WEF Residuals and Biosolids Conference 2017

Transcription

1 Challenges and Opportunities for Approaching Net Zero Energy and Improving Resiliency of a Unique Solids Handling Facility in California Irina Lukicheva, CH2M, Dan Frost, CCCSD, Nitin Goel, CCCSD, Peter Burrowes, CH2M, Summer Bundy, CH2M, Jamie Pigott, Carollo Engineers ABSTRACT Central Contra Costa Sanitary District (Central San) is in the process of developing a Comprehensive Wastewater Master Plan for their wastewater treatment plant in Martinez, California. The District collects and treats an average of 34 million gallons per day of wastewater from nearly half a million residents within a 145 square mile service area. Solids from primary treatment and waste activated solids are dewatered and then incinerated. Central San s incineration facility is currently one of two wastewater solids incineration facilities in California. The incineration facility became operational in the 1980s and included two multiple hearth furnaces and a centrifuge dewatering system. Waste heat recovered from the incinerator is used to drive steam turbines to generate air for secondary treatment aeration. As part of the Master Plan, Central San is evaluating future liquid and solids treatment alternatives that will address the aging infrastructure needs, increase plant resiliency against man-made or natural disasters, increase the efficiency of energy recovery at the plant, accommodate projected capacity needs, improve operational reliability, improve regulatory resiliency, and meet increasingly stringent water quality and air pollution control regulations. In addition, one of the main goals of the Master Plan was improvement of the Central San s energy recovery portfolio. The current practice of energy recovery in the form of waste heat recovery to steam to generate air for secondary treatment is no longer the most energy efficient and due to the delicate interconnections between liquids and solids treatment, it carries risks of not being able to supply sufficient air when solids and energy recovery processes are offline, for example in the event of the earthquake. As part of the Master Plan evaluation, Central San, CH2M and Carollo Engineers evaluated several alternatives for solids handling. These included advanced anaerobic digestion, fluidized bed incineration, drying, innovative thermal technologies, and combinations of the above listed processes. Mesophilic anaerobic digestion (MAD) of primary solids was chosen as a first processing step to generate biogas for energy recovery from the digestion of primary solids. Separate digestion of primary solids allows the plant to reduce the size of digestion required and allow for the import of high strength wastes such at fats, oils, and grease and liquid organic waste that can be co-digested in the MAD process to enhance biogas production and maximize power production and waste heat utilization. Digested primary solids combined with thickened undigested waste activated solids are to be dewatered using centrifuges. Drying of dewatered solids prior to incineration would be accomplished using a new dryer followed by incineration using a fluidized bed incinerator. The incineration process was selected to be continued as the most efficient method of managing solids that reduces energy demands, reduces greenhouse gas emissions, and does not require development of a new biosolids disposal management program, 650

2 thus helping address increasing concerns over the future availability and cost of biosolids disposal and reuse for facilities in the San Francisco Bay Area. Biogas produced from the digestion process would be used along with landfill gas in a cogeneration turbine to produce electricity. Heat recovered from the incineration in addition to the waste heat generated from the cogeneration turbine would be used to generate additional electricity using an Organic Rankine Cycle turbine. The overall energy conversion efficiency from solids into power in this case is significantly higher compared to the overall conversion efficiency for the current plant configuration. The project will generate about 3.7 MW projected electrical power requirements using biogas, landfill gas and heat recovered from incineration. Even with the increased conversion efficiency, the plant generated electricity may not be sufficient to meet the long term future plant demand, especially if the plant demand includes energy requirements for energy intensive recycled water processes such as reverse osmosis. To meet future recycled water power demands, Central San could consider some additional imported natural gas to the cogeneration turbine or can import electricity or produce green energy using alternative renewable energy generation technologies such as solar photovoltaic panels and wind turbines. KEYWORDS biosolids, incineration, energy recovery INTRODUCTION Central Contra Costa Sanitary District s (Central San) wastewater treatment plant is located in Martinez, California. Central San collects and treats wastewater for 470,000 residents and 3,000 businesses in central Contra Costa County. Central San operates and maintains over 1,500 miles of sewer, 19 pump stations, and 21 miles of force main that transport wastewater to Central San s treatment plant. Central San s treatment plant currently treats approximately 136,400 m 3 /d (30 mgd (million gallons per day)) dry weather flow. Central San s treatment plant process flow diagram is shown in Figure 1. The treatment processes include a Headworks with bar screens and grinders, influent pumping, pre-aerated grit removal, primary sedimentation, primary effluent pumping, activated sludge secondary treatment, and UV disinfection. The treated effluent is discharged into Suisun Bay or treated further for recycled water use. Tertiary filtration is used to produce Title 22 recycled water for on-site use, landscape irrigation, and other recycled water uses within Central San s service area. The current recycled water system operates at a peak day demand of approximately 6,800 m 3 /d (1.5 mgd) for utility water (2W) for plant use and 9,100 m 3 /d (2.0 mgd) for recycled water users, primarily for landscape irrigation. The solids treatment process flow diagram is shown in Figure 2. Solids are byproducts of the primary and secondary treatment processes. Waste Activated Solids (WAS) from the secondary treatment process are thickened with dissolved air flotation and blended with raw primary sludge in the Sludge Blending Tank (SBT). Lime is added in the SBT prior to centrifuge dewatering to improve performance of the dewatering polymer and to reduce the potential for formation of fused agglomerations of solids (clinkers) in Central San s multiple hearth furnaces. Dewatering polymer is added directly upstream of the centrifuges to improve cake dryness and reduce water 651

3 sent to the incinerators. Dewatered sludge (referred to as "cake") is pumped to one of the two multiple hearth furnaces. Incineration of the blended sludge requires supplemental fuel which is currently landfill gas. Ash remaining after incineration is re-used in soil amendment products but can also be landfilled when necessary. In the event that the cake cannot be incinerated, it is conveyed to the Emergency Sludge Loading Facility for off-loading to trucks for landfill disposal. Central San s incineration facility is currently one of two wastewater solids incineration facilities operated in California. The incineration facility became operational in the 1980s and included two multiple hearth furnaces and a centrifuge dewatering system. The facility also included an energy recovery system to recover waste heat from incineration and generate steam to run steamdriven aeration blowers. The excess heat from the incineration process off-gases is converted to medium grade steam using waste heat recovery boilers. Energy recovery has historically been an important Central San strategy and driver. The energy for the plant comes from four sources: heat recovered from incinerating plant solids, imported grid electricity (purchased from Pacific Gas and Electric (PG&E)), natural gas, and landfill gas. Landfill gas is primarily used as supplemental fuel for incineration, natural gas is primarily used in the cogeneration unit to generate electricity and steam, and natural gas is also used in the auxiliary boilers to generate the satisfy the remaining steam demand that is not met with the incineration and cogeneration unit waste heat recovery boilers. Approximately 95 percent of the treatment plant power demand is satisfied using onsite co-generation (~2.7 MW) with the natural gas turbine and the remaining power demand (~150 kw) is met with imported grid power (electricity). Figure 3 shows a breakdown of the sources of energy currently used at the plant. From 2014 to 2015, the total energy consumed from those sources was approximately 2,400 MMBTU/d. The generated electricity is mostly used for liquid treatment pumping, solids treatment and disinfection (Figure 4). In conventional treatment facilities, the secondary treatment process is the largest electricity consumer. However, because Central San relies on a steam system to power the aeration blowers, relatively little electricity is required for secondary treatment. Steam is generated from waste heat from the multiple hearth furnaces, waste heat from the cogeneration turbine, and steam made by the auxiliary boilers. Figure 5 shows the amount of steam that each steam-generating process contributes to the total steam production. Overall energy distribution through current plant is shown in the form of Sankey diagrams in Figure

4 Figure 1. Plant Process Flow Diagram 653

5 Figure 2. CCCSD solids processing process flow diagram 654

6 Figure 3 Central San energy source profile (heat and electricity). (Note: Calculated based on the heating value of natural gas, landfill gas and solids in MMBTU/d, and kw of energy from PG&E also converted to MMBTU/d) Odor Control HVAC Final Effluent Pumping Screening Primary Treatment Thickening Other Service Air Dewatering, 6% Secondary Treatment, 6% Other Solids Processes, 6% Incineration, 16% Influent Pumping, 13% Primary Effluent Pumping, 11% 3W water, 7% Filter Plant (Recycled Water), 7% UV, 10% Figure 4 Electricity usage profile by process area 655

7 Figure 5 Steam Generation Profile PROJECT GOALS AND DRIVERS Recognizing the importance of protecting its assets, Central San commissioned the Comprehensive Wastewater Master Plan (CWMP) to build on its planning efforts over the past few years. The following key "drivers" were identified to help guide the plan's direction as it relates to solids handling and energy management: Aging Infrastructure - Maintain performance and reliability of existing assets to ensure reliable wastewater collection and treatment. This includes repair or replacement (R&R) of aging equipment and some structures to extend their useful lives. Maximize Remaining Life of Incinerators: Through condition assessments, it was determined that the incinerators are in a good condition due to Central San s robust maintenance program and practices and have at least 20 years remaining physical life. Central San is also able to operate one of their two furnaces per year while performing preventive maintenance on the offline furnace. Although the incinerators are in good condition, several support solids handling support facilities (e.g. dewatering equipment) are beyond their useful life and require improvements in order to realize the full life of the incinerators. The CWMP focused on developing a long-term plan for transitioning from Central San s existing incineration facility while performing near-term improvements to maximize the remaining life of the incinerators. 656

8 Figure 6. Central San Sankey Diagram of Current Solids Processing and Energy Recovery 657

9 Capacity - Increase capacity, if required, to accommodate planned growth for the communities served within Central San s service area. As flows and loadings increase in proportion to population growth, some treatment processes may need to be expanded. Address Limited Incineration Capacity: The incinerators are approaching their design and permitted capacity. Currently, the solids produced during peak solids events cannot be processed through one incinerator and need to be stored for later processing. In the longer term, the average processing capacity of the incineration will also become limited resulting in the increased difficulty of solids management and the need to process some of the solids generated at the plant through an alternative parallel solids handling process or through off-site disposal. The CWMP evaluated options for reducing solids loading to the incinerators in addition to developing a phased implementation of the long-term solids handling improvements to address potential capacity triggers. Regulatory - Comply reliably with regulatory requirements, protect human health and the environment, and plan for anticipated future regulatory requirements. Reliably Meet Air Regulations: Over the last few years, Central San has been working on improvements to Central San s incineration air pollution control train and ash handling system to meet recent federal sewage sludge incineration rules (SSI MACT 129). To achieve reliable compliance with SSI MACT 129, the CWMP recommended considering air pollution control improvements to the wet scrubber air pollution control system. Plan for Increasingly Stringent Air Regulations: In the coming years, federal, state, and regional regulations for air emissions may become stricter. The Bay Area Air Quality Management District (BAAQMD) is currently considering implementation of a new rule for reduction of risk from air toxic emissions at existing facilities, which may require significant plant-wide improvements including additional air pollution control equipment for Central San s incinerators. Manage Greenhouse Gas Emissions: Currently, Central San successfully operates its existing solids handling processes and natural gas cogeneration facility such that it maintains greenhouse gas emissions to below the California Air Resources Board (CARB) Cap and Trade Threshold of 25,000 metric tons of CO2 equivalents. However, there are state greenhouse gas emission regulations and goals that may reduce this threshold or transform the way Central San operates its facilities. Additionally, Central San has relied on local landfill gas to provide supplemental fuel to the incinerators. However, the remaining life of the landfill is questionable and a reduction in available landfill gas supply would require more import of natural gas creating a significant challenge for Central San to stay below the CARB Cap and Trade Threshold of 25,000 metric tons of CO2 equivalents Increasing Regulations on the Biosolids Land Application and Landfilling: The majority of the wastewater facilities in the San Francisco Bay Area generating biosolids typically land apply Class B biosolids or send biosolids to a landfill. In the future, the amount of organics that could be sent to landfills is going to be limited, which directly impacts the amount of biosolids that can be sent to the landfill for alternative daily cover. This diversion of organics and ultimately biosolids from the landfills may trigger increased competition among biosolids producers for 658

10 land application and composting and lead to numerous facilities upgrading their solids processing to produce higher quality products and possibly look at thermal minimization technologies that do not produce significant amounts of biosolids that require disposal. These future limitations should be kept in mind during the planning of solids treatment facilities for Central San. Sustainability/Optimization - Minimize life-cycle costs, maximize benefits, and achieve economic stability through optimization, resiliency, resource recovery, and energy projects. Projects include optimizing existing treatment processes, completing energy efficiency improvements, recovering resources from solids processes, generating renewable power, increasing recycled water production, and completing improvements for reliability and resiliency against man-made and natural disasters. Improve Operational Resiliency: For many years, Central San has successfully operated a waste heat energy recovery system for its incinerators and cogeneration facility. Steam generated from the waste heat is used to drive steam-driven turbines that provide air for secondary treatment. Although this system was innovative for its time, the overall energy conversion is not as efficient as other energy recovery options that are now available. This interconnection between solids handling, energy recovery, and secondary treatment is unique but it also results in an operational vulnerability. Unexpected outages or disruptions in solids handling and/or steam production can create a ripple effect on the ability to provide oxygen for secondary biological treatment. Similarly, unexpected failures or changes in the air and steam systems can result in bypassing of the waste heat recovery system and air pollution control train resulting in a reportable compliance activity under the new SSI MACT 129 rule. Both Central San s Pump & Blower Building that house the steam-driven aeration turbines and the Solids Conditioning Building that houses the solids handling and cogeneration facility do not meet current seismic standards presenting an increased risk to Central San. Strive to Achieve Net Zero Energy: Central San also recently adopted an energy policy that is aimed at achieving net zero energy. The CWMP evaluated several solids and energy alternatives that provide better energy conversion efficiencies and that enable Central San to import high strength wastes that will be key to achieving net zero energy. To achieve net zero energy, the CWMP determined that other energy efficiency improvements and renewable energy production would be required in addition to more energy efficient processes and imported high strength wastes. ALTERNATIVES EVALUATION AND COMPARISON Electrical versus Steam Blowers The improvement of energy resiliency was the first step in solids and energy alternatives evaluation. In the current configuration, aeration air is supplied by steam-driven blowers. Steam is supplied from the waste heat recovery boilers for the multiple hearth furnaces and cogeneration system, in addition to steam from the auxiliary boilers. There were several drivers for replacing the existing steam-driven blowers: 659

11 The steam-driven blowers cannot turn down to meet low aeration demands. During these conditions, excess air is wasted. The steam-driven blowers are located in the Pump & Blower Building and the boilers and steam system are located a long distance away in the Solids Conditioning Building. An extensive amount of steam and condensate piping is routed through tunnels. These piping systems are old, corroding, and will require costly replacement. Additionally, the buried air piping from the blowers to the aeration tanks has significant leaks resulting in substantial air losses. The linkage between secondary treatment aeration and solids handling waste heat production creates a vulnerability for Central San. Disruptions in either system can have ripple effects across both the solids and secondary treatment systems, in addition to creating air emission compliance issues by triggering boiler bypass events. Decoupling these two systems can provide a significant improvement in operational resiliency. The waste heat steam system cannot supply enough energy for the blowers during peak aeration demands (hot weather and weekends) while still maintaining redundancy in the system. An additional auxiliary boiler is required to meet redundancy criteria. The anticipated future higher aeration demand for potential nutrient removal facilities required larger blowers, thus the existing steam blowers would have to be replaced with the larger steam blowers or with electrical blowers. Because of hydraulics issues, the existing two steam-driven blowers and the one back-up electric blower cannot be operated together, which limits capacity and operational flexibility. Central San s single back-up electric blower is not sized adequately for typical daily aeration demands. It is currently used as a temporary back-up when maintenance is required on the steam system or steam-driven blowers. An alternative considered was to use heat recovered from the incineration and co-generation facilities to generate electricity instead of the steam. Generated electricity could be used anywhere at the plant and in the case of solids treatment disruptions, electricity could be purchased from the grid or produced from on-site backup generators. Numerous incinerator facilities in the United States and Europe have investigated and constructed energy recovery for their multiple-hearth or fluidized bed incineration systems. They all convert heat to electricity using steam turbines associated with waste heat boilers or more recently using Organic Rankine Cycle (ORC) systems. Since the ORC systems are easier to operate and maintain and are typically more efficient compared to steam turbines, the ORC was chosen for further evaluation. ORC technology is similar to the steam turbines used at Central San, but instead of using water vapor, the ORC turbine uses vaporized high-molecular-mass organic fluid. Flue gas/waste heat from the incinerators and waste heat from the cogeneration facility will be sent to a thermal oil heat exchanger to recover energy as heated thermal oil. Heated thermal oil will then be sent to the ORC turbine to generate electricity that is connected to the plant electricity grid system. Two alternatives were developed to meet the needs of the aeration system: Continue with steam-driven aeration: This alternative would entail replacing the steam-turbine driven blowers with similar steam-driven blowers, but sized efficiently to meet the necessary 660

12 turn-down requirements. Several related improvements would be required for this approach: the air piping would need to be replaced to eliminate the leaks in the current system, the steam and condensate piping would need to be replaced because it is old and corroded, and the auxiliary boiler and waste heat boiler would need to be maintained and replaced as they age. An additional auxiliary boiler and an additional electric blower would also be required to meet redundancy requirements. Convert to electric-driven blowers: This alternative is to replace the existing steam-driven blowers and back-up electric blower with multiple small electric-driven, high-speed turbine blowers, housed in a new aeration building with new above-grade aeration air piping. Converting to electric, high-speed turbine blowers can provide several benefits: they are more efficient, have the capability to operate over the required range of air flows, and their operation would be independent of the steam system. The auxiliary boiler and waste heat boilers would be replaced with a thermal oil heat exchanger and ORC turbine designed to convert waste-heat from the furnaces into electricity for the plant. A lifecycle cost evaluation was completed to compare the alternatives. The projected aeration demands and energy consumption were used to develop the lifecycle cost estimates. Table 1 presents the results of the comparison. As shown in the results, the capital costs for the conversion to electrical blowers is higher; however, the reduction in annual O&M costs makes the lifecycle cost comparable to the steam-driven blowers. The electric blowers option was selected because the lifecycle cost was comparable and provided several benefits to treatment plant operations and was overall a more reliable alternative. Table 1. Net present value comparison of steam powered bowers vs. electric blowers. Cost New Steam Powered Blowers Electric Blowers Total Project Cost $32,832,200 $37,608,400 Total Annual O&M Cost (dollars annually) $2,691,600 $2,431,550 Net Present Value 1 $49,300,000 $47,600, NPV is calculated as sum of the total project cost and total O&M costs throughout the 20 years Alternatives Selection The alternatives evaluation process was conducted through a series of workshops with Central San staff and consulting teams. The first workshop targeted evaluation of the universe of alternatives shown in Figure 7, which is the broad field of possible solids stream, and energy process alternatives currently available in the industry. The following pass/fail criteria were used for the initial screening: Is the technology proven/scalable to meet liquid, solids, and air regulations? 661

13 Does the technology fit within site constraints? Does the technology maximize use of existing facilities? Figure 7. Universe of Solids Alternatives The screening criteria were applied to all alternatives and the remaining viable solids stabilization technology alternatives were carried forward for final screening and alternative development and evaluation. Assuming future solids disposal regulations will limit disposal options, only the proven technologies that result in production of Class A biosolids or ash were considered. In addition, the technologies that allow energy recovery were preferred. All emerging and embryonic hightemperature oxidation solutions were also deferred from the alternatives analysis due to the lack of proven performance. However, over the next several years, Central San will be exploring the feasibility of some of these technologies (e.g. Gasification, Hydrothermal Liquefaction, Supercritical Water Oxidation, etc.). The following alternatives for long-term planning were carried forward for the master planning evaluation: Alternative 1: Replacement of the MHFs with new fluidized bed incinerators (FBIs). Alternative 2: Mesophilic anaerobic digestion combined with a new dryer and one FBI in place of the MHFs. Alternative 3: Enhanced Anaerobic Digestion (thermal hydrolysis followed by anaerobic digestion). 662

14 In addition, a baseline alternative where Central San continues to use the existing MHFs was included in the evaluation. Due to concerns regarding air pollution control requirements, investment into the existing furnaces, and the age of the furnaces, long-term use of the MHFs was not considered to be a viable alternative. Central San agreed that the existing MHFs should be used for their remaining useful life and then the selected alternative in the Master Plan should be implemented. Unless an emerging technology shows promise and is determined to be more optimal than the selected alternative, the selected alternative will serve as the basis for MHF replacement whenever MHF replacement is required either at the end of its useful life or if the MHFs were to experience catastrophic failure. Final Alternatives Evaluation Following assumption were used for alternatives evaluation: For comparing operational costs among alternatives, it was assumed that the future electricity requirement for the plant would be 4,220 kw. Actual electricity requirement for the plant may be different due to variety of liquid treatment options that could be implemented in the future for nutrient removal and/or recycled water. All incineration alternatives assume that the heat recovery to generate steam currently practiced at Central San is replaced with a thermal oil heat exchanger and ORC turbine that generates electricity, as discussed previously. Provide an approach that minimizes greenhouse gas (GHG) production, maximizes cogeneration (to the GHG cap limit of 25,000 metric tons of CO2 equivalents), and prioritize the use of fuel source in the following order: biogas, landfill gas, and then natural gas. The remaining electrical demand was assumed to be satisfied by imported grid electricity. A separate evaluation was included to determine the feasibility of implementing solar and/or wind renewable power production to offset imported grid and natural gas usage. Table 2 shows flows and loads that were used as a base for alternatives evaluation. Table 2. Central San Projected Solids Flows and Loads Parameters 2035 Average Annual Years and Load Conditions 2035 Max Month 2035 Max Week 2035 Peak Day Primary Solids Load, (kg/d) 33,300 30,800 36,450 62,000 Primary Solids %VS 84% 84% 84% 84% WAS Solids Load (kg/d) 29,100 37,000 38,900 42,000 WAS Solids %VS 77% 77% 77% 77% Total Solids (kg/d) 62,400 67,800 75, ,

15 Alternative 1 (A1): Replacement of the MHFs with new fluidized bed incinerator (FBIs) Alternative 1 is the replacement of the existing MHFs with two FBIs. The end product from this alternative is ash produced at an average annual rate of 17,240 wet kg/day (19 wet tons/day). The process flow diagram for this alternative is shown in Figure 8. The following describes Alternative 1: The existing DAFTs would continue to thicken the WAS. Consistent with current operations, thickened WAS and primary solids would be blended and fed to centrifuge dewatering prior to incineration. Improvements to the solids blend tank and mixing system are required to reduce peak loadings to the incinerators and maintained a well-mixed solids feed. These improvements are necessary in the near-term for the current MHFs, which would continue to be operated while new incinerators are constructed. The use of new centrifuges would result in improved dewatering, which would result in a drier feed solids and reduce (or potentially eliminate) the required auxiliary fuel for incineration. In the future, two new FBIs would replace the existing MHFs. Two FBIs (one duty, one standby) are included to provide year-round solids stabilization, allowing one FBI to be out of service for routine and major maintenance similar to the current MHF arrangement. The evaluation assumed that both FBIs would be located inside the existing Solids Control Building. During pre-design, the construction of a new building could be further evaluated. Waste heat from the existing incinerators and future incinerators will be sent to the thermal oil heat exchanger and ORC. Cogeneration would operate with a combination of conditioned landfill gas and natural gas up to the cap and trade threshold. Alternative 2 (A2): Mesophilic anaerobic digestion combined with a new dryer and FBI in place of the MHFs Alternative 2 includes anaerobic digestion, thermal drying, and incineration. The end product of Alternative 2 is ash. The process flow diagram is shown in Figure 9. The following describes Alternative 2: As with Alternative 1, an improved or new sludge blend tank, new centrifuges, and an ORC conversion would be included. The anaerobic digesters are designed to digest primary sludge and WAS, and the existing MHFs would be phased out during the next 20 years and replaced with a single FBI when the existing MHFs are demolished. The digesters would be sized to operate as a stand-alone Class B biosolids process when the FBI is offline for maintenance or in emergencies. Increasing the digester capacity to achieve Class B treatment provided similar maintenance redundancy as included in Alternative

16 With the conversion of volatile organics to biogas in the digesters, the FBI would likely require additional fuel as compared to processing solids with an FBI only. To reduce the need for auxiliary fuel, a dryer using a portion of the heat recovered from the incinerator would be used to partially dry the dewatered digested solids prior to incineration. This alternative provides the opportunity to add fats, oils, and grease (FOG) and high strength waste (HSW) receiving facilities to augment biogas production in the anaerobic digesters. Cogeneration would operate with a combination of conditioned biogas, conditioned landfill gas, and natural gas up to the cap and trade threshold. Alternative 3 (A3): Enhanced anaerobic digestion (thermal hydrolysis followed by mesophilic anaerobic digestion) This alternative is based on producing Class A biosolids for beneficial reuse (fertilizer blending and land application). The purpose for treating to Class A biosolids standards in this alternative was to address concerns regarding California organics diversion goals from landfills and the impact on the feasibility and cost of disposing of Class B biosolids. The anaerobic digesters with thermal hydrolysis would be constructed when the MHFs have reached the end of their useful life. The process flow diagram is shown in Figure 10. The following describes Alternative 3: Mesophilic anaerobic digesters would be configured with thermal hydrolysis pretreatment. This process reduces the number of digesters required and has been shown to be cost competitive with conventional digestion. It is noted that other Class A treatment trains are also viable. This configuration was selected to provide a reasonable and comparable alternative. If this alternative were to be selected, the project s predesign phase would evaluate the appropriate Class A treatment train to meet Central San s goals. To provide the necessary pretreatment upstream of the THP units, this alternative relies on WAS co-thickening with Primary solids in three DAFTs, TWAS screening, and centrifuge dewatering. This alternative provides the opportunity to add fats, oils, and grease (FOG) and high strength waste (HSW) receiving facilities to augment biogas production in the digesters. Cogeneration would operate with a combination of conditioned biogas, conditioned landfill gas, and natural gas up to the cap and trade threshold. Dewatered Class A biosolids would be hauled off site and beneficially reused. 665

17 Figure 8. Alternative 1 Process Flow Diagram 666

18 Figure 9. Alternative 2 Process Flow Diagram 667

19 Figure 10. Alternative 3 Process Flow Diagram 668

20 Triple Bottom Line Plus Evaluation of Alternatives To select the preferred alternative, each alternative was evaluated using the triple bottom line plus method (TBL+). The triple bottom line plus method takes technical, financial, social, and environmental factors into consideration. The following summarizes the basis of the TBL+ scoring for each of the criteria. Technical Technical Objective #1 (T1) - The following reliability and performance criteria were used to evaluate each alternative: Reliability/Redundancy: A2 was the highest scored, due to the ability to have two different means of off-hauling stabilized solids from the plant either incineration with ash reuse or Class B biosolids disposal. Proven technology: All alternatives equal scoring for this criterion. Process stability: A1 was scored higher than A2 and A3 because it did not include anaerobic digestion, which is a biological process that does carry some additional process stability risk. Technical Objective #2 (T2) - The following efficiency criteria were used to evaluate each alternative: Financial Minimize expendables and number of equipment units: A1 includes the fewest number of units and received the highest score, A2 includes the second fewest, and A3 includes the most processes and received the lowest score. Ease of operation: A1 was scored the highest because this process is most similar to the existing incineration process. A2 adds digestion and biogas processes. A3 is an entirely new process and includes greater operational complexity than the other alternatives. Efficient use of space: A3 was scored lower than A1 and A2 due to the need for additional process space. Flexibility to meet future regulations and innovation: A1 and A2 were scored higher than A3, due to less control over potential biosolids regulations that may heavily impact A3. A3 final product is digested and dewatered cake, thus it is relying on the year-round land application of biosolids. Financial Objective #1 (F1) - Minimize Capital Costs: 669

21 Capital cost: All alternatives were scored equally because the planning level capital cost estimates are relatively similar for the three alternatives. Financial Objective #2 (F2) - Minimize Operations & Maintenance Costs: Social Annual O&M costs: All alternatives were scored equally because the planning level O&M costs are similar for the three alternatives. Social Objective #1 (S1) - The following criteria for protecting public health and safety were used to evaluate each alternative: Comply with regulatory requirements: All alternatives provide equivalent ability to meet this criteria. Resiliency for catastrophic events: A2 scored the highest because it includes two potential end products (ash or Class B biosolids) providing more resiliency in case of emergencies. Social Objective #2 (S2) - The following criteria for maintaining good public relations were used to evaluate each alternative: Limit odors to within fence line: A3 scored the lowest because it includes an additional solids process facility which would require additional odor control. Minimize noise/visual impacts: All alternatives provide equivalent ability to meet this criteria. Environmental Environmental Objective #1 (E1) - The following criteria for minimizing impact on the local environment were used to evaluate each alternative: Recycled Water Production: not applicable to the solids evaluation. Minimize impacts from sludge/biosolids: A3 scored lower than A1 and A2 due to the significantly increased number of truck trips required for biosolids disposal. Environmental Objective #2 (E2) - The following criteria for minimizing impact on the global environment were used to evaluate each alternative: Minimize Greenhouse gas (GHG) emissions: Estimated GHG emissions for each solids alternative are included in Appendix B. A3 scored the highest, followed by A2, and then A1. 670

22 Minimize energy use/maximize energy self-sufficiency: A3 scored the highest, followed by A2, and then A1. The results of alternatives comparison are shown in Figure 11. Alternative 2 scored the highest and was recommended for implementation. Alternatives Comparison in Terms of Energy Recovery The energy profile was analyzed to assess the efficiency of each alternative in recovering and beneficially reusing energy at the plant. The energy profile was evaluated for the following conditions: Current Operations: This represents the current facilities at Central San, including the existing MHFs with the current steam energy recovery system. This scenario caps the existing MHFs at a feed of 54.8 dptd, consistent with the existing Title V air permit. Baseline: The baseline condition is the current operation with the implementation of the aeration system upgrade, including removal of the steam system and addition of the thermal oil heat exchanger and ORC turbine generator. This scenario caps the existing MHFs at a feed of 54.8 dptd, consistent with the existing Title V permit, and sends projected excess loading of 11.9 dtpd to a solids handling demonstration facility. Alternative 1 - Replacement of the MHFs with new fluidized bed incinerator (FBIs) Alternative 2 - Mesophilic anaerobic digestion combined with a new dryer and FBI in place of the MHFs Alternative 3 - Enhanced anaerobic digestion (thermal hydrolysis followed by mesophilic anaerobic digestion) The sources of energy include the solids, landfill gas, natural gas, and PG&E grid power. Currently, energy produced from the solids feed is converted to heat, steam, and electricity. Energy profiles were prepared for each solids alternative to develop the design criteria for the energy facilities. The energy profiles were estimated considering the following: Projected energy demand is based on operating the liquid treatment train with nutrient removal. The efficiency calculation for current conditions is based on an annual feed rate to the MHFs of 54.8 dtpd (limited by incineration Title V permitted capacity), while the estimate for Alternatives A1 through A3 was for a feed rate of 60 dtpd (actual projected feed rate). The steam-driven blowers are replaced be electric blowers. Electric power would be generated using an ORC that recovers waste heat from the incinerators. 671

23 In all cases, the air emissions from the MHFs were held below 25,000 metric tons of CO2 equivalents (the GHG cap). It was assumed that electricity would be preferably generated by the cogeneration system rather than purchased from PG&E. The existing gas turbine will be replaced with a new 4.4 MW, gas turbine, which will be fueled with natural gas. In the case of Alternative 2, the turbine needs additional landfill gas, natural gas, or biogas to run above minimum loading to the turbine. All alternatives were assumed to consume landfill gas at the rate of 260 MMBTU/day. If landfill gas supply diminishes, it would be replaced by biogas, natural gas, or electricity, depending on the configuration of the plant at the time of the reduced supply. Figure 12 shows the solids conversion process and resulting efficiencies for the five conditions. E2 E1 E2 E2 S2 E1 E1 S1 S2 S2 E2 E1 F2 F2 S1 F2 F1 F1 F2 F1 S1 S1 S2 T2 T2 T2 F1 T2 T1 T1 T1 T1 Ideal Alternative Alt 1 - FBIs Alt 2 - Digestion + Dryer + FBI Alt 3 - Digestion Only T1 Provide Reliability and Performance F1 Capital Costs S1 Protect Public Health and Safety E1 Impact on Local Environment T2 Efficiency and Flexibility F2 O&M Costs. S2 Maintain Good Public Relations E2 Impact on Global Environment Figure 11 Cumulative criteria scoring summary solids alternatives 672

24 Figure 12 Solids conversion efficiency for each solids alternative. The efficiency of the conversion depends on the solids processes used: Current incineration process utilizes the energy contained in the solids to produce waste heat, then steam via boilers, and then energy via the steam turbine. This process is 5 percent efficient in converting the energy in sludge to energy for plant usage. The Baseline and Alternative 1 also utilize incineration to generate waste heat from the solids, which is converted to electricity using the thermal oil heat exchanger and ORC turbine. These alternatives improve the solids energy conversion efficiency from 5 percent to up to 9 percent. Alternative 2 utilizes the anaerobic digestion of solids to generate biogas and waste heat from incineration, which are converted to electricity. This alternative raises the conversion efficiency to 17 percent due to the generation of biogas and the higher efficiency of the newer cogeneration turbine that would be required. Alternative 3 utilizes anaerobic digestion of solids to generate biogas, which is converted to heat and electricity. This stand-alone digestion alternative has a 20 percent conversion efficiency and is the most efficient because it maximizes biogas production and does not require supplemental fuel like the incinerators would. Energy Recovery Comparison between Alternatives Figure 13 presents the energy production in terms of the energy value (BTUs) produced from each energy source (solids, landfill gas, natural gas, and imported grid power). Figure 14 presents the electricity produced by each energy source under each alternative, to meet an estimated 9.9 MW of total electricity demand for the future plant (all future solids and liquid upgrades included). The total demand of 9.9 MW is comprised of 4.2 MW for 673

25 wastewater processing and 5.7 MW for producing 20 mgd of recycled water through membrane and reverse osmosis processes. The following conclusions were drawn from the evaluation: Baseline Conditions: Several improvements can improve the overall efficiency of the existing MHF process such as the conversion of the waste heat recovery and steam systems with steam-driven blowers to a new electric blower system and waste heat to electricity using an ORC turbine. Additionally, more efficient cogeneration turbines can reduce the amount natural gas consumed for cogeneration. Lastly, a drier MHF feed cake would help to reduce the supplemental landfill gas required for the MHFs. Alternative 1: Alternative 1 benefits from the ability of the FBIs to process higher solids loads with less supplemental fuel, producing more energy. The reduced landfill gas amount can be conditioned and use in the cogeneration turbine and reduce natural gas combustion that produces anthropogenic greenhouse gas emissions. Compared to Alternatives 2 and 3, Alternative 1 produces the least electricity from solids and requires the most natural gas to generate electricity. Alternative 2: Alternative 2 generates more electrical energy from the same amount of solids as A1 due to the addition of anaerobic digestion upstream of incineration. Digestion plus incineration is more efficient at energy recovery than incineration alone. It requires less natural gas than Alternative 1 but requires more purchased electricity. Alternative 2 production and use of biogas results in more electricity derived from the solids and a reduce demand for imported natural gas compared to Alternative 1. It requires some additional grid electricity because the biogas and natural gas already maximize use of the cogeneration turbine capacity. Alternative 3: Alternative 3 produces the most electricity from solids due to the higher production of biogas with thermal hydrolysis pretreatment included. This alternative also requires the most imported electricity because the conditioned biogas and natural gas maximize use of the available cogeneration turbine capacity. This alternative uses the least amount of natural gas but it requires the most imported electricity. Alternative 3 does not have an ORC; therefore, the total potential for power production is lower than other alternatives. It is noted that since Alternatives 2 and 3 are below the greenhouse gas cap, they could be configured to include an additional turbine that could combust natural gas while remaining under the GHG cap. 674

26 Figure 13 Comparison of solids alternatives by energy source Figure 14 Electrical production by solids alternatives to meet electrical demand 675

27 Additional Energy Sources Central San would like to strive towards net zero energy. As described above, all alternatives would require the continued import of grid electricity to meet the plant s energy needs and some use of natural gas. Renewable energy potential (wind and solar) were evaluated to reduce the Central San s reliance on imported grid electricity and natural gas. Additionally, alternatives including anaerobic digestions were evaluated for accepting imported high strength wastes for co-digestion. Figure 15 shows how each alternative could incorporate additional renewable energy sources to closer approach net zero energy. The following are assumptions used for this evaluation: High-strength waste is calculated based on 20,000 gal per day of fats oil and grease (4.5% TS) imported to the plant. This amount was determined to be feasible given the service area. The maximum solar power potential was calculated for the installation of solar panels on all available Central San land and wet weather basins of approximately 130 acres. The peak solar production of this size of facility would be approximately 12 MW. The maximum wind power potential was calculated for the installation of wind turbines on all available Central San land of approximately 130 acres. The peak wind production of this size of facility would be approximately 1.9 MW. Figure 15. Electricity produced by source for additional energy alternatives 676

28 Summary of Energy Alternatives The following summarizes the energy evaluation findings: Central San cannot reach energy neutrality using incineration alone alternatives (Baseline and Alternative 1). Stand-alone anaerobic digestion (Alternative 3) offers the highest overall energy conversion efficiency. The second most efficient configuration is anaerobic digestion followed by fluidized bed incineration (Alternative 2). Anaerobic digesters play a key role in reducing GHG emissions by allowing imported carbon energy source (high strength waste). Net zero energy can be achieved for treatment process energy demands (excluding potential future recycled water power demands) if a high strength waste facility were installed with Alternatives 2 and 3 and sufficient high strength waste is available. With co-digestion and solar, net zero energy can be achieved for treatment process energy demands and up to 5 mgd of potential future high quality recycled water energy demands. Net zero energy for all future projected energy demands does not appear to be feasible. Central San will need to weigh recycled water opportunities and goals with net zero energy goals to determine an optimal balance. Further investigation and tracking of net metering rules and tariffs is important to understand how much renewable power can be wheeled across the grid and to confirm the payback for renewable energy production facilities. For large renewable power installations over 1MW, Central San would need to explore other opportunities with the electrical utility and possibly consider battery storage in order to maximize the use of renewable power being generated. CONCLUSIONS A universe of technologies for solids processing at Central San were evaluated. The broad list of alternatives was then narrowed to an optimal alternative using a triple bottom line plus evaluation. The future recommended facilities include: Conversion of the waste heat recovery and steam system to thermal oil heat exchangers followed by an Organic Rankine Cycle (ORC) turbine to generate power from waste heat. Electricity generated from the ORC turbine will be used to power new, more efficient electric blowers. Anaerobic digesters will be added upstream of incineration to improve the resiliency of the solids processing system and provide a means to recover energy from biogas. The digesters will be designed to process solids and other high strength wastes, such as FOG 677

29 or food waste, to create more biogas. Digested biosolids will be dried and then incinerated. As the MHFs near the end of their useful life (within approximately 20 years), they will be replaced with a single fluidized bed incinerator (FBI). The FBI is a more advanced incineration process that produces fewer emissions than the MHFs and requires less fuel augmentation for combustion. The upstream digesters will be designed to operate independently from the FBI to allow for scheduled FBI shutdowns or to be operated for Class B biosolids for emergencies. This arrangement avoids the need for a redundant FBI train. Energy improvements needed to strive towards net zero energy include: A 20,000 gal/d high strength waste facility to augment biogas production from the anaerobic digesters. A new, larger and more efficient cogeneration gas turbine will be installed to generate more electricity from the increased biogas production. Monitor energy incentive programs and re-evaluate net metering rules and tariffs for renewable energy. In anticipation of the solar energy facilities that would be required to offset future power demands, Central San will reserve land for potential solar and wind projects. 678