Fort Worth's Alternative Delivery Biosolids Management Solution

INTRODUCTION The City of Fort Worth's (City) Village Creek Water Reclamation Facility (VCWRF) implemented a major change to its biosolids processes using design-build-operate (DBO) delivery. During the summer of 2022, the facility started its new centrifuge dewatering and thermal dryer system and now produces a high-quality dried Class A product, resulting in significant cost savings, reduced odors, more regulatory certainty, flexibility, and greater public acceptance. PROJECT OVERVIEW AND BACKGROUND The VCWRF is a 166 million gallons per day (mgd) activated sludge treatment plant. Prior to the project, it had dewatered its anaerobically digested solids with belt filter presses (BFPs) and added lime to produce Class AB biosolids (Figure 1). Each year the facility land applied approximately 29,000 dry tons of material to approximately 40,000 acres of ranch and farmland in the North Texas area. After a comprehensive master planning process, the City made the decision to convert its biosolids operation to a thermal drying process. This was motivated by multiple factors, including: 1.Odor concerns from dewatered solids. 2.Regulatory uncertainties associated with the lime-stabilized Class AB biosolids previously produced by the VCWRF. 3.Public concerns over biosolids land application that are mostly odor related. 4.Increasing hauling distances to land application sites. 5.Increasing costs of biosolids management. The City chose to implement the project using a DBO approach, and contracted with the selected DBO contractor, Synagro, in December of 2019 after receiving multiple proposals. By the summer of 2022, the facility was successfully operating and producing Class A dried biosolids pellets. FACILITY AND ITS OPERATION The VCWRF biosolids facility is located on a separate property located a mile north of the main plant (Figure 2). Anaerobically digested biosolids are pumped to the biosolids site where they are stored in three storage tanks, the largest of which has a capacity of 5 MG. Biosolids are then dewatered by centrifuge, dried in one of the largest rotary drum dryers ever constructed for a biosolids facility with an evaporation capacity of 14,000 kg/h, and stored in silos for truck loading (Figure 3). Process wastewater is pumped back to the VCWRF for further treatment. Odorous air is treated by two bioscrubber systems for hydrogen sulfide and ammonia, and exhaust gas from the rotary drum dryer is treated by a regenerative thermal oxidizer. The existing dewatering belt filter press and lime facility is still available and maintained as a backup system. PROJECT DELIVERY AND LESSONS LEARNED The City decided to implement the project using a DBO delivery approach, funded in part by a Texas Water Development Board (TWDB) Clean Water State Revolving Fund loan. With the selected DBO approach, risk is shared between the facility owner and the DBO contractor (Figure 4), but the owner loses some control over the details of the work. This provides the DBO contractor with greater flexibility and control over cost and schedule, which for this project allowed the contractor to control cost and schedule to minimize change orders during an unprecedented economic climate: COVID pandemic, the 2021 Texas Freeze, and growing inflation. A dryer facility is more akin to an industrial process than a traditional wastewater facility. In addition, as a DBO the City was not provided with as much detail on the new dewatering and dryer systems during design as would traditionally be provided. Jacobs supported the City's review of the designs, provided construction observation, and reviewed acceptance test procedures and results to support the City's management of the contract and verify that the work was consistent with the DBO contract requirements. Together the project team worked with Synagro to address challenges as they arose, including those related to the material recycle loop, foaming, and other issues. The facility is designed to manage biosolids to 2040 (Table 1). Class A biosolids production was achieved in July of 2022, marking a major milestone for Fort Worth and the DBO team. At that time the facility began formal 'acceptance testing' which consisted of a contractually mandated evaluation to verify that the newly installed infrastructure was working as designed. The City and team continue to address various punch list items to shift fully into the operations phase of the project, but this marks a major milestone for the City and for the residents of Fort Worth. The DBO Contractor is responsible for all operation and maintenance activities at the biosolids facility, including those for existing dewatering infrastructure (belt-filter presses) as well as the new dryer facility. THE PAPER This paper will describe the thermal drying project and lessons learned including: 1.A brief overview of the dryer project's history and motivations, 2.Description of the new facility and its operation, highlighting some of the key features of the drying facility, including systems for heat recovery, efficient water usage, pellet recycling, and use of digester gas as a supplemental heat source. With the new biosolids product, benefits to the City include reduced product odor, significantly less trucking, wider diversity of product use options, better use of digester gas, and operating cost savings 3.Lessons learned through the project's unique delivery approach. For example, collaborating with the TWDB brought significant cost savings for the City but presented other unique challenges. The TWDB did not have pre-existing procedures for design build projects, so TWDB staff worked closely with City staff to satisfy the requirements of both entities. Lessons learned from this collaboration that can benefit other utilities on future TWDB-funded projects will be presented. For example, because of the significant schedule constraints on the project, the project team executed an early package for construction of the building foundations that allowed construction to begin sooner while the rest of the design was completed and reviewed. 4.Operation and maintenance overview of biosolids processing operations, including the operating term and undertakings that the DBO contractor will satisfy during the term of the contract. BENEFIT OF THE WORK The work presented in this paper will benefit WRRF Utilities who seek alternative methods of implementing biosolids management solutions, as well as consulting engineers and the contracting community.