The MBR Book: Principles and Applications of Membrane

Maintenance CIP was conducted through backflushing for 20–30 min using
150 mg/L NaOCl followed by an acid clean at pH 2.5 � 0.5. For hollow fibre membranes
the backflush (1.5 times the maximum operating flux) was applied intermittently
with 20–30 s pulses at applied at 5 min intervals. Flat sheet (FS) membranes
where backflushed continuously under gravity at a maximum hydrostatic head of
0.1 bar (the limit recommended by the supplier is 0.2 bar). Chemical usage per backflush
was between 2 L/m 2 for the HF membranes to 5 L/m 2 for the FS ones. Intensive
(recovery) cleaning was through soaking the membranes for 3–6 h in a high-strength
(0.1–0.5 wt%) NaOCl solution with pulsed aeration combined with suction for 5–10 s
every 20 minutes. Hypochlorite cleaning led to significant foaming due to formation
took place due to the organic matter present. The amount of chemical solution required
depended on the packing density of the membrane, but appeared to be in the range of
20–100 L/m 2 . Care was taken to drain the membrane of hypochlorite before applying
the acid wash of 0.5 mM sulphuric acid together with a 5 mM citric acid buffer, the mineral
acid being used to counter the alkalinity, the buffered acid pH being 2.8.
3.3 MBR design and operation
3.3.1 Reference data
Design 149
3.3.1.1 Aeration
Comparison of the pilot study data in Sections 3.2.1–3.2.6 (Table 3.20) demonstrates
the difficulty in attempting to generalise with design and operation of MBRs. A summary
of the data reveals substantial variation in some of the key performance parameters,
possibly because the conditions were not uniformly optimised. Extending the
analysis to available full-scale data from Chapter 5 (Table 3.21) provides a more reasonable
basis for an analysis and establishing appropriate operating conditions.
It has generally been observed from laboratory-scale studies that the attainable
flux increases with increasing aeration rate due to increased scouring. This is manifested
as an increase in the critical or sustainable flux (Section 2.3.7.1). In full-scale
plants increased aeration would be expected to produce an increase in sustainable
net permeability. Figure 3.3, based on the available data, would appear to indicate
that this is indeed the case. According to this figure there is a general tendency for
increasing permeability with increasing SAD m, though the data are very highly scattered.
Some of this data scatter can be attributed to obvious outliers, namely either
plant operating under sub-optimal conditions and/or small unstaffed plant where
blowers are more likely to be oversized to maintain permeability and so limit maintenance
requirements. Sustainable permeability also changes according to the nature
of aeration, that is the specifications of the aerator itself and the mode of application
(continuous or intermittent), and the cleaning protocol. Although physical cleaning
is to some extent accounted for by using net rather than gross flux in calculating permeability,
maintenance cleaning with hypochlorite permits higher permeabilities
to be sustained. A study of the data for flux for the two main technologies (Fig. 3.4,