THE VACUUM FILTRATION AND INCINERATION OF
SEWAGE SLUDGE USING CRUSHED COAL
AS A CONDITIONING AGENT
Ronald L. Pitzer, Graduate Student
Ronald F. Wukasch, Associate Professor
School of Civil Engineering
Purdue University
West Lafayette, Indiana 47907
Dennis B. Wells, Sanitary Engineer
Department of Public Works
City of Indianapolis
Indianapolis, Indiana 46621
INTRODUCTION
The City of Indianapolis operates two wastewater treatment plants.   The main plant
is the Belmont Wastewater Treatment Plant which provides preliminary and primary
treatment to 220 million gallons per day (mgd) and air activated sludge secondary treatment to 120 mgd.   The wastewater is chlorinated before discharge to the White River.
The smaller facility, the Southport Wastewater Treatment Plant, has the same treatment
capabilities with a primary sedimentation capacity of 84 mgd and a secondary treatment
capacity of 56 mgd.
The sludge disposal facilities for both plants are located at the Belmont Plant.   They
consist of dissolved air flotation units, gravity thickeners, rotary-drum vacuum filters,
and multiple-hearth incinerators.   The sludge disposal flow diagram is shown in Figure 1.
The primary and waste-activated sludges are pumped in a common force main from
the Southport Plant (No. 2 plant) to the dissolved air flotation units at the Belmont
Plant (No. 1 plant).   All of the Belmont Plant's waste-activated sludge is thickened along
with the sludges from the Southport Plant in the dissolved air flotation units.   The
Belmont Plant's primary sludge is pumped to either gravity thickener No. 1 or No. 2
and is mixed with the bottom ash slurry from the multiple-hearth incinerators.   The ash
and primary sludge thicken together and are pumped to either the No. 3 or No. 4 storage
tank where they are mixed with the floated sludge.   This sludge and ash mixture is
pumped to the vacuum filters where a cationic polyelectrolyte is added.   The filter cake
is incinerated and the ash is quenched with water.   The ash slurry is recirculated to the
gravity thickeners or is wasted to a dewatering lagoon and then landfilled.
The ash serves as a filter aid to the vacuum filtration process.   It is used in a ratio
of approximately one part of ash to one part of dry sludge solids.   Before the ash system
was initiated in 1973, the city used ferric chloride and lime to condition sludge.   The
chemically conditioned filter cake was of poor quality and required over 7000 gal of
No. 2 fuel oil per day to obtain complete combustion.   The ash system produced a drier
cake and a higher filter yield but it still required about 5000 gal of fuel oil per day to
incinerate to 150 tons of dry sludge solids produced per day.
This research project examined the use of crushed coal to fulfill two objectives:   to
perform as a filter aid for vacuum filtration of sludge; and to perform as a supplemental
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