Page 606 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Membrane Processes 561

that breaches may be detected if there is such an occurrence. Cross-flow velocity
Cleaning is another issue; to determine when replacement
is needed is yet another. Cleaning chemicals must be Forward wash velocity
ordered, stored, and utilized with special regard to safety
and regulations. Pumps must be maintained, sensors cali-
brated, computer outputs monitored, and samples obtained
and sent to a laboratory; also, reporting is required along
Permeate water side
with archiving data. Membrane
(a)
Cross-flow velocity
17.5.1 INTEGRITY TESTING
Integrity monitoring may be continuous, periodic, both, or
variable; some form was required by 15 states (in 2001). Reverse wash velocity
Assurance of membrane capabilities is tied to integrity
testing and monitoring. Most of this section was adapted
from Hugaboom et al. (2003). Their work was referenced to
six full-scale hollow-fiber MF and UF systems; some were Permeate water side
Membrane
dead end and two had cross-flow capability. Three had per- (b)
meate backwash and three used permeate and air. Five were
enclosed and one was immersed. FIGURE 17.23 Depiction of membrane surface washing. (a) For-
ward washing (typical) and (b) reverse washing (not typical).
17.5.1.1 Breaches
A breach is a ‘‘leak’’ in the system, that is, a small portion of reverse washing (opposite in direction to the cross-flow of
the feed water may reach permeate flow without passing the feed water) as shown in Figure 17.23b. Back-flushing, that
through the membrane. Types of breaches include pinpoint, is, causing the clean water to flow from the permeate side of
complete break, chemical or biological degradation, cross- the membrane to the feed-flow side, may help in dislodging
connection via macropores, mechanical failures (o-rings, deposited materials (Wiesner, 1993, p. 42).
gaskets, glued fittings), etc. In hollow-fiber bundles, individ-
ual fibers may break, for example, one fiber out of thousands.
17.6 PILOT PLANTS
17.5.1.2 Testing
A pilot plant study will help to resolve many questions of
Two methods of integrity monitoring are direct and indirect design and operation. Such studies are used almost routinely
evaluations. The direct methods include pressure decay, dif- as a basis for design and to establish operating protocol.
fusive air flow, bubble point testing, and sonic sensor testing.
Indirect methods include particle counting, particle monitor-
17.6.1 UTILITY OF PILOT PLANTS
ing, turbidity monitoring, laser turbidity monitoring, micro-
bial challenge testing, spiked integrity monitoring, and routine Questions for a pilot plant study may include the rate of
microbial testing. reversible and irreversible fouling, pretreatment method,
cleaning chemicals, cleaning protocol, removals of constitu-
ents, type of membrane, concentrate character and disposal
17.5.2 CLEANING
options, membrane integrity testing, monitoring parameters,
When water flux decline becomes significant, membrane and frequency. After a given study is completed, the pilot
modules are removed from service and washed using stored plant may have utility in resolving questions of operation as
permeate water and chemical solutions. The chemicals used they develop, in operator training, and for public tours.
depend on the cause of the fouling. Most selections are based
on trial and error, that is, what works. Chemical solutions 17.6.1.1 Pilot Plant Design
include alkaline (basic) soap solutions and acid solutions. A pilot plant may range from a single 51 mm (2 in.) diameter
Alkaline soap solutions are used to remove organic and par- module to a larger size, for example, 102 mm (4 in.) diameter
ticle fouling. Acid solutions are used to remove precipitated with several in parallel. Usually, at least two modules in
inorganics. parallel are set up that permit comparison of two different
Initially, membranes are rinsed with stored permeate water. membranes, or two conditions, for example, two source
Following this, an alkaline solution and then an acid solution waters, two pretreatment modes, two cleaning solutions, two
are recycled through membrane modules. Permeate water is operating pressures, etc. The more modules that are set up in
used to rinse the membrane between solution washes and parallel, the more that may be accomplished (and the higher
before placing the system online. the cost). Often, pilot plant skids are available from manufac-
Membranes are typically forward washed as shown in turer’s plant (perhaps for rent), or a pilot plant may be con-
Figure 17.23a. Some modular designs, however, allow for structed without too much effort, depending on the situation.