COMPOST
OPERATORS
FORUM
Increased
flow of yard
trimmings
led Sonoma
Valley
Compost to
evaluate
alternatives
to the turned
windrow
methodology
(left) to
handle more
materials on
the same
footprint.
Increasing Feedstock
Throughput On A
Smaller Footprint
Analysis of windrows and aerated
trapezoidal piles of varying depths
highlights options to increase capacity
on the same footprint.
Jan Allen and Will Bakx
HE ability to expand throughput
capacity at composting facilities is
often a space constraint issue. At
sites with limited land area to grow,
technology becomes the key factor in
the search for increased capacity and
expansion. Evaluating which approach is the most appropriate is a
matter of finding the balance between optimizing space efficiency,
technology, residence time, and cost.
For many composting operations,
T
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the initial size of the site is rarely the
ultimate development size. When an
operation starts, the owner often
doesn’t realize how large the program
will become or what the volume of the
waste stream will be. New diversion
programs come on line, and seasonal
variations in the waste stream lead to
larger waste flows that may not have
been anticipated. As a result, many
sites have had to grow reactively over
the years, instead of in an organized
or deliberate way. Critical components for successful facility management, such as adequate space for final screening and product storage,
are sometimes compromised as operators scramble for room for active
composting piles.
Sonoma Compost Company in
Petaluma, California is a yard trimmings composting facility owned by
Will Bakx and his partners. The site
is located at the Sonoma County Central Landfill. Turned windrows are
currently used. The facility has a set
amount of real estate and Sonoma
Compost needed to increase its processing capacity from its original size
because the county and most cities it
services began collecting yard trimmings weekly rather than biweekly,
dramatically increasing the amount
of incoming feedstock. In 2005, the
site often processed over 250
tons/day. Sonoma Compost decided
to work with CH2M Hill to conduct a
pilot study to evaluate if aerated static piles could significantly increase
the feedstock throughput on the
same physical footprint. The company also wanted to monitor the impact
on odors and leachate management
at the same time.
Space Efficient Processing Options
Outdoor windrow composting remains one of the most common technologies, primarily because of its
fairly low entry costs compared to
methods using aeration systems
and/or enclosures. The rate of composting typically is determined by
frequency of pile turning, moisture
addition, etc. Where windrow turners are used, the pile height and
SEPTEMBER 2006
grinding initially be- Figure 1. Graphical comparison of windrows
gins the process with
Static pile
Static pile
trapezoidal piles that
with walls
are 25 feet deep. The
material, as received, is
Trapezoidal pile
blended with sawdust
and bark dust and then
the piles are built with
excavators on a pad
that has air plenums
built into the floor. The
piles are aerated under
negative pressure, and
turned about every
three weeks with an excavator. Despite the 25Turned windrows
foot depth, a tremendous amount of air is pulled any series of pile shapes. Figure 1 is
through the pile. Approximately a graphical comparison of the three
45,000 cy of material are in each configurations. The pile in the rear
pile; sections within the pile repre- of the figure represents the deep
cross section described above, where
sent different batches.
Table 1 compares the volume of there is a sidewall to actually make
material that can be processed in a the cross section larger.
turned windrow, trapezoidal pile and
deep static pile. A one-acre footprint Estimated Residence Time,
is assumed for each composting Related Capital Allocations
The greatest capital investment
method (calculated by computing the
volume of a single pile and then ex- for a composting facility is in the acpanding that to a full acre footprint tive composting and curing processwith a series of piles to fill the acre). es. In turn, processing capacity of
According to the assumptions shown those two phases is determined by
in the table, 2,900 cy, 13,400 cy and pile volume and residence time. Dif19,000 cy of raw material/acre can be ferent feedstocks require different
processed using the turned windrow, residence times. In addition, resitrapezoidal pile and deep static pile dence time is determined by technology and the quality specifications
methods respectively.
When looking at this data, one’s for the product. Some technologies
first impression may be that the dif- can compress the residence time
ferences in volume have to do with more than others. Of course high
depth of the pile (7-feet, 12-feet and quality products usually require
20-feet for windrow, trapezoidal and more time.
Residence time for active comstatic piles respectively). But volume differences are not only at- posting and curing varies by feedtributed to pile depth. Other factors stock (the wide range is primarily a
include the amount of open pave- factor of technology used): Manure
ment between the piles and the side composting — 20 to 60 days; Food
slope to the piles. On a proportional residuals composting — 30 to 60
basis, there is 6.6 times more pile days; Biosolids composting — 40 to
volume on a given acre using 20-foot 84 days; Yard trimmings composting — 50 to 100 days. Yard trimstatic piles versus 7-foot windrows.
This exercise can be done with mings have so much wood and hardto-degrade cellulose and lignin, that
there is usually a longer residence
space efficiency for various
time than with more biodegradable
feedstocks. Generally speaking, active composting is usually the first
Deep
two or three weeks of the process,
Turned
Trapezoidal
Static
and may be as long as four weeks.
Windrow
Pile
Pile
Other parameters that come into
play
when evaluating the costs to
7
12
20
expand throughput on the same
18
100
200
footprint include storage, traffic and
149
149
149
logistics (machine shop, storm wa4
76
160
ter system, parking, etc.). Those ele135
125
109
10,395
132,000
392,400
ments require considerable space
385
4,889
14,533
but do not have the same cost factor
2,900
13,400
19,000
as active and curing phases of com1.0
4.7
6.6
post. In terms of storage, some facilities need capacity to store 270 days
width are determined by the equipment design.
To evaluate other technology options based on the amount of material that can be processed on a set footprint, calculations were done using a
turned windrow system as a baseline, and then compared to a trapezoidal pile and a deep static pile. The
following are descriptions of trapezoidal versus deep static piles:
Trapezoidal Pile: A trapezoidal
pile is usually an aerated static pile,
built with conveyors, wheel loaders
and in some cases, excavators. The
footprint difference between a
windrow and trapezoidal operation is
significant, starting with the height
of the piles, which can be upwards of
14 feet. A dairy using this method
had trapezoidal piles built 12-feet
deep by 60-feet wide and 120-feet
long. A negative air system, combined with a biofilter, was used for
process aeration and odor control.
Approximately 2,200 cubic yards of
material are in each pile. A windrow
system with center-to-center pile
spacing on the same footprint (60feet wide by 120-feet long), with a pile
height of 7-feet, can process 750 to
800 cy in total (provided the windrow
machine has maneuvering clearance
at each end of the 120 foot windrow).
The assumption is roughly three
windrows with two aisles (about 8feet wide) in the 60-foot wide space.
Each foot of windrow holds approximately 2.2 cy of material.
Deep Static Piles: Deep static
piles can be built with or without
aeration, and in any length and
width. Facilities use front-end
loaders, excavators, and conveyors
to build the piles. If above ground
piping is used, additional space is
required for removing reusable
pipes before moving a pile. Nonaerated systems and below grade aeration piping avoids this loss of active composting space. In one case,
a yards debris facility that does no
Table 1. Comparison of
technologies
Measurement
Height
Bottom width
Length
Top width
Nominal length
Cubic feet
Cubic yards per pile
Cubic yards per acre
Space efficiency (ratio to windrow)
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SEPTEMBER 2006
of compost production or longer.
With everything taken into account,
the area allocation for the active and
curing area may only be 16 to 22 percent — when added together — of
the entire site. And most of the capital budget will be spent on that 16
to 22 percent. (Storage typically is
45 to 60 percent of the acreage; traffic and logistics typically are 25 to 35
percent.) Given challenges with permitting, if capacity is really the issue for composting facilities, there
are some parts of the process that
can be exported (or relocated),
specifically the storage of the finished product.
Sonoma Compost Pilot
The pilot at Sonoma Compost
was set up to evaluate trapezoidal
aerated static piles. All aeration
pipes were above grade. Piles were
run on negative pressure, with process air treated in a biofilter. Going
into the pilot, Sonoma Compost decided to “hybridize” the trapezoidal
aerobic static pile (ASP) process by
creating three stages — ASP,
windrow, ASP. The primary reasons for the hybrid approach were
the anticipated higher cost of
achieving PFRP (process to further
reduce pathogens) in the initial
aerated static piles, and the benefit
of increased particle size reduction
in the windrow phase. Sonoma
Compost realized that meeting
PFRP in an ASP system would require an insulating layer of ground
yard trimmings — about 1.0 to 1.5
feet deep — on the piles in order for
all material to be exposed to 55°C
temperatures for three days. The
owners reasoned that meeting
PFRP during the windrow phase,
while concurrently achieving the
desired particle size for end product
screening (to minimize screen
overs), made more economic and
operational sense.
In the primary phase, where the
biological activity is the most active,
900 cubic yards of yard trimmings
were placed in a trapezoidal pile.
The airflow was set at 1,600 cfm. After three weeks, the pile was moved
to standard windrows. Piles were
turned once/week. After three
weeks, the piles went through the
third phase in the aerated static pile
for curing and to actively bring down
the moisture content to approximately 35 percent. The airflow for
this phase was found to be sufficient
at 900 cfm.
A key goal of the pilot was to find
out if this approach improved
throughput capacity. Clues to the
answer involved two variables —
the footprint of the piles, and the
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residence time. The loading capacity per running foot of an aerated
static pile was 10 cy, and the
windrows held about 2 cy. The aerated static piles also compressed the
overall residence time. The total
composting time for the aerated
static piles was 70 days compared to
98 days for windrow composting
achieving similar compost maturity.
When calculating the space required, clearances had to be factored
in for pulling out the above ground
piping. Allowing for that, the aerated static pile system could process
roughly twice as much per day on a
given plot of land — 30 cy/day versus 15 cy/day for windrows (in an
area of 200-feet by 80-feet). And if
the piping were put below grade
with aerated pavement, that production rate would go up considerably because the open space needed
for pulling out the pipes wouldn’t be
necessary. The production rate
would go up to 52 cy/day. However,
below ground piping was not a realistic option as the existing cement
pad would have to be torn up to install the pipes.
The finished compost quality
(based on compost maturity) from
the hybrid ASP system was equal to
that achieved with the windrows —
despite the shorter total composting
time with the ASP approach. In
terms of particle size reduction, using the trapezoidal ASP system,
without the benefit of turning with
the Scarab windrow turner, would
result in a larger final particle size,
and thus a much higher percentage
of screen overs (and a loss of end
product revenue). However, Sonoma
Compost found that the initial ASP
phase actually improved the ratio of
desired screened product to screen
overs. This was attributed to the initial ASP phase softening the larger
woody particles so that they broke
down better during agitation with
the turner.
In terms of odor emissions and
other environmental considerations, no malodors were generated
when the aerated static piles were
broken down after the initial active
composting phase. Additionally,
minimal leachate came out of the
aerated piles. The aeration system
was a positive factor in controlling
excess moisture after rainstorms.
(The pilot project was designed to
capture any leachate in an underground trap; the leachate was
pumped back onto the primary
static piles that still had to meet
PFRP temperatures as moisture
addition.)
Ironically, shortly after Sonoma
Compost committed to the pilot, it
learned that the county could make
an additional three acres of space
available adjacent to the existing
site. Sonoma Compost decided to
conduct the pilot anyway in order to
answer the questions about increased throughput, odor control
and ammonia and CO2 emissions.
The biofilter used in the pilot was
considered to be effective in treating
odors and reducing emissions.
At the present time, Sonoma Compost has decided to continue utilizing turned windrows for yard trimmings composting, taking advantage
of the adjacent space. Although more
material can be composted per acre
using trapezoidal piles, not enough
capacity is gained in total because of
the need to use above ground piping
(and there are increased labor costs
to remove the pipes).
Capital And Operating Cost
Comparisons
A further exercise was done by
CH2M Hill to compare both capital
and operating costs for windrow,
versus 12-foot deep and 15-foot
deep trapezoidal piles (aerated and
both 100-feet in length). A facility
designed with a volume capacity of
10,000 tons (estimated at 100
tons/day) was used as the baseline.
The following basic assumptions
were used to evaluate the windrow
and two trapezoidal pile systems:
Concrete slab surface under piles;
800 lbs/cubic yard density; $5/sq. ft.
pavement cost; 25 gallon/hour of
diesel use at $3/gallon; 50 kW electric rate at $0.06/kWhr; $30/hour
labor rate; seven year loan at 8 percent interest; $100/hour front-end
loader hourly cost; 350 cy/hour “re-
The aerated trapezoidal pile (above)
used in the Sonoma Compost pilot for
the first phase of composting
contained 900 cubic yards of material.
SEPTEMBER 2006
Table 2. Calculating production costs/ton of windrow
versus trapezoidal pile composting
Parameters
Capacity is gained via the increased
height of the piles (above), and by
eliminating space between the
windrows.
claim and rebuild rate” for managing the trapezoidal piles. The
windrow was assumed to have a 98
day residence time; the trapezoidal
piles have a 70 day residence time.
Table 2 provides the capital and
operating costs calculated for the
three scenarios. In terms of capital
costs, the windrow system has the
highest capital cost ($9.62/ton) due
mostly to the cost of concrete slab
needed for the acreage (7.34 acres
for the windrows vs. 1.85 and 1.50
acres for the 12-foot high and 15-foot
high trapezoidal piles respectively).
Conversely, machinery costs were
significantly higher for the trapezoidal piles. Capital costs for those
were $5.12 for the 12-foot pile and
$4.12 for the 15-foot pile.
The windrow system has the lowest operating cost — $2.76/ton/day
— versus $2.94/ton/day for both the
12-foot and 15-foot high trapezoidal
Windrow
Capital Costs
Tons/day
102
Acres required
7.34
Pavement
$1,597,944
Machinery
$267,857
Subtotal
$1,865,801
Annualized cost
$358,369
Capital costs/ton
$9.62
Operating Costs
Tons/day
102
Labor/day
$80
Pile reclaim and rebuild/day
$Power/day
$201
Subtotal
$281
Operating cost/ton
$2.76
Production cost per ton
$12.38
piles. The windrow system had the
highest labor cost ($80/ton/day versus $49/ton/day for the trapezoidal
piles) whereas it had zero cost for
pile “reclaim and rebuild” versus
$204/day for the trapezoidal piles.
When taken together, based on this
particular exercise, total per ton
production costs are calculated at
$12.38/ton for windrow, $8.07/ton
for the 12-foot high trapezoidal pile
and $7.06/ton for the 15-foot pile.
When comparing these numbers,
other factors need to be kept in
mind. For example, in the Sonoma
Compost trial, it was decided to add
Trapezoidal Pile
12’ High x 100’ Long 15’ High x 100’ Long
143
1.85
$402,102
$989,011
$1,391,113
$267,194
$5.12
143
1.50
$326,656
$791,209
$1,117,865
$214,711
$4.12
143
$49
$204
$167
$421
$2.94
$8.07
143
$49
$204
$167
$421
$2.94
$7.06
in an interim windrow stage primarily to reduce particle size (and
achieve pathogen reduction). Without that step, screen rejects were
significantly higher, which is a revenue loss for a company with highend compost markets.
Jan Allen is a Principal Technologist
with CH2M Hill in Seattle, Washington. Will Bakx is a partner in Sonoma
Compost Company (www.sonomacompost.com) in Petaluma, California.
This article is based on a presentation
by Jan Allen at the 2006 BioCycle West
Coast Conference in Portland, Oregon.
Reprinted From:
September, 2006
ADVANCING COMPOSTING, ORGANICS RECYCLING
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