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13.38 CHAPTER THIRTEEN

Backwash & Chemical Cleaning
Backwash

Irreversible
S~Loss
FIGURE 13.24 MF/UF TMP
- - - ~ --Initial TMP
control with periodic backwash-
Time D ing and cleaning.

Backwashing. MF and UF systems require periodic backwashing and chemical clean-
ing (cleaning-in-place, CIP) to maintain permeability, minimize the needed driving pres-
sure, and prolong the useful life of the membranes (see Figure 13.24). Depending on the
specific membrane system, backwashing uses unchlorinated water, chlorinated water, or
gas (e.g., air). Gas-backwashed systems first remove foulants from the membrane by trans-
ferring gas through the membrane from the filtrate to the feed side and then flushing the
foulants to waste using feed flow. Backwash is accomplished using relatively frequent (5-
min to several-hour) and short duration (3- to 180-s) backwashes, depending on the spe-
cific application and membrane system. For example, it would not be unusual to have a
system that backwashes for 15 s every 15 min or 90 s every 20 min. For municipal wa-
ter systems, backwashing is typically an automatically controlled process initiated on a
set frequency and set for a specific duration. Backwash can also be automatically initi-
ated based on TMP rise and/or other monitored parameters.
Chemically Enhanced Backwash (CEIl). Some MF and UF systems use chemically en-
hanced backwashes (CEBs) or "mini-cleans" to maintain permeability, control TMP, and
extend the cleaning frequencies. CEBs are usually less frequent than typical backwashes
and may include a short time for the membranes to soak in the chemical solution. Com-
mon CEB chemicals are sodium hypochlorite, citric acid, and caustic soda, and their con-
centrations and use are dependent on the specific membrane product and the targeted type
of foulant.
Cleaning System. MF and UF systems are typically cleaned without removing the mem-
branes from the system---called cleaning-in-place. MF and UF systems commonly include
chemical CIP systems used for periodic major cleanings when routine backwashes and/or
CEBs are not adequate. Most municipal systems have manually initiated, automatically con-
trolled chemical cleaning systems. Chemicals used, their concentrations, and CIP protocols
are specific for each membrane product and the type of foulants expected to be present. Com-
mon cleaning chemicals are sodium hypochlorite, citric acid, caustic soda, and detergents.
CIPs are often performed in multiple steps and typically include soaking time and can take
several hours to perform. Typically, a CIP system consists of one or more chemical solution
tank(s), a transfer and/or recirculation CIP pump, and other facilities and equipment.
Membrane Integrity Tests. Most systems include instrumentation that continuously
monitors filtrate quality and membrane performance. This instrumentation may include
turbidity monitors, particle monitors and counters, and other devices. For municipal sys-
tems, state regulations should be reviewed to determine specific requirements for system
monitoring and membrane integrity testing.