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BIOSOLIDS PROCESSING CHANGES TAKE “CONVENTIONAL” WISDOM

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Over the last few years, the City of Longmont, Colorado wastewater treatment plant (WWTP) has undergone significant changes. Prior to 2002, the WWTP used primary treatment and trickling filter/solids contact with nitrifying trickling filters for the liquid process train and a two-pronged approach to biosolids management. The solids generated in the TF/SC process City's were typically mixed and digested anaerobically, with a significant portion of the primary solids diverted to dewatering and composting at the City-owned aerated static pile facility. They operated a successful Class B, liquid land application program and produced a Class A compost that was used by the City and its citizens. By 2004, the liquid treatment process was converted to an activated sludge system; the compost facility was out of use, replaced by privatized hauling and composting; and the waste secondary solids were anaerobically digested and land-applied by a private operator.

A study of biosolids handling, treatment and reuse alternatives was undertaken for several reasons. First, the liquid stream process had increased biological solids production significantly. Also, the characteristics of the solids changed with the addition of activated sludge. Second, some of the structures currently in use date to the 1950's. Equipment age varies from process to process; however, the City was concerned about the remaining service life, operational capabilities, and overall effectiveness of these items. The operating costs and potential for odor issues in the solids handling system had increased. Finally, the City wants to strive to produce a biosolids product that is of the highest quality and meets all regulatory requirements, can be safely and effectively reused, and represents a resource to the community.

The study addressed not only the technical options available to the City, but also, a market assessment for various products, the perception of the costs of operating a Class A compost facility, and the operational complexity of all systems. This paper will discuss the wide range of alternatives evaluated for upgrading the solids processing facilities at the Longmont WWTP, which included solids thickening, dewatering, aerobic digestion, anaerobic digestion, both conventional mesophilic and temperature/process phased systems; digester pretreatment; chemical stabilization; thermal drying; and various product distribution options. The recommendations and current implementation plan include conventional anaerobic digestion to allow a robust agricultural land application program. Redundancy and flexibility have been designed in using both technical solutions and management alternatives. Equipment and facilities have back up systems, and the privatized composting option is being maintained. In today's biosolids world, as high-rate, sophisticated treatment processes such as thermal drying and advanced digestion are being used, we will address how the decision making process led to application of the tried and true mesophilic digestion process. The recommendations of the study and current design efforts under way include construction of a single, new mesophilic, anaerobic digester and supporting equipment.
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Document Type: Research Article

Publication date: 2007-01-01

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