Sludge vs. cake? Carbondale mulls what to do with its sewage
October 19, 2012
CARBONDALE – A rather pungent debate is under way as town sanitation officials ask themselves whether they should stick with sludge or convert to cake.
The Carbondale Sanitation Department has for some time been considering making the switch away from sewage sludge, which historically has been the gloppy end product of the sanitation system’s operations.
As an alternative, utility director Mark O’Meara said the town could end up further drying out its sludge to produce what is called “cake.” It contains less liquid, is more compact and more concentrated, and can be transported in dump trucks rather than tankers.
O’Meara said the town’s sewage historically has been treated and stored as sludge in a large tank. Once the load reaches 100,000 to 125,000 gallons, it is pumped into tanker trucks and taken to a local agricultural operation, where it is injected into the soils that grow hay.
The injection, he said, is done by what he calls an “aggator,” a tanker truck with an attachment that plows small furrows and injects sludge into the ground.
Four hoses attached to the sludge tank each terminate with a small, spade-like implement situated in front of a nozzle. The spade digs the furrow, and the nozzle injects the sludge.
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“It takes us a week to complete the cycle,” O’Meara said. The sludge-storage tank typically fills up every five weeks or so, and emptying it takes 30 to 35 tanker trips to and from the receiving site.
According to his research, O’Meara said hauling the cake would take only one dump truck trip.
So the conversion to cake appears to offer a chance to cut down on transportation costs and help reduce the department’s contribution to air pollution, O’Meara said.
And he believes the cake can be used for the same farming application as the treated sludge, O’Meara said, though he has more research to do on the subject.
Although he has not worked up a complete fiscal analysis of the change, O’Meara estimated that buying the new equipment to make the conversion from sludge to cake would cost $200,000 to $300,000.
The department will soon have to replace a worn-out tanker truck and the aggator that injects the treated sludge into the soil.
Replacement costs for that equipment, he said, would be about the same as buying the new equipment for the conversion. So the conversion to cake could be considered a revenue-neutral proposition.
Even if the cake could not be used for agricultural purposes, he said, his research shows that sending one dump truck to a landfill would still be cheaper than hauling the sludge in multiple trips.
“I think it’s a wise investment,” O’Meara said of the conversion. “Trucking is expensive.”
He said he hopes to have final cost estimates to take to the town trustees before the end of the year.