Page 421 - Water and Wastewater Engineering Design Principles and Practice

10-38 WATER AND WASTEWATER ENGINEERING

Summary:
3
3
Q design 43,200 m /d 0.5 m /s
Number of tanks 2
Width of each tank 6 m
L settler 24 m
Length of each tank 32 m
Side Water Depth including sludge 3.6 m
v approach 0.0116 m/s
v fc 0.0020 m/s
Reynolds number 19
5
Froude number 3.26 10
Launders 3 spaced evenly
Launder length 24 m
3
Weir loading 150 m /d · m
Sludge collector chain-and-flight
Comments:
1. Not all of the design recommendations were met, but overall the design is satisfactory.
2. The number of tanks and their size is considerably less than in Example 10-4 . Initially,
this appears to be a very favorable alternative. However, the cost of the settler modules
may offset the reduced cost for less tankage. In addition, there is no redundant settling
basin at the maximum design flow, so an additional basin would have to be provided.
An economic analysis is required to select the best alternative.

10-5 OPERATION AND MAINTENANCE
Perhaps the most important O&M activity is the optimization of the sludge withdrawal process.
The proper sweep cycle and duration are determined by trial. The concentration and characteris-
tics of solids are used to adjust the cycle. This is, of course, dependent on the turbidity and flow
rate of the raw water, so a range of conditions must be investigated.
Other operational characteristics, such as turbidity in the tank, equal hydraulic loading in the
tanks, and the number of tanks in service for a given flow rate must be observed with appropriate
adjustments. Maintenance is primarily focused on preventive maintenance on the collector and
observation for corrosion.

Hints from the Field. On new installations provided with plastic chain collection systems,

frequent tension adjustment in the first year after installation should be expected. Stainless steel,
(305 ss), or very heavy gauge plastic, is recommended for tube settlers to minimize corrosion
and/or deformation.
Pierpont and Alvarez (2005) offer the following suggestions for optimizing ballasted sedi-
mentation when surface water is characterized by high total organic carbon (TOC):

• Use larger tubes for the lamella settler: 90 mm instead of 40 mm.