Page 462 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Slow Sand Filtration 417

13.4 Effect of Additional Bed Depth on Life of Sand Required
Bed (13.3.2.6) 1. Estimate the kinetic energy of the raw water dis-
Given charge jets for estimated maximum flow at Empire
Let alternative initial bed depths of a sand bed be 1.0 m discharging at single points in each of the two filter
(3.3ft),and1.3m(4.3ft).Thesandbedscrapingfrequency boxes.
is about 1.0 month. The amount of sand scraped is about 2. Estimate the kinetic energy associated with free-fall
10mm each scraping. The sandbed isto beresanded when of water to the sand bed.
the depth is about 300 mm (1.0 ft). See Section 13.3.2.6. 3. Discuss the effect of the water fall to the sand bed
with no headwater and with about 500 mm of head-
Required
water.
Estimate the life of the sand bed before scraping in each
13.9 Design of a Sand Recovery System
case.
13.5 Depth of Sand Bed (13.3.2.6) Given
The scraped sand at Empire has been discarded by
Given
empting the buckets just outside the door at the top of
The depth of the sand bed is being considered for a new
the filter box. See Section 13.3.3.12.
slow sand installation. Assume the scraping rate of sand
is 5 mm=month. Required
Design a sand recovery system for the Empire slow
Required
sand filter. Consider storage of dirty sand, washing
Assume a convenient d 10 and UC. Then evaluate the
system, and storage of clean sand (if any).
pros and cons of different bed depths. For each depth
13.10 Design of a Pilot Plant Study
considered, include such concerns as
1. Life of the sand bed before rebuilding Given
2. Clean-bed headloss Mad Creek, a mountain stream, has been used for
3. Depth of the filter box decades by the town of Empire, Colorado for its drink-
4. Estimated cost of the different filter box depths, etc. ing water. A slow sand filter is to be constructed to
13.6 Operation of a Slow Sand Filter provide the town with filtered water.
Given Required
The piping layout for the slow sand filter at Moricetown, Design a pilot plant study plan for Empire. A small
British Columbia, is shown schematically in Figure community in your region may be selected if you wish
13.13. (in lieu of Empire). Set up an Excel data sheet for
Required collection and processing of data.
Make several copies of Figure 13.13 and show by 13.11 Critique of a Slow Sand Filter Design
colored highlighter type of felt-tip pen how the various Given
functions of the drainage system are executed. Assume that you have visited a slow sand plant in your
13.7 Effluent Weir Design region.
Given Required
Consider the slow sand filter at Empire, Colorado. Sup- Critique the design with respect to a goal to achieve a
pose that each of the two filter beds has a vertical pipe ‘‘passive’’ operation. [If you have not visited a plant,
telescoping overflow weir. The overall flow average to let the ‘‘required’’ be points for discussion.] Examples
3
both filters is taken as Q(Empire, avg) ¼ 946 m =day of points to consider are
(250.000 gpd). The maximum flow is taken as three 1. Pipe gallery. Examine the valve system for the
times the average flow. plant. Where are they located? How does the oper-
ator open and close valves?
Required
2. Effluent overflow. Is the tailwater weir eleva-
Determine the size of a sleeve pipe overflow weir cap-
tion at the proper level? Describe the control
able of handling the maximum flow.
for the tailwater elevation. Was there evidence
13.8 Sand Bed Erosion
of gas ‘‘boils’’ or did the operator comment on this?
Given
3. Flow measurement. Describe the flow measure-
Data given by Seelaus et al. (1986, p. 4) are
ment. Do you have recommendations?
3
3
Q(avg) 946m =day (250,000gpd) ¼ 0:010949m =s 4. Scraping. (a)What is the nature of the schmutz-
decke? (b) What is the frequency of scrapping?
2
HLR 0:26 m=h(6:5 mgad or 0:10 gpm=ft ) (c) How long does it take to dewater the headwater
2
2
A(total of filter beds) ¼ 153 m (1650 ft ) bed? (d) What is the duration required to scrape the
sand bed? (e) How much sand is removed? (f) Can
Number of filters ¼ 2
water be admitted to the sand bed from the under-
d(raw water influent pipe) ¼ 152:4 mm (6 in:) drain system?