Note: Descriptions are shown in the official language in which they were submitted.
CA 02438101 2004-04-13
-1-
Title: Membrane Supported Biofilm Modules Using Fibre Tows
Field of the invention
[0001] This invention relates to gas transfer membranes generally and
to membrane supported biofilm processes and apparatus in particular.
Background of the invention
[0002] U.S. Patent No. 5,116,506 to Williamson et al. describes a gas
permeable membrane which divides a reactor vessel into a liquid
compartment and a gas compartment. A biofilm is grown on the gas
permeable membrane on the liquid side of the membrane. The gas permeable
membrane is supported by the structure of the membrane itself. The biofilm is
chosen from bacteria to degrade certain pollutants by means of anaerobic
fermentation, aerobic heterotrophic oxidation, dehalogenation, and
hydrocarbon oxidation. This is accomplished by means of oxygen and
alternate gases (i.e., methane) through the gas permeable membrane to
certain bacteria growing on the liquid side of the gas permeable membrane.
Summary of the invention
[0003] It is an object of the present invention to improve on the prior art.
It is another object of the present invention to provide a gas transfer module
made using tows of a hollow fibre membrane. It is another object of the
present invention to provide a membrane supported biofilm module for
treating waste water, particularly wastewater having low COD concentration.
[0004] In one aspect, the invention provides a tow of hollow fibers. The
fibers are fine, for example with an outside diameter (OD) of 100 pm or less.
To facilitate building modules with minimal reduction in the effective surface
area of the fibres, the fibres are processed or used as tows over a
significant
portion, for example one half or more, of their length. Modules may be made
directly from the tows without first making a fabric. The tows may also be
made into open fabrics to facilitate potting, for example along the edges of
the
fabric, while leaving significant portions of the fibres as tows, for example
a
portion between the edges of the fabric. The modules made from tows may
be potted at both ends, or one end only with the other end left unpotted with
CA 02438101 2004-04-13
-2-
fibre ends open to permit exhaust gas to escape. A single header module may
have lower cost than a double header module. A single header module may
be inserted in a vertical configuration with the header at the bottom and the
fibres floating upwards. Such a module may be aerated from outside the
module to remove accumulations of trash and solids. Feed may also be
screened, for example through a 0.5 mm screen, to reduce trash in the feed
before it enters the reactor. Where the tow module is used in a downstream
stage of a multi-stage reactor, the upstream stage may also reduce the
amount of trash fed to the tow module reactor.
[0005] In another aspect, a reactor is provided for treating wastewater,
particularly wastewater having a low COD, for example 1,000 mglL or less or
500 mg/L or less. The reactor uses a module having an oxygen transfer area
of equal to or over 10 times the outer surface area of a biofilm attached to
the
fibres.
Brief description of the drawings
[0006] Embodiments of the invention will be described below with
reference to the following figures.
[0007] Figure 1 is a photograph of a membrane fiber.
[0008] Figure 2 is an elevation view of the fibres potted as tows.
[0009] Figure 3 is a photograph of a bench scale test module using
tows of fibres.
[0010] Figure 4 is a photograph of an open fabric made of tows.
[0011] Figures 5, 6 and 7 are graphs showing the results of
experiments with the module of Figure 3.
Detailed description of the embodiments
[0012] Figure 1 shows a textile polymethyl pentene (PMP) fibre with 45
micron outside diameter and 15 to 30 micron inside diameter. The fibre is
made by a melt extrusion process in which the PMP is melted and drawn
through an annular spinnerette. The raw polymer used was MX-001,
CA 02438101 2004-04-13
-3-
produced by Mitsui Petrochemical. The fibres are hollow inside but non-
porous with dense walls. Other fibres may also be used, for example
stretched microporous PE or PP fibres, treated to be hydrophobic, may be
used. The fibres may have various diameters and may be fine fibers having
outside diameters of less than 100 microns, for example between 30 and 100
microns or between 50 and 60 microns. Oxygen or other gases may travel
through the fibre walls.
[0013] Figure 2 shows a module with fibres arranged and potted in
tows of fibres. The tows are made of a loose collection of a plurality of
fibres,
for example between 1 and 200 or 48 to 96 fibres. The fibres may be lightly
twisted together or left untwisted. The fibres may be curled or crimped to
provide three dimensional structure to the each potted row. Curling may be
achieved by re-winding the fibres onto a bobbin while varying the tension on
the fibres. The individual fibres remain separable from each other in the tow.
Such a tow, when coated with a thin biofilm, for example of less than 1 mm
thickness, may provide ratio of gas transfer area through the fibre walls to
biofilm outer surface area (SAo,~ygen~SAbiofilm) of 1 or more or 10 or more.
Inert
fibres may be added to the tow to strengthen it if required. Each tow is
potted
into a plug of resin so that its ends are open at one face of the resin. The
plug
is glued into a plastic cap having a port which forms a header connecting the
port to the open ends of the fibers. There are 2 headers, one associated with
each end of the fibres, although modules with only an inlet header may also
be made. With two headers, air or other gases may be input into one header,
flow through the fibres and exhaust from the second header. Tows are potted
in a resin, such as polyurethane, and the potted ends are cut to expose the
fibre surface. Alternately, a fugitive potting material may be used to block
off
fibre ends, as described in U.S. Patent No. 6,592,759, or other potting
methods may be used.
[0014] Figure 3 shows a bench scale module made by potting 100
tows, each of 96 fibres as shown in Figure 1, into an opposed pair of headers.
The module was used to treat a feed water in a batch process. In the process,
r
CA 02438101 2004-04-13
-4-
the module was located in a tank filled to 4 L of synthetic wastewater. The
tank was drained and filled with fresh feed every 2 to 7 days. Air was applied
to the module at 30 mL/min. A biofilm of stable thickness grew on the module
for a period of over 6 months. The biofilm was essentially endogenous, its
rate
of growth generally equal to its rate of decay, except that a small part of
the
biofilm broke off and was discharged with some of the tank drains.
[0015] Figure 4 shows an open fabric made by weaving tows through
the shuttle of a loom and crossing the tows with an inert fibre only along the
edges of the fabric. The fabric is approximately 1.3 m wide, that is it has
active fibres of about 1.3 m long, and has inert fibers woven perpendicularly
to
the tows in a strip of about 2 cm along the edges. The tows remain
unrestrained between these strips. The resulting roll of fabric is cut into
sections of about 20-200 cm or 30-60 cm width to make individual sheets. The
sheets are cut along the woven edges to open the ends of the fibres and
potted with a 0 to 10 mm space between them into one or a pair of opposed
headers. Depending on the potting method used, the fibres may be cut open
either before or after they are inserted into the potting resin. 1 to 100 or 8-
20
sheets may be potted into a pair of headers to produce a module which is
placed inside a household septic tank. The module is fed with a 1/4 hp air
blower and creates a pressure drop of about 1 to 7 psi, or about 3 psi for an
8-
10 sheet module. With a typical household feed, a generally endogenous
biofilm grows on the individual fibre and tow surfaces. Biological treatment
in
the biofilm results in a reduction in the suspended solids and chemical oxygen
demand of the effluent, allowing the septic tile field to be reduced in size
or
eliminated.
[0016] In a batch process, the concentration of the wastewater
decreases towards the end of each processing period. Demand for oxygen
supplied to the biofilm also decreases and so the gas supply to the modules
may be reduced. Modules using fibres at least partially in the form of tows
allows a very high surface area for oxygen transfer and biofilm growth. Tow
modules are particularly useful in treating wastewater having a low COD, for
CA 02438101 2004-04-13
-5-
example 1,000 mglL or less, 500 mg/L or less or 300 mglL or less, because
they provide large surface areas. Pressure loss through the fine fibre lumens
is not limiting with the amount of air supply required to deliver oxygen to a
biofilm treating low COD wastewater. Although they may be useful for treating
other wastewaters as well, tow modules can be used where the initial feed
has a low COD or as a second or third stage behind other treatment
processes or apparatus that reduce the COD concentration of stronger
feedwaters. With municipal wastewater or other feeds, for example feeds
having a COD of 1,000 mglL or more, a two stage apparatus may be used. In
a first stage, membrane supported biofilm modules in the form of a fabric
sheet are used as described in U.S. Provisional Application No. 60/447,025,
which is incorporated herein in its entirety by this reference to it. The
outlet
from a reactor containing these modules is fed to a reactor containing tow
modules as described in this document which provides second stage
treatment. The inventors have observed that rapid reduction in COD from a
high COD wastewater limits the denitrification produced from a membrane
supported biofilm reactor. With a two stage process, the first stage may be
optimized for COD removal. The feed to the second stage has a reduced
COD and the second stage may be optimized to support nitrifying
microorganisms, for example of the species nitrobacter and nitrosomas, over
carbon degrading microorganisms to provide improved ammonia oxidation in
the second stage.
Example 1
[0017 A module similar to that shown in Figure 3 having 100 tows,
each tow having 96 fibres of dense walled PMP as described in relation to
Figure 1 was tested. The total surface area of the fibre was 0.54 m2. In the
module, each tow was individually potted into an upper and lower header. The
module was fed with a supply of air at a rate of 30 mL/min to the top header
and exhausted air out of the lower header. The module was suspended, with
the top header held in a clamp above the water surface and the bottom
header weighed down, in a container filled to a volume of 4 L of synthetic
°
CA 02438101 2004-04-13
-6-
wastewater. The module was operated in a batch mode. At the start of each
batch processing period, the container was filled with synthetic wastewater
having an initial COD of 1,000 mglL. Air was supplied to the module to
support a biofilm growing on the fibres for processing periods ranging from
between about 2 and 7 days while wastewater was neither added to nor
withdrawn from the tank. At the end of the processing period, the tank was
drained by opening a port at the bottom of it or by removing the module and
tipping the tank over. New wastewater was added to start the next processing
period. At various times, small segments of fibre were removed to measure
the thickness of the biofilm on them and measurements of the COD in the
wastewater were taken.
[0018] Figure 5 shows the thickness of the biofilm on the fibres over the
period of 180 days of operation. As shown in Figure 5, there was initially no
biofilm but after about 20 or 40 days a biofilm had developed having a
thickness that generally ranged between about 100 and 300 pm. For most of
the test run, no additional methods were used to control the biofilm thickness
and yet the biofilm thickness remained generally stable and acceptable. Small
portions of biofilm were observed to be shed from the module during at least
some of the tank draining operations, and biofilm control was otherwise
provided by endogenous growth of the biofilm. However, for a period of
approximately 15 days, the module was operated in a starvation mode. In this
mode, the tank was filled with tap water and air feed was continued. As shown
in Figure 5, the biofilm was reduced in thickness from about 250 pm to about
100 pm during the starvation period indicating that the starvation period was
effective at reducing the thickness of the biofilm.
[0019] Figures 6 and 7 show the removal rate of COD as a function of
time in Figure 6 and as a function of COD concentration in the wastewater in
Figure 7. Referring first to Figure 6, each vertical line within the figure
indicates the start of a new batch processing period. Accordingly, at the
times
indicated by the vertical lines, new wastewater having a COD of 1,000 mg/L
was added to the tank. As the batch progresses, the wastewater is treated
CA 02438101 2004-04-13
-7-
and accordingly its COD concentration reduces. As shown in Figure 6, the
COD removal rate tended to drop with time in each batch processing period
suggesting that the removal rate is related to the COD concentration in the
wastewater. Further, the removal rate in the batch between day 154 and day
159 approached zero indicating that further processing time would have
marginal value. In Figure 7, the COD removal rate is plotted directly against
the average COD concentration in the wastewater. As indicated in Figure 7,
the relationship between COD removal rate and COD concentration in the
wastewater is nearly linear with the removal rate being generally proportional
to the COD concentration.
Example 2
[0020] A module similar to that shown in Figure 3 having 100 tows,
each tow having 96 fibres of dense walled PMP as described in relation to
Figure 1 was tested. The total surface area of the fibre was 0.54 m2. In the
module, each tow was individually potted into an upper and lower header. The
module was fed with a supply of air at a rate of 30 ml/min to the top header
and exhausted air out of the other header. The module was suspended, with
the top header held in a clamp above the water surface and the bottom
header weighed down, in a container filled to a volume of 4 L with wastewater
from the second chamber of a septic tank. The characteristics of the
wastewater were as follows:
Total Chemical Oxygen Demand (CODs): 377 mglL
Soluble COD (CODS): 199 mglL
Ammonia Nitrogen (AN): 55.1 mg/L
Total Suspended Solids (TSS): 70 mg/L
The module was operated in a batch mode with batch processing periods of
approximately 24 hours to simulate actual reaction conditions in a septic
tank.
Air was supplied during these periods at the rate given above to provide
oxygen to the biofilm. After one processing period of 22 hours and 35
3
CA 02438101 2004-04-13
minutes in duration, a sample of the treated wastewater was analyzed and
results were as follows:
CODs: 140 mglL
CODS: 73 mglL
AN: 24.7 mg/L
TSS: 1 mg/L
A significant improvement in effluent quality was achieved. In particular, a
huge reduction in TSS was achieved. By visual observation, a large portion of
the TSS removed was in the form of colloidal matter.
