Page 54 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Water Treatment 9

FIGURE 1.2 Aerial photograph of WWTP, City of Colorado Springs, c. 1972. (Photo courtesy of City of Colorado Springs, Colorado
Springs, CO.)

settling. For disinfection, chlorine has been traditional in the chemical oxygen demand). The river has high value as a fishery
United States, while ozone is common in Europe. In munici- and for recreational use, and the filters provide a margin
pal wastewater treatment, as in Figure 1.1b, finer bar screens of safety.
have been used in lieu of comminutors; also, tertiary treatment
could be added. 1.4.1.1.2 Ignacio
The selection of unit processes depends upon the contam- Figure 1.3a is a photograph of an ABW system as
inants present in the source water and the objectives to be used for drinking water treatment at Ignacio, Colorado. Figure
achieved by treatment. The selection of technologies depends 1.3b is a cutaway perspective of the ABW system. The basic
on contextual factors; costs; preferences of the client; and the system is the same for water or wastewater, albeit the media
engineer’s vision, experience, and knowledge. Considerable may be different from one use to another. The traditional media
variation is possible within each treatment train in the selec- is sand with a depth of 305 mm (12 in.). A ‘‘cell’’ is 203 mm
tion of specific technologies. (8 in.) wide and its length is across the bed of the filter.

1.4.1 TERTIARY TREATMENT
In municipal wastewater treatment, additional unit processes
may include any or all of the following: filtration to reduce
particles, precipitation by lime to reduce phosphates, and
adsorption with activated carbon to reduce organic molecules.
If the goal is to produce potable water, then hyper-filtration
(reverse osmosis) may be added along with several other unit
processes.
1.4.1.1 Cases
Usually tertiary treatment situations have involved small
3
flows, e.g., 0.044 m =s (1.0 mgd). In some cases, such as in
providing water for irrigation, sand filtration without coagu-
lants has been used. The Parkson Dynasand moving bed filter
(a)
has been used in a number of such instances as has the ABW
traveling bridge filter of Infilco-Dregemont. These are ‘‘pack-
age’’-type technologies that can be added to any conventional
treatment train. These cases illustrate the systems context of
most water treatment projects and the role of political factors.
Both are inherent in any public project.
1.4.1.1.1 Aspen
(b)
TheCityofAspen,Colorado,installedanABWtravelingbridge
filter, developed for water treatment, to treat effluent from its FIGURE 1.3 ABW (automatic backwash) filtration system. (a)
conventional wastewater treatment train.Theeffluentdischarge ABW filter installation used for small community. (b) Perspective
permit for discharge to the Roaring Fork River required a drawing of ABW filter. (Photo courtesy of Infilco Degremont, Rich-
‘‘20=20’’ standard (20 mg=L suspended solids=20 mg=L bio- mond, VA.)