Abstract:
A membrane supported biofilm reactor uses modules having fine, dense, non-porous hollow fibers made from Polymethyl pentene (PMP) formed into a fabric with the PMP as the weft. One or more sheets of the fabric are potted into a module to enable oxygen containing gas to be supplied to the lumens of the hollow fibers. Various reactors using such modules are described.

Description:
This is an application claiming the benefit under 35 USC 119(e) of U.S. Provisional Application Ser. No. 60/447,025, filed Feb. 13, 2003. All of U.S. Ser. No. 60/447,025 is incorporated herein by this reference to it. 

   FIELD OF THE INVENTION 
   This invention relates to wastewater treatment and, more particularly, to a method and system for the treatment of wastewater, for example industrial or municipal wastewater. 
   BACKGROUND OF THE INVENTION 
   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. 
   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. 
   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 one side of the membrane and the biofilm is grown on the other side, which is 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. 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to improve on the prior art. It is 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 advantage 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% 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. 
   In one aspect, this invention provides a very fine dense hollow fibre made from polymethyl pentene (PMP), which has a high selectivity and diffusion coefficient for oxygen. Use of very small diameter fibre helps reduce module cost as established textile fine fibre technology can be used. A very large surface area can be provided to achieve high OTE. 
   In another aspect, this invention provides a fabric with a very large number of PMP hollow fibres providing sufficient surface area so that oxygen transfer does not become a limiting factor in controlling biological kinetics. The fabric is made 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 fibre to permit biofilm growth on its surface with minimal fibre breakage. 
   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 recirculation 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. 
   In another aspect, this invention uses air as a means of 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 form, such as worms, to digest the biofilm periodically. To speed up the biological digestion reactions, the air is preheated to raise the temperature of the bioreactor. 
   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. 
   In another aspect, this invention uses oxygen enrichment as a means of dealing with peak flows. Such oxygen enrichment may be determine by on-line COD monitors, or set according to time of day for municipal applications where diurnal flow and strength variations are well known. 
   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 of the hollow fibre. 
   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 wastewater strength. 
   In another aspect, this invention uses one or more of the apparatuses described above to digest primary and secondary sludge. 
   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 combinations or sub-combinations of features or steps described above with features or steps described below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will be described below with reference to the following figures. 
       FIG. 1  presents a picture of a fibre. 
       FIG. 2  presents a drawing of a fabric made from the fibre of  FIG. 1 . 
       FIG. 3  is a drawing of a module made from the fabric of  FIG. 2 . 
       FIG. 4  presents a picture of the module of  FIG. 3 . 
       FIG. 5  is a graph of results of tests on the module of  FIG. 4 . 
       FIGS. 8 and 9  are drawings of reactors excerpted from U.S. Ser. No. 09/799,524. 
   

   DESCRIPTION OF EMBODIMENTS 
     FIG. 1  shows a textile polymethyl pentene 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. Outside diameters of 30–50 microns may be used. The fibres are hollow inside but non-porous. Oxygen travels through the fibre walls by molecular diffusion. 
   In  FIG. 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.  FIG. 3  shows a module, in which a bundle or stack of sheets of fabric are potted at both ends in a header using potting materials such as polyurethane, hot melt or epoxy. A large sheet of the fabric may also be rolled or folded to produce a module rather than using separate sheets. The bundle is assembled together with spacers between the sheets of fabric which provide a gap between the sheets for aeration and substrate flow. These spacers may be 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. To produce the module of  FIG. 3 , a sheet of fibres is laid onto strips of adhesive located to cross the ends of the fibres. Spacing strips were then placed over the sheet, followed by additional strips of adhesive and an additional sheet of fabric. These steps were repeated several times. The resulting assembly was then sealed into a pair of opposed headers such that the lumens of the fibres would be in communication with a port in one or both headers. 
   Gas containing oxygen flows into at least one of the headers. The module may be operated in a dead end mode, with no outlet other than through the fibres. Alternately, the module may be operated in a cross flow manner with gas entering through one header, flowing through the membranes then exiting from the other header. The oxygen content and flow rate of the gas may be set to produce an oxygen transfer that provides aerobic conditions near the membranes and anoxic conditions near the substrate being treated. Multiple reactions, including carbon based organics, ammonia and total nitrogen reduction, may be performed in the biofilm, 
     FIG. 4  shows a picture of a module assembled as described above. The headers were about 2 metres apart. Additional spacers were used mid way between the headers to better preserve the sheet separation. A thin steel rod was attached to the edges of the fabric sheet in the right half of the module to address the folding which can be seen in the left half of the Figure. 
   Reactors similar to those describes in U.S. patent application Ser. No. 09/799,524, filed Mar. 7, 2001, may be used. For example the reactors discussed in an excerpt from U.S. Ser. No. 09/799,524 reproduced below may be used with the present invention. The entire text of U.S. patent application Ser. No. 09/799,524, filed Mar. 7, 2001, is incorporated herein by this reference to it. 
   In another embodiment of the invention, a biofilm is grown on a fabric woven from textile PMP dense wall hollow fibre. Oxygen bearing gas is introduced into 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 to nitrogen can take place. The result is simultaneous reduction of organic carbon, ammonia and total nitrogen. 
   In another embodiment, all the above features are used, except that high aeration velocity of 2–8 feet/second is used at the surface of the fabric to reduce the thickness of the biofilm. This is done once every day to once every week. Also, air may be used to periodically mix the contents of the bioreactor. 
   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. 
   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 digest the oxidized organics, thereby reducing the total amounts of solids generated. 
   In another embodiment of the invention, worms are used in an isolated section of the reactor to digest excess biofilm to reduce bio-solids generation. The worms are grown in a separate bioreactor. 
   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. 
   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. 
   EXAMPLE 
   Example 1 
   Chemical Oxygen Demand (COD) Reduction in a Membrane Supported Bioreactor 
   A bench scale bioreactor was designed using the experimental module presented in  FIG. 4 . Wastewater with a COD level of 1000 mg/l was introduced in a batch manner at daily intervals. A series of batch reactions were conducted to determine the rate of reaction and oxygen transfer efficiency.  FIG. 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 air. 
   Excerpt Form U.S. Ser. No. 09/799,524 
   Membrane Supported Biofilm Reactors for Wastewater Treatment 
     FIG. 8  shows a reactor  80  having a tank  82 , a feed inlet  84  to the tank  82 , an effluent outlet  86  from the tank  82 , a flow path  88  between the feed inlet  84  and effluent outlet  86  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 . 
   The planar elements  226  are sized to fit the tank  82  and fill a substantial amount of its volume. The planar elements  226  have no pre-manufactured or rigid frame and thus are preferably custom made to provide efficient use of the available space in the tank  82 . For example, planar 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  FIG. 8 . The planar elements  226  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 5 to 15 mm to allow for biofilm growth and wastewater flow between adjacent planar elements  226 . 
   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 (ie. near the inlet  84  and outlet  86 ) of the tank  82  for vertical movement of water and leaving minimal free 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. 
   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 apparatus  210  is held in the tank  82  by its headers  52  attached to a frame  90  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 flow 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 . 
   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  94  located above the water, a leak in any third apparatus  210  will not admit water into the manifold nor any other third apparatus  210 . Gas leaves each third apparatus  210  through its outlet conduit  218  which is connected to an exhaust manifold  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 manifold  95  may have become rich in volatile organic compounds 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. 
   Preferably, the gas is provided at a pressure such that no bubbles are formed in the water to be 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 meter 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. 
   Oxygen diffuses through the membranes  12 . The amount of oxygen so diffused is preferably such that an aerobic biofilm is cultured adjacent the planar elements  226 , an anoxic biofilm is cultivated adjacent the aerobic biofilm and the wastewater to be treated is maintained in an anaerobic state. Such a biofilm provides for simultaneous nitrification and denitrification. A source of agitation  96  is operated from time to time to agitate the planar elements  226  to release accumulated biofilm. 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. 
     FIG. 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  may also be used with suitable modifications to the second reactor  180 . 
   Each first apparatus  10  is held by its loops  30  wrapped around wires  100  or ropes attached to the tank  182 . The loops  30  and wires  100  restrain each first apparatus  10  in a position in the second reactor  180  whereby the planar element  26  of each first apparatus  10  is generally parallel to the flow path  188 . 
   The first planar elements  26  are sized to fit the tank  182  and fill a substantial amount of its volume. Like the third planar elements  226 , the first planar elements  26  have no pre-manufactured or rigid frame and are preferably custom made to provide efficient use of the available space in the tank  182 . The first planar elements  26  may range from 0.25 to 1 mm in thickness and are placed side by side at a distance of 5 to 15 mm to allow for biofilm growth and wastewater flow between adjacent first planar elements  26 . 
   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 space near the top and bottom of the tank  82 . Water to be 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. 
   Oxygen containing gas is provided to each first apparatus  10  through its inlet conduit  16  connected to a manifold  94  located above the water to be treated. With the inlet manifold  94  located above the water, a leak in any first apparatus  10  will not admit water into the manifold nor any other 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 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  26 , can be used to reinforce part of the planar elements  26  if they can not be mounted deep enough. 
   Alternatively, gas flow through the first element  10  is produced by applying a suction, preferably of not more than 10 kPa less than atmospheric pressure, 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 regardless of the depth of the first element  10 . 
   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  is operated from time to time to agitate the first planar elements  26  to release accumulated biofilm. A suitable source of agitation is a series of mechanical mixers  102 .