OPERATIONAL AND ECONOMIC MODELING AND
OPTIMIZATION OF MOBILE SLURRY SEPARATION
C. G. Sørensen, H. B. Møller
ABSTRACT. Economies of scale associated with mobile separation will reduce the costs of separation for each individual farm.
Mobile separation will help both larger and smaller animal production units, whose plant and animal production may be
unbalanced, to make efficient use of slurry separation.
A multiple-farm separation facility, comprising a decanter centrifuge mounted on a mobile trailer, has been adapted and
configured. The decanter separates the raw slurry into one fraction containing a high proportion of the dry matter and
phosphorus in the slurry, and into another fraction containing most of the nitrogen and potassium in the slurry. Its operational
performance in terms of capacity and separation efficiency has been tested and tools for operations management have been
devised.
Results shows that the separation efficiencies (%) obtained in tests were 37-68, 50-83, and 8-33 for dry matter (DM), total
phosphorus (TP), and total nitrogen (TN), respectively. The separation efficiency of TN was dependent on the DM content
of the manure, while the separation efficiency of TP was not.
The capacity of the separator averaged 19.0 m3 h-1, ranging from 6.0 to 40 m3 h-1. The average nominal time for assembling
dismantling, and cleaning the separator system, averaged 2.3 h, with a corresponding average labor requirement of 3.1
person-hours. The operating costs were strongly dependent on the amount of manure being treated annually, varying between
2.7 and 7.0 $ m-3 for typical operational constraints.
Keywords. Slurry separation, Manure, Mobile, Operational performance, Economics, Separation efficiency.
H
ouseholds, industry, and agriculture all contribute
to the discharge of phosphorus into recipients like
lakes, streams, and the sea. A number of legislative initiatives have reduced the discharge of
phosphorus from households and industry. As a result, the
phosphorus concentration in the recipients has been reduced
correspondingly. However a surplus of 10 kg phosphorus
ha-1y-1 in agriculture is still being observed, indicating that
the use of phosphorus needs to be reduced further (Jacobsen
et al., 2002). The Danish Action Plan for the Aquatic Environment III, 2005–2015, focuses on reducing the phosphorus
surplus (MFAF, 2004).
In Denmark, as in other European countries, there is a
limit on how much nitrogen from animal manure can be
applied per hectare. Beginning in 2003, Danish pig producers
have not been allowed to apply manure from more than
1.4 livestock units (LU) per hectare, where one livestock unit
is a unit of calculation, which corresponds to a maximum of
100 kg of nitrogen in the manure taken from storage (MFAF,
2002a). By use of conversion factors one livestock unit
correspond with 4.3 sows and 36 fattening pigs from 30 to
100 kg. If livestock farmers are required to balance phospho-
Article was submitted for review in December 2004; approved for
publication by the Power & Machinery Division of ASABE in October
2005.
The authors are Claus Grøn Sørensen, ASABE Member, Senior
scientist, and Henrik B. Møller, Researcher, Department of Agricultural
Engineering, Danish Institute of Agricultural Sciences, Denmark.
Corresponding author: Claus Grøn Sørensen, Department of Agricultural
Engineering, Danish Institute of Agricultural Sciences, Research Center
Bygholm, 8700 Horsens, Denmark; phone: +4589993023; fax:
+4589993100; e-mail: [email protected].
rus application with crop demand, only about 22 kg
phosphorus ha-1 y-1 can be applied to cereals (MFAF, 2002b).
Since sows and fattening pigs excrete 21 and 26 kg
phosphorus LU-1, respectively (Poulsen et al., 2001), the
maximum load of animal manure from fattening pigs is
approximately 1.0 LU ha-1. Balance in phosphorus application can be achieved either by increasing the available land
or through separating the slurry and exporting the solid
fraction to crop producers. The separation process transfers
the phosphorus and organic nitrogen content of the slurry to
a solid fraction with a high concentration of dry matter,
phosphorus, and organic nitrogen (Burton and Turner, 2003).
The solid fraction amounts to 10% of the original slurry
volume and may be transported at low costs to remote fields
that require phosphorus and nitrogen (Sørensen, 2003).
A number of different concepts for slurry separation have
been proposed. These concepts can be divided into two main
categories comprising low-technology systems (e.g., decanter centrifuge) and high-technology systems [e.g., Funki
Manura 2000 (Funki Manura Inc., Sønderborg, Denmark)]. Recent changes in the agricultural legislation in
Denmark have reduced the area requirements for farmers
who use separation technology. In the case of low-technology
systems, the area requirement is reduced by 25%, while for
high-technology systems, the reduction is increased to 50%
(Seadi and Møller, 2003). The high-technology concepts
separate nutrient-rich fractions with a separation efficiency
of more than 70% for the phosphorus and more than 70% for
the nitrogen, and with an average concentration of nutrients
at least 2.5 times as great as untreated slurry. The lowtechnology systems must be able to separate a nutrient-rich
fraction with a separation efficiency of more than 20% for the
nitrogen and more than 60% for the phosphorus, and with an
Applied Engineering in Agriculture
Vol. 22(2): 185-193
2006 American Society of Agricultural and Biological Engineers ISSN 0883−8542
185
average concentration of nutrients at least 1.5 times as great
as untreated slurry (MFAF, 2002b; Seadi and Møller, 2003).
Cost assessments show that initial investment and operating costs of effective separators are high, which indicates that
a high utilization in terms of amounts of treated manure is
required in order to reduce the unit costs (Møller et al., 2000).
In this context, increased harmony demands imposed by
legislation may contribute to a reduction in the number of
medium-sized livestock farms, unless it is possible to
develop a novel separation concept capable of treating
manure from a number of farms. Such a separation system
would be innovative in the sense that a mobile facility with
a high capacity and high separation efficiency is adapted for
use on small- and medium-sized farms regardless of the type
of slurry and the design of on-farm production facilities.
Mobile decanter centrifuges have been applied to the
separation of slurries in the Netherlands and adapted to Dutch
conditions (Jacobsen et al., 2002). The storage and the
livestock production systems in Denmark differ from those
in the Netherlands, which causes the optimal design of the
separator, as well as the operational planning, to differ from
the Dutch model. In addition, the composition of slurry in the
Netherlands is different from the composition of Danish
slurries (Burton and Turner, 2003), which will also cause the
variation in the slurry composition between farms to differ.
As the separation efficiency is influenced by the variation in
the composition of the slurry, differing demands will be
imposed on the centrifuges in the two countries.
The aim of this study was to develop and evaluate a
concept for the use of the decanter centrifuge. The technology was to be tailored to the needs for mobile separation of
slurry on a number of farms and the operational performance
of the facility was modeled as the basis for studying the
generalized capability and economic viability of the system
by systematic procedures. The objectives included evaluating the energy consumption, labor requirement, capacity, and
separation efficiency as indicators for the development of
equipment, operational management tools, and economic
assessments. The hypothesis was that the separator would be
robust, with an effective separation index (>60% P and >20%
total N) and capable of operating with low costs and high
capacity.
METHODS
SYSTEM DESIGN
A Pieralisi (Pieralisi Benelux B.V., Bleiswijk, The
Netherlands) Jumbo 3 decanter centrifuge was mounted on a
mobile trailer, which can be pulled by a tractor or truck. The
functional principle of the decanter is that the raw slurry is
subjected to a large centrifugal force in a high-velocity
rotating drum, typically 2000 to 4000 rpm. In the course of
this process, the particle components of the slurry settle
toward the periphery of the drum and subsequently may be
removed by a rotating screw. The decanter centrifuge is a
well-tested technology, which has been used for sewage
treatment both in industry and on municipal sewage disposal
plants, and in later years experience with slurry centrifugation has been gained (Møller et al., 2000, 2002).
The installed power unit in the trailer was a standard diesel
engine (80 kW) powering a generator coupled to the
centrifuge. Other installations included macerator, feeding,
mixing, and reject pumps, together with a conveyer for
transport of the solid fraction from the centrifuge to a
transport unit or storage space.
Different usage scenarios involve that the raw slurry is
separated from the outlet of a pig or dairy cattle house or from
a primary slurry tank. In both cases the liquid fraction from
the separation is deposited in a slurry tank while the solid
fraction is placed in a transport unit for final deposit at a
storage space or immediate transport to the field for
spreading.
In order to carry out the operational performance tests,
various measuring equipment was available as part of the
control console for the centrifuge. These included gauges for
measuring the electrical power take-off in terms of voltage
and amperage. A flow meter (Danfoss Mag 3100, Danfoss
Inc., Nordborg, Denmark) was mounted between the intake
pump of raw slurry and the centrifuge, and a fuel gauge
equipped with built-in time monitor was mounted on the
engine.
ON-FARM ANALYSES
The operation of the mobile separator during its use at
individual farms as well as in transit between farms was
investigated using detailed work studies producing basic
performance data (e.g., labor demand, energy consumption,
capacity). These farm-specific performance data were statistically analyzed together with the derivation of generalized
model parameters. The data were used in two ways: (1) in a
diagnostic way to determine the “current state” of the
separation system, or (2) in a prognostic way to predict the
state of the system given specific operational variables
including rated separation capacity, transport distance,
on-farm buffer capacity, etc. The latter data application
involved the use of work models (labor requirement/machine
capacity as a function of machinery size, transport distance,
etc.) using the data for unique work elements as building
blocks for such models (e.g., Auernhammer, 1976; Achten,
1997; Sørensen et al., 2003; Sørensen et al., 2005).
Detailed on-farm studies of the operational performance
were carried out on 16 preselected farms comprising 14 pig
farms and 2 dairy farms. Three of the pig farms were
equipped with an anaerobic digestion plant. On each farm,
the whole operation sequence of preparing the centrifuge for
operation, the actual separation process, and the dismantling
Solid fraction
Buffer tank
Animal house
Mobile separator
Storage of liquid
fraction
Pre−storage tank
Pre
Figure 1. Baseline system configuration for the mobile centrifuge operating at a farm.
186
APPLIED ENGINEERING IN AGRICULTURE
of the centrifuge for transport to the next farm were surveyed.
The labor content of each defined unique work element was
measured, together with the acquisition of information on
fuel consumption, electrical power requirement, intake of
raw slurry, and the amount of the separated solid fraction. The
weight of the solid fraction was determined by collecting the
solids in a container placed on weighing cells. Representative
samples were taken during separation from the solid fraction,
the liquid effluent, and untreated manure. In each test, five
samples of each fraction were taken and a subsample was
taken from the mixture. The samples were stored at -18°C
until analysis.
Parallel to the operational studies, samples were taken
from the input flow of raw slurry as well as from the output
flow of liquidized slurry, and the solid fraction. These
samples were subsequently analyzed for dry matter, nitrogen,
phosphorus, and potassium content.
LOGISTIC MODELING
The term logistics covers a wide range of meanings,
including business and industrial logistics (Blanchard, 1992).
In this study, logistics was concerned with the analysis and
optimization of the movement of the mobile decanter within
given boundaries of operation.
The on-farm slurry processing may be configured in a
number of ways, depending on local conditions as well as the
operational performance of the mobile centrifuge. Such
configurations will vary with respect to storage capacity at
the individual farm prior to separation. The storage capacity
will depend on the size of slurry canals, potential buffer
tanks, or the presence of one or more regular slurry tanks. It
is expected that the storage capacity for raw slurry must
exceed a certain number of days in order for the mobile
centrifuge to be able to handle an appropriate amount of raw
slurry at each visit to a farm. The logistic modeling is
intended to derive guidelines for the adaptation of the
separator capacity and operation to the storage capacity at the
farms. The following configurations are used as points of
reference:
separation from pre-storage tank (20 to 50 m3) to large
storage tank
separation from slurry canal (200 m3) to large storage tank
separation from buffer tank (ranging from 200 to 1500 m3)
to storage tank
separation from primary large storage tank (>2000 m3) to
secondary large storage tank (>2000 m3).
One model assumption is that the solution of separation
from one large storage tank to another large storage tank has
limited interest, as it implies that only half of the total storage
capacity can be used and subsequently only half of the slurry
amount may be separated and only at certain times of the year
when one tank is empty. Furthermore, it has been found to be
difficult to mix the contents of such large tanks, as the
different parts of the nutrients form sediments (Burton and
Turner, 2003). Another model assumption is that capacity
and labor input optimizations will dictate that a certain buffer
capacity is available with the buffer capacity being balanced
in relation to effective mixing and tank investment. Separation from slurry canals and pre-storage tanks seems to offer
an immediate solution, but operational performance measures and economy assessments may indicate otherwise.
Vol. 22(2): 185-193
TASKS AND OPERATIONS
The operation of the mobile separator involves a number
of tasks and operations. Operations include uniquely defined
work elements like unload hoses, prepare conveyor, etc.,
while tasks are the carrying out of one or more operations
during a certain time by a certain set of labor and equipment
(Goense and Blaauw, 1996). The mobile centrifuge unit
containing separator, auxiliary pumps, hook-up hoses, etc.,
are transported from farm to farm. Upon arrival at the farm,
a hook-up to the pre-storage tank, slurry canals, or storage
tanks are carried out. The mobile unit carries flexible hoses,
which are connected to the suction pumps, to the return pipe
to the slurry tank, and to the hose for draining-off the liquid
fraction to a dedicated storage tank. Current experimental
conditions employ no hook-up installations at the farm,
which is why laborious manual handling of the hoses is
necessary. Following the labor analysis, modeling was used
to demonstrate the benefit of more automated hook-up
procedures.
The studies of the operational performance were carried
out while the centrifuge was being prepared for, and is in
operation at, the individual farms. The targets of measurements were cleaning the unit, transport, preparation and
starting, operation, dismantling, and so on. The total labor
requirement is given by:
 t × 60
+
A = 
 v
b

+x×
c × 100


m a  + (( m c + m p + m d ) × (1 + q ))

(1)



where
A
= total labor requirement or nominal time per farm
visit (min)
t
= the driven transport distance between farms (km)
v
= the transport velocity (km h-1)
ma = the time for accessing and exiting the position for
hooking-up to the on-farm installation
mc = the time for post-operation cleaning of the
centrifuge and trailer (min)
mp = the time for pre-operation preparation and hook-up
to on-farm installations (min)
md = the post-operation disconnecting from on-farm
installations and dismantling of the unit (min)
q
= an addition for rest allowances (5%)
x
= the fraction of the actual operation time for
monitoring and operator availability (%)
b
= the size of the on-farm buffer capacity (m3)
c
= the effective capacity of the centrifuge while in
operation (m3 h-1)
OPERATING COSTS
The operating costs of the mobile unit include interest and
depreciation, maintenance, energy consumption, and labor.
Estimations of the costs were based on conventional methods
using annual interest, yearly depreciation and maintenance
of machinery (e.g. Witney and Saadoun, 1989). A general
degressive depreciation method with a rate of 15% was
assumed, based on the recorded value changes of agricultural
machinery (Laursen, 1993). The annual interest rate was set
to 6%.
187
The labor costs were based on contractually fixed hourly
wages of 25.8 $ h-1 (Sørensen et al., 2003), and the number
of hours was based on the labor requirement estimates.
The average annual total costs were given by equation 2
as a combination of capital costs and variable costs:
(
 i × 100 × 1 −(1 − w)n
C = I ×( 
−

1 − (1+ i ) n

+ (A × l )+ (O × (a + f ))+ u + h
) +(100 × (1 − w) × i))


n
(2)
where
C = the total annual cost,
I = the initial investment ($),
i = annual interest rate (%/100)
n = the number of years over which the machine will be
depreciated
w = the annual depreciation rate (%/100)
A = the estimated labor requirement derived from
equation 1 (h)
l = the labor cost ($ h-1)
O = the nominal operating time (h)
a = the maintenance cost ($ h-1)
f = the fuel cost ($ h-1)
u = the yearly machine insurance premium ($)
h = the yearly housing cost ($)
The mobile separator has a lifetime of 5 years. Maintenance and repair costs per hour included costs for both
materials and labor, and were based on normative data.
DECANTING PERFORMANCE AND SEPARATION EFFICIENCY
The performance of a decanting centrifuge depends on
several factors such as the G-value, the dewatering volume,
and the retention time. The term G-force or G-value (Møller
et al., 2002) is frequently used to define the force acting on
the solids. The G-force is defined as the multiple of the
gravitational constant that is obtained in the centrifuge and
is expressed in N.
An approximate formula for calculating the G-force at the
bowl periphery is:
G=
n2 × D
1800
(3)
where n is the bowl speed (rpm), and D is the bowl maximum
inner diameter (m), which is 0.470 m in the Pieralisi Jumbo 3.
Consequently, the centrifugal acceleration or G-value will
increase with the bowl diameter and bowl speed. The
dewatering volume of a decanter is considered as the total
content of the liquid zone in the cylindrical part of the bowl.
This volume may change in relation to the “weir plate”
diameter. For the Pieralisi Jumbo 3 decanter, weir plate
standard diameters are: 310, 300, 285, 280, and 273 mm.
During the tests, the centrifuge rotated at 2900 rpm, thus
causing a G-force of 2196 N derived from equation 3. During
the test, the weir plate diameter was 280 mm with a resulting
dewatering volume of 137 L.
The separation efficiency is defined as the total mass
recovery of solids and nutrients in the solid fraction as a
proportion of the total input of solids or nutrients (Svarovsky,
1985):
188
E1 =
U × Mc
Q × Sc
(4)
where
E1 = the index for the simple separation efficiency
U = the quantity of the solid fraction (kg)
Mc = the concentration of the dry matter (DM), total
phosphorus (TP), and total nitrogen (TN) components
in the solid fraction (g kg-1)
Q = the amount of raw slurry treated (kg)
Sc = the concentration of the components in the slurry
(g kg-1).
For example, given 100 kg raw slurry with a DM content
of 6% (60 g/kg), assume that by way of separation this is
separated into a liquid fraction (85%) with a DM content of
2.5% and a solid fraction (15%) with a DM content of 29%
(290 g/kg). The separation efficiency (eq. 4) can be
calculated as:
E1 =
15kg × 290 g / kg
= 0.725 = 72.5%
100kg × 60 g / kg
(5)
which means that 72.5% of the dry matter in the raw slurry
is recovered in the solid fraction. The quantity of solids in this
example is the 15 kg of the solid fraction (U in eq. 4).
RESULTS AND DISCUSSION
TASKS AND OPERATIONS
On average, the observed nominal time consumption for
preparation, dismantling, and cleaning was 2.3 h, while the
average labor consumption reached 3.1 person-hours, indicating the use of multiple simultaneous and periodic labor
units (table 1).
An important scenario is to predict the consequences of
streamlining the working procedures in preparing and
dismantling the centrifuge. The obvious modification would
be to install on-farm hose arrangements for immediate
hook-up to the mobile unit. By excluding the time and labor
requirement for handling the hoses, the nominal time and
labor requirement would be reduced by 19% and 26%,
respectively.
OPERATIONAL CAPABILITY AND COSTS
The operational capability of the centrifuge is expressed
as the input flow of raw slurry (table 2). Together with the fuel
consumption and other performance measures, the basis is set
for predicting capability and costs at different aggregation
levels.
By implementing the adapted model for machine performance and the adapted model for cost assessments in a
spreadsheet, simulations were carried out for the labor
requirement, mobile separator performance, and costs associated with the multiple farm use of the separation system
(fig. 2). The nominal time necessary for setting up the
centrifuge and operating it, and the potential yearly capacity,
may be estimated as a function of the amount of raw slurry
treated per visit of the centrifuge to individual farms.
Similarly, the unit costs are estimated as a function of the
annually treated amount of slurry.
A high yearly capacity requires a sufficient capacity. For
small buffer capacities below 500 m3, the unit costs are
increased and the yearly capacity reduced considerably. The
APPLIED ENGINEERING IN AGRICULTURE
Task
Table 1. Observed labor requirements
and model parameter estimation.
Nominal
Labor
Time[a]
Requirement[a]
(min)
(person-min)
Operation
Cleaning
Centrifuge flushing
Interior cleaning
Conveyor cleaning
Total (mc )
12.16 ± 2.5 (8)
12.67 ± 4.7 (4)
5.44 ± 1.9 (3)
30.26 ± 5.7
12.16 ± 2.5 (8)
12.67 ± 4.7 (4)
5.44 ± 1.9 (3)
30.26 ± 5.7
Transport
Accessing/exciting
(ma )[b]
Road transport[c]
Total[d]
5.71 ± 1.8 (8)
13.99 ± 6.2 (8)
19.70 ± 6.5
5.71 ± 1.8 (8)
13.99 ± 6.2 (8)
19.70 ± 6.5
Initiating assembling[e]
Unload/hook-up hoses[f]
Prepare conveyor
Prepare engine
Prepare pumps
System start
Total (mp )
11.16 ± 3.4 (10)
22.14 ± 7.8 (12)
7.03 ± 2.1 (17)
0.61 ± 0.5 (14)
1.28 ± 0.2 (4)
11.82 ± 6.0 (10)
54.04 ± 10.6
13.45 ± 5.6 (10)
42.42 ± 14.0 (12)
10.43 ± 4.0 (17)
0.61 ± 0.5 (14)
1.28 ± 0.2 (4)
11.82 ± 6.0 (10)
80.01 ± 16.7
Preparation
Dismantling
Prepare shutdown[g]
5.49 ± 4.0 (12) 9.46 ± 8.5 (12)
Conveyor dismantling 9.85 ± 5.2 (15) 18.88 ± 10.9 (15)
9.24 ± 2.8 (9) 17.46 ± 6.4 (9)
Unhook/load hoses[h]
Finishing dismantling[i] 7.90 ± 2.2 (12) 9.28 ± 3.0 (12)
Cleaning of the grounds 1.30 ± 2.8 (4)
1.30 ± 2.8 (4)
Total (md )
33.78 ± 8.0
56.38 ± 15.8
Aggregation
Total [j]
137.78 ± 15.9
186.35 ± 15.8
Data are means ± SD (SD = standard deviation), numbers in paren
theses indicate no. of observations.
[b] Accessing and exiting the position for hooking-up to the storage tank
with raw slurry.
[c] The measured transport velocity was 28 ± 4.0 km h-1 and the
measured transport distance was 6 ± 3 km.
[d] The total gives the measured time for accessing/exciting and transport
between farms.
[e] Include disconnecting tractor/truck, open trailer doors, activate
supporting legs, etc.
[f] Nominal time was 0.33 ± 0.1 (18) min/m of hose length and labor
requirement was 0.64 ± 0.2 (18) min/m of hose length.
[g] Include emptying pumps for water, connect tractor/truck, etc.
[h] Nominal time was 0.34 ± 0.1 (15) min/m of hose length and 0.62 ±
0.3 (15) min/m of hose length.
[i] Include lifting supporting legs, close trailer doors, etc.
[j] Total measured time for the whole operation sequence.
[a]
results recommend a buffer capacity in the range of 500 to
1000 m3, where the exact capacity must be determined by
balancing the reduced costs of the mobile separation against
the increased on-farm investment in storage tanks (fig. 2).
The results in figure 2 are the basis for analyzing scenarios
involving different configurations of the on-farm installations. In the case of the separator connecting to a pre-storage
tank with only limited capacity (e.g. 20 to 50 m3) the yearly
capacity will only reach the range of 31,000 to 52,000 m3
depending on the number of operating hours. Increasing the
pre-separation storage to 200 m3 by including submerged
slurry canals will increase the yearly capacity from 47,000 to
79,000 m3. A maximum capacity in the range of 55,000 to
93,000 m3 is reached for buffer tanks of 1000 m3 and more.
The annual manure production per LU reaches 17.2 m3
(DAAS, 2003), indicating that the mobile separator can serve
from 1802 to 5407 LU per year. As an example, an average
farm of 250 LU comprising 258 sows and 5696 annual
produced fattening pigs produces 11.8 m3 per day, which in
the case of a 500-m3 buffer capacity will require the service
of the mobile separator for each 42-day period.
Vol. 22(2): 185-193
Table 2. Derived model parameters and assessed
prerequisites for capability and costs prediction.
Default values for the reference scenario.
Parameters
Value
Centrifuge capacity, m3 h-1
Fuel consumption, l h-1[a]
Fuel consumption per unit of intake, l m-3
Power output, kWh m-3[b]
Investment, $[c]
Maintenance and repair costs, $ m-3[c]
Wage costs, $ h-1[d]
Housing, $ yr-1[e]
Insurance, $ yr-1[e]
Default values
Monitoring time, %
Buffer capacity, m3
Average driving distance, km
Yearly operating hours, h
19.00 ± 7.4
9.10 ± 1.1
0.58 ± 0.2
2.37 ± 1.8
395189
0.60
25.80
515
1718
100
1000
6.4
4000
[a]
[b]
The cost of fuel is $0.43 L-1 (DAAS, 2003).
The energy consumption is modest (1.77 kWh m-3) for the operation
of the centrifuge, the pumps, and the control unit. Additional,
auxiliary equipment requires 0.6 kWh m-3.
[c] The investment and maintenance costs were based on data from
Pieralisi Benelux.
[d] The labor costs were based on contractually fixed hourly wages
(Sørensen et al., 2003).
[e] The housing and insurance costs are based on normative data
(Poulsen and Jacobsen, 1997).
SENSITIVITY ANALYSIS
A number of variables influence the overall system
capacity of the centrifuge to varying degrees. Figure 2 shows
the impact of varying buffer capacities. Figure 3 shows a
detailed analysis of factors influencing the capacity of the
mobile unit as a function of selected factors varied within
±50%. The most significant influences are the centrifuge
capacity and the operating hours available. By changing the
centrifuge capacity by ±50% in the interval from 9.5 to
58.5 m3 h-1, the overall system capacity is either reduced or
increased by 47%, indicating a proportional relation. The
same impact is seen for similar relative changes of the
available operating hours in the interval from 2000 to
6000 hours yearly. In contrast, only marginal effects on the
system capacity are observed for changes in the value of the
parameters for transport distance and nominal time for
preparation and dismantling of the mobile unit during
individual farm visits. Changes of only 0.2% and 2% are
observed for changing the transport distance and the time for
preparation/dismantling ±50%, respectively. However, it
must be noted that some effects of the latter parameters will
be observed in the case of large transport distances and
smaller buffer capacities. If the buffer capacity only is
100 m3, a change of the preparation and dismantling time by
±50%, decreases and increases the system capacity by 11%
and 15%, respectively, and the general capacity is reduced by
27% compared with the 1000-m3 buffer capacity. For the
same scenario, an increase in the transport distance between
farms by a factor of 8 decreases the capacity by 18%, while
in the case of the reference buffer capacity the capacity is
reduced by 3%.
For non-adapted on-farm preparation and dismantling
procedures involving no targeted hook-up installations, the
yearly capacities as well as the unit costs of the decanter are
reduced by 0.7% for the 1000-m3 buffer capacity and by 5%
for the 50 m3 buffer capacity, indicating a relatively small
189
5.5
100
90
5.0
capacity at 3000 h
capacity at 4000 h
−3
4.5
70
$m
3
Yearly capacity, m x 10
3
80
60
4.0
capacity at 5000 h
unit costs at 3000 h
unit costs at 4000 h
unit costs at 5000 h
50
3.5
40
30
3.0
0
500
1000
1500
2000
2500
Buffer capacity, m3
Figure 2. Yearly capacity and unit costs of the mobile centrifuge as a function of the on-farm buffer capacity comprising dedicated buffer tanks and/or
slurry canals and pre-storage tank. The estimations are based on different assumptions such as the full-time monitoring of the decanter while in operation, transport distance of 6.4 km between farms and the available annual operating hours.
impact for sizeable buffer capacities. Other cost-influencing
factors are shown in figure 4.
The most significant influence on the costs is the decanter
capacity. By increasing the capacity by 50%, the unit costs
are reduced by 25% and by decreasing the capacity by 50%,
the unit costs are increased by 76%. The available operating
hours are the second most influential factor, while other
factors, in descending order of influence, include maintenance costs, monitoring time, and transport distance between
farms. In the case of the baseline scenario, the total costs are
3
System capacity, m3 x 10 y
divided into 72% variable costs and 28% capital costs.
Further, the variable costs are divided as follows: 63% for
wages, 28% for maintenance, and 9% for fuel, clearly
indicating the importance of wages and maintenance on
overall system costs.
For distances of 5 to 20 km between farms there is only a
minor increase in the costs, and even for 50 to 100 km
distances the cost reaches only 6%. The situation is different
for small on-farm buffer capacities (e.g., 50 m3) as the cost
increase for 100 km distances reaches 72%.
−1
100
operating hours available
(ranging from 2000 to 6000 hours yearly)
80
transport distance between farms
3
centrifuge capacity (ranging from 9.5 m h
preparation/dismantling of the mobile unit
−1
)
60
40
−60
−40
−20
0
20
40
60
Alteration, %
Figure 3. Sensitivity to changes in parameters influencing the systems capacity of the mobile unit with the reference scenario as default.
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APPLIED ENGINEERING IN AGRICULTURE
−3
$m
5.5
5.0
4.5
decanter capacity
available operating hours
monitoring time
transport distance in the case
of a 1000 m3 buffer capacity
transport distance in the case
of a 50 m3 buffer capacity
maintenance costs
4.0
3.5
3.0
2.5
2.0
−60
−40
−20
1.5
0
20
40
60
Alteration, %
Figure 4. Sensitivity to changes in parameters influencing the unit costs of the mobile separator with the reference scenarios as default.
UTILIZATION
The economy of slurry separation depends highly on the
capacity utilization, which is why the incentive is to treat as
much slurry on a yearly basis as possible in order to incur the
lowest costs. The operating costs were analyzed for different
amounts of treated raw slurry (fig. 5).
The unit costs vary between 2.7 and 7.0 $ m-3 depending
on the amount of processed slurry and different prerequisites
regarding buffer capacity and labor requirements for monitoring the separator during operation. By reducing the
supervision of the operation of the unit from 100% to 50%,
the costs are reduced by 7% to 11% for the small buffer
capacity, whereas the costs for the large buffer capacity is
reduced by 13% to 19%, both depending on the amount of
treated slurry. The increased reduction for the large buffer
capacity indicates the higher proportion of actual operating
time due to the larger buffer storage capacity.
SEPARATION EFFICIENCY
The chemical characteristics of the pig and dairy cattle
manure or anaerobically digested pig manure used in the tests
are given in table 2. Manure characteristics are affected by
animal categories but there is also a significant variation
within the animal categories.
The quantity of solids and the separation efficiency of dry
matter and nutrients were variable for different manure types
(fig. 6). The separation efficiencies (%) were 37-68, 50-83,
and 8-33 for DM, TP, and TN, respectively. The separation
efficiencies of total N and dry matter were dependent on the
dry matter content of the manure, while the separation
efficiency of total P was little affected by the dry matter of the
slurry.
The chemical characteristics of the solid fraction after
centrifugation are given in figure 7. The solid fraction
showed average concentrations of DM, TP, and TN, which
were 6-10, 9-11, and 2-2.9 times as great as the concentration
7.5
7.0
6.5
$ m −3
6.0
1000 m3 buffer capacity and 100% supervising
of the unit when in operation
5.5
1000 m3 buffer capacity and 50% supervising
of the unit when in operation
5.0
100 m3 buffer capacity and 100% supervising
of the unit when in operation
100 m3 buffer capacity and 50% supervising
of the unit when in operation
4.5
4.0
3.5
3.0
2.5
20
40
60
80
100
120
3
Quantity of raw slurry treated, m x 1000
Figure 5. Costs as a function of yearly decanter utilization. The reference scenario is a 1000-m3 buffer capacity and 6-km transfer distance.
Vol. 22(2): 185-193
191
Table 2. Average composition of finishing pigs, sows, dairy cattle, and
anaerobically digested manure from pigs before separation.
Finishing
Digested Pig
Pigs[a]
Manure
Sows
Dairy Cattle
Dry matter (%)
N-total (kg/t)
NH4-N (kg/t)
P-total (kg/t)
K (kg/t)
[a]
5.1 (2.4)
5.4 (1.2)
3.9 (0.7)
1.1 (0.6)
3.3 (0.6)
4.0 (2.4)
4.0 (0.8)
2.7 (0.4)
1.0 (0.7)
2.0 (0.7)
3.2 (2.5)
4.1 (0.6)
3.8 (0.7)
1.4 (0.9)
1.9 (0.3)
7.0 (0.8)
4.2 (0.4)
2.2 (0.3)
0.8 (0.1)
3.0 (0.6)
Standard deviation in parentheses.
Separation efficiency (%)
in untreated pig manure (sows and finishing pigs) and
digested pig manure. The solid fraction showed average
concentrations of DM, TP, and TN that were 3, 2.5, and
1.2 times as great as the concentration in untreated dairy
cattle manure. This clearly indicates that the centrifuge
concentrates the DM and TP content in the solid fraction by
centrifugation of pig manure, while the efficiency of
concentrating the DM and TP in the solid fraction from dairy
cattle manure is relatively lower. The efficiency of concentrating TN in the solid fraction is relatively low for all manure
types due to the fact that the centrifuge mainly transfers the
organic part of the nitrogen to the solid fraction, while most
of the dissolved NH4 + will stay in the liquid (Møller et al.,
2002).
CONCLUSIONS
A concept for the use of mobile separation has been
demonstrated and evaluated. The evaluation has included
operational and economic modeling and has been parameterized by on-farm analyses. The developed models were shown
to facilitate the evaluating of indicators, like operational
performance, separation efficiency, and economic effects of
different configurations as regards separator utilization,
operator routines, etc. The developed tools suite is considered to be well suited for supporting managers of mobile
separation systems.
Analysis of a mobile separation system in terms of labor
input and system performance showed an average separator
capacity of 19 m3 h-1 and an average fuel consumption of
9.1 L h-1. The yearly capacity varied between 30,000 and
90,000 m3, depending on available operations hours, transfer
distances between individual farms, degree of automation for
hooking-up to on-farm installations, etc.
The unit costs varied between 2.7 and 7.0 $ m-3 depending
on the amount of processed slurry and different prerequisites
regarding buffer capacity and labor requirements for monitoring the separator during operation. In order to maintain a
high yearly capacity, the buffer capacity must be in the range
of 500 to 1000 m3, where the exact capacity is determined
based on a balancing of the reduced costs of the mobile
separation against the increased on-farm investment in
storage tanks.
100
90
80
70
60
50
40
30
20
10
0
Digested manure
Sows
Finishing pigs
Cattle
Quantity Dry matter Total−N
of solids
(%)
NH4−N
Total−P
K
Mg
Cu
18
40
16
35
14
30
12
25
10
20
8
6
4
2
0
Ó
Ó
Ó
Ó Ó
Finishing pigs
Sows
Ó
Ó Ó
Ó Ó
Digested
manure
15
10
Dry matter content (%)
Concentration of nutrients (g/kg)
Figure 6. The average separation efficiency of the different manure categories. Error lines represent ±1 standard deviation.
P−total
Ó
N−total
K
Drymatter
5
0
Dairy cows
Manure type
Figure 7. Composition of the solid fraction after separation of the different manure types. Error lines represent ±1 standard deviation.
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APPLIED ENGINEERING IN AGRICULTURE
The most significant influence on the costs is the decanter
capacity. The available operating hours are the second most
influential factor, while other factors, in descending order of
influence, include maintenance costs, monitoring time, and
transport distance between farms. Only in the case of
non-adapted on-farm installations for buffer capacity may
the transfer distance significantly affect the overall system
performance and costs. Wages and maintenance make up
65% of variable costs, indicating the importance of these
costs on overall system costs.
The separation efficiency was high for TP, exceeding 60%
for all types of pig slurry, but it has not been possible to reach
the 20% of nitrogen for pig manure with low DM content
(<5%). The amount of solid fraction is highly variable
between manure types and is greatly affected by the dry
matter of the slurry; thus the amount of solid fraction from
dairy cattle slurry was 22.5% on average, while it was only
9.2% on average from slurry from finishing pigs.
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