Documents, Tools and Resources
Complete Mix Digester (Code No. 354i)
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 manure is 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 complete mix digester with a total solids (TS) concentration
between 2.5% and 10%.
Criteria
General Criteria:
Manure Characteristics. This practice is applicable
only to manures that are collected 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
complete mix digester shall contain 2.5 -10% 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 complete mix 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 complete mix 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 complete mix digester shall
be sized to retain 15 - 20 days of manure production and
water as needed for dilution.
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Configuration. The complete mix digester may be a square,
rectangular or circular tank. Conservation practice standards
are reviewed periodically, and updated if needed. To obtain
the current version of this standard, contact the Natural
Resources Conservation Service.
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Operating Depth. The operating depth of a complete mix
digester shall be 8 feet or greater.
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Floor. The complete mix digester floor may be conical
or 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 complete mix digester
below the permanent liquid level.
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Outlet. The complete mix 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.
Mixing. The digester shall be equipped with a mixing
device that is capable of breaking up floating solids or mats
that may develop and suspend settled solids on a daily or more
frequent basis.
Operating Temperature. The digester shall be maintained
between 35° and 40° C (95° - 103° F) with
an optimum of 37.5° C (100° F) and daily fluctuation
of digester temperature limited to less than 0.5° C (1° 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 complete mix 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 complete mix 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 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
Conservation practice standards are reviewed periodically,
and updated if needed. To obtain the current version of this
standard, contact the Natural Resources Conservation Service.
from the complete mix 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".
Total Solids Concentration. The influent total solids
concentration should be 2.5 - 10%, but as high as practicable
for the livestock production facility. Excessive water addition
increases digester size and construction cost without increasing
benefits.
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 complete mix digester
may require up to 50% 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 ruminant manure
digester effluent for solids separation treatment may be considered
due to the potential value of digested, separated solids for
bedding, or soil amendment. Non-ruminant digester effluents
contain only a minimal amount of recoverable solids.
Visual Screening. Vegetative screens or other methods
should be used to shield the complete mix 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 complete mix 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.
Mixing Equipment. The complete mix digester mixing
equipment should be designed by a competent designer familiar
with these systems. Supplier shall warrant the mixing equipment
for the intended use.
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. Complete mix digester 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 complete mix digester.
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Proper operating level of the complete mix 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. |
