Partial scouring air nozzle plugging causes uneven air flow pattern as shown in Fig. 1a. It is apparent that little air comes out to the surface especially in the right cassette. The membranes in the low air flow zone can develop sludge cake on membrane surface due to low mixed liquor supply to the membranes while loosing water as permeate. As a consequence, varying degree of caking occurs on membrane surface depending on the air flow rate to the membranes as shown in Fig. 2. Another consequence is that membranes can be bent due to either the cake in under aerated zone or the vigorous aeration in over-aerated zone as shown in Fig. 3.
It is very important to obtain evenly distributed scouring air for stable and reliable operation of MBR. All the potential causes listed here must be carefully reviewed during the design process, but the most common cause seems to be the insufficient removal or accidental leak of fibrous materials due to design flaw or improper screen installation/operation.
a) Before cleaning b) After cleaning
Fig. 1. Scouring air pattern before cleaning (left) and after cleaning (right) in Te Aroha WWTP in New Zealand (Mason, 2010)
Fig. 2. Varying degree of caking depending on scouring air flow (Mason, 2010)
Fig. 3. Used flat sheet membranes partially bent due to the improper operation especially with an excess scouring air. Spaces between panels are irregular. (Ferré, 2009)
© Seong Hoon Yoon