Pima County Biosolids Master Plan - Long Term Sustainable Solutions

ISSUE The Pima County Regional Wastewater Reclamation Department (County) partnered with HDR in mid-2020 to complete a Biosolids Master Plan (Plan) to develop a new roadmap for their biosolids processing. The sustainability of their land applied Class B biosolids which had fared for the past 38 years was tested by a sudden moratorium in December 2019 which was enacted to assess possible PFAS contamination resulting from biosolids. For 14 months, the County relied on landfill disposal, at significantly higher cost, before returning to bulk agricultural application after the moratorium was lifted in February 2021. In addition to external pressures, there have been a number of process changes at the Tres Rios Water Reclamation Facility (WRF) which is the County's centralized solids processing plant, including: - primary solids from a large scalping facility, Agua Nueva WRF, were redirected to the Tres Rios WRF headworks, placing higher than anticipated loads on the primary treatment system, - incorporation of a NuReSys® struvite abatement process, which improved cake dewaterability potential, and -ongoing implementation of side stream de-ammonification using ANITA„¢Mox which will require redirecting 20-50% of primary thickened sludge from the biological nutrient removal trains to the gravity thickeners. The combination of external pressures and process changes necessitated facility improvements and a new direction for the County's biosolids management practices. OBJECTIVES 1) Revisit previous assumptions and recommendations made in the last 2012 biosolids master plan. 2) Baseline existing conditions and develop projections for future biosolids loading. 3) Evaluate changes, recent and future, in the biosolids regulatory market. 4) Evaluate the cost effectiveness of centralized biosolids treatment versus construction of a new biosolids processing plant for the County's smaller sub-regional facilities. 5) Develop a combination of biosolids management strategies and technologies for achieving sustainable long-term sludge processing and residuals management. 6) Prepare recommendations and a capital improvement program (CIP) for implementation. BACKGROUND The County owns seven water reclamation facilities (WRFs), serving a population of roughly 682,000, and treating an average wastewater flow of 58 million gallons per day (MGD). The County's two largest water reclamation facilities, the Tres Rios WRF and Agua Nueva WRF, provide over 93 percent of the total treatment system capacity. The five smaller facilities, referred to as the sub-regional facilities, provide treatment for non-contiguous service areas surrounding the City of Tucson. In addition to providing 50 MGD treatment capacity, the Tres Rios WRF is the centralized sludge handling and solids treatment system for the entire County. The solids from the other six facilities are transferred to Tres Rios WRF for treatment and disposal. The Tres Rios WRF last went through a major facility expansion from 2010 & 2012, and the last biosolids master planning effort was conducted in 2012. METHODOLOGY To baseline existing conditions, the project included: - site visits, equipment inventory, and discussions with operators at all County treatment facilities - review of design documentation from the Tres Rios WRF 2012 expansion - review of multiple years of operational data - development of a calibrated BioWin model and mass balance To forecast future biosolids loading, the County provided population projections which were used in combination with the BioWin model to estimate loadings at the solids treatment processes. A thorough regulatory assessment was conducted to evaluate local, regional, and national trends as they relate to biosolids disposal. Multiple treatment technologies to produce Class A biosolids were evaluated, including mechanical drying such as belt and rotary drum dryers, solar drying, thermal hydrolysis, incineration, and gasification. Multiple vendors were consulted during the evaluation to make presentations to County engineering and operating staff regarding their technologies. Additionally, the study included an economic and feasibility study that compared the existing practice of consolidating solids treatment at Tres Rios WRF versus a new sub-regional solids processing facility. Guided workshops and decision analysis were performed with the County to identify the preferred management strategy and Class A biosolids treatment technologies, which included both monetary and non-monetary criteria. The results from the planning process were summarized in a comprehensive report along with budgetary level cost estimates for the CIP, and a roadmap for future implementation. RESULTS The decision analysis results showed a clear preference by the County to transition to Class A biosolids using a phased approach that still allows for continued bulk agricultural land application of Class B in the short term. The County's preferred technology for achieving Class A treatment was solar drying, noting that continued Class B land application is still the most cost effective solution but subject to future regulatory uncertainty. The results also demonstrated the County's preference to diversify the number of biosolids outlets, including participating in composting with third-party entities and exploring other disposal options such as cement kilns and mining reclamation. A key recommendation was for the County to initiate a regional biosolids market assessment to identify Class A, Class B, and compost end-users to inform the proportions of their final biosolids products. The budgetary cost for the solar drying facility was based on treating 100% of the County's biosolids, but this may be refined based on the results of the market assessment. Other process and equipment recommendations were also made, including expanding primary thickening capabilities to accommodate the ANITA„¢Mox implementation, updating controls of the waste activated sludge (WAS) thickening process, increasing WAS solids pumping capacity, and improving solids handling at the smaller sub-regional facilities. An economic analysis of centralized versus a new sub-regional solids processing facility showed that centralized treatment had the lowest life-cycle cost. CONCLUSION The transition to Class A biosolids greatly reduces the County's risk associated with future regulatory uncertainty while also providing a wider number of potential end-users. The most cost effective technology selected by the County for achieving Class A treatment is solar drying which leverages southern Arizona's climate. The Plan provides a roadmap that guides the County from current operations over the next five years through the phased adoption of a solar drying system, performing a biosolids market assessment, continuing to evaluate opportunities for composting, and soliciting a private partner for biosolids management.