Page 423 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

378 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

12.6.1.3 Pilot Plant System The first study for filtering secondary wastewater effluent,
Regarding pilot plant flow, there are no firm rules. A flow for illustrated in treatment train (3), was by Tchobanoglous and
a single treatment train, or filter, in the range, 20 < Q < 80 Eliassen (1970). Their pilot filter was set up with piezometers
L=min (5 < Q < 20 gal=min) is manageable in terms of having and sampling taps along its depth and also had one side that
flows large enough that coagulant flows may be metered and could be removed to examine deposits of solids. Uniform
measured accurately and yet the logistic demands, for sand, that is, UC 1.0 with ‘‘equivalent diameters’’ of 0.49,
example, for chemicals, contaminants-to-be-injected, etc., 0.68, and 0.98 mm were used as media to filter secondary
are not dominant factors. effluent with suspended solids concentrations 5–18 mg=L.
Particles sizes were bimodal at about 5 mm and about
12.6.1.4 Data Handling 90 mm average size. Zeta potentials were about 20 mV as
Data procurement and processing requires an organized a mean. The suspended solids were reduced about exponen-
approach. If attention is given to this phase of pilot plant tially within the first 50 mm (2 in.) of bed depth with decline
work the data generated can be processed in terms of final being unchanging with depth at Z > 50 mm for all three
plots and tables and archived in a form that permits easy sand sizes and for three filtration velocities, that is, 4.88,
2
retrieval. To minimize mistakes and to facilitate data process- 14.2, 24.4 m=h (2.0, 5.8, 10.0 gpm=ft ). Removal for the
2
ing, metric units are preferred. Conversions to U.S. Custom- 0.49 mm sand at 4.88 m=h (2.0 gpm=ft ) was only 0.40
ary units can be done easily by spreadsheet for any final fraction at depths Z > 50 mm, which was the highest and
results. A 24 h clock also reduces confusion, recorded to the was lowest, with removal about 0.2 for the 0.98 mm sand at
2
minute, for example, 2145 h. Data should be recorded on 24.4 m=h (10.0 gpm=ft ). They did not observe a moving
forms designed for the project at hand and transferred to a wave front as seen in filtration of metal flocs and determined
spreadsheet daily. Generally, a separate line should be pro- that the removal mechanism was straining in the top layer. As
vided for each clock time that a data set is recorded. one indication that the removal mechanism was straining was
that the headloss curves for each media increased with time
with the shape of a power function.
12.7 WASTEWATER FILTRATION
Not too much has been formalized about guidelines for filtra-
12.7.2 FORMS OF PRACTICE
tion of wastewaters, albeit by the late 1980s, the practice
became fairly widespread. Proprietary systems have assumed Filtration may be added to a biological treatment train for waste-
a prominent role, sometimes with a prior pilot plant study and waters, or it may follow chemical treatment. Designs have
other times a unit has been placed online with the idea of included denitrification as well by adding methanol to induce a
working out operational procedures in the course of water biological reaction in the filter. The procedure involves a short
production. backwash (3 min) at 4–8 h intervals to remove nitrogen gas.
Typical treatment trains for wastewater filtration given by
Tchobanoglous and Eliassen (1970) are as follows: 12.7.2.1 As a Unit Process within a Water
Treatment Train
1. Chemical treatment of raw sewage, then filtration, A common designation for treatment of wastewater following
followed by further treatment secondary biological treatment is ‘‘tertiary treatment.’’ In some
2. Chemical treatment of secondary effluent, then fil- cases, a conventional water treatment train is employed to follow
tration, followed by further treatment secondary wastewater treatment. The main idea is to remove
3. Filtration of secondary effluent with or without fur- particulates. Particular problems that are characteristic of waste-
ther treatment water as a source water include (1) biofilms may appear more
luxuriant than in ambient source waters and (2) the sludge is
Further treatment could include other unit processes, for more putrescent. In treatment, this means that cleaning must be
example , adsorption, ion exchange, oxidation, disinfection, frequent enough to control growths. Hosing, followed by a
etc. The third treatment train is the focus of this section, that disinfectant is usual when dealing with wastewaters that have
is, filtration of biological floc. The first two, are primarily for organic matter as a predominant characteristic.
removal of nutrients.
12.7.2.2 As a Stand-Alone Process Following
Biological Treatment
12.7.1 BACKGROUND
Filtration alone following biological treatment has been prac-
Filtration of wastewaters has to do with making the water ticed in two forms: (1) as cake filtration and (2) as depth
suitable for some further use, for example, cooling water, filtration. If the particles to be removed are smaller than the
irrigation of golf courses, irrigation of certain crops, industrial pores of the media, straining occurs and the particles are
water, and even as a precursor to further treatment that could retained on the media surface, forming a ‘‘cake.’’ Further
include drinking water. Rapid filtration may both reduce the removal is by straining by the cake, which increases in thick-
overall suspended solids loading to an ambient water and ness as particles are further retained and accumulated. Because
attenuate fluctuations. the headloss increases rapidly frequent backwash is required.