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TitleFixed Film Anaerobic Digester
TagsAnaerobic Digestion Sewage Treatment Biogas Manure Environmental Technology
File Size157.6 KB
Total Pages3
Table of Contents
                            Fixed Film Anaerobic Digester
	System Design
	Intergral Part of Waste Management System
	Digester Operation
	Biogas Productions
	Additional Benefits
	Bottom Line
                        
Document Text Contents
Page 1

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.

Page 2

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.

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