Page 90 - Membranes for Industrial Wastewater Recovery and Re-Use

70 Membranes for industrial Wastewater Recovery and Re-use

rinsing prior to membrane operation due to its propensity for complexation with
proteins. Other chemical bactericidal reagents that have been employed, and
may be included in proprietary formulations, include QACs, acidified
metabisulphite, hydrogen peroxide and peroxyacetic acid, the latter being
generated from hydrogen peroxide and peracetic acid. Some membranes,
notably fluorinated polymers and ceramic membranes, are steam sterilisable.
Most membrane plant is designed to allow cleaning in place (CIP). Fouled
cross-flow membranes systems are cleaned by recirculating a cleaning solution
around the concentrate side of the module by means of a small tank and
circulating pump. Each membrane module, or bank of modules if an entire bank
is to be cleaned at once, is normally provided with cleaning connections to allow
this. The connections are usually three-way L-port ball valves so that connecting
the cleaning solution circulation system automatically isolates the module(s)
from the high-pressure feed system. In dead-end systems CIP entails pumping the
cleaning solution in the reverse direction, i.e. from the permeate to the retentate
side. Submerged systems can also be cleaned ex situ by placing the entire module
in a tank containing the cleaning fluid.
The cleaning tank for an KO plant normally has a working volume equal to
the volume of the (empty) pressure tube(s) to be cleaned plus the volume of the
associated pipework. Tanks are typically of GRP or polypropylene, depending on
the cleaning chemical being used, and fitted with an electrically powered stirrer.
The circulation pump is normally rated to deliver a flow velocity of 0.06-0.08
m s-l at a pressure 1.2-4 bar, the exact pressure being dependent upon the
number of elements in the module. Precise recommended cleaning protocols
vary, with some suppliers recommending a 15-30 minute soak. It is normally
desirable for this stream to be fitted with a 5 pm filter upstream of the pressure
vessels. Disinfection can be carried out in the same way but with a higher
pressure (up to about 10 bar) to ensure that the disinfecting solution penetrates
to the permeate side of the membrane.

References
Ahmed, S.P. and Alansari, M.S. (1989). Biological fouling and control at RAS
Abu Jarjur KO plant - a new approach. Desalination, 74,69-84.
Applegate, L.E., Erkenbrecher, C.W. and Winters, H. (1986). Monitoring
control of biological activity in Permasep seawater RO plants. Desalination, 65,
3 3 1-3 59.
Arroyo and Fonade, C. (1993), Usc of intermittent jets to enhance flux in
crossflow filtration. J. Membrane Sci., 80,117-129.
ASTM D3739, Standard practice for calculation and adjustment of the
Langelier saturation index for reverse osmosis.
ASTMD4189, Standard test method for Silt Density Index (SDI) ofwater.
ASTM D4582. Standard practice for calculation and adjustment of the Stiff
and Davis stability index for reverse osmosis.