EVALUATION OF UNIT PROCESSES FOR MECHANICAL DEWATERING
OF ANAEROBICALLY DIGESTED SLUDGE AT
METRO CHICAGO'S WEST-SOUTHWEST SEWAGE TREATMENT PLANT
Bernard Sawyer, Research Chemist I
Robert Watkins, Research Chemist I
Cecil Lue-Hing, Director
Research and Development Department
The Metropolitan Sanitary District of Greater Chicago
Cicero, Illinois 60650
INTRODUCTION
The Metropolitan Sanitary District of Greater Chicago (District) collects and treats the
wastewater from Chicago and over 120 surrounding suburbs.   It serves an area of 852
square miles inhabited by 6 million people plus an equivalent industrial and nonindustrial
waste load of 4 million people for a total population of about 10 million.
The district's West-Southwest Sewage Treatment Works (W-SW STW) has a design capacity of 1200 mgd and currently utilizes its solids via the following:   heat drying and sale
as dry fertilizer, high-rate anaerobic digestion and application to land, and Imhoff digestion followed by sand bed dewatering, long-term storage, and distribution to the general
public.
The last method of solids utilization has been an extremely successful one for the
district in the past two years.   The general public has been so willing to utilize the solids
generated ("NU-EARTH") that the district currently cannot fill the demand.   It is believed that the success of this program is due to the fertilizer value of the solids and its
relatively dry state (>30% total solids).
The district has determined that this "NU-EARTH" program could be expanded by
including a portion of its liquid digested sludge.   This would require that the liquid slurry
(approximately 4-5% solids) from the digesters would have to be dewatered before long-
term storage to achieve a final solids concentration greater than 30%.   Preliminary studies,
however, indicated that sludge dewatering to a solids concentration of 15% would be required to avoid overly long storage times to achieve this level of dryness of Nu-Earth.
The engineering department of the district conducted a desk-top study to determine the
most cost-effective dewatering processes to be used in a testing program whereby a single
dewatering process would be selected for processing anaerobically digested sludge produced
at its W-SW STW.   As a result of this study, three processes were chosen for study via
pilot plant testing.   These were vacuum filtration, centrifugation and belt filter pressing.
The R&D department of the district, therefore, was charged with the responsibility of
testing these processes on a pilot scale at the W-SW STW.   This report details the results
and conclusions of that testing program.
Vacuum filtration has been used since the 1920s to dewater sewage sludges, with solid
bowl centrifuges coming into prominence in the 1960s, and belt filter presses in the 1970s.
Reviews of the performances of various mechanical dewatering devices have been prepared
by the Water Pollution Control Federation [1] and the U.S. EPA [2].   However, much of
this work concerns the dewatering of primary sludge, trickling filter humus, undigested
waste-activated sludge or combinations of the above, with limited information available on
the dewatering characteristics of an anaerobically digested primary and waste-activated
sludge mixture.
Mudgett [3] obtained a 28% cake and a 6.2 lb/ft2/hr yield, conditioning digested
sludge with 60 lb FeCl3/dry ton and 160 lb CaO/dry ton.   Schepman and Cornell [4]
achieved a 21% cake and a 3.0 lb/ft2 /hr yield using 160 lb FeCl3/ton and 240 lb/ton CaO.
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