Page 426 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Rapid Filtration 381

are the best place to obtain information. Demonstrations are 12.6 Optimizing Rapid Filter Design by Pilot Plant
set-up, as a rule, and knowledgeable representatives from the Given=Required
company are always present to explain and consult. As a rule, (a) Sketch a hypothetical h L versus time curve
proprietary systems are available as skid-mounted pilot obtained by pilot plant experiment along with a
plant units that may be shipped to any location. In most hypothetical concentration breakthrough curve.
cases, the units are ready to connect to raw water or waste- (b) Illustrate how the filter box depth may be deter-
water pipes or hose. mined by extrapolating the h L versus time curve
to the time at which concentration breakthrough
PROBLEMS occurs.
12.7 Negative Pressure in Depth Filtration
12.1 Theory of Depth Filtration
Given
Given=Required
A typical filter design showing headwater and tailwater
Discuss the relationship between the clogging rate in a
(the latter being the filtered water) was given by
filter and the transport efficiency.
McNamee et al. (1956, pp. 798, 799). The media is
12.2 Practical Design of a Rapid Filter
sand, 27 in. depth with 12 in. graded gravel with Wheeler
Given=Required bottom with a plenum perhaps 18 in. deep. The filtered
Design the operational components of a rapid filter. water reservoir is below the filters with the maximum
Include filter box sizing, media selection and depth, water level 12.0 ft below the headwater above the filter
surface-wash system, backwash storage, gravel support bed. A rate-of-flow controller maintains a constant efflu-
and tile under-drain system, wash-water trough and ent flow from the filter. The pipe from the filter bottom to
gullets, pipe sizing, valves, control devices and instru- the tailwater is ‘‘closed,’’ that is, no air gap.
mentation. Draw appropriate diagrams showing
Required
arrangement of all components.
Show the hydraulic grade line (HGL) from the head-
12.3 Backwash Protocol
water to the tailwater (a) at the start of the filter run, and
Given=Required (b) at the end of the run with the rate-of-flow controller
Outline the sequence of steps for backwash, specifying valve wide open.
duration of each step. 12.8 Headloss for Rapid Filter Backwash
12.4 Pilot Plant for Filtration of Wastewater Effluent
Given
Given=Required Suppose a ‘‘self-backwashing system’’ is to be designed
A physicochemical treatment scheme is proposed for a as part of a rapid-rate filtration system. The filter box is
municipal waste effluent. Most likely, this approach described as follows:
would be considered only for a small flow, for example,
a camp in a remote area, for example, 500 persons
D(filterbox) ¼ D(under-drains þ gravel support)
with water use estimated at 400 L=person=day giving
3
about 200 m =day (50,000 gpd). Most likely, the solids þ D(media) þ max: headloss þ freeboard
would be transported from the site. ¼ 600 mm þ 2000 mm þ 3000 mm þ 600 mm
(a) Outline a pilot plant program for process design ¼ 24 in: þ 66 in: þ 120 in: þ 24 in:
of the filtration step.
¼ 6200 mm (244 in: or 20 ft)
(b) What kinds of design results will the pilot plant
studies give?
(c) How will data be analyzed in terms of plots? The foregoing dimensions are approximations for guid-
(d) Show a data sheet which might be used. ance; they may be modified as desired. The bed is
(e) Sketch the experimental setup. anthracite with d 10 ¼ 1.0 mm, UC ¼ 1.3 and SG ¼ 1.68
(f) Consider how you would handle the solids. (as used by Amirtharajah and Cleasby, 1972, p. 58).
12.5 Filter Bed Hydraulics Required
Given=Required Determine the depth of filter box required in order to
Figure 12.39 shows a filter cross section along with a implement the self-backwashing feature of the design.
hypothetical hydraulic grade line (HGL) after some Assume the HLR(filtration-mode) 24 m=h (10
2
hours of operation. Use the sketch as the basis for the gpm=ft ).
following tasks. Hint: Assume, as a first try, and if needed, that the
2
(a) Sketch the HGL at the start of a filtration run. backwash velocity is 61 m=h (25 gpm=ft ). For filter
(b) Looking at the HGL as drawn, indicate where headloss, the intrinsic permeability, k, may be estimated
most of the solids have accumulated. from Figure E.2. Table CDE.2 provides a means to
(c) Show the negative head at point C. calculate k from K. An alternate approach is to utilize
(d) Show the headloss at the valve, point E, at the Table CDE.4, based on the Forchheimer equation.
beginning and end of the filter run. As seen, the problem is essentially three problems: (a) to