PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS 973
(3) lower initial plant cost, (4) ability to remove nitrogen and
phosphorus, and (5) a final sludge reduction to sterile ash in
a centrifuge-incineration process combined with a chemical
regeneration step to recover both the coagulant and the carbon.
While the estimated operational costs were high, the overall
costs during a 20-year plant life were considered to be signifi-
cantly less than costs for comparable biological facilities.
Ecodyne Corporation’s first complete physical-chemical
treatment plant in Rosemount, Minnesota, with 0.04 to
0.08 m^3 /sec peak capacity, consisted of bar screening, phos-
phate removal with sludge recirculation, dual media filtration,
carbon absorption to remove dissolved organics, secondary
filtration, and ammonia removal by ion exchange with zeo-
lite. The plant included facilities for regenerating the carbon,
recovering ammonia, and regenerating brine from the ion-
exchange system (Ecodyne, 1972).
Examples of more recent research include an exhaustive
review on the treatment of pulp- and paper-mill wastewa-
ter published by Pokhrel and Viraraghavan in 2004. This
includes the different processes involved with their effluents,
the different methods for treatment of these effluents, the
integration of biological and physico-chemical processes,
a comparison of them, and conclusions from this review.
An article by Van Hulle and Vanrolleghem (2004) presents
the development, calibration, and application of a model for
the simulation and optimization of a wastewater-treatment
plant. The constructed model proved to be able to predict
large variations in influent composition. This could be an
important tool for production scheduling when applied to
industrial wastewater-treatment plants. Recent research in
specific areas is included in for each process.
The number of water-treatment facilities in the United
States by treatment capacities is presented in Table 1. Many
of these facilities include some sort of physical-chemical
treatment technology. A diagram of alternative technologies
for wastewater treatment is shown in Figure 1; it includes
most of the processes to be discussed in the next section. As
environmental regulations, space availability, and cost fac-
tors affect the treatment of waste streams, more and more
physical-chemical treatment will be needed to meet these
constraints. This is an important research area that will con-
tinue to grow in the next years.PHYSICAL AND CHEMICAL PROCESSES USED IN
WASTEWATER TREATMENTThe following important unit operations and unit processes
involved in the physical and chemical treatment of wastewater
are discussed in detail:Flow equalization and neutralization
Chemical coagulation, flocculation, and sedimentation
Filtration
Gas stripping
Ion exchange
Adsorption
Flotation
Chemical processes
Oxidative, photochemical, and electron-beam processesFlow Equalization and NeutralizationBoth domestic and industrial wastewater flows show con-
siderable diurnal variation, and it is considered necessary
to significantly dampen these variations in inflow to relieve
hydraulic overload on both biological and physical-chemical
plants. This process will also smooth out variations in influent
characteristics.
Flow-equalization basins are basically flow-through or
side-line holding tanks, and their capacity is determined by
plotting inflow and outflow mass curves. These tanks are
generally located after preliminary treatment and should be
designed as completely mixed basins, using either diffused
air or mechanical surface aerators, to prevent settling of sus-
pended impurities. If decomposable organic matter is present
in the wastewater, aeration will prevent septicity. The pre-
aeration can also reduce the BOD on subsequent treatment
units.
Flow-equalization basins can also be used to neutral-
ize the acidity or alkalinity in incoming wastewater. The
neutralizing chemicals are added to the inflow wastewater
stream before entering the flow-equalization basins, and the
retention period in these basins provides sufficient time for
reaction. Any precipitates produced during neutralization are
separated in subsequent sedimentation basins.Chemical Coagulation, Flocculation, and
SedimentationThe use of chemical treatment appeared early in the devel-
opment of sewage- and wastewater-treatment technology.
Aluminum sulfate, lime, and ferrous sulfate, when used in
the manner usually adopted for water clarification, were
successful in producing an effluent of quality better than
that obtained by plain sedimentation. An effluent that is
generally fairly clear, with only very fine suspended or col-
loidal solids but with practically all of the dissolved solids
remaining, can be produced. Under most favorable condi-
tions and with skilled operation, SS may be reduced in an
amount of up to 90% and BOD up to 85%. However, theTABLE 1
Number of wastewater-treatment facilities in the United States (1996)Flow ranges m^3 /s Number of facilities Total existing flowrate m^3 /s
0–0.00438 6444 12.57
0.0044–0.0438 6476 101.78
0.044–0.438 2573 340.87
0.44–4.38 446 511.12
4.38 47 443.34
Other 38 —
Total 16204 1409.68
Source: Adapted from Tchobanoglous et al., 2003.C016_005_r03.indd 973 11/22/2005 11:25:15 AM