Polo, Christine

Christine has a BS and MS in Environmental Engineering and 15 years of experience supporting wastewater agencies in evaluating options for...

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About Polo, Christine

Christine has a BS and MS in Environmental Engineering and 15 years of experience supporting wastewater agencies in evaluating options for beneficial use of biosolids and biogas. She currently works at Carollo Engineers in the San Francisco Bay Area.

Polo, Christine

Titles from this speaker

Addressing Practical Barriers to Large-Scale Co-Digestion to Improve Feasibility

Abstract

APPLICABILITY The desire to reduce greenhouse gas emissions related to food waste disposal in landfills has driven great interest in co-digestion as one way to avoid methane emissions (see Figure 1). Utilities and regulators in New York, California, and other states with climate change mitigation goals understand the promise that co-digestion holds and the opportunity to leverage excess digester capacity in furtherance of those goals (see Figure 2). However, large-scale implementation of food waste co-digestion has been hampered by practical barriers that must be addressed before the process can achieve its promise and deliver on its benefits. DEMONSTRATED RESULTS AND OUTCOMES Our identification and understanding of both benefits and barriers has grown through several co-digestion projects including: - California Co-Digestion Capacity Analysis completed in 2019 for the State Water Resources Control Board - Characterization and Contamination Testing of Source Separated Organic Feedstocks and Slurries for Co-digestion at Resource Recovery Facilities completed in 2023 for the Water Research Foundation - Food Waste Pre-Processing and Co-Digestion Evaluation completed in 2023 for the City of Oxnard, California The benefits of large-scale food waste co-digestion paired with beneficial use of biogas include GHG emissions reduction from landfills, offsets of fossil-fuel based natural gas, reduction of purchased electrical power and/or natural gas for water resource recovery facilities (WRRFs), and potential revenue generation at WRRFs through tipping fees. The environmental benefits just in California exemplify the promise of these processes. If the state were to successfully implement co-digestion and biogas utilization for 35-50% of its 6.8 million wet tons of food waste projected for disposal in 2030, it could reduce landfill emissions by approximately 2 million metric tons of carbon dioxide equivalent (MT CO2e) - half of its legislated reduction goal of 4 million MT CO2e in 2030. The scale of these benefits drove the passage of California's Senate Bill (SB) 1383, which legislated organics diversion from landfills and went into effect in 2022. However, large scale co-digestion has not yet been realized in California, in large part because of remaining barriers to successful implementation and sustained operations. So what are these barriers and how can they be overcome? The projects listed above included identification of the physical and logistical elements required to implement co-digestion, determination of operational impacts and unintended consequences, environmental justice considerations such as truck traffic and odors, documentation of conflicting regulatory drivers, and estimates of capital and operational costs. Having identified these barriers and determined their scale, we suggest strategies to overcome the obstacles which have hampered large-scale implementation thus far. A few such strategies developed from these and other projects include: - Matching pre-processing technology to current municipal solid waste collection system (i.e. source separated vs combined waste) to avoid added costs for different collection schemes. - Regional facilities (combined or separate pre-processing and digestion facilities) that can distribute the burden of capital and operating costs broadly and offer opportunity to offset some costs through tipping fees charged by the entity operating the associated systems. - Public/private partnerships that reduce capital burden and risk for public utilities. - Collaboration with disparate regulatory agencies to develop cohesive strategies towards common goals, especially related to production and use of biomethane and biosolids. - Bench scale studies with expected feedstock to better assess and mitigate operational impacts, including biomethane production, nutrient loads in recycle streams, foaming potential, and dewaterability of digested sludge. (See Figure 3 for foaming potential and volume expansion in digesters with varying amounts of food waste.) We will present eight ways to implement these strategies to address practical barriers for consideration by utilities and regulators as they seek to facilitate co-digestion to meet their climate change mitigation objectives . By incorporating such guidance, utilities can incorporate sustainable co-digestion into their WRRFs as the Los Angeles County Sanitation Districts (LACSD) has done through it's regional food waste processing system. RELEVANCE TO AUDIENCE This presentation will identify practical barriers to large-scale implementation of co-digestion and offer strategies to overcoming these obstacles such that co-digestion becomes more feasible in the short-term and remains viable in the long-term.

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

SpeakerGupta, Rashi

Presentation time

14:10:00

14:30:00

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)

Gupta, Rashi, Polo, Christine, Charbonnet, Elizabeth

Author(s)R. Gupta1, C. Polo1, E. Charbonnet1

Author affiliation(s)Carollo Engineers, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159814

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count10

Description: City of Grand Junction and Mesa County's Collaboration to Compost Biosolids and Food...

City of Grand Junction and Mesa County's Collaboration to Compost Biosolids and Food Waste - Pilot and Feasibility Study

Abstract

APPLICABILITY Biosolids management often includes incineration, landfilling, or limited land application. However, biosolids contain high quality nutrients essential to plant growth and including them as a feedstock in the composting process can produce a quality compost product with limitless land application possibilities. Mesa County owns and operates a composting facility which primarily composts carbon-rich yard waste; a necessary ingredient for composting biosolids. Mesa County and the City of Grand Junction collaborated on an evaluation of the feasibility of composting biosolids and food waste at the Mesa County Composting Facility. The evaluation included a pilot study and an assessment of current and future feedstocks, capital needs, operational logistics, and the return on investment potential of adding these two feedstocks. The evaluation found that composting biosolids would improve the facility's ability to cover their operational costs through the revenues generated from tipping fees and the sale of a nutrient dense compost product. In addition, diversion of biosolids from the landfill to the compost facility was shown to substantially reduce greenhouse gas (GHG) emissions. As part of the evaluation, a survey of local compost users was conducted to understand the market potential of compost made with biosolids. It was recommended that outreach and education measures be taken to strengthen the local market. DEMONSTRATED RESULTS AND OUTCOMES Evaluation Background and Goals Driven by sustainability goals, facility capacity constraints, and a state led initiative to divert organic materials from landfills, Mesa County (County) and the City of Grand Junction (City) collaborated on an evaluation of composting biosolids and food waste at the Mesa County Composting Facility (the Facility). The Facility currently uses windrow composting to compost yard waste, and is interested in expanding its operation to include food waste and biosolids. The City currently disposes of biosolids at the landfill and is interested in the environmental benefits of composting biosolids. Methodology This evaluation studied the technical and financial feasibility, as well as the environmental benefits and risks, of composting biosolids and food waste at the County Facility. Additionally, the City conducted a biosolids composting pilot at an alternative composting facility to understand the process of composting biosolids. The evaluation included: - A site capacity and feedstock quantity analysis. - A business-case evaluation to determine the tipping fees and compost sale prices required for revenues to cover expenses. - An analysis of the GHG quantities generated when landfilling biosolids versus composting them. - A survey of local compost users. - An assessment of odors and process risks and mitigation tactics. Results For the baseline scenario, the capacity and feedstock quantity analysis determined that there is sufficient capacity and yard waste to compost food waste and biosolids with a Facility expansion (Figure 1). In the worst-case scenario, there would be a yard waste shortage, which could be addressed by expanding collection, purchasing woodchips, or diverting a portion of biosolids to another composting facility. The business-case evaluation included an evaluation of the capital and operating costs (Figure 2) for the expanded Facility over a 20-year life-cycle. Using tipping fees and compost sale prices from nearby composting facilities it was determined that revenues would be enough to cover the capital and operating costs if the Facility was expanded and composted yard waste, biosolids, and food waste. However, if only yard waste and food waste were composted, revenue would decrease substantially, making it difficult for the Facility to recover its costs. Since the Facility needs to expand to accommodate increasing yard waste quantities, this finding is a strong financial driver to co-compost biosolids and food waste. A GHG analysis using the BEAM*2022 model determined that GHG emissions could be reduced by 60 metric tons carbon dioxide equivalents (CO2e) per year by diverting biosolids from the landfill to the Facility. While the business-case and GHG evaluations demonstrated substantial financial and environmental benefits, composting biosolids and food waste poses odor and process risks. To address those risks, the County may consider switching to aerated static piles, gradually introducing food waste and biosolids into its composting program, and covering rows with a biofilter layer. A market survey of local compost users indicated that users are interested in purchasing compost from yard waste and food waste, but there is hesitancy around biosolids. To address this hesitancy, a public outreach campaign should be developed to inform the public about the quality and benefits of biosolids compost. The municipalities may also consider implementing a policy requiring each entity to purchase biosolids compost for use in its parks and on new developments. Outcomes This evaluation determined that it is feasible for the Mesa County Composting Facility to compost food waste and biosolids, given the right market and regulatory drivers, successful management of odor risks, and outreach to develop the local biosolids compost market. RELEVANCE TO AUDIENCE In many states, landfills are considered super emitters of greenhouse gases, and many communities are implementing, or plan to implement, policies requiring diversion of organic wastes from landfills. Driven by potential regulatory requirements, as well as sustainability and GHG emission reduction goals, many agencies are considering composting biosolids and food waste along with their yard waste. This presentation would benefit agencies that currently compost, or are interested in composting, biosolids and food waste. Attendees will benefit from the pilot study by gaining a better understanding of how to compost biosolids, the feedstocks required, and the environmental benefits of composting biosolids. Attendees will benefit from the business-case evaluation by gaining a better understanding of the technical operation and financial aspects of composting these feedstocks. Future composters must consider not only the feedstocks but also the market for the finished product. Attendees will gain an understanding of the measures needed to address potential process and market risks to ensure the successful implementation of this sustainable biosolids management strategy. Additionally, many wastewater treatment facilities and waste management facilities are run by, or in partnership with, a municipality. This study demonstrates the intra-agency partnership that is often needed to meet individual agency goals.

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

SpeakerPolo, Christine

Presentation time

08:30:00

08:50:00

Session time

08:30:00

10:00:00

SessionInnovations in Waste-to-Value Technologies: Carbon Management and Resource Recovery

Session number22

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicAcidogenic fermentation, Anaerobic Digestion, Biogas, business case evaluation, Circular Economy, Compost, Energy and Carbon Neutrality, Nitrogen Removal, Nutrient Removal, Phosphorus recovery, struvite, electrochemical, Phosphorus Removal and Recovery, Resource Recovery

Author(s)

Polo, Christine, Firl, Ashley, Richardson, Jennifer, Hyatt, Leanne, Woods, Sophie

Author(s)C. Polo1, A. Firl2, J. Richardson3, L. Hyatt1, S. Woods1

Author affiliation(s)Carollo Engineers, 1City of Grand Junction, 2Mesa County, 3Carollo Engineers, 1Carollo Engineers, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159754

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count20

Description: Energy Master Plans Showing the Path Forward for Resilient Onsite Power Generation...

Energy Master Plans Showing the Path Forward for Resilient Onsite Power Generation and Biogas to RNG Projects

Abstract

This paper was presented at WEFTEC 2023 in Chicago, IL.

This paper illustrates planning-level analyses that utilities can use to develop the most sustainable and resilient Energy Master Plans using digester gas (DG) and food waste co-digestion to find the most beneficial digester gas use options and integrate renewable energy sources including solar photovoltaics and energy storage. A novel approach using the Blue Plan-itÂ® modeling tool in combination with solar PV (Helioscope) and microgrid modeling (HOMER Grid) is presented.

SpeakerTasser, Christian

Presentation time

13:35:00

13:55:00

Session time

13:30:00

15:00:00

SessionOptimizing Energy Investments Part I: Energy Master Planning

Session locationRoom S501d - Level 5

TopicEnergy Production, Conservation, and Management, Intermediate Level, Sustainability and Climate Change, Utility Management and Leadership

Author(s)

Tasser, Christian

Author(s)C. Tasser 1; B. Gutierrez 1 ; C. Tasser 1; C. Polo 1;

Author affiliation(s)Carollo Engineers, Inc. 1; Carollo Engineers 1 ; Carollo Engineers, Inc. 1; Carollo Engineers, Inc. 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825158999

Volume / Issue

Content sourceWEFTEC

Word count18

Description: Pairing Table-top Analysis with Bench-Scale Testing to Assess Food Waste...

Pairing Table-top Analysis with Bench-Scale Testing to Assess Food Waste Co-digestion Feasibility

Abstract

Introduction Water Resource Recovery Facilities (WRRFs) have increasingly been investigating and implementing co-digestion of food waste to generate revenue and maximize biogas production. In California, an additional regulatory driver is Senate Bill (SB) 1383, which requires organics diversion from landfills to mitigate short-term greenhouse gas emissions. To meet these goals, the City of Oxnard applied for and obtained an EPA grant to study food waste pre-processing and co-digestion implementation at the Oxnard Wastewater Treatment Plant (OWTP). Carollo developed a feasibility study for co-digestion implementation, and Dr. Matt Higgins (Bucknell University) conducted a bench-scale study involving co-digestion, dewatering, and odor testing of the City's sludge and source-separated food waste from the City's Del Norte Materials Recovery Facility (MRF). The results of the bench-scale study were then used to refine the design criteria and operational assumptions used in the feasibility study. This session will cover the approach and key findings of the bench-scale and feasibility studies. The City and the Carollo/Bucknell team hope that this study can serve as an example to other WRRFs interested in implementing food waste co-digestion. Bench Scale Study Three months of samples of primary and waste activated sludge from the OWTP and source separated food waste from the Del Norte MRF were sent to the Bucknell Anaerobic Digestion Laboratory (see photo in Figure 1) at Bucknell University. The food waste was diluted and blended before delivery. Four anaerobic digesters were operated, Digester 1 as a control with no food waste addition, and Digesters 2- 4 at increasing food waste loading rates. The target food waste VS loading rates to each digester expressed as the mass of VS from food waste divided by the total mass of VS in the digester feed (food waste plus feed sludges) were: Digester 2: 25%, Digester 3: 35%, Digester 4: 50%. In addition to evaluating digestion performance, this study also evaluated the impacts of food waste co-digestion on foaming potential, dewatering, and cake odor production. A summary of the findings from the bench-scale study is provided below: -Food waste characteristics: --The food waste slurry TS and VS content averaged 5.8 %TS and 81 %VS, respectively. -Digester performance: --The food waste did not impact the digesters pH, alkalinity, and nitrogen and phosphorus concentrations in any appreciable way at the loading rates in this study, except for the digester upset. --As expected, food waste co-digestion increased the volatile solids reduction (VSR), biogas yield (see Figure 2 for a graph of biogas production for the 4 Digesters), and methane content of the biogas. The overall VSR for the control digester averaged 45.9%, while the VSR for the test digesters was 52.4-59.5%. From this, the VSR for the food waste itself was estimated as 72-77%. --A digester upset in Digester 4 occurred when it was fed a food waste VS loading of 60% relative to the total VS loading. Based on this, a conservative maximum food waste loading for stable digestion of 45% was selected for the feasibility study. This limit allows for significant food waste addition while maintaining digester stability. -Foaming potential: --The viscosity and foaming potential increased as food waste loading increased. --No clear pattern was observed for volume expansion. The lower two loading rates of food waste (Digesters 2 and 3) had a greater volume expansion than the control, while at the highest food waste loading (Digester 4), the volume expansion was less than the control. -Dewaterability: oAs the food waste load increased, the polymer demand increased and the cake solids concentration decreased. The lower TS concentration could have been due to a lower concentration of divalent cations, which was observed in the higher food waste loading digesters. -Cake odor potential: oFood waste addition did not negatively impact cake odor potential. As a result of these bench scale results, the design team was able to refine site specific design and operational criteria for the full-scale co-digestion cost and energy feasibility study, including identifying the maximum design food waste VS ratios for stable digestion and quantifying anticipated changes in biogas production and revenue, polymer demand and costs, dewaterability and biosolids production post co-digestion. These criteria were adopted for the following desktop feasibility study. Feasibility Study The purpose of the feasibility study was to determine the required facilities, costs, and other impacts of food waste co-digestion and whether the benefits from increased biogas production and cogeneration power generation are sufficient to cover the implementation costs of a co-digestion program. A summary of the findings from the feasibility study is provided below: The capacity assessment determined that the OWTP has sufficient capacity in all their processes to handle the increased loads, except when one digester is out of service. In that instance, the City could divert a portion of the food waste to the Agromin composting facility rather than building an additional digester. The capacity assessment also found that, with food waste co-digestion, the OWTP would approximately double its biogas production. -The condition assessment found that the City has already made or plans to make many of the improvements identified in the condition assessment. Most notably, the City plans to refurbish a digester which is currently out of service. -This feasibility study evaluated a wide range of technologies for pre-treatment of source separated food waste to produce a bioslurry suitable for co-digestion. Three alternatives were selected for a more detailed life-cycle cost, site layout, and non-financial evaluation. A hammer-mill at the Del Norte MRF with additional polishing with a sludge screen at the OWTP was the preferred technology selected for this study (see Figure 3 for a process schematic). -The co-digestion program alternative was evaluated against a baseline alternative, whereby the City continues their current operations. The City's food waste is currently managed by a third party which composts it at the Agromin composting facility. -The hourly energy analysis allowed the City to accurately estimate energy savings from increased biogas production from food waste co-digestion by taking into account fluctuations in biogas production as well as varying electricity usage charges during the winter and summer and during on-peak and off-peak hours. -The energy analysis found that food waste co-digestion would increase cogeneration power generation by 79 percent compared to the baseline alternative. However, the net plant power usage for the co-digestion alternative is only 23 percent lower than the baseline when accounting for additional power loads for food waste processing. -The results from the energy analysis were fed into the cost/benefit analysis which consisted of a 20-year life-cycle cost analysis including capital and O&M costs to compare the baseline and co-digestion alternatives (see Figure 4 for cost results). The analyses found that based on the unit cost assumptions made, the co-digestion alternative is more costly both in capital and O&M costs compared to the baseline, even when accounting for the increased biogas production and cogeneration power generation of the co-digestion alternative. -The analysis identified that one of the major O&M costs is the cost for food waste slurry hauling from the MRF to the OWTP. A sensitivity analysis indicated that the hauling cost can greatly impact the affordability of food waste co-digestion compared to the baseline scenario. In conclusion, this bench scale study and feasibility study found that, while the City of Oxnard has sufficient capacity for food waste co-digestion, the co-digestion alternative is more costly compared to the status quo of continued composting of the City's food waste. These results are likely due to the fact that the biogas is used for cogeneration and to the high hauling costs assumed in this study. While the results are specific to the City of Oxnard, the approach taken in this project of combining feasibility studies with bench-scale studies can serve as an example for other WRRFs. This project demonstrated how WRRFs interested in implementing food waste co-digestion can use cost-effective bench scale studies to gain confidence in the techno-economic evaluation of full-scale food waste co-digestion.

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

SpeakerPolo, Christine

Presentation time

8:30:00

9:00:00

Session time

8:30:00

11:45:00

SessionSession 11: Circular Water Economy

Session number11

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicSustainability and Resource Recovery

Author(s)

C. Polo

Author(s)C. Polo1, J. Hauser2, M. Higgins3, R. Gupta4, T. Rauch-Williams5, C. Tasser6,

Author affiliation(s)Carollo Engineers1; City of Oxnard2; Bucknell University3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2023

DOI10.2175/193864718825158784

Volume / Issue

Content sourceResiduals and Biosolids

Word count13

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Polo, Christine

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