One problem with some
reverse osmosis (RO) units
is chemical cleaning and
membrane replacement
costs due to fouling, scaling, and
chemical attack. In many cases these
problems can be traced back to inadequately
designed or improperly operated
pretreatment equipment. Inadequate
design may occur due to an
incomplete analysis of raw water
source, changing raw water quality not
anticipated in the initial design, or
expansion of final product water
capacity without pretreatment equipment
addition. The list could be more
exhaustive, but these are common
causes.
Figure 1. The boiler feedwater treatment system.
More frequently, downstream equipment
problems are caused by operational
practices. This is not necessarily
a reflection of operating personnel
capability. It is generally a result of
insufficient tools for equipment monitoring
and control, or “inadequate”
training/understanding of unit processes,
and how each affects the next one
downstream.
Process monitoring and
control methods
Jar tests and turbidimeters have
been the traditional instruments for
monitoring and controlling pretreatment
processes. In recent years, the
streaming current monitor (SCM) and
particle counters have replaced these
methods. These instruments provide
continuous, precise, and sensitive
measurement, and true process optimization
capability. Periodic jar tests
give a ball park idea of what coagulant
dosages should be, but are somewhat
“subjective” to the operator’s
visual interpretation. The SCM provides
continuous monitoring and
automatic control. Turbidimeters have
also been used to monitor filter performance,
but lack the sensitivity of
particle counters/ monitors. Silt density
index (SDI) has been the standard
test to predict membrane fouling.
However, it is not a continuous measurement
and is not always helpful in
troubleshooting and improving upstream
processes.
A case history
A large electric generating plant
installed a reverse osmosis system to
reduce demineralizer loading and minimize
regenerations. Feedwater pressure
buildup and frequent membrane
cleaning have been a persistent problem
since commissioning. The boiler
feedwater treatment system is shown
in Figure 1. Water source is a river with
an intermediate settling basin that minimizes
large turbidity swings to the
treatment process. Primary coagulant
was alum, but was later changed to a
polymer/inorganic blend. Turbidimeters
are used to monitor clarifier
and filter effluents. SDI tests were performed
for a time after the RO installation,
but were eventually discontinued,
as there was no correlation to cleaning
frequency.
Figure 2. Results of SCM trial to optimize polymer feed.
Particle counters/monitors were
installed to get “baseline” equipment
performance. The objective was to
improve removal of submicron particulate in the multimedia filters by feeding
a small amount of polymer filter
aid. Although the counters/monitors
used do not detect particles in the submicron
range, it is well known that colloids
are absorbed on filter media that
is properly conditioned with a filter
aid, or submicron particles are coagulated
to a size that are trapped in the
media. An SCM was used to optimize
polymer feed to ensure that the
absolute minimum was being used,
and would not carry through to the RO.
Polymer fouling can be a problem if
overfed.
The results are shown in Figure 2.
Without polymer feed, particle counts
in the multimedia effluent were over
120 counts/ml. With polymer, counts
dropped to less than 20/ml, with many
periods less than 5/ml. Polymer dose
was 0.4 ppm. An immediate improvement
was seen in the RO operation.
Cleaning frequency went from once
per week to once per month. Feedwater
pressure buildup was less than
2.0 PSIG after 30 days.
Currently, the plant is monitoring
three of the multimedia filters with
particle monitors, and the combined
effluent with a particle counter.
Polymer addition is being automatically
controlled with a streaming current
monitor using filter influent flow
and streaming current in a combined
algorithm.
Conclusion
The return on investment of using
sensitive and reliable control instrumentation
in a reverse osmosis pretreatment
system can be substantial.
Savings in chemicals, filter replacements,
membrane cleaning/replacement,
and demineralizer efficiency
can be easily documented. In the
power generation industry, the major
benefit is always boiler efficiency and
reliability.
Robert L. Bryant is the founder and
president of Chemtrac Systems, Inc.
located in Norcross, Georgia.
The
company is represented in Canada by
SPD Sales,
e-mail: sales@spdsales.com.
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