The MBR Book: Principles and Applications of Membrane

152 The MBR Book
No trend is evident in this data subset, though all but the highest (and probably
non-optimal) SAD p values lie within the range 20–40.
(b) HF systems tend to operate at lower permeabilities (generally �200 LMH/bar)
and are associated with lower aeration demands, sometimes achieved by employing
aeration intermittently. For these systems the permeability attained is roughly
linearly related to aeration demand (Fig. 3.2), with around 0.4 m 3 permeate generated
per Nm 3 air per bar, and SAD p values are generally between 15 and 30.
Conventionally MBRs, in common with all membrane processes, are designed on
the basis of operating flux. However, permeability more accurately reflects the fouling
condition of the membrane. Although the permeability of the virgin membrane
material varies somewhat from one membrane product to another, a low operating
permeability is generally indicative of a high degree of fouling. Given that permeability
is the ratio of flux to TMP, then operation within given boundaries of permeability
will necessarily determine the range of operational TMP values and vice versa.
According to Fig. 3.3 it follows that, for HF systems at least, the required aeration
rate for maintaining a given permeability (or TMP for a given target flux) can be estimated
from the permeability itself. Alternatively, flux can be correlated with aeration
rate from data depicted in Fig. 3.4; that is:
SAD � 0.0044J �0.708
for FS module
m net
SAD � 0.0052J �0.326
for HF module
p net
although clearly more data is required to ratify such correlations.
(3.18)
(3.19)
3.3.1.2 Cleaning
A summary of physical and chemical cleaning protocols for the pilot plant study
data (Table 3.22) and full-scale installation data (Table 3.23) reveals that physical
cleaning is predominantly by relaxation rather than backflushing. Pilot plant data
indicate downtime for physical cleaning to account for between 4% and 20% of the
operating time, with no profound difference between the two configurations. On the
other hand, maintenance cleaning every 3–14 days is routinely employed for some
HF technologies whereas chemical cleaning is usually limited to recovery cleans
alone for FS systems.
For both FS and HF systems recovery cleans are generally applied at intervals of
6–18 months depending on flux. Recovery cleans generally employ hypochlorite
concentrations between 0.2 and 1 wt% NaOCl (Table 3.9), usually adjusted to a pH
of �12. Both maintenance and recovery cleaning are brief and/or infrequent enough
to add little to the percentage downtime. For example recovery cleans at Porlock and
Brescia have been applied, on average, once every 9 and 15 months respectively. Even
for an overnight soak of 16 h, the total downtime for cleaning amounts to �0.25%
in such cases. For Zenon sewage treatment plants the maintenance cleaning cycle
normally incurs no more than 2 h of downtime and is employed once every one or