Section 10. SLUDGE CONDITIONING
THERMAL SLUDGE DRYING—A LOOK AT DRUM DRYER SYSTEMS
David L. Russell, Project Manager
Lockwood Greene Engineers Inc.
Atlanta, GA 30367
David A. Smith, Plant Superintendent
Wastewater Treatment Plant #45
North Tonawanda, NY 14120
INTRODUCTION
This is a study of thermal sludge dewatering using an atmospheric double drum dryer. This study
was performed in early 1984 at Amherst, New York's 19 Million Gallon Per Day advanced design
wastewater treatment plant.
The Town of Amherst, New York is a residential community just outside Buffalo, New York.
The recent Industrial Pretreatment survey identified no significant industrial users; the plant sludge
is exceptionally low in heavy metals, pesticides, and other organic contaminants and ideally suited
for landspreading or agricultural uses. Amherst produces about 3700 tons (dry weight) of sludge per
year. Sludge is dewatered 5 days per week at a 15 dry ton day rate (60-70 wet tons @ 21% solids).
Sludge is dewatered using a polymer fed for preconditioning, solid bowl Sharpies centrifuges and
a belt press. The dewatered sludge is sent to a sanitary landfill for ultimate disposal; current disposal
costs are $15.00 per wet ton, down from a $37.00 per wet ton cost in 1982. Current sludge dewatering
and disposal costs are around $ 150-$ 175 per dry ton of solids; landfilling costs represent about $75.00
per ton, and polymer conditioning costs represent about $22.00 per dry ton. Sludge disposal costs
are artificially low because competition from a new sanitary landfill in the area has depressed tipping
fees, but in the long run sludge disposal costs are expected to return to around $24.00 per wet ton.
The sludge disposal system is labor intensive. On a daily basis, excluding disposal costs, the labor
cost (exclusive of maintenance costs) represents almost 30% of the daily dewatering cost; when sludge
disposal cost is included in the total cost, the labor component drops to about 13.5% of the total.
Clearly, if a proposed sludge treatment system could offer savings on labor, chemicals and disposal
costs, that system might prove worthwhile.
After we discussed our interest in a drum dryer with Mr. Paul Miller, the Division Manager, of
Blaw Knox Food and Chemical Equipment, Inc., in Buffalo, he arranged a laboratory scale test of
the sludge dewatering capabilities of the drum dryer. The test results were encouraging, and an in
plant, two month lease of a small commercial scale drum dryer was arranged to allow the town to
evaluate the equipment performance, and develop their own cost data.
A drum dryer looks like its pictures in Figures 1 and 2. Sludge is fed into a pendulum feeder to
distribute it evenly in the area above the rollers. The drums are rotated in opposite directions by a
motor drive. Drum temperature is controlled by regulating steam pressure; adjustments are controlled
on the rollers where it is scraped off by a doctor blade shown in Figures I and 2. By adjusting the
drum speed, spacing, and the temperature of the drum (steam pressure), the operator can control
the amount of water removed from the sludge and can obtain any desired degree of dryness. Once
the operating parameters are set, system performance is constant, and the equipment is non-labor
intensive. Chemical and food industry experience has demonstrated that one operator can easily
run a bank of up to a dozen drum dryers and still have time for other duties. Most drum dryers are
rugged, a twenty-year operating life not uncommon, and some dryers are reportedly still in service
after 50 years.
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