Inverted air box aerator and aeration method for immersed membranes

Abstract

An aerator for immersed filtering membranes has an aerator shell with openings for discharging bubbles from its upper surface and a shape capable of temporarily containing a volume of air in fluid communication with the openings. The shell is open to tank water below it and located so that discharged bubbles will rise through an assembly of the filtering membranes. The shell may be wholly or partially made of parts of the assemblies of filtering membranes. A supply of air is provided to the air space in the aerators alternating between a high flow rate and a low flow rate in short cycles of between about 10 seconds and 100 seconds. A filtration system has an inlet for adding substrate and an outlet for retentate are located so as to create a horizontal flow of substrate through the tank. Membrane assemblies are located within the horizontal flow of substrate. Aerators as described above are provided and operated as described above.

1 . A process for aerating a membrane assembly comprising the steps of 
 a. providing 
 i. a membrane assembly having hollow fiber membranes held at their lower ends in a mass of potting material,  
 ii. side walls extending downwards from the mass of potting material; and,  
 iii. an aerator shell comprising the mass of potting material and the side walls and further comprising openings for discharging bubbles, the openings oriented vertically and adapted to transmit a gas through the aerator shell from below the mass of potting material to above the mass of potting material, the aerator shell adapted to contain a variable volume of the gas in fluid communication with the openings; and,  
   b. flowing a gas into the aerator shell at a rate that varies cyclically from a higher flow rate to a lower flow rate, the lower flow rate being in the range from no flow to one half of the higher flow rate, the cycles having a duration of 120 seconds or less.    
     
     
         2 . The process of  claim 1  wherein the rate of air flow during the period of low flow is 10% or less of the rate of air flow during the period of high flow.  
     
     
         3 . The process of  claim 2  wherein there is substantially no air flow during the period of low air flow.  
     
     
         4 . The process of any of  claim 1  wherein the period of high flow is between about ⅛ and ½ of the total cycle duration.  
     
     
         5 . The process of any of  claim 1  wherein the change between the high flow rate and low flow rate is performed in less than about 6 seconds.  
     
     
         6 . The process of  claim 5  wherein the change between the high flow rate and low flow rate is performed in less than about 3 seconds.  
     
     
         7 . The process of any of  claim 1  wherein the aerator is sized in relation to the duration and rate of air flow provided during a cycle such that air flows through the holes throughout each cycle.  
     
     
         8 . The process of any of  claim 1  wherein the aerator is sized in relation to the duration and rate of air flow provided during a cycle such that no air flows through the holes during at least part of the cycle such that liquid in the tank may flow into the holes.  
     
     
         9 . The process of  claim 8  wherein air flows through the holes of the aerator for between about ⅓ and ¾ of the cycle duration.  
     
     
         10 . The process of  claim 9  wherein air flows through the holes for about ½ of the cycle duration.