Membrane and Desalination Technologies - TCE Moodle Website

318 J. Paul Chen et al.
reduction of microbial content (82). Membranes are attractive as disinfection process because
it reduces dosage of aggressive chemicals such as chlorine and ozone. In addition, undesirable
disinfection by-products can be minimized or avoided. The interest in using membrane
as part of the disinfection process has intensified with the emergence of chlorine-resistant
pathogens. Chlorine-resistant Cryptosporidium parvum has been reported to cause outbreak
of diarrhoea epidemic in US and UK. Water supply authorities are looking to UF and
MF application to act as an absolute barrier to Cryptosporidium oocysts, which range from
4to6mm (83).
Chlorination of secondary effluent prior to membrane pre-treatment may extend membrane
run times between cleans. Over 90 h of MF operation were achieved with prechlorinated
secondary effluent compared to 42-h operation reported when secondary effluent was
not chlorinated (72, 73). Similar observations were reported with dosage of chloramine prior
to microfiltration pre-treatment (77). It was speculated that preoxidation due to chlorination
altered the chemistry of EPS produced by the microorganisms in the secondary effluent. This
could weaken the attachment of the EPS on the membrane and thus offset the detrimental
effect on the membrane flux. However, care must be taken to verify compatibility of
membrane with chlorination as some membranes are not tolerant to the aggressive action
of chlorine.
Although MF and UF membranes have been shown to be a viable option as feed
pre-treatment for RO, long-term operating and cost data are required to verify that membrane
pre-treatment is more cost effective than conventional pre-treatment.
From a fouling perspective, use of membrane pre-treatment has made it possible to
segregate flux loss due to colloidal fouling and biofouling from flux loss due to scaling
(84). Anti-fouling strategy can be developed with more focus on relevant type of fouling.
Although microbes can escape defective portions of pre-treatment membrane and cause
biofouling, the potential and degree of biofouling have been greatly reduced. This can reduce
the application of disinfection, increase operation interval between cleans and prolong
membrane life.
Overall, studies on membrane pre-treatment demonstrate the following advantages:
l Addition of chemicals is not required
l Effluent quality is independent of feed quality
l Operation at ambient temperature
l Forms an absolute barrier to pathogens
l Space efficient
The simplicity of membrane operation makes it an attractive option in the field of wastewater
reclamation.
8. MEMBRANE CLEANING AND FLUX RESTORATION
Fouling is almost an inevitable consequence of the nature of the RO process itself even
when good pre-treatment is employed. The challenge is therefore to reduce and control
fouling sufficiently to minimize the rate of RO flux decline and prolong membrane lifetime.