CA2438101A1

CA2438101A1 - Membrane supported biofilm modules using fibre tows

Info

Publication number: CA2438101A1
Application number: CA 2438101
Authority: CA
Inventors: Henry Behmann; Hidayat Husain; Pierre Lucien
Original assignee: Zenon Environmental Inc
Current assignee: GE Zenon ULC
Priority date: 2003-08-22
Filing date: 2003-08-22
Publication date: 2005-02-22

Abstract

A hollow gas transfer fibre is arranged in tows and potted into a module. The module may be used to treat a wastewater having a low GOD concentration by supporting a generally endogenous biofilm. The module may be used in a septic tank or as a downstream stage of a multistage treatment process or for single stage treatment, for example of low strength wastewaters.

Description

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 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, 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 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-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 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 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 °

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 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 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.

Claims

1. A module of hollow fibres arranged in tows of fibres potted in at least one header.

2. The module of claim 1 wherein the fibres have an outside diameter of 100 µm or less.

3. The module of claim 1 with an inert strengthening fibre added to the tows.

4. A process of treating wastewater by using the module of claims 1 to 3 to support and supply oxygen to a biofilm layer on the fibres.

5. The process in claim 4 operated with a generally endogenous biofilm.

6. The process of claim 4 operated as a batch process.

7. A septic tank with a module of any of claims 1 to 3 in it.

8. The process of claim 4 further comprising a step of starvation or periodic tank draining to control the thickness of the biofilm layer.

9. The process of claim 4 used to treat a feed water having a COD
concentration of 1,000 mg/L or less.

10. A multistage reactor for treating wastewater having an upstream stage and a downstream stage wherein the downstream stage has a module according to any of claims 1 to 3 used to support a biofilm.

11. The reactor of claim 10 wherein the upstream stage has a module of a sheet of gas transfer fibres formed into a fabric and used to support a biofilm.