SECTION 2-5
EQUIPMENT DESCRIPTION
FLOWMAX RO MANUAL REV: B 05/1999
USFilter
United States Filter Corporation
constructed of one of two classifications of membrane material, including cellulose acetate or
triacetate, or some type of plastic based material such as a polyamide or polysulfone based
material. These materials are then arranged into one of four element configurations including
flat plate, tubular, hollow fiber or spiral wound, with the latter two configurations being most
common in today's technology. Selection of the membrane material and element configuration
are based on numerous factors including water chemistry, space limitations, product water
quality requirements and system pre-treatment design. For this discussion, only the spiral wound
element configuration will be described, based on the design of the RO unit covered by this
Manual.
The spiral wound membrane configuration is constructed from a flat sheet membrane that is first
folded and sealed to form an envelop, with one opened end. Porous backing material, placed
inside the envelope separates the membrane sheets and forms a flow channel between them. The
opened end of the envelope is then attached and sealed around a plastic product tube that is
perforated, which allows the product water, or permeate, to pass into the product tube.
For compactness, the envelope of membrane material is then wrapped around the product tube in
a spiral wound fashion with a coarse plastic screen, referred to as a brine channel screen, being
included in the wrap, which creates a flow channel between the surfaces of the membrane where
the feed water enters the element and the reject, or brine flow, passes out of the element. The
element is then covered with an outer wrap of semi-rigid fiberglass for protection and to assist in
maintaining a uniform round shape. The final dimensions of the element are usually
approximately 40" long by either 4" or 8" in diameter.
The elements are then put into one or more cylindrical pressure vessels, which can contain one to
six membrane elements depending on the unit design.
Pressurized feed water is introduced into one end of the vessel. Some of the water, driven by the
feed pressure through the RO pressure vessel feed port, permeates through the membrane and
passes into the product tube and exits the pressure vessel from the pressure vessel product port as
product water, less most of the dissolved solids and all of the suspended solids. The remainder
of the water passes along the surface of the membrane with the rejected dissolved and suspended
solids and passes out of the pressure vessel as reject, or reject flow, from the pressure vessel
reject port.
The pressure vessels are generally arranged in stages when more product water is desired than
one vessel can produce. The staging of the vessels is designed to optimize the water flow
patterns across the surface of the membranes. This uniform water flow promotes good flushing
velocity across the membrane surface to prevent the accumulation of suspended solids on the
surface which would then foul the membrane and reduce the productivity. Multiple stages are
referred to as arrays.
A simple example of an array would be a unit containing three vessels, with two vessels plumbed
in parallel in the first stage and the third vessel plumbed in series after the first two vessels in the
second stage. In this case, the feed water to the unit is first split into two streams with each half
being directed into one of the first stage vessels. On entering these two vessels, some product
