Chemical Engineering August 2014 - 59

CT
blowdown
NaHSO3
Permeate
return to
process
UF or MF Sodium
softener
NaOH
High pH
RO unit
Reject to pond or
evaporator or
crystallizer (E/C)
FIGURE 1. Shown here is a generic outline of one emerging wastewater-treatment
technology, which is discussed in the text
and more constricted by deposits.
At facilities in arid locations that
have a large land area, evaporation
ponds may be sufficient to handle
the wastewater discharge. However,
these ponds must be properly lined
to prevent seepage of the wastewater
with its impurities into the underlying
soil. And permitting may or
may not be granted for such evaporation
ponds. Alternatively, at sites
that are strategically located, it
may be possible to have the wastewater
trucked off-site by a centralized
waste-disposal company.
If none of the above options are
available, thermal evaporation of
the waste stream may be the only
option to cost-effectively meet these
stricter discharge limits. At a recent
visit to a plant in the Southwestern
U.S., the author observed a brine
concentrator/crystallizer system
that treats the entire cooling tower
discharge stream. While the system
is manageable, the inlet flowrate at
full load is nearly 1,000 gal/min. The
energy requirements are quite large,
as are the regular maintenance costs
to remove accumulated solids from
the evaporation equipment.
To help minimize these two costs,
many cooling tower operators are
implementing treatment methods
that can reduce the overall volume
of the cooling tower blowdown
and other waste streams from the
plant. These may include microfilter
and reverse-osmosis (RO) reject,
plant drains, and service water
discharge. An emerging technology
for waste-stream volume reduction
and increased water recycle is highrecovery
reverse osmosis, as shown
in Figure 1. Keys steps in this
process are as follows:
* Microfiltration (MF) or ultrafiltration
(UF) to remove suspended
solids in the waste stream: This
is a critical process to prevent
suspended solids from fouling
RO membranes
* Sodium bisulfite (NaHSO3) feed
to remove residual oxidizing
biocides: This is also critical to
remove oxidizing biocides that
would degrade the water softener
resin and RO membranes
* A sodium softener to remove calcium
and magnesium: Otherwise
the downstream equipment would
suffer from calcium carbonate and
magnesium silicate scaling
* Sodium hydroxide injection to elevate
the pH above 10: The combination
of hardness removal
and pH elevation keeps silica in
solution
* Two-pass RO treatment
Under proper conditions, the RO
recovery rate may reach 90%. The
RO permeate is recycled to the
plant's high-purity makeup water
system or other locations. However,
while the process appears straightforward,
a number of lessons have
emerged regarding this technology
in actual application. One of the
most notable is that some standard
water-treatment chemicals, particularly
cationic coagulating and
flocculating polymers may foul MF,
UF and RO membranes. The difficulty
arises from the fact that most
membranes carry a negative surface
charge while many of the polymers
employed for water treatment
have a cationic charge. As a result,
residual polymer will coat the membranes.
A
similar phenomenon has been
observed with MF or UF systems
that are installed in makeup water
systems downstream of a clarifier.
Inexperienced designers and plant
personnel often do not recognize
that MF or UF are most effectively
used as a replacement for clarification
- not as a polishing process for
the clarifier.
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Chemical Engineering August 2014

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