Disposal of Digested Sewage Sludge
By Landfill
BRIAN W. MAR, Research Professor
Civil Engineering Department
University of Washington
Seattle, Washington
CHARLES HENRY
Municipality of Metropolitan Seattle
Seattle, Washington
INTRODUCTION
Sludge disposal in an economic and environmentally acceptable fashion is a universal
problem in waste water treatment. Solutions have ranged from direct discharge into
receiving waters to reclamation and reprocessing for its soil conditioning or energy value. A
number of seacoast cities have or are still directly discharging sludge in coastal waters. Both
Federal and State agencies have instructed the Municipality of Metropolitan Seattle
(METRO) to terminate the discharge of sludge into Puget Sound by 1972. In response to
this directive METRO enlisted the resources ofthe University of Washington, Department
of Civil Engineering, to participate in a joint sludge disposal demonstration project.
The initial concept of the demonstration project was to develop a process that would
employ wet digested sewage sludge to reclaim seashore land. This appeared to be attractive
since the treatment plant is located on the shores of Puget Sound and the City of Seattle had
expressed an interest in developing a parkway along the intertidal zone near the plant. Since
there are many coastal cities faced with sludge disposal problems and there was a great
interest by public agencies in acquiring and developing lands for parkways, open spaces,
and recreational areas, the concept of land reclamation with digested sludge appeared to be
a solution to a nationwide problem.
The initial construction of the M ETRO West Point treatment facility provided a four-
acre diked area in the intertidal zone adjacent to the plant site for future digester
construction. Digested sludge was originally discharged after screening with the primary
plant effluent to Puget Sound. Although METRO studies have shown that this practice has
no observable deleterious effects on the receiving water, this research program was initiated
to comply with State and Federal direction to seek an alternative method of disposal. The
research goal was to demonstrate the practicality of filling this lagoon with sludge as a
means of sludge disposal and land reclamation. Practicality was interpreted as being
economically feasible and environmentally acceptable. Offsite disposal was not considered
for this study, although some attractive alternatives can be suggested.
The West Point plant provides primary treatment of sewage from a combined sewer
system. Settled sewage is fed to a three unit high rate digester system. During periods of high
rainfall, the design peak wet weather flow of 325 million gallons per day is exceeded. Peak
flows up to 400 mgd have been successfully handled by the plant. Operation of the digesters
under high flow conditions is challenging and the resulting digested sludge has a one to three
percent solid content (weight). The solids will normally contain about 50 percent volatile
material, but the non-volatile content will increase during periods of high rainfall when
sediment from the combined sewers enters the system. During the demonstration period
approximately 25 tons per day of dry digested solids will be produced by processing 250.000
gallons per day of sludge from the digesters. Since the plant is located on the beach, large
quantities of beach sand are available for use as cover.
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