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City of Sedona Director of Public Works, Charles Mosley, presents alternatives for drying biosolids to the City Council.

SEDONA, AZ (November 12, 2009) - At the November 10, 2009 City Council meeting, Charles Mosley, Director of Public works outlined various options for replacing the biosolids drying beds at the city's Wasterwater Reclamation Plant.

Biosolid waste generated from the plant is currently aerobically digested and the water is removed either in a centrifuge or on an air drying bed.  The remaining sludge is then transported to the Gray Wolf landfill owned by Waste Management in Dewey, AZ.

The plant is currently equipped with 10 drying beds, each 90 feet in length, 40 feet wide, and 2 feet deep.  The beds typically last 10 years. 

The existing air drying beds are in need of repair and nearing their treatment capacity.  The most common concern with air drying beds is cracking and possible seepage of waste into the aquifer.

The city contracted with Carollo Engineers, an environmental engineering company that specializes in the planning, design, and construction of wastewater facilities to present alternative biosolid drying methods, their associated costs, and pros and cons.

Carollo presented the following alternatives:

Alternative 1B: Demolish the existing beds and replace them with new drying beds with slightly greater dimensions (94 feet in length, 42 feet wide, and 5 feet deep) and employ a decant system for separating lighter floating liquids from heavier liquids to enhance drying performance.  Alternative 1B was developed to meet new requirements by the Arizona Department of Environmental Quality.

Alternative 2: The same as Alternative 1B except that the beds would be retrofitted to accommodate a thermo drying system in the future that would include enclosing the beds in a greenhouse type structure where 95% of the energy required for drying the sludge is provided by solar energy.  A THERMO-SYSTEM reduces energy costs, reduces sludge volume and related transportation costs, and creates Class A biosolids that could possibly be sold (the city's current biosolids are Class B).

The THERMO-SYSTEM would last longer than the air drying beds (15 - 20 years), and cost $3.4 million.  Alternative 1B would cost $1.3 million for 10 years ($2.6 million for 20 years); however Alternative 2 has the additional benefits described above.  Carollo also said that ADEQ might impose more stringent environmental standards in the future that could potentially prevent the use of traditional air drying beds.

The council voted unanimously to choose Alternative 1B citing the lower upfront cost as the primary reason.