Page 109 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 85
concentrated or dilute form. Because of this complexity, priority should first be
given to the possibility of recovering part or all of the waste products for reuse
or sale. Frequently, money can be saved by installing recovery facilities rather
than more expensive waste-treatment equipment. If product recovery is not
capabIe of solving a given waste-disposal problem, waste treatment must be
used. One of the functions of the design engineer then is to decide which
treatment process, or combination of processes, will best perform the necessary
task of cleaning up the wastewater effluent involved. This treatment can be
either physical, chemical, or biological in nature, depending upon the type of
waste involved and the amount of removal necessary.
PHYSICAL TREATMENT. The first step in any wastewater treatment process is
to remove large floating or suspended particles by sources. This is usually
followed by sedimentation or gravity settling. Where sufficient land area is
available, earthen basins are sometimes used to remove settleable solids from
dilute wastewater. Otherwise, circular clarifiers with rotating sludge scrapers or
rectangular clarifiers with continuous chain sludge scrapers are used. These
units permit removal of settled sludge from the floor of the clarifiers and scum
removal from the surface. Numerous options for improving the operation of
clarifiers are presently available.
Sludge from primary or secondary treatment that has been initially con-
centrated in a clarifier or thickener can be further concentrated by vacuum
filtration or centrifugation. The importance of first concentrating a thin slurry by
clarifier or thickener action needs to be recognized. For example, concentrating
the sludge from 5 to 10 percent solids before centrifuging can result in a 250
percent increase in solids recovery for the same power input to the centrifuge.
Solid-liquid separation by flotation may be achieved by gravity alone or
induced by dissolved-air or vacuum techniques. The mechanisms and driving
forces are similar to those found in sedimentation, but the separation rate and
solids concentration can be greater in some cases.
Adsorption processes, and in particular those using activated carbon, are
also finding increased use in wastewater treatment for removal of refractory
organics, toxic substances, and color. The primary driving forces for adsorption
are a combination of (1) the hydrophobic nature of the dissolved organics in the
wastewater and (2) the affinity of the organics to the solid adsorbent. The latter
is due to a combination of electrostatic attraction, physical adsorption, and
chemical adsorption. Operational arrangements of the adsorption beds are
similar to those described for gaseous adsorption.
Three different membrane processes, ultrafiltration, reverse osmosis, and
electrodialysis are receiving increased interest in pollution-control applications
as end-of-pipe treatment and for inplant recovery systems. There is no sharp
distinction between ultrafiltration and reverse osmosis. In the former, the
separation is based primarily on the size of the solute molecule which, depend-
ing upon the particular membrane porosity, can range from about 2 to 10,000
millimicrons. In the reverse-osmosis process, the size of the solute molecule is
not the sole basis for the degree of removal, since other characteristics of the