Page 415 - Water and wastewater engineering

10-32 WATER AND WASTEWATER ENGINEERING

Summary:
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Q design 43,200 m /d 0.5 m /s
Number of tanks 6
Width of each tank 4 m
Length of each tank 55.5 m
L:W 13.8:1
Depth including sludge 3.6 m
L:D 28:1 without sludge depth; 18.5:1 with sludge depth
v f 0.0104 m/s
Reynolds number 8,000
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Froude number 1.1 10
Launders 3 spaced evenly
Launder length 18.5 m
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Weir loading 65 m /d · m
Sludge collector chain-and-flight

Comments:
1. Not all of the design recommendations were met. This is, in part, due to the use of the
pilot column data to set the overflow rate and the water depth. In general, exceeding the
guidelines is acceptable. When the guidelines are not met, consideration should be given
to the importance of the guideline in the function of the tank. In this case the depth of the
tank is quite shallow. Recognizing that deeper tanks are better for Type II settling, this
would be a reason for another design iteration. Likewise, the weir length is excessive for
the guideline, and an alternate scheme might be considered.
2. The design solution presented here is not the only one that is acceptable. For example,
4 tanks, 6 m wide, 3 m deep, divided into 2 channels with a baffle will also meet the
design criteria. An economic analysis is required to select the best alternative.
3. Numerous iterations may be required to balance the number of tanks, width, various
ratios and the Reynolds and Froude number recommendations. A spreadsheet is recom-
mended for the iteration process.

Design Criteria for Small- to Medium-Sized Plants
Frequently, small to medium-sized plants will operate for only one or two 8-hour shifts and store
water for the remaining period (Walker, 1978). Thus, the flow rates are higher than the estimated
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demand at the design life of the plant. For example, a 10,000 m /d demand could be met by oper-
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ating one 8-hour shift at a flow rate of (24/8)(10,000 m /d) 30,000 m /d. The 30,000 m /d flow
rate would be used for setting the dimensions of the tank. The decision of the operating schedule
is an economic one because the capital costs will be higher, but the operating cost for personnel
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and power will be less. If the operating schedule results in a flow rate above 40,000 m /d, then

the design criteria in Table 10-4 apply. The suggested design criteria in Table 10-5 may be used
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for flow rates less than 40,000 m /d.