Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a support medium for the biological
treatment of sewage and other effluents.
Biological treatment or disposal of sewage and other organic
effluents is normally performed by contacting the effluent with bacteria and
other microorganisms which grow and consume the organic content of the
ef1uent. Processes involving this basic principle vary widely in detail but
broadly they include:
~i) Anaerobic digestion where the effluent is contained within a tank
and the micro-organisms grow under anaerobic conditions by consuming organic
matter at a relatively slow rate, but with a high rate of conversion to
gaseous products, such as methane and carbon dioxide.
cii) Settling ponds where aerobic microbial action depends on rate of
air transfer into a relatively shallow pond containing the effluent.
Ciii) Activated sludge systems where the effluent is contained in a
tank or pond, to which air is supplied to carry out the digestion under
aerobic conditions and which permit a greater rate of digestion than (i) or
Cii). Effluent conversion is limited by the need to re-cycle some of the
microbial sludge and liquor to provide continued generation of the micro- ~- -
organisms.
Civ) Trickle filter systems where the active microorganisms grow on a -
support medium, effluent and air being circulated over the bacterial growth
to effect conversion of the effluent.
The present invention is concerned particularly, with the trickle
filter type of system but also extends to systems of the activated sludge or
extended aeration types.
Typically, trickle filter plants are composed of crushed rock ;:~
retained in a bed some 6 feet C2 metres) deep and up to 200 feet (60 metres)
wide. Effluent is sprayed over the top of the bed and air is allowed to
circulate through the bed from the top and open (or porous) base of the bed.
Microbial growth takes place on the surface of the rock. Such plants are
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limited in size, usefulness, and efficiency, by land use requirements, weight
of rock, air accessibility, susceptibility to flies, and other factors.
Recently plastic support media have replaced rock and stone in
trickle filter systems, giving the advantages of lighter weight, higher
surface to volume ratios, and greater open volume for better air penetration.
This has allowed the development of deeper, more efficient beds, and also
tota immersion treatments where the support medium is fully immersed in the
effluent and air or oxygen is pumped through the bed.
Such plastic support media are frequently constructed in the form
of solid, i.e., imperforate, sheets having a variety of corrugated patterns.
Some examples are the materials known as *Flocor tICI Aust. Ltd.) and *Surfpac
tnOw Co. Aust. Ltd.). Other types of support media include discs, rings,
spoked wheels and other shapes designed to facilitate microbial growth, with
maximum void space to provide for adequate air circulation.
According to the present invention there is provided a support
medium for the biological treatment of sewage or other effluent, said medium
comprising a plurality of superposed layers of a mesh-like material, the
sheets being disposed so as to avoid superposition of the apertures of at
least some adjacent sheets.
Simply by randomly laying one sheet on another, it has been found
that superposition of the apertures of a satisfactory number of adjacent
sheets is achieved. It should be appreciated, however, that this number can
be increased and the efficiency of the medium improved by d~liberately
arranging the sheets to avoid superposition of the apertures.
Thus, according to another aspect of the invention, there is a
support medium when used for the biological treatment of sewage or other
aqueous effluent, said medium comprising a plurality of superposed sheets
of a mesh-like material, the sheets being disposed so as to avoid super-
position of the apertures of adjacen~ sheets.
The preferred material for the mesh is a plastics material and it
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is specifically preferred to use a foamed plastics material. Some examples
of suitable materials of this latter type already manufactured are those sold
under the trade names "Trical" and "Netlon".
In the preferred arrangement of the medium, the sheets or layers
correspond in shape and area to the cross-section of the tank or other vessel
in which they are contained, i.e. each layer spans the full cross-section of
the vessel. It is, however, within the scope of the invention to use layers
which are of smaller area than the containing vessel.
Thus in another aspect of the invention there is provided apparatus,
such as a trickle filter, for the treatment of sewage or other aqueous
effluent the apparatus comprising a vessel which is at least partly filled
with the support medium.
Table 1 compares the density of packing ~i.e., a 'bulk density'),
calculated surface area per unit volume and void space of the total volume
of the packing for typical prior art materials used in trickle filter
applications, together with the same properties for support media in accord- - -
ance with the present invention.
TABLE 1
2-6 cm Rock FormedPlastic 2.5 x 2.5 x
particles PlasticWheels 3.5cm 2.5cm
Sheet Plastic Plastic
Mesh* Mesh*
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Packing Density
kgm/m 1450 37 150 69 81
Surface area
m /m 99 89 225 260 310
Void space % 45 80 77 73 ~ -
* Plastic mesh comprising 2.5 to 4.0mm oval cross-section meshes at quoted
spacings, made from polyethylene foamed to specific gravity of approximately
0.3 gm/cm
The particular advantages to be gained from use of a plastic mesh
packing medium will thus be seen to include: ease of installation (as no
complexities in assembly or packing are involved), high area per unit volume
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of packing, retention of adequate void space for aeration, and low weight of
the packed bed.
The data of Table 1 illustrate the advantages, relative to previous-
ly used materials, of employing a mesh support medium in bio-oxidation sew-
age and effluent treatment plants. In particular the mesh materials have
superior surface area while retaining ~avourable packing density and void
space characteristics.
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Although unfoamed, i.e., solid, plastics materials may be used to
form the mesh, the maximum advantage is obtained by use of a foamed plastic
mesh, which is of lighter weight than unfoamed materials and provides a higher
effective surface area due to the slight surface roughness of the foamed
material. Additionally, for totally immersed beds, the packing will float and
thus provide support for additional weight of the microbial growth.
EXAMPLE
In an experimental sewage treatment plant a tower approximately
lm x 2.8m in area and 2.8m high was packed with ta) a conventional filling of
plastic spoked wheels (supplied by Mass Transfer Ltd.) and (b) sheets of
2.5 x 3.5 cm (mesh openingj foamed plastic mesh. The sheets were cut to fit
the tower cross-section (approximately 2m square metres) and stacked horizon-
tally in the column, the cutting and disposition of the layers being such as
to avoid superposition of the apertures of adjacent layers. The relative
packing surface areas for the wheel- and mesh- packed column were 225 and
260m2/m3 respectively. The plant was operated, using each type of packing,
at a rate of about 41m3 per day on raw sewage of 300-500 ppm B.O.D. level us-
ing the extended aeration principle wherein the support medium carrying the
bacterial flock remains fully immersed in the sewage liquors, and air is pump-
ed into the tank so that it penetrates through the liquor and support medium
to effect bio-oxidation. The treatment plant worked equally well with both
types of packing, yielding an effluent of 120 - 150 ppm B.O.D. with 180 ppm ~ -
solids, which after settling and clarification gave effluent B.O.D. levels of
20-30 ppm in both cases. With such a small area difference it was not possible
to detect any improvement in performance from the mesh packing. Nevertheless
the experiment demonstrates that the support medium of the invention is cap-
able of at least the same performance as the prior art materials and thus
provides a support of lower cost and greater ease of use than the latter.
Those skilled in the art will appreciate that the invention describ-
ed herein is susceptible to variations and modifications other than those
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specifically described. It is to be understood that the invention includes
all such variations and modifications which fall within its spirit and scope.
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