Streamlined Multi-Facility Residuals and Biosolids Master Planning with TBL Optimization

Authors:

  • Vera Gouchev, Paul Knowles - Hazen and Sawyer

The recommended scenario, Centralized Thermal Hydrolysis at the County’s 120 mgd Yonkers Joint WRRF, as modeled in the tool is illustrated above.

Estimated cost savings of approximately $6.5 million per year, as disaggregated above, are derived from reduction of disposed volume and mass through increased digester solids destruction and dewaterability, production of Class A biosolids, and utilization of biogas for onsite energy generation.

The Twenty-Year plan has a relatively stable Payback time that generally stays below 10 years regardless of increase in electricity cost, or a doubling in residuals hauling disposal cost.

As regional utilities are faced with tighter regulations, and simultaneous goals for economic and environmental sustainability, it is critical that they identify synergies between multiple facilities in order to improve efficiency and performance. To address this, a streamlined approach to multi-facility master planning for residuals and biosolids management has been developed. This approach identifies municipality or utility specific triggers that would necessitate improvement, and then employs a triple bottom line analysis to provide recommendations.

Examples of different types of triggers include requirements to produce Class A biosolids, meet sustainability goals, and/or achieve nutrient reduction. A multi-facility Residuals Handling Evaluation and Optimization Tool was utilized to identify the most suitable approach for meeting a trigger requirement in a streamlined manner to facilitate quick decision making. This tool allows decision makers to rapidly evaluate the Triple Bottom Line performance of different multi-facility improvement and consolidation scenarios, and to analyze the sensitivity of economic payback time to variations in future cost and process assumptions. This approach also allows decision makers to weight outcomes according to multiple user priorities such as sustainability goals or the desire for reduced operating complexity.

This paper describes our approach to multiple-facility planning in order to develop a County-wide long-term (Twenty-Year) Residuals Management Master Plan for Westchester County (NY) Department of Environmental Facilities (DEF).

Westchester County
Westchester County DEF owns and operates seven Water Resource Recovery Facilities (WRRFs), with a combined capacity exceeding 190 MGD, and three staffed-pumping stations. These facilities currently produce 120,000 cubic feet of screenings and grit, and approximately 19,000 dry tons of biosolids annually for beneficial use and disposal. Hazen and Sawyer was retained by DEF to determine the most efficient, flexible, and cost-effective methods to process and beneficially dispose of used wastewater residuals County-wide and at the facility level for five-year and twenty-year planning horizons.

Method
The main focus of the Five-Year plan was to identify and implement easily actionable changes that would provide large cost savings. Analysis of consolidated facility costs indicated that liquid sludge and dewatered cake hauling is the majority of expenses for residuals processing. As such, liquid volume reduction was established as a priority for the Five-Year plan. A scenario was modeled for the Five-Year plan that recommended two steps: 1) co-digestion of liquid sludge from multiple WRRFs at a facility with excess digester capacity; and 2) installation of dewatering equipment at the co-digestion facility with sufficient capacity to dewater the combined digestate. The plan would result in only dewatered cake being hauled for beneficial use and/or disposal from County facilities. Co-digestion of high strength organic waste and installation of a Combined-Heat and Power (CHP) plant to beneficially use digester gas for onsite energy production was included as a component of this scenario to improve environmental and economic performance.

The Twenty-Year plan considered how the County could best take advantage of emerging state-of-the-art processes to provide further Triple Bottom Line improvements and to equip the County for future uncertainty around cost and regulation. Four ‘triggers’ were identified that would necessitate changes to current processing methods: Class-A Biosolids Requirements, Limited Disposal Option, Sustainability Mandates, and Nutrient Reduction Requirements. The developed tool was used to analyze 18 competing process improvement and consolidation scenarios, incorporating combinations of the following technologies at specific plants: Thermo-Chemical Pretreatment, Thermal Hydrolysis, High Strength Organic Waste Co-Digestion, and Thermal Drying. Utilization of additional digester gas using new or existing CHP capacity was considered as a component for each scenario. The highest ranked scenarios were subjected to a sensitivity analysis to establish the stability of payback time projections against variations in future cost and process assumptions for residuals hauling and disposal cost, electricity, and Class A biosolids disposal costs. The analysis included scoring of scenarios against County priorities such as economics, operability, risk, and community interests. The normalized and weighted final scores represent how well each scenario/technology fulfills County objectives.

For more information, please contact the author at vgouchev@hazenandsawyer.com.

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