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MEMBRANE PROCESSES 13.43

Membrane Cleaning System

With few exceptions, all membrane systems are subject to fouling by one or more source
water components and therefore require periodic cleaning. Membrane cleaning is usually
performed without removing membranes from the pressure vessels or the system (i.e.,
cleaned in place). A cleaning system is designed to prepare and recirculate chemical so-
lutions through some of or all membrane modules at low pressure.
The cleaning system can also be used to feed special membrane posttreatment chem-
icals (not to be confused with membrane system posttreatment) that sometimes are used
to improve membrane performance. The cleaning system also serves to prepare and trans-
fer membrane storage solutions, or preservatives, used to prevent microbial growth and
in some cases to prevent freezing when the membrane system is shut down for extended
periods, typically more than a week.
The cleaning system for an RO or NF system should be designed to accommodate all
cleaning and membrane storage solutions expected to be used at the plant. Cleaning sys-
tems typically consist of the following basic components:
• Tanks with mixers for holding cleaning solutions.
• A pump providing sufficient head to circulate the solution from the tank through the
membrane system and back to the tank.
• A cartridge filter to intercept any suspended solids present in the cleaning solution and
prevent them from clogging the module flow passages.
• Piping and valves to transfer the solution to and from the train feed, permeate, and con-
centrate headers. The design should consider the use of removable piping spools or
hoses to provide positive air break in the cleaning system connection points. An air
break prevents chemical solutions from entering the product header piping and being
transported to downstream product facilities when the membrane system is being
put back into service or when one train is being cleaned while other trains remain in
operation.
• Instrumentation and control for proper system operation, including tank level, temper-
ature switches, and a pH monitor.
• A cleaning solution heating device (not always required), such as an immersion heater,
to increase solution temperature and improve the efficiency of cleaning, particularly
when cleaning with detergents.
• A cleaning solution cooling device (usually not required), such as a heat exchanger ex-
ternal to the cleaning solution tank or cooling coils inside the tank, if it is determined
that the maximum solution temperature needs to be controlled. The need for cooling
increases when the cleaning solution makeup water is at relatively high temperature or
the heat energy expected to be transferred to the cleaning solution from the cleaning
pump while recirculating is too great.
Typical design allows for separate cleaning of each stage of a multistage train. With
large-capacity trains, individual sections of a stage can be designed to allow separate clean-
ing, minimizing cleaning tank volume and pumping requirements.

Residuals Disposal

The designer must consider the least expensive, acceptable method of disposing of wastes
generated in both pretreatment processes and residuals resulting from membrane treat-
ment processes.