Membrane and Desalination Technologies - TCE Moodle Website

590 J.P. Chen et al.
4.3.2. Physical Cleaning Methods
Physical cleaning methods depend on mechanical forces to dislodge and remove foulants
from the membrane surface. Physical methods used include forward flushing, backward
flushing, backwashing, vibrations, air sparge, and CO2 back permeation.
FORWARD FLUSHING
The purpose of a forward flush is the removal of a constructed layer of contaminants on the
membrane through the creation of turbulence. A high hydraulic pressure gradient is in order
during forward flush. When forward flush is applied in a membrane, the barrier that is
responsible for dead-end management is opened. At the same time, the membrane temporarily
performs cross-flow filtration, without the production of permeate.
BACKWARD FLUSHING
Permeate is always used for a backward flush and the backward flush pressure is about
2.5 times more than the production pressure. The pressure on the permeate side of the
membrane is higher than the pressure within the membranes. The pores of a membrane are
flushed inside out, resulting from the higher pressure on the permeate side of the membrane
than that within the membranes.
AIR/WATER FLUSH
Air is added to the forward flush to form air bubbles, causing turbulence which removes
fouling from the membrane surface. The benefit of the air flush over the forward flush is that it
uses a smaller pumping capacity during the cleaning process.
PERMEATE BACK PRESSURE
The pressure added on the product side reverses the normal water transport across the
membrane. At the same time, the materials dislodged from the surface are carried away by the
feed to brine flush simultaneously.
VIBRATION
The permeator is vibrated by a pneumatic hammer device attached to the pressure vessel.
The materials dislodged from the surface are carried away by the feed to brine flush
simultaneously.
CO2 BACK PERMEATION
This method is suitable for hollow fiber configuration. A high-pressure CO2 gas is
introduced into product side through the internal fiber bores and removed through the fiber
walls. The foulants lifted from the surface are carried away by a flush stream.
Seven RO cleaning methods were compared for cleaning a hollow fiber: simple forward
flush, reverse flow, permeate back pressure, vibration, air drain and water refill, air sparge,
and CO2 back permeation (66). The results showed that the reverse flow technique (in which a