Civil Engineering Reference
In-Depth Information
Lagoons typically are earthen basins with 4- to 12-foot (1.2- to 2.7-m) sidewater
depths, covering from 0.5 to 15 acres (0.2 to 6.0 ha), which are equipped with inlet
control devices and overflow structures. The best design practice is to place the inlet
and outlet structures as far apart as possible. Residuals are added until the lagoon is
filled with solids, and then it is removed from service until the solids have dried to
the point at which they can be removed for final disposal.
If necessary, the residuals can be pumped for relatively long distances to a lagoon
site. The distance between filter plants and lagoons, while adding to the cost of a
pipeline, is a minor cost item in total water filtration plant costs. At York, Pennsylvania,
residuals are pumped 2 miles to a lagoon, and at Appleton, Wisconsin, the sludge
lagoons are 2.1 miles (4.03 km) from the softening plant. The Louisville (Kentucky)
Water Company, which clarifies and softens Ohio River water, has three lagoons lo-
cated 7 miles (13 km) from the major treatment plant.
Alum Sludge Lagoon Operation and Design.
Alum sludges have proved difficult to
dewater in lagoons to a concentration at which they can be landfilled. Neubauer re-
ported a detailed study of a lagoon receiving alum sludges from a 32-mgd (121-ML /
d) plant in Rochester, New York.
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The 400 x 300-foot (122
91-m)-wide lagoon
had been in operation for 3 years. Core samples indicated that the solids concentration
increased from about 1.7 percent at the sludge interface to a maximum of 14 percent
at the lagoon bottom. The average solids concentration of the sludge was 4.3 percent,
with a majority of the sludge having less than 10 percent solids concentration. The
lagoon did not produce a sludge suitable for landfill disposal without further dewater-
ing.
Other plants have reported removing thickened alum sludge by dragline or clam-
shell and dumping the sludge in thin layers on the lagoon banks to air-dry; dumping
the thickened sludge on land disposal areas or on roadsides; or transporting the thick-
ened sludge to a specially prepared drying bed.
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In general, alum sludges do not consolidate under water, but they do dry readily
when exposed to air and when drainage through the soil occurs. When lagoons are
built above ground, the berms should be from 10 to 15 feet (3 to 5 m) high and far
enough from property lines so that, if necessary, their top elevation can be raised. This
can be done by removing dried sludge from the interior of the lagoons for use as
embankment material. Lagoon berms for larger plants or those with softening should
be about 12 feet (4 m) wide at the top to facilitate the use of equipment for lagoon
cleaning.
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Two or more lagoons should be provided for alternating use, to allow between 6
months and 1 year for decanting, evaporation, and drainage. Such a drying period
reduces the sludge to the consistency of the soil at the site.
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The State of Kansas Bureau of Water Supply has sludge storage lagoon design
criteria requiring water treatment plants to have as a minimum the following storage
lagoon capacity:
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Two cells to be provided.
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Clarification sludge: Each cell to be designed for storage of 16 cu ft of sludge /
MG (0.12 m
3
/ ML) of raw water treated during an 18-month period.
•
Softening sludge: Each cell to be designed for storage of 85 cu ft of sludge / MG
(0.64 m
3
/ ML) treated during an 18-month period for the first 100 mg / L of total
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