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88 Mrmbranes for lndustrial Wastewater Recovery and Re-use
The extent of recirculation can be increased by using chemical treatments
either to adjust the pH of the water or to sequester minerals and prevent
deposition, in the same way as reagents are used to ameliorate scaling in reverse
osmosis plant (Table 2.15). In most cases, the Rc is optimised based on water
availability, water quality, and treatment costs. Typically, evaporative
recirculating cooling towers that use reclaimed water operate with Rc values
ranging from 1 to 3. An advantage of higher Rc values is that reduced quantities
of make-up water are required and there is less blowdown water to be treated or
discharged. It should be noted that the concentration of dissolved constituents in
the blowdown water increases with increasing Rc. Therefore, one trade-off
associated with higher Rc levels is the increasing costs of disposal or treatment of
the blowdown water.
The impact of recirculation on water quality in a recirculating cooling system
using reclaimed water as a source water is shown in Fig. 3.11. This system is
operated with a Rc of about 2. A comparison of the conductivity of the source
water to that of the recirculating stream reveals that the concentration of
dissolved solids increases about two-fold in this system. When blowdown water
is removed from this system it is returned to the wastewater reclamation facility.
Higher Rc values for this source would yield a blowdown water with a higher salt
content that might preclude discharge to the wastewater reclamation facility.
3.1.7 Cooling tower water quality issues
Industrial cooling tower operations are susceptible to four potential water
quality problems: (1) scaling, (2) biological growth, (3) fouling of the heat
5000 1
5 4000
a
3.
.& 3000
Cooling Water
.L
2000
3
W
C
6 1000
0’
Jan-00 Jul-00 DeoOO Jun-01 Dec-01 Jun-02
Date
Figurr 3. I1 Comparison of the conductivity of source ivater and rrcirculatrd wntrrfor n cooling systrm
operatrd by Rnythron Corporationin St. Petersburg, Floridn (Knighton, 2002)
