Cleaning of reverse osmosis (RO) and nanofiltration (NF) membranes
“Each cleaning situation is different; therefore,
specific cleaning recommendations are dependent
on the type of foulant.“
“Most effective cleaning allows longer system
operating time between cleanings and results in
.the lowest operating costs“
(DOW FilmTec Technical manual)
?When to clean
All membranes must be cleaned periodically; as a minimum at least
one routine clean per year is required. Although the symptoms
of contamination are not always immediately detectable, the
membranes should be cleaned whenever there are variations of 10-
15% of normalised data for the parameters of: differential pressure,
product flow or permeate quality.
?What equipment is needed to clean
Most RO/NF systems include a clean-in-place (CIP) system. The
basic design consists of a tank for preparation and mixing of
cleaning solutions, heating element and a centrifugal pump for
re-circulating the cleaning solution throughout the system. It
should also incorporate control instrumentation for the process
parameters (flow, pressure, temperature) and points for sampling.
It is also advisable to install a simple filtration system (cartridge
filters) in the recirculation loop in order to prevent suspended solids
from re-entering the system and damaging the membranes.
?How to clean
The general cleaning procedure for RO /NF membranes comprises
four main stages:
preparation of cleaning solution*
recirculation of cleaning solution in the system**
removal and rinsing of the used cleaning solution***
start of production once the process is complete.****
Organics and biofouling:
Feed water
with high organic matter content is the
cause of these deposits. The presence
of such compounds also facilitates
biofilm formation on the surface of the
membranes, and is the most commonly
occurring type of fouling.
Colloidal Fouling: Colloids present
in the feed water should normally
be removed by the pre-treatment
system; failure or poor design of pretreatment
systems may lead to colloidal
membrane fouling.
Inorganic scale deposits: The most
common types of scale found in
membranes are calcium carbonate
and calcium sulphate with bariumand
strontium sulphate occurring less
frequently. Scale can often be attributed
to incorrect antiscalant selection or poor
antiscalant control. In addition it is also common to find iron and
silica deposits.
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