Page 99 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 79

Table 3.1 Comparison of water use in cooling systems (Asano et aL, 1988; Blackson and
Moreland, 1988; Burger, 1979; Kemmer, 1988; State of California, 1980; Tay and Chui, 1991)

Type Application Water source Water use
Once-through Large power facilities or Fresh, saline, 75-225 m3 MWh-l.
non-contact cooling refineries adjacent to a water or reclaimed water Evaporative consumption:
source 0.5-2%
Direct contact Processing of inert material Freshwater; degree Depends on process
cooling when chemical reaction of purification
quenching needed, such as related to process
in primary metals industry conditions
Non-contact, open, Widely used where water Fresh, saline, or 0.5-4 m3 MWh-I.
recirculating cooling resources are limited and reclaimed water Evaporative consumption:
alternative water sources 80-95%
such as reclaimed water
are available (such as
inland regions)
Closed recirculating Water circulated in closed Ultrapure water Water velocities:
system loop with negligible 1-1.5 m s-l Evaporative
evaporation or exposure consumption: < 5%
to atmosphere: Used for
critical cooling applications

cooling system where process heat from the warmed water is transferred to the
air via evaporation so that the water can be reused. Process cooling is
accomplished through indirect contact of the process streams and cooling water
in heat exchangers. Closed recirculating systems consume minimal water, but
require higher purity water to prevent operational problems. Therefore, due to
the economics of providing supplemental water treatment, closed recirculating
systems are only used in cases where cooling efficiency is critical, such as in steel
production and microelectronics. Currently, on a global basis, the majority of
thermoelectric power plants rely on either once-through cooling or open
recirculating cooling systems. The degree of water treatment required for cooling
water applications depends on the system. Typically, minimal treatment is
required for once-through cooling, whereas scale and corrosion control are
needed for open recirculating systems. Closed recirculating systems require
control of salts and minerals, similar to requirements for boiler operation.
In once-through processes, heat is transferred to water which is then
returned to its source. For once-through cooling systems, the cooling water
inlet temperature is the temperature of the source water, whereas the outlet
temperature is 5-2 5°C higher than the inlet. Thus discharge of this water can
introduce temperature gradients within the receiving water if inadequate mixing
is available. Recirculating systems transfer the heat from the warmed water to the
air through evaporation; therefore, outlet temperature and inlet temperatures are
similar. As shown in Table 3.1, recirculating systems require less than 10% of the
quantity of water used in once-through cooling systems. The reduction in water
use has an impact upon the volume of water discharged from the cooling system
and results in a net decrease in the quantity of thermal discharges.