Page 604 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Membrane Processes 559

are high, the relationship between water flux and pressure is pretreatment system, chemicals for pH adjustment, gas-strip-
valid. If, however, the process deviates from these conditions ping tower, storage of permeate water, blending tank, pump-
substantially, water flux may become independent of pressure. ing to distribution system, sensors, actuated valves, pumps for
The asymptotic portion of the curves that depict the water flux backwash, laboratory, offices, computer control system, etc.
vs. pressure relationship, as illustrated in Figure 17.21 in the
‘‘diffusion-controlled region,’’ is considered a result of con-
17.4.1 PRETREATMENT
centration polarization (Cheryan, 1986).
Most systems require some kind of pretreatment to remove or
17.3.8.5.4 Effect of Temperature reduce levels of whatever ‘‘foulants’’ may exist in the feed
In general, increasing the temperature of the feed water will water. As noted, foulants are in four categories, that is, (1)
increase flux in both the pressure-controlled and the diffusion- particles, (2) organics, usually NOM, (3) bacteria, and (4)
controlled regions of operation, as indicated in Figure 17.21. mineral substances such as silica. In addition, substances
In the pressure-controlled region, temperature affects water could be present, for example, chlorine and other chemicals,
flux due to its effect on viscosity. In the diffusion-controlled that could damage some membranes.
region, temperature affects the diffusivity of the feed
solutes, thereby affecting concentration polarization effects 17.4.1.1 Cartridge Filters
(Cheryan, 1986). For particles, cartridge filters may be the most economical
choice to reduce particles to tolerable levels for a given
17.3.8.5.5 Effect of Feed-Water Solute Concentration membrane. The cartridge filter assembly for a plant may
A higher solute concentration in the feed flow can signifi- consist of a bank of such cartridges placed (in parallel) in a
cantly decrease permeate flux and increase fouling by decreas- pressurized stainless steel vessel. For redundancy, at least two
ing solute diffusivity, and thus increase concentration such vessels should be used. The main issue is the frequency
polarization effects. At the same time, increasing feed con- of replacing the cartridge elements. For example, some sur-
centration may increase fouling by exceeding solute solubi- face waters may render cartridge filters ‘‘blinded’’ after only
lity’s within the concentration polarization boundary, thus 1–2 days of use. In other cases, for example, well waters,
precipitating solutes on the membrane surface. Concentrated where their use provides a margin of safety for expensive
organics near the surface of the membrane can form a gelat- membranes, the change frequency may be several months.
inous layer or what is known as a gel layer (Cheryan, 1986).
The gel layer can enhance the collection of particles and the 17.4.1.2 Microfilter
growth of microorganisms on the surface of the membrane. For higher raw-water particle concentrations, MF may be
integrated into the treatment train ahead of NF=RO. Again,
17.3.8.5.6 Cross-Flow Velocity pretreatment may be required ahead of MF.
The fluid shear at the membrane surface is proportional to the
cross-flow velocity. Deposited materials at the membrane 17.4.1.3 Conventional Treatment
surface are resuspended in proportion to this shear stress, As noted, pretreatment may be required ahead of MF, and
being resisted by the adhesion between the foulant and the especially ahead of RO. For the case of Westminster, CO, a
3
membrane and the cohesive forces within the foulant material. 57,000 m =day (15 mgd) MF plant was constructed (in 2002)
Higher cross-flow velocities decrease the effects of concen- to handle turbidities >20 NTU by coagulation, flocculation,
tration polarization by reducing the boundary layer thickness and plate settling (without filtration).
at the surface of the membrane. A cross-flow velocity of
1–4m=s will produce sufficient shear to resuspend most 17.4.1.4 Other Pretreatment
deposited materials (Cheryan, 1986). In the case of spiral- Other foulants, for example, NOM, bacteria, minerals, etc.,
wound membrane elements, mesh-like materials are used for each require treatment specific to the situation at hand. For
feed-flow spacers to promote turbulence. These spacers, how- example, NF or RO may be constructed following a conven-
ever, create stagnant flow zones behind the spacers and may tional drinking water treatment plant to remove excess total
enhance deposition of particle matter (Champlin, 1998). organic carbon (TOC), or say, arsenic. In some cases, special
pilot plant studies, or even research projects, may be neces-
sary to determine the means of handling a given issue. Sea-
17.4 DESIGN
water has an abundance of biotic matter that will readily foul
Only two aspects of process design are covered here: pretreat- membranes; subocean floor intakes have been reported as a
ment and layout. Design of a plant includes, however, many means to condition the feed water (Furukawa, 2006).
ancillary aspects to make it work. A membrane skid is
common and may be installed in a building sized to accom-
17.4.2 MEMBRANE LAYOUTS
modate as many skids as needed for present and future flows.
Some of the ancillary components may include header pipes, NF=RO membrane modules are laid out commonly in a ‘‘tree’’
tubes, gages, etc. The support facilities may include pumps arrangement. In most circumstances, the fraction of feed water
to pressurize the system, cleaning tanks with containment, that may be filtered by an element is 0.10 R < 0.30.