Stephens, Nicole

Nicole is a process engineer working in all phases of water, wastewater, biosolids, and bioenergy projects. She enjoyes supporting planning and research efforts focused on evaluating new and emerging technologies and triple bottom line-based alternative selection.

Titles from this speaker

Assessing the GHG Emission Tradeoffs of Energy Recovery with Thermal Drying

Abstract

INTRODUCTION The Town of Cary, NC Utilities Department serves a population of roughly 175,000. Cary is a regional leader in sustainability and climate change mitigation with an ambitious goal of reducing town wide GHG emissions by 25 percent by 2025 and by 100 percent by 2040. A high-level study conducted in 2011 showed water and wastewater services account for approximately 60 percent of Cary's municipal GHG emissions. More recently, in conjunction with GHG emission accounting research led by the Stantec Institute for Water Technology and Policy, Cary developed a comprehensive emissions inventory for their three water reclamation facilities (WRF), one water treatment facility (WTF), and one major pump station to both measure progress related to emission reduction activities and to incorporate GHG impact analyses in alternative selection processes. One of the WRFs included in the inventory is the 18 million gallon per day (MGD) Western Wake Regional Water Reclamation Facility (WWRWRF), commissioned in 2014 and discharging to the Cape Fear River. The WWRWRF solids handling facility includes belt filter press thickening of waste activated sludge (WAS) and thermal drying via two BioCon dryers with a combined capacity of 1,489 wet pounds per hour. The dried biosolids are processed through a pelletizer and distributed as a Class A product for land application. The facility's original design included an energy recovery system (ERS) that would combust dried biosolids to generate heat for the drying process, thereby offsetting natural gas consumption. The ERS was removed from the design with the intent to revisit installation in the future. In October 2022, Cary commissioned Stantec to assess the GHG emissions impact of installing an ERS using the newly developed GHG inventory tool. The purpose of this paper is to provide an overview of Cary's GHG emission inventory tool, understand is applicability for utilities of all sizes and treatment technologies, present the results of the GHG emission impact analysis and provide considerations for incorporating GHG evaluations in facility planning and alternative selection processes. METHODOLOGY Cary's GHG was developed for a baseline year of 2021 and is an Excel-based calculator specifically focused on water and wastewater facility emissions. The calculator sources reference data and equations from the Local Government Operations Protocol, IPCC Guidelines, EPA GHG Emission Factors Hub, EPA Waste Reduction Model, and the Biosolids Emissions Assessment Model (BEAM). The tool was developed with a user-friendly interface and can be quickly updated annually to monitor emissions over time or applied as a decision-making tool by comparing capital improvement or operational optimization alternatives. Energy recovery systems accept and combust dried biosolids from a dryer and recover the waste heat from the combustion process for dryer operation. The end product is ash, which can either be disposed via a landfill or beneficially reused as a construction material amendment. The heat produced by the ERS offsets the natural gas required to heat the dryer. WWRWRF's 2021 GHG inventory was used to assess the theoretical GHG impact of installing an ERS. Installation of an ERS is anticipated to impact GHG emissions in five areas: 1.Reduced natural gas combustion by the dryer 2.Reduced GHG offset from biosolid land application 3.Reduced end-product hauling 4.Increased electricity consumption from ERS equipment motors 5.Increased direct emissions from combustion processes There are several energy recovery technologies available to WRFs. However, this evaluation assumed the Veolia ERS system would be installed as it is designed to integrate with the BioCon dryers already in operation at the WWRWRF. The design criteria assumed a throughput of 1,489 lb/hr, consistent with the existing dryer system nameplate capacity and energy consumption estimates were provided for the full system capacity and are summarized in Table 1. The GHG impact from the ERS was assessed for 2021 only and during that year the WWRWRF dryer throughput averaged 636 lb/hr. Electricity consumed by the ERS system was estimated by multiplying the expected energy draw at design capacity by the ratio of the actual and design loads. The ash production rate is approximately 40 percent of 2021 dried biosolids production rate and the average hauled distance for ash disposal was assumed to be 30 percent of that of biosolids land application. Based on Veolia-reported values, the ERS is assumed to provide 68 percent of the dryer's heat demand and the remaining is provided by natural gas combustion. While the ERS does require some consumables for flue gas treatment such as urea, emissions associated with its transport were not included as consumption is estimated at less than one tote per year. In addition, no changes in facility or administrative costs were assumed. There is not current guidance on GHG emission estimates from an ERS. Therefore, solids combustion emissions were calculated using the 'Combustion' tab in BEAM, which is specifically intended for solids incinerators. BEAM calculates Scope 1 emissions from solids combustion as well as the biogenic CO ‚‚ emissions that arise from the conversion of the organic matter in the solids to CO ‚‚. Per LGOP and IPCC guidance, biogenic emissions can be estimated and monitored but do not need to be included in a facility's inventory. RESULTS Results of the analysis are presented in Table 2 and Figure 1 and show the existing and theoretical (with ERS) emissions in 2021. The results of this analysis show that installation and operation of the ERS system would have increased 2021 solids handling related GHG emissions by 36 percent. Nearly all of the ERS combustion emissions are as nitrous oxide, which is particularly difficult to estimate, with significant variability in emission factors. As such, actual emissions may vary based on how well the energy recovery system is maintained and operated. The difference in GHG emissions is also heavily influenced by the elimination of land application practices, which currently provide Cary with a significant emission offset. This is demonstrated in Figure 1, which shows there would not be any 'negative' emissions without land application practices. As previously mentioned, Cary may beneficially reuse the ash product for construction materials, but this practice is not expected to provide the same magnitude of offset since it does not directly increase carbon sequestered biomass. While a more complete evaluation of all financial, environmental, and social impacts of the technology alternative is recommended, this work demonstrated Cary's ability to quickly and effectively quantify the GHG emission impact of a technology alternative. In addition, this evaluation revealed the tradeoffs within the circular economy as an alternative intended to reduce reliance on fossil fuels may result in reduced production of value beneficially reused biosolids.

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

SpeakerStephens, Nicole

Presentation time

10:00:00

Session time

11:45:00

SessionSession 11: Circular Water Economy

Session number11

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicManaging Biosolids in the Carolinas, CWE

Author(s)

N. Stephens

Author(s)N. Stephens1, S. Lobo2, D. Forney3, 4,

Author affiliation(s)Stantec1; Town of Cary2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2023

DOI10.2175/193864718825158802

Volume / Issue

Content sourceResiduals and Biosolids

Word count12

Impact of COVID-19 and Viruses On A UV Disinfection System Design Dose

Abstract

INTRODUCTION
Waterborne transmission is known to be a key exposure pathway for enteric viruses and human norovirus (hNoV) is the number one cause of acute gastroenteritis globally (Ahmed, 2014). While the presence of enteric viruses in waterways and associated health effects are by no means emerging, monitoring and regulation of viruses in our water systems is of emerging interest for the protection of public health and the environment. The EPA and other regional regulatory agencies have been investigating the need for, and potential consequences of, regulating viruses in wastewater effluent â€“ either in lieu of or in addition to current microorganism criteria for the fecal bacteria indicator organisms Escherichia coli (E.coi) and enterococci. Several considerable drivers exist in favor of enteric virus regulations including potentially higher environmental persistence than bacteria, higher resistance to treatment, and higher infectivity; all of which point to virus-based control being more protective of public health. However, significant hurdles have thus far prevented the successful deployment of virus regulations including: - variability in mechanisms and effectiveness of disinfection technologies, - knowledge gaps around health-based thresholds, - lack of feasible enumeration and quantification methods, - consensus on the most appropriate indicator organism, and - the perceived cost burden to an infrastructure system that is already substantially underfunded. Perhaps most importantly, 'there is no clear-cut epidemiological evidence linking viral GE (gastroenteritis) outbreaks from exposure to bathing waters that do meet criteria based on fecal indicator bacteria'' (Dorevitch 2016). While virus effluent limits are yet to be promulgated through wastewater discharge permits, consideration for regulations and their resulting impact on current and future wastewater disinfection systems have long proceeded the SARS-CoV-2 pandemic (Pandemic). Further, the pandemic has prompted significant pubic interested in the presence and threat of pathogens in waterways. The wastewater industry has proven its adaptability through implementing influent monitoring to predict coronavirus outbreaks and the Pandemic has ushered in a new perspective on both virus monitoring and potential regulation. As a result, disinfection projects occurring during the pandemic have been privy to the influence of virus-based control considerations, irrespective of official discharge regulations.
South Platte Renew (SPR) owns and operates the 189 megaliter per day (MLD) (50 million gallons per day [MGD]) SPR Water Resource Recovery Facility (WRRF) in Englewood, Colorado. Increasingly stringent nutrient limits being promulgated by the state of Colorado are driving the implementation of a new chemical phosphorus removal system and the conversion from chloramine to UV disinfection under the Chemical Phosphorus Removal and UV Disinfection Project (Project). The purpose of this paper is to present a case study highlighting how both heightened disinfection awareness resulting from the Pandemic and consideration for other non-regulated enteric pathogens influenced the Project design criteria, and therefore capital and operating costs, for the new UV disinfection system.
DISCUSSION
The state of Colorado's (State) design criteria guidance document, WPC-DR-1, requires a UV design dose of 30 millijoules per centimeter squared (mJ/cm2) assuming a UV transmittance of 65 percent. However, the Project conservatively selected a design UVT of 60 percent based on historical operating data. Collimated beam tests (CBT) were then performed by two UV equipment vendors (Trojan and Wedeco) to validate the UV dose necessary to achieve effluent E.coli limits. The results of the tests are presented in Figure 1 and show that at doses greater than 10 mJ/cm2, the effluent E. coli concentrations are reduced below SPR's effluent limit. Therefore, the state-required design dose of 30 mJ/cm2, based on validation using MS2 as the surrogate organism, is overly conservative by a factor of 2 to 3. Figure 1: Collimated Beam Test Results The State allows projects to request a design dose variance given sufficient seasonal operating data and/ or collimated beam testing. Given the CBT results presented above, the Project engaged the State regulators in the preliminary design phase to request a lower design dose of 20 mJ/cm2. The Project team compiled and presented information to support the argument for a lower UV design dose as follows.
1. The use of MS2 as a surrogate organism for UV system validation is not representative of E.coli-based disinfection control. The dose response curve presented in Figure 2 shows MS2 is an overly conservative surrogate organism and the same log inactivation is achieved for E.coli and T1 at less than 10 mJ/cm2.
2. The State provides tiered disinfection dose criteria for oxidant-based disinfection (such as chlorine) with increasing levels of upstream treatment (Figure 3). The SPR WRRF includes trickling filters and solids contact, nitrifying trickling filters, and tertiary denitrification filters and therefore the disinfection design dose should consider the impacts of increased degrees of upstream treatment.
3. There is no significant difference in the inactivation of enteric pathogens achieved for a dose of 20 compared to that of 30 mJ/cm2. Figures 4 and 5 summarize literature reporting the dose required to achieve 1, 2, and 3 log-inactivation of waterborne enteric bacteria, protozoa, and viruses of potential concern. For bacteria and protozoa, both doses easily achieve 3-log inactivation for all pathogens. Adenovirus, MS2, and rotavirus show similar resistance against both doses and therefore higher design doses do not provide a significant increase in disinfection projection. Figure 2: Dose Response Curve for Different Surrogate Organisms Figure 3: WPC-DR-1 Tiered Disinfection Dose Requirements for Chlorine Figure 4: Proposed and Required Dose Efficacy Against Various Pathogens
RESULTS AND CONCLUSIONS
While regulators and the Project team agreed the data presented supported the argument that a design dose of 20 mJ/cm2 would be protective of public health based on both existing bacterial permit criteria as well as other enteric pathogens of concern, the regulator denied the requested variance. The primary factor influencing their decision was the SARS-CoV-2 pandemic and associated public interest and trust in public water systems. The Project's final design criteria are summarized in Table 1. The new UV disinfection system will be retrofitted into the existing chlorine contact tanks and consists of two UV disinfection channels with four UV banks per channel (Figure 5).
The higher design dose requirement resulted a system that is approximately 50 percent larger than that of the requested dose increasing both capital and annual operating cost. In the near term and during low flow conditions, turndown limitations may result in the system having to overdose thereby increasing operational costs. Long term O&M cost impacts are also expected as the larger system requires additional maintenance and parts replacement for bulbs, ballasts, and wipers. The Projects is closing out the final design phase at this time and will be in construction at the time of final manuscript submission. Table 1: SPR Disinfection Design Criteria Figure 5: Rendering of SPR's UV System

The wastewater industry has proven its adaptability through the implementation of influent monitoring to predict coronavirus outbreaks and the SARS-CoV-2 pandemic has ushered in a new perspective on both virus monitoring and potential regulation. As a result, disinfection projects occurring during the pandemic have been privy to the influence of virus-based control considerations, irrespective of official discharge regulations impacting the UV design dose for secondary effluent applications.

SpeakerStephens, Nicole

Presentation time

14:00:00

14:15:00

Session time

13:30:00

15:00:00

TopicAdvanced Level, Disinfection and Public Health, Municipal Wastewater Treatment Design

Author(s)

Stephens, Nicole

Author(s)Zeena Kohr1; Jamie Safulko2; Nicole Stephens3; Cody Charnas4

Author affiliation(s)South Platte Renew, Englewood, CO1; South Platte Renew, Englewood, CO2; Stantec Consulting Services Inc., Denver, CO3; Stantec Consulting Services Inc., Denver, CO4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158643

Volume / Issue

Content sourceWEFTEC

Word count13

Political and Technical Tools on the Trajectory To Decarbonizing Water

Abstract

INTRODUCTION
Despite lingering disputes over the validity, consequences, and severity of climate change, progress toward greenhouse gas (GHG) emission reduction continues to be made in industrialized and developed nations. Political, technological, and social challenges continue to present significant hurdles and risk. However, the quest toward global decarbonization is increasingly becoming a question of not if, but when and how. There is growing acceptance and adoption of the need to limit global warming to 1.5Â°C by achieving net-zero carbon emissions by year 2050 (Matthews, et al., 2018); climate change initiatives such as the Paris Agreement largely operate under that deadline. Addressing the matter of 'how,' on the other hand, is far more complex and political division has caused the resources available to the water industry to lag behind global trends. As such, the Stantec Institute for Water Technology and Policy has undertaken a two-phase effort to investigate the existing resources available to water industry professionals related to climate change mitigation and to summarize the purported scientific bases for political and technical resources required for setting and achieving emission reduction goals.
The purpose of this paper is to document the results of a comprehensive literature review and investigation of (1) the socioeconomic policy pathway to a net-zero 2050 and its impact on the water industry, (2) the technical underpinnings and applicability of existing GHG computational tools for water utilities, and (3) identified gaps and needs to promote meaningful (having a quantifiable environmental impact) GHG emission reduction in water.
PHASE 1: The Policy Pathway to a Net-Zero 2050 DISCUSSION Several political mechanisms exist to address pollutant emissions, all with their own benefits and drawbacks. These include emission caps or reduction requirements, efficiency standards, subsidies, and emission taxes or non-compliance penalties. The structure of a pollution reduction program dictates how inevitable economic and social burdens are distributed between polluters, consumers, and society. Within the context of global decarbonization, solutions must seek to facilitate both an economic and social transition that results in a stronger economy and promotes social equity (National Academies of Sciences, Engineering, and Medicine, 2021) and is decoupled from environmental cost. The first phase of this work, conducted in 2021, focused on understanding the political tools available to address pollutant emissions given that there are no such regulations currently in place at the federal level. By drawing on experience and lessons learned from carbon reduction efforts around the world, as well as historically successful pollutant elimination programs (e.g., air emissions controls), we identified key programs and policies on the trajectory of decarbonization (Figure 1) and considered the resulting impact on, and the role of, the water industry. In addition, we considered the opportunities that exist to engage in decarbonizing the water industry that are both economically favorable and socially equitable. Figure 1: Decarbonizing the Water Sector RESULTS AND CONCLUSIONS Several key policy advancements may shape the trajectory of decarbonization. First, there is a need to address conflicting applications of the Clean Air Act to GHGs on a federal level and the resolution will dictate the regulatory options available to the federal government. Next, North America could address climate change through decarbonization with one, or a combination of, three policy approaches:
1. Do nothing nationally and continue business as usual.
2. Implement pollutant elimination regulation via efficiency standards and emission caps.
3. Implement market-based instrument such as tax incentives/ disincentives and/or cap-and-trade programs.
Findings of this work suggested that even with a lack of national regulatory action in a 'business as usual' scenario, organizations can be expected to continue voluntarily committing to GHG emission reduction through corporate climate initiatives and regional trade markets, as both are shown to be economically favorable in business case studies and peer review literature. Furthermore, standards and emission caps are unlikely to occur in solitude as they fail to take advantage of revenue generating and reallocation opportunities. Therefore, the promulgation of market-based cap-and-trade programs is likely and provides opportunities to utilities, consultants, contractors and equipment suppliers in the water industry (Table 1). Table 1: Water Opportunities in Decarbonization
PHASE 2: Technical Tools and Gaps In 2013, Stantec (then MWH Global) and the Water Research Foundation published the 'Toolbox for Water Utility Energy and Greenhouse Gas Management' that documented available tools for water agencies to measure energy and GHG emissions, their underlying models, and capability to support process optimization. In addition, the document identifies the strategies necessary to establish a set of best practices that could be 'harmonized across utilities and geographic regions'. Since that publication, much has evolved in terms of the political landscape and technical tools available to the water sector, yet little has been done to standardize the most appropriate and technically sound approach to computing and reporting emissions and/ or setting science-based emission reduction targets. The second phase of this work will be completed by mid-2022 and includes a comprehensive literature review and analysis of available computational models and tools for estimating GHG emissions. In addition, the Stantec Institute for Water Technology & Policy is conducting an analysis of the scientific basis for existing tools and their applicability to the water industry and will identify gaps and needs associated with advancing decarbonization efforts in water. The result of this work will include a compendium of available tools and a recommendation for the standard method to be applied across the water industry for measuring GHG emissions, establishing emission reduction targets, and monitoring and reporting progress.

This paper documents the results of a comprehensive literature review and investigation of (1) the socioeconomic policy pathway to a net-zero 2050 and its impact on the water industry, (2) the technical underpinnings and applicability of existing GHG computational tools for water utilities, and (3) identified gaps and needs to promote meaningful (having a quantifiable environmental impact) GHG emission reduction in water.

SpeakerStephens, Nicole

Presentation time

11:00:00

11:25:00

Session time

10:30:00

12:00:00

TopicFundamental Level

Author(s)

Stephens, Nicole

Author(s)Nicole Stephens1; Art Umble2; Mehran Andalib3

Author affiliation(s)Stantec Institute for Water Technology & Policy, Stantec Consulting, Inc., Raleigh, NC1; Stantec Institute for Water Technology & Policy, Stantec Consulting, Inc., Denver, CO2; Stantec Institute for Water Technology & Policy, Stantec Consulting, Inc., Boston, MA3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158565

Volume / Issue

Content sourceWEFTEC

Word count11

Technoeconomic Evaluation of Alternatives for TTHM Compliance in Class A Reuse Effluent

Abstract

INTRODUCTION Total trihalomethanes (TTHMs) are disinfection byproducts formed through chlorine-based disinfection processes and are an increasing threat to wastewater, reuse, and drinking water systems. Recent regulations for direct and indirect reuse systems require reduction of TTHM levels. However, little guidance is available on appropriate technologies and methods for effective TTHM mitigation. Both removal technologies and alternative disinfection approaches offer possible solutions. This paper describes the comprehensive evaluation of TTHM mitigation alternatives that was conducted for a 15 megaliter per day (MLD) water reclamation facility (WRF) producing Class A reuse-quality effluent. Historical WRF operational data showed TTHM concentrations regularly exceed permitted limits. The permitting requirements for disinfection and TTHMs provide the basis of compliance for all alternatives considered. The facility's Aquifer Protection Permit (APP) specifies a TTHM alert level (AL) and discharge limit (DL) for TTHMs and fecal coliform. Alternatives were developed based on the treatment goal of reducing TTHM levels below the AL of 80 micrograms per liter (ug) and include peracetic acid (PAA), chloramines, air stripping, and source control. Figure 1: THM Speciation with Alert Level and Discharge Limit PAA DISINFECTION PAA can be a cost-effective alternative to chlorine-based disinfection systems. Existing disinfection doses and performance can indicate the efficacy of PAA disinfection, but there is little industry experience with achieving the disinfection performance required at this WRF. Bench testing was recommended to identify an appropriate dose range (Figure 2). While the bench testing showed good recovery of PAA through the contact period (Table 1), the maximum dose tested of 4 mg/L and 40 minutes contact time did not provide the effluent pathogen reduction required to comply with regulatory discharge limits. Figure 2: PAA Bench Test Results for 2 ppm Dose Table 1: PAA Recovery in Bench Tests Bench testing was inconclusive with respect to the design dose. However, recovery rates suggest higher doses would not reliably achieve disinfection performance targets as available residual PAA provided little disinfection between 20 and 40 minutes of contact. Additional bench or full-scale demonstration testing would be required to reduce uncertainty in design dose and refine cost estimates. CHLORAMINE DISINFECTION Under the proper ratio, chloramines bind up chlorine in a combined form and eliminate the pathway of TTHM formation. Chloramine disinfection requires a longer reaction time and CT target (total chlorine residual multiplied by reaction time). Disinfection kinetics can be complex and site specific; however, California's Title 22 regulations establish requirements for tertiary filtered disinfected water irrespective of free or combined chlorine. These requirements include a minimum CT of 450 mg-min/L in addition to total coliform limit of <2.2 MPN per 100 mL, corresponding to a minimum modal contact time of 90 minutes based on peak dry weather design flow. The limits from California's regulations were used as a conservative planning level basis and the expanded contact chamber footprint is conceptually shown in Figure 3. Figure 3: Conceptual Expansion Required for Chloramine Disinfection AIR STRIPPING Air stripping is a treatment method that effectively removes volatile organic compounds (VOCs) from a contaminated water source. The process forces air through contaminated water through the water via air stripping towers or aeration tanks. Several process alternatives for air stripping were considered including air stripping towers (3 or 6 tower configuration), surface aerators within a tank, and floating spray aerators within a tank. Vendors offer performance guarantees at the stated percentages to ensure that effluent water meets the permit's requirement, 80 ug/L, under a set of defined influent conditions (e.g., flow rate, TTHM speciation, temperature), as shown in Table 2. Table 2: Air Stripping Alternatives Performance Comparison SOURCE CONTROL ALTERNATIVES An evaluation of DBP precursors is also being performed to inform the identification and evaluation of potential source control alternatives. The results of this evaluation are pending and will be incorporated into a final presentation. RESULTS PAA was considered infeasible due to excessive risk and was eliminated for consideration. Variations of chloramine disinfection and air stripping were progressed to conceptual design for capital and annual operating cost estimates. The introduction of ammonia nitrogen and formation of NDMA from chloramines are considerable issues for the WRF. The net present values (NPV) for each alternative were estimated and air stripping towers were determined to be the most cost-effective alternative. This solution requires the smallest footprint (even in the two-pass system), has the lowest life cycle cost, and provides a confident performance guarantee given the plant sees its projected flows. This work serves as a valuable reference for utilities with TTHM concentrations exceeding current or anticipated effluent limits in drinking water, reuse, and wastewater applications. Table 3: Alternatives Cost Comparison Summary

A comprehensive evaluation of alternatives for total trihalomethane (TTHM) mitigation for a water reuse application. Alternatives included optimization of existing chlorination practices, reducing total organic carbon and therefore TTHM precursors upstream of disinfection, using alternative disinfectants that do not produce TTHMs including peracetic acid and chloramines, and removal of TTHMs from the finished water using air stripping.

SpeakerStephens, Nicole

Presentation time

08:30:00

09:00:00

Session time

08:30:00

10:00:00

SessionPeracids Challenge Traditional Disinfectants

Session number503

Session locationRoom 338

TopicDisinfection and Public Health, Intermediate Level, Research and Innovation

Author(s)

Stephens, Nicole, Stiles, Colton, Hadacek, Tyler

Author(s)N.R. Stephens1, C. Stiles1, T. Hadacek1

Author affiliation(s)1Stantec, TX, 1, 2Stantec

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159536

Volume / Issue

Content sourceWEFTEC

Word count13

Test Drive Before You Buy: Making Data-driven Life Cycle Cost Decisions

Abstract

INTRODUCTION The City of Chattanooga (the City) owns and operates the 870-megaliter per day (ML/d) Moccasin Bend Wastewater Treatment Plant (MBWWTP) that discharges treated effluent to the Tennessee River. Over the last 20 years, the City has been making strategic investments in planning level studies and innovative technologies to engage in the circular economy. Recently, site constraints and a limited capital improvements budget drove the City to investigate how thermal hydrolysis (TH) pre-treatment paired with intensified anaerobic digestion could optimize the capacity of existing assets thus avoiding costly anaerobic digestion (AD) expansion work in the future with potential economic paybacks through resource recovery. A previous investigation of TH+AD versus conventional AD indicated a narrow margin in capital and annual operating costs, however conservative planning criteria resulted in a recommendation to forego TH and expand the existing digester complex. Preliminary calculations suggested more aggressive planning criteria for a TH+AD alternative could result in a more favorable 20-year life cycle cost and opportunity for long-term return on investment through resource recovery. However, limited full-scale proof of such aggressive criteria presented unacceptable risk and required application of uncertainty factors that would again render TH+AD economically unfavorable. Therefore, the City elected to make a small ($500,000) investment in a pilot-scale demonstration study to generate site-specific data and a comprehensive life cycle cost evaluation. The objective of this paper is to discuss how the City used the pilot-scale study to de-risk a conceptual level life cycle cost evaluation and avoid approximately $50,000,000 in capital and operational expenditure over a 20-year planning period. METHODS The City completed a TH+AD pilot study to inform cost-estimating efforts associated with implementation of TH+AD at the MBWWTP. The results of this pilot study were used together with analysis of historical operating data, previous planning efforts, and industry experience to conduct a comprehensive, life cycle cost-based business case evaluation (BCE). The primary components of the BCE included flow and load projections, conceptual design of alternatives, pilot-scale testing to empirically verify the potential benefits to anaerobic digestion and sludge dewaterability, detailed capacity analysis of the existing facilities, and calculation of the 20-year life cycle cost of alternatives. The evaluation proposed and compared three alternatives: Alternative 1: Conventional Anaerobic Digestion Alternative 2: Partial-Plant TH and Intensified Anaerobic Digestion Alternative 3: Full-Plant TH and Intensified Anaerobic Digestion Site-specific operating data derived through the pilot evaluation are summarized in Table 1 and were supplemented with additional vendor testing, literature, and industry experience. These data were used to develop conceptual designs for the three alternatives including major equipment size, type, quantity, configuration, and performance. Table 1: Summary of Pilot-Derived Data Cost estimators developed a Class 5 Opinion of Probable Construction Cost (OPCC) based on the conceptual designs and price information provided by equipment vendors. The Class 5 OPCC included construction costs (direct costs and general costs), taxes, bonds, insurance, and 25 percent construction contingency and is summarized in Table 2. Escalation, estimate contingency (2.5 percent), engineering, and administrative costs are not included in the OPCC but are included in the Total Project Cost Estimate (TPCE) used in the value business case evaluation. Table 2: Capital Cost Estimates Annual O&M costs were estimated assuming 2034 annual average flow conditions (Table 3). Electricity costs were based on the number of duty units in operation at 2034 annual average solids loading conditions. Chemical costs included pre-THP dewatering polymer and biosolids dewatering polymer. Operational and maintenance labor costs were based on experience at other THP facilities and the City's hourly wage rates. Biosolids hauling costs were estimated based on the projected wet tons produced annually and vendor quoted costs for handling Class A and B biosolids. Table 3: Annual Operating Cost Estimates Finally, results were incorporated into a qualitative alternatives assessment to incorporate non-monetary considerations associated with the triple bottom line into the decision-making process. The team worked collaboratively with multiple City stakeholders to evaluate and rank each alternative under 11 criteria (Table 4). Table 4: Qualitative Evaluation Matrix RESULTS 20-year life cycle costs were prepared assuming design and construction of all alternatives occur over four years, beginning in 2020. The cumulative annual life cycle cost of each alternative was calculated (Figure 1). The estimated annual expenditure was similar for each alternative, with TH-based alternatives being slightly less due to significant savings in biosolids hauling costs. Alternative 2 was shown to have the lowest life cycle cost. However, concerns with partial-plant TH were identified early in the evaluation including inability to achieve Class A biosolids. As a result, Alternative 2 was eliminated. Alternative 3 had a lower life cycle-cost than Alternative 1 and a more favorable score in the categorical comparison. Therefore, Alternative 3 was selected as the recommended alternative. Figure 1: Cumulative Life Cycle Cost CONCLUSIONS The City's small investment in a pilot-scale demonstration study resulted in meaningful, site-specific data that increased confidence in planning criteria and life cycle cost estimates that were ultimately used to make important capital planning decisions. As a result, the City anticipates approximately $50M in capital and annual operating cost avoidance over the 20-year planning period. Several additional recommendations were proposed based on findings made throughout the evaluation. Recommendations are summarized in chronological order below. 1. Further de-risk planning and budgeting efforts by conducting a solids sampling campaign confirm or revise the blended sludge assumptions made under this evaluation. 2. Review and update flow and load projections and re-evaluate facility build-out capacity. 3. Progress Alternative 3: Full-Plant TH+AD to Preliminary Design (30 percent). Conduct value engineering, a Class 3 cost estimate, and an alternative delivery method evaluation. 4. Evaluate secondary treatment capacity considering current performance, future performance under projected flows and loads, current and future nutrient regulations, and opportunities to retrofit existing infrastructure for sidestream treatment. 5. Design and construct full-plant THP and intensified anaerobic digestion incorporating findings from recommendations 1 through 4. 6. Design and construct sidestream treatment (likely in conjunction with TH project). 7. Evaluate energy management alternatives in a small study after TH and digester system commissioning. 8. Implement selected energy recovery based on results of evaluation and available funding.

This paper was presented at the WEF Residuals and Biosolids Conference in Columbus, Ohio, May 24-27, 2022.

SpeakerStephens, Nicole

Presentation time

10:45:00

11:15:00

Session time

8:30:00

11:45:00

Session number08

Session locationGreater Columbus Convention Center, Columbus, Ohio

Topicbusiness case evaluation, Capital Planning, Pilot Scale

Author(s)