In Search of Digester Capacity for Less Cost? Two-Phase and Three-Phase Digestion Hold Much Promise

The 91st Avenue WWTP in Phoenix, Arizona currently treats over 150 mgd of wastewater from the Phoenix metropolitan area. The biosolids from it's nitrifying/denitrifying activated sludge facility are thickened and anaerobically digested in 16 high-rate digesters. As the population of the Phoenix metropolitan area continues to grow and waste activated solids from satellite treatment plants are directed to the 91st Avenue WWTP, the flows and loads continue to rise. The facility is currently constructing a liquid stream 30-mgd expansion and is planning for the next expansion to be on-line by 2005, estimated to cost 118M. The next expansion was originally master planned to include two new 110-ft diameter digesters.With approximately 13 additional conventional high-rate digesters required to treat build-out loadings, the City decided to embark on a search for digestion capacity. The main objective of the search or study was to find a way to optimize the capacity of the existing digesters, thus pushing the construction of two new digesters (and reducing the overall number of digesters at build-out) further into the future. With this objective in mind, the study team evaluated six alternatives that had potential to improve digester capacity. In addition, the study team also reviewed the alternatives with respect to their ability to “do more for less.” Volatile solids reduction, dewaterability, facility impact and operability were additional criteria that were evaluated. Literature review and full-scale plant interviews helped the team rank and narrow down the alternatives to two alternatives that held the most promise for what the City wanted to accomplish – no new digesters. The two alternatives that survived the test were two-phase mesophilic-mesophilic digestion and three-phase mesophilic-thermophilic-mesophilic digestion.To fully assess the facility, cost and operations impacts, the team proceeded with a detailed impact evaluation. The team investigated the retrofit requirements for the digesters, heating system, transfer and overflow piping, downstream dewatering and odor control facilities, as well as sidestream centrate loadings from the centrifuge dewatering facility. Both alternatives were estimated to provide favorable cost savings as compared to the cost of the baseline digestion alternative.As a result of this study, the City is moving forward with the design of phased digestion. Three-phase meso-thermo-meso digestion has been selected and holds not only the key to “doing more for less”, but a positive step towards achieving Class A biosolids and a cleaner burning energy source.