Achieving Class A Biosolids with Staged Thermophilic Anaerobic Digestion

INTRODUCTION: The Metropolitan Sewer District of the Greater Cincinnati (MSDGC) owns and operates the Little Miami Wastewater Treatment Plant (LMWWTP). Figure 1 is an aerial view of the LMWWTP. The LMWWTP has annual average capacity of 27 mgd with a design peak wet weather capacity of 100 mgd. The current solids processing system dewaters raw sludge for disposal in landfill. MSDGC is in the process of designing and building a new solids processing system that will produce Class A biosolids and beneficially use the biogas from anaerobic digestion. The new solids processing system consists of sludge receiving, screening, thickening, anaerobic digestion, dewatering, biosolids storage, and truck loadout. Biogas produced from the anaerobic digesters will be treated and beneficially used to fuel hot water boilers, engine driven cogeneration units, and gas turbine driven aeration blowers. Initially, the temperature phased anaerobic digestion (TPAD) process with batch pasteurization was selected. Figure 2 is a rendering of the solids processing system with the initial TPAD process. As the design developed, the process was changed to staged thermophilic anaerobic digestion (STAD) without batch pasteurization to reduce capital costs, while still able to produce Class A pathogen reduction biosolids. Figure 3 is a solids process flow diagram with the STAD process. Changing the process from TPAD to STAD was made early in the pre-construction phase of this progressive design build (PDB) project. METHODOLOGY: Starting with the planning phase, MSDGC established a business case with the primary goal of producing Class A biosolids for beneficial use through land application. Another goal was to beneficially use all the biogas from anaerobic digestion on-site. MSDGC chose to hire a design-builder to deliver the new solids processing system using the progressive design build (PDB) method. The PDB method was selected to provide flexibility for MSDGC's input during the design and for more construction cost certainty from a guaranteed maximum price (GMP) before design is completed. Early in the design phase, the construction cost was estimated to be higher than the planning level, project budget. MSDGC worked with their design-builder to value engineer the design to reduce construction costs. One cost saving item to reduce the construction cost is by changing the digestion process from TPAD with batch pasteurization to STAD. The TPAD system was based on two 2.4 Mgal digesters in series: a thermophilic digester followed by batch pasteurization and a mesophilic digester. The STAD system is three 1.2 Mgal thermophilic digesters in series. The last digester is also serving as storage tank to dewatering. Changing from TPAD to STAD increased the number digesters but decreased the total volume from 4.8 Mgal to 3.6 Mgal. The change to STAD eliminated the need for four batch pasteurization tanks and associated mixing, pumps, and heat exchangers. Having all three digesters operate at thermophilic temperatures eliminated the need for cooling heat exchangers required with TPAD. To achieve the necessary pathogen reduction to produce Class A biosolids, the technical team applied features from several successful operations of STAD installations. Table 1 lists the similarities and differences to LMWWTP of several successful thermophilic anaerobic digestion installations. All these installations achieved Class A biosolids with mixed plug-flow digesters in series. Figure 4 shows the configuration of the three mixed, plug-flow, thermophilic digesters in series. Each of the digesters in the LMWWTP project were designed to exceed the minimum time and temperature requirements set by the biosolids regulations. Sludge stabilization and pasteurization will be achieved at the same time with STAD. A minimum retention time of one day and corresponding temperature of 55 degrees C will be exceeded under all operating conditions. Raw sludge will be fed at the top of silo-shaped tanks and withdrawn at the bottom. The feed will be heated to over 55 degrees and maintained throughout the digester. A minimum hydraulic retention time of 8 days will be achieved even with one digester out of service. When all digesters are operating, there will be triple redundancy with three-staged thermophilic digestion. When one digester is out of service, there will be redundancy with two-staged thermophilic digestion. During commissioning, the biosolids with be tested to verify Class A pathogen reduction is achieved. FINDINGS: The flexibility of the PDB method enabled the technical team to design the new solids processing system to save construction costs by changing the anaerobic digestion process from TPAD to STAD. Solids processing systems at other water resource recovery facilities have been achieving Class A biosolids with STAD. The LMWWTP STAD process is expected to produce Class A biosolids. During commissioning the necessary testing will verify the biosolids quality. SIGNIFICANCE: The STAD process offers a lower cost alternative and expected to achieve Class A biosolids. The PDB method provides flexibility for owners and engineers to design to budget.