Apr 12, 2006

Power Station Utilizes RO Membranes

Pretreatment to remove suspended
solids from raw makeup
water is a requirement for
the potable water industry, but it is
also a critical application in many
process industries, including steamgenerating
electric utilities.

In the 1980s and 1990s, reverse
osmosis exploded in popularity as a
retrofit technique ahead of existing
demineralizers at power stations. RO
membranes, whose pore sizes are
only angstroms in diameter, will
remove most dissolved ions from
water, thus greatly reducing the load
on downstream ion exchange units.

At Kansas City Power & Light Co.’s
(KCPLC) La Cygne, Kan., generating
station, an RO system was placed in
the Unit 1 (820 MW, supercritical boiler)
makeup water system in the 1980s.
As part of a major upgrade in the
1990s, an RO unit and downstream
ion exchange system replaced the
original flash evaporator in the Unit 2
(720 MW, drum boiler) makeup train.
Both RO systems were designed for
75% recovery with a maximum product
water flow rate of 200 gpm.

Testing necessary

Even though both La Cygne makeup
water systems were fitted with RO
units, they continued to operate with
the original clarifier/sand filters for
suspended solids removal. By the early
2000s, combined chemical costs for
the two clarifiers had easily exceeded
$100,000 annually, with labor and
routine equipment repair costs adding
considerably to that amount.

When each clarifier operated properly,
effluent turbidity could be lowered
to around 0.3 nephelometric
turbidity units (NTU). However,
upsets in lake water chemistry or
chemical feed equipment malfunctions
periodically caused excursions
in clarifier performance, such that
effluent turbidities might exceed 1
NTU. In these cases, there would be
quick fouling of RO prefilters and an
increase in RO membrane differential
pressures. The Unit 1 clarifier was particularly
troublesome in this regard.

In autumn 2004, based on reliable
information from colleagues within
the power industry, KCPLC tested a
Pall Aria 4-ft microfilter (MF) in the
Unit 1 makeup water system to ascertain
if it would produce cleaner water
for RO feed, and how this would affect
downstream equipment. Whereas
most RO systems for power plant
applications utilize spiral-wound membranes,
the microfilter at La Cygne is
of hollow-fiber configuration, in which
each module contains thousands of
spaghetti-sized hollow-fiber tubes. To
produce the 300-gpm flow required
by Unit 1 and auxiliary systems,
24-membrane modules were necessary.

The microfilter process, like RO,
operates via cross-flow filtration, in
which the raw water flows parallel to
the membrane surface. Water that
passes through the membranes and
is purified is known as permeate. Not
all water passes through each membrane,
as a small portion at least
must flow along the surface to carry
away the suspended solids. This
stream is known as the reject. The
membranes in the unit KCPLC tested
are configured such that the raw
water flows from outside to in, with
the reject flowing along the outside
surface of the fibers.

No water lost

Raw water enters tank T-1 for feed
to the membranes. A level control
gauge in the tank modifies inlet valve
operation so the tank maintains a
constant level. Pump P-1 (rated at
20 hp) moves the raw water to the
membranes. This pump is controlled
by a variable frequency drive (VFD)
to adjust the output based on the
flow rate requested by the operator.The feed to the membranes passes
through a basket strainer to remove
any large solids that might otherwise
foul the membrane surfaces. The
permeate flows directly to an existing
storage tank, while the reject
flows back to tank T-1.

Thus, no water is lost during normal
operation. The standard mode of
operation for the system is 25 minutes
of water production followed by
a one-minute air scrub/reverse flush
(AS/RF) to remove solids that collect
on the membrane surfaces.

When the AS/RF sequence initiates,
pump P-1 stops, and pump
P-2 (also rated at 20 hp) feeds water
from tank T-2. This tank contains previously
filtered water to which sodium
hypochlorite has been added via
pump P-3, which takes simple feed
from a drum of hypochlorite. Air
valve 7 opens to allow air to scrub
the membranes while the chlorinated
water flows inside out through the
membrane surfaces. Pump P-2 is
also powered by a VFD to allow the
operator to adjust reverse flush
flow rate as necessary.

Once this process is complete,
pump P-1 reactivates and flushes the
system for a short period followed by
a return to permeate production. At
the beginning of the new production
cycle, tank T-2 fills with clean water
while pump P-3 injects fresh sodium
hypochlorite to the tank. The controls
also include a timer that periodically
backwashes the inlet strainer with
feed from tank T-1.
The only significant cost to operate
the unit is the electricity that
powers the P-1 and P-2 pumps. This
cost is negligible compared to what
KCPLC spent on the clarifier and
sand filters. The heart of the control
system is a dedicated PLC mounted
on the pump skid, which KCPLC controls
from a personal computer in the
Unit 1 laboratory.

KCPLC set the flow rate, AS/RF frequency,
strainer backwash frequency
and other parameters from this PC.
The PLC acts upon any command
changes instantly, and this provides
excellent flexibility for adjusting water
flow to meet plant requirements.

Results and lessons learned

Makeup water for the boilers is
taken directly from Lake La Cygne,
where the typical turbidity ranges
from 5 to 15 NTU. KCPLC was given
performance criteria that indicated
the microfilter would remove particles
down to 0.1 micron in size and produce
an effluent turbidity of less than
0.1 NTU. Within an hour after system
start-up, effluent turbidities had
dropped to a range of 0.027 to 0.036
NTU, where they have consistently
remained. KCPLC found that the cartridge
pre-filters ahead of the Unit 1
RO, which normally have to be
replaced every two to three weeks,
did not have to be replaced once
during the initial three-month test.

MF membrane pore sizes are
larger than those of RO membranes,
which require much less pressure to
push water through the membranes.
Typical membrane inlet pressures on
the KCPLC system range from 10 to
20 psi. The minimal pressure requirement
allows membrane construction
of coarser but much more durable
materials, in this case polyvinylidene
fluoride. This aspect proved to be
very important. KCPLC found early
on during the test that even with
regular AS/RF, membrane differential
pressures (DP) would gradually
increase from day to day.

As an experiment, in the spring of
2005, KCPLC began treating the raw
water feed with a small but continuous
dosage of sodium hypochlorite
to maintain a 0.2 to 0.5 ppm
chlorine residual in the membrane
permeate. This did wonders for membrane
cleanliness, and the gradual DP
increases ceased, in fact dropped, to
near start-up levels (8 psi at 300 gpm
flow rate) for several months.

Subsequently, KCPLC has, at times,
seen the membrane DP increase
gradually in spring and summer.
During these events, KCPLC increases
the AS/RF frequency to better scrub
the membranes. If the membrane DP
climbs too high (30 to 35 psi), or the
microfilter needs to be out of service
for an extended period, KCPLC cleans
the membranes with a dilute solution
of sodium hydroxide followed by a
rinse and then a cleaning with a citric
acid solution followed by another
rinse. KCPLC adds the chemicals
manually to tank T-1 and then circulates
the solution through the membranes
using pump P-1.

From March to October 2005,
KCPLC operated continuously before
taking the unit off for cleaning. The
DP did not recover to original values;
although, KCPLC was able to continuously
produce water until February
2006, at which time, with the aid of
Pall personnel, KCPLC performed a
more vigorous cleaning of the membranes.
Subsequently, KCPLC set up
a schedule to clean the membranes
on a quarterly basis.

Other than a faulty inlet valve that
the vendor replaced promptly, the
reliability of the system has been
superb. Results were so impressive
that KCPLC purchased the unit and
installed it in a permanent location in
February 2005. Operation since has
been very steady, and payback for the
MF will be in less than three years.

About the author

Brad Buecker is plant chemist at Kansas
City Power & Light Co.’s La Cygne, Kan.,
power station. He can be reached at
[email protected].