Page 195 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.31

installed have multiple bed units and a degasifier. Manufacturers must be consulted for the
specific system, resin selection, and the required equipment.
Where applicable, the use of weakly acidic and weakly basic resins minimizes chemical
costs and reduces losses to waste because of high regenerative capacity.
There are some problems, such as microbial growth, associated with water softeners.
Sanitization is usually accomplished during regeneration. Iron buildup in the unit could
pass through to downstream purification equipment unless operating personnel constantly
monitor the water quality.

MEMBRANE FILTRATION AND SEPARATION

Membrane filtration and separation is a general term for a water purification process that
removes contaminants from feedwater by means of a thin, porous barrier called a mem-
brane. When used as a filter, a membrane is capable of removing impurities of a much
smaller size than other types of filters. Filters of this nature are often called ultrafilters and
nanofilters.
A semipermeable membrane limits the passage of selected atoms and/or molecules in
a specific manner. Membrane filtration and separation, when used to produce pure water,
is characterized by having the feedwater flow parallel to the membrane (often called tan-
gential flow). Not all of the feedwater is recovered. Many of the membranes used are also
available as both depth filters and, in single thickness, disk filters. There are two general
categories of membrane filtration: reverse osmosis using a semipermeable membrane and
filtration using ultrafiltration and nanofiltration membranes.

Reverse Osmosis
Reverse osmosis (RO) is a continuous, steady-state process that uses a semipermeable
membrane for removing solutes, such as dissolved ions from a solution. This membrane
is called semipermeable because it allows the solvent to diffuse, or pass through, but is
impervious to the solute.
In the natural osmosis process, when two solutions of different concentrations are sepa-
rated by a semipermeable membrane, water molecules from the less concentrated solution
will spontaneously pass through the membrane to dilute the more concentrated solution.
This continues until a rough equilibrium is achieved. The driving force is a difference
of pressure, called the osmotic pressure or concentration gradient, that exists across the
membrane and is based on the degree of concentration of contaminants. The pressure in
the stronger solution is lower than that in the weaker solution. This pressure is what drives
the flow of solvent. The flow or flux will continue until the osmotic pressure is equalized,
which then results in a higher pressure on the concentrated solution side that is equal to the
osmotic pressure.
Reverse osmosis is the flow of solvent in the direction opposite to that of natural osmo-
sis. If enough pressure is applied to the more concentrated solution, which in these discus-
sions is water, pure water is diffused through the membrane leaving behind the bulk of the
contaminants. These contaminants are continuously flushed to drain, thereby removing
them from the system. The purified water is called permeate and the contaminant-containing
water is called reject or the reject stream. In some cases, the reject stream is referred to
as salt. The performance characteristics of the selected membrane determine how large a
system is required. The flow rate is measured in membrane flux, which is a measurement of
the flow rate of permeate that will pass through a given area of the membrane at a specific
temperature and pressure. The ratio of purified water flow to the feedwater flow is called
recovery. Most applications require a minimum 40-percent recovery rate to be considered

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