Page 137 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

to design instead of operation, the same dividends can be 5.6 Comminutors
3
realized. As a note, plots of this nature do not substitute for For a sewage flow, Q ¼ 0.308 m =s (7 mgd), size a
knowledge gain from pilot plant tests. They provide guidance comminutor. Provide a drawing of your design showing
for what to explore in pilot plant studies. For example, once approximate dimensions of the comminutor and chan-
the appropriate K is determined by pilot plant studies, the nel. Suppose the screen becomes ‘‘half-clogged.’’ Show
mathematical model may be applied as indicated here. Of on your spreadsheet how this affects your solution.
course, some spot checking of the mathematical model Describe problems caused by this condition.
would be advisable. 5.7 Hydraulic Profile for Headworks
Show the hydraulic profile for the headworks of a
wastewater treatment plant.
PROBLEMS
5.8 Microscreen Design with Incomplete Data
Bar Screens Reference: Excelt spreadsheet file for microscreen
design, Table CD5.7. The left side of Table CD5.7 pro-
5.1 Traditional Approach to Bar Screen Sizing
vides data on the headloss coefficient, K; these data
For a bar screen design for a municipal wastewater treat- were generated for the purpose of generating frequency
3
ment plant, let Q(max sewage flow) ¼ 0.396 m =s (9.0 of occurrence of K’s, plotted in Figure 5.13. The right side
3
mgd) and let Q(max storm flow) ¼ 0.44 m =s (10.0 mgd).
provides an algorithm for design, i.e., sizing for A(net),
5.2 Spreadsheet for Traditional Approach to Bar Screen
based upon different ‘‘scenarios.’’ For the above context
Sizing
(a) Select a microscreen mesh size (or opening size
Design a spreadsheet to accomplish all of the design tasks
in mm).
illustrated in Example 5.1 for any flow. Assume values for
(b) Explore the effects of uncertainty regarding the
Q(max sewage flow) and Q(max storm flow) and bar sizes
coefficient, K, with respect to the effect on head-
and bar spacing. Apply the criteria for velocity through the
loss.
screen. The spreadsheet should provide a design that 3
(c) Suppose Q is increased from 0.308 m =s (7 mgd) to
meets the criteria stated. Also, calculate headlosses. 3
0.616 m =s (14 mgd) for an existing microscreen.
5.3 Scenarios on Spreadsheet for Traditional Approach
Determine the associated headloss.
to Bar Screen Sizing
Assumptions for ‘‘baseline’’ scenario: Flow, Q ¼
Modify your spreadsheet to address different scenarios of 0.308 m =s (7 mgd), v ¼ 6.28 rad=min.
3
operation. In other words, the design is fixed. Therefore, 5.9 Microscreen Modeling
apply the spreadsheet in #2 to explore the effects of
The Denver Marston Water Treatment Plant treats
different scenarios of flow. These might include some
drinking water drawn from the adjacent Marston Lake
unexpected storm flows, or, by contrast, very low sani-
(near Quincy Avenue on the south side of Denver). The
tary flows (to simulate a draught, for example, such as the
plant experiences algae blooms that interfere with
one in California in the 1980s).
coagulation and filtration. Suppose that microscreening
5.4 Half-Clogged Bar Screen Added to Traditional
is a proposed treatment process for removing the algae.
Approach to Sizing
A manufacturer has provided a pilot plant which you
For a sewage flow, Q ¼ 7 mgd, size a bar screen system.
will use as the basis for a design. For this context, or a
Provide a drawing of your design showing approximate
similar one with which you are familiar, (a) Outline an
dimensions of the bar screen and channel. Suppose the
experimental program that you might propose. (b) State
screen becomes ‘‘half-clogged.’’ Show on your spread-
dependent variables. (c) Identify the independent vari-
sheet how this affects your solution. Describe problems
ables. (d) Would you do any bench scale testing? (e)
caused by this condition.
Would you visit any microscreen plants? (f) Would
Assumptions for ‘‘baseline’’ scenario: mathematical modeling have a place? (g) Describe
plots that you would generate from the pilot plant oper-
Flows are: Q(avg: sewage flow) ¼ 7 mgd ation. (h) Would you apply mathematical modeling for
Q(max: storm flow) ¼ 12 mgd any aspect of your design? (i) Describe how you would
arrive at a final sizing for a full-scale design.
5.5 Case Study on Cleaning Frequency for Bar Screen 5.10 Variables and Scenarios in Microscreen Design
The Marcy Gulch WWTP in Colorado has a 6 mm As a choice in a design exercise, the 50% frequency
(1=4 in.) bar screen (Parkson, Inc.). In this case, the would be a reasonable choice for input to a design
screen in a moving screen that makes an incremental spreadsheet, which would then be the basis for explor-
movement up after a period of screenings accumulation, ation of design outcomes using different input ‘‘scen-
which may be perhaps 2–3 min. How would you deter- arios’’ (combinations of independent variables). The
mine the frequency of screen renewal, if you were able to spreadsheet should include several such ‘‘scenarios,’’
travel to the site and take measurements? What would be comprising different flows, i.e., Q, and other uncertain-
your criteria? Determine for reference the headloss for ties concerning the design, e.g., substance to be
the clean screen. removed. Each situation is unique, however, and testing