Page 86 - Membranes for Industrial Wastewater Recovery and Re-Use

66 Membranes for Industrial Wastewater Recovery and Re-use

include the membrane material and feedwater bacterial and nutrient
concentration (Characklis and Marshall, 1 990).
The principle effect of biofilm formation on membrane process performance is
to reduce membrane permeability. In the case of cellulosic membranes the
bacteria may ingest the membrane itself, causing irreversible degradation and
significant loss of salt rejection. Since the elimination of biofouling is only
possible through complete removal of microorganisms, the only convenient
effective pretreatment is dosing with a liquid biocide. However, the most effective
of these (chlorine and its compounds) are oxidative and will degrade most
reverse osmosis membranes, the principal exception to this being cellulosic
materials. UV irradiation, and even pre-sterilisation, are of limited efficacy since
they generally are unable to completely prevent biofilm formation and cannot
act directly upon the biofilm once it has formed. The main emphasis, therefore, is
on control of the biofilm (where possible), and on its periodic removal through an
appropriate cleaning protocol.

Organic matter
There are essentially three key categories of organic foulants:

0 proteins,
0 carbohydrates, and
0 fats, oils and grease (FOG).

As with biological matter, pretreatment to remove organic matter is rarely
feasible, and it is more usual to adopt an appropriate backflush and, in
particular, cleaning strategy so as to ameliorate the worst effects of organic
fouling. Proteins and carbohydrates form a part of the extracellular polymeric
substances (EPS), as well as natural organic matter (NOM). Proteins may be
colloidal or dissolved, and are least soluble at their isoelectric point, which tends
to arise at pH values of 4-5. Their removal is therefore most effective at extremes
of pH. Carbohydrates include starches, polysaccharides, and fibrous and pectin
materials. Fouling by these materials is very sensitive to flux, and their removal
sensitive to the precise cleaning protocol adopted. FOG forms part of the
suspended matter, and is particularly problematic since these substances
generally have a high affinity for the more hydrophobic membranes, and
polysulphone in particular, and are not readily removed by normal backflushing
and cleaning methods. Their effective removal is normally only achievable at
high temperatures and/or by the use of organic solvents. Organic solvents are
only an option for ceramic or highly inert polymeric membranes (Section 2.1.3).
A summary of foulant impacts and pretreatment options for reverse osmosis
are listed in Table 2.1 5.

2.4.4 Backflushing and cleaning
An essential distinction must be made between the intermittent backflush cycle,
in which the fouled membrane is physically cleaned by flowing the product