Abstract:
An apparatus for treating water has a contact zone, a separation zone and a collection zone. The contact zone and the separation zone are located adjacent to each other in a tank. Feed water containing pressurized dissolved air enters and releases bubbles into the separation zone. The bubbles contact the contaminants to form bubble-contaminant complexes that float upwards and spread over the surface of the tank. Bubble-contaminant complexes move to the collection zone. A filtered permeate is withdrawn from the water in the tank through an immersed membrane filtration module located within the separation zone of the tank.

Description:
[0001]    This is an application claiming the benefit under 35 USC 119(e) of U.S. application Ser. No. 60/434,396; filed Dec. 19, 2002. The disclosure of U.S. application Ser. No. 60/434,396 is incorporated herein by this reference to it. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to water treatment, and more particularly to filtering water to provide water with reduced concentrations of particles or other contaminants, to dissolved air flotation and to immersed membrane filtration.  
         BACKGROUND OF THE INVENTION  
         [0003]    Raw water contains contaminants including suspended, colloidal and dissolved particles. Membrane filtration systems have been used with a pretreatment stage located upstream of a membrane filtration stage to reduce the amount of contaminants reaching the membranes. For example, clarifiers have been used to remove easily settled solids upstream of a membrane filter. Dissolved air filtration systems have been used as a pretreatment stage upstream of a media filter.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the present invention or inventions to improve on the prior art. Other objects of the invention or inventions include providing a water treatment process or apparatus. The one or more inventions may consist of combinations of one or more of the elements or steps described in this document. The summary below discusses various features that may help the reader understand the one or more inventions, but is not intended to define any invention.  
           [0005]    The inventors have noticed that pretreatment stages may beneficially reduce the rate of fouling of a downstream filter, but that each additional stage tends to increase capital cost and plant space. Each additional stage can also add to the complexity of the system. In the present invention, however, dissolved air flotation is combined with an immersed membrane filter. The dissolved air floatation removes various contaminants such as colloids which tend to foul membranes rapidly. The dissolved air flotation stage and membrane filtration stage are integrated together to provide a reduction in capital cost and plant space compared to a system having two distinct stages. In particular, the membrane filters are located inside of the separation zone of a dissolved air flotation stage. The membranes are located towards the bottom of the separation zone below the area where bubbles aggregate at or near the surface of the separation zone. Effluent from the integrated system is withdrawn as permeate through the membranes. Although the membranes are located within the separation zone, they nevertheless operate in an area of reduced concentrations of contaminants. Operating in this area, membrane fouling is reduced.  
           [0006]    In some aspects, the invention provides an apparatus for treating water having a contact zone, a separation zone and a collection zone. The separation zone further has an aggregation zone at or near the surface of the separation zone and a filtration zone directly below the aggregation zone. An inlet port is provided near the bottom of the contact zone for receiving a feed water containing pressurized dissolved air or otherwise pre-treated for a dissolved air filtration process. A pathway adapted to convey floating bubbles connects the top of the contact zone to the aggregation zone of the separation zone. A removal device, for example a skimmer, is operatively connected between the aggregation zone and the collection zone to move objects floating in the aggregation zone to the collection zone. An immersed suction-driven membrane filtration device is located in the filtration zone for withdrawing a filtered permeate from the filtration zone. The apparatus is adapted to perform dissolved air flotation and membrane filtration processes generally continuously and simultaneously.  
           [0007]    In other aspects, the invention provides a method of treating water, for example to remove colloids or other contaminants to provide water with reduced concentrations of contaminants. Feed water containing pressurized dissolved air, or otherwise pre-treated as for a dissolved air flotation process, flows into a contact zone. Bubbles are released into and rise through the contact zone and contact contaminants to form bubble-contaminant complexes. The bubble-contaminant complexes move from the contact zone to a separation zone. The bubble-contaminant complexes aggregate at or near the surface of the separation zone. The aggregated bubble-contaminant complexes are removed from the separation zone. A filtered permeate is withdrawn from an area of the separation zone below where the bubble-contaminant complexes are aggregated through an immersed membrane filtration module. These steps may be performed generally continuously and generally simultaneously. Solids retained in the contact zone or separation zone may be removed from time to time or, optionally, generally continuously. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which show exemplary embodiments of the present invention and in which:  
         [0009]    [0009]FIG. 1 is a schematic diagram of an integrated dissolved air flotation and immersed membrane filtration apparatus according to a first embodiment of the present invention.  
         [0010]    [0010]FIG. 2 is a schematic diagram of an integrated dissolved air flotation and immersed membrane filtration apparatus according to a second embodiment of the present invention.  
         [0011]    [0011]FIG. 3 is a schematic diagram of a water treatment system incorporating the integrated dissolved air flotation and immersed membrane filtration apparatus of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0012]    Referring first to FIG. 1, an integrated dissolved air flotation and immersed membrane filtration apparatus, or reactor, according to a first embodiment of the present invention is shown generally at  10 . The reactor  10  has a tank  12  that is open to the atmosphere, filtration means  14  and, optionally, a skimmer  16 .  
         [0013]    The tank  12  has three zones, including a contact zone  18 , a separation zone  20 , and a collection zone  22 . The separation zone  20  further has an aggregation zone  19  above a filtration zone  21 . The contact zone  18  and the separation zone  20  may be at least partially separated by a separating means, for example, a divider  24  that extend part of the way to the top of the tank  12  and leaving a path between the top of the contact zone  18  and the aggregation zone  19 . The tank  12  has an inlet port  26  that is fluidly connected to a feed water line  28 . The inlet port  26  may be located near the bottom of the contact zone  18 .  
         [0014]    The filtration means  14  may be an immersed suction driven membrane module. The filtration means  14  is located in the filtration zone  21 . The filtration means  14  may have pores that are sized for ultra-filtration or micro-filtration. For example an immersed hollow fiber membrane such as those sold under the trade mark ZEEWEED™ by Zenon Environmental Inc., such as ZW500 or ZW1000 modules, may be used. ZW 500 modules may be aerated intermittently to inhibit their fouling with minimal disturbance to the aggregation zone  19 . ZW 1000 modules may also be aerated intermittently or used without aeration. Suitable modules are also described in Canadian Patent No. 2,227,692 which is fully incorporated herein by this reference to it. Permeate (filtered water) is removed from the filtration means  14  through permeate line  30  which may be connected to a permeate pump, gravity outflow, a siphon or other source of suction.  
         [0015]    Still referring to FIG. 1, the reactor  10  is adapted to generally simultaneously and generally continuously perform dissolved air flotation and membrane filtration processes. For the dissolved air flotation process, feed water is introduced into the contact zone  18  of tank  12  through inlet port  26 . Coagulation chemicals and pressurized dissolved air are both added to the feed water upstream of the inlet port  26 . As the untreated feed water flows through the inlet port  26 , tiny air bubbles are produced in vast quantities in the contact zone  18 . In the contact zone  18 , at least a portion of the bubbles contact and adhere to at least a portion of the contaminants in the feed water to form bubble-contaminant complexes. The bubble-contaminant complexes flow across to the aggregation zone  19  of the separation zone  20 . The aggregation zone  19  has relatively quiescent conditions that allow the bubble-contaminant complexes to aggregate at the surface of the tank  12  where they dewater and form a float blanket. The optional skimmer  16 , or another removal means or device, collects the bubble-contaminant complexes from the surface of the tank  12 , and moves them into the solids collection zone  22 . One alternative means in place of the skimmer  16  is a weir or lowered edge of the tank  12  between the aggregation zone  19  and collection zone  22  that allows bubble-contaminant complexes to overflow into the collection zone  22 . In this case, bubble-contaminant complexes may be made to overflow generally continuously, by having a sufficient feed rate compared to flow rates out of the tank  12 , or from time to time when the filtration means  14  is backwashed. Waste is removed from the collection zone, for example through a waste outlet  32 .  
         [0016]    For the membrane filtration process, suction on the inner surface of the filtration means  14  draws filtered permeate through the membrane wall. Permeate is removed from the filtration means  14  through permeate line  30 . Contaminants not removed to the collection zone  22  are rejected by the filtration means  14  and accumulate in the separation zone  20 . These accumulated contaminants can be removed from time to time by pumping them out or by wholly or partially draining the separation zone  20  through a retentate drain  60 . Optionally, accumulated contaminants can be removed through the retentate drain without draining the separation zone  20 , or with draining of the separation zone  20  to a lesser degree, by generally simultaneously increasing the supply of feed water or backwashing the filtration means  14 . The filtration means  14  may be backwashed from time to time to dislodge contaminants and may be removed through a retentate drain  60  or pumped out from time to time.  
         [0017]    Referring now to FIG. 2, another integrated dissolved air flotation and immersed membrane filtration apparatus, or second reactor, is shown generally at  100 . The second reactor  100  is similar to the reactor  10  but with the modifications as described below. A second filtration means  114  comprises one or more membrane modules that substantially cover the cross sectional area of the separation zone  20 . Suitable modules are shown and described in PCT Publication No. WO 01/36075 which is fully incorporated herein by this reference to it. An aerator  40  is operated only periodically either during or directly before or after the second filtration means  114  is backwashed. In a manner similar to that described in WO 01/36075, the backwashing and aeration cause an air lift and increase in water level in the separation zone  20  which entrains contaminants retained in the filtration zone  21  and overflows then into the collection zone  22 . More persistent contaminants, if any, may also be pumped out from time to time or removed through the retentate drain  60 .  
         [0018]    Referring now to FIG. 3, a water treatment system is generally shown at  200  incorporating the reactor  10  of FIG. 1. As in a conventional dissolved air flotation process, coagulation chemicals are added to the untreated feed water upstream of the module  10 . Generally, two conditions are desirable to enhance flotation: (1) charge neutralization of the contaminants; and (2) production of hydrophobic contaminants. Adding one or more positively charged coagulation chemicals, such as polyaluminum chloride, tends to produce these conditions. The pH of the feed water may also be adjusted. Desired coagulant and pH conditions can be predicted by jar tests.  
         [0019]    Untreated feed water is introduced into a static mixer  32  through line  34 . Coagulant is added to the static mixer  32  through line  36 . The water is thoroughly mixed to ensure rapid dispersion of the coagulant. Optionally, a coagulation aid may also be added to the static mixer  32  through line  36 . The negatively charged suspended and colloidal contaminants are substantially neutralized in the static mixer  32 . Optionally, an acid or alkali may also be added to the static mixer  32  through line  36  to maintain the pH at a desired level.  
         [0020]    The water is withdrawn from the static mixer  32  through line  38 , and enters a flocculation tank  40 . The water in the flocculation tank  40  is gently mixed to promote particle collisions, resulting in the formation and growth of flocs. Large flocs are not needed for flotation, since the bubble-contaminant complexes require a density that is less than water to be able to rise to the surface of the tank  12 . Accordingly, in known manner, the residence time in the flocculation tank  40  is chosen to produce flocs of a size so they can be efficiently removed by the dissolved air flotation process.  
         [0021]    The water is withdrawn from the flocculation tank  40  through line  28 , and is introduced into the contact zone  18  of tank  12  through inlet port  26 . A percentage of permeate is recycled back to the inlet port  26  through line  42 . Optionally, unfiltered water from the filtration zone  21  may be recycled back to the inlet port through line  42 ′. Air under pressure is injected through line  44  into the recycled permeate. The recycled permeate is then passed through a pressurization pump  46 . The pressurization pump  46  sends the permeate to an air saturation tank  48 , where the permeate is saturated with air taken from the atmosphere and pressurize to high pressures (i.e., about 60 psi to about 100 psi). A needle valve  50  in a line between the saturation tank  48  and the line  28  may be used to control the flow rate of the recycled permeate. A sudden drop in pressure after the needle valve  50  causes the formation of fine air bubbles ranging from about 1 μm to about 100 μm in size as the recycled permeate is mixed with the untreated feed water and passed into the reactor  10 . Saturators, spargers or other means known in the art may also be used to produce the fine bubbles.  
         [0022]    It is to be understood that what has been described are preferred embodiments of the invention for example and without limitation to the combination of features necessary for carrying the invention into effect. The invention may be susceptible to certain changes and alternative embodiments without departing from the subject invention, the scope of which is defined in the following claims. In particular, but without limitation, alternatives for numerous aspects of the DAF system or process described as occurring outside of the tank  12  in FIG. 3 are known in the art and may be used with the present invention. Also, the configuration of the tank  12  may be modified and the various zones 18 to 22 may be distributed across multiple tanks or vessels.