Case Study 10 - Centrifuge

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Solid Separation Systems for the Pig Industry Case Study 10 Centrifuge

Case Study 10

CENTRIFUGE DECANTER

Contents

CASE STUDY 10 CENTRIFUGE DECANTER .......................................... 10-1

10.1

Description of the System....................................................................... 10-3

10.2 Manufacturers / Distributors.................................................................. 10-5

10.3 Information Sources................................................................................. 10-6

10.4 Performance Data ..................................................................................... 10-610.4.1 Industry data on the dual gear centrifuge.......................................... 10-610.4.2 Payne (1990) - On-farm piggery trial Western Australia ................. 10-710.4.3 Abery (1994) - Piggery wastewater separation, Corowa, NSW ......10-810.4.4 Piccinini and Cortellini (1987) - Separation of animal wastewaters. 10-1010.4.5 Moller et al. (2000) - Separation of animal wastewaters................. 10-13

10.4.6

Sneath et al. (1988), Sneath (1988a&b) - Piggery wastewater......... 10-14

10.4.7 Miner et al. (1983) - Separating anaerobic lagoon sludge............... 10-15

10.5 Running Costs and Maintenance ........................................................10-16

10.6 Practical Operating Issues..................................................................... 10-17

10.7 Piggery Case Studies.............................................................................. 10-17

10.8 Summary Selection Criteria...............................................................10-1810.8.1 Solids removed.....................................................................................10-1810.8.2 Capital cost ........................................................................................... 10-1910.8.3 Operating costs and returns ............................................................... 10-19

10.8.4

Ease of operation..................................................................................10-1910.8.5 Solids management options ...............................................................10-20

10.9 References ................................................................................................ 10-20

List of Figures

Figure 10-1 Schematic diagram of centrifuge decanter............................................. 10-3Figure 10-2 - Relationship between influent TS concentration and removal efficiency -

Piccinini and Cortellini (1987)................................................................................ 10-11

List of Photographs

Photograph 10-1 Centrifuge Decanter (side view)..................................................... 10-3

April 2002 FSA Environmental Page No.10-1


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Photograph 10-2 Centrifuge Decanter (end view) .....................................................10-4

List of Tables

Table 10-1 - Industry data provided by Westfalia Separators..................................... 10-7

Table 10-2 - On-Farm piggery trial - Payne (1990): data for liquor ............................ 10-8Table 10-3 - On-farm piggery trial Payne (1990): data for solids ................................ 10-8

Table 10-4 - Piggery wastewater separation trial - Abery (1994)................................ 10-9Table 10-5 Solid removal efficiency of piggery wastewater................................... 10-11Table 10-6 - Solid removal efficiency of anaerobically digested pig wastewater... 10-12Table 10-7 - Separated solids chemistry for raw piggery wastewater and

anaerobically treated piggery wastewater ...........................................................10-12Table 10-8 - Livestock wastewater trial - Moller et al. (2000)..................................... 10-13Table 10-9 Capital and operating costs of Centrifuge case study.......................... 10-18



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FIGURE 10-1 SCHEMATIC DIAGRAM OF CENTRIFUGE DECANTER

A is wastewater inlet, B the bowl, C the screw, D the conical section, E the beach, F the solids outlet, Gthe level regulating discs, r1, r2 and r3 are the inside radius, the radius of settled solids, and the liquidradius respectively (Sneath et al., 1988)

PHOTOGRAPH 10-1 CENTRIFUGE DECANTER (SIDE VIEW)

10.1 Description of the System

Solid bowl centrifuges use the force developed under fast rotation to separate theliquid from the solid fraction. Decanter or scroll centrifuges differ from the earlierbasket centrifuges in adding a helical screw conveyor, capable of continuouslydischarging the separated solids from the bowl (Albertson et al., 1991). The conveyorrotates at a slightly higher or lower speed than the bowl, conveying the solids from

the stationary zone where the wastewater enters, to the dewatering beach where thesolids are discharged. The scroll pushes the collected solids along the bowl wall and



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up the dewatering beach, located at the tapered end of the bowl. The liquid flowsaround and through the conveyor, over an adjustable weir towards the liquiddischarge end. Decanter centrifuge models can rotate in counter-current, orcontinuous concurrent mode. Those with a concurrent design typically operate atlower speeds, depending on the machine size and separated solids properties.

PHOTOGRAPH 10-2 CENTRIFUGE DECANTER (END VIEW)

The solids content of the separated solids are determined by the length of thedewatering beach, and the differential between the speed of the bowl and conveyor(Albertson et al., 1991). By controlling the differential speed, optimum solidsresidence time in the centrifuge and the desired water content of the separated solidscan be obtained. Newer models control the speed of the bowl as a function of theconveyor torque, with eddy current brakes also used. The best performance isachieved when the flow rate and solids concentration of the influent arestandardised.

A recent dual-gear drive development provides for the automatic adjustment of therotation of the scroll relative to the bowl, to account for minor fluctuations in theflow rate and solids concentration of the wastewater (Westfalia Australia). Thispatented 2-gear drive system reduces the degree of supervision required, andreduces the likelihood of equipment failure due to torque overloading (machinechoking). Increased throughput is also possible since the automatic torque-relatedscroll speed adjustment can allow for the feed rate to be increased without thedanger of plugging.

Sludges with a high proportion of fine and hydrous particles are more difficult to

separate. Sludges prone to flowing will resist being conveyed up the slope (beach) tothe solids discharge point. Hence most sludges are preconditioned prior to



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centrifugation using gravity-thickening and the addition of organic polyelectrolytesto produce heavier particles for capture.

The key features of the Westfalia Australia centrifuge decanter are listed below. Themanufacturer presets many of the components although the operator can control

some:

Bowl diameter

Bowl length

Bowl rotational speed

Beach angle

Beach length

Pool depth

Scroll rotational speed

Scroll pitch

Feed point of the sludge

Feed point of the chemicals Condition of the scroll blades

10.2 Manufacturers / Distributors

Dual-gear system Westfalia Separators42-47 Northgate DriveThomastown Victoria, 3074

Phone: 03 9463 1999Facsimile: 03 9464 5455Email: [email protected] site: http://www.westfalia-separator.com

Alfa Laval Australia Pty. Ltd.Locked Bag 40Blacktown Business CentreBlacktown, NSW, 2148

Phone: 02 8822 2700Facsimile: 02 8822 2799Email: [email protected] site: http://www.alfalaval.com

TEMA Engineers Pty. Ltd.PO Box 4335Milperra DC NSW, 1891

Phone: 02 9792 3555

Facsimile: 02 9792 3134Email: [email protected]

http://www.westfalia-separator.com/http://www.alfalaval.com/http://www.alfalaval.com/http://www.westfalia-separator.com/


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10.3 Information Sources

The information presented in this case study is derived from the following sources:

Manufacturers product information (including performance test data).

Site inspection of units at Weston Bioproducts Moorooka (Feb 2001).

Payne (1990) - On-farm performance of piggery wastewater treatmentsystems in Western Australia.

Abery (1994) Trial of various separator devices at Bunge Meat Industries,Corowa, Australia.

Piccinini and Cortellini (1987) - Livestock wastewater trial, Italy.

Moller et al. (2000) - Livestock wastewater trial, Denmark.

Sneath et al. (1988) - Piggery wastewater trial, Britain.

Miner et al. (1983) - Anaerobic lagoon wastewater dewatering, Singapore.

10.4 Performance Data

10.4.1 Industry data on the dual gear centrifuge

Solids recovery data provided by Westfalia Australia for digested municipal sludge(Table 10-1) indicates that performance increases as the feed concentration increases.The polymer dose remained relatively constant at around 4.34 g/kg. Recovery of the

solid fraction was highest (98.3%) at the highest TS concentration of the feed (4.6%).However, to accommodate the higher TS concentration of the feed the flow rate andtherefore the capacity of the centrifuge was reduced from 35 m3/hr to 30.2 m3/hr.The dual gear function patented by Westfalia automatically adjusts the flow rate tomaintain the TS concentration of the separated solids relative to the feed TSconcentration. A high TS concentration of separated solids from municipal sewagetreatment plants is desirable, as landfill is the common disposal option with chargesbased on weight.

Polymer dosing was not used for the separation of solids from piggery and cattlewastewaters. Raw primary wastewaters are the most readily thickened, with

activated sludge waste considered the most difficult (Albertson et al., 1991). Therelatively high TS concentration of the feed indicates that the piggery wastewaterwould have been gravity-thickened prior to solids separation (Table 10-1). No solidsrecovery calculation was provided, but the TS content of the separated solids wasvery high (30-35%), rendering it readily stackable. The wetter solids produced fromthe cattle wastewater may indicate the poorer thickening properties of ruminantwastewaters, but the solids content of the separated solids was still sufficiently highto render it stackable (25-30%). In the absence of solids recovery data for the piggerywastewater, further interpretation of the results is difficult.



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TABLE 10-1- INDUSTRY DATA PROVIDED BYWESTFALIA SEPARATORS

Capacitym3/h

Feed% TS

Polymerdose g/kg

TS %Separated

solids

Solidsrecovery %

Digestedmunicipalsewagesludge

35.035.140.030.330.330.2

2.62.72.73.03.04.6

4.54.43.64.34.34.5

36.734.937.938.538.537.8

96.797.396.697.597.598.3

PiggerywastewaterCattle

manure

L/hr/m23-4

3-4

3-6

6-9

Not used

Not used

30-35

23-30

n.a.

n.a.

10.4.2 Payne (1990) - On-farm piggery trial Western Australia

The aim of this trial was to compare the performance of a number of mechanicalsolid separation devices installed in commercial Western Australian piggeries.Performance characteristics of the different systems could then be compared toindicate their effectiveness in reducing the pollutant loading of piggery waste.Separator systems evaluated included stationary and centrifugal screens, and one

decanter type centrifuge. The decanter used (manufactured by Bird) was not dualgeared. Adjustment of the inflow rate to match the influent TS concentration withthe required TS concentration of the separated solids was under operator control.The decanter centrifuge achieved the highest solids removal percentage, andproduced the driest solids fraction.

However, the TS removal of 37% (Table 10-2) is substantially lower than thatachieved for the municipal digested sludge of 97%(Table 10-1). The separated solidsfraction had a similar TS content, averaging 35.4% (Table 10-3). The low TSconcentration of the wastewater feed would in part account for the poorer solidsrecovery, as would the selection of a higher separated solids TS concentration.

Gravity-thickening prior to solids separation and adjusting the flow rate to produce aslightly wetter solids would improve the TS removal efficiency. The TS content ofthe separated solids could be reduced to 25% without unduly altering the handlingcharacteristics. At this TS content, the material would still be stackable.

The low recovery values for nitrogen (TKN) and ammonia are as expected, given thata high proportion of the nitrogen is excreted in the urine (soluble). The relativelyhigh total phosphorus (TP) recovery reflects the efficiency of the decanter inremoving the very fine organic particles, which contain the phytate P fraction notreadily metabolised by pigs (Giusquiani et al. 1998). The recovery of the volatilesolids (VS) fraction (42% based on wet weight) indicates that a substantial reduction

in the odour generation potential of the liquid fraction has been achieved. However,



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on a volumetric basis, the reduction as indicated by the Biochemical Oxygen Demand(BOD) is much less, at only 25% (Table 10-2).

TABLE 10-2- ON-FARM PIGGERY TRIAL - PAYNE (1990): DATA FOR LIQUOR

Fraction type andunits of measure

TS SS VS TKN NH4+ TP BOD

Concentration(% wet wt. or mg/L)Incoming InfluentOutgoing effluent

% wetwt

1.71.1

% wetwt

1.20.7

% wetwt

1.40.8

mg/L

1,8011,590

mg/L

850782

mg/L

335238

mg/L

76955867

Mass (kg/hra)Incoming influentOutgoing effluentbSolids

25311093

18310380

20211983

27.023.63.4

12.811.61.2

50.335.41.5

115.487.228.2

% Removed fromliquid influentb 37 44 42 13 9 30 25

a an estimated average throughput of 15 m3/hrb based on solids stream mass at 1.48% of incoming wastewater streams

TABLE 10-3- ON-FARM PIGGERY TRIAL PAYNE (1990): DATA FOR SOLIDS

TS% wet

wt

SS% wet

wt

VS% wet

wt

TKNmg/L

NH4+

mg/LTP

Mg/LBODmg/L

Concentrationwet wt basis% dry wt basis

35.4(100)

31.789

29.483

4,7321.34

8480.24

2,9650.84

17,9015.05

10.4.3 Abery (1994) - Piggery wastewater separation, Corowa, NSW

The aim of this study was to compare the efficacy of vibrating screens, sedimentation(gravity thickening), centrifugation and dissolved air flotation in series and/or inparallel in reducing the solids loading on anaerobic ponds. The trials wereconducted at a commercial piggery, but no details of the age-class or diet of the pigswas given. Piggery wastewater was pumped into a sedimentation tank (capacity120,000 litres) prior to testing the separation equipment. The sedimentation tank was

square in cross-section, 6 m wide and 3 m deep, with one corner sloping to 4 m tofacilitate the flow of solids to the pump. A steel mesh cage was used to remove largeobjects prior to solids separation. The centrifuge was a Sharples P2000 Decanter,with four options on the adjustment of the plate dam (weir) for selection of solidswater content. For this trial the second driest solids setting was selected. Operatorcontrol was required to match the influent TS content with the flow rate to achievethe required TS content of the separated solids.

A polyacrylamide cationic flocculant (10% MW 15-20 million powder costing $6 perkg) was tested prior to centrifugation. SNF Australia Pty. Ltd. conducted on-sitetests for the selection of the polymer. The flocculant was added at the sedimentationstage prior to centrifugation. The residence time of the wastewater in thesedimentation tank was 2 hours. A MEX-P portable sludge level detector was used



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to locate the interface between the settled particles and the clarified liquid prior togravity thickening. No details have been provided on the agitation system used toresuspend the solids, or on the pump used to feed the solid separators.

Assuming that the raw wastewater had a TS concentration of 0.8% (quoted in the

vibrating screens trial with no pre-treatment), the sedimentation step alone thickenedthe wastewater to 2.05 and 3.10% (Table 10-4). Results for gravity thickening withincreasing rates of polymer dosing are inconsistent. The lowest polymerconcentration of 7.2 mg/L achieved a TS concentration of 5.4%, whereas the secondhighest rate of 19.4 mg/L achieved a concentration of 1.7%.

Flocculation involves the formation of a series of molecular bridges between particles(Rushton et al., 2000). The polymer chain is adsorbed onto one particle, and whenanother comes into close proximity the extended polymer chain is adsorbed onto it.The flocculation reaction is irreversible, and high rates of shear force easily break themolecular bridges. Polymer concentration is critical, as is low shear rate mixing to

increase the collision rate of particles. Overdosing can lead to the formation of anadsorbed polymer layer, stabilising the suspension and making it difficult toseparate. The results in Table 10-4 for polymer concentrations of 8.5 mg/L andabove suggest that the combination of insufficient mixing during the sedimentationstep, excessive shear force applied to resuspend the solids in the wastewater and/oroverdosing, has interfered with the formation of large flocs.

TABLE 10-4- PIGGERY WASTEWATER SEPARATION TRIAL - ABERY (1994)

Flocculant Sedimentation tank CentrifugeConc

mg/L

Cost

$/ML

Solids%

Over

Flow

red.%

Solids

red.%

Solids%

out

Solid

% TS

Liquid

% TS

Flow

red.%

Solids

red.%

007.28.513.614.619.420.9

0.000.0043.450.881.387.5116.3125.7

0.530.300.470.200.280.390.280.34

15.219.417.614.415.716.315.115.5

42.153.264.747.857.164.144.261.8

3.102.055.371.472.674.651.723.89

38.726.730.732.831.231.031.533.6

1.971.302.070.691.201.900.851.25

3.33.011.52.34.99.52.88.2

39.736.661.552.255.059.250.867.8

However, the data does show that gravity thickening alone (settling), to produce aTS concentration of above 2 %, can achieve a solids reduction of 40% (Table 10-4).Increasing the flow rate into the decanter at this TS concentration would improve therecovery percentage, producing wetter solids that would still be stackable. The TScontent of the separated solids was 39%, the driest achieved during the trial.Increasing the concentration of polymer improved the solids reduction percentage,despite the less than optimal management of the flocculation process. Improving themanagement of the flocculation step and matching the flow rate into the decanterwith the TS concentration of the feed would improve the performance of the system.Overall solids reductions were approximately 20% without the polymer and

increased to about 40% with the use of a polymer. Until the management of the



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flocculation step is improved, conclusions on the cost-effectiveness of polymeraddition should not be drawn.

10.4.4 Piccinini and Cortellini (1987) - Separation of animal wastewaters

The aim of this study was twofold:

To compare the performance of a range of mechanical solid separationsystems commonly used in Italian rural industries, and

To investigate how the TS concentration and wastewater type affects theperformance of each device

The devices compared included stationary, rotating and vibrating screens, andhorizontal type centrifuges. Data from the horizontal centrifuges has been included

here, to indicate the effect of gravity thickening and polymer addition onperformance. The wastewaters tested were raw piggery, raw cattle andanaerobically digested piggery wastewater. Data for the raw and anaerobicallydigested piggery wastewaters are included in Table 10-5 and Table 10-6 respectively.

Two mobile horizontal centrifuge units were tested, Alfa Laval model AVNX 414 Brated at 31 G and Pieralisi model FP 600-RS. The volume of influent and effluent wasmeasured using metal tanks with a capacity of 2.2 m3. The separated solids werecollected and weighed. Cationic polyelectrolyte was added to the feed for eachcentrifuge in accordance with the manufacturers recommendations, decreasingconcentrations as the TS concentration of the wastewaters increased. For the raw

piggery wastewater, TS concentrations were 1.0, 2.3, 4.7 and 7.3 %. The respectivepolymer concentrations were 35.1, 4.4, 3.9 and 1.1 kg/t of TS. For the digestedpiggery sludge TS concentrations were 1.4, 4.5, 5.6, and 6.2 %. The respectivepolymer concentrations were 8.5, 2.4, 4.5 and 1.6 kg/t of TS.

The data presented in each of the tables lists the maximum and minimum values,and the arithmetic mean. Averaging the results does not adequately describe therelationship between the influent and centrifuge performance. However, referenceto the maximum and minimum values in the table and to the figure depicting TSremoval percentage against feed TS concentration provides more meaningfulinsights (Figure 10-2).

The figure indicates that the relationship between the TS concentration (g/kg) of thefeed with and without polymer and the removal efficiency of the centrifuges is notlinear. Without polymer a TS concentration of 4.5% (45 g/kg) produces the bestsolids recovery of 60.5% (Table 10-5). The addition of polymer substantiallyenhances the recovery of the feed at the lower TS concentrations, but gravitythickening alone to 4.5% produces a better solids recovery than the combination ofgravity thickening to 2.3% and adding 4.4 kg/t TS of polymer. The results for theanaerobically digested wastewater show a similar trend, but the addition of polymeris not as effective in improving solids removal. This is as expected, given thatdigested wastewaters do not flocculate as well as raw wastewaters (Albertson et al.,1991).



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TABLE 10-5 SOLID REMOVAL EFFICIENCY OF PIGGERY WASTEWATER

Wastewater characteristicsg/kg, mg/L or mg/kg

Removal %Without flocculant

Removal %With flocculant

mina(1.0%)

maxa(4.5%)

meana min max meana min max

TS (g/kg)VS (g/kg)TSS (g/kg)VSS (g/kg)COD (mg/L)TKN (mg/L)NH4+ (mg/L)

TP(mg/kg)

9.75.64.23.2

13,91016401268249

75.963.469.960.770,080347315162178

44.647.557.251.131.216.710.764.7

15.017.837.235.27.83.45.4

57.9

60.565.271.762.844.032.414.668.4

59.365.173.369.257.532.420.868.6

38.149.863.162.144.525.012.750.9

81.686.990.773.280.257.435.080.5

a 4 pig wastewaters used in trial had % TS concentrations of 1.0%, 2.5%, 4.5% and 7.5%

Piccinini and Cortellini (1987)

FIGURE 10-2- RELATIONSHIP BETWEEN INFLUENT TS CONCENTRATION ANDREMOVAL EFFICIENCY - PICCININI AND CORTELLINI (1987)

Centrifugation was effective in removing TSS (Table 10-5: 72% removal withoutpolymer and 91% for the 4.5% TS concentration feed). This is also reflected in thehigh recovery of phosphorus, bound as phytate in the finer wastewater particles, andin the reduction in the COD. The removal percentage maxima were 44% withoutpolymer and 80% with for the raw wastewater, and 60% without and 70% withpolymer for the digested wastewater.



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TABLE 10-6- SOLID REMOVAL EFFICIENCY OF ANAEROBICALLY DIGESTED PIGWASTEWATER

Wastewater Removal %Without flocculant

Removal %With flocculant

mina

(1.4%) maxa

(5.5%) meana

min max meana

min Max

TS (g/kg)VS (g/kg)TSS (g/kg)VSS (g/kg)COD (mg/L)TKN (mg/L)NH4+ (mg/L)

TP(mg/)

14.17.15.43.8

13,7001,5301,158

238

62.443.155.439.5

61,5802,9701,351

998

43.445.553.7

41.325.47.9

32.3

6.33.6

11.2

5.31.73.7

14.8

67.071.080.0

59.639.312.0

49.9

58.462.270.9

61.533.717.1

33.1

14.513.131.8

52.58.1

10.7

16.0

80.285.891.0

70.049.023.5

50.2a 4 digested pig wastewaters used in trial had % TS concentrations of 1.4%, 4.5%, 5.5% and 6.2%

- Piccinini and Cortellini (1987)-

According to the authors, the addition of polymer to the wastewaters did notsignificantly affect the chemical composition of the separated solids. Hence, the datawere combined and presented in the one table (Table 10-7). For both the raw andanaerobically digested separated solids, the TS concentration is very high (range of22.4 - 27.3% for the raw wastewater and 21.2 - 25.2% for the anaerobically digestedwastewater). At these concentrations the solids would be stackable, readily storedand composted. The difference between the minimum and maximum values for the

recovery of solids from both the raw wastewater and the anaerobically digestedwastewater emphasises the need to adjust the TS concentration of the feed and theflow rate for cost-effective centrifugation (Table 10-7). The solids recovery valuesvaried from 12 to 244 kg of solids per cubic metre of raw wastewater, and from 12 to264 kg of solids per cubic metre of anaerobically digested wastewater.

TABLE 10-7- SEPARATED SOLIDS CHEMISTRY FOR RAW PIGGERY WASTEWATERAND ANAEROBICALLY TREATED PIGGERY WASTEWATER

Raw pig wastewater a Digested pig wastewater a

mean min Max mean min max

TS %VS (%TS)TKN (%TS)Total P (%TS)

Quantity(kg solids /m3 of waste)

22.480.3

3.42.4

119.0

16.369.9

2.30.9

12.0

27.383.9

5.13.5

244.0

21.272.93.11.4

147.0

17.369.1

2.60.8

12.0

25.278.0

4.21.6

264.0

a Data with and without polymer addition were combined by the authors



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10.4.5 Moller et al. (2000) - Separation of animal wastewaters

The paper compared the efficiency of different solids separators in removing asmuch as possible of the nutrient content of livestock wastewaters in the solidfraction. The aim was to produce a sufficiently dry solid for ease of transportation to

regions with low animal densities. The mechanical devices tested included astationary screen, three different screw presses, a 2-stage separator, a belt press andtwo different decanter type centrifuges. The conventional mass balance calculationused to measure separation efficiency was adjusted to provide a reduced efficiencyindex. The equation developed gave a value of 1 for complete TS removal, and 0 forno removal. Capital costs, the interest rate, the period of depreciation, maintenanceand repair costs, as well as electrical usage were also used to calculate the economicaspects of solids separation for each of the systems tested.

Both pig and cattle wastewaters were tested. However, the data used for thedecanter calculations were from two different studies, with no information provided

on the characteristics of the wastewaters (Table 10-8). The authors based theeconomics of separation on a pig farm with an annual production of 4,000 tons ofanimal wastewater, corresponding to the annual production of 8,000 pigs. Theyconcluded that the cost of wastewater treatment with a decanter centrifuge was fivetimes more expensive than treatment with a stationary screen. However, if theremoval efficiency for total phosphorus is set as a key factor, then the cost is only25% higher. The decanters are more effective in removing the fine particles, andproduce a drier solids fraction. As expected, none of the equipment could transferdissolved nitrogen (ammonium nitrogen) from the liquid to the solid fraction, butdecanters were the most effective in transferring the organic nitrogen fraction.

The authors concluded that mechanical screening systems were not effective inremoving total phosphorus and organic nitrogen from wastewaters. On this basisthey favoured screw press and decanter systems, despite their higher capital costs.The cost of manure treatment with a screw press and a decanter centrifuge were$1.25 and $6.30 per tonne respectively, for a farm producing 4,000 tons of wastewaterannually. They concluded that farms with a larger livestock production, or contractmanure separator operators servicing several farms would reduce the cost oftreatment substantially.

TABLE 10-8- LIVESTOCK WASTEWATER TRIAL - MOLLERET AL. (2000)

Reduced separation efficiency indexDecantertype

Wastewatertype

Energy kWhtonne-1 TS TN TP

SKSNh450 CattlePig

4.02.9

0.360.64

0.110.19

0.400.43

Pieralisi Anaerobicallydigested

2.2 0.74 0.14 0.82

Data for the centrifuges only are presented (Table 10-8). Although not indicated bythe authors, we assume that the sludge was gravity thickened and that polymer wasnot added prior to solids separation. The efficiency of solids separation, both for TS,and total phosphorus (TP) is very high. Given that 1 represents 100% removal, thevalues of 0.64 and 0.40 respectively for the pig wastewater indicate that a highproportion of the finer particle fraction of the wastewater was removed. The low



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total nitrogen (TN) value is to be expected, given that a high proportion of thenitrogen is in the soluble phase as ammonium nitrogen. Only the organic nitrogenfraction, locked up as protein in the finer particles of the wastewater, would havebeen removed (Giusquianii 1998).

10.4.6 Sneath et al. (1988), Sneath (1988a&b) - Piggery wastewater

The authors of these papers found that whilst the potential of decanters tosubstantially reduce the solids content of wastewaters was well documented, theperformance of the machines over the range of TS contents characteristic of pigwastewaters was not. Neither did the literature adequately document how changesin the operating settings of the centrifuge affected the properties of the separatedliquid and solid fractions. In a series of three papers the authors examined theperformance of a decanting centrifuge (Sneath et al., 1988), the economic impacts of

centrifugation on wastewater storage (Sneath, 1988a), and the economic effects onaerobic methods of odour control (Sneath, 1988b).

The decanting centrifuge used in this study was an Alfa Laval NX 314. The beachand weir configurations of the centrifuge could be adjusted through 8 positions.During this study three positions were compared, regulating the volume of liquidretained in the bowl at 8.8, 12.5 and 18.9 litres respectively. No details on the pigtype or production system producing the wastewater was given, except that the rawwastewater had a flow rate of 8.75 t/h and a TS content of 7%. For the trials, thewastewater was diluted to provide a thick raw wastewater (about 4.5% TS) anddilute (about 2% TS) feed for centrifugation. Three flow rates (10, 6.5 and 4 t/h) were

also used, but the rotational speed of the bowl was not adjusted.

The authors found that neither the liquid level in the centrifuge nor the rawwastewater flow rate had any significant effect on the TS content of the separatedsolids. However, as the TS content of the wastewater decreased, the TS of theseparated solids increased (P= 0.001). In this paper removal or separation efficiencywas calculated as the ratio of mass of TS or of suspended solids (SS) in the separatedsolids to the mass of TS or SS in the raw wastewater. In general, the amount of TSremoved in the separated solids was directly proportional to the TS concentration inthe wastewater. Within the range of wastewater TS concentrations and flow ratestested, no evidence of the reduction in solids removal percentage described byPiccinini and Cortellini (Section 10.4.4 and Figure 10-2) for high TS concentrations offeed were observed. Similarly, the reduction in the liquid mass after centrifugationwas directly proportional to the TS content of the raw wastewater.

The effect of centrifugation was to remove particles that were smaller than 0.15 mm.The results of wet sieving indicated that 53% of the raw wastewater particles werelarger than 0.15 mm (the pore diameter of the sieve used). Indeed, in the centrifugedliquid fraction (centrate) only 2% of the particles were larger than 0.02 mm. Removalof particles as small as this explains the improved removal of organic nitrogen andtotal phosphorus as presented in Table 10-5, Table 10-6 and Table 10-7.



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In summary, the decanter was able to remove 61% of the TS with an influentwastewater TS concentration of 8%. This produced solids of with a TS content of27%. Reducing the influent concentration down to 2% TS reduced the separationefficiency down to 43%. The TS content of the separated solids was 30%. Thedecanter was also able to remove up to 73% of the SS contained in the raw

wastewater diluted to 1.9% TS and up to 65% of the SS contained in the rawwastewater at 8% TS. The corresponding TS content of the solid fraction producedwas 30 and 27% respectively. Reducing the raw wastewater flow rate to thecentrifuge from 10 to 4 t/h increased the separated solids TS content by up to 3% andincreased the solids removal efficiency. The decanter used did not have a dual gearfacility, hence all adjustments to the flow rate for the maintenance of the highersolids content of the separated solids were under operator control.

With respect to the economics of operation, Sneath (1988a) concluded that the costsof centrifugation could only be justified for an 8000 head piggery if the solids couldbe sold for Aus$70 per tonne. Installation and capital costs were Aus$106,000.

However, when odour became the key determinant centrifugation was rated as moreeffective than aeration for a 30 day storage requirement for an 8,000 head herd(Sneath 1988b). The centrifuge is not considered cost-effective for herds of 2,000 head.In all cases, except when wastewater from a small herd needs storing for only 5 days,the cost of treating and storing piggery wastewater reduces as the TS content of thewastewater increases.

In the Australian context, wastewater is stored for much longer than 30 days. Evenfor herds of less than 8,000 head, if odour generation is a key impediment todevelopment, then the improved TS, total phosphorus and total nitrogen removalefficiencies may justify the higher capital costs.

10.4.7 Miner et al. (1983) - Separating anaerobic lagoon sludge

The objective of this trial was to evaluate the performance of a decanter centrifuge todewater and desludge an anaerobic pond. The anaerobic pond was receiving solidsfrom a piggery, with the sludge occupying over 90% of the pond volume. A two-stage variable speed eccentric screw pump (Mono Pump model MD82 with a 3.7 kW5 hp variable speed drive) and a pump suction PVC line of 6m in length and 75 mmdiameter was used to extract the sludge. The pressure line from the pump to thedecanter was a 100 mm diameter flexible hose. For the polymer feed studies Zetag92 as a 0.1% aqueous solution was fed directly into the inlet pipe of the decanterusing a variable speed peristaltic pump from a 0.380 m diameter vertical cylindricaltank. Flow rate, bowl speed, differential speed of the bowl and scroll, and beachlength within the centrifuge were varied.

The sludge pumped from the lagoon had a TS concentration of 7 to 10%. Theeccentric screw pump was able to handle this highly viscous liquid, and was self-priming. Solids produced from the decanter ranged from 14.7% to 30.5% TS. Therewas no relationship between polymer dosage and solids dryness. Solids recovery

from the decanter ranged from less than 60% to over 99.5% depending on theoperating conditions being evaluated. Without the use of the polymer solids



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recovery rates were greatly reduced unless the flow rate of the feed was reduced toless than one third of the hydraulic capacity of the decanter. Increasing the bowlspeed from 1150 RPM to 2200 RPM whilst maintaining a differential speed of 3 or 3.5RPM and a beach length of 80 mm at a flow rate of 0.8 to 1.8 m3/hr increased the TScontent of the solids from 19 to 24%. Increasing the flow rate from 1.9 to 3.7 m3/hr

increased the TS concentration of the solids from 23 to 28%. As with the previousstudy, varying the beach length and the differential speed did not affect the qualityof the separated liquid (centrate). However, the quality of the liquid was verysensitive to the use of polymer and the feed flow rate. The best quality was achievedwith a flow rate of less than 1 m3/hr, producing a suspended solids concentration ofbelow 10,000 mg/L.

In practice the solids produced were stackable, and sufficiently stable to avoid odourgeneration and vermin attraction. The decanter, pumps and supporting equipmentcost Aus$157,000, with an hourly operating cost of Aus$30 based on 1800 operatinghours per year. Power cost at a tariff rate of Aus$0.15/kWh was Aus$2.10/hr.

Labour requirements were estimated at Aus$33.00 per hour. At a flow rate of 1.0m3per hour, feed TS of 10% and with no polymer the dewatering costs are Aus$392/tTS, equating to Aus$8.80 per pig marketed (assuming each pig contributes 13.5 kgdry, digested lagoon sludge. At a flow rate of 3.0 m3/hr, feed TS of 10%, polymerused at 2 kg/t the dewatering cost is Aus$290/t dry solids and $3.80 per pigmarketed. The author concluded that desludging piggery ponds with a decanter wasfeasible, but only economic where there is no opportunity for local land applicationfor farming purposes. Where land is limiting and odour generation is a key issue,decanters can produce a readily handled, inoffensive solid and a relatively odourless,clarified liquid.

10.5 Running Costs and Maintenance

Decanter type centrifuges do not function efficiently on feeds with a low and widelyfluctuating TS concentration. The presence of particles larger than 3-5cm, may alsoblock and damage the equipment. Hence the use of a sump for gravity thickening,and a coarse screen to remove particles larger than 5cm would improve separationperformance and reliability. The sump should include an agitation system, toresuspend the solids after gravity thickening. Wastewaters thickened to 3-5% TSconcentration will still be pumpable, once the sump is agitated to resuspend the

particles in solution.

For most piggeries, the decanters will not be operating continuously. At the end ofeach operating cycle, flush water should be used to reduce the solids build-up on thescroll and weir mechanisms between cycles. Otherwise maintenance is restricted tolubricating the main bearings every 100 hours of operation, checking the gearbox oil,lubricating the conveyor bearings and checking the v-belts and solids dischargebrushes for wear every 1,000 hours. Foundation bolts and vibration dampers shouldbe checked every 4,000 hours. After about 5-6,000 operating hours the scroll mayshow signs of wear. Westfalia Australia has a scroll exchange program, where clientscan purchase a reconditioned replacement and receive a credit on their old scroll.

Alfa Laval has service kits available for purchase, consisting of those parts mostlikely to be replaced at intermediate or at major equipment servicing.



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10.6 Practical Operating Issues

Provided that the TS concentration is uniform and within the preferred TSconcentration range, the flow rate into the decanter and the weir plate can beadjusted during commissioning to set the throughput for the required separated

solids TS content and solids recovery performance. Once set, if the flow rate and TSconcentration of the feed remains uniform, very little adjustment aftercommissioning will be required. A major advantage of the patented dual gearsystem (Westfalia Australia) is that the machine automatically adjusts the scroll ratefor minor fluctuations in the flow and TS concentration of the feed, to maintain thedesired separated solids output. Hence, the requirement for uniform feed propertiesis not as exacting as for other decanter makes. The weir mechanism is alsocomparatively simple, allowing a relatively unskilled operator to adjust thethroughput and separated solids properties of the decanter on-site.

In practice, the Westons Bioproducts factory undertakes an annual major service onthe centrifuges (replacing bearings), and a weekly lubrication schedule. Thecentrifuges remove starch using only gravity thickening, removing 80% of the starch

particles having a diameter of 60-200 . The TS concentration of the separated solidsis 52-54%. The settling tanks used for gravity thickening have a capacity of 27,000 Lwith a maximum 2 hour residence time (modified rainwater tanks). The dual gearcentrifuges are the easiest to maintain (Westfalia Australia), given the automaticadjustment for variations in the feed flow and TS concentrations. For decanterswithout this feature the flow rates and TS concentration of the feeds must bemonitored and adjusted by the operator.

10.7 Piggery Case Studies

Four piggery case studies have been analysed. These are a 200-sow and a 2000-sowunit operated under low flushing (5 L/SPU/day) and high flushing (25 L/SPU/day)regimes. Capital and operating costs were estimated using data supplied by themanufacturer. It was assumed that power costs $0.13/kWhr and labour costs are$25/hr. Table 10-9 provides summarised capital and operating costs. A coarsescreen (5mm) is assumed to also be installed to remove larger solids particles.



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TABLE 10-9 CAPITAL AND OPERATING COSTS OF CENTRIFUGE CASE STUDY

Item Units 200-sowlow-flush

200-sowhigh flush

2000-sowlow-flush

2000-sowhighflush

No of pigs SPU 2134 2134 21340 21340Flushing L/SPU/day 5 25 5 25Hosing L/SPU/day 1 2 1 2Total effluent a ML/yr 9 25 85 250Effluent flow (24hr)

L/s 0.27 0.79 2.7 7.9

Solids content ofeffluent

% TS 3.1% 1.2% 3.3% 1.2%

Solids t/yr 270 290 2800 2940Data ALDEC Centrifuge DecanterFlowrate L/s 0.6 1.4 4.4 11.7Operation hrs/day 11.7 13.7 14.6 16.3

hrs/yr 4,270 5,000 5,340 5,950Solids Removal b % 30 20 30 20

t/yr 80 58 840 590Capital cost c $ 115,000 115,000 152,500 235,000

$/ML treated/yr

13,460 4,600 1,780 940

$/t solidsremoved /yr

1,430 1,980 180 400

Operating Cost kWhr/yr 70,500 94,920 98,820 110,030$/yr (power) 9,170 12,340 12,850 14.300Labour hr/day 0.5 0.5 1.0 1.0$ /yr (labour) d 4,560 4,560 9,130 9,130

$/yr (main) e 2,000 2,000 4,000 5,000Total $/yr 15,730 18,900 25,970 28,430

$/ML treated 1,840 760 300 114$/ t solidsremoved

195 325 31 48

a Total effluent includes flushing water, hosing water, manure and drinking water wastage.b While the manufacturer claims a higher solids removal percentage, this figure is adopteduntil better data is available.c Capital cost includes a shed to cover the centrifuge and a manure collection sump withpumps and agitator. A coarse screen is also installed to remove larger particles.d Labour for monitoring and maintenance costed at $ 25/hre Routine maintenance of pumps and agitators

10.8 Summary Selection Criteria

10.8.1 Solids removed

The solids removal efficiency of centrifuges improves substantially if the feedwastewater has a high TS concentration. Most researchers either gravity settled thewastewater or used a polymer. The TS removal efficiency on a mass balance basismeasured by Payne (1990) was only 37%. However, this was due to a lower influentTS concentration of 1.7% (not pre-thickened) and the separated solids had a high TSconcentration (35%). Abery (1994) tested the performance of a centrifuge with and



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without flocculant using gravity settling. He showed that gravity settling alone,without flocculant reduced the TS by about 40%. The centrifuge only removed 35-40% of the settled solids, giving an overall reduction in TS of about 20%. With theaddition of a polymer during the gravity settling phase, overall solids reductionsimproved to about 40%. Piccinini and Cortellini (1987) showed removal efficiencies

(concentration basis) ranging from 15-60% and 38-82% with and without the additionof a polymer respectively. These wide variations were due to wide variations ininfluent concentration (1-7.6%). Sneath et al. (1988) was able to remove 61% of the TSwith an influent wastewater TS concentration of 8%. Reducing the influentconcentration down to 2% TS reduced the separation efficiency to 43%.

The removal efficiency used for these case studies is assumed to be 20% for thewastewater with 1.4% TS concentration and 30% for the wastewater with 3.1% TSconcentration. No gravity thickening was factored into the case studies.

10.8.2 Capital cost

From Table 10-9, the capital cost could range from $115,000 for a 200-sow piggeryand $152,500 to $235,000 for a 2000-sow piggery. Capital costs include a centrifuge,coarse screen, shed, pumps, sumps and agitators. Higher capital costs are associatedwith the higher throughput of wastewater. The throughput could be lowered bygravity thickening the wastewater to obtain a TS concentration of 5%. The removalefficiency is likely to increase to approximately 50%, thus reducing the capital costper tonne of solids removed.

10.8.3 Operating costs and returns

From Table 10-9, the operating costs could range from $760 to $1,840 per ML ofeffluent treated for a 200-sow piggery to $115 to $305 per ML of effluent treated for a2000-sow piggery. Operating costs per tonne of dry solids removed range from $195to $325 for a 200-sow piggery and $31 to $48 for a 2000-sow piggery. The lower costsreflect economies of scale with larger piggeries. Operating costs include power,labour and routine maintenance of pumps and agitators. Removal efficiencies couldbe increased if the wastewater was firstly gravity thickened and a TS concentration of5% was fed to the centrifuge. This would also reduce the throughput of wastewaterand lower operating cost per tonne of solids removed. With the TS removalefficiency increased to 50%, the operating cost would be substantially reduced.

10.8.4 Ease of operation

Provided the centrifuge is feed is at a constant flow rate and TS concentration, oncecommissioned centrifuges require very little supervision and maintenance. The dualgeared decanters are even easier to operate, with the differential between the bowland scroll speeds automatically adjusted to compensate for minor fluctuations in theflow rate and TS concentration of the feed. The solids could be gravity fed into a

hopper for batch collection, or onto a conveyor with minimal risk of odourgeneration or seepage.



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10.8.5 Solids management options

Decanter centrifuges produce the very dry solids, with the potential of removing thegreatest proportion of total nitrogen and total phosphorus. In practice provided thatthe solids content is over about 20%, the solids will be stackable, readily handled for

composting. In practice the production of very dry solids is at the expense of therecovery of the finer particle fractions containing most of the organic nitrogen andphosphorus. Producing solids with a higher TS content (over 25%) would only beadvantageous if long distance off-site transport was required.

10.9 References

Abery R. 1994. An evaluation of methods of effluent treatment at Module 5,Corowa. Bunge Meat Industries, Corowa, NSW.

Giusquiani P.L., Concezzi L., Businelli M. and Macchioni A. 1998. Fate of pig sludgeliquid fraction in calcareous soil: Agricultural and environmentalimplications. Journal of Environmental Quality 27:364-71.

Moller H.B. Lund I. and Sommer S.G. 2000. Solid-liquid separation of livestockslurry: efficiency and cost. Bioresource Technology. 73. 223-229.

Payne R.W. 1990. On-farm performance of pig effluent treatment systems in use inWestern Australia. Final Report to the Research Advisory Committee,Western Australia Pig Industry Compensation Fund. Department ofAgriculture, Western Australia.

Piccinini S. and Cortellini L. 1987. Solid-liquid separation of animal slurries. inAgricultural Waste Management and Environment Protection. Vol(1).pp219-29. Proceedings of 4th International Scientific Centre of FertilisersSymposium. Braunschweig, Federal Republic of Germany 11-14 May 1987.

Rushton A., Ward A.S. and Holdich R.G. 2000. Solid-liquid filtration and separationtechnology. Second edition, WILEY-VCH.

Sneath R.W., Shaw M. and Williams A.G. 1988. Centrifugation for separatingpiggery slurry 1. The performance of a decanting centrifuge. Journal of

Agricultural Engineering Research. 39, 181-190.

Sneath R.W. 1988a. Centrifugation for separating piggery slurry 2. Economic effectson slurry storage. Journal of Agricultural Engineering Research. 39, 191-197.

Sneath R.W. 1988b. Centrifugation for separating piggery slurry 3. Economic effectson aerobic methods of odour control. Journal of Agricultural EngineeringResearch. 39, 199-208.

A il 2002 FSA E i l P N 10 20

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Case Study 10 - Centrifuge