Documents, Tools and Resources
Plug Flow Digester (Code No. 363i)
Definition
A constant volume, flow through, controlled temperature tank designed for methane
production and recovery in conjunction with a separate waste storage facility
at a livestock production operation with manure of suitable consistency for
this process.
Purpose
To produce and recover methane as an energy source while minimizing odors.
Conditions Where Practice Applies
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where ruminant manure is scrape collected from concrete
surfaces weekly or more often.
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where methane production and recovery are components
of a planned livestock waste management system.
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where existing waste impoundment(s) can be modified to
the requirements of this standard or for new construction.
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where the influent manure can be collected and delivered
to the plug flow digester with a total solids (TS) concentration
between 11% and 14%.
Criteria
General Criteria:
Manure Characteristics. This practice is
applicable only to ruminant manures that are collected by solid
scraping equipment. Manure should be fresh, less than 7 days
old, with minimal amounts of soil, sand, stones or organic
bedding material. Clumps of long straw should be excluded and
managed separately.
Total Solids Concentration. Manure influent
to the plug flow digester shall contain 11-14% total solids.
Water or wastewater, other than needed for dilution to achieve
design total solids concentration, shall be excluded.
Rainfall Runoff. Runoff water, clean or
contaminated, shall not be allowed to enter the digester.
Safety. If the plug flow digester will create
a safety hazard it shall be fenced and warning signs posted
to prevent children and others from using it for purposes other
than intended.
Regulatory. Local, state and federal laws
shall be complied with; there may be regulations that affect
animal population, distance separations, storage volumes, storage
periods, land requirements for nutrient application, fencing,
or visual screening in excess of recommendations contained
herein.
Digester Criteria
Tank Characteristics. The plug flow digester
tank shall be corrosion protected metal or reinforced concrete,
above or below ground, with allowances for manure entry and
exit, heat pipe entry and exit, and special consideration for
solid concrete covers or attachment of secured, inflatable
covers. Structural digester components shall meet the criteria
of Practice Standard 313, Waste Storage Facility, and the following:
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Design Operating Volume. A plug flow digester shall be
sized to retain 18 - 20 days of manure production and water
as needed for dilution.
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Configuration. The plug flow digester may be either a
straight, flow through rectangular tank or, a rectangular
or circular tank divided in half that forces flow down
the tank, around the end of the center wall and back to
an outlet. The ratio of the length to the width of a rectangular
plug flow digester shall be between 3.5:1 and 5:1.
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Operating Depth. The operating depth of a plug flow digester
shall be 8 feet or greater.
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Width to Depth. The ratio of the width to a depth of
a plug flow digester shall be less than 2.5:1.
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Floor. The plug flow digester floor shall be flat.
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Temperature Control. The tank shall be equipped with
a heat exchanger designed to maintain the digester at the
operating temperature. The heat exchanger within the digester
shall be black iron, steel, copper, or aluminum (Galvanized
iron will not per permitted) located below the normal operating
fluid level. Tanks shall be equipped with temperature sensors
for monitoring internal temperature. The tank surface,
walls, and floor shall be insulated as required by local
climatic conditions to reduce heat loss and maintain the
design operating temperature. Each completed design shall
include a summary of the heat balance computations for
the heat exchanger and the digester tank at design operating
conditions for the mean low winter and mean high summer
temperatures.
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Inlet. Inlets shall be of any permanent type designed
to resist corrosion, plugging, freeze damage, and prevent
gas loss. The inlet shall enter the plug flow digester
below the permanent liquid level.
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Outlet. The plug flow digester shall be equipped with
an outflow device such as an underflow weir, that will
maintain the operating level, maintain a gas seal under
the cover, prevent gas loss, and release effluent directly
to separation facility or waste storage facility.
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Embankments and disturbed areas surrounding the facility
shall be treated to control erosion.
Operating Temperature. The digester shall
be maintained between 35 o and 40 o C (95 o -103 o F) with
an optimum of 37.5 o C (100 o F) and daily fluctuation of digester
temperature limited to less than 0.5 o C (1 o F). Special circumstances
may allow higher operating temperatures.
Operating Level. The minimum distance provided
between the maximum operating level and the top of the digester
wall for plug flow digesters shall be as follows:
floating or inflatable tops............6 inches
solid top..................................18 inches
Cover. The cover shall be designed to capture
and convey the methane to a designed gas outlet. The cover
shall also collect and direct precipitation to a designed outlet
while exposed to site climatic conditions for its design life.
Capital cost, repair technique, and warranty life should be
considered when selecting a cover.
Gas Outlet. A digester gas outlet shall be
installed to safely convey biogas to its intended use or to
a flare where it can be safely burned.
Weather Protection. In areas of extreme wind
or excessive snow, appropriate structures may be necessary
to protect a secured inflatable digester cover from damage.
Waste Storage Facility. The waste storage
facility shall meet the requirements of Practice Standard 313,
Waste Storage Facility. The volume of the digester shall not
be considered in determining the storage requirement of the
waste storage facility.
Considerations
Location. The plug flow digester should
be located as near the source of manure as practicable and
as far from neighboring dwellings or public areas (minimum
distance of 91 m (300 ft)) as possible; proper location should
also consider slope, distance of manure transmission, vehicle
access, wind direction, proximity of streams and floodplains,
and visibility. The plug flow digester should be located near
a suitable site for energy utilization equipment. Short distances
for the transmission of methane through buried pipe are preferable.
Location of the waste storage facility should consider elevation
and distance from the plug flow digester to take advantage
of gravity flow.
Collection/Mix Tank. A collection/mix tank
may be required to accumulate manure, settle foreign material
and pretreat influent waste to the appropriate total solids
concentration. A volume equal to two days of manure collection
is recommended.
Location of Inlet and Outlet. The inlet and
outlet devices shall be located as far apart as practical to
minimize "short circuiting".
Gas Collection Cover. Gas collection cover
can be a floating cover, secured inflatable fabric cover or
a solid cover.
Cover Design. A variety of digester cover
designs can be considered to meet the needs of the farm. A
secured, inflatable cover allows for 4 - 12 hours of biogas
storage. A solid cover does not allow for biogas storage.
Insulation. A design heat loss calculation
should be completed and certified by a competent designer.
Four feet of earthen backfill to within 1 foot of the top of
the digester will usually provide adequate insulation. In cold
climates, the surface of the digester and a portion of the
side walls may require additional insulation. Above ground
digesters will require insulation.
Gas Utilization. The most beneficial use
of the biogas energy must be investigated and selected. A plug
flow digester may require up to 40% of the biogas heat value
to maintain the design temperature in the winter in cold climates.
Heat can be recovered in cooling water from internal combustion
engines or must be produced for the digester.
Effluent Tank. An effluent tank to hold
digester effluent for solids separation treatment may be considered
due to the potential value of digested, separated solids for
bedding, or soil amendment.
Visual Screening. Vegetative screens or other
methods should be used to shield the plug flow digester from
public view and improve appearance.
Plans and Specifications
Plans and specifications shall be prepared in accordance with
the criteria of this standard and shall describe the requirements
for applying this practice to achieve its intended use.
Heating System. The plug flow digester heating
system should be designed by a competent designer familiar
with these systems. The completed drawings shall include a
summary of the design parameters and performance limits for
the heating system.
Cover. The cover manufacturer shall warrant
the cover for the intended use and design life, provide maintenance
instructions, and certify that the cover is properly installed.
Gas Use. Plug flow gas use system should
be designed by a competent designer familiar with this equipment.
Supplier shall warrant the gas use equipment for the intended
use.
Operation and Maintenance
An operation and maintenance plan shall be developed that
is consistent with the purposes of the practice, its intended
life, safety requirements, and the criteria for its design.
The plan shall contain operation and maintenance requirements
including but not limited to:
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Proper loading rate of the plug flow digester.
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Proper operating level of the plug flow digester.
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Digester temperature control.
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Estimates of methane production and recovery.
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Identification of a plan for safe use or flaring of biogas.
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Environmental considerations for handling/utilization
of effluent.
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Cover and other component maintenance.
Conservation practice standards are reviewed periodically,
and updated if needed. To obtain the current version of this
standard, contact the Natural Resources Conservation Service. |
