Anaerobic
Sludge Digestion Process
Anaerobic sludge digestion
is widely used to stabilize concentrated organic solids removed from settling
tanks, biological filters and activated sludge plants. Digestion renders
the sludge suitable for reuse purposes such as land fill and garden mulch
(local authority permitting).
EPCO Australia designs
systems and equipment such as:
Digester
Roofs
Digestion
tanks are normally covered with concrete or steel roof structures of
different shape and structural design. Dome shaped covers are often
used both in steel or concrete construction, steel covers can be fixed
or floating. Floating covers in primary digestion tanks have a number
of advantages. The more uniform gas pressure provides a high degree
of operating safety and the maintained relationship between liquid level
and mixing devices fitted on floating covers results in greater operating
consistency.
Dry seal type
gasholder covers are generally used in large treatment plants where
gas storage is separate from the digesters. The piston is provided with
a balancing system. The main seal is made from a special synthetic rubber.
Concrete ballast is sized for the desired operating pressure. The gasholder
is available as a single-lift or two-lift design. Gasholders may also
be of the wet seal single lift type.
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| Liverpool
Digester cover prior to lifting into its tank. |
Sludge
Mixing
Sludge mixing
in the primary digestion tank is carried out to mix the incoming sewage
with the digesting sludge and to maintain a uniform liquor temperature
in order to optimise bacterial activity. Mixing also disperses any scum
that may form on the surface of the digester liquor.
Mixing in
the digester tank can be carried out by mechanical mixing or gas diffusion
type mixing systems. Selection of the mixing system is dependent on
the size and type of treatment system and the means of subsequent sludge
disposal.
 |
 |
| Typical
lance mixing system |
Digester
cover with ballasts installed |
Sludge
Heating
Heating of primary digesters
maintains sludge temperature. The heat exchanger can be located inside
or outside the digestion tank. Internal heating systems are normally combined
with the sludge mixing function of the plant such as internal or external
draft tubes. Modern practice uses an external heat exchanger. Design
parameters to be followed in sizing heat exchangers are of paramount importance,
as overheating beyond narrow limits can cause perpetual sludge caking
and associated maintenance will inhibit the operation of the digestion
process whereas inadequate heating will not achieve the intended goals.
Waste
Gas Burning
Surplus gas produced in
the sludge digester is burned to reduce plant green house gas emissions.
The unique burning system of our Groth range of gas flares contain flame
retention vortex vanes and when coupled with the ¬downdraft preventer˙
in the shield the result is an updraft and an air mixing action that
provides a highly efficient, smokeless and odourless burn and virtually
eliminates the possibility of the flame being blown out. The wind shield
is designed to control outside winds up to 200 MPH, and operate efficiently
in heavy rains.
