Charlotte Water (CLT Water) is the largest public water and wastewater utility in the Carolinas, serving more than one million customers in the City of Charlotte and greater Mecklenburg County. Charlotte Water currently owns and operates five major water reclamation facilities (WRFs), treating an average of 325 million liters per day (ML/d) or 86 million gallons per day (MGD) of wastewater and is in design for a sixth plant, the Stowe Regional Water Resource Recovery Facility (WRRF). Stowe Regional WRRF will treat flows from the western portion of Mecklenburg County and will also accept and treat flow from two neighboring towns in a bordering county. The Stowe Facility will have the capacity to process 68 ML/d or 15 MGD of wastewater, which will eventually grow to 114 ML/d or 25 MGD. CLT Water has a phased regionalization program in place for consolidation of biosolids treatment at McAlpine Creek Wastewater Management Facility (McAlpine Creek WWMF) as shown in Figure 1, which includes solids conveyance projects and the design and construction of a thermal hydrolysis process (THP) system for pre-treatment of solids prior to anaerobic digestion. CLT Water completed a Biogas Production and Utilization Study for the McAlpine Creek WWMF in September 2020. This study projected expected biogas production at McAlpine Creek WWMF taking into account the regionalization of solids treatment and the installation of THP. The combined regionalization and digestion improvements are anticipated to increase biogas production to approximately 1,930 normal cubic meters per hour (Nm3/h) or 1,200 standard cubic feet per minute (scfm). These projected biogas production values were used to generate a bioenergy model to evaluate multiple alternatives for beneficial biogas utilization including Combined Heater and Power, Steam Generation for THP, Compressed Natural Gas (CNG) fueling, and Renewable Natural Gas (RNG) pipeline injection. The bioenergy model, shown in Figure 2, considered capital and lifecycle costs of each alternative and produced dynamic comparisons of the different alternatives based on varying assumptions and user inputs. A non-economic evaluation was also used to compare the biogas utilization options, including safety, risk and O&M considerations, bioenergy utilization, private development opportunity, and public relations. The outcome of the study was a recommendation to implement RNG pipeline injection as this would allow for the largest percent utilization (99%) of the biogas and the most flexibility to operations, while aligning closely with the City of Charlotte's Strategic Energy Action Plan (SEAP). Following completion of the Study, CLT Water proceeded with a competitive sale process, viewing the biogas as a personal property to be sold as a commodity to a buyer to be converted to RNG and soliciting sealed bids from project teams to implement the RNG strategy. As a condition of the sale, the potential buyer was required to provide a 'turn-key' solution for the utilization of the biogas by providing the buyer's own capital, facilities, design, construction, operation, management, and permitting services necessary to convert the biogas into RNG. The buyer will perform conditioning of the biogas and resell the resulting product gas as RNG, which will create Renewable Identification Numbers (RINs) that can be monetized via the federal RIN market as well as state level credit markets. The buyer will have the rights to all environmental credits/attributes such as renewable energy certificates, low carbon fuel standard credits, and other similar entitlements associated with the RNG. Figure 3 shows an arrangement of the proposed biogas utilization program. CLT Water conducted the following two solicitation steps in order to enter into an agreement for the sale: 1)Request for Proposals (RFP): Utilized to short-list qualified companies taking into consideration vendor qualifications and experience, demonstration of safe RNG operation practices, quality, delivery, workmanship, and any applicable environmentally preferable attributes associated with the company's proposed solution for RNG. The short-listed companies were qualified to receive an Invitation to Bid. 2)Invitation to Bid (ITB): sealed bid sale where short listed companies have an opportunity to provide CLT Water their best price for the purchase of unconditioned biogas within a defined cost framework. This procurement approach encouraged competition in the determination of the biogas value and efficiency of proposed solutions. CLT Water has completed the RFP and ITB stages, selected the highest responsive, responsible bidder, and are currently undergoing final contract negotiations which include a land license for the facility construction inside CLT Water property and an offtake agreement for the delivery of biogas. Design of the facility is expected to commence in Spring 2025 and is anticipated to be sized for 1,930 Nm3/h (1,200 scfm). The system will be capable of removing hydrogen sulfide (H2S), volatile organic compounds (VOCs), siloxanes, and carbon dioxide (CO2) to meet pipeline specifications. The presentation will summarize the history of the biogas utilization study, development of the RNG implementation approach, and highlights of design efforts completed at the time of the conference.

Municipal organic wastes, such as sewage sludge and food wastes, are typically stabilized in conventional anaerobic digesters producing biogas and biosolids. These two products are tried and true recoverable resources that have been captured and reused across the world. This study proposes adding volatile fatty acids, VFAs, to this list of recoverable resources via fermentation. This proposal consists of modifying existing digesters to two-phase digesters that includes a thermophilic acid digester followed by a gas mesophilic digester that would produce higher valued VFAs, biogas, and potentially Class A biosolids. Figure 1 depicts the conventional solids handling system in NYC WRRFs and the proposed two-phased digestion system. All experiments were conducted in batch reactors at the City College of New York (CCNY). Gravity thickened combined primary and secondary sludge were collected from the Wards Island WRRF, municipal food waste slurry from the Newtown Creek WRRF, and food waste from the University cafeteria. Each of the two feedstocks were fermented in separate acid reactors for a 2-day SRT. Despite the practice at Newtown Creek WRRF combining thickened sludge with food waste slurry prior to anaerobic digestion, the two feedstocks were fermented in separate acid reactors because food waste produces higher concentrations of VFAs with lower concentrations of ammonia. Thus, the two feedstocks were fermented separately with different VFA/ammonia ratios which has the potential to replace glycerol in a BNR facility, or to reroute a recovery process for alternate byproducts. This study was conducted in three phases: (1) characterization of fermentation products from the two feedstocks, sewage sludge and food waste, (2) comparison of the VFAs produced to glycerol as sources carbon to support denitrification, and (3) preliminary approach for VFA recovery. Phase 1: Characterization of Fermentation Byproducts from Sewage Sludge and Food Wastes Sewage sludge from Wards Island WRRF and food waste are fermented in lab scale batch reactors for 2 days contact time at 38°C at the following two conditions: (1) maintain a pH of 7 with reagent-grade sodium hydroxide (NaOH) addition, and (2) without pH control. Table 1 to Table 3 shows the characteristics of fermented combined sludge and fermented food waste when fermented with and without pH control. Results from experiments fermenting combined sludge and CCNY food waste show VFA production increased when pH was maintained at 7 for both organic wastes. In the case of food waste, VFA production per unit volatile solids fed (VFA/VS) increased eight-fold from 100 mg CODVFAs/g VS to 830 mg CODVFAs/g VS when maintaining the pH at 7 while with no pH control, the pH dropped from 5.0 to 3.6, causing an insignificant VFA production due to inhibition. Further indication of inhibition was the concentration of soluble COD (sCOD) remaining constant with the control. In contrast, sewage sludge fermentation retained the pH constant at 5.6 while the sCOD increased 8-fold to 8,000 mg/L because of active fermentation and significant release of ammonia from the combined sludge. The additional ammonia contributed to alkalinity, thereby increasing the buffering capacity and resisting acidification caused by VFA production and accumulation, as shown in Table 2. The quantity of ammonia released from fermenting sewage sludge compared to food waste was measured at 37-40 mg N/g VS compared with 12 mg N/g VS, respectively. Table 3 shows the Newtown Creek food slurry behaved more like the fermentation of Wards Island combined sludge, namely, that pH control to 7 had minimum impact on further production of sCOD. This may be a result of a more diverse food waste that was combined in the slurry and significant fermentation that had occurred prior to reaching Newtown Creek WRRF. Phase 2: Denitrification Studies Comparing VFAs from Fermented Sewage Sludge and Glycerol Batch experiments were performed to determine the efficacy of VFAs in the fermented organic waste filtrate satisfying carbon demand for denitrification. The efficacy of fermented sewage sludge and food waste were determined by comparing specific denitrification rates (SDNR) and CODconsumed/Nremoved ratio to glycerol specifically formulated for denitrification. The experiments were conducted at CCNY in 2-liter jars equipped with paddle mixers and polystyrene floats as a barrier between the sample and atmosphere. Mixed liquor from Wards Island WRRF BNR tanks were dosed with 20 mg-N/L sodium nitrate (NaNO3) and 200 mg/L COD from (1) fermented sewage sludge filtrate, (2) fermented food waste filtrate, or (3) glycerol. The carbon sources were prepared by centrifugation followed by filtering the supernatant through a 0.45 um pore size filter. A control of mixed liquor was included and dosed with 20 mg-N/L of NaNO3 without adding carbon source. Samples were taken at specific intervals to measure the reduction of nitrate (NO3-N) and nitrite (NO2-N), collectively as NOx-N, over a 2-hour period. The data from the duplicate batch experiments are plotted in Figure 2. Figure 2-A shows the profile of NOx-N throughout the duration of the experiment. A regression analysis was performed using the linear part of the profile, within the first 40 minutes. The slopes of the linear regressions developed in Figure 2-A were used to calculate SDNR values and were shown in Figure 2-B. The SDNR based on the fermented sewage sludge filtrate, fermented food waste filtrate, and glycerol carbon source were determined to be on average 5.45, 5.95, and 4.83 mg N/g VSS/hour, respectively, indicating that fermented sewage sludge and fermented food waste supports denitrification as effectively as the specifically formulated glycerol. The carbon demands as COD/N ratio of fermented sewage sludge, fermented food waste, and glycerol were 9.66, 6.75, and 12.2 g COD/g N, respectively. COD/N ratio indicates that VFAs from fermentation of combined sludge and food waste achieved similar denitrification capacity as glycerol but at a lower dosage rate. Glycerol consumption averages 13,000 lbs COD/day (~1600 gal/day) for mainstream flow of the Wards Island WRRF (as reported by DEP). Wards Island WRRF generates 185,000 lbs TS/day of sewage sludge solids which when fermented could potentially produce 26,000 lbs COD/day of VFAs. Therefore, 50% of VFAs produced could replace the glycerol consumed at Wards Island WRRF. Considering food waste fermentation, only 21,000 lbs TS/day is required to satisfy the same glycerol consumption. Hence, there will be excess availability of VFAs that could be repurposed for alternate high value products. Phase 3: Preliminary Approach for VFA Recovery Bench scale VFA extraction from the fermented sludge is conducted based on the modified two-step flash evaporation and vacuum distillation process in a batch system. This process would make VFA a higher value product by concentrating and purifying it for efficient transport. The extraction process also isolates ammonia from the fermented sludge, which would remove the nitrogen load when applied for denitrification. The bench scale VFA extraction apparatus depicted in Figure 3 consists of a 2-L pressure vessel that carries the centrate from thickened fermented sludge. The vessel is heated to 65oC using a heating mantle at atmospheric conditions. When the target temperature is achieved, the vessel is sealed from the atmosphere and a vacuum is applied at 100 mbara from the vacuum pump. At these temperatures and pressures, the solution in the pressure vessel is heated beyond the boiling point, thus causing rapid boiling that mimics flash evaporation. The sudden boiling volatilizes both VFAs and water from centrate which is diverted and bubbled through the alkaline scrubber containing NaOH solution. The scrubber solution is also subjected to the temperatures and vacuum as the pressure vessel to prevent water vapor from condensing and diluting the scrubbing solution. The high pH of the scrubber solution would bind the VFAs in the scrubbing solution while the water is evaporated out of the scrubber solution, hence concentrating VFAs within the scrubber solution. NaOH addition is included to maintain the high pH. The water that escapes the alkaline scrubber is condensed in a water-cooled condenser and the condensate is collected while any residual gases not removed by the scrubber and condenser are exhausted out the vacuum pump. This VFA extraction system is in the preliminary phase and requires further work to determine its efficacy. Various operational parameters and their impacts on recovery efficiency are to be studied such as fermentation centrate characteristics (VFA speciation and concentration, pH), and system temperature and pressures. Other methods for VFA condensation are to be studied such as cold-column condensation and mechanical vapor recompression to manipulate the temperatures that can affect condensation.

At wastewater treatment and water reclamation facilities, thickening of solids is critical to maximize process efficiency. Facilities that co-thicken, mixing waste activated sludge (WAS) and primary sludge (PS), rely on a single, crucial solids treatment process. Many facilities have aging historic assets, such as dissolved air flotation thickeners (DAFTs), and are looking for creative ways to extend the life of existing systems, improve performance, and increase ease of operation. The LOTT (Lacey, Olympia, Tumwater, Thurston County) Clean Water Alliance is located at the south end of Puget Sound in Olympia, WA. LOTT operates the Budd Inlet Treatment Plant (BITP), an advanced biological nutrient removal facility with and average flow of 13 million gallons per day (MGD) and peak flows of over 70 MGD. BITP relies on four large concrete DAFT tanks for co-thickening WAS and PS prior to anaerobic digestion. These DAFTs, constructed in 1982, perform reasonably well, but much of the associated mechanical equipment has reached the end of its useful life. While a simple solution to replace all equipment in-kind was possible, LOTT was interested in creative ways to take older thickening technology and revitalize it by blending it with more efficient, modern technology. This paper provides an overview of planning and pilot testing to upgrade a DAFT solids thickening system by creatively blending new technology with existing infrastructure to maximize value and improve operation. The information presented provides a methodology and key pilot results demonstrating how to apply alternative technology to upgrade a more traditional thickener. Key topics include: - Evaluating the existing DAFTs in comparison with newer thickening technologies such as rotary drum thickeners (RDTs) and gravity belt thickeners (GBTs). - Determining how to refurbish and reuse existing DAFTs with more modern technology such as self-aspirating recirculation (SAR) pumps. - Developing a full-scale pilot to test SAR pumps and determine treatment capacity and thickening performance when applied to the existing DAFT hydraulics. The full-scale pilot was conducted over multiple years to verify performance and long-term capacity. - Final design, construction, startup, and testing of the complete system that validated the successful integration of modern technology with classic, well understood thickening equipment. The goal of this overview is to provide design engineers, planners, and operators, with a means to carefully assess critical existing thickening assets and creatively apply technology to bring an 'older' system into peak condition. Technology Selection The LOTT DAFTs were originally rated for approximately 15,000 lb/d solids removal (WAS only), with a target thickened solids (THS) concentration of 3.5 percent. Each of the four tanks is approximately 46 ft long by 16 ft wide. The tanks include a traditional internal flight system, with upper flights that carry solids to a beach for collection in a hopper and lower flights/cross collectors that remove residual settled solids. Each system included a centrifugal recirculation pump with a large pressurization tank that was used to dissolve air into the recirculated liquid and mix it with influent solids. Figure 1 provides a photo of the original pressurization tank system along with an internal view of a DAFT prior to the recent upgrades. In 2015, LOTT began the process to review options to either replace the aging DAFTs or upgrade them. The DAFTs were suffering from mechanical failures and excessive maintenance time to operate the pressurization tanks. In addition, LOTT was looking at long-term loading and performance needs. Table 1 provides a summary of the design conditions during the initial evaluation. LOTT determined that the DAFTs needed to produce a 6-8 percent THS and be able to accommodate a firm capacity of 80,000 lb/d to serve for the next 20-25 years. The review process included two broad options: (1) replace the existing system with a mechanical thickening option such as an RDT, or (2) refurbish the existing thickening system to provide long-term reliability. The RDT technology appeared promising, as it would use an efficient technology that is commonly installed in new wastewater facilities and allow LOTT to reclaim process space in the plant. The existing DAFT tanks, while unwieldy and aging, perform well and the large tank volume provides a buffering capacity to maintain consistency and resiliency across various solids loads. Refurbishment of the DAFTs was the preferred approach, assuming two key conditions could be met: 1)Complete replacement of all internal components (including flights, drives, shafts, chains, sprockets, cross-collector, etc.) and repair of any damaged baffles or concrete. 2)Replacement of the complicated pressure vessel system with technology that would be reliable and easier to operate. To address the first condition, an assessment of the existing DAFT tanks was performed and a vendor was selected (Brentwood Industries, LLC) to provide all new internal components that would be procured by LOTT for installation in each DAFT tank. The second condition was addressed by a proposal to replace the pressurization system with simpler, easy to operate, SAR pumps. The proposed SAR pumps (Nikuni America, Inc.), as shown in Figure 2, are utilized by World Water Works (WWW) in their package and custom DAFT units. The technology provides a more compact and efficient means to transfer air into solution, as the SAR pumps act as both the recirculation pump and the pressurization system. A small rotometer allows air to flow into the volute of the pump, driven by the venturi effect of the impeller, where it is compressed into the discharge. The challenge with this approach is that LOTT's DAFTs are particularly large units and there was no simple way to ensure the size or number of pumps necessary to meet the required solids loading. SAR Full-Scale Pilot Testing To determine the design of the SAR pumps, DAFT 4 was utilized as a full-scale pilot system. In 2017, a single SAR pump, Nikuni's largest model M80FP, was procured and installed by plant staff as a substitute for the existing pressurization tank and recirculation pump. Figure 3 provides a photo of the full-scale layout. The SAR pump operates at a constant speed, 280 gpm flow rate with a 70-80 psig discharge pressure. Flow from the pump is directed to an injection pipe manifold in which the following occurs: - Large residual air bubbles are removed from the line via a vent valve. - The remaining entrained air passes across a knife gate valve throttled to burn the residual head from the pump. The large pressure drop across the valve releases the entrained and dissolved air in a fine bubble foam. - Influent solids (WAS and PS), mixed with polymer, are injected into the liquid recirculation line, where they mix with the fine bubbles prior to entry into the DAFT. The pilot system was operated with the help of WWW for over a year to verify operation and to test the capacity. LOTT staff stress tested the DAFT in 2019 with solids at higher and higher loading rates until the primary criteria (6 percent THS for effluent solids) was no longer achievable. The system achieved approximately 12,000 lb/d, which fell short of the required design criteria. At this point, the pilot study was modified to determine if a second SAR pump in parallel could improve the results. Working with WWW, a customized header with a series of ball valves was developed to better control the air from the SAR pump. A second SAR pump was installed in parallel with the previous model and stress tested in late 2020 and early 2021. Figure 4 provides a photo of the second pilot system. Dual SAR pump pilot testing achieved loadings above 25,000 lb/d per DAFT, exceeding the necessary design criteria and providing critical information to integrate SAR pumps into the existing DAFTs. Table 2 provides a summary of the key pilot results. Construction and Full-Scale Operation Given the success of the SAR pump pilot, design and installation of the full upgrade for the remaining DAFTs was completed over the course of 2023-2024. The pilot work had validated the necessary SAR pump capacity, ease of operation, and piping design that allowed for consistent, steady results. Figure 5 provides a picture of the final SAR system, integrated into the existing DAFT tanks based on lessons learned from the pilot work. The system was fully operational in October 2024 and is producing consistent effluent solids in the 6-8 percent THS range. The work in this paper demonstrates a methodology, operational approach, and potential design criteria to creatively apply the alternative SAR pump technology to existing DAFT systems to allow them to operate effectively and efficiently for decades to come.

Solids handling has become an increasingly important aspect of the wastewater recovery industry as we deal with new pressures on hauling costs, limitations on historical outlets, drivers for improved energy recovery, and emerging regulatory pressures. The industry is being asked to reduce the total mass of biosolids products while also considering implementation of high temperature and/or pressure processes to comply with potential regulations of emerging contaminants. These new processes require additional handling within the facility. This presentation will focus on the elements of solids handling 'outside the box' to incorporate a wide range of advanced solids technologies including thermal hydrolysis, thermal drying, pyrolysis, gasification and supercritical water oxidation. The conversation around each element will include how it relates to upstream and downstream processes, the impact on O&M efforts and some practical implementation considerations for each. Elements that will be discussed include solids screens, low pressure cake conveyance, high(er) pressure cake pumping, high temperature elements, and final product management. A primary challenge facing WRRFs in the implementation of advanced solids treatment is how to best incorporate these processes into their existing plant and also ensure that the solids feed to these processes meet the often-specific requirements of these treatment approaches. The primary elements and impacts that will be discussed are: 1. Solids Screens -- how does the removal of stringy or fibrous materials either benefit or negatively impact dewatering, drying or processes with reduced flow path areas. 2. Low Pressure Cake Conveyance -- how can WRRFs best balance containment, agitation, energy consumption, cake consistency and downstream process integration while ensuring a consistent and reliable solids feed to advanced solids treatment processes. 3. High(er) Pressure Cake Pumping -- how can higher pressure feed requirements be safely and reliably implemented through intentional design incorporating online monitoring and passive safety features, operational controls, and standard operating procedures that ensure long term performance. 4. High Temperature Elements -- how can WRRFs maintain safe and reliable operation with systems that require skills sets that are vastly different than those traditionally required in the wastewater industry while achieving energy efficiency and operational high performance. 5. Final Product Management -- how can realistic reliability be achieved while transporting and handling solids with increasingly abrasive properties while also ensuring safe atmospheres, high levels of pipe integrity and consistency in transport to final beneficial use destinations. As the wastewater recovery industry moves into the future, a detailed understanding of how to integrate these elements into the existing treatment process is critical to long-term success. At the heart of the discussion is how to best 'manage the interfaces' to bring the whole facility together. This requires careful consideration during the design phase as well as careful thinking during implementation to ensure that operations and maintenance staff are educated on these new elements and are equipped with the tools needed to effectively use these new facilities to achieve new goals.

In June 2023 the EPA passed the Biogas Regulatory Reform Rule, also referred to as BRRR throughout the biogas industry. The BRRR constitutes new set of requirements for registering biogas to Renewable Natural Gas (RNG) projects seeking to obtain RINs through the federal Renewable Fuel Standard Program. In practical terms the BRRR will bring about significant changes for planned, in-construction, and operational RNG assets. One of the major issues cited amongst RNG practitioners is the lack of clarity in the rules around the technical aspects of the regulation. This paper lays out some of the major aspects of BRRR relevant to Biogas Producers (i.e. WRRFs) and provides the latest clarity available from EPA decision making, as well as several case studies of RNG projects taking steps for compliance under the BRRR. Some of the major relevant aspects of the BRRR include: - Requiring each individual entity within the RNG supply chain to be separately registered with the EPA and undergo Quality Assurance Protocols (QAPs). - Requiring all existing and in-construction projects to register by specific timelines to be granted more lenient compliance under Alternative Measurement Protocols (AMPs). - Requiring flow metering equipment for biogas before upgrading to comply with specific metering technical specifications laid out 40 CFR 80.155(a)(1). - Requiring Btu measurement of biogas before upgrading to be measured with independent, dedicated Gas Chromatography (GC) equipment meeting technical specifications laid out 40 CFR 80.155(a)(2). - Providing a pathway for 'mixed digestion' in which a digester treating a mix of municipal sludge and imported food waste can retain a significant portion of D3 RIN classification. TIMELINES As shown in Figure 1, there are major timelines for various types of RNG projects to file registration, to file Alternative Measurement Protocols (AMPs), and receipt of approval by the EPA. Some of the major deadlines and time thresholds include: - Apr 1, 2024 -- window opens for legacy and in-construction projects to file registration allowing compliance with the old rules and AMPs for technical equipment compliance. - July 1, 2024 -- RNG projects must have begun construction by this date to be eligible for in-construction status when consideration for granting of AMPs. - Oct 1, 2024 -- window closes for legacy projects to file registration and AMPs. - Jan 1, 2025 -- EPA to issue decisions on AMPs filed by the Oct 2024 deadline and all legacy projects must have approved registration. This abstract is being written in Oct 2024 just as existing projects have reached the deadline for filing AMPs, with official decisions due back on Jan 2025. At the time of presentation of this paper, there should be more technical clarity available from the EPA based on the decisions rendered in response to these AMPs. BIOGAS PRODUCERS One of the fundamental shifts brought about the BRRR is a requirement for each individual entity within the RIN supply chain to have registration and compliance responsibilities. This new paradigm with each defined entity is shown in Figure 2. It should be noted that 'Biogas Producers' which include municipal WRRFs are defined entities within the supply chain. This is a departure from the old framework in which a single entity, typically a commercial developer or broker, was the sole pathway registrant. This change was intended to enhance transparency and reduce data manipulation within the RIN ecosystem. In practical terms, it means that WRRFs will be required to jump through more hoops to participate in biogas to RNG projects. New activities will include joining data exchange platforms, conducting EPA compliant record keeping, and being subject to quality assurance audits. BIOGAS MEASUREMENT The most direct impact on those designing and constructing RNG projects are the new BRRR technical requirements on biogas measurement, which includes both flow rate and Btu content of 'Biogas' as defined by the EPA. This is also the area in which the most technical clarification is still outstanding. The EPA defines biogas as a gas product that is: 1.At least 52% methane 2.Has not been treated or refined to the extent that is contains 75% methane or greater This has raised many yet unresolved concerns about the practical definition of biogas that will ultimately be accepted by the EPA. For example, there is ambiguity as to whether new required measurements of 'biogas' can occur after treatment steps like H2S removal, compression, moisture removal, and/or siloxane removal. Constructing biogas measurement equipment further upstream in the biogas train has a higher likelihood of meeting EPA biogas definitions but also creates more difficulties in measuring accurately and keeping measuring equipment from degrading. - Filing of AMPs -- legacy or in-construction RNG projects can avoid letter of the law technology compliance if they can demonstrate 'high installation costs' or 'physical difficulty' installing the new equipment. This is done by filing an AMP to demonstrate how alternative equipment to be used meets or exceeds requirements stated in the BRRR. -Metering -- new requirements were intended to increase granularity and timing of data collection. In practice, all biogas meters must comply with standards called out in 40 CFR 80.155 which preclude most biogas metering technology currently used in the industry. As a result, many meter manufacturers have submitted AMPs to get meters approved as alternate technologies. Facilities will also need to submit AMPs to prove they are using acceptable metering technology. A list of currently approved metering AMPs are shown in Table 1. - BTU Measurement -- this is perhaps the most challenging and most ambiguous technical requirement. The rule calls for separate and dedicated GC equipment to continuously measure 'biogas', with continuous measurements defined as once every 15 seconds. Very few, if any projects have provisions for raw biogas GC meters, and there are significant cost and practicality constraints for adding them, especially if 'biogas' cannot be refined to remove corrosive contaminants like H2S and moisture. No clarity is yet available on what will be accepted via AMPs on this item, but some direction is expected via the Jan 2025 responses to AMPs filed for legacy projects. MIXED DIGESTION In a tidbit of good news related to the BRRR, there was a new protocol set forth to allow for 'mixed digestion' which refers to municipal sludge digester that treat predominantly sludge but also import a portion of organic waste such as FOG or manufactured food slurry. Under old frameworks these digesters would be precluded from receiving D3 RINs despite the fact that the majority of their feedstock is cellulosic. Under new rules mixed digesters treating a majority of sludge have the following options to retain D3 RINs: - Accept 25% D3 RINs and 75% D5 RINs based on very conservative analytical derivations of biogas yields per new EPA standards. - Development of a 'baseline' condition for biogas production from only sludge, and that baseline will determine the ceiling amount of D3 RINs that can be generated, with the remainder being counted as D5. As of the writing of this abstract, no project has successfully been registered under this new mixed digester standard. This means many technical aspects of this rule are up for interpretation, especially in regards to requirements for setting baselines. The presentation will elaborate on any recent clarity brought forth on this item. CASE STUDIES Two case studies will be presented, both which are biogas to RNG projects considered to be 'in-construction' by EPA definitions. The paper will present actions taken and available results to bring these projects in compliance with BRRR. City of Mesa, AZ -- 300 scfm (nameplate) triple mass membrane RNG system slated for startup and commissioning in November 2024. Capital Region Water, Harrisburg, PA -- 400 scfm (nameplate) triple mass membrane RNG system slated for startup and commissioning in October 2025.

Over the last fifty years, anaerobic digestion has become common practice at medium to large water resource recovery facilities (WRRFs) in the United States, providing a sustainable source of biogas while achieving significant improvements in sludge stabilization. With more installations, facilities are being presented with a larger array of biogas end use choices to meet their cost and sustainability goals. After four decades of operating a cogeneration facility to produce electricity and heat, Metro Water Recovery (Metro) and Carollo Engineers, Inc. (Carollo) completed a Biogas Utilization Study in 2021 to consider new beneficial uses of biogas at the Robert W. Hite Treatment Facility (RHWTF) that serves Colorado's Denver metropolitan area. The study compared replacing the existing cogeneration system with new cogeneration equipment versus installing biogas upgrading equipment to generate renewable natural gas (RNG) for pipeline injection. Through a detailed comparison of the biogas utilization alternatives that included financial analysis, operational considerations, and sustainable attributes, Metro selected RNG as the biogas utilization pathway which would provide the highest social, environmental, and financial benefit to Metro. In 2022, Metro and Carollo began design of the Biogas Utilization Project, including a biogas upgrading system for pipeline injection of RNG and a Boiler Mechanical Facility to replace the heat currently supplied by the cogeneration system. The initial phase of this project included further analysis into the available technologies for biogas upgrading. Most WRRFs in the United States have selected membrane or pressure swing adsorption technologies for the separation of methane and carbon dioxide due to their cost effectiveness at relatively small biogas flow rates. Although these approaches do not have technical limitations at higher flow rates, amine wash and water wash were determined to be more appropriate from a financial and physical layout perspective at Metro's design biogas flow of 3,000 standard cubic feet per minute (scfm). As part of the preliminary design effort, the team collected information on these two systems and conducted a triple bottom line alternatives analysis to support an informed technology selection. The detailed information included: - Equipment budgetary proposals (including capital and operating costs, equipment layouts, and equipment lead times) from multiple manufacturers. - Site visits to operating equipment installations. - Phone interviews with past and current RNG system operators. - Conceptual system layouts. - Conceptual design and construction costs. - Greenhouse gas (GHG) emissions. - Local natural gas utility's RNG quality requirements. A comparison of the water wash and amine upgrading systems is summarized in Table 1. Based on Metro's evaluation of the two technologies, site visits, and feedback from reference installations, the design team determined that both the amine and water wash upgrading systems could reliably produce RNG that meets the natural gas utility's quality requirements. While water wash systems contain fewer mechanical components and higher levels of redundancy (based on the equipment manufacturer's two-train proposal), the amine system offers slightly higher methane capture, a higher purity tail gas stream that may be more amenable to future beneficial use, lower GHG emissions, and the ability to recover a significant portion of the input heat to offset natural gas required for heating the digesters and buildings on site. Based on this evaluation, Metro selected the amine upgrading system for final design. In an amine upgrading system, raw biogas is boosted and conveyed into a counter-flow reactor tower where the carbon dioxide is solubilized into the amine solution at relatively low pressure (12 pounds per square inch). The methane flows out the top of the reactor tower while the carbon dioxide-rich amine solution is pumped to a stripping tower. Heat is added to the stripping tower to break the bond between the carbon dioxide and amine in solution, releasing carbon dioxide gas and regenerating the amine solution, which is pumped back to the reactor tower. The heat used by the amine upgrading system will be supplied by new hot water boilers that are being constructed as part of the project to heat the digesters and buildings (replacing heat that was previously supplied by the cogeneration system). The carbon dioxide separation process requires a high temperature to regenerate the amine solution. While the heat is often supplied by steam, in this case, the design includes superheated water pressurized to 125 pounds per square inch gauge (psig) and heated to 275 degrees, avoiding separate steam boilers, reducing overall system complexity, and improving redundancy. Up to 90 percent of the heat required by the amine upgrading system can be recovered and used elsewhere on the plant-site. However, the recoverable heat is 'low grade' in that it is not hot enough to be usable at the temperatures supplied from the amine skid for traditional digester process or building heating applications. To utilize heat recovery from this system, Metro and Carollo evaluated the use of industrial water-to-water heat pumps to upgrade the low-quality recovered heat to higher quality heat for use in the digestion process, the largest heat demand at the RWHTF. To better understand the costs and benefits of a heat pump heat recovery system, the team developed a 20-year net present value and estimated GHG emissions reductions associated with the heat recovery system. The capital cost of the heat pump system was developed by the construction manager at risk (CMAR) contractor, Moltz Construction, Inc. The total construction cost for the heat recovery system, including the building it is housed in, is estimated to be $9.2 million. If this system were to qualify for Inflation Reduction Act (IRA) funding (which is anticipated based on discussions with Metro's tax advisor), the capital cost would be reduced to approximately $6.4 million. While the heat pump system has a significant electrical demand, its high coefficient of performance means the energy output is more than five times the electrical energy input. The heat pumps will directly offset approximately $1.5 million in annual natural gas usage in the boilers and reduce GHG emissions by 30 percent compared to solely using natural gas for digester heating (Figure 1). The calculated 20-year NPV of the heat recovery system is approximately $3.1 million with a payback period of roughly 15 years. With the IRA's capital cost reduction (assumed to be a 30 percent discount) the 20-year NPV increases to $5.9 million and the payback period decreases to approximately 10 years. A graph comparing the 20-year NPV with and without the IRA reduction is shown in Figure 2. Construction of Phase 1 of the Biogas Utilization Project began in October 2024. Phase 1 includes construction of the Boiler Mechanical Facility, which will house the hot water boilers required to supply heat to the digesters, the amine upgrading system, and other process and building heating demands at the RWHTF. Construction of Phase 2 will begin in 2026 and includes demolition of the cogeneration facility and installation of the amine upgrading and heat recovery systems. At an inlet design flow rate of 3,000 scfm, the RWHTF's biogas upgrading system will be the second largest RNG system and the first installation of an amine upgrading system at a WRRF in the United States. Figure 3 presents a rendering of the proposed facilities. WRRFs are under increasing pressure to recover resources and make sustainable decisions related to energy use. This project showcases how a large regional WRRF pivoted from cogeneration to pipeline injection of renewable natural gas with a focus on maximizing resource recovery and minimizing natural gas use. This novel application of amine upgrading paired with a heat pump system provides a road map for other utilities to follow. The project's use of triple bottom line evaluations provides a solid business case for decision making.

Introduction Metro Vancouver (MV) is facing a tripling of sludge production by 2050 due to wastewater treatment plant (WWTP) upgrades and population growth. To address that challenge, MV needs to diversify its solids management options and is considering Hydrothermal Liquefaction (HTL) as a potential alternative to complement land application. HTL uses temperature and pressure to convert wet biomass into biocrude, which is then refined into low-carbon transportation fuels. HTL of wastewater solids leverages existing infrastructure to aggregate organic feedstock and create biofuels (Figure 1). While HTL of municipal sewage sludge has been proven successfully at bench scale (Marrone et al., 2018; Liu et al., 2022 & 2023), larger scale HTL has only been proven with other feedstocks. MV is building a continuous flow HTL demonstration facility that uses wastewater sludge as its feedstock in British Columbia (BC), Canada (Figure 2). It will process 2 dry tonnes/day of undigested primary and secondary sludge, targeting 7,500 hours of cumulative and 1,000 hours of continuous operation. Design is complete and operation will begin in 2026. Project partner Parkland Refinery will co-process the biocrude. Data from the demonstration will be used to refine the business case, evaluate the technical and financial viability of HTL as an alternative for solids management, and inform the decision to implement HTL at full-scale. ...Innovation Through the design and fabrication phase of the MV HTL demonstration project, the design teams have resolved multiple data gaps to scale up from bench-scale testing, including: - Treatment options for the aqueous effluent byproduct - Byproduct re-use and disposal options, characterization and application of waste disposal regulatory requirements - Characterization of odorous air streams and treatment options - Characterization of biocrude product and -Fire protection design requirements, risk assessment, and refinement Recent work published by PNNL on the effects and fate of PFAS compounds will also be incorporated into the testing plan for the MV HTL demonstration facility (Olarte 2024). ...Business Case Seiple et al. (2020) have shown that wastewater sludge can be a cost-effective feedstock for transportation biofuels using HTL. MV's decision to build the demonstration facility was based on the business case for full-scale HTL compared to business-as-usual (BAU) of new digesters and biosolids land application. However, with many knowledge gaps in using HTL at WWTPs, mainly around managing the HTL aqueous and solid byproducts, MV took a conservative approach for the business case. - The BAU scenario used optimistic assumptions: --Biosolids management costs stay the same as present, whereas availability of land application sites may decline due to changing public perception and regulatory restrictions, which could lead to disposal with 15% higher cost. --Inflation stays at 2% for land application costs, vs. a more realistic 4%. - The HTL scenario used conservative assumptions: --Biocrude yield at low end of published range, resulting in less revenue. --Biocrude is sold at a discount compared to fossil crude, due to difference in attributes. --HTL aqueous byproduct is treated by catalytic hydrothermal gasification (CHG), which is effective but costly. MV is investigating other less costly processes to treat or valorize the aqueous byproduct, which were not considered in the business case. Data from the demonstration plant will be used to support this investigation. --Solid byproduct must go to hazardous waste disposal at a high cost, with no revenue from resource recovery. Preliminary data analysis suggests other viable pathways for reduced disposal cost or re-use. --No revenue from sale of carbon credits. --No monetization of HTL's additional benefits, such as a smaller footprint than digesters, and destruction of contaminants such as PFAS. Key assumptions are summarized in Table 1. Table 2 summarizes the business case results. While revenue for BAU is higher due to value of RNG compared to biocrude, BAU O&M costs are higher due to biosolids management costs. The net result is a lower annual operating cost (OPEX) for HTL. Capital cost (CAPEX) of HTL is higher than BAU. The resulting lifecycle savings for full-scale HTL outweigh the cost of the HTL demonstration, despite the conservative assumptions. A sensitivity analysis on several parameters will be presented in the paper. The greenhouse gas (GHG) emissions of the two scenarios were compared. In BAU, RNG displaces natural gas in building heating. The HTL biocrude is refined into fuel that displaces diesel in transportation. The carbon intensity of the finished fuel from HTL was estimated using GHGenius version 4.03, a lifecycle analysis model for transportation fuels in Canada. While HTL has higher operating emissions than BAU, the greater avoided emissions result in a net GHG benefit for HTL (Table 3). The finished fuel from HTL biocrude would generate credits under the BC Low Carbon Fuel Standard, trading at an average of US$352/tCO2e over the past year (Table 4). For a 31,000 tCO2e/year reduction, the carbon credits have a value of US$222 million over 20 years. This would greatly improve the HTL business case, but was not considered in the current analysis. There are more than 15,000 publicly-owned wastewater recovery facilities (WRRFs) sized at 17 MegaLitres per day (ML/d) [4.6 million gal/d] or larger in the USA. Using HTL to process the sludge from all of these WRRFs would eliminate an estimated US$3.29B/y in sludge disposal costs while producing 3.67 GL/y [0.97 Bgal/y] of biocrude intermediate, equivalent to 0.3 percent of crude consumption in the USA (Seiple, 2020). ...Next Steps The HTL demonstration project will provide critical information to fill knowledge gaps and refine the business case for full-scale implementation. The project team has developed: - A detailed 12-month operational and testing plan to assess the impact of feedstock variability and HTL process settings on biocrude and byproduct yield and quality. - An extensive byproduct work plan to determine options for pre-treatment, resource recovery and disposal during the demonstration. - Business case workshops to engage key stakeholders to establish financial and non-financial success factors, and analyze the BAU and HTL scenarios using Multi-Objective Decision Analysis, with or without monetization of factors such as value of carbon. Highlights of these plans will be presented in this paper. Ultimately, the commercial viability of HTL depends on producing a continuous supply of biocrude with consistent quality, as well as identifying economic and technically feasible outlets for the aqueous and solid byproducts. ...Significance As utilities face challenges with land application and landfilling of biosolids, HTL offers a solution that greatly reduces the volume of solids to be managed, destroys contaminants, and creates valuable low-carbon fuels. In a relatively conservative industry, demonstration of HTL at a relevant scale is a critical step to inform business case analysis toward full-scale implementation.

BACKGROUND Jacobs operates two wastewater treatment plants for the City of Vancouver Washington, across the Columbia River from Portland Oregon. The West Side WWTP is a 28 MGD capacity plant that uses a biological nutrient removal process, followed by centrifuge dewatering and incineration. The project team is led by Travis Capson, with Sam Mielcarek as operations manager and Paul Muirhead as maintenance manager. Optimizing polymer dosage for solids thickening or dewatering is an especially challenging problem in our industry, with at least a dozen potential variables to consider when trying to maximize dewatered solids at the lowest overall cost: feed solids concentration, flow rate, polymer dosage and concentration, temperature, feed solids characteristics and blends, machine settings, etc. The site operations staff already fully understood the complexity of the challenge but was forced to spend extra effort tracking and making frequent changes to various settings in the system every day, all while balancing the competing demands of polymer usage cost vs. the cake solids dryness and fuel cost at the incinerator. DISCUSSION A machine learning model that matched the biggest drivers of polymer dosage against actual dewatered cake solids content values multiple times per day, over several years, was created. This involved leveraging recent upgrades to the SCADA control systems designed by Jacobs at the plant to support the capture of real time data for accurate dosage predictions. Starting in May 2023, push notifications for dosage recommendations went live to field staff. Gradually at first, but with increasing confidence, the staff have been following the predictive recommendations and seeing reduced polymer usage and utilization throughout FY 24 is shown in Figure 1. The predictive data science approach to polymer dosage control is savings around 10%-20% - an improvement on a budget of over $700,000 per year in polymer. The intent behind this optimization strategy is to reduce polymer consumption while maintaining appropriate cake solids concentrations for downstream processes and off-hauling. The plant has indicated a target concentration of 26-28% as optimal based on their experiences and the figure below provides a summary of where the system operates below range and when it has operated within range historically. Although It is understood that polymer may not be the only factor for contributing to differences in cake TS concentrations, for simplicity, prior to deployment of the optimization of a preliminary opportunity assessment was done to check these ranges as a function of polymer dose and simplify this to 'overdosing' and 'underdosing'. Based on the historical data going back to January 2021, 54% of the time the system operates within the expected range, 30% of the time above the expected range (overdosing 30% of the time), and 15% of the time below the expected range (underdosing 15% of the time). and thus the opportunity stemmed from being able to cut the 30% overdosing value to being within the desired Cake TS range. A multi-headed convolutional neural network (MH-CNN) approach was used with a temporal component with lookback features (as predictors, response, control variable -- polymer feed) and lookforward features (response and control variable) technique. For each prediction, the model looks back at a period of data and based on what it observes predicts what will happen over the lookforward period. This allows for the MH-CNN to do its own lag engineering by identifying the phemonena within the lookback window are most predictive for lookforward dynamics an schematic illustrating this architecture is shown in Figure 3. In addition to the data provided from the facility, an acoustic sensor was installed on the external wall of pipe carrying post-centrifuged cake. The theory behind this was to use the vibrations and noise measured at different frequencies when concentration of cake varies in order to predict cake TS. This was a non-intrusive install and sensor was glued to the pipe at the discharge point of schwing pump. The location of this was set such that it was installed close to a wall to minimize readings of ambient and non-relevant noise signals. Figure 4 shows the location of this installation. The site has robust sampling plan of measuring total solids at various locations in the process and this data is used to validate and continuously improve the results of the models. Figure 5 shows the results of the model's ability to predict cake TS concentrations. The test set shows a Pearson's R correlation value of 0.59 with a p value less than 0.05 indicating a statistically significant correlation. The variable importance figure shows the importance of the relative features used to train the model where WAS feed rates and the installed acoustic sensor showed significant importance on the model's ability to predict cake TS concentrations. This case-study demonstrates significant progress in the successful implementation of AI at a treatment facility. The strong engagement with operators has advanced their skillset such that they are submitting abstracts to their local operator's conferences at their own volition.

Background: The paper will present a case study on thickening impacts when implementing biological nutrient removal (BNR) at the Salt Lake City Water Reclamation Facility (SLCWRF) as part of a $750+ million-dollar plant upgrade. The 48 MGD trickling filter / activated sludge plant is being upgraded to a BNR facility using the Westbank process. The new facility is being constructed in the area that previously housed the sludge drying beds (Figure 1). The upgrade will include new primary sludge thickeners that will be operated as fermenters to provide volatile fatty acids (VFA's) to the BNR facility. The existing rotary drum thickeners (RDT) will continue to be used for waste activated sludge (WAS) thickening. Primary Sludge Thickening: SLCWRF currently uses gravity thickening for primary sludge, but the existing thickeners do not meet current seismic codes and are not sized appropriately to be operated as fermenters. The existing gravity thickeners are also not located in an ideal location at the site in comparison to where the new BNR facility will be located. For initial construction two 67-foot-diameter primary sludge thickeners will be used, designed for the maximum month total suspended solids load. This is based on requiring one in service for the annual average condition, and two for the maximum month condition. Under average conditions, the solids retention time (SRT) will be 6 days with two primary sludge thickeners in operation and 3 days with one out of service. These SRTs are considered adequate for volatile fatty acid generation for the biological phosphorus removal process. The site layout also accounts for space for two future gravity thickeners to allow for future plant expansion. The new gravity thickeners are under construction as shown in Figure 2. WAS Thickening Four existing RDTs, as shown in Figure 3, were installed in the last ten years and will continue to be used to thicken the WAS from the new BNR process. Each RDT is rated for 400 gpm and the building includes space to add a fifth RDT to allow for future expansion. The RDTs are used to increase the WAS concentration from an average of 0.5 percent total solids (TS) to approximately 5 percent TS. RDT filtrate will be pumped to the new WAS gravity thickener supernatant pump station, using the existing filtrate pumps. After being combined, the two supernatant streams will be pumped to the primary clarifiers. The RDT sludge will be pumped to the existing digesters for stabilization, using the four existing sludge pumps. Reusing the existing WAS thickening, however, resulted in several challenges for the new BNR facility. These challenges were due to existing issues with the rotary drum thickeners as well as challenges integrating it with the new BNR facility. A first challenge related to the original design intent to implement surface wasting from the BNR which would result in very thin WAS and a high volume that would exceed the capacity of the RDTs. The low TS in the WAS (less than 0.5%) would also derate the capacity of the RDT further exacerbating the capacity issue. Options such as gravity pre-thickening of the WAS were considered initially but was ultimately ruled out since WAS doesn't settle well; this would add additional cost and the gravity pre-thickening may also promote phosphorus release. A second challenge related to the first challenge was meeting design throughput of the existing RDTs (400 gpm) with the current dry polymer set up. When using dry polymer, the RDT's could only run at less than half of the design capacity. Testing with emulsion polymer, however, showed that the design capacity of 400 gpm could be achieved, but this would result in much higher polymer costs. At current WAS loads, the capacity of the RDTs operating with the lower cost dry polymer at less than half design capacity was sufficient for operations, but this would not provide sufficient capacity for design future loadings. A third challenge was maintenance issues with the existing rotary drum thickeners. This included uneven wear on trunnion ultra-high molecular weight polyethylene roller bearings that caused pre-mature failures. After initially replacing only the failed bearings it was found that this would result in other bearing failures. Now when one bearing fails, all are replaced. Several RDTs experienced catastrophic failures involving horizontal cracking in drums at trunnion rollers. The repairs to the RDTs included upgraded materials for stabilizer rollers, sprockets, chain, and base tensioner. Because of these challenges, it is desired to have future unit(s) with bearings outside of wetted parts. Because of the capacity and maintenance challenges, several additional thickening alternatives were considered. This included replacing all of the existing RDTs with a different manufacturer, adding a 5th RDT of a different manufacturer, replacing RDTs with Volute Presses and replacing RDTs with DAFs. The results of the evaluation found that adding a fifth RDT would be the lowest cost option, and that design future capacity could be met with the RDTs if polymer was switched from dry to emulsion polymer. To better evaluate alternatives, SLCWRF pilot tested an alternative RDT and a volute thickener. This pilot testing showed similar performance results as existing RDTs with emulsion polymer. The pilot test also found that capacity was derated when operating with dry polymer but data with dry polymer was limited. After completing the study and pilot testing, several recommendations were developed for future WAS thickening. First, SLCWRF will integrate use of emulsion polymer to increase capacity when needed. Second, a fifth RDT will be added for capacity, when needed, likely with an alternative RDT manufacturer. Finally, SLCWRF will re-evaluate thickening technology when the existing RDTs reach the end of their useful life. Conclusion The experience from Salt Lake City shows the importance of thickening when considering advanced treatment such as BNR. The impacts on technology selection or using existing assets will be important for other utilities which may be considering similar process upgrades.