Membrane and Desalination Technologies - TCE Moodle Website

Membrane Systems Planning and Design 349
be adjusted to maintain the pH within an acceptable operating range to minimize the
hydrolysis (i.e., chemical deterioration) of the membrane. The addition of chlorine or other
disinfectants may also be used as pretreatment for primary disinfection or to control
biofouling. However, because some NF/RO membrane materials are readily damaged by
oxidants, it is important that any disinfectants added upstream be quenched prior to contact
with such membranes.
A number of different chemicals may be added as pretreatment for MF or UF, depending
on the treatment objectives for the system. For example, lime may be added for softening
applications; coagulants may be added to enhance removal of TOC with the intent of
minimizing the formation of DBPs or increasing particulate removal; disinfectants may be
applied for either primary disinfection or biofouling control; and various oxidants can be used
to precipitate metals such iron and manganese for subsequent filtration. However, as with NF
and RO membranes, it is important to ensure that the applied pretreatment chemicals are
compatible with the particular membrane material used. As with conventional media filters,
pre-settling may be used in conjunction with pretreatment processes such as coagulation and
lime softening. While a MF/UF system may be able to operate efficiently with the in-line
addition of lime or coagulants, pre-settling in association with these pretreatment processes
can enhance membrane flux and increase system productivity by minimizing backwashing
and chemical cleaning frequency.
Although chemical pretreatment is generally not associated with cartridge filters, if the
filters are compatible, disinfectants may be added upstream of MCF systems to maximize
the time for primary disinfection (i.e., CT). Because cartridge filters are designed to be
disposable and are generally not backwashed, pretreatment chemicals such as lime or
coagulants could rapidly foul the cartridges and thus are not applied upstream these
systems.
With any form of chemical pretreatment, it is very important to understand whether any
chemical under consideration for use is compatible with the membrane material. In addition
to irreversible fouling and/or physical damage to the membranes, the use of an incompatible
chemical may void the manufacturer’s warranty. Some chemicals such as oxidants can be
quenched upstream, while others such as coagulants and lime cannot be counteracted prior to
membrane exposure. In general, most NF/RO membranes and some MF/UF membranes are
not compatible with disinfectants and other oxidants. However, some MF/UF membranes that
are incompatible with stronger oxidants such as chlorine may have a greater tolerance for
weaker disinfectants such as chloramine, which may allow for a measure of biofouling
control without damaging the membranes. Certain types of both MF/UF and NF/RO membranes
require operation within a certain pH range. Coagulants and lime are incompatible
with many NF/RO membranes but are typically compatible with most types of MF/UF
membranes. Polymers are incompatible with NF/RO membranes, and generally not compatible
with MF/UF membranes, as well, although this depends to some degree on the charge of
the polymer relative to the charge associated with the membrane. A polymer with a charge
opposite to that of the membrane is likely to cause rapid and potentially irreversible fouling. It
is critical to consult with the membrane manufacturer prior to implementing any form of
chemical pretreatment.