Sosa-Hernandez, Ornella

Ornella Sosa‑Hernandez works at Clean Water Services, contributing to wastewater treatment operations and utility initiatives.

Titles from this speaker

Description: Developing A Bench Scale Test To Evaluate Operational Digestion Stability and...

Developing A Bench Scale Test To Evaluate Operational Digestion Stability and Capacity

Abstract

Introduction
Anaerobic digestion is the backbone of the solids stream process for the Rock Creek Water Resources Recovery Facility (WRRF). Despite its importance, operational status is limited to periodic laboratory samples and a small number of online values. The total feed flow and overall gas generation of the five digesters are tracked online while discrete measurements of volatile acids (VA), alkalinity, pH and volatile solids content are performed routinely per digester to satisfy typical operational requirements. The inability to measure individual feed flow rates and track gas production from each digester can slow the identification of failures and the conditions that cause upset events. Figure 1 shows that even though, two digesters are operated under the same loading conditions, instability can occur due to mechanical failures and our ability to recognize them and respond is limited by discrete measurements that might take several days to be completed. To improve knowledge of the digestion process, Clean Water Services (CWS) is working on the development of a bioassay that can help monitor stability which would benefit operations and the Capital Improvement Program, offering guidance to troubleshoot limitations and helping understand maximum loading conditions for co-digestion. Biochemical Methane Potential (BMP) tests are broadly used to determine the anaerobic degradability of substrates and to obtain a qualitative evaluation of process kinetics (Angelidaki et al., 2009). However, limited conclusions are obtained from BMP tests about the stability of continuously operated systems (Koch et al., 2020). The bioassay CWS is developing is a short version of a BMP test and analogous to an assay developed by Conklin et al., in which the maximum aceticlastic methane production rate (Vmax,ac) is measured and used to estimate the Acetate Capacity Number (ACN). The ACN is the ratio of Vmax,ac to a full-scale digester aceticlastic methane production rate, which can be used as an indicator of process upsets caused by VA accumulation and additional capacity of the system to convert organic matter to methane (Conklin et al., 2008). The goal is to develop a bench scale test that is replicable and easily implementable at the CWS facilities to: - Gain insight on digestion performance and health by identifying conditions, such as mechanical failures, that can cause upset events. - Determine capacity limitations and/or requirements for future construction projects. - Evaluate impacts from the co-digestion program. Bioassay Development Initial Test Trial Initially, a bench test was implemented using available resources (shown in Figure 2-a) to measure the additional aceticlastic methanogenic activity. The feasibility of the test and the potential value of the results for operations were evaluated. Each experiment consisted of feeding acetate to one bottle containing freshly collected digestate from an active digester and comparing its gas production rate to that from another bottle containing only digestate. Despite challenges to determine the initial rate of substrate utilization for aceticlastic methanogens, and limited methane measurements, consistent results were obtained during the first trial. The results suggested that the ratio of biogas production from acetate and digestate could correlate to the Volatile Solids Loading Rate (VSLR) of the full-scale digester but that testing conditions, limited data and the number of experiment replicates could mask differences in digester operational conditions.
Refinement of Bioassay Test A commercial system (AMPTS II Light -BPC Instruments) was later purchased to improve the experimental set up and control, to increase the number of replicates, and to facilitate data gathering (Figure 2-b). Initial validation tests are being performed using fresh digestate from the two digesters located at the Durham WRRF which have online measurement of the individual feed and gas production. Replicability and reproducibility of the experimental conditions are being evaluated using the commercial system. Table 1 includes a description of the factors being tested to establish a consistent method. The indicators that are consistently measured are: - Biogas generation per mass of added acetate - Rate of biogas production in an acetate fed bottle (rac) - Rate of biogas production in a bottle containing only digestate (rb) - Ratio of the two measured biogas production rates (rac/rb) The response to different acetate concentrations was evaluated. The results are observed in Figure 3 which help determine the experiment length. A test that can generate indicators in a short time can be valuable for operations to respond to critical conditions. As observed in Figure 3b, full acetate utilization in comparison to the experiment with digestate can be obtained in less than a day when feeding acetate at <25 mM while the rac is measurable within the first hours of the experiment with consistency independent from the acetate concentration. Consistent gas recovery from carbon addition as acetate has been obtained during the tests that are being conducted weekly. As observed in Figure 4, the gas recovered from mass of acetate seems to not be sensitive to the variability or accumulation of acetic acid & VA in the full-scale digester. The background biogas production rate (rb) is used as a surrogate to the current operation of the digester. This is important because individual gas flow measurements are not available at the Rock Creek WRRF. Comparability of the available bioassay results and gas production of the Durham full-scale digester has been obtained (Figure 5). The ratio between the gas production rates (rac/rb) in these early tests has been compared to the digester VSLR. Figure 6 shows a good correlation between these two parameters despite the intrinsic variability of the TVS measurement. The ratio should represent available aceticlastic capacity which would be limited by VS loading conditions or the relative activity of the biomass that may be influenced by other factors apart from loading, such as temperature, detention time, and the variable loading composition. Additional results from the tests described in Table 1 will be provided in the full paper that include the repeatability and reproducibility of the test as well as relationships to detention time and feed characteristics.
Next steps The testing plans for 2022 consist of: - Bench scale tests will be performed at the Rock Creek WRRF and results will be compared to full-scale parameters and to the results obtained from the validation tests performed at the Durham WRRF. - Testing will be continued at the Durham WRRF and extended to Digester 2 to validate current operational benchmarks. - As an additional valuable result from the test, gas recovery from other substrates for co-digestion will be evaluated. The two digesters at Durham WRRF are currently fed FOG and can be used as a model for new substrate evaluation as well as to determine how the recovery obtained in the batch test compares to that obtained in a continuously fed full-scale digester.

Clean Water Services is developing a bioassay that is easily implementable to monitor digestion stability and understand the impact of shifting loading conditions. The bioassay measures the aceticlastic methanogens activity from the full-scale digesters. The test was found to be repeatable and results can be obtained within a day which is important for operations to respond to upset events. The indicators of digester health match well with full-scale digester performance metrics.

SpeakerSosa-Hernandez, Ornella

Presentation time

08:35:00

08:50:00

Session time

08:30:00

10:00:00

TopicIntermediate Level, Biosolids and Residuals, Facility Operations and Maintenance, Laboratory Practices

Author(s)

Sosa-Hernandez, Ornella

Author(s)Ornella Sosa-Hernandez1; Peter Schauer1; Edgar Sancho-Fausto1

Author affiliation(s)Clean Water Services1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158505

Volume / Issue

Content sourceWEFTEC

Word count13

Effect of Fat, Oil, and Grease (FOG) on Digested Sludge Dewaterability

Abstract

Clean Water Services investigated the impact that co-digestion with Fat, Oil and Grease (FOG) has on the dewaterability of digested sludge. During a 4-month evaluation, more FOG was fed to one of the two anaerobic digesters at the Durham AWWTF while fed thickened primary and secondary sludge evenly. The digester feed, FOG stream and digested sludge were characterized by measuring parameters such as proteins, lipids and carbohydrates, orthophosphate, and cations concentrations. Dewaterability was assessed through analysis conducted by Dr. Matthew Higgins at Bucknell University. The results indicated that FOG addition had no negative impact on the digested sludge dewaterability. The polymer demand was more affected by the thickened primary and secondary sludge VS loads than FOG loading and cake solids percentage seem to improve with higher FOG VS load proportion. The presence of charged compounds in the digestate such as phosphate and cations impacted some of the dewatering characteristics.

SpeakerSosa-Hernandez, Ornella

Presentation time

10:50:00

11:10:00

Session time

10:30:00

11:30:00

SessionUnderstanding Constituents of FOG and Impacts on Digestion and Dewatering

Session number312

TopicBiosolids and Residuals, Energy Production, Conservation, and Management, Facility Operations and Maintenance, Research and Innovation

Author(s)

O. Sosa-HernandezP. Schauer

Author(s)O. Sosa-Hernandez1; P. Schauer1;

Author affiliation(s)Clean Water Services, OR, OR1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2020

DOI10.2175/193864718825157901

Volume / Issue

Content sourceWEFTEC

Word count12

Living Capital Planning Tools

Abstract

INTRODUCTION Clean Water Services (CWS) operates four WRRFs that are expanded and improved through a ~$40 million yearly capital budget. The goal of the CWS capital planning approach is to have a level capital outlay year to year. Capital expenditures are reprioritized yearly to achieve this goal and to ensure that processes are expanded only as they reach capacity. Typical master planning approaches are updated only about every five years. A 'living' capital plan allows CWS to adapt to changes in critical assumptions that drive planning timelines and proactively manage capital expenditures. This paper presents the CWS approach to creating and managing a living capital plan and summarizes the benefits. OVERVIEW The first integrated capital planning tool was developed for the Durham WRRF in 2020 with Carollo and Jacobs, the consultants for the most recent Durham master plan project (Figure 1). As part of the ongoing West Basin master plan, work is underway with Carollo to develop the tool for the three interconnected West Basin facilities, which include Rock Creek, Hillsboro and Forest Grove (Figure 1). The initial pieces of the tool are developed in collaboration with consultants but the final product is owned and managed by CWS. The goal of the living capital planning tool is that it's updated every year and should improve and evolve over time. OUTCOMES FLOW AND LOAD PROJECTIONS Flow and load projections are developed from base assumptions in the flow and load spreadsheet. The process of building the spreadsheet provided CWS with a comprehensive understanding of all the critical assumptions used to generate the projections, making it easier to evaluate those assumptions over time. Alternate planning scenarios were also created based on differing growth assumptions for the Durham WRRF, and flow diversion alternatives for the West Basin WRRFs. Historic flows and loads are evaluated annually and compared to projections. Tracking observations against planning numbers makes it evident when revisions are necessary (Figures 2 and 3). While the intention is not to revise the projections every year, the spreadsheet tool and yearly evaluation process is meant to make the effort for revising projections more efficient when needed. PROCESS PERFORMANCE AND DESIGN CRITERIA Design criteria, which set the capacity of the unit processes (e.g. solids loading rate), and process performance criteria, which are assumptions based on operational observations (e.g. percent removal, solids capture, sludge volume index) are also evaluated yearly. The data validation and process capacity spreadsheets gather process performance assumptions and design criteria from the master plan and compare them to historical observations. This comparison determines when unit processes are approaching capacity triggers and identifies when unit processes should be optimized due to declining performance. One example of a process performance parameter (capture in the primary solids thickening process, Figure 4) that drove the capital planning timeline for secondary treatment (Figure 5) is illustrated here. Lower solids thickening capture sends additional solids loads to the secondary process, decreasing its treatment capacity. Just this single assumption changed the secondary capacity trigger year by 4 years (Figure 5). With projects at four WRRFs to phase, improving this 4-year discrepancy in secondary capacity allows capital outlay to be phased more effectively. MASS BALANCE SPREADSHEET VERUS PROCESS SIMULATOR CWS uses both a process simulator and a spreadsheet mass balance model to create solids projections for its West Basin WRRFs. The mass balance spreadsheet can be used to quickly evaluate scenarios that require a lower level of sensitivity and to identify scenarios that could have a major impact to timelines. This simpler, quicker approach is beneficial with the three interconnected West Basin treatment facilities as running a process simulator with all three WRRFs connected is complex and prohibitively slow. However, the process simulator generates more detailed predictions within each individual facility that are beneficial for informing comprehensive planning evaluations. Table 1 compares CWS experiences using these tools. CWS aims to extend both approaches to its East Basin capital planning system to take advantage of their complementary strengths. LESSONS LEARNED The living capital planning approach means that planning is not confined to periodic, consultant-lead master planning efforts. Tracking of flows and loads and other critical assumptions yearly against master planning values has provided valuable insights into when capacity and capital planning shifts are needed. It also daylights areas of conservatism or risk as capacity triggers are reached. Even if a utility is not running models or doing the projections themselves, tracking assumptions against reality is beneficial and achievable for any utility. Other benefits: Having ownership over the data and tools gives CWS the independence to quickly evaluate new scenarios (e.g. impact of a new large industrial user or shifts in population growth). Daylighting and tracking assumptions also aids in documenting the information that drives capital planning decisions. The integrated tool makes capital planning information more accessible to a wider range of CWS stakeholders from executive managers to operators.

This presentation presents the Clean Water Services approach to creating and managing a living capital plan and summarizes the benefits. The goal of our approach is to daylight areas of conservatism and risk as capacity triggers are reached and to routinely review unit process performance and plant influent flows and loads to catch and address deviations as they arise.

SpeakerMenniti, Adrienne

Presentation time

16:30:00

16:50:00

Session time

15:30:00

17:00:00

SessionLeveraging Automation and Analytics for Better Situational Awareness and Optimization: Part II

Session number224

Session locationRoom 350

TopicEnergy Production, Conservation, and Management, Facility Operations and Maintenance, Intelligent Water, Intermediate Level, Nutrients

Author(s)

Menniti, Adrienne, Sosa-Hernandez, Ornella, Barker, Leila, Schauer, Peter

Author(s)A. Menniti1, O. Sosa-Hernandez1, L.K. Barker1, P. Schauer1

Author affiliation(s)1Clean Water Services, OR

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159537

Volume / Issue

Content sourceWEFTEC

Word count5

Navigating a Digester Upset Event Using a Bench-scale Digestion Stability Test

Abstract

INTRODUCTION Anaerobic digester upsets are rare at Clean Water Services (CWS) Water Resources Recovery Facilities (WRRF), but when they occur, they can limit solids treatment and cause costly, plant-wide disruptions. Operations staff work hard to prevent them by regularly monitoring process data, ensuring reliable measurements, and applying conservative standards. Historically, identifying causes of instability has been difficult at the Rock Creek WRRF, which operates five digesters of varying sizes and mixing configurations. Discrete measurements of key parameters - such as Volatile Acids (VA), alkalinity, pH and Volatile Solids (VS)- meet routine needs but take time to complete and lag behind actual process changes. Online measurements are limited to a single feed flow meter and until recently, a single overall gas generation value. Gas monitoring has been increased in response to data collected from tests described in this paper. Limited process information makes preventing and recovering from upsets challenging. CWS operations and research teams work together to analyze available and new information and make data driven decisions that ensure digester stability. DIGESTION STABILITY TEST CWS' research group developed a bench-scale Digestion Stability Test (DST) (Sosa-Hernandez et al., 2022) that has been used to monitor the digesters since 2022 (Sosa-Hernandez & Schauer, 2023) and provide additional information to Operations about digestion health. DST consists of measuring the ability of the microbial community of an active digester to use acetate. The results of DST are: - Rate of biogas production in an acetate fed reactor (rac), is the acetate utilization rate which represents active methanogenic biomass and the digester current loading capacity. rac would change with biomass growth or reduction caused by loading or operational shifts. - Rate of biogas production in a reactor containing only digestate (rb) is comparable to full-scale gas production. rb indicates organic loading changes, and can be used to validate full-scale trends. - Acetate capacity (rac/rb), represents the availability of the microbial community to take in higher organic loads. The hypothesis is that rac/rb would decrease if the hydraulic or organic loading to the digesters increase or if an inhibitory event occurs. DIGESTER UPSET EVENT The digesters are fed sequentially using one pump and control valves. When a digester reaches its set volume, the pump stops, its valve closes and the next valve opens. Due to level differences between digesters, shorter units can experience overloading due to valve leakage. CWS considered installing individual feed flow meters to improve feed control, but maintenance needs and complex installation require further evaluation. This section describes an event caused by equipment failure that was not noted through typical means. DST was critical in providing information to identify potential causes of stress, validate full-scale observations and gas flow measurements, and monitor microbial health during recovery. Figure 1 shows DST parameters and gas productions for active digesters (RC-3, RC-4, RC-5, RC-6), and Figure 2 presents process data during the event, which consisted of the following periods: A)RC-4 gas increase. - Gas system connections and the reliability of individual gas flow meters were questioned due to recent installation and inconsistencies with total gas usage. - DST measurements indicated increased loading to RC-4. Increased rb matched full-scale observations, and increased rac, indicated higher activity or growth of biomass; however acetate capacity was maintained stable at a value of 2, indicating ability to adapt to loading changes and maintain capacity. - RC-4 had been activated within the previous four months, raising concerns about valve placement. Valves and seals were inspected, confirming integrity. - Other process parameters (VA, Alkalinity, VS reduction, pH) showed no abnormalities. Without additional data, operations may not have recognized the issue in time, risking delayed response and potential failure. - Flame arrestors were cleaned and gas meters were calibrated to ensure accuracy of gas values. - RC-4 gas production continued rising, while the gas production of taller digesters RC-5 and RC-6 gradually decreased. B)Valve replacement and actuator issue for RC-4. - The increased gas production prompted valve reinspection, confirming leakage and requiring repair work. The valve was replaced and RC-4 was returned to typical feeding regime. Afterward, RC-4 gas spiked again while other digesters dropped. Incomplete valve closure due to actuator placement issues directed all digester feed to RC-4. - In addition to increased gas flow, operators and laboratory staff noted increased odor before other process parameters showed a response to the overload. Temperature control became erratic with higher solids load. - DST acetate capacity dropped then to 1, which is an indication of overloading or suppressed microbial acetate utilization (Figure 1.B). This correlated to the observed VA accumulation and reduced alkalinity (Figure 2.B). - To prevent complete failure, a reactive response was required after valve actuator correction. Feeding was stopped for RC-4 and sludge was transferred to and from other digesters for re-seeding and to supply alkalinity. Operations monitored CO2, H2S and pH daily, and DST was conducted to verify microbial activity. - RC-4 acetate capacity increased to 2.8 within 8 days indicating successful recovery and resiliency. Feed was started at 20% of typical feed and then increased gradually to support biomass growth, which was confirmed by rac trending upward (Figure 1.C). C)RC-4 reactivated and gas discrepancies. - After stable operation was regained for RC-4, gas differences were observed with RC-5 and RC-6. - Laboratory measured gas, rb were equivalent for RC-5 and RC-6, at the beginning of period C(Figure 1), suggesting equivalent loading and pointing to inaccurate full-scale gas values. Other DST measurements indicated no shifts in loading. - As observed in Figure 3, rb values match full-scale daily average gas flows except when gas from RC-6 was captured in RC-5 due to shared gas piping. To verify, the gas connection upstream of flow meters was closed, which made RC-5 and RC-6 gas trends to realign as shown in Figure 1.C. VALIDATING OPERATIONAL BENCHMARKS Figure 4 shows a comparison of DST's acetate capacity in comparison to calculated volatile solids loading (VSL) and hydraulic detention time (HDT). Since individual feed flow meters are not available, the VSL and HDT were back-calculated using gas productions and an assumed gas yield of 6.3 CF/lbVS. Gas yields for the digester feed are routinely measured full and bench-scale, and were assumed to be consistent through the monitored period. As observed in Figure 4, an acetate capacity of 2 is a good indicator of higher than typical loading or that the digesters could be at risk of approaching hydraulic overload. This event helped confirm that a VSL benchmark of 0.2 ppd/ft3 is conservative enough to maintain microbial activity, however, the hydraulic capacity may be limiting. LESSONS LEARNED -Managing risk: Traditional operating parameters do not give early warnings of conditions that lead to digester stress. Using DST alongside helps monitor microbial health, especially when digesters are operated near capacity. Gas productions complemented with microbial activity can prompt earlier investigation. -Trusting instrumentation: This event helped gain confidence in full-scale gas flow meters. DST results are useful for operators to validate gas production trends and work to understand intricacies of gas system connections. -Enhancing process information: When used in conjunction with typical process metrics, DST indicators provide insight to the causes of instability. For example, to distinguish mechanical from biological causes. Regular measurements of microbial activity daylight the effects of operational changes on biomass growth and health. This is particularly important during start-up or reactivation of upset digesters, to establish procedures that ensure a smooth transition. This event highlighted the value of cross-functional dialogue between different groups within CWS, helping establish a framework for interpreting digestion data. This improves decision-making under uncertainty, strengthening organizational resilience. This troubleshooting example builds confidence and provides guidance to respond to future challenges and proposed operational changes.

This paper was presented at the WEF Residuals, Biosolids, and Treatment Technology Conference in Kansas City, MO, May 11-14, 2026.

Presentation time

16:00:00

16:30:00

Session time

15:30:00

17:00:00

SessionConventional Anaerobic Digestion Is Still Relevant!

Session locationKansas City Convention Center

TopicDigestion

Author(s)

Sosa-Hernandez, Ornella, Wegener, Kevin, Schauer, Peter

Author(s)O. Sosa-Hernandez1, K. Wegener1, P. Schauer1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2026

DOI10.2175/193864718825160247

Volume / Issue

Content sourceResiduals, Biosolids and Treatment Technology Conference

Word count12

Description: Partnering with Local Sources of High Strength Wastes Through a Co-digestion...

Partnering with Local Sources of High Strength Wastes Through a Co-digestion Program

Abstract

Introduction Clean Water Services (CWS) is pursuing an opportunity to use available digestion capacity of the Rock Creek Water Resources Recovery Facility (WRRF) by developing a Co-digestion Program. This program serves two purposes: (1) it allows CWS to better serve the district by creating and strengthening relationships with surrounding industries that can provide High Strength Wastes (HWS) and, (2) it increases the overall biogas generation to a quantity that allows CWS to consider Renewable Natural Gas (RNG). The Durham WRRF currently uses Fats, Oils and Greases (FOG) for co-digestion which are highly variable and create multiple issues that represent a significant investment and a strain on staff morale. For Rock Creek WRRF, the preference is to utilize HSW from surrounding industries that are more consistent and require simpler handling strategies than FOG. Multiple groups within CWS are collaborating in this program to systematically identify, characterize and select wastes that can contribute to gas production goals for RNG without compromising digestion capacity and stability. Criteria for selection is being developed, which include management strategies that have minimum impacts to maintenance and the operation of the digesters, in addition to cost recovery, and staff satisfaction. A key aspect to the development of this program is the focus on a fast response time by both working directly with the industries, and by conducting in-house testing. The testing method was developed to provide information to the team about the biogas generation potential of HSW, possible impacts to the digestion process and operation, and facility requirements in a timely fashion. Program development 1.Defining the Capacity of the Existing System The first step of the evaluation is to determine the capabilities of the existing infrastructure to take on additional organic loading. Using values developed during the facilities planning effort, initial capacities can be estimated for HSW. Within the allowable volume defined by this effort, the biogas shortfall from the total RNG goal must be met. This has allowed us to stablish a minimum biogas yield per gallon of HSW required. The initial value of 4.4 ft3/gallon is used to select potential sources. Other operational conditions have been evaluated which allow us to understand how flexible our gas yield criterion is and the quality of the wastes we would be required to accept under those conditions. The capacity and biogas generation values will be periodically re-evaluated based on operation, and to determine if more HSW can be fed to the digesters in the future. A parallel effort to evaluate anaerobic digester stability is being used and to understand the current capacity of the digesters (Sosa-Hernandez & Schauer, 2023). With a defined capacity and yield, the HSW volumes can be forecasted to stablish volumetric requirements for permanent feeding facilities. 2. Identification and Evaluation of Potential Sources Individuals from CWS leadership, operations, business opportunities and the industrial pretreatment groups have worked together to identify potential candidates for co-digestion materials. The initial candidates are current CWS industrial contributors that have additional biodegradable streams which disposal and transportation represents a significant. If a candidate is considered promising, tours of the source facilities are conducted to further the relationship, understand their practices and processing, and obtain samples. These tours also allow the team to identify other HSW that would not have been considered as potential candidates by the industry. A basic characterization of the HSW samples and qualitative assessment of potential management strategies is performed. A short-duration biogas potential test (BPT5) that was developed in-house is performed to determine the biogas yield per Chemical Oxygen Demand (COD), volatile solids (VS) and volume of each HSW sample. Each of these tests is conducted for 5 days and provides a single data point that corresponds to a conservative estimate of biogas generation, and does not account for slowly degradable components of the material. The preferred co-digestion feedstocks should be relatively easy to digest, and the gas yield should be measured within the duration of the test. The BPT5 procedure also helps identify any toxicity impacts, and suggest handling and feedings strategies that would be required full-scale. Additionally, the gas yields obtained with the BPT5 are being compared to the results obtained with in-house materials such as primary and waste activated sludge, to understand their impact and degradability. A test that is easily implementable and can provide information in a short time frame is ideal for this program, as we can offer relatively quick feedback to the industrial contributor and start negotiations. The gas yields shown in Figure 1a, were used to estimate the average contributions to the gas production goal as well as the available volumetric capacity presented in Figure 1b. As observed in Figure 1, all the HSWs presented meet the volumetric gas yield criteria in various degrees, however this is not the only criteria that would be used for selection. The physicochemical characterization of the HSW samples is used to project potential impacts on the overall treatment system including biosolids generation and nutrient loading return to the liquids process. For example, the solids concentration and nitrogen contributions of these wastes are also being considered as presented in Table 1. 3.Assessing Impact through Full-Scale Testing Full-scale trials are also being performed to validate expectations in terms of handling concerns, as well as to reaffirm commitment with partners and start a working relationship. This requires the implementation of temporary receiving facilities and handling strategies. Another consideration for selection of HSW is staff acceptance and satisfaction, which is influenced by their knowledge of the new incoming product. Staff is included in the setup and equipment of the temporary facility, and training is provided to be able to safely handle the foreign material. The information gathered throughout this evaluation is presented in Table 1, which can help make decisions as to which are the most suitable candidates. This provides the necessary background to conduct negotiation for tipping fees with the industries. 4.Negotiations and Product Acceptance In addition to providing a product that has consistent quality and quantity, an ideal partner would provide information to CWS on a frequent basis. CWS would provide reliability, offer a financial incentive, and would help align their company values with certifications. These benefits and the information generated through this evaluation is considered during the negotiation process prior to acceptance. Beverage Industry A (which results are presented in Figure 1 and Table 1), has provided an HSW that meets our gas yield criteria and is very easy to handle. Their disposition has made them a very suitable candidate and the partnership has been solidified through a contract that was signed in October of 2023. We have started feeding the Rock Creek digesters with this waste using temporary feeding facilities and have accepted other of their HSW that were feasible even though they require higher investment. Conclusions Through a systematic evaluation, CWS has defined the capacity of the existing system, identified, and evaluated potential HSW sources, and conducted full-scale testing to assess real-world impacts. The focus on fast response times, collaboration among internal groups, and negotiations with industrial partners, exemplified by the successful contract with Beverage Industry A, highlights the proactive and comprehensive approach taken by CWS. This initiative not only aligns with sustainability goals but also demonstrates a commitment to innovation and partnership.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerSosa-Hernandez, Ornella

Presentation time

13:30:00

14:00:00

Session time

13:30:00

16:45:00

SessionResource Recovery and Sustainable Practices

Session number06

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAnaerobic co-digestion, High Strength Waste, Partnerships

Author(s)

Sosa-Hernandez, Ornella

Author(s)O. Sosa-Hernandez1, K. Wegener1, P. Schauer1

Author affiliation(s)Clean Water Services 1; Clean Water Services 1; Clean Water Services 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159460

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count13

Strengthening Partnerships with Local Industries through a Co-digestion Program

Abstract

Introduction Clean Water Services (CWS), a water resources management utility that serves Washington County, Oregon, is developing a Co-digestion Program to utilize available digestion capacity at the Rock Creek Water Resources Recovery Facility (WRRF) and increase biogas generation. This program allows CWS to better serve the district, by strengthening relationships with surrounding industries that can provide High Strength Wastes (HSW). We anticipate needing a variety of HSW sources to meet gas production goals, which requires a streamlined evaluation approach. A key focus of this program is to improve our interactions with the industries by having a fast response time. This can be accomplished by both working directly with our local contributors and by conducting in-house testing. In addition to providing a product that has consistent quality and quantity, an ideal partner would provide information to CWS on a frequent basis. In exchange, CWS can provide reliability, offer a financial incentive, and can help align company values to obtain business certifications. Identification and Evaluation of Potential Sources The initial candidates are current CWS industrial contributors that have additional biodegradable streams which treatment, disposal and transportation represent a significant cost or do not align with their sustainability goals. If a candidate is considered promising, tours of the source facilities are conducted to further the relationship, understand their practices and processing, and collect samples. A short-duration biogas potential test (BPT5) that was developed in-house is performed to determine the biogas yield of each HSW sample. The preferred HSW should be relatively easy to digest, and the gas yield should be measured within the duration of the test which is conducted for 5 days. This provides a conservative estimate of biogas generation and does not account for slowly degradable components of the material. The BPT5 procedure also helps identify toxicity impacts, and suggest feedings strategies that would be required full-scale. Figure 1 shows the results obtained with the HSW that have been tested so far. Because the evaluation can be conducted over a short period of time, multiple samples from a single source can be tested. Understanding the potential variability of the source and its impact on the digestion process is an important factor considered prior to acceptance. A basic physicochemical characterization of the HSW samples is also used to infer potential impacts to the overall treatment system, which include biosolids generation and nutrient loading return to the liquids process. Full-scale trials are also being performed to validate expectations in terms of handling concerns, as well as to reaffirm commitment with partners, and address potential operational issues. This requires the implementation of temporary receiving facilities and staff training. The information gathered throughout this evaluation is presented in Table 1. The data provides necessary background to conduct negotiation for tipping fees with the industries. Acceptance of High Strength Wastes and Negotiations Based on an evaluation of the hydraulic capacity of our full-scale digesters and biogas production goals, we have generated a gas yield criterion that is used to select HSW. As shown in Table 1, the HSW meet the gas yield criterion of 4.4 ft3/gal in various degrees, however this is not the only criteria that would be used for selection. Other criteria for acceptance include: management strategies that have minimum impacts to maintenance and operation, number of truckloads of HSW to be received, cost recovery, and staff satisfaction. The Durham WRRF currently uses Fats, Oils and Greases (FOG) for co-digestion which are highly variable and create multiple issues that represent a significant investment and a strain on staff morale. For Rock Creek WRRF, HSW that are more consistent than FOG and have less handling complexity are preferred. Beverage Industry A (which results are presented in Figure 1 and Table 1), has provided an HSW that meets our gas yield criteria and is very easy to handle. Their disposition has made them a very suitable candidate and a contract was signed in October of 2023. We have started feeding the Rock Creek digesters with this waste using temporary feeding facilities and have accepted other of their HSW. They have benefited from this program by reducing their transportation costs, and by being able to maintain sustainability certifications. Conclusions The evaluation approach provides prompt information to the team and the industries about the biogas generation potential of HSW, possible impacts to the digestion process and operation, and facility requirements. The focus on fast response times, collaboration among internal groups, and negotiations with industrial partners, exemplified by the successful contract with Beverage Industry A, highlights the proactive and comprehensive approach taken by CWS. This initiative not only aligns with our sustainability goals but also demonstrates a commitment to creating partnerships and innovation. The manuscript will include: (1) the results obtained so far, and that have supported our HSW selection, (2) the lessons learned throughout the evaluation process, and (3) examples of how the fast response and information exchange have improved our relationship with local industries.

Clean Water Services (CWS) is developing a Co-digestion Program at the Rock Creek WRRF to use its available digestion capacity. The program aims to strengthen relationships with local industries to obtain High Strength Wastes (HSW) for increased biogas generation, potentially leading to Renewable Natural Gas production. Influenced by operational experiences at the Durham WRRF, CWS is moving away from Fats, Oils, and Grease, and is collaborating internally to systematically select and test HSW.

SpeakerSosa-Hernandez, Ornella

Presentation time

13:30:00

13:50:00

Session time

13:30:00

15:00:00

SessionMaking Money with Biogas: Co-Digestion and RNG

Session number601

Session locationRoom 356

TopicBiosolids and Residuals, Energy Production, Conservation, and Management, Intermediate Level, Municipal Wastewater Treatment Design

Author(s)

Sosa-Hernandez, Ornella, Wegener, Kevin, Schauer, Peter

Author(s)O. Sosa-Hernandez1, K. Wegener1, P. Schauer1

Author affiliation(s)1Clean Water Services, OR

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159535

Volume / Issue

Content sourceWEFTEC

Word count10

Description: Understanding the Stability and Capacity of Full-scale Anaerobic Digesters Using a...

Understanding the Stability and Capacity of Full-scale Anaerobic Digesters Using a Bench-scale Test

Abstract

Introduction Despite being essential to treat the solids stream at Water Resources Recovery Facilities (WRRF) around the world, anaerobic digesters are typically operated with imprecise and limited process information. Instead, industry standard values with generous safety factors are used to buffer any perturbations. Understanding the risk of upset is particularly important when the digesters are operated close to their design capacity, under variable loading conditions, or co-digesting extrinsic feedstocks. Clean Water Services (CWS) operates digesters at two facilities. The Durham WRRF runs two digesters that receive imported Fats, Oils, and Greases (FOG) for co-digestion which increases the variability of the loading conditions. The Rock Creek WRRF digesters are not fed co-digestion materials routinely but have limited online measurements. Total feed flow and overall gas generation of the five-digester system are the only values tracked online, each by a single meter. Discrete measurements of physicochemical parameters such as volatile acids (VA), alkalinity, pH and volatile solids (VS) content are performed routinely and individually to satisfy typical operational requirements. However, the status of the digesters can often be misrepresented when using only grab sample values. Furthermore, the diagnosis of upset events can be slowed by miscalculated loading rates, restricted visual inspections and the use of laboratory analyses that take time to be completed. CWS has refined and is using a bioassay to monitor the Durham and Rock Creek digesters. This bioassay was developed based on the understanding that a method that measures the capability of the microbial community to use a key intermediate, such as acetate, can help determine how close a digester is to failure (Conklin et al., 2008). Monitoring the ability to complete this metabolic step allows staff to troubleshoot and take corrective action prior to upset events, and better understand operational conditions that can affect the health of the digesters and limit their capacity. The bioassay has shown reproducible results during development, and validation against operational conditions (Sosa-Hernandez & Schauer, 2023). This presentation will 1) include updated validation of bioassay results against full-scale operation, and 2) provide example insights into the digestion process. Weekly bioassays have been conducted at the Durham WRRF starting in September of 2021. Testing was extended to Rock Creek in June of 2022 with the intention of identifying responses of the indicators to usual operational conditions. The goal is to capture routine changes that are hypothesized to burden the digesters microbial metabolism, some of which include: FOG loading changes, digester restarts, and mechanical difficulties. Bioassay Implementation The bioassay consists of measuring the ability that fresh digestate samples from active digesters have to utilize additional acetate. Figure 1 shows real experimental results and the bioassay indicators, which are: - Rate of biogas production in an acetate fed bottle (rac) - Rate of biogas production in a bottle containing only digestate (rb) - Ratio of the two measured biogas production rates (rac/rb) - Biogas generation per mass of added acetate or biogas recovery - Acetate depletion time Changes to volatile solids loading rate (VSLR) and hydraulic detention time (HDT) of a full-scale digester can impact the rates of biogas production (rac and rb) and the ratio rac/rb. Hypothetically then, the ratio rac/rb represents the excess capability of the digestate to use acetate which could decrease when volatile acids accumulate due to organic overloading or due to low detention times that could cause biomass wash-out. Responses to Volatile Solids Loading Rate and Hydraulic Detention Time Figure 2 shows the relationships between rac/rb and the VSLR and HDT of the Durham digesters which are fed FOG. A correlation, although weak, is observed, which suggests an impact of operational conditions on the capacity to use excess acetate. The variability of the results could be influenced by our inability to properly measure characteristics (e.g. volatile solids) of the digester feed and flows, which worsens with co-digestion feedstocks like FOG. Physicochemical parameters that are tracked routinely (VA, Alkalinity, pH, etc.) show no signs of instability during the monitored period, despite the loading fluctuations and operating close to the operational benchmarks shown in Figure 2. A validation of the risk of failure when operating at industry standards might be required, that accounts for operational history and other microbial health indicators. Additionally, the organic loading capacity of FOG fed digesters might not be understood using solely biogas production from additional acetate, and other anaerobic digestion intermediaries could be used to evaluate this hypothesis. Responses During Digester Re-start and Mechanical Limitations The results obtained during the following events will be included in the presentation and manuscript: a)The Rock Creek digesters were put offline for cleaning and restarted in late 2022.The bioassay allowed us to determine the time at which a newly restarted digester and the system reached steady state. The changes in the ratio rac/rb can be used to determine how quickly fresh feed loading can be increased to the digesters, and to observe impacts to the other digesters. b)Overflowing of the Rock Creek digester contents has been observed on different occasions. Bioassay results indicated that the ability of the biomass to use the additional acetate was not perturbated during these events, which helped understand that the causes of overflowing were not related to organic overloading. This focused troubleshooting on mechanical issues that caused temperature to decrease, and provided confidence that the digester could quickly return to steady state if proper operating conditions could be resumed. Comparison of Digesters and Type of Feed Figure 3 presents a comparison of the results obtained from both the Durham and Rock Creek digesters, and the daily VSLR at which these are operated. The Durham digesters are operated under significant loading variability on a daily basis, and the ratio does not seem to be sensitive to these changes. This could also indicate a more robust biomass that is able to take shock loads or that the changes, despite their magnitude, are consistent on the long term which help maintain stability. Co-digestion with FOG could cause a microbial population shift that may relieve some of the burden over aceticlastic methanogens, but can affect earlier degradation steps (Ram Mohan et al., 2022). Due to their consistent loading, the Rock Creek digesters could be more susceptible to overloading if changes occur on a short timeframe. Conclusions and Next Steps We were able to identify responses of the bioassay indicators to different operational conditions. The insight gained from these results include: - The ratio rac/rb, which represents the capacity to use additional acetate, could be an indicator of the ability to take in higher loads. - Daily and highly variable loadings from FOG do not seem to impact the ability of the biomass to use additional acetate, which also seems to be increased. This could indicate greater capacity to take in higher organic loads, robustness of the microbial community or that the aceticlastic methanogens are more available. - The indicators could be used to distinguish process failures that are caused by mechanical problems from those that are caused by organic/hydraulic overload. These can also help identify long term impact of mechanical failures on the microbial metabolism. - A higher loading rate at which the capacity to utilize acetate is inhibited has not been identified using the bioassay. This suggests that conventional loading rate criteria may be overly conservative. The digesters have been generally stable during the monitored period, as denoted by other process parameters and observations. Monitoring of our digesters will continue during suspected risk of failure. Additionally, this study offers the opportunity to collaborate with other utilities that experience process difficulties which can help test our hypotheses elsewhere.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerSosa-Hernandez, Ornella

Presentation time

08:30:00

09:00:00

Session time

08:30:00

11:15:00

SessionTroubleshooting & Optimization

Session number26

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAnaerobic Digestion, Co-Digestion With Other Organic Sources, Performance Indicators

Author(s)

Sosa-Hernandez, Ornella

Author(s)O. Sosa-Hernandez1, P. Schauer1

Author affiliation(s)Clean Water Services 1; Clean Water Services 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159422

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count14

Using a Bench Scale Test to Evaluate Anaerobic Digesters Stability and Capacity

Abstract

Limited process information is available to describe the stability and capacity of anaerobic digesters at the Clean Water Services (CWS) facilities. To supplement this information, a bioassay has been implemented to monitor the digesters operated at the Durham and Rock Creek facilities. Weekly bioassays have been conducted over the past two years. The results have allowed us to evaluate the responses of the bioassay indicators to usual operational conditions, such as changes to loadings from fats, oils and grease addition, digester startup after cleaning, and mechanical difficulties. The indicators show a response to changes to loading and detention time and the type of feed that could help understand the risk of failure, and help distinguish mechanical from biological causes of foaming events.

Clean Water Services is using a bioassay to understand the risk of failure and the capacity limitations of the full-scale digesters, as well as the impact that different operational and mechanical changes have on the health of the digesters. The responses of the bioassay indicators to usual operational conditions were evaluated. A method that monitors the microbial community behavior can offer better insight on digestion health.

SpeakerSosa-Hernandez, Ornella

Presentation time

08:30:00

08:50:00

Session time

08:30:00

10:00:00

SessionIntensifying and Optimizing Anaerobic Digestion

Session locationRoom S503 - Level 5

TopicIntermediate Level

Author(s)

Sosa-Hernandez, Ornella

Author(s)O. Sosa-Hernandez 1; P. Schauer 1 ; O. Sosa-Hernandez 1;

Author affiliation(s)Clean Water Services 1; Clean Water Services 1 ; Clean Water Services 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825159150

Volume / Issue

Content sourceWEFTEC

Word count13

Description: Using the Capillary Suction Time Test as a Tool to Support Operation of the...

Using the Capillary Suction Time Test as a Tool to Support Operation of the Dewatering Process

Abstract

Introduction The capillary suction time (CST) test is an established method of evaluating the dewaterability of stabilized wastewater solids and has been used primarily to evaluate different flocculants, flocculant demand, or to determine the differences in dewaterability between solids generated under differing conditions. Many variables exist in modern dewatering processes which offer opportunities for optimization. Flocculant make down systems are now tunable with regard to mixing energies, make down concentrations can be easily changed, and the system may or may not include an aging or feed tank. Often times there are options to dose flocculant at multiple points in the solids feed ahead of dewatering equipment. Feed solids concentration and rheology can also have an effect and pre-dilution of the feed offers another control point to gain efficiency. Clean Water Services (CWS) (Washington County, Oregon, USA) is undergoing the development of CST testing procedures to evaluate, predict, and recommend full-scale dewatering process optimization strategies. Procedure The CST apparatus itself is a simple device that measures electrical continuity between two sets of contacts. The contacts rest on a piece of filter paper, a conditions solids sample is applied to the center of the filter, and water is drawn out across the paper. A timer is initiated when the water reaches the first set of contacts at radius 1, and stopped when it reaches the second set of contacts at radius 2. The lower the time the more easily water is withdrawn from the solids, and the more dewaterable the sample is considered to be. CWS utilizes an OFITE (Houston, Texas, USA) CST apparatus (Part No. 294-50). Although simple in concept, the CST test requires a well thought out procedure and careful technique to improve the accuracy and repeatability of the results. The manufacturers instruction manual contains procedure recommendations including using a syringe or pipette to transfer the conditioned sample to the sample holder, thoroughly cleaning and drying the sample blocks and electrodes between tests, and repeating a minimum of three times and averaging results. Additional notes state that temperature and solids concentration have significant effect on results, and that 5 repetitions per test are recommended. The consequence of these procedural nuances is that it will take a significant amount of time to evaluate a single solids sample at a single flocculant dose through 3-5 repetitions. Over the course of the time required, two major effects can be seen to skew results. First, when evaluating anaerobically digested solids the initial sample will be at a temperature of approximately 37 C for a mesophilic system. Depending on sample size and ambient lab temperature the sample may cool as much as 10 C over the run of 5 repetitions. Second, the made down flocculant solution, whether prepared in the lab or taken from the full-scale system will be aged up to one hour over the trial run. The goal with any bench test is to duplicate as exactly as possible the full-scale system. The procedure under development at CWS attempts to achieve this in multiple ways. The test is considered to be a field test, meaning that it is performed as close to the dewatering process as possible. The flocculant solution is sampled as close to the dosing point as possible to account for activity changes in the piping run from the make down system to the dosing point. The solution is used in the test as quickly as possible, ideally under 1 minute, and a fresh sample is taken for each test. A 4 L dewatering feed sample minimizes temperature changes. In the test itself a 60 mL syringe is capped and inverted, plunger removed, and a 5 mL sample of solids and corresponding polymer dose are concurrently pipetted into the syringe. The plunger is inserted enough to make a seal, the syringe is placed tip up, the cap loosened and the plunger depressed to a volume of 30-40 mL. The cap is tightened and syringe is shaken 30 times in a back and forth motion to effect flocculation. The cap is removed and the conditioned solids are pressed to the end of the syringe, then displaced into the sample holder. Results Initial results of procedure development and investigation of effect of flocculant solution aging illustrate the inherent variability in the test and increases in CST time from flocculant that has been aged on the lab bench. In Figure 1, a dose curve of CST time is shown for the same solids sample conditioned with the same fresh flocculant and run on side by side CST instruments. Although the curves have the same shape, there is chance variability in the CST at a dose of 14 lb. active/DT. The effect of solution aging can be pronounced as shown in Figure 2. Here, two separate runs are shown where the dose remains at 20 lb. active/DT but the solution has aged on the bench up to 30 minutes. Summary The CST test may prove to be a useful tool to support operational decision making and to evaluate the effect in dewaterability of solids treatment processes. The method is however, sensitive to technique and procedure and the nuances of the analysis as a representative bench test must be considered if the resulting data are to be useful. The future of this work, to be completed by time of manuscript include a full procedure, evaluation of effect of flocculant aging, solids sample temperature, flocculant solution concentration, and dosing point. These data will be accompanied with a correlation to full scale operational performance.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerMaher, Chris

Presentation time

08:30:00

11:15:00

Session time

08:30:00

11:15:00

SessionTroubleshooting & Optimization

Session number26

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicChemical Treatment, Dewatering Optimization, Laboratory Methods And Analysis