Documents, Tools and Resources
Fixed Film Anaerobic
Digester
by Ann Wilkie
 A
state-of-the-art, fixed film anaerobic digester designed specifically
to meet the needs of the typical Florida dairy farm has been
installed at the Institute of Food and Agricultural Sciences
Dairy Research Unit (DRU) at the University of Florida. The
DRU is located in Hague, Alachua County, Florida. The goal
of the project is to demonstrate use of fixed- film anaerobic
digester technology at a working dairy to simultaneously treat
wastewater and produce energy in the form of methane gas. This
holistic manure treatment system not only stabilizes the wastewater,
but also produces energy, controls odors, reduces pathogens,
minimizes environmental impact from waste emissions, and maximizes
fertilizer and water recovery for reuse.
 Florida
dairies use large volumes of water for barn flushing, resulting
in large amounts of dilute wastewater. The most common manure
management system utilizes short-term holding ponds for flushed
manure wastewater storage, with subsequent pumping to sprayfields
to supply fertilizer nutrients and irrigation water for production
of forage crops. Although effective for nutrient recycling,
these systems can produce strong odors. The partial decomposition
of organic matter by anaerobic microorganisms is the primary
cause of odor in dairy livestock manure. The land application
of manure slurry volatilizes the malodorants and creates a
high odor nuisance potential.
Being a completely closed system, however, a fixed- film digester
allows more complete anaerobic digestion of the odorous organic
intermediates found in stored manure to less offensive compounds.
Analyses conducted using human odor panels showed a substantial
decrease (94 percent) in flushed dairy manure odor after fixed-film
anaerobic digestion at three-day hydraulic retention time (HRT).
By contrast, short-term storage (three days) of flushed dairy
manure was shown to exacerbate manure odor by 77 percent.
System Design
The basic fixed-film digester design consists of a tank filled
with plastic media on which a consortia of bacteria attach
and grow as a slime layer or biofilm hence the name fixed-film
digester. The media is fully submerged and wastewater flow
can be in either the upflow or downflow mode. As the wastewater
passes through the media-filled reactor, the attached and suspended
anaerobic biomass convert both soluble and particulate organic
matter in the wastewater to biogas, a mixture of mostly methane
and carbon dioxide. The biogas produced can be collected and
used either directly (e. g. for heating water) or in an engine
generator to provide electricity.
Immobilization of the bacteria as a biofilm prevents washout
of slower growing cells and provides biomass retention independent
of HRT. Since the bacteria are not continuously washed out
along with the effluent, a substantial microbial biomass develops
within the reactor. Because there are more bacteria for a given
reactor volume compared to conventional suspended-growth designs,
less time is needed to degrade the wastewater, allowing operation
at short HRTs typically in the range of two to six days. Fixed-film
digesters are ideally suited for treating large volumes of
dilute, low-strength wastewaters such as those generated by
Florida dairy operations (< 1 percent total solids), because
large numbers of bacteria can be concentrated inside smaller
digesters operating at shorter HRTs than would be needed to
achieve the same degree of treatment with conventional suspended-growth
anaerobic reactors. Also, fixed-film digesters have a smaller
footprint than conventional suspended-growth digesters an important
factor where land availability is limited.
Intergral Part of Waste Management System
The Alachua fixed-film anaerobic digester is designed to be
an integral part of the overall waste management system for
the 500-milking cow dairy and will serve as a model for the
Florida dairy industry. As constructed, the complete digester
system consists of a 100,000-gallon, fixed-roof digester tank;
a biogas collection and flare system; an influent feed pump;
a recycle pump; a desludging pump; a liquid level control structure;
and a mechanical building for housing pump controls, an air
compressor for powering the pneumatic feed pump, and biogas
utilization equipment. Vertically arranged, three-inch diameter
corrugated polyethylene drainage pipe, commonly used in septic
tank drain fields, is installed as the media in the digester
(four zones of 4 ft pipe). This widely available material offers
a low-cost solution to providing sufficient surface area in
the digester for microbial attachment. By simply changing the
position of a damper valve, flow direction through the media-bed
can be switched between upflow and downflow modes.
The milking herd at the DRU is confined to free-stall barns,
which are hydraulically flushed on an automated schedule. The
milking parlor apron is equipped with an udder washer which,
together with the milking parlor wash-down wastewater, flows
to a wastewater channel. In summer, misters in the free-stall
barns help to keep the cows cool and also contribute additional
water to the waste stream. Currently, the cows in the barns
are bedded on sand. The wastewater initially flows down the
collection channel to a sand-trap, where some of the sand is
recovered for reuse. After the sand-trap, the wastewater flows
through a channel to a mechanical separator, which removes
large fibrous solids. The wastewater flows across a settling
basin and then over a weir into a sump from where the digester
is fed. Solids removed by the separator and from the settling
basin are land applied.
Digester Operation
The digester tank is set on a custom-designed slab that has
a conical bottom for easy removal/ recycling of sludge. The
influent wastewater is pumped from the sump using a pneumatic
pump. The influent flow is monitored by the number of actuations
of the influent pump. The influent line runs through the wall
of the tank and into a central feed duct. In upflow mode, the
wastewater flows down through the feed duct into the area below
the media bed, then up through the media zones and back to
the feed duct, where it flows out the effluent line into the
level control structure.
A portion of the effluent is recycled through an open impeller
centrifugal pump back to the influent line. The digested effluent
flows down through an exterior culvert that houses the level
control structure and to a storage pond to be land applied
in accordance with the farm's nutrient management plan. A desludging
pump removes sludge via a line leading from the apex of the
conical bottom and recycles it back to the influent line through
the recycle pump. The recycle line is equipped with a meter
for continuous flow monitoring.
Biogas leaves the top of the digester tank through a two-inch
PVC line and passes through a sediment trap to a mass-flow
meter, prior to flowing through a pressure relief valve and
on to the flare. Both pressure and vacuum emergency relief
valves are located on top of the tank. The tank also has several
sampling ports for obtaining mixed liquor samples at various
radii from above, below and within each media zone.
Biogas Productions
The unit is operating at a three-day HRT at ambient temperature
and producing 6,000 ft3 of biogas per day at 80 percent CH4
:20% CO2 . The biogas produced from the digester is being flared
to reduce odors and emissions of methane, which is a potent
greenhouse gas.
Potential options for biogas utilization at the DRU include:
Production of hot water for use in the milking parlor; Generation
of electricity for on-farm use; Absorption refrigeration for
milk cooling; and Vehicular fuel. However, the total amount
of biogas that can be reliably produced in winter conditions
is not yet known. For now, the biogas will continue to be flared
until the best energy recovery option can be selected based
on accurate biogas production information.
Additional Benefits
In addition to minimizing offensive odors and producing usable
energy, anaerobic digestion has several other important benefits.
One advantage is nearly complete retention in the effluent
of the ferti lizer nutrients (N, P and K) that were in the
raw manure entering the digester. Also, a broad spectrum of
microbial pathogens can be destroyed by anaerobic digestion.
This may have particular significance for animal health in
recycle flush systems. The ability of the fixed- film digester
to provide recyclable water for barn flushing is especially
valuable as water becomes an increasingly precious limited
resource.
 Pretreatment
by mechanical screening followed by high- rate methanogenesis
in a fixed-film reactor is a novel concept in animal waste
management. The primary benefit of separation of solids from
liquids is the production of two fractions that are inherently
more manageable than the original slurry. Minimizing waste
stream solids avoids clogging problems and/or impaired biofilm
activity. The benefits in terms of ease of materials handling,
production of a highfiber by-product, and reduction in digester
volume requirements are substantial. Although digested wastewater
solids produce biogas, fixed- film digesters achieve the highest
treatment efficiency with diluted wastewater, or low-solids
concentrations. Fibrous solids have potential use for bedding,
refeeding and horticultural applications.
Bottom Line
The financial attractiveness of on-farm anaerobic digestion
is likely to improve considerably as environmental concerns
assume greater importance. A complete assessment of economic
feasibility must take account of all avoided costs and the
value of intangible environmental externals, such as odor control
and reduction of volatile emissions. Environmentally sound,
sustainable practices are essential for staying in business
over the long run. In order to coexist with their neighbors,
intensive livestock operations must manage both effluents:
the odors as well as the manure. The alternative may be to
relocate the enterprise, or cease operations entirely. The
fixed-film anaerobic digester offers an innovative technology
option for waste management without limiting the enterprise.
The process can be adapted to an individual situation and incorporated
into the existing manure management scheme. The Alachua digester
is destined to play a significant role in promoting the use
of fixed-film anaerobic systems for processing of flushed dairy
and swine manures.
This project was supported by a grant from the Florida Energy
Office. Ann Wilkie is an associate professor with the Soil
and Water Science Department at the University of Florida,
Gainesville. She can be reached by e-mailing acwi@gnv.ifas.ufl.edu.
JOURNAL OF COMPOSTING & ORGANICS RECYCLING
419 State Avenue
Emmaus, PA 18049-3097
610- 967- 4135
www.biocycle.net 
Reprinted From: September, 2000 |
