The MBR Book: Principles and Applications of Membrane

238 The MBR Book
Figure 5.25 Membrane tank at the commissioning phase, Brescia
to 42 MLD capacity through the installation of additional membrane cassettes after
about 1 year of operation.
Plant design and operation The MBR system, which is protected by a fine screen
upstream, consists of a denitrification tank followed by one nitrification tank which
precedes four independent filtration trains. Each train is capable of producing
438 m 3 /h of permeate, yielding an overall installed capacity of 42 MLD from a total
membrane area of 73 000 m 2 (hence 24 LMH mean flux at peak load). In each train
permeate is withdrawn from two membrane tanks (Fig. 5.25), each having a capacity
of about 115 m 3 . Each tank thus holds 20 ZW500c cassettes (Fig. 5.26), giving 160
cassettes in total. Each cassette contains 22 membrane elements providing an area of
456 m 2 per cassette (20.7 m 2 per element). Fine bubble aeration takes place at
between 6000 and 11 000 Nm 3 /h depending on the organic loading. An MLSS concentration
of 8–10 g/L is maintained by an SRT of 17 days and an HRT of 7–8 h.
The plant is equipped with fully automated operation and maintenance (O&M)
cleaning. It is operated with a filtration cycle of 10.5 min production and 90 s relaxation:
no backflushing is employed at this plant. Since this is one of the more recent
Zenon plant, intermittent aeration is used, cycling 10 s on/10 s off by switching the
aeration between two banks of membranes. Coarse bubble aeration of the membranes
is conducted at a rate of 6 Nm 3 /h per membrane element overall (i.e. 12 Nm 3 /h during