Anaerobic Sludge Digester
This unit operation employs microorganisms to convert industrial waste water into readily disposable digested sludge. Anaerobic digestion is a bacterial process that breaks down organic materials within waste in the absence of oxygen. Itis generally run in closed tanks (1). Generally, biomass consisting
of sewage or processing wastes is mixed with water and fed into
the digester without air. The waste stream generally contains
fats, oils, greases. These waste streams are produced in
processes involving the the manufacture of detergents and soaps,
as well as within the petrochemical industry. Municipal sewage
treatment facilities also use sludge digesters. The generalized
equation for anaerobic sludge digestion is:
Organic Matter + Combined Oxygens --------->Anaerobic Microbes +
+ Energy for life processes + CH4 + CO2 + Other gases
The combined oxygens consist of CO3-2, SO4-2, NO3-1, and PO4-3.
A example of an anerobic digester process used in industry:
The following link provides additional information on Waste Digester Design
Biogas is produced when organic matter is degraded in the
absence of oxygen. The biogas from anaerobic digester contains
about 60% methane and 40% carbon dioxide, with trace amounts of
nitrogen, hydrogen, and hydrogen sulfide. In addition, anaerobic digestion also produces methane, a valuable fuel, making it a
more attractive as a waste water alternative. (1) Separation of
metals from incoming wastes prior to digestion can also be a
source of income. It is also important to note that the anaerobic
digestion process is not instantaneous, thus requiring larger
vessels because it takes time for the bacteria population to grow
Generally, any recyclable or unwanted materials are
separated from incoming wastes. If the waste is already very wet,
the addition of water is not necessary. The average ratio of
solid to liquid for a typical digester is 15-25%, but certain
technologies can handle solid concentration as high as 30% (2).
The digester operates around 35-27 degrees C for varying
periods of 10-20 days (2). The time period depends on external
temperature fluctuations, and waste composition.
After the biological degradation is complete, the solid
residue is removed and usually screened to remove oversized and
unwanted items like glass or plastic.
In order to understand the usefulness of the anaerobic
sludge digester, consider the following example of a municipal
facility involving the remediation of a grease-cap. In this 100
million-gal/day municipal facility, only 1.6 million gallons were
being utilized to the presence of a 10-ft layer of scum and
grease at the top of the unit, consisting of approximately 20%
total solids. Physical removal of this layer was estimated at 1
million dollars. (Huban-Plowman, 77). Due to this high cost,
other alternatives were sought such as microbial degradation ofthe scum.
Anaerobic digestion was used to remove this grease cap, and
after 45 days, the sludge composition was reduced from 20% solids
to less than 1.5%. The cost was less than $50,000 with no
physical removal required. (Huban-Plowman, 78). The digester
therefore reaps enormous economic and safety benefits due to the
introduction of microorganisms.
Anaerobic digestion has many advantages to it. It can make
landfills easier to manage by removing potentially hazardous
organic wastes. It avoids the generation of biogas in landfills
and can recover this biogas for further use. (2) It contributes
to recycling goals set by industries and government agencies, and
the end product can often be used as soil conditioner.
Some typical problems to be considered in the use of the
digester focus on the microbial population stability. The
stability of the digester depends on the type of strain selected
for a given process. Certain strains of bacteria will produce
better yields of digested sludge than others. (Huban-Plowman,
79). Bioaugmentation procedures are currently being researched
and refined to aid in this process.
One of the biggest costs incurred in the anaerobic digester
involves selecting the appropriate strain of bacteria and
conducting tests to determine degradation efficiency. The heat
input to the digester will also have certain costs per year
associated with it. The measurement apparatus costs would be
minimal, consisting of thermocouples used to measure temperature,
and flowmeters on the influent and effluent lines.
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