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The development was initiated and has been heavily funded by the 
Environmental Protection Agency. The Federal Highway Administration funded 
large-scale pilot plant studies to verify process conditions and operability 
and to produce sufficient tonnages of rock for highway surfacing tests. The 
City of Philadelphia co-funded with EPA the design, construction and operation 
of a large $3,000,000 demonstration plant which although moderate in cost, is 
adequate to handle the sludge load for a city of 200,000 persons. 
The basis of the process is to mix and dilute sludge with 2 to 4 parts of 
shredded municipal solid waste; this solid waste helps to dry and burn-out the 
sludge. The solid waste then fuses with the sludge ash in the kiln to produce 
an insoluble rock product. The process and the rock product have been named 
ECOROCK (Figure 1) because of their economic and ecological soundness. 
In the ECOROCK process, sludge pathogens and parasites are thermally 
destroyed, along with toxic organic substances; toxic metals are permanently 
locked-up in the rock. Emissions and odors, which normally occur when sludge 
is burned by itself, do not occur. Because the sludge is diluted and spread 
over the solid waste, drying and burning are facilitated; controlled complete 
oxidation occurs; and the deep bed of hot highly silicious particles of solid 
waste ash captures and entraps emissions. The burning temperature in the kiln 
is kept below 1800°F by using air. The hot partly agglomerated ash particles 
that reach the end of the kiln after burning then drop into a small box 
furnace where a flame burner maintains a temperature 200-400°F above the kiln 
temperature. The hot ash particles quickly melt, fuse together, and flow by 
gravity from the furnace onto an air-cooled metal belt conveyor. As the melt 
is slowly air-cooled, it fuses into the rock product that is then crushed for 
use as a road paving aggregate. Extensive 6 year long road tests by the 
Federal Highway Administration have shown this resulting material to be a 
tough, durable aggregate with good tire skid resistance properties. 
The energy from the hot combustion gases generated in the rotary kiln are 
captively used to dry the incoming sludge/solid waste feed stream. This is 
done in a conventional rotary-drier located ahead of the kiln. Over 90% of 
the energy requirements for the process are supplied by the burning waste 
