Adjustable variable bubble size aeration for submerged membrane air scour

In a wastewater treatment facility including membrane bioreactor (MBRs) with membrane units submerged in mixed liquor suspended solids (MLSS) and with air diffusers positioned below the membrane units for aeration and/or air scouring of the membranes, the improvement comprising:

• a series of the submerged air diffusers, each being elongated in length and having a width, the diffusers being arranged parallel to one another and at a desired spacing, and at a preselected distance below the membranes,
• a supply of air, to each of the diffusers, so as to produce bubbles issuing from pores of the diffusers when air is delivered under pressure,
• a frame in which the diffusers are mounted for pivotal movement of the diffusers such that the width dimension of each diffuser can be tilted relative to horizontal and such that the series of diffusers can be tilted in unison, from an angle of about 0° to any desired tilt angle,
• the air diffusers at a selected inclined tilt angle allowing evolving fine bubbles at lower sides of the angled diffusers to coalesce with other bubbles as they rise, to form coarser bubbles, while bubbles from upper sides of the diffusers remain essentially as fine bubbles,
• whereby bubbles of finer and coarser sizes can be produced from each diffuser, such that smaller, slower-rising bubbles can supply aeration requirements as needed while coarser, faster-rising bubbles can supply desired mixing and energy for air scour of the membranes as needed, and whereby selection of a desired bubble size distribution can be made by selection of an angle of inclination.