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Building upon the momentum of a public private partnership (P3) that delivered energy neutrality to Victor Valley Wastewater Reclamation Authority (VVWRA)'s Water Resource Recovery Facility (WRRF), VVWRA and Anaergia teamed to implement California's first WRRF-based pipeline renewable natural gas (RNG) project. This initiative demonstrates the critical role WRRFs of all sizes can play to combat climate change, increase resilience, and create alternative revenue streams.
Background: VVWRA operates a 14MGD advanced treatment facility in Victorville, CA. VVWRA sought to better utilize biogas, offset electricity costs, minimize energy price fluctuations, and achieve net-zero operations. Typically, facilities of this size have limited ability to accept outside waste, and produce insufficient biogas to offset energy demand. To achieve these goals and mitigate risk to the Authority, VVWRA partnered with Anaergia to design, build, and finance upgrades to increase digestion capacity and deploy biogas utilization infrastructure. Anaergia provided its Omnivore high-solids digestion package to retrofit a 300,000-gallon anaerobic digester, forgoing the need for additional tank volume. These upgrades significantly increased digestion capacity and solids destruction, allowing for the introduction of energy-dense organic wastes for co-digestion. In addition to digester retrofits, high-strength waste (HSW) receiving was installed, enabling receipt of up to 20,000 gallons per day. As biogas production increased, Anaergia procured two 800-kW dual-fuel combined heat and power (CHP) modules, which began operation in 2016. The system was sized to utilize all WRRF biogas and meet 100% of VVWRA heat and electricity needs. VVWRA realized significant cost savings from offset utility consumption and benefits from enhanced resilience.
Objective: The first phase of this partnership demonstrated operational and economic benefits of an organics-to-energy program for VVWRA. Further, it highlighted demand for alternative outlets for organic waste disposal, driven in part by landfill diversion mandates such as California Senate Bill SB1383. These mandates aim to reduce methane emissions from organic waste in landfills, which can instead be leveraged to generate renewable energy and offset fossil fuels. As a result, VVWRA sought to maximize use of remaining excess digester capacity to increase co-digestion of external HSW and expand on benefits realized thus far – namely, delivery of valuable WRRF infrastructure upgrades, increased biogas production, and ultimately increased financial benefit under a P3 arrangement. A key consideration in further expanding biogas production was determining the most valuable use of biogas, and the most economical way to fuel the cogeneration engines.
Application: Anaergia worked with VVWRA on cost-benefit analysis to determine the most economically favorable beneficial use of increased biogas. At the scale achieved from an expanded co-digestion program, it was concluded that all biogas should be conditioned and upgraded to RNG for injection into the Southwest Gas pipeline. To maximize value to VVWRA, biogas would no longer be used to fuel the cogeneration system, which would instead run-on utility natural gas. Based on RIN and LCFS credit values vs. commodity prices, analysis confirmed that value of RNG sales more than offset the cost of natural gas purchases for CHP. The conclusions are consistent with findings of similar biogas projects throughout California and North America: when sufficient biogas production scale is achieved, maximizing biogas to pipeline RNG creates greatest value, even where CHP systems are in place. This is compounded by higher operating expenses for biogas CHP compared to biogas upgrading. This approach has added benefit of generating carbon-negative fuel, an invaluable tool in achieving carbon-neutrality goals and mitigating climate change. Project Overview: The public private partnership (P3) includes WRRF upgrades for expanded co-digestion (new liquids receiving station, high-solids mixers in each digester) and biogas utilization facilities (biogas conditioning, RNG membrane upgrader, and utility interconnection). Ancillary upgrades (e.g., dedicated digester feed lines) were provided to support overall WRRF operations and address capital improvement needs. The project tripled VVWRA co-digestion and associated biogas production. Under the P3, Anaergia provided design, construction, and financing. Anaergia owns and operates the biogas utilization facility, located on a parcel leased by Anaergia from VVWRA. Anaergia receives all biogas produced at VVWRA and provides conditioning (to remove contaminants such as H2S, VOCs, and siloxanes) and upgrading to produce pipeline-quality RNG. Carbon-negative RNG is 99% methane and delivered to the Southwest Gas point of receipt for pipeline injection and sale, offsetting fossil fuels. Anaergia manages interconnection and offtake agreements, as well as registration with state and federal renewable fuel standard programs to maximize renewable energy credits and RNG value. VVWRA will continue to own and operate co-digestion and ancillary upgrades. Given the well-established co-digestion program, VVWRA maintains responsibility for coordinating organic feedstock (charging $0.05/gal for HSW). VVWRA continues to benefit from energy resilience and electric grid independence via cogeneration using natural gas. In addition, VVWRA receives new revenue streams via lease payments from Anaergia, additional tipping fees, and RNG sales revenue sharing. Construction of RNG infrastructure began in 2020 and was completed in 15 months, sending RNG to Southwest Gas as of late 2021. Lessons Learned: Pairing existing public infrastructure with private investment provides an effective and responsible means to expand resource recovery capabilities, opening the door for ambitious and innovative initiatives. This partnership demonstrates a replicable model to transform wastewater treatment facilities into energy-neutral or -positive resource recovery centers. Private investment can revitalize existing infrastructure to support climate goals and advance a circular economy while limiting risk to agencies and ratepayers. The P3 model enables rapid project delivery at scale to provide meaningful environmental solutions. P3s are appropriate to provide much-needed infrastructure and create new revenue for WRRF capital needs and rate stabilization. Risk to agencies and ratepayers is limited: an experienced industry partner is responsible for project delivery and performance. WRRFs can focus on their core mission while sharing in the benefit of investment and revenue from resource recovery. Bioenergy projects need not impede – and can significantly improve – WRRF operations. Anaergia's compact biogas utilization facilities encompass less than one-quarter acre. Potential impacts to nutrient removal, solids handling, and dewatering are evaluated during development to identify necessary mitigation measures (e.g., Omnivore retrofits or sidestream treatment). Third-party financing may also be leveraged to address deferred maintenance and capital improvement needs beyond core project scope. Overall, P3 is an effective and appropriate tool to deliver infrastructure upgrades. RNG can maximize project value and climate impact. Anaergia analysis confirms it is generally most economically favorable for all biogas to be upgraded for pipeline RNG pipeline. RNG value is sufficient to justify investment in new facilities and increased utility natural gas consumption for existing systems. RNG value is driven primarily by federal and state renewable energy credits, which monetize the potential of carbon-negative RNG to reduce methane emissions, offset fossil fuels, and advance carbon-neutrality.

Introduction
As public agencies make commitments to reducing their climate footprints, the importance of rigorous carbon accounting in wastewater treatment grows. Often, these efforts have focused on plant energy balance, primarily focused on reducing aeration energy use or achieving energy neutrality. While aeration is indeed the largest energy demand at wastewater treatment plants, a plant's greatest climate impact may instead stem from the direct release of greenhouse gasses (GHGs) (Vasilaki et al. 2019). Specifically, methane and nitrous oxide (N2O), two gasses produced during biological treatment, have much higher impact per unit of mass than carbon dioxide released during electrical generation. According to the IPCC, the global warming potential of one unit of methane (on a 100-year timescale) is 21 times higher than CO2, equal to 21 CO2 equivalents (CO2e), while N2O is 310 times more potent than CO2 (IPCC 1996). Off-gas testing for these two GHGs can be expensive and technically challenging, but process models can be used to estimate their production. The present study uses process models to estimate and compare the GHG impact (in CO2e) of various wastewater treatment processes so that utilities can make informed and sustainable choices when selecting process upgrades or retrofits.
Materials and Methods
To compare the overall climate impact of common wastewater treatment technologies, a predictive model for GHG emission was developed and paired with the established IWA ASM2d process model. The GHG model normalized nitrous oxide & methane release, along with energy and chemical (alum, polymer, and methanol) demand, into CO2-equivalent GHG emissions units as defined by the IPCC. This model considered both direct emission of gasses and emission of GHG dissolved in effluent. Model inputs included (1) the dissolved nitrogen gas mass balance, (2) the relationship between dissolved oxygen and nitrous oxide production in biological nitrogen removal (BNR), and (3) the electricity and (4) chemical usage trends. This model was applied using commercial simulator software (MassFlow, UnU Inc.), first to validate the model, and subsequently to compare simulated GHG emissions from 7 representative BNR processes. Simulated conditions were based on a flow of 20,000m3/d and influent characteristics as per table 1. Simulated processes included the Modified Ludzack-Ettinger (MLE) and Anaerobic/Anoxic/Oxic (A2O) conventional activated sludge processes, a membrane bioreactor (MBR) process, integrated fixed film activated sludge (IFAS), biological aerated filter (BAF), aerobic granular sludge (AGS) and Partial Nitritation/Annamox (AMX) processes. GHG production was calculated per unit of total nitrogen removed, which assumed complete conversion of influent ammonium into nitrogen gas. Sidestream nitrogen sources (i.e. centrate from sludge dewatering) were also considered in the process model. Results and Discussion The model-predicted GHG generation per unit of total nitrogen removal ranged from 15.2-15.5 CO2e/kgTNremoved for the MBR, IFAS and ANAMMOX processes, and 11.2 CO2e/kgTNremoved for the BAF process (Fig. 1). In all cases, total GHG impact was dominated by N2O emission (69.2%), while the contribution of methane (7.2%) and CO2 embodied in electrical energy for aeration (22.1%) was minor (Fig 2). Carbon embodied in process chemicals such as coagulants, flocculant polymers and methane (required for denitrification) were nearly negligible at only 1.5% of total GHG impact. Simulated CH4 emission was similar for most processes apart from BAF and anammox (Fig 3). The Anammox process was the largest N2O emitter at 59.8 gN2O-N/kgNHx-Nremoved, and BAF was lowest at 18.4 gN2O-N/kgNHx-Nremoved (Fig 4). Total GHG generation ranged from 18.4-59.8 gN2O-N/kgNHx-Nremoved in all processes. Excluding Anammox, all other processes' emissions matched previously reported ranges of 22.6-48.6 gN2O-N/kgNHx-Nremoved (Vieiral et al. 2019). N2O emissions from BNR processes are affected by pH, water temperature, stripping factors, solids retention time, and dissolved oxygen (DO), but the effect of reactor DO is proportionally large. Previous ASM2d modelling work by Massara et al. (2016) found that emission of N2O peaked at DO concentrations between 1.0-1.5 mg/L, while a later paper by the same author found even higher N2O emissions at even lower DO conditions (0.8-1.1 mg/L). In this study, the BAF process, which was simulated based on installation data from the Proteus biofilter (Tomorrow Water, Anaheim, CA) demonstrated the lowest N2O generation, and subsequently the lowest total GHG impact. This result was driven primarily by high dissolved oxygen concentrations in this process (5-6 mg/L). The complete conference paper will also include empirical data validating the modelled N2O removal in fullscale Proteus biofilters. This study found that while the single-stage Anammox process uses significantly less energy and chemical carbon than other processes, it nevertheless demonstrated the highest GHG impact of all processes modeled, primarily as a result of N2O production in low-DO, high-nitrite conditions within the granule/biofilm. The complete conference paper will compare these results with both modelled and empirical GHG emissions from a two-stage Anammox process, where the partial-nitritation reaction occurs at high-DO conditions, to test whether N2O emission in Anammox systems can be minimized by adjusting the process configuration. This change in process configuration has the potential to shift AMX from the most GHG-intensive option, to one of the least intensive. MBR demonstrated a similarly high GHG impact to Anammox in this model, though this was driven primarily by CO2 emissions embodied in the high electrical energy inputs required to aerate MBRs running at high MLSS, which leads to a degradation in alpha factor.
Overall, this study demonstrates that holistic comparisons of the climate impacts of various treatment processes require integration of (1) existing process models, (2) emissions factors for process inputs (electrical energy and process chemicals), and (3) models of direct GHG emissions (N2O and CH4). Results from this integrated approach suggest that nitrous oxide emissions dominate total GHG impact for most common BNR treatment processes, while electrical energy use remains important in some cases (such as MBR). Results of the GHG emission model were sensitive to changes in process DO, influent nitrogen loading and an empirical N2O emission factor. Since this factor was selected based on off-gas testing results published in previous studies, continued collection of empirical N2O production data from full-scale installations is needed to continue calibrating and improving the underlying GHG emissions models based on real-world data.

Introduction
In late 2018, Geosyntec Consultants, Inc. (Geosyntec) was retained by a confidential pharmaceutical client to perform an independent review of zero liquid discharge (ZLD) systems at two pharmaceutical manufacturing facilities in in India. Geosyntec worked with a local firm who provided on-site observation and evaluated process data from each facility to document the wastewater flow balance and confirm controls were in place. Geosyntec provided project management, technical support and peer review of the final report. The Central Pollution Control Board (CPCB) of India defines ZLD as the installation of facilities and systems which will enable industrial effluent for absolute recycling of permeate and converting solute (dissolved organic and inorganic compounds/salts) into residue in the solid form by adopting a method of concentration and thermal evaporation. ZLD is a rising technology in India because of regulatory pressure to select ZLD. Analysis carried out by Global Water Intelligence showed that over 30 environmental clearance reports granted to industrial projects by the Ministry of Environment, Forests and Climate Change (MoEF&CC) since February 2020 showed that all projects have ZLD conditions attached as part of the project development. Other industries in India, such as food and beverage, steel and textile have incorporated ZLD, even with the high energy consumption, operational costs for equipment cleaning and the need for skilled manpower. In pharma, ZLD was getting attention due to the global concern of antibiotics in the environment and antimicrobial resistance (AMR). The discharge of antibiotics and Active Pharmaceutical Ingredients (APIs) in water bodies have been critically investigated in India for over the last 10 years. As of now, the AMR Industry Alliance has called on pharma facilities to reduce the quantity of API released into the environment and evaluate risk by comparing the predicted environmental concentration (PEC) to the predicted no-effect concentration (PNEC). Academia and researchers, working to mitigate antibiotic resistance, believe the PEC should be extremely stringent at the fence line, some suggesting that the effluent limit should equal the PNEC. In January 2020, a full year after our evaluation, MoEF&CC announced very strict standards on concentrations for 121 antibiotics found in the waste discharged by pharmaceutical factories into rivers and the surrounding environment. Pharma facilities and common effluent treatment plants (CETP) with membership of bulk drug and formulation units were subject to the rule. The rule also incorporated ZLD as the technology standard. India became the first country in the world to introduce such standards, meant to reduce chances of creating drug-resistant bacteria, and were ahead of the US and EU in developing numerical permit standards. However, the numerical values were eliminated in a revision in the rule published in August 2021, and the rule is silence to the presence of antibiotics in the effluents and the requirements of ZLD. Treatment Alternatives to Address the Discharge of Antibiotics Many advanced treatment technologies are available to reduce the concentration of antibiotics and other APIs in the final effluent. ZLD is an approach to water treatment where all water is recovered and contaminants are reduced to solid waste. While many water treatment processes attempt to maximize recovery of freshwater and minimize waste, ZLD is the most demanding target since the cost and challenges of recovery increase as the wastewater gets more concentrated. Salinity, scaling compounds, and organics all increase in concentration, which adds energy costs associated with managing these increases. ZLD is achieved by stringing together water treatment technologies that can treat wastewater as the contaminants are concentrated.
Goals and Objectives
The overall goal was to document that each facility maintained in-plant source control and wastewater treatment systems in place to minimize or eliminate the release of treated wastewater and any antibiotics to the environment. Objectives consisted of confirmation that the two plants are: designed and operated as ZLD with no treated wastewater discharge to the environment; well maintained and operated with trained and qualified personnel; and adequate redundancy is provided for standby equipment and storage volumes.
Project Methods/Approach
The Client and Geosyntec conducted the work in a collaborative fashion with the local India team and US team. Our Client's manufacturing platform at the time includes a number of facilities which are in India. Plant A and Plant B, which was the subject of the evaluation, are considered bulk API facilities and designed for antibiotic production. Our approach to perform the work consisted of reviewing: - Sources of wastewater generation; - Wastewater flow generation; - Treatment system review; - Process flow diagram and hydraulic balance; - Permit limits and conditions; - Residual waste handling and disposal; and - Unit operations evaluation , including evaporator, crystallizer and reverse osmosis units. Based on our review, our findings were developed and compared to legal and permit requirements and to determine if the facilities were designed and operated as zero liquid discharge with no treated discharge to the environment. Evaluation of Plant A and Plant B Pant A and Plant B are similar in design. Figure 1 and Figure 2 present the hydraulic balance and ZLD flow diagram for Plant A, respectively. Figure 3 presents the hydraulic balance for Plant B. Table 1 and Table 2 present the flow evaluation of Plant A and Plant B, respectively. All flows are presented as 1000 liters per day (KLD). The treatment technologies evaluated for the high total dissolved solids (TDS) wastewater included solids removal, steam stripping, multiple effect evaporator (MEE), agitated thin film dryer (ATFD) and salt collection and disposal. Condensate from the MEE and ATFD are combined with the low TDS streams generated from equipment washing, utility blowdown, etc. Subsequential process units consist of primary and biological treatment, membrane bioreactor polishing and reverse osmosis in series for utility reuse. The paper and presentation will also include other design and operational issues, water chemistry objectives for reuse in utilities and cooling tower makeup, membrane fouling challenges, RO recovery, brine management and disposal alternatives and energy consumption and operational cost. Conclusions The ZLD facility at both Plant A and Plant B are designed and operated as ZLD with no treated wastewater discharge to the environment. Both plants are well maintained and operated with trained and qualified staff and adequate redundancy is provided for standby equipment and storage volumes. In addition, periodic reviews are completed and ensure proper operation and consistent performance.

Background: The City of Toronto Water Department, Ontario, Canada (Toronto Water) serves nearly three million customers who rely on over 1,500 employees to keep their operations running at their peak. Toronto Water is responsible for drinking water supply and four water treatment plants, wastewater collection and an additional four wastewater treatment plants, as well as stormwater management. While managing these assets and the related processes, a group of self-described 'hard-core' data users were experiencing challenges with effectively utilizing Toronto Water's supervisory control and data acquisition (SCADA) and laboratory information management system (LIMS) data on a regular basis to execute critical tasks.
Selection Process: In June 2017, a cross-functional 'Operational Intelligence Group' was formed within Toronto Water, comprised of plant engineers, central process engineers, and members of the process controls systems group. The group performed market research and became aware that data management software solutions were available that could improve efficiencies and support their complex workflows. The group interviewed vendors and other large utilities from around the world finding that complex data warehouses systems were commonplace, and typically require a high level of effort to sustain. The group also drew from their own understanding of unsuccessful large deployments of enterprise systems to create requirements for a new data management solution. The requirements included a software application that would transform their existing data systems into a single, powerful view of Toronto Water's operations. The software had to support hundreds of active users. Another consideration was a large IT organization with strict cyber security measures in place. Above all, an application that fostered power users was desired, but one that did not require a high level of effort to maintain. A unique aspect of the process included establishing internal governance, which was endorsed by all managers and department heads involved with the targeted data systems. Unanimous endorsement ensured that sufficient resources would be made available to satisfactorily fulfill the roles and responsibilities of the implementation project – including Lead Business Owner, Supporting Technical Leads, Advisory Group, and Application Owners. After thorough evaluations, the group concluded that nothing could provide the level of functionality of the top solution and in February 2018 the group made a unanimous decision to acquire eRIS, a SUEZ owned application. An initial rollout project began in Spring 2019 which was completed two years later in May 2021. By selecting a solution that was widely used commercially 'off-the-shelf', and met all required functional needs, Toronto Water started using (and benefitting) immediately. An extensive library of integrations, alarm capabilities, integrated electronic logbook and a modern, intuitive mobile application streamlined adoption.
Phase One Initial Deployment: Toronto Water initiated the project in 2019 at one water and one wastewater plant. The Toronto Water and SES team developed 20 key reports, performed a rollout of plant operational logbooks, and captured manually collected data into a dedicated data repository. They also created a data validation process to ensure manually collected data was reviewed and corrected prior to including in reports. Support and training were keys to success. Customized training was developed in an e-learning platform, which was interactive and online, and was customized to Toronto Water's exact data sources. New users could easily complete the training as allowed by their workloads and shift schedules. Toronto Water created an internal web tool to serve up quick start guides and a wide range of beginner to power user training resources. As of December 2021, Phase One has been completed and Phase Two is currently ongoing.
Phase Two Expansion: Subsequently to the initial project, Toronto Water engaged the SES team to assist with expanding the application to include all remaining Toronto Water treatment plants. Eight (8) additional plant logbooks were deployed along with additional plant operational reports. Other projects were initiated to improve data integration and sharing including one project to automatically transfer daily operational data to a regional partner and to a co-developed data repository and website. This solution eliminated the manual daily process with a fully automated, reliable data transfer tool. Future deployments will include a mobile application for use outside the office, a digital whiteboard, and lockout-tagout tracking tools.
Findings/ Project Outcomes: Throughout the project implementation, Toronto Water not only improved operation and business decision making, but they also experienced a clear return on investment (ROI). Each treatment facility has an employee who is responsible for monthly regulatory reporting. The ability to extract all required process data with enhanced analytics, automated reporting functions, and powerful calculation tools has resulted in faster, easier, and cleaner data and reports. This turnaround has been a key driver in staff-hours saved. The project owner quantified the annual value of staff time saved is nearly $600,000 CAD, driven in large part by staff utilizing the application to access complex process data. The Phase One Initial Deployment was awarded a City of Toronto Excellence Award for both the value and execution of the project. Digital Water does not need to be as complex as artificial intelligence to be highly valuable to utilities. Prior to deriving value from complex analytical solutions, utilities would benefit from enhancing their enterprise data management to better understand and trust their data quality and control expectations for different datasets. Key activities such as managing data governance is critical in case of staff turnover and ongoing validation of key operational advisory systems.
Learning Objectives: ###1. Upon completion, participants will have a clear understanding of how a large utility successfully implemented data governance to build a foundation for digital water programs.
2. Upon completion, participants will better understand the differences between data warehouse systems and dynamic data hub applications, including level of effort required for deployment, security, cost, and data ownership issues.

INTRODUCTION
In the wastewater industry, energy and operation costs are major factors in selection of a disinfection technology. Ultraviolet Light Emitting Diode (UV-C LED) systems based on a solid-state technology, generally tends to operate with low electrical consumption and less relative space to conventional technology. A comparison of LED UV to UV systems applied to wastewater indicating that there was large (over 50 percent) cost savings for this technology when applied to wastewater systems. UVC LED Technology currently is on a rapid development scale. UVC LED units were produced 2 years ago have changed dramatically the output and decreasing the production costs. Testing of three different UV-C LED water disinfection pilot-scale units at the U.S. Environmental Protection Agency (EPA) Test and Evaluation Facility in Cincinnati, OH. The pilot-scale testing included collimated beam testing and flow-through testing for both drinking water and municipal wastewater. During the first two studies the test runs targeted MS-2 bacteriophage and other organisms. The focus of the third test was on Legionella inactivation.
BACKGROUND
In much the same way that visible light emitting diodes (LEDs) have revolutionized the general lighting industry, UV-C LEDs are now viewed as the 'next generation' in UV-C disinfection and are expected to rapidly replace conventional mercury arc lamps in water disinfection applications. LEDs are made from crystalline compound semiconductors that emit light when energized. UV-C LED system integrators purchase many single-packaged UV-C LEDs and combine them into a larger UV-C LED reactor which is then equipped with electronics, thermal management system, optics, and housing to create a finished UV-C LED system or module. Point-of-use (POU) UV-C LED systems for the treatment of potable water are already commercially available and market competitive, but larger-capacity UV-C LED systems for municipal water and wastewater disinfection are still under development.
RESULTS<b/>
The primary objectives of this project were to evaluate the disinfection performance, reliability, and energy use requirements of three small-scale UV-C LED water disinfection units (e.g., one at 10 gallons per minute (gpm) and two at 20 gpm rated sizes) for a variety of test conditions. Test conditions included treating water to achieve public supply drinking water, municipal treated secondary wastewater, combined sewer, and reuse water requirements. Both Collimated beam and flow through testing was conducted to validate the removal of MS-2 and Legionella. MS-2 Testing Results of MS-2 Testing can be divided into two group. Collimated beam testing was Collimated Beam Results MS-2 bacteriophage was added to the dechlorinated water to conduct this test. The Ultraviolet Transmittance (UVT) was 98.7% at 280 nm. Collimated beam tests were performed at 280 nm with results shown in Fig. 1. Reuslts of testing indicate a 0.5 log removal of MS-2 per 5 mJ/cm2 increase in dose. These results match with data presented and used for development of NWRI response curves. Extrapolating the data, a 5 long inactivation would indicate that a 5 log removal of MS-2 could be achieved at a dose of 50 mJ/cm2 if the removal remained linear. Typically the inactivation curve is more geometric, therefore the predicted dose would probably be more closely to 100 mJ/cm2. Flow-Through Results Due to the piping configuration at the EPA T&E Facility, the flow rate to the unit was limited to 22.7 liters per minute (6 gpm). Flow-through testing was only conducted with MS-2 bacteriophage.. The test water was inoculated with MS-2. The MS-2 was fed at a rate of 93 mL/min. The water flowed through the unit for 3 minutes and then inlet and outlet samples were collected at 8-minute intervals. The results for both pilot unit 1 and 2 are shown in Figure 2. At 22.7 liters per minute (6 gpm), the pilot unit 1 could provide a 3-log removal at a dose of 85 mJ/cm2. Pilot unit 2 achieved a 4-log removal at 85 mJ/cm2 and with a flow rate of 15.14 liters per minute (4 gpm). Based on these flow rates, the power input would be in the range of 200-250 kilowatt-hours per million gallons (kWh/MG).
LEGIONELLA TESTING
Legionella Testing was conducted by USEPA to validate the dose response that could be achieved four different SerioGroups at three wave lengths. UVC-LED disinfection efficacy were evaluated for four Lp strains representing three clinically significant serogroups (sg1, 4, and 6) in drinking water utilizing both a collimated beam (at 255, 265, and 280 nm) and a point-of-entry (POE) treatment set-up (280nm). Understanding the inactivation differences between Lp serogroups in drinking water, especially for UVC-LED POE applications, will help determine the most effective remediation strategies needed to target specific isolates during contamination events. The focus of the testing was to examine the inactivation of legionella at 280 nm a that was the wavelength of the unit shown in Figure 3. However, for the POE tests, only sg6 displayed the highest reduction (mean ± standard deviation) of 5.0 ± 0.5 log10 CFU/mL compared to 3.5 ± 0.5, 3.3 ± 0.2, and 3.6 ± 0.1 log10 CFU/mL for sg1, sg1 DW, and sg4, respectively.
DISCUSSION<b/>
Using the flow-through pilot test results, the project team developed scale-up factors to project energy use values for UV-C LED water disinfection. Because of the flowrate limitations, the energy use projections for the UV-C LED system are preliminary, and we provide the energy use values in relatively wide ranges. We project the energy use for UV-C LED disinfection of drinking water is in the 65-250 kWh/MG range. The projection is for a UV-C LED system treating drinking water with 90% transmittance at a UV dose of 40 mJ/cm2. The energy use projections for UV-C LED disinfection systems are comparable to conventional UV mercury vapor lamp systems. The energy use requirements for disinfecting drinking water are approximately 60-120 kWh/MG for medium-pressure mercury vapor lamp systems and 50-200 kWh/MG for low-pressure mercury-vapor lamp systems.
CONCLUSION
A summary will be provided of the advances of LED UV, focusing on its ability to reduce microbial transfer across several applications. There are often limits in place that prevent fully utilizing newer technology. When capital and operating costs come down, availability of products, new possibilities open up to apply technology. UV LED technology can be seen to following the same path. Test results collected by USEPA on the use of UVC-LED on MS-2 as well as Legionella have been shown to be able to achieve similar results to that in low pressure UV applications. System can be applied to point of use applications cost effectively and improvements in design will allow for deployment in larger flow systems.

BACKGROUND
It has been shown that after secondary treatment, anaerobic digestion is tied for the second highest consumption of electrical power at an average water resource recovery facility (WRRF; SAIC, 2006). Despite that, there has been limited research on how to optimize digester mixing. Many design engineers and regulators take the approach that it is better to err on the side of excess mixing power. The implicit assumption is that excessive mixing has little or no detrimental effect. That may not be a good assumption. In a landmark study on digester foaming, Miot et al (2013) showed that reducing the fraction of time the digester mixing system was in operation in an egg-shaped digester reduced foaming and variability in the daily biogas flow without reducing the total biogas flow. Eventually the best results were achieved with the mixing system turned completely off. There is also much anecdotal evidence that at least some digesters function well without external mixing. Occasionally, digester mixing systems break down and the error is only discovered much later often without impacting digester performance. In other cases, owners and operators may not have the budget available to fix a mixing system that fails unexpectedly or prematurely and are forced to operate the digester without mixing, sometimes for years or even decades, without obvious signs of loss of performance. Collectively these cases raise the question of whether anaerobic digesters can be operated without external mixing, relying only on the natural mixing provided by the biogas generated in the digester (and mixing from the heat loop) or if it is only possible under certain conditions.
METHOD
Digestate in a WRRF is a non-Newtonian, shear-thinning fluid, in that its apparent viscosity reduces at higher shear rates (Taylor et al., 2020). As the shear rate is increased from near zero, there is initially a sharp drop in apparent viscosity, but eventually there is little change with increasing shear rate. One theory for optimizing digester mixing is to operate the digester at a point where there is a limited response to changes in shear rate without approaching the point of diminishing returns (Krugel & Stallman, 2018). Presumably operating under these conditions would limit the variability of viscosity throughout the digester. The authors suggested two distinct rates of response (slopes in the curve) that might be targeted for successful operation: -0.33 and -0.50 Pa.s2, as shown in Figure 1. The general applicability of these targets remains to be confirmed experimentally. Rheological testing of a sludge sample at different solids concentrations allows a series of curves, such as shown in Figure 2, to be developed. This means that the shear rate and the corresponding minimum mixing intensity can be determined at each different concentration. Figure 3 shows the minimum mixing intensity for three different plants as a function of digester solids concentration. Note that the rheological testing at Plant A used two different cup and bob sizes in the rotational viscometer, and those produced slightly different results, confirming the limitations of current test protocols. Both Plants A and B digest primary sludge only, but their measured viscosities are quite different: at around 4% total solids, the estimated minimum mixing power for both plants are similar, but it is obvious that at higher concentrations the requirement for Plant A increases much faster than for Plant B. Plant C, digesting only waste activated sludge (WAS), has much higher sludge viscosity and requires much more mixing intensity. This confirms that rheological properties vary significantly from between WRRFs, so that it is necessary to test the rheology at each WRRF. Pre-treatment, such as thermal hydrolysis, may be used to reduce sludge viscosity. Pretorius et al. (2018) showed how to calculate the mixing power provided by the biogas flow in digesters. The procedure can be reversed, to estimate the minimum gas flow required to provide the mixing intensity calculated from sludge rheology. The gas flow can be used to estimate the minimum volatile solids loading rate required, based on an assumed volatile solids reduction (VSR) percentage and a gas yield (m3/kg VSR or cf/lb VSR). This is not a new approach: the USEPA (1976) stated that if a volatile solids loading rate of 6.4 kg/m3.d (0.4 lb/cf.d) could be maintained, natural mixing would be sufficient. Depending on feed sludge concentration and volatile fraction, that loading rate would translate into an HRT of approximately 6.5 days, confirming that this is an aggressive loading rate. The VSR depends on the hydraulic retention time (HRT) in a once-through digester. The HRT, in turn, is directly related to the volatile solids loading rate. An alternative way to present the conditions required for natural mixing to suffice is as a maximum HRT.
RESULTS
Of the three plants evaluated, only the Plant A data yielded viable results for relying on natural mixing. The Plant B data, determined for a recuperative thickening project, started at a relatively high solids concentration of 3.8% where a viable solution would require a specific digester design, such as a deep digester or a high aspect ratio. For Plant C the viscosity throughout the tested range (2.9% and above) was too high to yield a viable solution. Figure 4 shows the minimum volatile solids loading rate required for natural mixing as a function of digester solids concentration, based on the rheological data presented in Figure 2. The figure suggests that up to a solids concentration of approximately 2.5%, natural mixing should suffice without excessive volatile solids loading rates. The figure also shows that for this particular sludge, the criterion recommended by the USEPA (1976) of 6.4 kg/m3.d (0.4 lb/cf.d) is quite conservative and would allow natural mixing to suffice up to a solids concentration of almost 4%. Figure 5 shows that maximum allowable HRT for the same rheological properties. The figure suggest that a target HRT of 15 days can be maintained up to a concentration of approximately 3%. The full manuscript will include a discussion of the impact of SWD and digester shape on the mixing requirements. Other factors, including grit accumulation and blending of the feed into the digester contents will also be addressed.
CONCLUSION
A method is presented that allows a comparison to be made between the mixing provided by biogas generation in the digester and the mixing required to ensure a relatively even digester viscosity to be maintained. The results show that site-specific sludge viscosity determines whether natural mixing by itself could be sufficient. The method would allow significant power savings at WRRFs to be realized. Eliminating or significantly reducing the digester mixing power would allow WRRFs to make significant progress towards achieving energy neutrality or positivity.