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In 2002, the City of Phoenix began the 160 MGD Improvements Project at the Union Hills Water Treatment Plant (UHWTP), designed to increase the production reliability of the plant. As part of this project, the City planned to construct a new solids handling facility (SHF) to dewater process residuals generated by the existing and proposed plant processes. The UHWTP currently disposes of process residuals at the nearby Union Hills WTP Subsurface Injection Site, however use of the Subsurface Injection Site may be limited in the future.

Under the original design criteria of the improvements project, the proposed SHF would be designed with the capacity to dewater (using centrifuges) process solids generated by the “firm event”, defined as incoming raw water with a turbidity of 10 NTU (98 percentile of turbidity in raw water). However, during the preliminary design phases of the project, it was determined that the proposed SHF required to process a “firm event” would have additional unused capacity during typical raw water quality events. The average raw water turbidity is 1.6 NTU. Based on the latent capacity of the proposed SHF during normal operation and the close proximity of the City's Cave Creek Water Reclamation Plant (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 Wastewater Treatment Plant (WWTP) for subsequent processing and dewatering.

To determine the feasibility of the Joint SHF, several alternatives were evaluated from a process and economic perspective, relative to the baseline alternative of providing separate solids handling facilities for the water residuals and biosolids. The joint facility analysis evaluated three potential locations (UHWTP On-Site, UHWTP off-site at Subsurface Injection Site, CCWRP). After the site location was selected based on anticipated regulatory and site constraints, three process configurations (combined, separate, flexible) were developed and evaluated. These configurations were also evaluated versus the “total baseline alternative”, defined as a dedicated SHF at the UHWTP (for treatment of residuals only) in conjunction with continued transfer of CCWRP biosolids to the 91st Avenue WWTP for subsequent processing. Under the “combined” process configuration, residuals and biosolids liquid streams would be combined/mixed together prior to entering the 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 would allow liquid residual and biosolids streams to be combined prior to dewatering, or 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.
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Document Type: Research Article

Publication date: 01 January 2005

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