1307WATER REUSE
A commonly accepted definition of water reuse is that the
water that will be further utilized will still be under the
control of the first user. An early example of water reuse
involved the sale of secondary wastewater effluent from the
Back River Treatment Plant in Baltimore to the Bethlehem
Steel Sparrows Point Plant. The Sparrows Point Plant was
served by wells, which were close to brackish Baltimore
Harbor. Excessive pumping of the wells led to saltwater
intrusion, and low saltwater concentrations made the well
water unsuitable for some operations. Steel rollers pit
when applied water contains 5 mg/l chloride ion. Wolman
recommended that the treated wastewater effluent be used
for some rough operations, such as quenching. An 8-mile
pipeline was installed to carry the effluent to the industrial
plant. Drawdown on the wells was reduced significantly,
and the city of Baltimore received revenue for a previously
discarded product.
Water reuse can be regarded as a conservation strategy.
Here the discussion will deal with reclamation of used water.
Desalination, or desalting, is covered in another chapter (see
“Desalination”). Guidelines and design standards have been
developed for water reclamation and water reuse as alterna-
tives to disposal of surface receiving waters. Table 1 lists
water-reuse applications. It should be noted that other uses
will be found.
All water is reused. It may be used and used again in its
journey from deposition as rain to eventual mixing with the
sea. Available water is not always near the point at which it
is required. Economics of supply is important, and pumping
cost may dictate the source chosen.
In many cases, water for industrial processes must be of
much higher quality than that used for drinking. In water-
poor areas reused water has long been accepted, even for
drinking. In general, however, public-health authorities have
not looked favorably on this use. A poll found that 50% of
the people surveyed did favor reclaimed water for drinking.
Water usually is not immediately available for reuse.
With more stringent regulations governing effluent qual-
ity coming into effect, these higher-quality discharges may
become more practical for water reuse.
Conventional primary and secondary wastewater treat-
ment alone are not normally sufficient to prepare wastewater
for any but the coarsest uses. Further, or tertiary, treatment
will be required. Secondary, or biological, treatment will
remove about 90% of influent suspended solids and BOD
(biochemical oxygen demand). BOD and COD (chemicaloxygen demand) do not themselves affect suitability of the
effluent for reuse but are indications of the unstable nature
of the effluent. These will be reduced in the natural decom-
position cycle.
Some compounds are not removed in conventional
wastewater-treatment processes. These substances, termed
“refractory,” range from simple inorganic compounds to
large synthetic organics such as pesticides, herbicides, and
surfactants. In an essentially closed system, high concentra-
tions of unwanted materials would soon result from continued
recycling. In Table 2, components of typical tap water are
compared with those of a typical secondary effluent.
Stream quality or other water sources might be improved
enough by removal of BOD and suspended solids. In some
cases it is possible to save large amounts of water by process
changes and improved housekeeping, and water reuse might
not be necessary.
If used water is not yet suitable for reuse after conven-
tional treatment, it may be blended with clean water and the
mixture rendered acceptable. However, in many cases tertiary
treatment will be required. A common definition of tertiary
treatment is any treatment in addition to secondary treatment
by which further removals are achieved. A classification of
wastewater-treatment processes is presented in Table 3.
Chlorination, ozonation, and application of ultraviolet
light are not considered tertiary processes. Chemical precipi-
tation, by itself, is primary treatment. However, applied after
secondary treatment, it can give a high-quality effluent. An
insoluble compound is formed and the resulting precipitate
sweeps other material from suspension. Neutralization of
charge on colloids and agglomeration of particles, including
microorganisms, give a clear effluent. Phosphates are removed
by addition of soluble aluminum or iron salts. Lime is usually
added to produce hydroxide floc.
Filtration may be necessary for further particulate
removal after chemical precipitation. In this case filtration
is not straining out of the suspended matter. The primary
mechanism is adsorption of the suspended matter onto the
surface of the granular filter media. Sand, anthracite, and
garnet are commonly used filter materials. A mixed-media
filter has a greater effective depth than does a comparable
sand filter. When a filter clogs, it may be backwashed by
water alone or a mixture of water and air. A filter acts as a
fluidized bed during backwashing.
Diatomaceous-earth filtration is also practiced. A layer of
diatomaceous earth is placed over a relatively loose septum toC023_005_r03.indd 1307C023_005_r03.indd 1307 11/18/2005 1:04:50 PM11/18/2005 1:04:50 PM