Membrane and Desalination Technologies - TCE Moodle Website

Membrane Separation: Basics and Applications 317
studies and site experience have led to better understanding of process parameters, allowing
process optimization making membrane processes more technically feasible.
Water Factory 21 has demonstrated that MF pre-treatment can provide good pre-treatment
for the RO processes with multiple benefits over conventional pre-treatment (75). The
benefits of MF pre-treatment include increased RO flux and overall efficiency, prolonged
operation time between cleans, and reduction in operating and chemical costs. Following this
successful demonstration at Water Factory 21, the industry is moving from lime clarification
towards MF and UF as a pre-treatment. There are multiple pilot projects evaluating MF as
a pre-treatment to the RO process. Pilot study performed in San Francisco utilizes MF
as pre-treatment to RO for desalination of municipal wastewater for horticultural reuse.
The average turbidity removal was 99.4 0.4%. The average silt density index (SDI) was
1.15 0.53.
Pilot plant studies conducted at Canary Islands (Spain) showed that microfiltered secondary
effluent from Tías WWTP contained below 1.0 mg/L of suspended solids and turbidity
below 1.0 NTU. Total and fecal coliforms were also absent from the microfiltered water. The
SDI of the microfiltered water was below 3.0. Average removal achieved for BOD5, COD and
TOC were 81, 40 and 27%, respectively. The MF achieved water recovery of about 85% (76).
A study was performed to investigate MF pre-treatment performance in treating a broad
range of water. Results showed that the performance of MF remains satisfactory when
subjected to very cold water (0.2 C), water with high iron content and water with high
organic load and biofouling potential (77). Acid wash has to be included in the operation
procedures when treating high iron content water to prolong run time between chemical
cleaning. The addition of acid keeps the iron content in the water in a reduced and dissolved
form, preventing precipitation and scaling on the membrane.
A lab-scale evaluation of pre-treatment for RO recycling of secondary effluent from
refinery demonstrated that UF is able to provide good pre-treatment for subsequent RO
process. UF was capable of removing over 98% of suspended solids and colloids. Partial
removal (30%) of COD was also achieved. The removal efficiency was consistent and
was independent of influent water quality and operating conditions (78). Similar results
reported by Qin and coworkers (79) demonstrate that an appropriate UF pre-treatment
could reduce fouling of RO membrane and increase the flux of RO membrane by
30–50%.
Another pilot study on UF–RO membrane treatment of industrial effluent (pulp and paper
mill effluent) showed that UF permeate flux increased with temperature of feed water. The
fluxes were 1.44 and 1.84 times higher at 30 and 40 C, respectively, compared to flux at
20 C. The improvement in flux was attributed to decreased wastewater viscosity with
increased temperature. However, at higher temperatures, more organic matter was able
to diffuse through the membrane. Comparison of UF pre-treatment at feed pH 2.4, 5.3 and
7.0 showed that higher flux is achieved at pH 7.0 (80).
MF and UF can reduce biofouling tendency in RO membrane as they pose a physical
barrier to these microorganisms. MF is capable of removal of protozoa ( 10 mm), coliform
( 1 mm) and cysts ( 0.1 mm). The pore size of UF is smaller and thus can further remove
viruses ( 0.01–0.1 mm) (81). It has been reported that UF showed more than five-log