Environmental Engineering Reference
In-Depth Information
stant ocean temperature and hence damping the extent
of global climatic fluctuations; second, greatly contribu-
ting to heat regulation of all living organisms and espe-
cially to the maintenance of homeothermy in higher
animals.
Yet, despite such strong molecular coherence, water
has retained high mobility. Its low viscosity, merely
1 mPa
s, means that living organisms need not spend
much additional energy in pumping their body fluids
and that waterborne transportation has the lowest energy
cost (measured as J/kg
m). And water's high dielectric
constant (78.5 at 25
C) makes for easy transport of
nutrients in animate bodies (as ions remain separate)
and releases more of the metabolized energy for growth
and reproduction. Biospheric energetics would be very
different if biota contained much less of this unusual
compound. But there is a price to pay: because primary
production is predicated on a highly uneven trade-off of
CO
2
and H
2
O (section 3.1), water needs for photosyn-
thesis are very high. Even the most efficient C
4
species
needs 20-30 t/GJ, and many C
3
plants require over
100 t of water to produce 1 GJ of phytomass. Since edi-
ble phytomass is usually no more than half of the synthe-
sized total, the rates for plant foods are mostly 60-200
t/GJ.
As an order-of-magnitude estimate, an average of 150
t/GJ for the annual global harvest of some 40 EJ of food
and feed results in worldwide agricultural consumption
of at least 6000 km
3
of water. The best available esti-
mates indicate that about two-fifths of this need are
supplied by irrigation. Most of this use belongs to the
consumptive category because water is lost through
evapotranspiration and carried to markets with harvested
and increasingly exported crops. In contrast, the largest
category of water use by energy sector is in a non-
consumptive process, for condensing steam in thermal
electricity-generating plants. Traditional once-through
cooling, whereby surface water takes a single pass
through a condenser and is returned to streams, lakes,
or bays, has minimum consumptive (evaporative) losses
but clearly needs access to continuously large volumes of
water and this will affect its quality.
About 29 t/GJ of electricity are needed with a 10
C
rise, 58 t/GJ with the maximum 5
C rise. Thermal
power plants in the United States relied almost solely on
once-through cooling until the late 1960s. By that time
about 5% of the country's total annual freshwater runoff
passed through power plant cooling systems, and the
practice began to cause considerable concern about the
effects of thermal pollution on aquatic life. The increased
temperature of discharged water may be beneficial to
some aquatic species, but its deleterious effects include
lower levels of dissolved O
2
and changed species compo-
sition due to reduction or demise of heat-intolerant
organisms; many microorganisms (phyto- and zooplank-
ton), eggs, larvae, and small fish are also sucked into
intake water pipes or crushed against intake filters.
Amendments to the U.S. Federal Pollution Act of
1972 began a rapid shift to closed-loop cooling systems,
and large wet cooling towers, common in Europe for
several generations, offered an effective solution (fig.
11.6). In some arid parts of the world (beginning in
South Africa) utilities began installing dry cooling towers
that lower water temperature in large heat exchangers
without evaporation. Cooling towers have higher con-
sumptive water use compared to once-through arrange-
ments. This is due to the necessity to replenish water
lost to evaporation, drift (droplets entrained in the mov-
ing air), and bleeding (releasing some water in order to
limit the dissolved solids). But this makeup water adds
