Page 85 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 65

effective threshold chemicals, including phosphonates. have become available.
Polycarboxylic acids, such as polyacrylates and polymalonates, operate by
blocking crystal growth sites preventing the growth of nuclei into crystals whiIst
chelating chemicals react with potentially insoluble cations like calcium to form
a soluble complex. Dosage rates are typically of the order of 5-10 mg 1-1 and,
whilst some indication of the efficacy of these reagents may arise from the scale
inhibition mechanism, it is generally the case that only pilot trials yield reliable
information as to their suitability for a particular duty.
Where scale-inhibiting chemicals are unable to cope with the concentrations
involved, and where chemical dosing is to be avoided for some reason, then
pretreatment of the water is necessary to remove the scale-forming salts. This
may be by sodium cycle ion exchange softening, ion exchange dealkalisation,
lime or lime-soda softening or even a nanofiltration process if divalent ions are to
be selectively removed. Some scalants are particularly recalcitrant. Dissolved (or
“active”) silica, evaluated using ASTM D4993, is not readily removed by
pretreatment and there are proprietary reagents that have been developed
specifically to inhibit its precipitation. However, modern RO membranes are
reasonably tolerant, and raw environmental waters rarely require more than
filtration and dosing as pretreatment for membrane permeation.

Mircroorganisms and nutrients
Bacteria are ubiquitous and thrive in the high surface area environment of a
reverse osmosis membrane element, where they form biofilms. They are
naturally transported towards the membrane surface under the force of the
permeate flow, and are supplied with nutrients both from the feedwater and,
under some circumstances, the pretreatment chemicals. Reports of the latter, as
discussed by Flemming (1992), include flocculants (Graham et al., 1989),
phosphorus-containing scale inhibition chemicals (Ahmed and Alansari,
1989), sodium thiosulphate, as used for quenching chlorine (Winters and
Isquith, 1979), and even chlorine itself, which was found to degrade organic
matter sufficiently for it to become biologically assimilable (Applegate et al.,
1986). Although they can form substantial biofilm layers in potable and
wastewater applications, bacteria require only an extremely low nutrient load to
survive in biofilms and can exist even in ultrapure water systems, where their
management becomes particularly vexing, Biofilm formation is thus
unavoidable in most membrane operations, and it is likely that all membrane
fouling is associated with biofilm formation, although the biofilm itself is not
necessarily onerous.
Biofilm formation results from the rapid formation of an organic film, normally
within the first few minutes of operation, followed by microbial adhesion and
further entrapment of dissolved and suspended solid matter which co-deposits
with the microorganisms. The conditioning film can be existing organic matter
from the feedwater, such as NOM, or microbial products, namely extracellular
polymeric substances (EPS). The rate of microbial deposition and the overall
biofilm thickness depend on hydrodynamics, with biofilm thickness apparently
decreasing with increasing turbulence (Ridgeway, 1988). Other key factors