Those tanks are digesters $22.1 million tanks part of Back River treatment plant 'Compost piles in the sky'

September 14, 1992|By Patrick Gilbert | Patrick Gilbert,Staff Writer

No, those golden, egg-shaped towers commuters se sparkling in the early morning sunlight from the Beltway or Eastern Avenue aren't symbols of a new fast-food franchise or the spires of an Eastern-rite church.

They are human-waste digesters, $22.1 million worth of aluminum-plated compost piles in the sky. The Back River Waste Water Treatment Plant and local newspapers have been inundated with calls from the curious.

"They're quite spectacular, aren't they?" said George G. Balog, director of Baltimore's Department of Public Works. The city owns the plant on Back River in Essex.

The new digesters, which will begin operation tomorrow, are expected to save the city hundreds of thousands of dollars annually in maintenance and operation costs, Mr. Balog said. They are more efficient than the older ones, and the sludge won't smell as bad.

The plant, on a 455-acre site, serves about 1.3 million people, mostly in Baltimore and Baltimore County.

The new tanks are made of reinforced concrete. A frame is built around the concrete and covered with insulated, anodized aluminum panels.

The design for the two 121-foot-high digesters was pioneered in Europe after World War II but never caught on in the United States. Smaller versions are operating in Los Angeles and Western Maryland, "but this is the only one with this much capacity in the United States," Mr. Balog said.

The new digesters can hold 3 million gallons and were built aboveground because it is less expensive, said Robert T. Mohr, chief of the waste water facilities division. The underground tanks, which will still be used, hold 1 million gallons each. The underground tanks are cheaper to build, but the maintenance and operations costs are higher.

"When we decided to increase our digester capacity, it was a matter of spending a few bucks more at the front end to build the aboveground digesters rather than to spend all the money on maintenance and operation costs at the other end," Mr. Mohr said.

The new above-ground digesters use the same process to break down sludge as do the six underground digesters. Sludge is the residue left after everything that goes into the sewer system is treated.

Wet sludge is pumped into the digesters, heated to 95 degrees and stored to allow bacteria in the sludge to break down the substance, Mr. Mohr said. That produces methane gas, most of which is used for fuel to heat other buildings at the treatment plant. The rest is sold to Baltimore Gas and Electric Co. and turned into natural gas.

The main difference between the two types of digesters is in design and efficiency, Mr. Mohr said. The above-ground digesters -- each of which is actually an egg-shaped body with two elongated inverted cones on each end -- prevent a buildup of gritty, non-organic materials in the sludge. The grit passes out the bottoms of the tanks, along with the remainder of the sludge.

In the underground cylindrical tanks, the grit builds up in the corners and bottoms of the tanks, and reduces the tanks' capacity.

"We have to clean out two of the underground tanks each year at a cost of about $25,000 a year each," Mr. Mohr said. "And we have to shut down the tanks for about two weeks during the cleaning."

In addition, fatty, oily and greasy substances in the underground tanks rise to the top of the liquefied sludge and form what is called a scum blanket. The blanket, like the grit deposits, can build up, reduce the tank's capacity and gum up the operating parts of the digester, Mr. Mohr said.

"The top of the cylindrical tank is approximately 80 feet across, so you get a much larger scum blanket than in the new digesters, where the top is only about 20 feet in diameter," Mr. Mohr said.

Sludge can be stored for up to 15 days in the new digesters, as opposed to 10 days in the underground tanks. The longer storage period means that more of the sludge is broken down by bacteria.

"We pay $40 a ton to haul the processed sludge away, so the more sludge that is broken down and turned into gas, the less sludge we have to pay to remove," Mr. Mohr said. "And that means more savings."

Once the sludge is removed from the new tanks, it goes through another process that spin-dries it in much the way a clothes dryer operates. That drains the sludge. What remains is trucked to farms outside the metropolitan area and is used for fertilizer.

Mr. Balog said he knew that the 121-foot towers were going to be noticed, so considerable time went into designing the digesters to make them pleasing to the eye.

"We've received quite a number of calls from people asking what on earth these things are," Mr. Mohr said. "Almost all of the callers have complimented us on the aesthetics of the digesters."

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