Chemical Engineering November 2011 - 52

Solids Processing
Environmental Manager
Feed
Modern
Concepts
in
Technologies offer
new alternatives
for water treatment
Brad Buecker
Kiewit Power Engineers
W
ater and steam are vital
components of virtually
all plants in the chemical
process industries (CPI).
And, a very important operation is
the treatment of makeup water to produce
the purity required for certain
applications, most notably for use in
heat exchangers and steam turbines.
An important consideration in water
treatment is that many industrial facilities
are, or will soon be, dealing with
tightening restrictions on wastewater
discharges. These may include cooling
tower blowdown, rainwater runoff and
other aqueous discharges.
As with other technologies, the processes
utilized to produce high-purity
water have been greatly improved over
the last decades. This article examines
several of the most modern techniques
for makeup water production. These
technologies are also being employed
to reduce wastewater volume and to
recover pure water for reuse in the
plant. (For more on reuse strategies,
see Strategies for Water Reuse, Chem.
Eng. September 2009, pp. 34-39.)
Think twice about a clarifier
Typically, the general makeup-treatment
process involves: 1) large solids
removal by screens or settling basins;
2) suspended solids removal; and 3)
dissolved solids removal to produce the
Membrane
FIGURE 1. Crossflow
Permeate
filtration is demonstrated
in this schematic
Reject
Makeup Water Treatment
FIGURE 2. The spiral-wound membrane is the
most common configuration for RO membranes
water needed for the process. We will
focus on the last two process steps.
Clarification and media filtration
were commonly used in the past for
suspended solids removal. Where lime
softening is needed to reduce raw
water hardness, clarification is still
a viable process. However, when the
primary focus is removal of suspended
solids only, micro- or ultra-filtration offers
a very reliable alternative. My own
personal experience is with microfiltration
as a clarifier/filter replacement for
makeup water pretreatment to an 800MW
supercritical utility boiler [1].
All major microfilter designs utilize
hollow-fiber membranes. A very common
design uses pressurized systems,
where thousands of spaghetti-like
membranes are packed into pressure
vessels. The number of pressure vessels
then determines system production
capabilities.
This microfiltration process,
like
others of its type, operates in what
can be thought of as a combination of
crossflow and dead-end modes. Raw
water flows parallel to the membrane
surface in a crossflow pattern (Figure
52 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2011
1), but unlike reverse osmosis (RO)
where most impurities are carried
away with the concentrated stream
(called the reject or concentrate),
suspended solids collect on the membrane.
Water that passes through the
membranes and is purified is known
as permeate and, of course, is sent onward,
while in many designs the reject
returns to the unit inlet.
So, this begs the question of how
suspended solids are purged from the
system. Typically, a regular backwash
with an accompanying air scrub dislodges
particulates and discharges
them to a waste stream.
One microfiltration system has, in
five years of operation, consistently produced
water with turbidity levels below
0.04 nephelometric turbidity units
(NTU). The results on downstream
RO operation have been predictable in
the best sense of the word, where the
frequency of cartridge filter replacements
went from weeks to months; and
where microfiltration combined with
an improved chemical-treatment program
corrected a microbiological fouling
problem that once plagued the RO
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Chemical Engineering November 2011

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