Sludge Conditioning and Renewable Natural Gas at the Capital Region Water AWTF

On the heels of a major BNR upgrade completed in 2016, Capital Region Water (CRW) in Harrisburg, PA set out on making significant improvements to its solids and energy streams. CRW operates a 22 MGD Advanced Wastewater Treatment Facility (AWTF) with a pair of mesophilic digesters dating back to last major plant expansion in the 1970s. These digesters are currently undergoing refurbishment including grit removal, new covers, and mixing. With BNR and fresh digesters in place, CRW is now setting its sights on sustainability goals by targeting a project to enhance energy recovery from its solids resources. To help identify the most beneficial energy recovery project, CRW engaged with an engineering team led by Arcadis in 2018 to undertake a comprehensive solids and energy planning effort targeting upgrades. The CRW plant already had an operational history of recovering its biogas for energy using CHP engines. The plant has been operating a set of 600 kW Waukesha 'Enginators' since 1984 to make electricity and recover hot water for heating (see Figure 1). Interestingly, CRW does not utilize the CHP electricity internally, but instead transmits its generated power to the local utility and receives full delivered unit pricing via an interconnection agreement. These engines are reaching the end of their useful lives, however, and the CRW staff initially envisioned an in-kind replacement of the CHP engines with upgrades to heat recovery. The solids and energy planning effort was aimed at looking at a wide range of options both in conjunction with and in lieu of CHP. For example, the plant currently co-thickens primary sludge and WAS in gravity thickeners, leading to significant seasonal deterioration in thickening performance (see Figure 2) especially after the implementation of BNR. One of the improvements examined was the separation of WAS for mechanical thickening to improve sludge feed concentrations to the digesters. This, along with the following list enhancement items were examined: Separated mechanical WAS thickening WAS lysis for enhanced digestion New CHP engines with upgraded heat recovery Biogas to Renewable Natural Gas (RNG) Acceptance of High Strength Waste (HSW) Backup power generation with CHP Upgrades to biogas compression and transmission The key tool used to evaluate all these options and their interacting effects was holistic solids and energy flow model (see Figure 3). With this tool, any potential combination of processes or loading rates was quantitatively analyzed for annualized cost, greenhouse gas (GHG) reductions, net power generation, and capital cost. Along with determining an operationally sound and cost-effective treatment facility, processes were also examined to meet goals of maximizing value of the plant's energy resources and supporting the cost effective Class B biosolids land application program already in place. There were some very significant site specific factors identified early in the planning process. One item was the existence of a natural gas 'peaker' facility directly adjacent to the AWTF fence line in the area of the digesters. This facility is operated by the local natural gas utility. Engagement with this utility revealed a positive and cooperative position regarding connection of RNG to their system. This meant that CRW had not only identified a partner for RNG, but also would have an interconnection pipeline requirement of essentially zero. Another site specific item for this project was the abundance of high quality HSW in the area due to the presence of local food producers such as Hershey Creamery and Utz Potato Chips. These producers have previously shown high interest bringing their material to the AWTF, making the acceptance of HSW a high priority for CRW. With these site specific conditions in mind, Figure/Table 5 give the results of some of the most prominent scenarios from solid and energy flow model tool. One of the major conflicts facing this facility and many others is the illogical incentive structure under the current Renewable Fuel Standard (RFS) where digesters receiving HSW get penalized as RINs are downgraded from D3 to D5. This was confounded by the acute drop in D3 RIN pricing occurring in late 2019 during the time of the study. Significant sensitivity analysis on both D3 and D5 RIN pricing was done using the flow model tool, allowing many potential future RIN pricing scenarios to be considered. Ultimately, the decision to target both RNG and HSW acceptance was made. While accepting HSW reduced RIN value potential, it also brings in tipping fees and avoids the higher sensitivity swings related to D3 RIN prices. Some other conclusions were as follows: The replacement of CHP engines with new equipment and upgraded heat recovery did not provide an annualized savings to the plant. This included maintaining the existing interconnection agreement at $0.075/kWh and taking a capital credit for offsetting backup power generation. The main driver was the high cost of biogas conditioning and the O&M support package which is difficult to overcome for newer engines at this scale. The addition of enhanced sludge thickening by separately mechanically thickening WAS did not provide annualized savings to the plant. However, this was viewed as a fundamental investment in solids infrastructure, providing operators more control over thickening and eliminating the difficulties with seasonal bulking and performance deterioration in the gravity thickeners. Savings from other parts of the project would offset this long term investment in improved operations. The addition of WAS lysis beneficial on an annualized costs basis, paying for its capital by increasing biogas recovered as RNG and reducing biosolids to be managed. Implementation of lysis was selected, with major driver being current dewatering difficulties. Currently the AWTF must produce 20% TS cake as part of their land application which has been a struggle. With lysis, a cake TS increase of 2-3% is expected which would significantly improve dewatering operations for land application. Based on the results of this work, CRW has moved forward with implementation of the recommended project with the design completed and construction slated to begin in early 2021. The project includes a new Thickening and Lysis Building to house new WAS thickeners as well as thermal-alkaline WAS lysis. Heated and lyzed WAS will be blended in-line with received HSW in a new feed line to the digesters. A new biogas to RNG skid will be implemented with the neighboring natural gas peaker facility taking custodial transfer of the product gas for injection into the pipeline. The engineering team has also recently identified a local PA 'green gas' program in the Philadelphia area willing to pay premium prices for RNG. Partnering with this type of offtake can remove headaches related to the RFS by avoiding swings in RIN market prices and value conflicts around accepting HSW. By partnering with local food producers and natural gas utility, this project serves as model for how cooperation and regionalization can significantly uptick the value that can be obtained for energy resources at a WRRF. This also demonstrates the value of effective solids and energy planning as CRW staff were able to quickly and confidently move forward with implementation, and take on long term operational improvements such as thickening in combination with money savings aspects like HSW and RNG.