CA2438432A1 - Membrane supported biofilm reactor for municipal and industrial wastewater treatment - Google Patents

Abstract

Gas permeable hollow fibres are made into a fabric sheet and potted between headers to form a module, The module maybe used to support and supply gases to a biofilm growing in a substrate of wastewater.

Description

Title: Membrane Supported Biofilm Reactor For Municipal And Industrial 9Nastewater Treatment Field of the invention [0001] This invention relates to gas transfer modules and processes, to water treatment and, more particularly, to a method and system for the treatment of industrial and municipal wastewater using a membrane supported biofilm.
Background 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. The rate of oxygen transfer is limited by the need for transfer from the bubbles to the water and then to the suspended microorganisms.
[0003, A second type of 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.
[0004 The membrane supported bioreactor concept involves growing biofilm on the surface of a gas permeable or transfer membrane. Oxygen containing gas is supplied on the one side of the membrane and biofilm is grown on the other side, which is exposed to the substrate. Oxygen transferred through the membrane is absorbed by the biofilm in the form of very fine bubbles. This process has not become commercially viable. For example, existing common hollow fibre porous membranes tend to wet resulting in a drastic drop in oxygen transfer rates. More recent efforts have focused on "sandwich° type fibre, with porous inside and outside surface and a very thin middle dense membrane layer that avoids wetting. The cost of this membrane is very high and the membrane is not suitable for a wastewater application.

Summary of the invention [0005) it is an object of this invention to provide a gas transfer module.
Another object is to provide a wastewater treatment process or apparatus using a membrane supported biofilm. Some aspects of the invention are described below.
[0006) A very fine, i.e. outside diameter of 100 microns or less, dense hollow fibre is made from polymethyl pentane (PMP), which has a high selectivity and diffusion coefficient for oxygen. Use of very small diameter fibre results in low cost and established textile fine fibre technology can be used. A very large surface area can be provided to achieve high oxygen transfer efficiency (OTE).
[0007) A fabric with a very large number of PMP or other hollow fibres is used to ensure that oxygen transfer does not become a limiting factor in controlling biological kinetics. The fabric may be made, for example, by weaving with the PMP fibre as weft and an inert fibre as warp to minimize the damage to the fibre while weaving. The fabric provides strength to the fine fibres to permit biofilm growth on its surface with minimal fibre breakage.
[0008) Modules may be built from fabric sheets with very high packing density to permit good substrate velocities across the surface without recircuiation of large volume of liquid. The modules enable a supply of oxygen containing gas, such as air, to the lumen of the hollow fibre without exposing the lumens to the wastewater. Very long fibre elements, for example between 1 and 3 metres or between 1.5 and 2.5 metres are used and potted in the module header to provide a tow cost configuration.
[0009) Air may be used as a means of controlling the biofilm thickness to a desired level. Treatment with acid, alkali, oxidant, or enzyme, or anaerobic treatment may be used periodically prior to aeration to weaken the biofilm and to improve the efficacy of air in completely or partially removing the biofilm. Other methods of biofilm control include in-situ digestion, periodic ozonation followed by digestion, periodic alkali or acid treatment followed by digestion, periodic enzyme treatment followed by digestion, and use of a higher life forms, such as worms to digest the biofilm periodically. To speed up the biological digestion reactions, the air supplied to the module may be preheated to raise the temperature of the bioreactor.
(0010] Plug flow or multistage continuous stirred or batch tank reactors may be used to conduct biological reactions at the highest possible substrate concentrations for a given feed. This maximizes mass transfer of organic carbon compounds and ammonia in the biofilm, eliminating this process as a potential limitation to reaction rates.
[0011] Oxygen enrichment may be used as a means of dealing with peak flows. Need for such oxygen enrichment may be determined by on-fine COD monitors, or set according to time of day for municipal applications where diurnal flow and strength variations are well known.
[0012] The module and bioreactor design may be used 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 of the hollow fibre.
(0013] Air or enriched air may be used to supply oxygen. The selection of enriched air and level of oxygen present in such air may be determined by the wastewater strength.
[0014] This invention may be used to digest primary or secondary sludge.
Brief description of the drawings [0015] Embodiments of the invention will be described below with reference to the following figures.
[0016] Figure 1 presents a picture of a fibre.
[0017] Figure 2 shows a plan view of a module having multiple sheets of fabric between a pair of headers.

(0018] Figure 3 shows a fabric sheet of fibres and details of the connection between the sheet and the headers of Figure 2.
(0019] Figure 4 presents a picture of a module.
(0020] Figure 5 is a chart of COD reduction by treating wastewater with a module having a single sheet of fibres.
(0021] Figure fi is a schematic representation of a reactor.
(0022] Figure 7 is a schematic representation of the tank of the reactor of Figure 6 with modules installed.
(0023] Figure 8 is a side elevation view of the tank of Figure 7.
(0024] Figure 9 is an enlarged view of a portion of the tank of Figure 7 showing module mounting structure in greater detail.
(0025] Figure 10 is a front view of the portion of the tank shown in Figure 9.
Detailed descriation of the embodiments (0026] Figure 1 shows a textile polymethyi pentene fibre with 45 micron outside diameter and 15 to 30 micron inside diameter. In Figure 2, the fibre is woven in a fabric, with PMP fibre running horizontally, and an inert fibre running vertically to provide support to the fine PMP fibre. Figure 3 shows the module, in which a bundle of fabric sheets is potted at both ends in a header using potting materials such as polyurethane, hot melt or epoxy. The bundle includes spacers which provide a gap between the sheets for aeration and substrate flow. These spacers may include plastic strips or hot melt layers.
The gap between sheets may range from 3 mm to 15 mm depending on the nature of the wastewater. The length of the module may range from 1 m to 5 m, with 1.5 m to 2.5 m as optimum for the fibre or Figure 1. Figure 4 shows a picture of an experimental module.
[0027] In an embodiment of the invention, a biofilm is grown on a fabric woven from textile PMP dense wall hollow fibre. Oxygen bearing gas is introduced in the lumen of the fibre. 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 water, and ammonia to nitrates. The surface of the biofilm is maintained under anoxic conditions such that conversion of nitrates and nitrites to nitrogen can take place. The result is simultaneous reduction of organic carbon, ammonia and total nitrogen.
[0028] In another embodiment, all the above features are used, except that a high aeration velocity of 2-8 feetlsecond is used at the outer surface of the fabric to reduce the thickness of the biofilm. This is done once every day to once every week. Also, air, for example from aerators outside of the modules, is used to periodically mix the contents of the bioreactor.
(0029] In another embodiment of the invention, a number of bioreactors are installed in series to provide flow patterns approaching plug flow. This results in higher reaction rates and better utilization of oxygen.
[0030] In another embodiment, ozone gas, introduced in the fibre lumen is used to oxidize a part of the biofilm to make it digestible. Oxygen is then provided to the lumens to digest the oxidized organics, thereby reducing the total amounts of solids generated and to control the biofilrn thickness.
The reactor may be treated in this way one section at a time.
[0031] In another embodiment of the invention, worms are used to digest excess biofilm to reduce bio-solids generation. The worms may be applied to a section of the reactor at a time. The worms are grown in a separate bioreactor.
[0032] In another embodiment of the invention, different oxygen levels are used in different stages of the bioreactor by oxygen spiking to meet different levels of oxygen demand and to achieve high bioreactor loadings.
[0033] In another embodiment of the invention, the elements or modules are stacked in a vertical configuration. Flow of aeration from outside the modules or of water in the tank may be from top to bottom or bottom to ,. CA 02438432 2005-O1-28 top. This minimizes the capital required for aeration and the operating cost of air.
[0034] Referring to Figure 6, a reactor 110 has a tank 112 with one or more membrane supported biofilm modules 114 installed inside of it. The modules 114 may be constructed of one or more hollow fibre sheets 115 as described above. The sheet 115 rnay be constructed of hollow fibers 117 and inert fibers 119 as shown in Figure 3. The module 114 has a gas inlet header 116 fed with air, or another oxygen containing gas, through a blower 118. Gas passes from the inlet header 116 to the inside of one or more gas transfer membranes 120. A portion of the gas passes through the membranes 120 while another portion, and possibly some gasses taken up from the tank 112, flow to an outlet header 122 of the modules 114 and to an exhaust 124. The gases leaving the exhaust 124 may be post-treated or discharged to the atmosphere.
(0035] Feed water enters the reactor 110 through a feed valve 126 and feed pump 128. The feed is filled to a feed fill level 130 above the modules 114. After a batch of feed has been treated, a drain valve 131 is opened to drain, the tank 112 of treated water. The treated water may flow to a municipal sewer to the environment, discharged directly to a receiving stream, to another stage of a membrane supported biofilm reactor, or to another sort of reactor for further processing. With appropriate modifications, the reactor may also be operated in other configurations such as plug flow or continuously stirred tank reactor, depending on the application and strength of the wastewater.
[0036] A biofilm 132 grows on the outside of the membranes 120. To control the thickness of the biofilm 132, one or more aerators 134 are provided below the modules 114 and connected to an aeration blower 136 through an aeration valve 138. The aeration blower 136 can be operated to provide bubbles when the tank 112 is full of water. The bubbles rise through the module 114 and physically remove some of the biofilm 132 from the membranes 120. The tank 112 is generally open to the atmosphere and ,.. CA 02438432 2005-O1-28 contains liquid at generally ambient pressure but has a lid 146 which may be closed from time to time to provide an enclosed space.
[0037] Referring to Figures 7 and 8, the tank 112 and modules 114 of an embodiment are shown. The embodiment of Figure 7 is a pilot reactor treating 1 cubic meter per day of industrial wastewater having a COD of over 1,000 mgJL, typically 7,000 mg/L. The feed is treated to reduce its COD
concentration to 300 mgJL as required for discharge into the municipal sewer that it outlets to. The tank 112 has a fill volume of 1.8 m3. Fifteen modules are provided in the tank 112, each module 114 containing six sheets 115 of 3.6 m2 surface area of a woven fabric of PMP fibers 117, woven as tows, providing the membranes 120: The fibres 117 are 1.8 m long and extend between the inlet 116 and outlet 122 headers of the modules 114. Total number of PMP tows per sheet is 1968, and fibres per sheet are 94464. Also, polyester yarn is woven perpendicular to the PMP fibre, and the total number of yarn per module is 1912. Air pressure drop in fibre lumen is in the range of 5 to 10 psi. Total biofilm area per module is 17 m2, and ratio of biofilm area to oxygen transfer area is 0.9.
[0038] Referring again to Figure 7, the modules 114 in the reactor 110 can be mounted in such a way that the tension of the sheets 115 extending between the headers 116, 122 can be adjusted. A rigid structure 150 is provided adjacent the modules 114, and one or both of the headers 116, 122 are movable relative to the rigid structure 150.
[0039] In the embodiment illustrated, the rigid structure 150 comprises a pair of side plates 152 that extend along the distal side surfaces of the outermost modules 114 of the stack of modules 114. As best seen in Figures 9 and 10, the modules 114 are attached to the side plate 152 by means of a mounting bracket 154 extending transversely between the side plates 152 at either end of the modules 114. The mounting brackets 154 are provided with grooves 156 shaped to receive T-shaped tongues 158 extending from surfaces of the headers 116, 122, opposite the sheets 115.

-[0040, The module 114 can be secured to the mounting brackets 154 by sliding the tongues 158 of the headers 116 and 122 into the grooves 156 of the brackets 154. The mounting brackets 154 can be secured to the side plate 152 by, for example, a bolt 160 passing through an aperture 162 engaging the plate 152 and a threaded hole 164 in an edge surface of the bracket 154.
[0041] The aperture 162 can be slot-shaped, so that the bracket 154 with the attached header 116, 122 can be shifted horizontally to increase or decrease the tension of the sheets 115. An eccentrically mounted cam member 166 can be provided between the head of the bolt 160 and the plate 152, with an outer diameter surface in engagement with an abutment surface 168 fixed to the plate 152. Rotating the cam member 166 can force the opposed brackets 154 further apart or allow them to draw closer together, thereby adjusting the tension of the sheets 115 in the modules 114.
[0042] The tension adjustment mechanism can be provided on only one end or on both ends of the modules 114, and can be modified to provide individual tension adjustment for each module 114 or for sub-groups of modules 114. Other mounting methods may also be used to allow modules 114 to be removed or tensioned. Similarly, other fibre densities or yarn densities may be used.
Examples:
Example 9: Chemical oxygen demand (COD7 reduction in a membrane su,~ported bioreactor [0043] A bench scale bioreactor was designed using an experimental module as presented in Figure 4 except that only a single sheet of the fibres was used. Wastewater with a COD level of 1000 mg/l was introduced in a batch manner periodically. A series of batch reactions were conducted to determine the rate of reaction and oxygen transfer efficiency. Figure 5 presents the results. it can be seen that 80-90% reduction of COD was obtained. Oxygen transfer efficiency during these series of tests ranged from 50 to 70%, as measured by the exit concentration of oxygen from the module.

_g_ Reactor volume was 30L. The length of the sheet was 0.57 m and height 0.45 m, providing a total biofilm area of approximately 0.5 m2 with both sides of sheet available for biofilm growth. Total oxygen transfer area was 1.0 m2.
Inlet air flow was 25 ml/min at a pressure of 34.5 kPa.

Claims (12)

1. Use of Polymethyl pentene fibre as a medium for supplying oxygen to a biofilm.

2. Use of PMP fibres in a fabric to provide support for biofilm growth

3. Potting of a bundle or sheet of the fabric into modules to supply and remove air while keeping air and wastewater separate.

4. Providing a gap of 2 mm to 20 mm or 3 mm to 15 mm between fabrics.

5. Making the module 0.25m to 3m long to optimize cost and pressure drop.

6. Using plug flow, multistage batch or multistage continuous stirred tank reactors with one to five stages.

7. Installing a number of modules in parallel in a reactor and the reactors in series in a multistage reactor format.

8: Using periodic aeration to remove excess biofilm

9. Using ozone oxidation of biofilm by introducing ozone gas in the lumen of the fibre, followed by supplying oxygen in the lumens to promote aerobic digestion to minimize biosolids production or to control biofilm thickness.

10. Using worms and other higher life forms to digest the excess biofilm by introducing a broth containing such higher life form.

11. Using in-situ aerobic digestion to minimize biosolids production.

12. Using oxygen spiking in all or part of the system during periods of high organic loading on the system.