Alternate: We're on the Road to Somewhere in Paradise: A Roadmap for Novel and Sustainable Biosolids Management at the Sand Island WWTP, Honolulu, Hawaii

Introduction and Background The Sand Island Wastewater Treatment Plant (Sand Island) is located on the idyllic shores of Oahu with deep blue waters laying before it, lush green mountain of jungle vegetation behind it, Pearl Harbor to its left, and the bustling high-rises of downtown Honolulu and Waikiki to its right. Sand Island is owned and operated by the City and County of Honolulu (CCH). Sand Island is currently a 90 MGD primary treatment with UV disinfection and biosolids treatment plant with ocean discharge for its treated effluent and thermally dried pellets for its biosolids distribution. CCH is under a federal mandate to upgrade Sand Island to meet full secondary treatment water quality standards by 2035. Sand Island is undergoing its upgrade in two phases. Phase 1 is under construction to build ~20 MGD of Membrane Bioreactors (MBR) capacity and additional mesophilic anaerobic digestion capacity to handle the waste activated sludge (WAS) along with the projected increase in primary sludge due to the continued population growth within the CCH service area through 2035. Phase 2 is in progress transition into the detailed design phase and will complete the full secondary treatment capacity to 90 MGD including wet weather storage to carry Sand Island through a 2045 planning horizon. The selected secondary treatment process for Phase 2 is Aerobic Granular Sludge (AGS) and the combination of Primary sludge, MBR WAS and AGS WAS, a never-before-seen sludge cocktail in our industry and a unique blend to consistently process on a small island with limited space and options for biosolids product distribution. This is the backdrop for a one-of-a-kind solids alternatives analysis in paradise. The existing solids handling treatment at Sand Island involves conventional mesophilic digestion, dewatering, and drying. The existing solids handling system will require additional treatment capacity to process the increased solids load when full secondary treatment is in service. With this task at hand we set to work on pitting tried and true biosolids processing strategies for Sand Island against new effective options adaptable to the Hawaii market to produce sustainable biosolids and energy to fuel Sand Island. This will also lower the utilities' impact of the Island of Oahu and its limited and imported natural resources. Solids Treatment Alternatives Four distinct alternatives for the solids treatment upgrades were identified as potentially viable solutions and were evaluated on their merits to economically enhance process performance, redundancy and reliability. - Alternative 1: Conventional mesophilic anaerobic digestion (AD) + Thermal Drying (TD) - This alternative is the baseline case and uses the existing solids treatment process philosophy of mesophilic anaerobic digestion at a 20-day solids retention time (SRT) followed by rotary drum drying, producing Class A dried pellets for beneficial reuse (Figure 1) expanded to the new Primary Sludge + WAS solids projections through 2045. - Alternative 2: Thermal Hydrolysis Process (THP) + AD + TD - This alternative modifies the existing treatment process to include sludge pre-conditioning for THP followed by mesophilic anaerobic digestion, producing Class A biosolids and reducing the volume of solids undergoing digestion, followed by dewatering, rotary drum drying, and producing Class A biosolids for beneficial reuse (Figure 2). - Alternative 3: THP + AD + Combined Heat & Power (CHP) (No Dryer) - This alternative modifies Alternative 2 to eliminate the drying operation. THP produces Class A biosolids cake that can be land applied. However, the land application market for Class A dewatered cake in Oahu requires further analysis. Dewatered cake can also be accepted at H-POWER (Covanta Honolulu Resource Recovery Venture) to produce power. By eliminating thermal drying a surplus of biogas becomes available for heat and power generation through a CHP system with a very favorable return on investment (ROI) of 5 years due to the resource limitations of Hawaii (Figure 3). - Alternative 4: Thermophilic Anaerobic Digestion (TPAD) + CHP (No Dryer) - This alternative would modify and augment the anaerobic digestion process currently under construction at Sand Island with batch thermophilic anaerobic digestion at 55°C (131 °F) followed by mesophilic anaerobic digestion at 35 °C (95 °F) (TPAD) to produce a low odor Class A biosolids cake eliminating the need for thermal drying to achieve Class A Biosolids. However, the land application market for Class A dewatered cake in Oahu requires further analysis. Dewatered cake can also be accepted at H-POWER (Covanta Honolulu Resource Recovery Venture) to produce power (Figure 4). Capital, Operations and Maintenance (O&M) Costs Class 5 Opinion of Probable Construction Cost (OPCC) estimates include the costs associated with construction, engineering, administration, and contingency were performed. The total capital cost with high and low range sensitivity analysis estimates for Alternatives 1, 2, and 3 is depicted in Figure 5. Alternative 4 is still under evaluation and will be complete by the time of presentation. O&M costs include the costs associated with energy and consumables use, labor, and the maintenance for the improvements identified and detailed in each alternative. Figure 6 depicts the annual O&M costs associated with Alternatives 1, 2 and 3. Alternative 4 is still under evaluation and will be complete by the time of presentation. Net Present Value Analysis NPV analysis was performed to evaluate life cycle costs over a 20-year horizon which includes the cumulative capital and O&M costs associated with each alternative (Figure 7). Key Takeaways This presentation will provide insights on - Impacts of each alternative on the existing solids treatment train - Mass and Energy Balance analysis summary associated with different alternatives - Life cycle cost analysis for each alternative over 20-year planning horizon - Comparison of alternatives based on the following: -- Economic Criteria: Capital and O&M Cost -- Non-Economic Criteria: O&M Complexity, Technology Maturity/Reliability, Health & Safety, Product Application, and Footprint. - Furthermore, driving factors associated with higher costs in noncontiguous state such as Hawaii in comparison to Mainland states will be discussed.