Dry Another Day

Introduction As markets and regulations change, so must our approach to biosolids resource recovery. Loudoun Water's Broad Run Water Reclamation Facility (BRWRF), located in Ashburn, VA was first constructed in 2008 and has a total current capacity of 11 million gallons per day. Loudoun Water is currently constructing an expansion of the liquids treatment process to a design flow of 16.5 mgd. The BRWRF sets a new world-wide technology standard for environmental protection and water reclamation. Waste activated sludge is centrifuge thickened, combined with primary sludge for stabilization in anaerobic digesters, and centrifuge dewatered prior to land application as a Class B biosolid product. To anticipate future growth, regulatory changes regarding PFAS, and a shift in appetite from Class B to Class A products, the BRWRF has undertaken planning efforts to expand several treatment processes. As part of the evaluation, Loudoun Water determined that a thermal rotary drum drying system will best prepare the BRWRF for flexibility in future expansion and upgrades as well as provide short-term solids treatment capacity. The approach for identifying and evaluating treatment processes and technologies for the BRWRF involved: -Defining the near-term and long-term boundary conditions -Exploring the world of options relevant to biosolids treatment processes -Evaluating integrated technology solutions that can serve Loudoun Water's near- and long-term needs -Converging on a selected path forward As illustrated in Figure 1, the overall approach is analogous to a sieve analysis whereby each step facilitates convergence towards integrated alternatives that are most applicable to the BRWRF. Boundary Conditions Hazen established boundary conditions to ensure project decisions meet the near-term and long-term needs of the BRWRF for the following criteria: -Environmental discharge -Flexibility for future end use -Biosolids -Energy -Reliability -Resource recovery -Treatment capacity Hazen developed the near-term (up to 2040) and long-term (2040 to 2070) boundary conditions with input from Loudoun Water to ensure that decisions align with the BRWRF-specific vision, and position Loudoun Water for a flexible future. Figure 2 is a graphical depiction of the near-term boundary conditions, and Figure 3 is a graphical depiction of the long-term boundary conditions that the BRWRF will maintain flexibility for in the future. Viability Screening Hazen developed a world-of-options of innovative to established biosolids treatment technologies. The list was narrowed down to technologies viable for BRWRF through a multi-criteria evaluation that scored the technologies on six overarching themes. The biosolids treatment overarching themes included: appetite for innovation, product marketability, sidestream impacts, biosolids/energy co-management approach, long term applicability, and reliability/redundancy. Figure 4 and Figure 5 show the results of the multi-criteria evaluation at this screening stage. Applicability Screening Following the viability screening phase, Hazen evaluated the technologies further for applicability to the BRWRF using the criteria shown in Table 1. Among the screened stabilization technologies, thermal hydrolysis (THP) was initially recommended for sludge pretreatment. THP increases the production of digester gas, reduces the final mass of biosolids for disposal, reduce digester volume and footprint, increase the dewaterability of biosolids, produce a Class A biosolids, and will support the long-term vision for resource recovery and movement towards energy neutrality. During piloting, it was found that the biosolids pretreatment process released high concentrations of nitrogen, phosphorus and COD to the return stream in forms that were not bioavailable for removal in the liquid treatment train, increasing final recalcitrant nutrient concentrations in the effluent. At that time, the project team came to the consensus that THP was not a technology to be evaluated further due to the stringent effluent limits imposed on BRWRF. The screened advanced biosolids treatment technologies were evaluated based on anticipated regulatory requirements regarding emerging contaminants and final product management. Loudoun Water required biosolids treatment to produce Class B biosolids at a minimum; however, flexibility for PFAS destruction in the future was an important consideration. As PFAS is a concern that is developing globally, technologies for PFAS destruction are still undergoing research, and Loudoun Water had a preference to implement technologies that are established in the industry. SCWO was eliminated due to the lack of case studies in which the technology has been implemented successfully. Advanced thermal processing has been implemented in several locations for the purpose of reducing biosolids volume; however, the success of each technology in PFAS destruction is still undergoing R&D. Current results show several data points where PFAS is non-detect in the biochar product. Research is on-going to validate destruction of PFAS in the gas emissions following the thermal oxidation of the produced syngas. Nutrient reclamation was a consideration that was driven by end product marketability. Biosolids fertilizer pellets was considered because it is an enhancement to BRWRF efforts in increased resource recovery. Ultimately, this processing approach was eliminated as the end product did not appear to provide significant economic benefits as compared to the cost required to implement the technology. The two technologies that scored the highest the applicability screening phase include conventional mesophilic digestion and thermal drying. As shown in Table 1, conventional mesophilic digestion is a technology with which the staff are familiar with operation and maintenance. It can be used in conjunction with downstream advanced biosolids treatment technologies to produce a higher than Class B quality end product. Thermal drying was determined to be the most reliable technology that exceeds the near-term minimum boundary condition (production of Class B biosolids) and has potential for expanded beneficial reuse alternatives. Thermal drying can also serve as a prerequisite step to advanced thermal processing. Advanced thermal processing is an enhancement that was determined to be non-critical for implementation as part of the near-term planning. However, it may be considered in the future as a viable long-term technology. Benefit to Cost Ratio Evaluation Hazen performed a benefit-to-cost ratio evaluation on the final two alternatives that were being considered for implementation. The first alternative, in which the entire solids load is digested, is described schematically in Figure 6. The second alternative, including thermal drying, is described schematically in Figure 7. Loudoun Water assigned a level of importance to each criterion shown in Table 2. The most important criteria were ease of operations and maintenance, future flexibility, and regulatory compliance. Figure 8 shows the weighted scoring for the two alternatives evaluated. Although the baseline alternative scores highest in ease of operations and maintenance, the PS only digestion + thermal drying alternative offers greater regulatory compliance and future flexibility than the baseline alternative, which ultimately gives the PS only digestion + thermal drying alternative the highest weighted MCE score. After determining the weighted scores, Hazen calculated the benefit-to-cost ratio for the two alternatives. The benefit-to-cost ratio is the total weighted MCE score (shown in Figure 8) divided by the 20-year net present value. A higher benefit-to-cost ratio reflects a better alternative because more value will be provided for a lower cost. The PS only digestion + thermal drying option was found to have a higher benefit-to-cost ratio compared to the baseline option. Converge The evaluation of technologies converged on a near-term solution that addresses the issue of insufficient capacity, satisfies the near-term (and many of the long term) boundary conditions for BRWRF, and allows for future implementation of long-term solutions. Detailed design will proceed with the near-term solution of digesting only primary solids at a 12-day SRT (minimum), including an excess primary solids bypass only to be used under peak conditions, and constructing a thermal drying system. Because Class A biosolids are achieved in the proposed design, Loudoun Water can defer the decision of how to manage primary sludge digestion beyond the capacity of the existing digesters to a future stage of improvements. Consideration of long-term solutions will advance when regulations and R&D for emerging contaminants develop further.