure"

Environmental

'Ma nage me nt

Contents lists available at ScienceDirect

Journal of Environmental Management

journal homepage: www.elsevier.com/locate/jenvman

Research article

Centrifuge separation effect on bacterial indicator reduction in dairy manure

Zong Liu a, Zachary S. Carroll b, Sharon C. Long b'c, Aicardo Roa-Espinosa d, Troy Runge a' *

a Dep. of Biological Systems Engineering, Univ. of Wisconsin-Madison, 460 Henry Mall, Madison, WI53706, United States b Dep. of Civil & Environmental Engineering, Univ. of Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53705, United States c Wisconsin State Laboratory of Hygiene and Dep. of Soil Science, Univ. of Wisconsin-Madison, 1525 Observatory Drive, Madison, WI 53706, United States d Soil Net LLC, 560 Enterprise Drive, Belleville, WI 53508, United States

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ARTICLE INFO

Article history: Received 31 August 2016 Received in revised form 6 January 2017 Accepted 12 January 2017

Keywords: Dairy manure Liquid/solid separation Pathogen indicator reduction Centrifuge speed Polyacrylamide

ABSTRACT

Centrifugation is a commonly applied separation method for manure processing on large farms to separate solids and nutrients. Pathogen reduction is also an important consideration for managing manure. Appropriate treatment reduces risks from pathogen exposure when manure is used as soil amendments or the processed liquid stream is recycled to flush the barn. This study investigated the effects of centrifugation and polymer addition on bacterial indicator removal from the liquid fraction of manure slurries. Farm samples were taken from a manure centrifuge processing system. There were negligible changes of quantified pathogen indicator concentrations in the low-solids centrate compared to the influent slurry. To study if possible improvements could be made to the system, lab scale experiments were performed investigating a range of g-forces and flocculating polymer addition. The results demonstrated that polymer addition had a negligible effect on the indicator bacteria levels when centrifuged at high g forces. However, the higher g force centrifugation was capable of reducing bacterial indicator levels up to two-log10 in the liquid stream of the manure, although at speeds higher than typical centrifuge operations currently used for manure processing applications. This study suggests manure centrifuge equipment could be redesigned to provide pathogen reduction to meet emerging issues, such as zoonotic pathogen control.

© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Large livestock farms often process manure before land application to recycle water for manure flushing, floor and sand washing (Sarkar et al., 2006). This not only reduces water usage at the farm but reduces the volume of manure that needs to eventually be land applied. Large-scale centrifugation is commonly used for dairy manure liquid/solid separation, offering high separation efficiency compared to other mechanical separators (Hjorth et al., 2010).

The emphases of most environmental studies concerning manure management have been on the effects of nutrient recovery and water quality. Nevertheless, it is important to monitor and to reduce bacteria in manure because many outbreaks of gastroenteritis related to livestock operations have been reported (Massé et al., 2011). Manure from large farms is often centrifuged for

* Corresponding author. E-mail address: trunge@wisc.edu (T. Runge).

slurry dewatering and the filtrate may be reused within barn operations (Hjorth et al., 2010). However, recycling water from manure can potentially cause disease outbreaks in both animal herds and humans unless the manure is handled and treated appropriately. More than 150 pathogens have been identified in manure which have subsequently been demonstrated to potentially cause zoonotic infections (Pell, 1997; AWWA, 2006). Pathogens may enter the human food chain as a result of land application of contaminated organic manures. The health risk is highest if manure is applied to ready-to-eat crops such as fruits and vegetables eaten raw (Nicholson et al., 2005). Therefore, management and treatment for pathogen reduction is an important practice in waste management quality control on large farms which also reduces potential risks of exporting pathogens to offsite users.

Not surprisingly, manure mechanical separators have been demonstrated in a limited number of studies to have low pathogen reduction efficiency (Hjorth et al., 2010). The most efficient and reliable pathogen reduction treatment for manure separated liquid is microfiltration, but such technology is rarely used on actual farms

http://dx.doi.org/10.1016/jjenvman.2017.01.022

0301-4797/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

because of the complexity and cost of these systems (Burton, 2007). However, high speed centrifugation is a common laboratory practice used for harvesting suspended bacteria in the field of microbiology. In this paper, the "speed" of centrifuge refers to the relative centrifugal force which is the acceleration measured in multiples of the gravitational acceleration constant, g. Bacteria are harvested using a wide variety of forces ranging from 1000 to 12,000 x g (Peterson et al., 2012). Centrifugation at greater than 5000 x g has been reported to cause a loss of viability in some bacterial strains (Pembrey et al., 1999). Industrial-scale centrifuges used for manure liquid/solid separation are typically operated at around 2000 x g, although they can be operated at a higher centrifugal force. These centrifuges can spin at approximately 2500 to 3500 rpm (Floerger, 2003), and because of their size, result in high g forces which are functions of centrifugal radii. For example, a full-scale centrifuge with a 1 m radius of rotation operating at 3500 rpm would exert g forces over 13,500 x g. Typical decanter centrifuges can be operated at over 4000 x g and solid-bowl centrifuges can be operated at over 10,000 x g (Beveridge, 2000).

We hypothesized that a higher centrifuge speed than what is currently used in most farm centrifuges would be able to enhance manure solid separation efficiency and to further reduce the pathogen levels in the separated liquid stream. The bench-scale centrifuge used in this study has a maximum operating relative centrifugal force of 10,000 x g. Therefore, a longer retention time (10 min, which is typically used in cell harvesting in microbiology labs) than field centrifugation was used. The results obtained can be converted to the equivalent higher centrifuge speed at short retention time when applied to large-scale centrifugation separations following Eq. (1) (Cvjetkovic et al., 2014).

T1/K1 = T2/K2 (1)

where T is the time of centrifugation, and K is the k-factor for the centrifuge rotor, which is the relative pelleting efficiency of a given centrifuge rotor at its maximum rotation speed.

On large farms, physical separation methods such as centrifu-gation are often enhanced by chemical addition (Vanotti et al., 2002; Amuda and Alade, 2006; Garcia et al., 2007). These chemicals bind and separate the smaller particles for efficient concentration of solids and nutrients (Zhang and Westerman, 1997). The use of polyacrylamide (PAM) polymers, their homopolymers, and their acrylamide/acrylic acid copolymers, alone or in combination with various inorganic salts, has proven to be effective in concentrating solids and nutrients in the separation process (Vanotti et al., 2002; Garcia et al., 2007). However, there is a lack of studies relating polymer properties such as charge density and molecular weight, as well as dairy manure characteristics such as total solid levels, chemical oxygen demand (COD), and particle size to separation efficiency. Furthermore, there is especially a lack of knowledge on their effects on pathogen sequestration into the solids or inactivation in the planktonic phase, in combination further referred to as pathogen reduction in the liquid stream.

The goal of this study was to systematically study the effects of slurry dewatering by centrifugation with and without polymer addition, to better understand the contribution these practices have on reducing the pathogen content of recycled farm water. In this study, the relationship between centrifuge speed, addition of PAM, and the microbial quality of the liquid stream after separation was performed using a factorial experimental design. Indicator microorganisms including total coliforms and E. coli were monitored as an index of the bacterial pathogen population in the samples. In addition, the centrifugation speed for optimal solids and pathogen reduction in the liquid stream was evaluated taking into account economic considerations.

2. Materials and methods

2.1. Farm scale study samples

The manure samples from a working centrifuge on a farm manure processing system were collected at a dairy farm in Rock County, Wisconsin where an industrial manure centrifuge separator was used. Centrifuge feed of raw manure was at a rate of 130 gallons per minute; the operation bowl speed was set at 2787 rpm. The after centrifuge liquid stream (centrate) was then sent to sand separators at a rate of 90 gallons per minute. This farm recycles the separation liquid from the centrifuge for sand bedding washing and barn flushing.

Manure samples before and after large-scale centrifugation were taken every 2 h over a 24-h period to eliminate the influence of daily farm operations and understand the short term variability. These samples were frozen in sterile containers immediately and then sent to the Marshfield Agriculture Research Station, University of Wisconsin-Madison, for total solids and total dissolved solids contents analysis according to standard manure testing protocols described by Peters et al. (2003). Additionally, samples after sand separation and washing were also collected, kept in 125 mL containers at 40 C and tested for bacteria indicator levels within 24 h. This farm discontinued using centrifuge separation a few weeks after samples were taken. To evaluate large-scale centrifuge effects on bacteria population, dairy manure samples before and after centrifuge separation were taken from another dairy farm in Dane County, Wisconsin, and enumerated for bacteria indicator cell numbers. These two large-scale centrifuges evaluated in this study were the same model and running under similar conditions, and the manure samples were both undigested raw manure. The sampling and testing procedures followed the protocol described by Liu et al. (2016b).

2.2. Bench scale study samples

The manure samples for bench scale experiments were collected at a large dairy farm in Manitowoc County, Wisconsin. The polymer reagents (cationic polyacrylamide) were supplied by Soil Net LLC, Belleville, WI. Manure samples were collected at the dairy farm from a mesophilic anaerobic digester in sterile containers and delivered in coolers. The samples were stored at 4 0C until analysis, which was performed within 24 h.

2.3. Isolation of the bacteria strain

An E. coli strain was isolated from manure samples and used as a control organism for the pathogen reduction centrifuge experiments. The manure samples were collected at a dairy farm from the anaerobic digester in sterile containers. These samples were stored at 4 0C, and the E. coli isolation was initiated within 24 h following the procedure described by Liu et al. (2016a).

2.4. Polymer characterization and screening

Molecular weight (MW) and charge density (CD) are two key features that determine the effectiveness of the polymers used in wastewater and dairy manure treatment industry. To understand the effect of MW and CD of the polymers on dairy manure solid separation, different polymers with varying CD and MW were compared. The cationic polyacrylamide (CPAM) tested were 1000-, 1100-, and 1400- series which varied in molecular weight from relatively low (AL), medium (SAL), and high (VAL), and in charge density levels as shown in Table 1.

Table 1

Cationic Polyacrylamide (CPAM) properties (A. Roa-Espinosa, personal communication, 2014).

Polymer name Charge density level (relatively) Molecular weight (g/mol) Mol. Wt. level (relatively)

1000AL Low 4.5 x 106 Low

1000SAL Low 6.0 x 106 Medium

1100AL Medium 5.0 x 106 Low

1100SAL Medium 6.0 x 106 Medium

1100VAL Medium 9.0 x 106 High

1400AL High 4.0 x 106 Low

1400SAL High 5.5 x 106 Medium

1400VAL High 8.5 x 106 High

2.5. Manure separation using polymers-jar test

To compare the effectiveness of polymers for manure solid separation, dairy manure samples collected after 21-day digestion from a plug-flow mesophilic anaerobic digester were tested using the polymers listed in Table 1. The separation efficiency parameter used in this study was separated solid settling time using a jar-test method. The solid settling time/velocity test used identical 400 mL beakers. The manure samples were mixed using stir plates for 1 min at 300 rpm. After mixing ceased, the settling time was measured using a stopwatch. The settling distance was measured at fixed time intervals using a calibrated graduated scale (minimum 0.5 mm) marked on each beaker. The effects of polymer addition on clarification were assessed by calculating the clarified fraction. The manure clarified fraction was defined as the clarified volume to total volume as a percent:

Clarified fraction = (clarified volume/total volume) x 100%

2.6. Centrifuge experiments

The manure samples were shaken and mixed well and then evenly divided into two portions in sterile beakers. Fifty parts per million (final concentration) of CPAM 1000SAL was added to one of the beakers, which was mixed using a sterile stir bar at 300 rpm for 3 min. Six aliquots (for centrifugations at six different speeds) of 45 mL of each manure sample (with and without CPAM added) were made in 50 mL sterile Corning Centrifuge Tubes with Cen-triStar Caps (Corning, NY). A duplicate of each sample was also made. These tubes with and without CPAM added were then centrifuged at 0 x g, 2000 x g, 4000 x g, 6000 x g, 8000 x g, and 10,000 x g for 10 min each using an Allegra 25R Centrifuge (Beckman Coulter, Brea, CA). The highest centrifuge speed tested in this study was 10,000 x g because that is the maximum speed of a typical industrial-scale centrifugal separator that would be used in this application (Records and Sutherland, 2001). The supernatant of each tube was sampled immediately for E. coli, total coliform, total solids, and chemical oxygen demand (COD) determination following the methods described below. The tubes of 0 x g centrifugation speed were refrigerated at 4 0C for 30 min before analysis while setting up the experiments. This was conducted to study the polymer effects on manure treatment without centrifu-gation. A control of overnight E. coli culture inoculated into duplicate 45 mL tubes of buffer media was also conducted at each centrifuge speed.

2.7. E. coli enumeration

E. coli and total coliform densities were determined using the Colilert Quanti-Tray 2000 (IDEXX Laboratories, Inc. Westbrook, ME) enumeration method descried by Liu et al. (2016a).

2.8. Manure solids content determination

The total solids contents of manure samples from the bench scale experiments were measured gravimetrically following with Standard Methods 2540 (APHA, 2005). About 30 mL of well-mixed sample aliquot was pipetted into an aluminum tray and weighed before and after drying at 105 0 C.

2.9. Chemical oxygen demand (COD) determination

HACH Digestion COD vials (high range) (Loveland, CO) were used for COD determination of each liquid sample before and after centrifuge. Each sample was diluted to 20 times; 2 mL of each sample was added to the COD vial. The digestion was programmed to run 2 h at 150 0C including quantity calibration verification (QCV) controls. The COD result of each sample was obtained using a HACH DR3900 spectrophotometer.

2.10. Statistical analyses

The statistical analysis in this study was conducted using the R program version 0.98.1091 (Rstudio, Boston, MA). A two-way ANOVA (analysis of variance) was fitted with two category factors (centrifuge speed and whether polymer was applied) to compare the mean total solids content in each treated manure sample. A pair-wise comparison was also conducted to understand the difference between each factor levels (six speeds, and whether polymer was added or not) using TukeyHSD (Tukey Honest Significant Differences) with 95% family wise confident interval. The same method was also used to analyze COD, total coliforms, and E. coli data.

3. Results and discussion

3.1. Farm scale centrifuge system evaluation

As seen in Fig. 1, total solids content before centrifuge separation of the first sampling event were much higher compared to the rest of the samples. The centrifuge tended to have higher total solids removal rate when the input stream was higher in solids content and had a total solids removal efficiency average of 27%. The total solids removal efficiency by mass-balance analysis was 30% on average. Overall, the total solids removal efficiency of the centrifuge operating on the study farm was relatively low compared to typical centrifuge separation systems (M0ller et al., 2000). Possible reasons of the lower separation efficiency might be the high dissolved solids concentration (73.9%) in the centrifuge feed due to liquid recirculation within the farm, relatively low centrifuge speed, and/ or short retention time. In addition, high bacterial indicator concentrations were found in the separated sand after using diluted centrate for sand washing (higher than 5000 Most Probable Number of both total coliforms and E. coli per gram of sand, with about 10% moisture, respectively).

PM PM PM PM PM AM AM AM AM AM AM PM Fig. 1. Solids content in dairy manure before and after centrifuge separation.

The results of high bacterial indicator concentrations in flushing water and bedding sand at the dairy farm in Rock County suggested that the large-scale centrifuge separation on this farm had negligible effects on bacteria reduction, which may pose a risk of disease outbreaks in the herd when the separated liquid is recycled. The follow-up experiments from the dairy farm in Dane County showed less than 10% reduction of bacteria indicators after large-scale centrifuge separation compared to the samples before centrifuge separation (See data in Supplementary Material). The findings in this study agreed with Hjorth et al. (2010) and Liu et al. (2016a) which demonstrated that most manure separation technologies based on mechanical operations have low pathogen reduction efficiency.

3.2. Lab scale experiments

3.2.1. Manure solids settling efficiency using selected polymers

Dairy manure with high solids (higher than 4%) content can take several weeks to months to separate into high-solid and low-solid phases in settling tanks without any flocculent/coagulant addition or without accelerated separation such as centrifugation. Cationic polyacrylamide (CPAM) was used in this study to enhance dairy manure liquid/solid separation. To select a suitable polymer to flocculate the manure, screening tests were conducted with a variety of polymers (Fig. 2). The polymer screening results suggest that several CPAM polymers were able to improve the manure settling characteristics. One polymer (SAL 1000) was selected for the rest of the polymer separation experiments according to the simple screening results. As shown in Fig. 2, no separation was

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Fig. 2. Manure settling experiments: Clarification fraction with different cationic polyacrylamide (CPAM) added.

observed over the 1 h settling experiment without CPAM addition. Separation efficiencies as high as 28% were achieved within 1 h when polymers were added.

Comparing the clarification fractions, 1000SAL with medium molecular weight and low charge density yielded the best result which agrees with other studies investigating wastewater treatment (Wong et al., 2006). The relatively low molecular weight 1000AL and relatively high charge density 1100AL/1100SAL were also very effective at facilitating manure solids settling. High molecular weight CPAM may be operating through the bridging floc-culation mechanism in addition to charge neutralization. Flocculated materials which are formed via bridging flocculation stay apart when broken up, since polymer tails and loops bridging across two or more particles are physically severed by the shearing forces (Yan et al., 2004). Because suspended solids particles in dairy manure are typically negatively charged, CPAM can act as coagulants that neutralize or reduce the negative charge on the particles, similar to the effect of alum or ferric chloride (Hjorth et al., 2010). The charge neutralization drastically reduces the repulsive force between colloidal particles, which allows the Van der Waals forces to aggregate colloidal and fine suspended materials (Amirtharajah and O'melia, 1990). However, as shown in Fig. 2, two of the high charge polymers in the VAL series were observed to yield the lowest clarification fraction among the tested polymers. It is possible that when the total charge density of the applied polymer is in excess of the necessary treatment dose, a charge reversal can occur and the particles will again become dispersed (Dempsey et al., 1984). In addition, the VAL series CPAM are more viscous than SAL series CPAM. The addition of high viscosity VAL CPAM would increase the viscosity of the slurry, which can lower separation efficiency (A. Roa-Espinosa, personal communication, 2014).

3.2.2. Centrifugal speed effects on solids content, COD, and bacterial density

Bench-scale centrifugation study was performed to better understand the effects of speeds and chemical additives on manure solids and bacteria reduction. Because of the low solids separation and bacterial indicator level reduction efficiencies of large-scale centrifuge, higher speed centrifugation and longer retention times were applied in lab scale experiments. The effect of polymer addition on solids content, COD, and bacteria was investigated under different centrifuge speeds in this study. From the results shown in Fig. 3, 30 min of settling without centrifugation reduced more than 20% and 40% of total coliform and E. coli densities in liquid phase when PAM was added, respectively. A minor decrease in solids content (8%) and a negligible effect on COD (2%) reduction were observed when PAM was added with no centrifugation (Fig. 4).

Total solids content, COD, total coliform, and E. coli densities in the liquid phase were all reduced after centrifugation. None of these parameters showed significant differences at all speeds between the samples with and without polymer added. The results demonstrated that the PAM used in this study had a negligible effect on manure separation when centrifuged at 2000 x g or higher speed for solids content reduction. However, other research results suggested that PAM addition can enhance solid/liquid separation in industrial-scale centrifuges (Sneath et al., 1988). One possible reason is that for the consideration of economic performance, large-scale centrifuges usually have a short retention time because of the high volumetric feed rate (Sommer et al., 2013). Ten minutes centrifugation at 2000 x g might be too long to observe the polymer effects. Additionally, these results may be different in industrial-scale centrifugation on large farms where particles travel longer distances compared to the 50 mL centrifuge tubes. Centri-fugation at 2000 x g reduced about 50% more solids content

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2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000

Centrifuge speed (xg) Centrifuge speed (xg)

Fig. 3. Total coliform and E. coli in manure with increasing centrifuge speed.

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□ With РАМ ♦ Without РАМ

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Centrifuge speed (xg) Centrifuge speed (xg)

Fig. 4. Solids content and COD in manure with increasing centrifuge speed.

compared to pre-separation samples in both manure only and manure with PAM added. The centrifuge speeds higher than 2000 x g tested in this study yielded less than 10% reduction in solids content compared to the 2000 x g speed conditions. The centrifuge speeds of 8000 x g and 10,000 x g had no additional effect on solids reduction relative to the 6000 x g conditions. Total coliform and E. coli reduction are expected to occur alongside the organic material reduction during the process as a result of an increase in microbiological competition for substrate (Viancelli et al., 2013).

COD reduction in wastewater is typically achieved through biological treatments or physio-chemical treatment (Satyawali and Balakrishnan, 2008). The results in this study demonstrated that high speed centrifugation can slightly reduce liquid phase COD; but the reduction efficiency was not as great as conventional treatments such as fungal treatment, bacterial treatment, activated carbon adsorption, and ozone oxidation. Also, no difference was observed in COD reduction by adding PAM as shown in Fig. 4. These results suggest that most of the organic matter in manure is present

as small particles or in a dissolved form. Enhancement of floc formation through the addition of PAM did not aid in liquid phase COD reduction. In addition, the results from ANOVA analysis (Table 3 in supplementary material) were consistent with the results shown in Fig. 3 and 4 which demonstrate that high speed centrifugation was effective (significant, P < 0.05) for solids and COD reduction in animal manure. In contrast, the effect of polymer addition was not significant (P > 0.05).

3.2.3. Bacterial reduction in manure samples compared to buffer solutions

Centrifugation is a commonly used laboratory practice for harvesting suspended bacteria such as E. coli in suspended cultures. However, few studies have focused on the remaining bacteria levels in the supernatant after high speed centrifugation. The results in Fig. 5 demonstrate that high speed centrifugation has a positive effect on pathogen reduction. Two logio (99%) reduction was achieved for both total coliforms and E. coli when centrifuged at 10,000 x g. ANOVA analysis (Table 3 in supplementary material)

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Centrifuge Speed (xG) Centrifuge Speed (xG)

Fig. 5. Log10 reduction of total coliform and E. coli with increasing centrifuge speed.

revealed similar trends for solids and COD reduction, namely that centrifugation speed had a significant effect (P < 0.05) for bacteria indicators reduction but polymer addition did not. Nearly linear removal curves can be derived from the log10 reduction data, indicating that higher centrifugation speed may achieve greater liquid phase pathogen reduction results, although it might not be feasible in industrial-scale practice. In contrast, the cultured E. coli enumerated in buffer media solutions yielded only about one log10 reduction and the pathogen indicator reduction reached a plateau when the centrifuge speed reached 6000 x g and higher.

The results from this study revealed that a significant amount of bacteria remain in the supernatant of the buffer only control treatment after high speed centrifugation. Therefore, it can be hypothesized that an interaction of manure solids prior to exposure to centrifugal forces could act to remove pathogens from the liquid phase. In the centrifugation separations, the driving force is a result of the difference in density between the solid particles and the suspending liquid. Most separable particles in manure samples are larger and heavier than E. coli cells, and thus easier to separate. E. coli cells then can be more efficiently separated when these larger particles are present. Experimental results indicate that the particles formed in the presence of PAM had little effect on sequestering E. coli cells into the solid fraction of the system. Simply the presence of manure solids was enough to affect E. coli removal. These E. coli cells are possibly entrapped within the centrifuge pellets, therefore the pathogen densities would be reduced in the liquid stream. Additionally, bacterial cells may suffer shear damage from shear forces and collisions with suspended particles when centrifuged at high speed. Depending on the species, the cell damage rate could be up to 12% when centrifuged at 13,000 x g in culture media as a result of collisions and shear forces (Heasman et al., 2000). Thus, it is possible that when heavier particles such as fibers and insoluble ash are present in the system, a higher cell damage rate would be expected.

3.3. Economic considerations

In 2000, the US EPA estimated the typical polymer cost to be $2.65 to $91.15 per million gallons processed with centrifugal dewatering (USEPA, 2000). Also, energy cost is one of the major concerns when using high speed centrifugation at industrial scales. It is estimated that the electricity costs is about 5%—50% of the total

centrifuge cost, depending on the volume of manure treated (M0ller et al., 2000). Therefore, optimizing the centrifugal speed for the best separation results and minimum energy cost is necessary. According to Records and Sutherland (2001), the total power input required by a decanter centrifuge follows Eq. (3):

Pt = Pp + Pf + Ps + Pb (3)

Where PT is the total power required by the decanter centrifuge and PP is the power required to accelerate the input material to the bowl speed at the discharge radius; PF is the power required to overcome the friction; PS is the power consumed by conveying the energy and PB is the power for braking. To simplify the scenario for the large-scale centrifugation, in this study we only considered the energy cost of the power required to accelerate the input material to the bowl speed at the discharge radius, although the idling power can be a major component of the energy cost at lower speed.

Pp= Qf Pf u2r2 (4)

In Eq. (4), the power required to accelerate the input material to the bowl speed is related to the angular velocity u and the decanter discharge radius r. Qf is the material feed rate and pf is the material density. In this study, the centrifuge speed was measured by G-force, where:

G = u2r/g (5)

So the power required to accelerate the input material to the bowl speed is:

Pp= Qf PfgrG (6)

For large-scale centrifuges on farms, the power required to accelerate the input material to the bowl speed PP is in a linear relationship with the G-force applied. It is estimated that a fully grown 1400 lbs lactating cow could produce approximately 18.7 gallons of manure daily (Lorimor et al., 2004). Therefore, for a typical large size farm in Wisconsin of 4000 fully grown 1400 lbs lactating cows will produce about 27.2 million gallons of manure annually. A large-scale centrifuge processing 1 gallon liquid manure at 2000 x g, the energy consumption is about 0.075 kWh. Assuming $0.10 per kWh is the average electricity rate; the cost of separating dairy manure generated on a large farm annually based on different

Centrifuge Speed (xG)

Fig. 6. Reduction rate of total solids content, total coliform, E. coli, and energy cost with increasing centrifuge speed. 'Annual energy cost to treat manure (by total volume generated on farm).

centrifuge speed could be very high. In Fig. 6, all the reduction curves have been normalized as a ratio compared to the maximum reduction in order to compare to each other. From this figure, pathogen indicator densities and solids contents are reduced significantly at 2000 x g and 4000 x g. In contrast, high speeds after 6000 x g gain negligible additional reduction while the energy input for each additional 2000 x g is about $204,000. Therefore, higher centrifuge speed and/or longer retention time may be considered during manure processing.

4. Conclusions

Large-scale centrifuge manure separation on a Wisconsin local dairy farm yielded low solids and bacterial indicators reduction efficiencies in the liquid fraction. Therefore, effects of centrifuge speed with and without polymer addition were investigated in the second phase (bench scale) of this study using a systematic factorial experimental design. The impacts of both high speed centrifugation and polymer addition on lowering the bacterial indicator levels in the liquid stream of the manure were measured. CPAM significantly increased manure liquid/solid clarification fraction without centri-fugation but had negligible effects on solids reduction and indicator bacteria reduction when centrifuged. High speed centrifugation has a notable impact on solids reduction and indicator reduction at speeds of up to 6000 x g. However, higher speeds yielded minor additional reduction. Future studies using a higher centrifuge speed in large-scale centrifugation for pathogen reduction should be investigated. The results from this study suggested higher speed centrifugation and longer retention time could be considered during manure solid/liquid separation if improving pathogen reduction is a concern on the farm. Further investigation using mass balance approaches (i.e., quantifying system loads of E. coli and tracking their fate in both separated liquid and solid phases) in conjunction with polymers known to have biocidal activity could lead to improved and more effective manure processing and recycling approaches.

Acknowledgements

This work was supported by the United States Department of Agriculture-National Institute of Food and Agriculture (USDA BRDI Grant number 2012-10006-19423).

Appendix A. Supplementary data

References

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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2017.01.022.