The Metro Wastewater Reclamation District (Metro District) provides wholesale wastewater transmission and treatment service to 55 local governments in the Denver metropolitan area. Solids processing at the Central Treatment Plant (CTP) consists of dissolved air flotation (DAF) thickening
of waste activated sludge (WAS), anaerobic digestion of blended primary sludge and thickened WAS, dewatering with high-solids centrifuges, land application at the Metro District's METROGRO Farm located approximately 100 miles east of Denver, and composting of a portion of the digested biosolids.
Prior to this project, the Metro District had 10 anaerobic digesters operated in parallel in a high-rate mesophilic digestion process. Based on flow and load projections, a capacity expansion of the process was necessary to meet the needs of the Metro District. CH2M HILL was retained by
the Metro District to evaluate ways to meet its future sludge digestion needs and provide additional digestion capacity through the design year 2020. Following evaluation of numerous alternatives, the decision was made to construct two additional digesters (Digesters 11 and 12) to provide
the additional needed capacity. In addition, the decision was made to provide the flexibility to operate the digestion process in either a single-stage mode or a two-phase (acid/gas) mode. Key components of the construction project included: • Two new anaerobic digesters, designed
to operate as either acid, methane, or combined modes as part of either a single-stage or two-phase process. • A new Digester Control Building, improvements to the existing Digester Mall, and improvements to the existing anaerobic digesters. • Improvements to the hot water
system. Hot water is created by waste heat from the CTP's Cogeneration system, with three natural gas fired boilers serving as back up. • Improvements to the digester gas handling system, including four new waste gas flares. • Acid sludge pumps and digested sludge pumps. •
Process Control and instrumentation upgrades. The innovative two-phase process separates the two biological steps of anaerobic digestion into separate vessels for maximum efficiency. The two-phase system consists of highly loaded "acid phase" digesters followed by lightly loaded "gas phase"
or "methane" digesters. The first stage provides hydrolysis and acidification pretreatment, and the second stage maximizes gas production. The two-phase process is anticipated to result in improved volatile solids reduction, higher gas production, and improved process stability. The new digesters
were placed into operation in late 2005 and the digestion process was converted to two-phase in February 2006. This paper will briefly summarize the main components of the project and explain the two-phase digestion process and how it is operated. It will focus primarily on describing the
lessons learned in starting up the new facilities and the process optimization efforts completed after startup. It will relate the impacts of a separate DAF thickening and primary sludge thickening project at the CTP on the digestion process and also describe the effects of two-phase digestion
on digester foaming. Comparative data gathered over several years will be presented for operation of the digestion facilities in both single-stage and two-phase operation.
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