JOINT SOLIDS HANDLING FACILITY FEASIBILITY STUDY FOR WATER TREATMENT RESIDUALS AND BIOSOLIDS IN PHOENIX, ARIZONA

Based on the recommendations of the North Phoenix Capacity Increase Program (NPCIP) study completed in late 2001, the City of Phoenix authorized a series of improvements to the Union Hills Water Treatment Plant (UHWTP) designed to increase the production reliability of the facility. As part of this project, the City planned to construct a solids handling facility (SHF) to process residuals generated by the existing and proposed plant processes.Under the original design criteria, the proposed SHF would be designed with the capacity to process the solids generated by the design “firm event.” However, during the preliminary design phases of the project, it was determined that the facilities required to process a “firm event” would have additional capacity during typical raw water quality events. Based on the latent capacity of the proposed SHF during normal operation and the close proximity of the City's Cave Creek WRP (CCWRP) to the UHWTP, it was proposed that CCWRP biosolids and UHWTP residuals could potentially be processed at a Joint SHF. Currently, CCWRP primary and secondary biosolids are discharged to the sanitary sewer system and ultimately conveyed to the 91st Avenue WWTP for subsequent processing and dewatering.Before proceeding with the preliminary design, the City of Phoenix requested that a feasibility study be performed to determine the practicality of designing and operating a Joint SHF.To determine the feasibility of the Joint SHF, several alternatives were evaluated from a process and economic perspective. As a point of comparison, a “total baseline treatment alternative” was developed. The “total baseline treatment alternative” included a dedicated SHF at the UHWTP (for treatment of residuals) in conjunction with continued transfer of CCWRP biosolids to the 91st Avenue WWTP for subsequent processing. The joint facility analysis evaluated three potential locations (UHWTP On-Site, UHWTP Off-Site, CCWRP).As part of the evaluation process, various disposal methods for residuals and biosolids were explored. The primary alternatives for residuals and biosolids disposal are land injection (residuals only), land application, and landfilling. Due mainly in part to its less expensive cost (relative to landfilling), the preferred disposal option for solids is land injection/application. An additional disposal alternative, amalgamation of residuals and biosolids (pre or post dewatering), was also evaluated to maximize the quantity of solids available for land application.Once the location was determined, three process configurations (combined, separate, flexible) were developed and evaluated versus the baseline alternative. Under the “combined” process configuration, residuals and biosolids liquid streams would be combined/mixed to generate a homogeneous solution prior to entering the mechanical (centrifuge) dewatering process. The “separate” process configuration allows residuals and biosolids process streams to remain completely separate throughout the dewatering process. Liquid residual and biosolids streams would be stored and processed independently and the resultant cakes transferred to dedicated hauling vehicles in separate loading bays. The proposed “flexible” process configuration essentially incorporates the advantages of both the combined and separate process configurations into a single alternative. If desired, the flexible process configuration allows liquid residual and biosolids streams to be combined into a homogeneous solution prior to dewatering. The configuration also provides the ability to store and process the liquid streams independently.The cumulative capital and O&M costs associated with each alternative were calculated over the 25-year study period. Based on the evaluation criteria, the “separate” and “flexible” joint facility alternatives were more economical (i.e. had a lower cumulative cost) than the “total baseline treatment alternative” over the study period. However, the capital cost associated with constructing a Joint SHF still did not fall within the overall UHWTP 160 MGD Improvements Project budget. Consequently, additional alternatives were evaluated including a Phased Joint SHF and an Interim (residuals only) SHF.The Phased Joint SHF would be designed to minimize initial capital costs of the Joint SHF. Phase 1 construction would include only the facilities required to process UHWTP residuals. Phase 2, which would provide the ability to process UHWTP residuals and CCWRP biosolids at a Joint SHF, would be constructed sometime between 2007 and 2017, based on updated flow projections. In an effort to further reduce initial capital costs, an Interim, “bare bones” facility (for UHWTP residuals only) was also investigated as a possible alternative.Results of the analysis completed as part of this study indicate that both the Phased Joint SHF and Interim Dewatering concepts are economically viable alternatives to the “total baseline treatment alternative.” However, although the combination of an Interim SHF for UHWTP residuals and continued transfer and processing of CCWRP biosolids at the 91st Avenue WWTP requires the lowest initial capital expenditure, the most economical option over the study period would be to construct the Phased Joint SHF at the UHWTP Subsurface Injection Site. Furthermore, this alternative provides the City with the maximum current and future design, process, and operational flexibility.