WEF Residuals and Biosolids Conference *To whom correspondence should be addressed.

Transcription

1 What s the Best Use of Your Digester Gas? Well, it Depends Ian McKelvey 1 *, Eron Jacobson 1, Peter Zemke 1, John Smyth 2 1 Brown and Caldwell, 701 Pike St, Suite 1200, Seattle, WA King County Wastewater Treatment Division, 201 South Jackson Street, Seattle, WA *To whom correspondence should be addressed. imckelvey@brwncald.com ABSTRACT A series of biogas utilization studies were performed for King County s Wastewater Treatment Division to determine the performance of the County s existing systems and identify preferred alternatives for utilizing biogas in the future. Although the County s treatment plants operate within close proximity to each other (within 25 miles) and are subject to the same financial and policy pressures, the results of the studies demonstrate how sensitive the preferred use of biogas at wastewater treatment facilities can be to site-specific conditions at a given facility. By completing the reviews using the same standardized approach for each treatment plant, the results for each plant can be compared to each other and an understanding of why a utility may select one means of biogas utilization over another can be understood. KEYWORDS Digester gas, biogas, biomethane, combined heat and power (CHP), biogas upgrading, renewable identification numbers (RINs), pipeline quality gas, effluent source heat pumps INTRODUCTION Increasingly, wastewater treatment agencies are seeking opportunities to reduce energy consumption at their wastewater treatment plants (WWTPs), both by reducing the consumption of energy and by more efficiently recovering energy from the wastewater treatment process. For those facilities that include anaerobic digestion of their residual solids, the methane rich digester gas (or biogas) produced during the solids stabilization process is a prime source of renewable energy available for recovery. One such agency seeking to maximize resource recovery is King County s Wastewater Treatment Division (WTD). King County owns and operates three regional wastewater treatment plants in the greater metropolitan Seattle area that each utilize anaerobic digestion for solids stabilization. The County has a strong tradition of beneficially using the recovered biogas at these plants but recently hired Brown and Caldwell to determine the performance of the existing biogas utilization systems and identify a preferred alternative for optimizing biogas usage in the future at each facility. These studies were performed sequentially with the first taking place in 2012, the next in 2016, and the last still in progress. 795

2 The process used to generate and evaluate alternatives is not unlike the process used by many other communities. The baseline condition and performance of existing systems was established, alternatives were brainstormed and screened to a short-list, and the short-listed alternatives were evaluated in detail based on established financial, environmental, and operational objectives. What is unique about these studies is differences in the results can be attributed to factors outside of the evaluation process or the objectives used for the evaluation. In other words, because the same process was used to evaluate alternatives, the same agency was involved at all three plants, and the same objectives for a future biogas utilization program were used, the differing results from each study indicate that the preferred use of biogas at a facility is due to site-specific conditions. This paper documents the existing biogas utilization systems at each of the WTD s three regional wastewater treatment plants and describes the approach and methods used to evaluate biogas utilization alternatives. The results are presented and conclusions are drawn indicating that there is no biogas utilization alternative that is more universally beneficial over all others; the preferred use of biogas at wastewater treatment facilities is ultimately dependent on site-specific conditions. DESCRIPTION OF EXISTING SYSTEMS King County s wastewater treatment system serves 1.7 million people in the metropolitan Seattle region. The County s service area covers 424 square miles, providing trunk sewer conveyance and regional wastewater treatment to 34 wholesale customers. King County has been providing regional wastewater treatment services since 1965 and in 2015 treated an average of 178 million gallons of per day (mgd) of sewage. Regional wastewater treatment occurs at three regional wastewater treatment plants: South Treatment Plant (South Plant) in Renton, Washington, West Point Treatment Plant (West Point) in Seattle, Washington, and Brightwater Treatment Plant (Brightwater) near Woodinville, Washington. Figure 1 is a map showing the WTD s service area and treatment plants. These facilities are different sizes, utilize different treatment approaches, and are fed from different types of sewer systems. One similarity between all three facilities is that they all use mesophilic anaerobic digestion to produce Class B biosolids and biogas. A summary of the characteristics of the three regional treatment plants is presented in Table 1. To initiate a study of biogas utilization alternatives at each plant, a baseline of the existing biogas utilization and heating systems at each facility was completed. This included a site visit, interviews with plant operators, and evaluation of data provided from King County. This baselining effort established the performance and condition of the existing systems and provided biogas production and heat demand parameters needed to develop alternatives. The historical and current use of biogas at each facility is summarized in the following sections. 796

3 Figure 1. King County WTD s Service Area and Treatment Plants. 797

4 Table 1. King County WTD s Regional Treatment Plant Characteristics Characteristic South Plant West Point Brightwater Location Renton Seattle near Woodinville Sewer type separated combined separated Design capacity - Maximum 144 mgd 215 mgd 41 mgd month - Peak capacity 325 mgd 440 mgd 100 mgd Average annual 69 mgd a 92 mgd 17 mgd flows (2015) Secondary conventional high-purity oxygen membrane bioreactor treatment method activated sludge activated sludge Solids thickening dissolved air flotation gravity belt gravity belt process Solids stabilization mesophilic anaerobic mesophilic anaerobic mesophilic anaerobic process Number of active digesters (active digestion volume) digestion 4 (10.4 million gallons) digestion 5 (9.5 million gallons) digestion 3 (3.8 million gallons) Biosolids centrifuge centrifuge centrifuge dewatering process Annual biosolids produced (2015) 13,400 dry tons b 11,600 dry tons 3,300 dry tons a South Plant also treated 24 million gallons of hauled septage in b South Plant also treated raw sludge from the Vashon and Carnation WWTPs in 2015, totaling 140 dry tons. South Plant South Plant has provided secondary treatment since it began operation in 1965 but has only included solids treatment since 1987 (prior to that, residual solids were transferred to West Point). As part of the enlargement that implemented solids treatment, a biogas scrubbing system was implemented which upgrades the plant s biogas to a pipeline-quality biomethane product for sale to the local natural gas utility. To maximize the amount of biogas available for use in this system, South Plant originally used effluent-source heat pumps to meet its process and space heating demands. These systems were expanded when the plant underwent a third enlargement in Due to a number of factors, the County implemented a hot water boiler and a cogeneration system in 2002 and 2004, respectively. The two primary drivers were the dramatic increase in electricity prices in the western states in the early 2000 s and the loss of efficacy of the plant s heat pumps when the original R12 refrigerant was banned and replaced with R134a refrigerant. 798

5 As a result, the plant now uses the gas-fired hot water boiler as its primary source of heat. Table 2 lists the biogas and heating equipment at South Plant. Table 2. South Plant Biogas Utilization and Heating Equipment System Type Installed Capacity Gas scrubbing High-pressure water solvent 1987 and ,667 scfm a Heat pumps Effluent source, centrifugal compressor 1987 and at 6.0 MMBtuh each b Hot water boiler Fire tube MMbtuh Cogeneration Combined cycle turbines at 3.5 MW c a Installed capacity. Operators indicate actual capacity may be up to 30% lower. b Five units are installed but only one remains operational. c The system also has a 1.0 MW steam turbine that has not been used to date. In terms of the size of the biogas utilization system, hourly biogas production at South Plant averaged 920 standard cubic feet per minute (scfm) in 2015 with 95% of biogas production ranging between 700 and 1150 scfm (see Figure 2). Figure 3 shows the plant s heat demand between 2010 and Average daily heat demand was 5 million British thermal units per hour (MMBtuh) during this period with a peak average week of 8 MMBtuh and a minimum average week of 1 MMBtuh. Specific end uses of the plant s biogas and sources for the plant s heat demand were documented during the study in Figure 4 indicates that the majority of the gas produced at South Plant was either scrubbed and sold to the natural gas utility (Puget Sound Energy, PSE) or burned in the boiler, while Figure 5 shows that most of the plant s heat needs are met by the hot water boiler. 799

6 Figure 2. Histogram of hourly biogas production at South Plant in 2015 (Bin size: 5 scfm). Figure 3. Estimated average daily heat demand at South Plant between 2010 and

7 Figure 4. Biogas use at South Plant in Figure 5. Heat supply sources at South Plant in Due to the age of the existing gas scrubbing system and the lack of redundancy in the existing heating system, the biogas utilization study for South Plant focused on the best system for utilizing all of the plant s biogas while reliably satisfying the plant s heat demand. West Point West Point was constructed as a primary-only treatment facility with anaerobic digestion in The unreliable power supply to the plant led to the plant historically using biogas-fired engines to drive the plant s raw sewage pumps (RSP) and effluent pumps; heat from the engines was recovered to meet the plant s heat demands, supplemented by boilers. In the 1980 s, with the additions of Digesters 4 and 5, the County also added a combined heat and power (CHP) system consisting of three 1.2 megawatt (MW) reciprocating internal combustion (IC) engines. With the addition of the secondary treatment process in 1995, the plant s energy profile was changed and the biogas utilization systems were modified. The effluent pumping station engines were replaced with electric motors (but the RSP engines were replaced in-kind) and gas-fired hot water boilers were installed as the plant s primary source of heat. In 2011, a new CHP facility was installed to replace the aging existing system. Table 3 lists the current biogas and heating equipment at West Point. 801

8 Table 3. West Point Biogas Utilization and Heating Equipment Type Installed Capacity Raw sewage pump engines Reciprocating internal combustion at 440 bhp Combined heat and power Reciprocating internal combustion at 2.3 MW a Hot water boiler Fire tube at 6.9 MMBtuh, 1 at 15.5 MMBtuh a Upstream gas conditioning is only sized for operation of one engine at a time Hourly biogas production at West Point averaged 1050 scfm in 2014 with 95% of biogas production ranging between 820 and 1250 scfm (see Figure 6). Figure 7 shows the plant s estimated heat demand in 2014 with an average heat demand of 9 MMBtuh, a peak average week of 15 MMBtuh, and a minimum average week of 4 MMBtuh. Based on this gas production and heat demand, the plant s biogas consumption in 2014 is shown in Figure 8 while the sources of heat are indicated in Figure 9. These figures indicate that the plant primarily utilizes the CHP facility as a source of heat and beneficial end use for the plant s biogas. Figure 6. Histogram of hourly biogas production at West Point in 2014 (Bin size: 5 scfm). 802

9 Figure 7. Estimated daily heat demand at West Point in 2014 Figure 8. Biogas use at West Point in Figure 9. Heat supply sources at West Point in Due to the need to operate the CHP engines to meet contractual requirements and the decision to continue to operate the raw sewage pump engines to eliminate concerns regarding backup 803

10 electrical power, the biogas utilization study at West Point focused on best utilizing the surplus gas being flared in enclosed waste gas burners. Brightwater The Brightwater plant is the newest treatment plant in WTD s system, beginning full operation in The plant meets its process and space heating demands utilizing gas-fired hot water boilers (two at 17.2 MMBtuh). A CHP system was removed from the design of the plant to save capital costs and as a result, any biogas produced in excess of the boiler fuel demand is flared. Between November 2015 and November 2016, average hourly biogas production at Brightwater was 290 scfm with 95% of biogas production falling between 210 and 390 scfm (see Figure 10). During this same period, the average heat demand was 5 MMBtuh with peak average week of 9 MMBtuh and a minimum average week of 2 MMBtuh (see Figure 11). Figure 12 shows the breakdown of biogas consumption at Brightwater indicating that just over half of the biogas produced is used to fuel the boilers. Figure 10. Histogram of hourly biogas production at Brightwater from November 2015 to November 2016 (Bin size: 2 scfm). 804

11 Figure 11. Estimated daily heat demand at Brightwater between November 2015 and November Figure 12. Biogas use at Brightwater Between November 2015 and November With the lack of a beneficial use outside of the gas fired boilers, the study at Brightwater is focused on the best use of all of the plant s biogas while meeting the plant s heat demand. 805

12 Table 4 summarizes the biogas production and heat demand parameters for all three treatment plants. With this data and the baselining of the existing systems complete, the studies next turned to the development and evaluation of alternatives. Table 4. Biogas Production and Heat Demand at WTD s Three Regional WWTPs Parameter South Plant West Point Brightwater Average gas production (scfm) Gas production range a (scfm) Average heat demand (MMBtuh) Minimum average week heat demand (MMBtuh) Peak average week heat demand (MMBtuh) a 95% of total production observed EVALUATION OF BIOGAS UTILIZATION ALTERNATIVES To compare the biogas utilization alternatives for each facility, it was first necessary to identify the County s biogas utilization objectives. Objectives for each plant were developed in a workshop setting with plant operations, process, and reliability-centered maintenance staff; WTD resource recovery, project planning and delivery, and project management staff; and Department of Natural Resources and Parks (DNRP) policy staff. The objectives were divided into three categories: financial, environmental, and operational. To reflect the relative importance of the objectives compared to each other, a weighting was applied such that objectives considered more important than others received a higher weight. The objectives identified were largely the same for each plant. Financial objectives were focused on minimizing the life-cycle cost of any end use. Environmental objectives were based on a compilation of goals identified in the County s energy plans and strategic climate action plans. Operational objectives focused on minimizing risk while maximizing system reliability and flexibility. A summary of the objectives used in all three studies is provided in Table 5. Table 5. Objectives Used for Biogas Utilization Alternatives Analysis Objective Relative Weighting FINANCIAL Maximize the program s net present value a 8 Minimize sensitivity to commodity price changes b 0 ENVIRONMENTAL Reduce use of and expenditures for energy 1.6 Reduce greenhouse gas emissions 1.6 Increase production of renewable energy 1.6 Increase consumption of renewable energy 0.6 (not in the WP or BW list) Reduce quantity of digester gas sent to the flares 0.6 (not in the SP or BW list) Invest in alternative fuel transit and fleet vehicles

13 OPERATIONAL Maximize system redundancy and reliability 1.3 Maximize system operational flexibility 0.6 Minimize WTD labor requirements 1.6 Minimize reliance on outside contracts 0.3 Minimize technical risk 1.6 Minimize air quality treatment requirements 0.6 a South Plant study broke this objective out into its individual components (i.e., capital costs, operation and maintenance costs, revenues, and grants) b Provided for information only. A detailed sensitivity analysis for each alternative was performed. WP = West Point, BW = Brightwater When used with a 1 through 5 scoring scale, the weighting of objectives allowed for a maximum score of 100 points. The relative weighting reflected the reality that financial objectives held more sway over the environmental and operational objectives but was not the only important factor. Within the environmental factors, the weighting shows the County s commitment to reducing energy consumption and greenhouse gas emissions. Lastly, the operational objectives were weighted such that technical risk, the reality of limited staffing, and system reliability were more critical than operational flexibility, the need for third party maintenance, or the risk of air quality regulatory changes. As Table 5 shows, the objectives and their relative weighting were largely the same for all three studies. This reflected that the County s goals for a biogas utilization system were largely independent of the specific treatment plant generating the biogas. Once the objectives for a biogas utilization system were defined, alternatives were brainstormed with plant staff and evaluated based on these objectives. The alternatives considered for each plant are listed in Table 5 below. Ultimately, the alternatives for each plant were screened down to three alternatives for a more detailed evaluation using the objectives identified in Table 5. The alternatives carried forward for a more detailed consideration are identified in bold text in Table 6. Table 6. Biogas Utilization Alternatives Considered at WTD s Three Regional WWTPs South Plant West Point Brightwater Status quo (boiler + refurbished Status quo (flare Status quo (boilers) scrubbing system) surplus gas) Status quo to produce renewable compressed natural gas (rcng) Status quo + improved boiler controls Existing boilers + gas scrubbing for pipeline injection 807

14 Status quo with sale to a third party New gas scrubbing system New gas scrubbing system to produce rcng New gas scrubbing system with sale to a third party New boilers + refurbished scrubbing system New boilers + refurbished scrubbing system to produce rcng New boilers and gas scrubbing system New boilers and gas scrubbing system to produce rcng New heat pumps + refurbished scrubbing system New heat pumps + refurbished scrubbing system to produce rcng New heat pumps and gas scrubbing system New heat pumps + gas scrubbing system to produce rcng Operate existing CHP facility full-time (refurbished gas scrubbing system) Operate existing CHP facility full-time (new gas scrubbing system) Convert CHP system to biogas operation and run full-time New biogas IC engines New biomethane IC engines + refurbished gas scrubbing system New biomethane IC engines and gas scrubbing system Status quo + upsize existing CHP conditioning Additional CHP with one IC engine Additional CHP with two IC engines Additional CHP with microturbines Biogas upgrading for pipeline injection Biogas upgrading, trucked off site Biogas upgrading, trucked off site with storage Convert RSP engines to electric motors New heat pump Convert electric motors to gas-fired engines Add biogas storage Organic Rankine cycle Convert standby generator to CHP Existing boilers + gas scrubbing for vehicle fuel Heat pumps + gas scrubbing for pipeline injection Heat pumps + gas scrubbing for vehicle fuel Heat pumps + gas scrubbing, trucked off site IC engines IC engines + heat pumps Microturbines 808

15 The alternatives considered generally focused on a few groups of technologies: Combined heat and power: Systems that generated both electrical power and heat were considered at all three plants. In particular, reciprocating internal combustion (IC) engines were considered at all three plants but microturbines and modifying South Plants existing combined-cycle turbines were also considered. Biogas upgrading: Systems that separated the methane in the biogas from other constituents to produce a biomethane product were also considered at all three plants. Specific technologies included high-pressure water solvent systems, pressure swing adsorption systems, and membrane separation systems. End uses for the biomethane that were considered included pipeline injection and renewable compressed natural gas (rcng) vehicle fuel use Heating: Because meeting plant heating demands is inextricably linked to biogas utilization options, various means of meeting heating needs were considered, both in terms of using the biogas directly for heating (e.g., hot water boilers) or using another energy source in order to maximize the availability of biogas for another end use (e.g., electric heat pumps). Detailed evaluation of the three selected alternatives at each plant (plus the status quo alternative kept for baselining purposes) was completed next. This detailed evaluation included preliminary siting, a Class 5 construction cost estimate, a greenhouse gas emissions inventory, life-cycle cost analysis, and a sensitivities analysis for each alternative. The results from these efforts were used to reevaluate the scoring for each objective and select a preferred alternative. A brief summary of the analysis for the South Plant and West Point plants follows. The Brightwater study is currently ongoing and this detailed analysis is in progress. South Plant Because of the age of the existing systems, the status quo for South Plant required the significant rehabilitation of the existing gas scrubbing system and the addition of new boilers to meet the plant s redundancy needs (to be located in a new building). This resulted in a relatively high capital cost for a status quo alternative. In addition, this alternative assumed that the new boilers would continue the plant s existing practice of using high-btu fuel (i.e., natural gas or biomethane) to improve system reliability. In one alternative, a new gas scrubbing system would be paired with boilers. For the new gas scrubbing system, a pressure-swing adsorption system was assumed as the base technology based on the design team s familiarity with the system and the relatively low cost compared to other biogas upgrading technologies considered (e.g., water solvent, membranes). To conserve construction cost, the study assumed a new PSA system could be installed in the same space 809

16 occupied by the plant s existing gas scrubbing system (see Figure 13). To minimize greenhouse gas emissions and avoid the high energy consumption required to upgrade biogas, the boilers selected would operate on low-btu (i.e. minimally conditioned) biogas. For the alternative in which heat pumps would be used rather than boilers to meet the plant s heat demands, a new building would not be needed. Instead, the new heat pumps would be installed in the same location as the plant s existing heat pumps. High-temperature dual-stage heat pumps with screw compressors were assumed to minimize the operational challenges the plant had experienced previously with their single-stage centrifugal compressor heat pumps. The CHP alternative at South Plant assumed three 1.2 MW IC engines would be installed in a new building. To fuel the new CHP system, a new gas conditioning system would be included to remove moisture, hydrogen sulfide, and siloxanes. The existing CHP system at South Plant runs on only high-btu fuels and therefore does not already include this equipment. Figure 13. Preliminary equipment layout of a new biogas upgrading system at South Plant Table 7 summarizes the major results from this analysis. The sensitivities analysis performed found that the results were most sensitive to the price of the generated biomethane or power (including associated Renewable Identification Numbers [RINs] or Renewable Energy Certificates [RECs]). In particular, the sales price the County could negotiate with the 810

17 purchasing utility could ultimately determine whether any selected alternative would have a positive net present value. Table 7. Results from detailed analysis of South Plant Biogas Utilization Alternatives Parameter Status Quo Scrubbing + Scrubbing + CHP System Boilers Heat Pumps Construction $10 million $11 million $12 million $19 million cost (2013 dollars) Net present -$5 million -$2 million -$3 million -$2 million value (2013 dollars) Greenhouse gas emissions (metric tons of eco2 per year) -3,000-8,000-10,000-2,000 Figure 14 graphically demonstrates the results when comparing the alternatives using the objectives outlined in Table 5. Both gas scrubbing alternatives scored highly within 1 point of each other. 811

18 Figure 14. Results from the South Plant Biogas Utilization Study West Point At West Point, the selected alternatives focused on how to best utilize the surplus gas leftover after the needs and capacity of the existing systems had been met. As a result, the status quo did not carry any capital cost with it; the plant would continue to use biogas in the RSP engines, boilers, and CHP system, and would flare any surplus biogas. The first alternative considered adding one additional engine to the plant s existing CHP facility. The new engine would be smaller than the existing engines (1.5 MW versus 2.3 MW) allowing for improved flexibility in performing with variable fuel quantities. Figure 15 provides an example of how a third engine could be added to the existing CHP facility. The lack of any major structural work made this a relatively cost effective alternative. The second alternative consisted of two new engines to further improve the plant s ability to consume biogas as the gas production and heat demand fluctuated. Two 700 kw engines would not be able to fit within the space for the existing CHP system however, so an additional space for the system would need to be constructed within the plant. The third alternative was for a new biogas upgrading system (i.e. gas scrubbing system) that would upgrade the surplus biogas to pipeline quality biomethane for sale to a third party. Because of the plant s location, injection into the local natural gas utility grid is not viable. As a result, this alternative assumed the upgraded gas would be compressed and trucked via CNG tube trailer to the third-party purchaser directly. To facilitate trucking off-site, the system would be located in the old Chlorine Handling Building, near the existing truck parking area. This 812

19 allows for the County to also consider converting a portion of their biosolids hauling trucks to CNG-fueled engines and running them on the rcng. Figure 15. Preliminary layout for one additional IC engine at West Point For the candidate technology, a 400 scfm BioCNG membrane system was assumed based on the appropriate size for the plant s needs, the low height profile (as compared to a water solvent system), and the convenience of having the entire biogas upgrading system provided by a single vendor. Table 8 summarizes the major results from this analysis. One may note the extremely small values for greenhouse gas emissions reductions for the CHP alternatives. This is because the electrical utility that serves West Point (Seattle City Light) has a very small carbon profile (less than 2% of power supply is from traditional fossil fuels). The sensitivities analysis performed found that the NPV results were extremely sensitive to the value of the electricity produced by the CHP system and the amount of surplus biogas available. In particular, the County s current agreement with Seattle City Light has very favorable rates and 813

20 if these rates can be extended to the new system, the NPV of one additional engine would be positive. But if lower rates are assumed, the value of this alternative quickly becomes negative. Similarly, if the amount of surplus biogas varies significantly, the alternatives would either become cost prohibitive (at lower than expected biogas flow rates) or would all become net positive in terms of cost (if more biogas is available than expected). Table 8. Results from detailed analysis of West Point Biogas Utilization Alternatives Parameter Status Quo One IC Engine Two IC Engines Gas Upgrading Construction - $5 million $8 million $7 million cost (2016 dollars) Net present -$2 million $1 million -$3 million -$8 million value (2016 dollars) Greenhouse gas emissions (metric tons of eco2 per year) ,200 Figure 16 graphically demonstrates the results when comparing the alternatives using the objectives outlined in Table 5. The single IC engine alternative scored the highest. Figure 16. Results from the West Point Biogas Utilization Study Brightwater 814

21 Detailed evaluation of the selected alternatives at Brightwater is still in progress but a description of the alternatives can be provided. Because the boilers at Brightwater are still in good working condition and meet the plant s needs, the status quo for this study would be to continue utilizing biogas to meet the plant s heating needs and flaring the remainder. As a result, there would be no capital cost associated with the status quo but also no benefit in terms of the County s energy goals. One alternative under consideration would add a biogas upgrading system to produce a biomethane product from the gas remaining after the plant s heat demand had been satisfied. The biomethane would be injected into the local natural gas utility s transmission grid but sold to a third-party buyer for the value of the associated RINs. Another alternative would install a similar biogas upgrading system but would pipe the biomethane directly to a nearby refuse hauler CNG fueling station instead of injecting biomethane into the natural gas transmission system. By selling the biomethane directly to the end user the County would avoid the contractual and quality requirements imposed by the natural gas utility but would pay for this in terms of additional market risk (e.g., only having one possible user for the product) and capital cost for rcng storage. This alternative will also consider the use of effluent-source heat pumps to reduce the heat required from the boilers, increasing the amount of biogas available for upgrading. The layout of the treatment plant lends itself to low-temperature (i.e., degrees Fahrenheit) heat pumps because the higher temperature process heating needs (e.g., sludge heating) can easily be separated from the lower temperature space heating needs. The last alternative under consideration at Brightwater is to install a new CHP facility as was originally planned during detailed design of the plant. Because the original plan for the plant included this facility, the expectation is that integration of this type of facility into the plant should be more straight-forward than that for the other alternatives under evaluation. There are no results to report to date on the detailed evaluation; the evaluation is expected to be completed during the summer of RESULTS AND CONCLUSIONS Despite the fact that the same evaluation process was used in both studies and that the County s objectives for biogas utilization is largely the same at both facilities, the results of the studies have led the County to move forward with differing approaches for South Plant and West Point. At South Plant, the study found that biogas upgrading with direct utility pipeline injection was the preferred approach for the County and has resulted in a project to replace the existing scrubbing system with a new biogas upgrading system and to install both hot water boilers and effluent source heat pumps. The high value for the RINs associated with the biomethane and the improved greenhouse gas emissions profile led the County in this direction but the plant s familiarity with the biogas upgrading technology and the availability of existing infrastructure 815

22 for pipeline injection made this alternative viable. For West Point, the value of the RINs and the environmental benefits would still be present for a biogas upgrading alternative, but ultimately the lack of natural gas infrastructure at the plant led to this alternative not being very viable. Instead, the plant s long history of operating biogas-fired engines and the current rate the plant receives for RECs made an expanded CHP system the preferred alternative for the County. Thus, site specific and historical factors end up playing a significant role in selecting a preferred use of biogas at any given wastewater treatment plant. The study for Brightwater has not been completed yet but site-specific factors can already be seen playing an effect on the direction of this study too. Brightwater s location is such that injecting biomethane into the natural gas utility is a viable alternative and the proximity of a large CNG refuse hauler refueling station could make a dedicated vehicle fueling alternative possible. In addition, the site-specific layout of process and space heating needs at the plant makes the use of a hybrid heating supply system that utilizes boilers for higher temperature needs and heat pumps for lower temperature needs a possibility. The results of these studies shed light on how a consistent approach to evaluating biogas utilization alternatives with similar objectives can still result in widely different outcomes, even for facilities in relatively close physical proximity operated by the same utility. For utilities considering options for implementing a biogas utilization system, this conclusion emphasizes the importance of site-specific evaluations rather than applying a one-size-fits-all solution to every facility. There are many good uses for digester gas but these studies show that ultimately the best use for your digester gas will depend on your specific situation. ACKNOWLEDGEMENTS The authors gratefully acknowledge the cooperation of King County s WTD staff that provided invaluable input, review, and direction for these studies: - South Treatment Plant: Mike Wohlfert, Curtis Steinke, Pete Carter, Rick Butler, Dave Jurgens, Lou Broadhead, Lester Van Gelder - West Point Treatment Plant: Eugene Sugita, Showell Osborn, Dave Truman, Al Williamson, Steve Huang, Jim Belcher, Steve Zamperin, Charles Wenig, Chris Boyle, Kate Osborn - Brightwater Treatment Plant: Carol Nelson, Andy Strehler, Bruce Kessler - King Street: Bob Bucher, Pardi Sukapanpotharam, Carl Grodnik, Jessie Israel, Dave Broustis, Felix Brandli, Sue Hildreth, Amanda Connell 816