Page 227 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

operation, and to the time and money available. As a rule, a If the depth can be changed, then the effect of u can
study requires a proposal that describes the purpose (why); the be investigated also (u is not a relevant variable,
scope in terms of objectives (what), work to be done, esti- however, since the reactor is not homogeneous).
The dissolved air portion of the design can be inves-
mated time, and budget; and the personnel to be involved. A
tigated by varying pressure and airflow and Z D ,or
party who has budget authority must commit to the study.
L(saturator), to investigate their respective effects on
Example 8.8 suggests some considerations on how a pilot
dissolved gas concentration, such as [O 2 ].
plant study might be conducted.
4. Plots: The relationships expected include C as a func-
tion of [gas]=C o , and of the hydraulic variable, vo. For
Example 8.8 Pilot Plant Study for Design of an Air the dissolved air design, [O 2 ] will be plotted as a
Flotation System function of P, Q a . From these relationships the basin
can be sized and the dissolved air system can be sized
(which would include water pump to give the needed
Outline a pilot plant study to design a DAF system to remove
a flocculent suspension. Identify both relevant independent pressure, the air compressor, and the vessel size).
variables and relevant dependent variables to be measured.
Indicate how you would use the results in sizing the dis- Discussion
solved air portion of the system and in sizing the basin. Designing a pilot plant study is an art since many judg-
ments must be made and experience helps to converge on
Solution the key issues. Any pilot plant study could become a full
The dependent variables must be identified first, and then research study with useful design knowledge being gener-
the independent variables. Before going ahead, however, ated. Budget and time are always limited, however, and so
how a flotation basin functions will be reviewed. the study should develop priorities.

1. Identify dependent variables: The flotation unit must 8.4.3 CASE:BIRMINGHAM
remove a portion of the flocculent suspension. The
primary dependent variable then will be the con- The Frankley Water Treatment Plant provides water to the
centration of the suspension in the effluent, C. City of Birmingham, in the United Kingdom and is operated
2. Identify independent variables: The dependent vari- by Severn Trent Water (data abstracted from Schofield, 1996).
ables include The plant capacity is 450,000 m =day, serving a population of
3
2.1 Flotation variables 1.15 million persons. Investigations started in 1987 to look at
3
Q, flow of water into the system (m =s)
treatment options related to a redevelopment plan to provide
C o ,influent suspension concentration (mg=L)
new treatment facilities to ensure compliance with a European
[gas i], concentration of gas i, such as oxygen,
Community Drinking Water Directive. Based on pilot plant
i.e., [O 2 ], in (mg gas=L)
C o ,influent suspension concentration (mg=L) studies, ferric sulfate was selected as the coagulant for the
[gas]=C o , ratio of dissolved gas concentration to removal of color and organics at a pH of about 5.5. The
influent solids concentration; oxygen would surface loading rate was set at v o 11 m=h. The maximum
2
be the most convenient gas to measure as an flotation area was set at 100 m , with maximum width,
2
indicator (mg ss=L=mg gas=L) w 8.5 m, with d 1.5 m. With A(plan) ¼ 100 m , and
2
3
v o , overflow rate (m =m =s) v o ¼ 11 m=h, Q(flotation cell) ¼ 26,400 m =day, requiring 17
3
u, detention time in basin (s) cells (which was set at 20 cells for the design).
w, L, d, width, length, and depth of flotation The flow scheme started with two raw water streams pass-
basin, respectively (m) ing through static mixers, where lime and=or carbon dioxide
2.2 Variables that affect dissolved gas concentra-
tions, i.e., [gas]: are added for pH control. Ferric sulfate is added just upstream
Q a , flow of air into the pressurized gas transfer from a measuring weir for each inlet channel, with the coagu-
vessel (gr=s) lant mixing provided by the turbulence below the weir. The
P, pressure of water (and the gas bubbles) at flow is then distributed to the 20 DAF cells. Each DAF unit
diffusers (atm) has a flocculation basin comprising three compartments each
The fundamental independent variables include with vertical variable speed paddle wheels. The detention time
1
[Co=[gas] and overflow velocity, v o . Detention time, was 30 min, and with 25 < G 80 s . The flocculated water
u, is not a factor (except Z D is a primary variable). is transported to the contact zone of the DAF basin located
3. Pilot plant study: Variables to be measured in a pilot midway in length such that the cell is divided into two half-
plant study include those listed in 2, with [gas]=C o cells where the flow is directed upward to the contact zone. In
and v o being calculated. The pilot plant study would the contact zone, the saturated water at 400 kPa pressure is
seek to determine C as a function of [gas]=C o and v o .
injected through a series of needle valves.
The effect of [Co=[gas] would be examined first.
A surface float is removed periodically by raising the
Then when the ‘‘optimum’’ [gas]=C o is established,
that value would be used and held constant as the water level a cell and then scrapping the float to a collecting
effects of v o are investigated. Since the pilot plant trough where it is removed as ‘‘sludge.’’ The sludge is
further dewatered to 25% solids by means of a filter press so
will have fixed dimensions, the only way to vary v o
is to change Q (holding other variables constant). that it is acceptable for landfill disposal. The final phase in