Rajagopalan, Ganesh

Ganesh Rajagopalan is a National Resource Recovery Practice Lead with AECOM, based in California.

Titles from this speaker

Description: Alternate: Evaluating Feasibility of Implementing Co-digestion at San Francisco...

Alternate: Evaluating Feasibility of Implementing Co-digestion at San Francisco International Airport

Abstract

Background: This study was performed to evaluate feasibility of implementing co-digestion at San Francisco airport facility (SFO) in California. The overall objective was to obtain preliminary feasibility information regarding diverting organic wastes from the terminal restaurants to its wastewater treatment plant anaerobic digester to enhance biogas production for beneficial use. The study included preliminary regulatory, technical, economic as well as triple bottom-line evaluation. The key objectives of the study included evaluating i) how much organic waste can be added to the airport's anaerobic digester without impacting the digester stability, ii) can all of this capacity be met with the organic wastes generated at the airport, iii) how much biogas will be generated during co-digestion, iv) what are the potential beneficial uses for the quantity and quality of biogas produced that are in alignment with the airport's sustainability goals as well as current and impending regulations, v) what are the potential impacts (e.g. increase in wet cake, polymer demand, metals in biosolids) of this process, and vi) what is the preliminary cost estimate for this program. Regulatory Review: Regulatory evaluation included review of SFO's existing wastewater treatment plant permit requirements (e.g. effluent discharge, emissions) as well as recent changes to co-digestion related regulations. A review of the permits indicated i) discharge limits for ammonia, organics and select metals, and emissions limit for ammonia, select organics and hydrogen sulfide (H2S). The bench scale studies were designed to partially address impacts of co-digestion on these limits. Among the new/emerging regulations, the 'Advanced Clean Fleets' (ACF; Release #23-13) regulation put forward by California Air Resources Control Board (CARB) and approved by the State in 2023 requires all the heavy and medium duty vehicles to be phased out and replaced with zero emission vehicles (ZEV) within the next 20 years. This will have a significant impact on the use of biogas for vehicle fuel. Further, a proposed CARB amendment to suspend Low Carbon Fuel Standard (LCFS) incentives, if implemented, will further discourage pipeline injection of treated biogas (Renewable Natural Gas (RNG)). The impact of these and other regulations were taken into consideration during evaluation of beneficial use alternatives for biogas. Bench Scale Testing: A bench scale test was performed using the TWAS from the airport full scale sequential batch reactors and the organic waste from the airport restaurants to address various objectives of this study. The organic wastes were collected as mixed waste which was then extracted using a Thor process for addition to bench scale reactors. The wastes and TWAS were characterized for organic and inorganic constituents, including metals concentration. The data indicated that the organic content (COD) and degradable solids (VS) were much higher in the food waste than those for TWAS. The COD of the TWAS and organic wastes were approximately 48,000 and 245,000 mg/L respectively. The total and volatile solids of the TWAS were 4.6 and 82%, respectively and the total and volatile solids of the organic waste were 21 and 88%, respectively. Four 10 Liter bench scale digesters filled with 8 L of TWAS and appropriate amount of food waste was used in the bench scale studies (Figure 1). One of the digesters (Control) received only the TWAS. The other three digesters (R1, R2 and R3) received food waste at 20, 40 and 60% of volatile solids as the TWAS volatile solids content. On a volume basis the food waste added was 4.3, 8.3 and 12.25% of the TWAS volume in the three digesters, respectively. The digesters were operated in the anaerobic digestion lab (ADL) at Bucknell University (Lewisburg, PA) at 37 oF. Various operating parameters and biogas quality and quantity were measured during the four-month study. The bench scale data indicated that the control digester (i.e. TWAS-only) produced approximately 6,000 mls/day of biogas whereas the digester with the highest amount of food waste produced approximately 14,000 mls/day of biogas Figure 2). In addition to total biogas production, the biogas yield, i.e. the cubic feet of biogas produced per pound of volatile solids reduced also were higher for the food waste added digester. The biogas yield of the control and the highest food waste added digesters were 35.5 and 47 cu.ft./lb VS reduced, respectively. These data indicated that, the food waste was digested more readily than the TWAS and produced significantly higher amount of biogas at a much smaller volume added. Addition of food waste increases the solids added to the digester. This, in turn, may increase the amount of dewatered cake generated after digestion. Hence, dewatering studies were conducted using the digested sludge from the bench scale study. The data indicated that the net wet cake mass may decrease by about 7% at the loading used in R1 digester and increase by approximately 8 and 12% at the loading used in the R2 and R3 digester, respectively (Figure 3). Projections for Full Scale Operation: The bench scale study data was then used to project various parameters for the airport's full scale co-digestion operation. The evaluations indicated that nearly 1,900 tons of organic waste is required annually for the co-digestion process at the desired (food waste at 60% VS of TWAS VS) loading rates. The airport generates approximately 4,500 tons of organic waste each year. Hence, enough organic waste is available for co-digestion. Further, sufficient volume is available in the digester to receive the food waste is diluted up to 7% solids content for ease of pumping. This operation is estimated to produce approximately 38 to 45 standard cubic feet per minute (scfm) biogas. Based on bench scale data, addition of this amount of food waste may increase the dewatered cake produced by 1.3 wet tons per day and the polymer demand for dewatering by approximately 19.5 lb/day. A preliminary evaluation of regulatory compliance, limited to the data from the bench scale study, indicated compliance requirements for the tested parameters may be met after food waste addition to the digester. However, more detailed evaluation through a larger scale study and detailed discussions with regulatory agencies may be required during the next phases of the study to ensure compliance with regulatory requirements. Evaluation of End Use Alternatives: The above information/data were used to evaluate beneficial use alternatives for the biogas produced through co-digestion. The alternatives evaluated include i) combustion-based combined heat and power (CHP) generation technologies such as internal combustion engines (IC Engine) and microturbines, ii) non-combustion CHP technologies such as fuel cell systems and linear generation technology, iii) use of biogas for vehicle fuel, and iv) pipeline injection of treated biogas. The airport does not prefer combustion-based technologies due to environmental concerns. Further, the current and impending CARB regulations do not favor the use of biogas for vehicle fuel or pipeline injection in the long term. The non-combustion CHP technologies, while relatively new to the market, have made significant advancements in the recent years. Further, they produce much lower greenhouse gas emissions. Hence, these non-combustion technologies (fuel cell and Linear Generation) are recommended for further evaluation in the next phase for potential installation at the airport. Preliminary Cost Evaluation: A preliminary cost evaluation was performed for the implementation of the proposed program. The airport is currently evaluating its options for food waste collection, sorting and processing. Hence, the evaluation did not include food waste collection, extraction, storage and pumping to the digester. The preliminary cost estimate for the rest of the constituents (CHP, biogas treatment and installation, etc.) is approximately $2.95 M. The increased annual O&M cost is approximately $180,000/year. The estimated revenue from the savings in electricity is approximately $ 270,000/year. Triple Bottom-Line Evaluation: Further, a triple bottom-line cost-benefit analysis (financial, social and environmental) was performed for the proposed technology. The evaluation indicated that in the next twenty years the proposed technology will avoid a total of approximately 19,500 tons of CO2e emissions, mainly from diverted waste and fertilizers offset. It is also estimated that the proposed technology will mitigate emissions of approximately 5,200 pounds of contaminants through avoided natural gas purchases. No avoided emissions due to electricity generation is anticipated since SFO already purchases clean energy from San Francisco Public Utility Commission.

This paper was presented at the WEF Residuals & Biosolids and Innovations in Treatment Technology Joint Conference, May 6-9, 2025.

SpeakerRajagopalan, Ganesh

Presentation time

13:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionConsiderations for Long-Term Biosolids Planning

Session number5

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicAnaerobic co-digestion, Biomethane, Lipids/proteins/carbohydrates ratio, TWAS, Manure, SSO, Biosolids treatment, Circular Economy, Co-Digestion With Other Organic Sources, Market Analysis, Master Planning, PFAS/Emerging Contaminants, Resiliency, Resource Recovery, Sustainability, Triple Bottom Line Plus

Author(s)

Rajagopalan, Ganesh, Higgins, Matthew, Hummel, Michael, Mahoney, John, Cooke, Erin

Author(s)G. Rajagopalan1, M. Higgins2, M. Hummel3, J. Mahoney4, E. Cooke5

Author affiliation(s)AECOM, 1Bucknell University, 2Stok, LLC, 3Tanner Pacific, 4San Francisco International Airport, 5

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159745

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count12

Description: Bench-Scale Study for Optimization of Co-Digestion of Creamery and Poultry Waste for...

Bench-Scale Study for Optimization of Co-Digestion of Creamery and Poultry Waste for City of Petaluma

Abstract

The City of Petaluma is planning to implement a co-digestion program using creamery waste and poultry waste to increase the current TWAS-only biogas production of 30 to 40 scfm to approximately 100 scfm to cost effectively implement a biogas to biofuel (vehicle fuel) program. Approximately 1,500 gpd of creamery waste and 6,600 gpd of poultry waste are available for digester addition. Prior to implementation of the full scale program, the City performed a bench scale study using four 10 L digesters to determine i) if the available creamery and poultry wastes can increase biogas production to target levels, ii) the impact of co-digestion on digester stability and plant operations and iii) the optimum organic waste loading between its two 550,000 gallon digesters to maximize the renewable energy credit (Renewable Identification Number or RIN credit) as well as the overall economics of the program. The bench scale study data indicated that the addition of the organic wastes increased volatile solids reduction from approximately 30 in the TWAS only digester to about 65% in the digester with the creamery and poultry waste addition. The unit biogas production increased by 30 % due to organic waste addition. The data indicated that addition of available organic wastes can result in the production of 125 to 150 scfm biogas. However, such an addition will increase the digester ammonia levels to about 4,500 mg/L and the composite centrate ammonia levels to about 2,300 mg/L. Although the digesters operated without any upsets at such high levels of ammonia, this may potentially cause challenges for the plant to meet its nutrient discharge limits. A loading scenario involving operating one of the digesters with only thickened waste activated sludge (TWAS) and operating the second digester with about 5,000 gpd TWAS and all of the creamery waste and only the DAF waste (~2000 gpd) was recommended. This loading is estimated to produce about 93 scfm biogas, keep the ammonia level at approximately 1,170 mg/L and maximize the Renewable Identification Number (RIN) credit. A preliminary economic evaluation indicated a simple payback period of 10 years for this project.

City of Petaluma is planning to implement a co-digestion program using creamery waste and poultry waste to increase the current TWAS-only biogas production of 30 to 40 scfm to approximately 100 scfm to cost effectively implement a biogas to biofuel (vehicle fuel) program. A bench scale study was performed to identify the quantity of the wastes and distribution to its digester to maximize biogas production, minimize unintended consequences and maximize renewable energy credit. The paper summarizes the results of the study.

SpeakerRajagopalan, Ganesh

Presentation time

14:15:00

14:30:00

Session time

13:30:00

14:30:00

SessionCo-digestion: Advancements and Lessons Learned

Session number108

TopicBiosolids and Residuals, Energy Production, Conservation, and Management, Research and Innovation

Author(s)

Ganesh Rajagopalan

Author(s)G. Rajagopalan1; M.J. Barnes1; M.J. Higgins3; L. Cameli2; M. Pierce2; S. Beightol3;

Author affiliation(s)Kennedy Jenks Consultants, Irvine, CA 1City of Petaluma, Petaluma, CA 2Bucknell University, Lewisburg, PA 3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2021

DOI10.2175/193864718825157982

Volume / Issue

Content sourceWEFTEC

Word count16

Description: Co-digestion of Organic Wastes in Anaerobic Digesters: Potential Benefits,...

Co-digestion of Organic Wastes in Anaerobic Digesters: Potential Benefits, Unintended Consequences and Lessons Learned from Multiple Studies

Abstract

Background Water reclamation facilities (WRFs) have increasingly been investigating and implementing co-digestion of organic materials as a method to generate revenue through tipping fees and to increase biogas (and energy) production. Co-digestion provides an excellent opportunity to divert organic wastes from landfills thereby reducing greenhouse gas (GHG) emissions. Further, co-digestion helps beneficially reuse this organic matter through renewable energy production and facilitates recycling nutrients and organic matter to soils, creating a sustainable reuse cycle. However, successful implementation of co-digestion program requires a clear understanding of organic waste characteristics and pre-processing requirements, digester stability during co-digestion, upstream and downstream process impacts, operational challenges, biogas quantity and quality, permitting requirements and economics. This presentation will share the author's experience from multiple bench scale, pilot and full-scale studies on potential benefits, unintended consequences and lessons learned for implementation of co-digestion program. Methods/Approach The study will present results from studies performed for multiple clients as well as research studies using variety of organic wastes (e.g. food waste, FOG, poultry, creamery, whey wastes). The client projects range from full-scale demonstration of food-waste extraction to bench scale studies using industrial organic wastes. The research projects include those funded by Water Research Foundation to evaluate un-intended consequences of co-digestion as well as evaluation of biogas quality during co-digestion. The presentation will address extraction of organic materials from Municipal Solids Wastes (MSW), comparison of source separated organics (SSO) and MSW extraction approaches for co-digestion, polishing requirements for the MSW extracted organics, digester stability/performance at pre-determined volatile solids loading rates (e.g. pH, volatile solids reduction, COD, alkalinity, foaming potential), biogas quantity and quality at various loading conditions using different organic wastes, dewatering performance of co-digester sludge (e.g polymer demand, percent solids), centrate quality, and dewatered cake characteristics. Further, the presentation will include key lessons learned (operations, regulatory, etc.) from a three-year full-scale food waste extraction and co-digestion demonstration project. Most of the co-digestion studies were performed in mesophilic anaerobic digesters. While the scope of evaluation varied among the studies, in general, a holistic evaluation was performed in these studies to better understand the impact of co-digestion. Digesters without organic wastes (sludge-only) and with pre-determined amounts of organic wastes were operated to selectively identify the impact of organic waste addition. Results Select Results from these studies are present below. Pre-processing of Organic Wastes An organic extrusion process (OREX, Anaergia Corporation) that uses differences in viscosities of organic and inorganic materials was used in the extraction of organic materials from MSW during a full-scale demonstration study at Silicon Valley Clean Water (SVCW) facility, Redwood City, CA. The OREX extracted organics had a solids content of approximately ~35%. The volatile solids content and COD were approximately 83% and 325,000 mg/L, respectively. The soluble COD was approximately 7,000 mg/L. The extracted organics was then polished using a paddle finisher to remove residual impurities (plastic gloves, bags, spoons, etc.) and dilute the pulp to a percent solids content of ~6% to facilitate transfer to the digester. The percent volatile solids and total COD of the polished waste were approximately 88% and 60,000 mg/L, respectively. A preliminary economic evaluation to compare SSO program and MSW extraction of organics indicated that, MSW extraction is more economical for this facility. However, the results can vary at other facilities based on site-specific conditions. Figure 1. (a) Municipal Solid Waste, (b) OREX extracted organic waste and (c) polished organic waste. Organic Waste Loading and Biogas Yield The volatile solids loadings varied from ~0.04 to 0.2 lb VS/cu.ft./day in the studies performed using a variety of organic wastes. The ratio of organic waste to sludge VS loading varied from ~10 to 100%. Results from these studies indicated that, in most cases addition of organic wastes to the digesters not only increased the total biogas production, but it also increased unit biogas production. In most cases, the digesters did not experience any process upsets. The pH did not vary by more than 0.2 to 0.5 units in most cases. The volatile solids to alkalinity ratios were well below 0.1 (a general criteria for digester stability), except in one case using poultry blood waste. However, the high ratio due to the addition of poultry blood waste did not upset the digester performance. Figure 2 shows the biogas yield in two bench scale studies using pre-processed food waste and raw organic waste. Figure 2. Specific methane yield during co-digestion of (a) pre-processed organic waste and (b) crushed raw organic wastes in sludge from two WRRFs. X-axis shows ratio of organic waste VS to sludge VS. Biogas Quality As part of a Water Research Foundation study, biogas quality during co-digestion of variety of organic wastes were evaluated for six different treatment plants Figure 3. Summary of select VOC levels in biogas from sludge and selectively from organic wastes from participating WRRFs In general, co-digestion increased the concentration of VOCs in biogas using some of the organic wastes, lowered the siloxane levels, and increased or decreased ammonia and H2S levels depending on the organic waste. The extent of these changes varied from plant and the type of organic wastes. Dewatering, Cake Odor and Centrate Quality In general, at an organic waste VS to sludge VS ratio of up to 25 to 30 %, a decrease in the net mass of dewatered cake was observed. When the amount of organic waste added increased more than 30%, the mass of dewatered cake increased compared to sludge-only digestion. The polymer demand generally increased during dewatering of co-digested sludge. Headspace total volatile organic sulfur compounds (TVOSCs) levels were measured to identify cake odor production. In general, addition of organic wastes at up to 25% of sludge VS appeared to increase cake odor. At higher organic waste loading the cake odor production significantly decreased compared to sludge-only digestion. In many cases, addition of organic wastes increased the levels of mono-valent salts and ammonia. The bivalent cation levels decreased due to organic waste addition. Lessons Learned Key lessons learned include: -A discussion with local permitting agencies prior to initiation of co-digestion will help tailor the scope and expedite the program -A clear communication with generators of organic wastes is critical to ensure the quantity and quality of organic waste supply -A Technology Impact Analysis to document and implement various process and infrastructure improvements needed prior to implementation of full-scale co-digestion program is required -A hot water system is required to effective pumping of FOG waste and prevent its deposition in grease pits.

This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.

SpeakerRajagopalan, Ganesh

Presentation time

15:45:00

16:15:00

Session time

13:30:00

16:45:00

SessionSession 14: Co-Digestion

Session number14

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicCase Studies/Lessons Learned