|
Anaerobic Digesters
(This
portion of the web site is co-sponsored by the U.S. Department
of Energy and
the Blue Moon Fund.)
Anaerobic
digestion processes take place in a warmed, sealed airless container
(digester)
that creates the ideal conditions for the bacteria to ferment
the organic material
in oxygen-free conditions. The digestion tank needs to be warmed
and mixed thoroughly to create the ideal conditions for the
bacteria to convert organic matter into
biogas.
The
biogas is composed of 60 to 70% methane (CH4), 30-40%
carbon dioxide (CO2),
and trace elements of other gases. The biogas (once cleaned)
can be utilized to:
•
Produce both electric power & recycled heat through an
engine generator set (i.e. combined heat and power)
•
Produce heat through a boiler
•
Be injected into a natural gas pipeline (requires extensive
cleanup)
•
Converted to a compressed or liquefied clean transportation
fuel
The
other by-product from the anaerobic digester is the effluent
which consists of
the non-digestible products. The effluent consists of solids
(fiber) and the liquid filtrate
which are put through a press to separate. The effluent retains
the nutrients while the odor is removed. The effluent streams
can be utilized for:
•
Solids (fiber)… compost, animal bedding, pellet/granule
fertilizer, fuel pellets, medium density fiberboard, decking
•
Liquid (filtrate)… liquid fertilizer
•
heated to 86°F -105°F (30°C - 41°C) - optimal
approx. 98°F (37°C)
•
residence time is typically 15 - 30 days
•
tends to be more robust and tolerant than the thermophilic process
•
gas production is less and larger digestion tanks are required
compared to the thermophilic process
•
sanitization (if required) is a separate process stage
•
heated to 122°F - 135°F (50°C - 57°C ) - optimal
approx. 130°F (54°C)
•
residence time is typically 12 - 14 days
•
offer higher methane production, faster throughput, better pathogen
and virus
kill
•
requires more expensive technology, greater energy input and
a higher degree of operation and monitoring than mesophilic
process. The information in this market sector
is divided in the following sections:
Basic Digester
Designs
1.
Batch-Type Digesters are the simplest to build. Their
operation consists of
loading
the digester with organic materials and allowing it to digest.
The retention
time
depends on temperature and other factors. Once the digestion
is complete, the effluent is removed and the process is repeated.
2.
Continuous Digesters are constantly or regularly fed.
The organic material moves through the digester either mechanically
or by the force of the new feed pushing out digested material.
Unlike batch-type digesters, continuous digesters produce biogas
without the interruption of loading material and unloading effluent.
- There
are three types of continuous digesters: vertical tank systems,
horizontal tank or plug-flow systems, and multiple tank
systems.
- Proper
design, operation, and maintenance of continuous digesters
produce a steady and predictable supply of usable biogas.
They may be better suited for large-scale operations. Biogas
from the anaerobic digester(s) at the WWTFs contains approximately
60% methane, which can be used as a “free”
fuel to generate electricity.
Factors in
Selecting the Type of Digester
•
type and quantity of substrates (feedstocks)
•
substrate total solids (TS) concentration
•
facility location
•
the existing (or planned) manure handling system at the facility
•
other factors (i.e. bedding materials)
Digester Technologies
Covered
Lagoons
•
treat and produce biogas from liquid manure with less than 3%
solids
•
require large volumes of manure
•
require depths > 12 feet with fitted cover to collect biogas
•
are compatible with flush manure systems in warm climates
•
may be used in cold climates for seasonal biogas recovery and
odor control
•
normally low cost with proven reliability
•
are engineered, heated, and round/rectangular tanks (above or
in-ground)
•
treat slurry manure with 3-10% total solids
•
require less land than lagoons
•
are compatible with combinations of scraped and flushed manure
•
good for mixed substrates (feedstocks)
•
tolerates some bedding
•
are engineered, heated, and rectangular tanks (mostly in-ground)
•
heated to 100
•
treat scraped dairy manure with 11-13% total solids
•
cannot treat swine manure due to its lack of fiber
•
steady gas output with no climate limits
•
tolerates some bedding
•
not good for mixed substrates (co-digestion)
Two
Staged Mixed Plug Flow Digesters (GHD Design)
•
In-ground concrete vessel
•
heated to 101
•
two chamber operation (acid and methanogenic)
•
treat scraped dairy manure with 11-13% total solids
•
good for dairy manures, can handle mixed substrates (co-digestion)
•
tolerates some bedding
•
consist of a tank filled with
plastic media
•
contain media that supports a thin layer of anaerobic bacteria
called biofilm (or fixed-film) that when waste manure is passed
through the media, biogas is produced
•
are best suited for dilute waste streams typically associated
with flush manure handling or pit recharge manure collection
•
can be used for both swine and dairy wastes (separation of dairy
manure is required to remove slowly degradable solids)
Digester
Tech Factors Affecting the Digester Operations
•
Type of substrates (feedstock)
•
The concentration of the substrates
•
The temperature (mesophilic / thermophlic)
•
pH and alkalinity levels (pH between 6.8 and 8 or production
decreases)
•
Retention Time (flow of substrate through the system)
•
Digester loading rate
|
