Note: Descriptions are shown in the official language in which they were submitted.
CA 02438444 2003-08-22
Title: Hollow Fibre Membrane Supported Biofilm Module
Field ofthe invention
[0001a 'this invention relates to membrane supported biofilm modules
which may be used, for example; in water treatment.
Backaround of the invention
[0002 Currently, most wastewater treatment plants use an activated
sludge process, based on biological oxidation of organic contaminants in a
suspended growth medium. Oxygen is supplied from air using bubble type
aerators. Efficiency of these systems is poor resulting in very high energy
use. Tank size is large as chemical oxygen demand loadings are low
because of low biomass concentration. The result is high capital and
operating cost.
j0003a A second type of established biological oxidation process uses
biofilms grown on a media. The wastewater is circulated to the top of the
reactor and trickles down. Air is supplied at the bottom. The rate of oxygen
transfer is limited by the biofilm surface area, and the operating cost is
high
because of wastewater pumping requirements. Other versions of this process
are also available, but all of these result in high operating costs.
[40041 Recently, development work has been done on a membrane
supported bioreactor concept. This process involves growing biofilm on the
surface of a permeable membrane. Oxygen containing gas is supplied on
-~- --~ - ~---~ ~ ~ -- - -~ ~ ~~~--- ~ ~~t~ne side of tt~ membrane and the
biofilm is grown on the other side, which is
2a exposed to the substrate. Oxygen transferred through the membrane is
absorbed by the biofilm as it is available in the form of very fine bubbles.
This
type of process has not become commercially viable.
Summar5:~ ~af-the-;i~r;:e:aio::
[0005 It is an object of this invention to improve on the prior art. It is
another object of the invention to provide a membrane material suitable for
use in membrane sunaorted bio~lms used in water treatment applications It is
CA 02438444 2003-08-22
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another object of this invention to provide methods and apparatus suitable for
treating water, for example industrial and municipal wastewater; using
membrane supported bioreactor technology. It is another object of this
invention to provide a hollow fibre membrane and module and to use them in
a membrane supported biofilm reactor. The inventors have observed that a
membrane and module with a high gas transfer rate and adequate surface
area would allow a membrane supported biofilm reactor to provide an
operating cost advanfiage over other processes used in the art. For example,
a savings in operating cost may be achieved using a membrane with an
oxygen transfer efficiency (OTE) of over 50% or in the range of 50% to
70°Ib
or more. The inventors have also observed that a module of hollow fibre
membranes may provide a large surface area but that commercially available
hollow fibre membranes tend to wet which results in a drastic drop in their
oxygen transfer rates:
r0006~ Membrane supported biofilm modules are a subset of
membrane based gas transfer modules generally. In the ~efd of gas transfer,
membrane materials and methods of manufacture have attempted to provide
high oxygen transfer rates and high selectivity: For example, silicone has an
excellent gas permeability of about 2,000,000 ec~mm/mz~24hr~Bar with a
dense wall structure that prevents water penetration. However, silicone is
very
expensive. Husain and C8fe proposed in U.S. 6014T7,025 a module using
poly-methylpentone (PMP). PMP has a gas permeability of about 70,000
cc~mmlm2~24hr~Bar in dense wall form. While this is significantly less than
silicone, PMP may be melt spun into a hollow fibre. Buy using PMP in hollow
fibre form, an effective module can be made. However, PMP is still expensive
compared to more common materials. For example, it is currently about 10
times as expensive as common polyolefins such as polyethelene (PE) and
polypropylene (PP). These substances have poor gas transfer rates in dense
watt form of only about 4,000 to 8,000 cc~mmlm2~2~hr~Bar. These materials
may be made in a microporous form with much increased gas permeability,
but the pores tend to wet out in practice when immersed in water containing
surfactants or proteins rese~lting in a significant decrease of flux rate.
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_ __ ._--h~-.ry-~:ws~w: li7YGi1'ti0l~, Ca ii7G~tFiO ro-r uas., far a>cam~a:a,
~a
support a biofilm for water treatment, is made using hollow fibres. The fibres
are made of a polymeric material: The fibres have a small outside diameter,
such as 100 Ir.m or less, and substantial hollow area, for example 30% or
more or 40°Jo or more; so as to have a thin wall.
[0008] The fibres have a dense asymmetric, variable porosity or
homogeneous wall which does not permit water flow, but has gas permeability
over the standard values mentioned above. The increased permeability
results from altering the method used to make the fibres. The method of
making the fibres also typically reduce the selectivity of the membranes, for
example selectivity to carbon dioxide or nitrogen (O~/N2, OzICO2) may be as
!ow as 6 or less or 1.5 or less. This lack of selectivity may make the fibres
unsuitable far gas separation but the inventors have found that high
selectivity
is not required, and may even be undesirable, in many water treatment
applications. In particular; where air is used as a source of oxygen to
support
a biofilm growing in water, there are minimally adverse or even useful partial
pressure gradients of carbon dioxide or nitrogen across the membrane wall.
The fibres can be woven, knitted or otherwise make into a fabric,
[0009 The polymer used for tha fibres may still be a highly gas
permeable polymer such as PMP. However, the fibres may also be made of
less expensive polymers, for example polyolefins such as PE or PP. The
polymer is extruded as hne hollow fibres by melt spinning under certain
conditions and subject to post-treatment whereby the permeability to gases is
increased, for example to 20;000 or 30,000 cc~mmlm2~24hr~Bar for a PE or
PP fibre. The gas permeability is increased by adjusting the spinning, process
parameters to increase crystallinity and enhance molecular chain alignment to
obtain a row or stacked lamellar structure. Permeability is then increased by
lamella separation without forming through pores. The use of fine hollow
fibres allows the thickness of the polymer wall to be low, for example a0 ~.m
or
less, which is several times less than what would be required to make a film
handleable. The ftne fibres may themselves be difficult to handle on their
own,
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but may be combined into units such as threads ar taws for handling, which
may include forming textile sheets. Despite the optional use of inexpensive
polymars°as--#he'-base 'rr~teFials,-a~ed~rate oxygen transfer
capability iS
provided such that air can be used as a feed gas without limiting the growth
of
the biofilm and with acceptable pressure loss due to air flow through the .. _
._ . _ _.....
module.
X0010] In one aspect, this invention provides a very fine dense hollow
f r~..which av nx may not have a high ePipctivitv,h«tdr,P~_ha P a hinh
c~i~usEOn coe~ic~ent for oxygen. An appropriate quantity of~re surface area
can be provided to achieve high OTE.
[0011 In another aspect, #his invention provides a fabric with a very
large number of hollow fibres providing sufficient surface area so that oxygen
transfer does not become a limiting factor in controlling biological kinetics.
The fabric rnay be rrmade with fhe fiber tows as weft and an inert fibre as
warp
to minimize the damage to the fibre while weaving. Other methods of
preparing a fabric may also be used. The fabric provides strength to the fine
fibre to permit biofilm growth on its surface with minimai fibre breakage.
~0092~ In another aspect, the invention provides a module built from
this fabric with very high packing density to permit good substrate velocities
across the surface without cecirculation of a large volume of liquid. The
modules enable oxygen containing gas to be supplied to the lumen of the
hollow fibre without exposing it to the wastewater. Long fibre elements are
used and potted in the module header to provide a low cost configuration.
r0013~ !n another aspect, this invention uses air as a means of
2b controlling the biofilm thickness to an optimum level. Other methods of
biofilm
control include in-situ digestion, periodic ozonation followed by digestion,
and
use of a higher life farm, such as worms, to digest the biofilm periodically.
To
speed up the biological digestion reactions, the air may be preheated to raise
the temperature of the bioreactor.
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[0014] In another aspect, this invention provides a plug flow, or
multistage continuously stirred tank reactors to conduct biological reactions
at
high substrate concentrations. This maximizes mass transfer of organic
carbon compounds and ammonia in the biofilm, eliminating this process as a
potential limitation to reaction rates.
[0015) tn another aspect, this invention uses oxygen enrichment as a
means of dealing with peak flows. Such oxygen enrichment may be
determine by an-line COD monitors, or set according to time of day for
municipal applications where diurnal flow and strength variations are weft
known.
[0016) In another aspect, this invention uses the module and bioreactor
design to conduct other biological reactions on the surface of the fabric. An
example is biological reduction of compounds such as sulphates in water
using hydrogen gas supplied to the lumen c~# the hollow fbre.
[001Tj In another aspect; this invention uses either air or enriched air to
supply oxygen. Selection of enriched air and level of oxygen present in such
air is determined by the wastewaterstr-engtl~.-
.____.____________.._.__.__._.___._.___. _______.__.
(0018) In another aspect, this Invention uses one or mare of the
r..~Y,n~nW ~oeo rleerrih~ ahnvp tl1 ('~lf7fPCt hrll'nFiN Dr secondary sludae.
[0019] The features of these various embodiments may be combined
together in various combinations or sub-combinations. The description above
is intended to introduce the reader to aspects of the invention, embodiments
of which will be discussed below. In addition to various combinations of
features described above, the invention may also involve combinatians or
sub-combinations of features or steps described alcove with features or steps
described below.
Briief descriation of the drawings
[0020a Ernbodirnents of the invention will be described below with
reference to the fioltowing figures.
[0021) Figure 1a is a cross-sectional view of a fibre.
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[0022] Figure 1b shows a bundled group of fibres of Figure la.
(0023] Figure 2a presents a front view drawing of a fabric sheet made
from the fibre of Figure 1.
x0024] Figure 2b is a side view of the sheet of Figure 2a.
[0025] Figure 2c illustrates a method of making the sheet of Figure 2a.
(0026] Figure 3a is a top view of a module using the fabric sheets of
Figure 2a.
(002T] Figure 3b is a front view of an upper portion of the module of
Figure 3a.
[4028] Figure 3c is an enlarged top view of a portion of the module of
Figure 3a.
(0029] Figures 8 and 9 are drawings of reactors excerpted from US
Serial No. 09/799,524.
Description of Embodiments
[0030] Referring fio Figure 1a, a hollow fibre 10 according to the present
invention is prepared with melt Spinning. The spinning process parameters
are adjusted to increase crystallinity and enhance molecular chain alignment
to obtain a row or stacked lamellas structure. Spinning or post-treatment
steps
that can be used or controlled include the spinning speed or drawing ratio,
the
quenching conditions such as temperature or air flow rate, post annealing, if
any, stretching and heat setting: Permeability increase is obtained by lamella
separation of the row lamellas stfucture.
('Ofl31] Methods of making such fibres 10 are known in the art. For
example, U.S. Patent No. 4,664,681 to Anagau et al: describes; in examples 4
and 6, processes for melt.-spinning and post-processing PE and PP to
produce acceptable fibres. Processes described in "Melt-spun Asymmetric
Poly (4-methyl-1-pentene) Hoiiow Fibre. Mc~mlt~ran~"~.toumaJ of MRrnhranR
Science, 137 (1897) 55-E31, Twarowska-Shmidt et al., also produce
acceptable fibres of PMP and may be adopted fo produce fibres of polyolefins
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such as PE or PP. These references are incorporated herein in full by this
reference to them.
[0032] Suitable fiibres 1p may also be formed by methods other than
melt spinning. Such other methods may include, for example, but not limited
to, meltblown extrusion, flash spinning, and electrospinning. In meltblawn
extrusion, fibres are formed by extruding molten polymer through spinneret
orifices. As the filaments exit from the orifices, they are attenuated by high
temperature, high velocity air streams before being deposited into a conveyor
belt to dry.
[0033 In flash spinning, pure solvent droplets and highly saturated
polymerlsolvent mixtures ace decompressed through a spin orifice. As the
pressurized solution is allowed to expand rapidly through the orifice, the
solvent is "flashed off~ instantaneously leaving behind a three-dimensional
film-fibril network.
95 j0034~ Electrospinning uses an electric field to draw a polymer melt or
polymer solution from the tip of a capillary to a collector. A voltage is
applied
to the polymer, which causes a jet of the solution to be drawn toward a
grounded collector. The fine jets dry to form polymeric fibres, which can be
collected on a web. By choosing a suitable polymer and solvent system, fibre
diameters can be varied and controlled.
[0035 Referring to Figure 1a, in the illustrated embodiments a melt
spinning method, modified as discussed above, is used to make the fibres 10
with an outside diameter 12 of 100 ~m ar less. The hollow area (or lumen 14)
of the fibre may be between 25% and 40% or 30% or may be 40% or more.
For example, a PE fibre having an outside diameter 12 of between about 50
to fi0 wm and an inside diameter 16 of 30 p.m or more, resulting in a wall
thickness 18 of 15 p,m or 10 p,m or less and a gas permeability of over 20,000
or 30,000 cc~mm/m2~24hr~Bar or more, may be used. Plainly melt spun, or
otherwise formed, PMP fibres of the same or similar dimensions may also be
used.
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[0036] The fibres 10 may be combined into fbre units 19 such
asindividual fibers 10, threads, yarns or tows 20, for example, of 48 or 98
fibres each, either twisted-or-t~ntwistzd-~Figt~re ~.b)r.~ro!ag~r- inert
fibres 22,
such as polyester yam may be included in the units 19. The fibres are non-
porous but oxygen, or other gases, may travel through them by diffusion,
dissolution-diffusion, Knudsen's flow, advection, or Poiseuille flow.
~8037j In Figures 2a and 2b, the fibre units 19 are wovera~into a fabric
,~liect ~3-wiil-r-thro~-units-'!O-rt~svesira.~y ~.vi~nrff~llli -anl~~ illl~!'-
~'-flhrfal2 ',~~ rllnl'lina
vertically to provide support to the units 19. Figure 2b shows a basic two
10 dimensional woven structure while Figure 2c illustrates the steps involved
a
process for making the sheet 26. Unit 19 type, unit 19 bundle size, spacing
between unit 19 bundles and .percent of fibre or unit 19 in each direction can
all be tailored to meet the mechanical or thermal requirements of each unique
application. Further, surface roughness of the shat 26 can be contolled to aid
15 in biofilm control. For example, it may be easier to control the bioflm on
a
sheet 26 with a rough or textured surface, for example, one in which the
height of the surface undulations roughly matches the desired range of biofilm
thickness, which may be, for example, from 200 to 1,000 microns.
[0038] The fabric sheets 26 rnay also be made by other methods such
20 as braiding, stitching or knitting, such as warp knitting. Warp knitting is
desirable, far example, when small units 19 or tows 20 or even individual
strands of fine fitare 10 are used as the weft since the process is gentle on
the
weft. The fabric sheefs 26 may be patterned, as in pattern knitting, if
desired,
to provide areas with fewer fibres or holes to enhance flow through the
25 sheets.
rD039J In one aspect of the present invention, the fibre units 19 provide
a support surtace fior the growth of a bio~lm 30 {Figure 2b). The number of
hollow fibre units 19, and the number of fibres 10 per unit 19, may be
adjusted
to provide a desired surface area for 0~ transfer compared to surtace area of
30 biofilm 30 or to the planar surface area of the fabric sheet 26. The planar
surface area of the sheet 26 is simply the sheet length multiplied by its
width,
CA 02438444 2003-08-22
multiplied by two (since the sheet has two sides). The surtace area of the
biofilm 30 is the total area of the biofilm 30 exposed to the liquid in the
reactor,
which may be generally the same as the planar area of the sheet 26 {for a
substantially two dimensional sheet configuration), as seen in Figures 2a and
2b.
[0040] The surface area far 02 transfer is the total area of the hollow
fibre units in the sheet exposed to the biofilm, which is approximately equal
to
the product of the circumference and length of the fibre 10, multiplied by the
number of the fibres 10 in the sheet 28. The inert fibres 22 crossing the
hollow fibres 10 in the sheet 26, and areas of tight fibre 10 to fibre 10
contact
may reduce this theoretical surface area somewhat.
[Q049a - AIttTVUgirthe~urface area=af~the#oivfitm=is=generally-the same as
the planar area of the sheet 26, it may be slightly larger for very rough
fabrics.
Varying fabric roughnesses may also be used to affect the thickness of the
biofilm 30 or how readily the biofilm 30 can be reduced or controlled: High
o~~,$~,gr~ ~,~'~~~~~~,~er a~rr,i~~51'~'r~n~~e ~ ~w°e"~~ra, r~ir
i~e'~'a~nr"~ieea
water with a high concentration of COD, for example, 300 mglL or more, lower
SA O21SA biofilm ratios, for example, between 1.6 or 2 and 10 are sufficient,
and may be preferred to reduce module cost. An SA t7alSA biofilm ratio in the
range of about 1 to 8 or 1.6 to 6 can provide satisfactory results in most
treatment applications.
[0042] Figures 3a, 3b, arid 3c show a module 40 in which a set of
parallel sheets 2B are potted with a gap 42 between them in a header 44.: Two
headers 44 rnay be used as shown when a bleed of exhaust air is desired.
One header 44 may also be used with exhaust bled through opposed open
ends of the fibres 10 or with the other ends of the fibres 10 closed for dead
end operation. The gap 42 may be between 2mm and 10 mm thick, or
between 3 mm and 15 mrn. The chosen gap 42 may depend on the water to
be treated or the choice of method to control biofiim thickness. For example,
a
module 40 of tensioned sheets 26 may have a gap 42 of 6 mm when used
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with aeration to control bIofilm 30 thickness. Tension may be provided by
mounting the headers 44 to a rigid structure, which may include parts of a
tank, with one or both headers 44 movable relative to the structure. The
sheets of fabric 26 are potted in the headers 44 using potting materiats 46
such as polyurethane, hot melt or epoxy. A large sheet of the fabric may also
be rolled or folded to produce a module 40 rather than using separate sheets.
[0043] The module 40 is assembled together with spacers 48 between
adjacent sheets 26 of fabric which provide the gap 42 between the sheets 2fi
for aeration and substrate flaw. These spacers 48 may be plastic strips ar hot
melt layers. The length of the module 40 may range from 1 m to 5 m or
between 1 m and 3 m.
[00441 To produce the module 40 of Figure 3, a sheet 26 of fibres 10 is
laid onto strips 50 of adhesive located to cross the ends of the fibres.
Spacing
strips 52 are placed over the sheet, followed by additional strips 50 of
'15 adhesive and an additional sheet of fabric 2~. These steps can be repeated
several times. The resulting assembly can then be sealed into a pair of
opposed headers 44 such that the lumens 14 of the fibres 10 are in
communication with a port 54 in one or both headers 44. The ends of the
fibres 10 are cut before potting fio open them. Alternately, sheets 26 may be
separated, be glued to spac~er_s at~hair .edge..s ~n$ jnsel_te~t. jnta
a.header
cavity, Additional glue is then placed around this assembly ~c~ peal .it :tn
the
header cavity.
[00451 In use, an oxygen-bearing gas 60, which may be air, flows into
at least one of the headers 44. The module 40 may be operated in a dead
end mode, with no outlet other than through the walls of the hollow fibres 10.
Hiternaieiy, me rnoauie may ~C Uf.JCIGttGU 111 CI 41V,7J 11V~~ 1«uW mr~ rwu,
y,.v ,,",,
entering through one header, 44 flowing through the fbres 10 and exiting from
the other header. The oxygen content and flow rate of the gas 60 may be set
to produce an oxygen transfer that provides aerobic conditions near the fibres
10 and anoxic conditions near the substrate being treated. Multiple reactions,
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including carbon based organics, ammonia and total nitrogen reduction, may
be performed in the biofllm 30.
[0046 Reactors sim!!ar to those described in U.S. Patent Application
Serial No. 091799,824, filed PUlarch 7, 2001, may be used. Iror example the
5 reactors discussed in an excerpt from US 091799,524 reproduced below may
be used with the present invention. The entire text of U.S. Patent Application
Serial No. 09J799,524, filed March 7, 2001; is incorporated 'herein by this
reference to it.
[004Tj in another embodiment of the invention, a bloftlm !s grown on a
fabric woven from textile PMP dense wall hollow fibre. Oxygen bearing gas is
introduced into the lumen of the fiibre. Aerobic reactions take place at the
surface of the fibre, where the highest levels of oxygen exists. These
reactions Include conversion of organic carbon compounds to carbon dioxide
and wafer, and ammonia to nitrates. The surtace of the biofilm is maintained
15 under anoxic conditions such that conversion of nitrates to nitrogen can
take
place. The result is simultaneous reduction of organic carbon, ammonia and
total nitrogen.
[0048] tn another embodiment, a!! the above features are used, except
that high aeration velocity of 2-8 feetlsecond is used at the surface of the
20 fabric to reduce the thickness of the biofilm. This is done once every day
to
once every week. Also, air may be used to periodlcaily mix the contents of
the bioreactor.
[0049 In another embodiment of the invention, a number of bioreactors
are installed in series to provide flow patterns approaching plug flow. This
25 results in higher reaction rates and better utilization of oxygen.
[0050 In another embodiment, ozone gas, introduced in the fibre
lumen 14; is used to oxidize a part of the biofilm to make it digestible.
Oxygen
is then provided to digest the oxidized organics, thereby reducing the total
amounts of solids generated.
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!!f-GI11VU~G7 T3111~V1i~17ttiW t'vf'liw-irrwa~ttion;-w.ormo-.m,ro-wss~1-in_a,n
isolated section of the reactor to digest excess biofilm to reduce-bio-solids
generation. The worms are grown in a separate bioreactor.
(0052 In another embodiment of the invention, different oxygen levels
5 are used in different stages of the bioreactor by oxygen spiking to meet
different levels of oxygen demand and to achieve high bioreactor loadings.
r0053~ In another embodiment of the invention, the elements are
stacked in a vertical configuration, with flow taking place from top to bottom
or
bottom to top. This reduces the capital required for aeration and the
operating
cost of air. Numerous other embodiments may also be made according to the
invention.
[0054] Air may be used as an oxygen bearing gas 60 input to the
modules 40. Even though N2 or CU2 selectivity may be tow, the partial
pressure gradient between N2 or CU2 on the inside of the fibres and the
15 biofilm or surrounding substrate is not large. Accordingly, substantial
amounts
of Nx or C02 do not diffuse into the substrate. In contrast, CO2 may back-
diffuse into the fibre lumens 14 particularly with a low selectivity membrane.
The exhaust gas, collected at the exhaust header, may be bubbled through a
lime slurry to recover the C02 and prevent discharge of COz. The flow rate of
20 COZ enhanced gas to be treated is much smaller than for a conventional
wastewater treatment process allowing far more efficient control of C02
emissions.
[0055) Back diffusion; particularly in fibres of low selectivity, is also
useful in other applications. for example, the modules 40 may be used to
25 oxygenate water used to grow fish in a fish farming operation. Even with a
fibre 1fl of low selectivity, the Nz gradient towards the water that the fish
live in
is small or even negative. Accordingly, U2 is added to the water without
causing Nz saturation in the water even when air is fed into the modules: The
ability to use air, rather than oxygen as required when bubbles are used,
30 results in a significant cost reduction: Depending an the concentration in
the
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fish water, ammonia may back-diffuse into the fibre lumens 14 which helps
maintain non-toxic ammonia levels in the fish water.
Excerpt form U5 Serial No._091799;524
iUtembrane Supported Biofii!rr~ Reactors for Wastewater Tr~eafinent
5 [0058 Figure 8 shows a reactor 80 having a tank 82, a feed inlet 84 to
the tank 82, an effluent aufitet 86 from the tank 82, a ttow path 88 between
the
feed inlet 84 and-effluent-c~utlet.8fi.and a~plurality of the third apparatus
210.
The third apparatus 210 is shown as an example only and the second
apparatus 110 or first apparatus _10 may also be used with suitable
modifications to the reactor 80.
~wws,~ TL. y.l~.......a. wn~,r.nraWn '~(~e. lore e~i~Q!'~ '~'t~..~Jf.~E'lLl.
lank f~7 anr! fill 9
substantial amount of its volume. The planar elements 226 have no pre-
manufactured or rigid frame and thus are pr~eferabiy custom made to provide
efficient use of the available space in the tank 82. For example, planar
15 elements 226 may range from 0:5 m to 2 m wide and 2 to 10 m deep. The
planar elements 226 are preferably arranged in the tank 82 in a number of
rows, one such row being shown in Figure 8. The planar elements 22fi may
range from 0.5 to 2 mm in thickness and adjacent rows are placed in the tank
82 side by side at a distance of 3 to 15 rnm to allow for biofilm growfh and
24 wastewater flow between adjacent planar elements 226.
[0458] The tank 82 is longer than it is deep and it is preferred to
encourage a generally horizontal flow path 88 with minimal mixing. This is
done by leaving some space near the ends (i.e, near the inlet 84 and outlet
86) of the tank 82 for vertical movement of water and leaving minimal free
25 space at the top, bottom and sides of the tank 82: A baffle 90 may also be
placed upstream of the effluent outlet 86 to force the flow path 88 to go
under
it. A sludge outlet 92 is provided to remove excess sludge.
[0089] The flow path 88 is generally straight over a substantial portion
of the tank 82 between the feed inlet 84 and effluent outlet 86. Each third
30 apparatus 290 is held in the tank 82 by its headers 52 attached to a frame
90
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and by its weight 68. The headers 52, frame 90 and weights 68 restrain each
third apparatus 210 in positions in the reactor 80 whereby the planar element
226 of each third apparatus 210 are generally parallel to the flow path 88.
Preferably, a plurality of planar elements 226 are spaced in series along the
flow path 88 so that the reactor 80 will more nearly have plug flow
characteristics. Wastewater to, be treated may be partially recycled from the
effluent outlet 86 to the feed inlet 84. Such a recycle can increase the rate
of
gas transfer by increasing the velocity ,of wastewater along the filow path
88,
but it is preferred if the recycle ratio is small so as to not provide more
nearly
mixed flow characteristics in the reactor 80.
[0060 Oxygen containing gas is provided to each third apparatus 210
through its inlet conduit 216 connected to an inlet manifold 94 located above
the water to be treated. With the inlet manifold 04 located above the water, a
leak in any third apparatus 210 will nat admit water into the manifold nor any
ather third apparatus 21D. Gas leaves each third apparatus 210 through its
outlet conduit 218 which is connected to an exhaust rttanifold 95. Although it
is not strictly necessary to collect the gases leaving each third apparatus
210,
it does provide some advantages. For example; the gas in the exhaust
manifnlrl 4S may haves hPnnmp rich in vnlatila nmanin rnmnn«nrla which may
create odour or health problems within a building containing the reactor 80.
These gases are preferably treated further or at least vented outside of the
building.
[006~t~ Preferably, the gas is provided at a pressure such that no
bubbles are formed in the water to lae treated and, more preferably, at a
pressure of less than 10 kPa. This pressure is exceeded by the pressure of
the water to be Treated from one metre of depth and beyond. Preferably at
least half of the area of the Third planar elements 226 is below that depth.
The
water pressure thus prevents at least one half of the surface of the
membranes 12 from ballooning.
[0062 Oxygen diffuses through the membranes 12. The amount of
oxygen so diffused is preferably such that an aerobic biofilm is cultured
CA 02438444 2003-08-22
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adjacent the planar elements 226, an anoxic bio~film is cultivated adjacent
the
aerobic bioftlm and the wastewater to be treated is maintained in an anaerobic
state. Such a biofttm provides for simultaneous nitrification and
denitrificatian.
A source of agitation 98 is operated from time to time to agitate the planar
elements 226 to release accumulated biofilrn. A suitable source of agitation
is
a series of coarse bubble aerators :98 which do not provide sufficient oxygen
to the water to be treated to make it non-anaerobic.
r4063~J Figure 9 shows a second reactor 180 having a tank 182, a feed
inlet 184, an effluent outlet 186, a flow path 188 and a plurality of the
first
apparatus 10. The first apparatus 10 is shown as an example only and the
second apparatus 110 or third apparatus 210 rnay also be used with suitable
modifications to the second reactor 180.
f0064~ Each first apparatus 10 is held by its Loops 30 wrapped around
wires 100 or ropes attached to the tank 182. The loops 34 and wires 100
restrain each first apparatus 10 in a: position in the second reactor 180
whereby the planar element 26 of each ftrst apparatus 10 is generally parallel
to the flow path 188.
X0063) The first planar elements 26 are sized to fit the tank 182 and fii!
a substantial amount of its volume. Like the third planar elements 226; the
first
planar elements 2fi have no pre-manufactured or rigid frame and are
preferably custom made to pro~ride efficient ase of the available space in the
Tank 182. The first planar elements 26 may range from 0.25 to 1 mm in
thickness anct are placed side by side at a distance of 5 to 15 mm to allow
far
biofilrn growth and wastewater flow between adjacent first plan_a~ elements
26.
~0068~ The tank 182 is deeper than it is long and it is preferred to
encourage a straight and generally vertical flow path 188 over a substantial
portion of the tank 182 with minimal mixing. This is done by leaving minimal
._
space near the ends and sides of the tank 82 but a substantial amount of
.__.....,~,_..~_;.._.......a.t...u...y, ,.i+f,n f~nl~ ft7 VV~tp~ tn hc~
treated may be
partially recycled from the effluent outlet 186 to the feed inlet 184 but it
is
preferred that the recycle rate be small.
CA 02438444 2003-08-22
~ 16-
(x067] Oxygen containing gas is provided to each first apparatus 10
through its inlet conduit 16 connected o a manifold 94 located above the
water to be treated. Wtth the inlet manifold 94 located above fihe water, a
leak
in any first apparatus 1O wilt not admit water into the manifold nor any ether
5 first apparatus 210. The outlet conduits 18 are clipped in a convenient
place,
for example to the inlet manifold 94, above the surface of the water to be
treated. Preferably, the gas is provided at a pressure of less than 10 kPa and
the planar elements 26 are located more than 1 m deep in the tank 182. In
this way, the gas pressure is exceeded by the pressure of the water to be
10 treated which prevents the membranes 12 from ballooning. Glue lines (not
shown), preferably not effecting more than one half of the area of the planar
elements 28, can be used to reinforce part of the'planar elements 28 if they
can not be mounted deep enough.
[0068 Altemativefy, gas flow through the first element 10 is produced
16 by applying a suction, preferably of not mare than 10 kPa less than
atmospheric pn=ssure, to the outlet conduits 18: The inlet conduits 16 are
placed in fluid communication with the atmosphere. By this method, the rate
of gas diffusion across the membrane 12 is slightly reduced, but no
reinforcement of the membrane 12 (for example, by glue lines) is required
20 regardless of the depth of the fiirst element 10.
(0069] Oxygen diffuses through the membranes 12 preferably such that
an aerobic biofilm is cultured adjacent the planar elements 26, an anoxic
biofilm is cultivated adjacent the aerobic biofilm and the wastewater to be
treated is maintained in an anaerobic state. A second source of agitation 196
25 is operated from time to time to agitate the ftrst planar elements 26 to
release
accumulated bioflm. A suitable source of agitation is a series of mechanical
mixers 102.
