Co-Digestion to Class A Biosolids — Altoona Water Authority's Path to Water Resource Recovery

Recognizing the operational and financial benefits of converting their aerobic solids treatment process to an anaerobic co-digestion process, the Altoona Water Authority (AWA) is constructing an integrated anaerobic digestion and biogas-fueled thermal biosolids drying complex at their Westerly Wastewater Treatment Facility (WWTF). AWA is implementing this project, scheduled for a September 2022 completion, using a collaborative project delivery method through Pennsylvania's Guaranteed Energy Savings Agreement (GESA) legislation. What began as a feasibility assessment of high strength waste (HSW) co-digestion evolved into a cradle-to-grave water resource recovery strategy that incorporates waste recycling and reduction to achieve a sustainable biosolids management solution for AWA and the surrounding region. This presentation describes how the collaborative development process was used to connect new treatment assets to the revenues and savings associated with the regional waste and biosolids disposal markets. The Westerly WWTF is an 8 MGD (average daily flow) Biological Nutrient Removal (BNR) facility. Through a 2018 GESA procurement, AWA selected an an Energy Services Company , Energy Systems Group, to design, build, and guarantee financial outcomes associated with the new assets. The GESA law requires that the project's revenue and savings equal its associated debt service and operating costs. The initial premise was a co-digestion complex that would convert biosolids and HSW into renewable energy in the form of biogas. The financial value would be in the form of tipping fee revenue and savings and/or revenue associated with the conversion of biogas to usable energy. It became clear during the early assessment phase that that AWA's Class B biosolids disposal practice (agricultural land application) had become a costly, unsustainable alternative as their costs to dewater, store, test and transport, and land apply as escalated in recent years. Achieving Class A biosolids became a project requirement, with biogas-fueled thermal drying as the technology to achieve that outcome. The design challenge was thus connecting the front end HSW co-digestion to the back end solids drying with a 100% renewable thermal energy source. The process modeling established that there would be sufficient biogas to expand the dryer capacity threefold beyond the AWA plant sludge, creating the potential for a regional facility with merchant sludge revenue. The collaborative delivery process accommodated a regional assessment in parallel with the design progression, not as a separate feasibility study that would have delayed the schedule. Other municipal authorities in the region were identified with the potential of transferring biosolids to the WWTF fore drying. While presenting potential challenges to drying due to the differing cake solids properties, the potential revenue from the biosolids tipping fees was important in the cash flow for the project. Specifically these facilities were producing different types of sludge and biosolids that could potentially foul the dryer. These included primary sludge, waste activated sludge (WAS), a high purity oxygen WAS that has properties similar to a primary sludge, as well as a primary sludge with a high concentration of cellulosic fiber. Based on the results of drying these various biosolids in a pilot dryer, different strategies were developed to manage these multiple feeds. These include mixing biosolids from multiple sources upstream of the dryer, hauling the sludge with the high concentration of cellulosic material into the facility as thickened WAS and anaerobically digesting it prior to dewatering and drying, and metering multiple biosolids feeds into the dryer by way of biosolids cake pumps. Not only did this approach provide the AWA with the flexibility to fashion a project in real time that addressed its evolving needs, it provided a performance guarantee that limited risk and helped this innovative project become a reality. Figure 1 is a tabular summary of the projected revenue and savings associated with the project. Figures 2 and 3 show the digester complex construction progress in April and September 2021, respectively. The project will result in an annual positive cash flow for AWA. In addition to new hauled waste tipping fee revenue, it will generate sufficient renewable biogas to provide the thermal energy demands of the treatment processes, buildings, and thermal drying. Lastly, it is providing the region a stable biosolids disposal option, with predictable long term biosolids disposal costs.