Modeling Cost, Energy, and Total Organic Carbon Trade-Offs for

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Environmental Modeling

Modeling cost, energy, and total organic carbon trade-offs for stormwater spreading basin systems receiving recycled water produced using membrane-based, ozone-based, and hybrid advanced treatment trains Jonathan L. Bradshaw, Negin Ashoori, Mauricio Osorio, and Richard G Luthy Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00184 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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Environmental Science & Technology

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Modeling cost, energy, and total organic carbon trade-offs for

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stormwater spreading basin systems receiving recycled water

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produced using membrane-based, ozone-based, and hybrid

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advanced treatment trains

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Jonathan L. Bradshaw †‡, Negin Ashoori †‡, Mauricio Osorio †‡1, and Richard G.

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Luthy †‡*

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† Department

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94305-4020

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‡ ReNUWIt,

of Civil and Environmental Engineering, Stanford University, Stanford, CA

National Science Foundation Engineering Research Center for Re-inventing

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the Nation’s Urban Water Infrastructure, Stanford, CA 94305-4020

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* Corresponding author. Mailing address: The Jerry Yang and Akiko Yamazaki

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Environment & Energy Building, 473 Via Ortega, Room 191, MC 4020, Stanford

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University, Stanford, CA 94305. E-mail address: [email protected].

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Telephone number: 650 721-2615. Fax number: 650 725-9720

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1

Present address: EKI Environment & Water, Inc., Burlingame, CA 94010

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Abstract

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To address water scarcity, cities are pursuing options for augmenting groundwater

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recharge with recycled water. Ozone-based treatment trains comprising ozone and

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biologically activated carbon potentially offer cost-effective alternatives to membrane-

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based treatment, the standard process for potable reuse in numerous countries.

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However, regulations in multiple states effectively limit the extent to which ozone-based

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treatment alone can produce recycled water for groundwater recharge. To investigate

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the trade-offs between treatment costs and regulatory constraints, this study presents

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methods for modeling and optimizing designs for 1) producing recycled water using

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membrane-based treatment, ozone-based treatment, and hybrid treatment trains

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comprising ozone-based treatment with a membrane sidestream, and 2) delivering that

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water to stormwater spreading basins. We present a case study of Los Angeles, CA, to

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demonstrate the model’s application under realistic conditions, including regulations that

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limit spreading recycled water based on its concentration of total organic carbon and

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extent of dilution. While the membrane-based treatment train exhibits economies of

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scale, we demonstrate how regulatory constraints create a diseconomies of scale effect

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for hybrid treatment systems because larger scales necessitate a higher proportion of

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recycled water undergo membrane treatment. Nevertheless, relative to membrane-

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based treatment, we identify opportunities for ozone-based or hybrid treatment trains to

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reduce treatment costs and energy use by up to 57% and 59%, respectively, for

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systems with up to 1 m3/s (23 million gallons per day) mean water recycling rate,

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potentially lowering the barrier for decentralized water recycling systems. This modeling

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approach could inform planning and policy regarding recycled water projects for 2 ACS Paragon Plus Environment

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groundwater recharge through spreading basins and, with additional modification, other

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potable reuse applications.

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Abstract/ Table of Contents art

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1 Introduction

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To address current and future water supply challenges, cities worldwide are pursuing

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opportunities to cost-effectively augment potable water supplies, including recharging

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groundwater using recycled water.1–5 While full scale, planned potable reuse projects

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and managed aquifer recharge projects operate worldwide, project costs remain a

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primary barrier to their wider adoption.6–13 Aiming to reduce the cost of producing

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recycled water, researchers and water professionals are investigating alternatives to

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membrane-based treatment, often referred to as full advanced treatment. Typically

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comprising microfiltration (MF), reverse osmosis (RO), and ultraviolet light with

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advanced oxidation (UV/AOP), membrane-based treatment is currently used for potable

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reuse in, for example, Singapore,14 Australia,15,16 Belgium,17 and California.18 Although

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water engineers in numerous countries generally consider membrane-based treatment 3 ACS Paragon Plus Environment


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the standard technology for planned potable reuse projects,6,19 it may not be the most

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fitting treatment process when considering the overall financial, social, and

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environmental impacts.20,21 In particular, the RO process’s high cost, energy-intensity,

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and concentrate management challenges motivate using alternative advanced

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treatment processes.6,22

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Especially for indirect potable reuse purposes, a leading alternative to membrane-based

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treatment replaces the MF-RO processes with ozone and biologically activated filtration

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(BAF), such as biologically activated carbon (BAC).6,23–25 In indirect potable reuse

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projects, recycled water enters an environmental buffer (e.g., an aquifer or surface

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water body) before entering the public water distribution system. Full-scale planned or

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de facto potable reuse projects using O3-BAF-based treatment trains operate

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worldwide, e.g., in Switzerland, Australia, Texas, and Georgia (United States),23,26 and

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water utilities recently led pilot projects using similar treatment in California27–29 and

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Virginia.30 Some researchers and practitioners have concluded that O3-BAF-based

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treatment can produce recycled water of comparable quality as membrane-based

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treatment, aside from dissolved solids remaining intact,22,30 which includes meeting

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public health criteria developed by the California Department of Public Health, the

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National Water Research Institute, and other agencies in the United States and

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Australia.31

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Despite O3-BAF’s potential as an alternative to membrane-based treatment, regulations

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may limit the practical contributions of O3-BAF within potable reuse projects. While there

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are federal guidelines for potable reuse in the United States, there are no formal federal

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regulations. In the absence of federal regulation, 14 states have developed unique 4 ACS Paragon Plus Environment

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potable reuse policies.24 Of these states, practitioners expect California to be a potable

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reuse leader in both installation and regulation. In 2017–2027, analysts predict

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California will install 59% of new recycled water capacity in the US.32 Practitioners

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generally consider California to have the most formalized potable reuse regulations33

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and see these regulations as potentially serving as a template for other states.33

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California requires that recycled water used for subsurface injection or surface water

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augmentation must undergo membrane-based treatment.34 Elaborated in Section 2.2.1,

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California regulations do not specifically require membrane-based treatment for the

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surface spreading of recycled water; rather, regulations limit the proportion of

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groundwater recharge from recycled water based on its total organic carbon (TOC)

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concentration. Regulators use TOC as an indicator of the public health risks from

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various trace organic chemicals.6,35 Because membrane-based treatment removes

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more TOC than O3-BAF treatment does, this regulatory constraint creates a practical

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trade-off for implementing O3-BAF treatment: membrane-based treatment costs more

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than O3-BAF, but regulations allow more recycled water for spreading if membrane-

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based treatment is used. Gerrity et al.31 identify O3-BAF’s lower TOC removal rate as a

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major limitation for potable reuse.

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Given water professionals’ growing interest in O3-BAF as an alternative potable reuse

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technology and its practical trade-offs due to regulations, our study presents system

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analysis methods to investigate how O3-BAF processes can be incorporated into multi-

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supply spreading basin systems (i.e., systems in which spreading basins receive both

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stormwater and recycled water). System analysis can be important for understanding

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how new water technologies could efficiently fit into water resource management 5 ACS Paragon Plus Environment


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plans.36–38 To facilitate planning of these systems, our study’s scope includes the

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practical engineering and economic factors that water planning entities typically

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consider during a recycled water engineering study, such as a feasibility or master

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planning study, on which planning entities often rely when making high-level planning

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decisions.

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As prior studies39,40 detailed, there is substantial literature on various aspects of

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planning groundwater recharge projects or recycled water infrastructure (e.g., 41–52) for

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urban water supply, with emphasis typically on non-potable reuse, direct potable reuse,

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siting groundwater recharge projects, or allocating water supplies without considering

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infrastructure life cycle costs. Relevant to our project’s scope, Bradshaw and Luthy39

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and Bradshaw et al.40 present methods to model and optimize multi-supply spreading

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basin systems receiving stormwater and membrane-based treated recycled water.

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Additionally relevant, Plumlee et al.53 presents generalized costs equations for several

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water recycling unit processes, including O3, BAC, MF, RO, and UV/AOP. However, we

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did not identify any studies that model groundwater recharge systems receiving

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recycled water produced using an O3-BAF process or model the relevant trade-offs

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between O3-BAF and membrane-based treatment. Our current study addresses this

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gap by developing the following new modeling frameworks and analyses to facilitate

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planning of multi-supply spreading basin systems:

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

Modeling costs and TOC removals for three different treatment trains (Figure 1),

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(a) the membrane-based treatment train comprising MF-RO-UV/AOP-soil aquifer

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treatment (SAT), (b) the ozone-based treatment train comprising O3-BAC-


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UV/AOP-SAT, and (c) a hybrid treatment train comprising ozone-based treatment

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train with a MF-RO sidestream between the BAC and UV/AOP processes

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

Quantitatively assessing how various concentrate management technologies

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affect the unit life cycle costs of producing recycled water with the above

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treatment trains

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

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Modeling how California’s regulations could limit groundwater recharge using recycled water produced using the above treatment trains

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Optimizing multi-supply spreading basin systems that meet recycled water regulations while minimizing infrastructure unit life cycle costs.

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To illustrate how this modeling framework applies under realistic conditions, we present

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a case study for the Hansen Spreading Grounds (Hansen) in Los Angeles, CA (LA).

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The City of Los Angeles has been piloting the ozone-based train, and considering the

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hybrid treatment train, to produce up to 1.5 m3/s of recycled water for delivery to

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Hansen.27 The hybrid treatment train’s MF-RO sidestream offers the opportunity to

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reduce dissolved solids and, as further discussed in Section 2, reduce TOC

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concentrations below California’s 0.5 mg/L regulatory benchmark for surface spreading

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of recycled water. Previous studies28,39,40 provide additional context for recycled water

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and spreading basin systems in LA.

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(a) Membrane-based treatment train

(b) Ozone-based treatment train

(c) Hybrid treatment train

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Figure 1. Process flow diagrams for the recycled water treatment trains evaluated in

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this study, (a) the membrane-based treatment train comprising microfiltration (MF),

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reverse osmosis (RO), ultraviolet light with advanced oxidation process (UV/AOP), and

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soil aquifer treatment (SAT); (b) the ozone-based treatment train comprising ozone (O3),

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biologically activated carbon (BAC), UV/AOP, and SAT; and (c) a hybrid treatment train

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comprising O3, BAC, UV/AOP, and SAT with a sidestream of MF and RO between the

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BAC and UV/AOP processes.

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2 Materials and Methods

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2.1

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Our model builds on the engineering and economic framework developed by Bradshaw

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and Luthy39 and Bradshaw et al.40 Collectively, those studies presented a specialized

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network flow model for multi-supply spreading basin systems receiving membrane-

Modeling and Optimization Approach


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based treated recycled water and dynamically available stormwater. The system

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boundary comprises 1) the production of recycled water, 2) the pipeline and pumping

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infrastructure to convey recycled water to spreading basins, and 3) the infiltration of

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stormwater and recycled water through existing spreading basins. The model identifies

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design and operation variables that minimize the unit life cycle costs (i.e., the

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infrastructure life cycle costs per unit of groundwater recharge) subject to engineering

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constraints. Tables S1 and S2 summarize the model and case study inputs,

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parameters, and decision variables.

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Due to a lack of available information, our study does not account for the post-treatment

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of recycled water, such as decarbonation or adding lime to stabilize the water after the

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MF-RO-UV/AOP processes. In literature from research and engineering practice, initial

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planning studies typically exclude these post-treatment steps, apparently because their

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costs are expected to be relatively small. For example, for their membrane-based

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Groundwater Replenishment System, Orange County Water District reports that post-

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treatment costs account for approximately 5% of capital costs and 1.6% of operation &

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maintenance costs.54The absence of relevant models describing the costs of post-

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treatment processes represents a data gap that could be addressed in future work. Our

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systems model is a mixed integer nonlinear program with multiple nonconvex terms, for

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which existing optimization approaches could not guarantee finding a global solution.

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Section S2 details our modified optimization approach to find the global minimum unit

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life cycle cost.



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2.1.1 Ozone-based and hybrid treatment train performance background

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As mentioned in Section 1, Gerrity et al.31 detail how the ozone-based treatment train

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can meet various public health criteria, including removal of trace organic chemicals,

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disinfection byproducts, and pathogens. That study identified the removal of total

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organic carbon and nitrogen as potential limitations to the ozone-based treatment train,

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particularly if the wastewater treatment process does not include nutrient removal. In

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our study, we assume all wastewater undergoes conventional nutrient removal, which

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typically provides sufficient nitrogen removal.55

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In our model, the ozone-based and hybrid treatment trains’ performance depends on

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site-specific conditions and operational design parameters, particularly, influent TOC

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concentrations entering the water recycling process, O3 dose, and BAC empty bed

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contact time (EBCT). While the influent TOC concentration depends on the upstream

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wastewater influent and wastewater treatment processes, which are outside of our

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study’s scope, our study accounts for decisions regarding O3 dose and BAC EBCT.

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Ozonating treated wastewater effluent can form the regulated disinfection byproducts N-

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nitrosodimethylamine (NDMA) and bromate, with the trade-off that higher O3 doses

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increase bromate formation but facilitate BAC’s reduction of trace organic

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chemicals.25,56–58 Prior studies have demonstrated that operating the ozone-based

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treatment train with an ozone dose of approximately 0.7–1.0 mg O3 per mg influent TOC

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and a BAC EBCT of approximately 15–40 minutes generally provides an appropriate

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balance between reductions in TOC and other contaminants while sufficiently controlling

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NDMA and bromate.27–29,56 Multiple existing water recycling facilities operate O3-BAC


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processes with similar parameters.23,58,59 The UV/AOP, SAT, and MF-RO processes

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can further reduce TOC, NDMA, and bromate.6,25,28,56,60–62 Section 2.2 discusses how

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these treatment and design considerations applied to the LA case study.

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2.1.2 Treatment train and concentrate management costs

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Based on Plumlee et al.’s53 data for generalized costs of water recycling unit processes,

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we developed parametric cost equations to model the treatment train’s life cycle cost

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(Table S3). For each process in our treatment trains except SAT, Plumlee et al.

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proposed that capital and operation & maintenance (O&M) costs exhibited power law

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relationships as a function of the water recycling facility (WRF) capacity. To describe the

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MF and RO processes’ capital and labor costs in our model, we selected power law

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relationships, which we confirmed provides a close fit for the cost data (Figures S1–S3).

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However, to describe every other capital and O&M cost, we selected a linear

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relationship, which we found provides an equivalent or closer fit than a power law

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relationship does (Figures S1–S6) and reduces the optimization problem’s complexity.

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We applied Plumlee et al.’s methods to describe how O3 and BAC costs depend on

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ozone dosing and BAC EBCT, design choices that affect treatment performance, as

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described in Section 2.1.1. Following Plumlee et al.’s guidance, we assumed the unit

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process costs are additive, though some practitioners suggest that O3-BAC process

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may reduce costs of the MF-RO processes.63 We also followed Plumlee et al.’s

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assumption that the MF and RO processes would operate with 90% and 75% recovery

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rates, respectively, for an overall recovery of 67.5%, i.e., 90% * 75% = 67.5%.64 These 11 ACS Paragon Plus Environment


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recovery rates are conservative, with other studies estimating overall recovery rates of

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75–80%.63,65 Consistent with the existing model’s assumption that our system

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comprises existing spreading basins and infiltrating recycled water does not materially

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affect spreading basins’ costs,39 we assume the SAT process does not incur additional

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costs.

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In addition to the recycled water treatment train costs, our analysis incorporates general

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costs of managing RO concentrate. For inland locations or other places where existing

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infrastructure for ocean disposal is unavailable, the absence of cost-effective and

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environmentally suitable concentrate management can be a major barrier to recycled

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water projects, further incentivizing a treatment process, like O3-BAF, that does not

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produce a concentrate.21,66 The actual cost and feasibility of various concentrate

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management technologies depend on numerous project-specific geographic

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characteristics, e.g., influent water quality, availability of land, geologic conditions,

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proximity to the ocean, and environmental policies and regulations.67 Nevertheless, for

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the context of using FAT to produce potable water from treated wastewater, the

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National Water Research Institute63 offers general reference costs for several

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concentrate management technologies. These technologies include (costs expressed

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here in 2011 national average values per unit of product water) deep well injection

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($0.052/m3), evaporation ponds ($0.11/m3), land application ($0.085/m3), brine line to

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ocean ($0.085/m3), and zero liquid discharge ($0.57/m3). For illustrative purposes, we

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adapt those reference costs to consider how concentrate management influences the

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costs of designs that include various proportions of MF-RO treatment.


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2.2

Los Angeles Case Study

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Our case study consists of examining three different system scenarios to connect

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recycled water to the Hansen Spreading Grounds (Figure 2 and Table S2), to which LA

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water utilities are investigating various opportunities to deliver recycled water.27,65 These

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scenarios, which we evaluate over a 30-year project lifetime, include three separate

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system configurations with connections to Hyperion, LA-Glendale, and Tillman WRFs,

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respectively. For our study, we assume all three treatment plants produce secondary or

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tertiary-treated water with nitrogen removal. While these treatment levels are common,

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of these three WRFs, Hyperion does not currently feature nitrogen removal. Existing

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work suggests adding conventional biological nitrogen removal may increase unit life

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cycle costs by 0.04–0.06 $/m3 and energy intensity 0.4–0.5 MJ/m3, but it would also

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improve the performance of the membrane-based treatment through reduced

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fouling.21,69 Consistent with LA’s planning, our case study assumes all designs include

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ultimately discharging RO concentrate to the ocean using existing infrastructure.

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In our model, three hydraulic parameters constrain Hansen’s operational capabilities.70

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First, Hansen’s exiting intake infrastructure allows diverting up to 17 m3/s of stormwater

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from the Tujunga Wash, a channel adjacent to Hansen. Second, water can infiltrate at

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Hansen up to 4.3 m3/s, the hydraulic loading rate (i.e., year-round long-term infiltration

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rate). Third, when the water delivery rate to Hansen exceeds the hydraulic loading rate,

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up to 1.7 x 106 m3 of water may pond in Hansen. In our model, Hansen may receive

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stormwater or recycled water as long as those constraints are satisfied.

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Figure 2. Map of system scenarios evaluated in LA case study. Color of the “System

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scenario and flow direction” arrows correspond to system scenarios plotted in Figures 5

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and 6. Data sources: 70–73.

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2.2.1 Treatment train design and regulatory constraints

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As mentioned in Section 2.1.1, the design of O3-BAF systems requires balancing

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several water quality considerations, namely, concentrations of TOC, bromate, and

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NDMA. Potable reuse regulations in California, Florida, Massachusetts, and

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Pennsylvania as well as federal guidelines place limits on TOC.24,34,74–76 In California

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regulations, TOC constrains the recycled municipal wastewater contribution (RWC,

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sometimes called the recycled water contribution) when recharging aquifers supplying

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drinking water.34 In the context of our spreading basin system, the RWC is the ratio of

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recycled water delivered to the sum of recycled water and diluent water delivered to the

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spreading basin (Equation 1), where diluent water meets certain water quality,

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monitoring, and reporting requirements and is not of wastewater origin. In our study,

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stormwater is diluent water. Regulations specify the RWC cannot exceed the ratio of 0.5

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mg/L to the TOC in recycled water (Equation 2). For example, if the recycled water

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contains 1 mg/L TOC, then the maximum permissible RWC is 50%, indicating the

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spreading basin must receive at least one unit of diluent water for each unit of recycled

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water it receives. Similarly, any recycled water TOC concentration less than 0.5 mg/L

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indicates regulations would not limit on how much recycled water a spreading basin

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may receive.

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𝑅𝑊𝐶 =

𝑅𝑒𝑐𝑦𝑐𝑙𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 𝑅𝑒𝑐𝑦𝑐𝑙𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 + 𝐷𝑖𝑙𝑢𝑒𝑛𝑡 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑

(𝐸𝑞 1)

289

290

mg 0.5 L 𝑅𝑊𝐶 ≤ 𝑇𝑂𝐶

(𝐸𝑞 2)



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In these regulations, the recycled water TOC concentration refers to the concentration

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after all credited treatment process steps, including SAT, the effectiveness of which is

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quantified by demonstration studies and by monitoring groundwater at a permitted

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compliance point.34 Therefore, predicting the TOC value to compute the RWC requires

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quantifying 1) the TOC in the treated wastewater effluent entering the water recycling

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process and 2) the removal of TOC throughout the recycled water treatment train,

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discussed further below.

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Concentrations of organic carbon in wastewater vary due to numerous location-specific

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factors. For context of typical organic carbon concentrations, in their survey of treated

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effluent from 12 wastewater treatment facilities in California and one in Nevada, Khan et

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al.77 reported dissolved organic carbon (DOC)—generally comparable to TOC in treated

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effluent from well-managed wastewater treatment plants—present in a range of 4.9–

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13.7 mg/L. Lee et al.78 reported similar values in their survey of 10 wastewater

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treatment facilities in Switzerland (3), Australia (2), and the United States (5): a range of

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4.7–15.0 mg/L except for one Australian sample with 26.4 mg/L.

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To describe the TOC concentrations at each of the WRF our case study considers, we

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rely on Khan et al,77 which reported 10-day mean DOC concentrations of 8.7 mg/L, 9.2

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mg/L, and 10.9 mg/L for Tillman, LA-Glendale, and Hyperion, respectively, in the treated

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wastewater effluent—i.e., the influent to the water recycling process. These values fall

310

within the typical range of TOC concentration found around the world, as discussed

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above. Based on available data, the model assumes constant mean TOC

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concentrations; however, the model could incorporate TOC concentrations that vary

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with time if relevant data were available. More recent work finds a lower median TOC 16 ACS Paragon Plus Environment

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concentrations at Tillman, 7.8 mg/L;79 however, more recent concentrations at LA-

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Glendale and Hyperion are unavailable.80 Consequently, our case study uses Khan et

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al.’s reported concentrations for dataset consistency across the three WRFs. Khan et

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al.’s concentrations for these three WRFs are also generally representative of recent

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TOC and DOC concentrations found in pilot studies in Los Angeles,29 San Diego,63 and

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Hampton Roads, VA,30 as well as recent lab studies on O3-BAC using treated effluent

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from wastewater treatment plants.56

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To maximize groundwater recharge with recycled water, water planners seek to operate

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O3-BAF systems that maximize TOC removal while controlling the formation of bromate

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and NDMA. For our case study, we selected treatment parameters based on a pilot

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study in LA that includes processing Tillman’s tertiary-treated effluent through O3-BAC-

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UV/AOP and a column filed with soil from Hansen to simulate SAT. Using an optimal

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ozone dose of 0.7 mg O3/ mg TOC, BAC EBCT of 15 minutes, and a soil column

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retention time of 30 days—a duration LA selected to reflect their expected SAT

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compliance point—this pilot study found O3-BAC reduced TOC by approximately 25%,

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and SAT removed an additional 50–60% TOC for an overall TOC reduction of 65–

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75%.27,79 As discussed in Section 2.2, both this O3 dosing and BAC EBCT represent

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typical design values. Additionally, these TOC reductions are generally consistent with

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other lab and field studies studying O3-BAC and SAT under similar operating

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conditions.25,29,59,60,60,81 Following the Tillman pilot study, for our case study we selected

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a dose of 0.7 mg O3/ mg TOC, a 20 minute BAC EBCT, and assumed an overall 70%

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reduction of TOC throughout the O3-BAC-UV/AOP-SAT treatment train. 17 ACS Paragon Plus Environment


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The MF-RO processes provide greater TOC removal than the ozone-based treatment

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train. For the membrane-based treatment train, we assume a 99% TOC reduction

338

based on a typical design.65 In the hybrid treatment train, the MF-RO sidestream

339

provides additional TOC removal based on the proportion of flow processed through the

340

sidestream. However, we are unaware of performance information describing how the

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overall TOC reduction for flow undergoing all six processes of O3-BAC-MF-RO-

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UV/AOP-SAT treatment train. Given this information gap, for the hybrid treatment train’s

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flow through the MF-RO sidestream, we assume a 99% TOC reduction based on a

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typical MF-RO-UV/AOP treatment train design, discussed above. This reduction may be

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slightly conservative: pilot studies in San Diego demonstrated between 99.1% and

346

99.6% TOC reductions through MF-RO-UV/AOP and O3-BAC-MF-RO-UV/AOP,

347

respectively.63,82 Although SAT may further reduce TOC, we could not find relevant

348

studies regarding TOC reduction in an O3-BAC-MF-RO-UV/AOP-SAT treatment train;

349

consequently, our model does not include additional TOC reduction from SAT following

350

the MF-RO-UV/AOP processes. In sum, we assume water processed through the O3-

351

BAC-UV/AOP-SAT main stream undergoes a 70% TOC reduction, and water processed

352

through the MF-RO sidestream (for a complete process O3-BAC-MF-RO-UV/AOP-SAT)

353

undergoes a 99% TOC reduction.

354

2.2.2 Additional case study parameters

355

In addition to the parameters discussed above, our case study applied existing models

356

for project finance, conveyance cost, stormwater availability, and spreading basin

357

performance.40 To account for the regional and temporal cost differences represented

358

among the various cost data sources, we adjusted all case study costs to the Los 18 ACS Paragon Plus Environment

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359

Angeles-Anaheim region and to the year 2011 to be consistent with Plumlee et al.53 We

360

adjusted capital costs using Engineering News-Record Construction Cost Index83 and

361

O&M costs using the Consumer Price Index.84

362

On a comparable basis to our financial modeling approach, LA estimates the economic

363

value of recharged water is $1.2/m3 (in 2011 dollars), signifying that LA is more likely to

364

pursue projects with unit life cycle costs below this threshold value.85 This value can be

365

considered a benchmark for evaluating infrastructure projects like our study

366

investigates.

367

3 Results and Discussion

368

This section discusses how our model can inform water planning by quantifying cost,

369

regulatory, and energy trade-offs among different water recycling treatment process

370

designs and concentrate management conditions. We conclude this section with a

371

discussion of how this work can apply to future water infrastructure research and

372

planning.

373

3.1

374

Treatment Train Trade-offs Under Different Concentrate Management Conditions

375

Applying the treatment system design conditions and generalized treatment costs

376

discussed in Section 2.1, Figure 3 presents recycled water production costs under two

377

different concentrate management conditions: (a) concentrate management incurs no

378

additional expense—e.g., if the concentrate is discharged using an existing ocean

379

outfall or other existing wastewater effluent disposal method—and (b) the cost of



Page 20 of 41

380

producing recycled water includes managing concentrate using zero liquid discharge,

381

the most expensive concentrate management technology National Water Research

382

Institute53 presents. Zero liquid discharge is an increasingly popular process that

383

eliminates the liquid waste stream from RO concentrate—e.g., using vapor compressors

384

to concentrate and crystallize brine—with residual solids suitable for resource recovery

385

or disposal.86 Figure S7 presents the results for additional concentrate management

386

options. These figures show that the costs of each treatment train exhibit economies of

387

scale. The concentrate management method affects the unit life cycle costs for both the

388

membrane-based and hybrid treatment trains. Additionally, for the hybrid treatment

389

train, the unit life cycle costs depend on the proportion of water produced through the

390

MF-RO sidestream. Specifically, the plots illustrate how two factors increase unit life

391

cycle costs with the proportion of MF-RO:

392

1. Per unit production capacity, the MF-RO processes have higher capital and O&M

393

costs than the O3-BAC processes (Figures S1–S6, Table S2). Moreover, the MF

394

and RO processes create a retentate and concentrate waste stream,

395

respectively, that detracts from the volume throughput of product water (i.e.,

396

permeate). Specifically, the MF-RO process’s overall 67.5% recovery rate means

397

that producing 1 unit of permeate requires the upstream O3-BAC process to treat

398

approximately 1.48 units of water, with 0.48 units becoming a waste stream.

399

Together, these factors lead to a wide range of unit life cycle costs (Figure 3a)

400

depending on how much MF-RO the treatment train uses. For example, to

401

produce 1 m3/s of recycled water using the hybrid treatment train, unit life cycle


Page 21 of 41


402

costs more than quadruple from $0.16/m3 to $0.76/m3 as the proportion of

403

recycled water processed through MF-RO increases from 0% to 100%.

404

2. Managing the RO concentrate stream may incur additional expense, further

405

amplifying the costs associated with the MF-RO sidestream. As mentioned in

406

Section 2.1.2, zero liquid discharge (Figure 3b) costs $0.57 per m3 of MF-RO

407

permeate. Compared to the condition where concentrate management does not

408

add expense, producing 1 m3/s using membrane-based treatment with zero liquid

409

discharge, for example, nearly doubles production unit life cycle costs ($1.17/m3

410

with zero liquid discharge vs. $0.60/m3 with no added concentrate management

411

expense).

412

The plots illustrate how, as concentrate management costs increase, costs for

413

membrane-based treatment (which includes using MF-RO for 100% of product water)

414

increase faster than costs for the hybrid treatment train (which includes using an MF-RO

415

sidestream for a varied proportion of product water). For example, producing 1 m3/s of

416

recycled water through membrane-based treatment incurs a unit life cycle cost of

417

approximately $0.60/m3 when concentrate management does not add expense (Figure

418

3a). Under this same concentrate management condition, the $0.60/m3 unit life cycle

419

cost is comparable to a hybrid treatment train producing 1 m3/s with the MF-RO

420

sidestream generating 75% of the product water. If production costs instead included

421

zero liquid discharge (Figure 3b), the unit life cycle cost of this same hybrid treatment

422

train configuration would increase to approximately $1.05/m3, approximately 11% less

423

than the $1.17/m3 unit life cycle cost for membrane-based treatment. This example

424

further emphasizes the hybrid treatment train’s potential economic advantage over 21 ACS Paragon Plus Environment


Page 22 of 41

425

membrane-based treatment in situations where water recycling may require expensive

426

concentrate management.


Page 23 of 41


427 (a) No additional concentrate management expense

(b) Zero liquid discharge

428 429

Figure 3. Unit life cycle costs for producing recycled water under two concentrate

430

management conditions: (a) concentrate management incurs no additional expense and

431

(b) the cost of producing recycled water includes managing concentrate using zero

432

liquid discharge. Each plot includes the costs for i) membrane-based treatment and ii)

433

hybrid treatment train with varying percentages of microfiltration and reverse osmosis

434

(MF-RO). In the legend, percentages refer to the proportion of the hybrid treatment

435

train’s product water treated through MF-RO. Hybrid treatment train designs with 0%

436

MF-RO are equivalent to the ozone-based treatment train. Costs are prepared in 2011



Page 24 of 41

437

United States national average values. Each marker represents an increment of 0.044

438

m3/s production capacity.

439 440

3.2

Full System Design with Regulatory Constraints in Los Angeles Case Study

441

The results of our LA case study demonstrate how recycled water systems could be

442

planned under realistic engineering and regulatory conditions. Figure 4 shows the

443

optimal system unit life cycle costs for producing recycled water using either membrane-

444

based treatment or the hybrid treatment train, assuming no additional concentrate

445

management expense. All system scenarios include maximizing the volume of

446

stormwater diverted into the spreading basin over the 30-year project lifetime, a mean of

447

0.61 m3/s. In our case study’s context, regulations credit the stormwater as diluent water

448

towards determining the system’s RWC, discussed in Section 2.2.1.

449

For each of the systems in our case study, membrane-based treatment reduces

450

recycled water’s TOC concentrations below 0.5 mg/L, signifying that regulations would

451

not constrain using membrane-based treated recycled water for spreading. In contrast,

452

after the ozone-based treatment train (O3-BAC-UV/AOP-SAT), the recycled water from

453

the WRFs exceeds 0.5 mg/L TOC, and, consequently, regulations would limit the extent

454

of spreading recycled water. Specifically, mean deliveries of ozone-based treated

455

recycled water to Hansen would be limited to 0.15 m3/s, 0.14 m3/s, and 0.11 m3/s for

456

Tillman, LA-Glendale, and Hyperion systems, respectively. However, as Figure 4

457

illustrates, greater deliveries are possible through hybrid treatment with greater use of


Page 25 of 41


458

the MF-RO sidestream to reduce the recycled water’s TOC, with the trade-off of higher

459

costs.

460

The dynamics of this trade-off—between the costs of producing recycled water and

461

regulatory limits on spreading it—create a general diseconomies of scale effect for the

462

hybrid treatment train. That is, the unit life cycle costs increase with system scale

463

because a higher proportion of recycled water must be produced using MF-RO. In

464

contrast, we observe economies of scale with membrane-based treatment (Figure 4)

465

and the conveyance system (Figure S8), a finding consistent with prior studies.39,40 As a

466

result of these behaviors, the hybrid treatment train’s economic advantage over

467

membrane-based treatment generally decreases as scale increases.

468

Among our various case study system scenarios, there are two reasons why the

469

individual WRFs’ influent TOC concentrations lead to different unit life cycle costs when

470

using the ozone-based and hybrid treatment trains. The first reason is that costs for

471

ozonation increase linearly with influent TOC concentrations. In our case study, unit life

472

cycle costs modestly increase 8.1 x 10-4 $/m3 per mg/L of influent TOC. Given the

473

WRFs we evaluate have influent TOC concentrations between 8.74 mg/L and 10.96

474

mg/L, differences in ozonation costs contribute to less than 1% of production unit life

475

cycle costs. The second, more substantial, reason why influent TOC concentrations

476

affect unit life cycle costs is that the RWC—the regulatory constraint on how much

477

recycled water is permissible for spreading—increases as the inverse of TOC

478

concentrations. Consequently, WRFs with higher influent TOC concentrations, such as

479

Hyperion, require a higher proportion of costly MF-RO treatment to achieve comparable

480

RWC values as WRFs with lower influent TOC concentration, such as Tillman. In the 25 ACS Paragon Plus Environment


Page 26 of 41

481

case study, the mean unit life cycle cost to produce 0.08–0.5 m3/s of recycled water

482

increases by 0.028 $/m3 per mg/L of influent TOC; over this production range,

483

Hyperion’s production unit life cycle costs are a mean of 16% higher than Tillman’s. In

484

sum, regulatory constraints on TOC in recycled water are the primary driver for the

485

differences in the optimal production costs possible among our case study’s different

486

systems.


Page 27 of 41


(a) Tillman system

(b) LA-Glendale system

(c) Hyperion system

487

Figure 4. Optimal unit life cycle costs for producing recycled water applying different

488

treatment trains in the (a) Tillman system, (b) LA-Glendale system, and (c) Hyperion

489

system. Each plot includes the costs for i) the membrane-based treatment train and ii)

490

the hybrid treatment train with varying percentages of microfiltration and reverse

491

osmosis (MF-RO), assuming no additional concentrate management expense. In the

492

legend, percentages refer to the proportion of the hybrid treatment train’s product water

493

treated through MF-RO, which range from 0–70% in the Tillman system, 0–74% in the

494

LA-Glendale system, and 0–82% in the Hyperion system. Hybrid treatment train designs



495

with 0% MF-RO are equivalent to the ozone-based treatment train. Each marker

496

represents an increment of 0.044 m3/s production capacity.

Page 28 of 41

497 498

The hybrid treatment train’s diseconomies of scale substantially change the shape of

499

the optimal cost curves for Tillman and LA-Glendale systems. When these systems

500

instead use membrane-based treatment, the economies of scale for both production

501

(Figure 4) and conveyance (Figure S8a) result in overall system economies of scale

502

(Figure 5a). In contrast, the production diseconomies of scale for the hybrid treatment

503

train counterbalances the conveyance economies of scale (Figure S8b), resulting in a

504

relatively constant system unit life cycle cost (Figure 5b): means of $0.92/m3 and

505

$1.0/m3 for Tillman and LA-Glendale systems, respectively. Compared to the

506

membrane-based treatment systems, ozone-based and hybrid treatment train systems

507

have up to 57% and 54% lower unit life cycle costs for Tillman and LA-Glendale,

508

respectively, with greater economic advantage associated with smaller scaled systems.

509

In contrast, for Hyperion systems, the difference between membrane-based treatment

510

and the hybrid treatment train is dampened by both higher conveyance and production

511

costs compared to the Tillman and LA-Glendale systems. To produce 0.08–0.2 m3/s of

512

recycled water, the hybrid treatment train system costs up to 22% less than the

513

membrane-based treatment system. However, for producing more than 0.2 m3/s, unit

514

life cycle costs for the hybrid treatment train system remain within 7% of the membrane-

515

based treatment system, with membrane-based treatment systems exhibiting lower unit


Page 29 of 41


516

life cycle costs than hybrid treatment train systems for mean production rates above 0.5

517

m3/s.

518

(a) Membrane-based treatment

(b) Hybrid treatment train

519

Figure 5. Optimal unit life cycle costs for total system costs in the Los Angeles case

520

study. Plots include the costs for a) membrane-based treatment and b) the hybrid

521

treatment, assuming no additional concentrate management expense. Each marker

522

represents an increment of 0.044 m3/s system capacity.

523 524

In the context of LA’s groundwater recharge planning, our findings indicate that LA may

525

consider a broader range of potential recycled water projects as economically feasible. 29 ACS Paragon Plus Environment


Page 30 of 41

526

As mentioned in Section 2.2, LA estimates the economic value of recharged water is

527

$1.2/m3 (2011 dollars). While several of the small-scale membrane-based systems

528

exceed this threshold, all but one of the hybrid systems for LA-Glendale and Tillman fall

529

below it, suggesting that the hybrid treatment train—even with current regulatory

530

constraints—may enable water planners to consider smaller, decentralized recycled

531

water projects as economically feasible.

532

Similar to its economic advantage, the hybrid treatment train’s energy efficiency relative

533

to membrane-based treatment also decreases with scale (Figure 6). While the marginal

534

energy intensity of membrane-based treatment and ozone-based treatment remains

535

relatively constant—approximately 4.9 MJ/m3 and 1.6 MJ/m3, respectively, in our

536

model—the energy intensity for hybrid treatment trains starts at approximately 2.0

537

MJ/m3, 59% less than membrane-based treatment, and increases with scale as more

538

MF-RO treatment is required to comply with state regulations. Additionally, similar to the

539

unit life cycle costs of ozonation, the energy intensity of ozonation increases linearly

540

with influent TOC concentration. Overall, these factors cause the hybrid treatment train

541

to be less energy-intensive than membrane-based treatment over two specific

542

production ranges, 1) Tillman and LA-Glendale systems with mean production rates

543

less than 0.4 m3/s and 2) Hyperion systems with mean production rates less than 0.2

544

m3/s. The systems’ different conveyance designs, which are effectively independent of

545

the selected treatment train, causes the remaining differences between the systems’

546

energy intensity. Following from the differences in pipeline length and net elevation

547

change (Table S2), the Hyperion system’s conveyance has the highest energy intensity

548

(3.9–5.3 MJ/m3) followed by the LA-Glendale system (2.4–3.2 MJ/m3) and Tillman 30 ACS Paragon Plus Environment

Page 31 of 41


549

system (1.2–1.7 MJ/m3). Discontinuities in Figures 6b and 6c occur when there is a

550

change in the conveyance system’s integer variables: energy intensity changes abruptly

551

with changes in pipeline diameter or number of pump stations.

552 553 (a.) Hybrid treatment train production

(b.) Hybrid treatment train systems

(c.) Membrane-based treatment systems

554

Figure 6. Energy intensities of recycled water infrastructure in the Los Angeles case

555

study for (a) recycled water production through the hybrid treatment train, (b) hybrid

556

treatment train systems, (c) and membrane-based treatment systems. Each marker

557

represents an increment of 0.044 m3/s system capacity. 31 ACS Paragon Plus Environment


Page 32 of 41

558 559

3.3

Discussion

560

3.3.1 Implications for water management and engineering practice

561

This study presents and demonstrates a modeling framework that advances the

562

understanding of the trade-offs between membrane-based, ozone-based, and hybrid

563

treatment trains to produce recycled water for groundwater recharge. In particular, we

564

demonstrate how regulations limit the opportunity for ozone-based and hybrid treatment

565

train to contribute to multi-supply spreading basin projects. The potential advantage of

566

ozone-based and hybrid treatment trains relative to membrane-based treatment is

567

sensitive to both TOC concentrations entering the water recycling process and the scale

568

of the recycled water project, with lower TOC concentrations and smaller scale projects

569

offering greater potential advantage. Our findings highlight the opportunity for the

570

ozone-based or hybrid treatment trains to offer substantial economic and energy

571

savings for small- or medium-sized systems (i.e., ≤ 1 m3/s, or 23 million gallons per day,

572

mean water recycling rate), potentially lowering the barrier for decentralized water

573

recycling configurations, an ongoing focus of water infrastructure research (e.g.,

574

2,36,41,43,49,87,88).

575

available, concentrate management costs amplify potential advantages of the ozone-

576

based or hybrid treatment trains.

577

We expect our systems approach to be most useful during the early stages of planning

578

recycled water infrastructure systems or during policy development. Because our model

579

provides insights between the costs and water quality trade-offs associated with

For locations where low-cost concentrate management options are not


Page 33 of 41


580

different advanced treatment trains, water planners can better understand the

581

opportunities, and limitations, of incorporating ozone-based or hybrid treatment.

582

Similarly, this modeling framework can inform the impact on project economics of

583

specific water quality constraints, such as California’s 0.5 mg/L TOC regulatory

584

benchmark for spreading recycled water, modifying water reuse regulations to focus on

585

biodegradable dissolved organic carbon (BDOC) rather than TOC,35 or limiting

586

concentrations of dissolved solids, a particularly important consideration for aquifers

587

with high risk of concentrating salts.

588

3.3.2 Potential areas of future applications and research

589

Although our study focuses on multi-supply spreading basin projects, our framework

590

could serve as a basis for modeling other recycled water contexts, such as surface

591

water augmentation or direct potable reuse. For example, a treatment train comprising

592

O3-BAC-MF-RO-UV/AOP, analogous to our hybrid treatment train with 100% of product

593

water treated through MF-RO and without SAT, is being evaluated for potential

594

application in direct potable reuse89 and will be used to augment surface water

595

reservoirs in the Pure Water San Diego Program.90 Future research could build on our

596

modeling framework to investigate the salt balance of recycled water infrastructure

597

designs, an important consideration for water reuse in both urban and agricultural

598

settings.91,92 A salt balance can be challenging because indirect potable reuse schemes

599

are often part of partial closed-loop water supply systems, with numerous other stocks

600

and flows of salts.

601

Future experimental and field studies could clarify some uncertainties related to

602

recycled water project costs. As mentioned in Section 2.1, the absence of models 33 ACS Paragon Plus Environment


Page 34 of 41

603

describing post-treatment processes represents a data gap. In addition, our modeling

604

framework assumes unit process costs are additive, but some practitioners suggest that

605

the O3-BAC process may lower costs of the MF-RO processes by reducing membrane

606

fouling and permitting the operation of membranes at higher fluxes.63 Research that

607

elaborates on these potential process synergies could better inform the types of trade-

608

offs evaluated in our study and may show that hybrid treatment trains have a greater

609

economic advantage we have demonstrated.

610

4 Supporting Information

611

Additional details, including plots and tables, regarding this study’s modeling and

612

optimization methods, water recycling costs, concentrate management alternatives, and

613

case study results.

614

5 Acknowledgements

615

Project support was provided by the National Science Foundation’s Engineering

616

Research Center for Re-inventing the Nation’s Urban Water Infrastructure (ReNUWIt,

617

NSF ERC 1028968) and the Stanford Woods Institute for the Environment. J.L.B. was

618

supported by the U.S. Department of Defense through its National Defense Science &

619

Engineering Graduate Fellowship Program and by Stanford University through a David

620

& Lucille Packard Foundation Fellowship.

621

We thank John Kenny at Trussell Technologies, Inc., for providing data and other

622

materials used in this study.


Page 35 of 41


623

The map featured in this manuscript was created using ArcGIS® software by Esri.

624

ArcGIS® and ArcMap™ are the intellectual property of Esri and are used herein under

625

license. Copyright © Esri. All rights reserved. For more information about Esri®

626

software, please visit www.esri.com.

627 628

References

629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659

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