Desalination 518 (2021) 115300

Available online 16 August 20210011-9164/© 2021 Elsevier B.V. All rights reserved.

Operational optimization of closed-circuit reverse osmosis (CCRO) pilot to recover concentrate at an advanced water purification facility for potable reuse

Han Gu a,*, Megan H. Plumlee a, Michael Boyd b, Michael Hwang c, James C. Lozier d

a Orange County Water District, Research & Development Department, 18700 Ward Street, Fountain Valley, CA 92708, United States of America b Desalitech (DuPont Water Solutions), One Gateway Center, Suite 809, Newton, MA 02458, United States of America c Jacobs Engineering Group, 2600 Michelson Drive, Suite 500, Irvine, CA 92612, United States of America d Jacobs Engineering Group, 1501 West Fountainhead Parkway, Suite 401, Tempe, AZ 85282, United States of America

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• Closed-circuit reverse osmosis (CCRO) was piloted to treat RO concentrate.

• Pilot is operationally sustainable from a two year study in a water reuse facility.

• Adaptive control strategies were employed to manage fluctuations in water quality.

• CCRO treating RO concentrate could increase the overall RO recovery to 91%.

A R T I C L E I N F O

Keywords: Closed circuit reverse osmosis Pilot study Operational optimization Fouling and mineral scaling mitigation Water and wastewater treatment Potable water reuse

A B S T R A C T

Closed-circuit reverse osmosis (CCRO) was piloted to treat RO concentrate from a potable reuse facility to in-crease the RO recovery beyond 85%. The study determined optimum operating conditions and maintenance requirements for sustained performance at maximum recovery including flux, cross-flow velocity, and membrane cleaning intervals. The CCRO pilot included a “side conduit” to displace spent concentrate without depressu-rizing the membrane elements. Performance was evaluated in terms of recovery, clean-in-place (CIP) frequency, and permeate quality. Adaptive control strategies were implemented to manage feed water quality fluctuations by operating in variable recoveries where cycle-to-cycle recovery was controlled by concentrate conductivity, feed pressure and volumetric recovery. An important contribution of this study is the long-term pilot dataset collected over two years of operation, which showed the treatment of RO concentrate by CCRO to be technically

Abbreviations: AWPF, Advanced Water Purification Facility; CIP, Clean-In-Place; CCRO, Closed-Circuit Reverse Osmosis; ED, Electrodialysis; FO, Forward Osmosis; FR, Flow Reversal; GPM, Gallons Per Minute; GWRS, Groundwater Replenishment System; MD, Membrane Distillation; MPV, Membrane Pressure Vessel; MF, Microfiltration; MGD, Million Gallons per Day; OC San, Orange County Sanitation District; OCWD, Orange County Water District; PF, Plug Flow; PV, Pressure Vessel; RO, Reverse Osmosis; ROF, Reverse Osmosis Feed; ROC, Reverse Osmosis Concentrate; SC, Side Conduit.

* Corresponding author. E-mail address: hgu@ocwd.com (H. Gu).

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Desalination

journal homepage: www.elsevier.com/locate/desal

https://doi.org/10.1016/j.desal.2021.115300 Received 19 April 2021; Received in revised form 8 August 2021; Accepted 9 August 2021

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feasible and operationally sustainable. The pilot operated continuously with a CIP interval greater than two months and produced permeate that met potable reuse requirements. At theoretical full scale, CCRO could in-crease the facility RO recovery from 85% to 91% (92% was demonstrated in a short-term run). At the future expanded plant capacity of 130 million gallons per day (MGD) (5.7 m3/s), this corresponds to a production increase to 139 MGD (6.1 m3/s).

1. Introduction

Reverse osmosis (RO) is a widely used membrane treatment process to purify municipal wastewater for potable reuse [1–3]. Operated in multistage configuration, RO can achieve up to 85% recovery in potable reuse applications. Constraints in mineral scaling and electrical energy consumption at a typical advanced treatment facility prevent higher recovery rates [4–6]. The rejected concentrate contains inorganic salts, organics, and microbes that must be discharged via permit to inland discharge sites, evaporation ponds, deep well injection, or ocean outfall [2,7,8]. Disposal can be costly, particularly for inland facilities. Alter-natively, the RO concentrate could be treated further to generate more product water and simultaneously minimize the volume of the concen-trate waste stream, with the potential to increase the overall recovery of a potable reuse facility to greater than 90–95% [2,8]. Water recovery from concentrate is becoming more economically favorable as the cost of membrane treatment technologies has dropped and the value of water has increased due to greater demand resulting from population growth and water scarcity associated with climate change [9–11].

Increasing overall RO recovery can help maintain permeate pro-duction when less wastewater effluent is available to treat due to drought conditions or water conservation. For example, the Orange County Water District (OCWD) (Fountain Valley, CA, USA) Ground-water Replenishment System (GWRS) currently treats 100% of the sec-ondary wastewater effluent available from a wastewater treatment facility operated by the Orange County Sanitation District (OC San) (Fountain Valley, CA), approximately 118 million gallons per day (MGD) (~5.2 m3/s). Given OCWD’s scheduled final GWRS expansion which will increase the plant advanced treatment capacity from 100 MGD (4.4 m3/s) to 130 MGD (5.7 m3/s), OCWD will begin relying on an effluent supply from a second OC San facility due to lack of sufficient flows from the first facility because of increased water conservation over recent years. Further, OCWD is facing some uncertainty in the avail-ability of sufficient flows from the second facility (~153 MGD [6.7 m3/ s] total requirement). If the addition of a concentrate treatment system after the primary 3-stage RO were able to increase the overall recovery from 85% to 91%, the District could generate 9 MGD (0.4 m3/s) of additional RO permeate (an increase from 130 MGD [5.7 m3/s] to 139 MGD [6.1 m3/s]) and reduce the concentrate disposal from 23 MGD to 13.8 MGD (1 to 0.6 m3/s) (Fig. S1). In other words, the plant could still maintain 130 MGD (5.7 m3/s) of permeate production even if the wastewater source (influent to GWRS) were to drop to 143 MGD (6.3 m3/s), ensuring a more resilient treatment capacity (Fig. S1).

Addition of a fourth RO stage to treat RO concentrate is not without challenges. Recovery beyond the optimal level of the applied RO feed water antiscalant can result in concentrations of mineral salts beyond their solubility limits and beyond the ability of the antiscalant to inhibit precipitation onto the membrane surface and feed spacer (i.e. mineral scaling) [4,6,12].

Recently, several RO-based batch and semi-batch technologies have been successfully demonstrated for high recovery operation in industrial wastewater and municipal water reuse applications. These include flow reversal (FR)-RO, pulse flow RO (PFRO), and closed-circuit reverse osmosis (CCRO) [13–16]. In these systems, the RO membranes switch between periods of filtration mode and periods of flushing mode [15,17]. The FR-RO technology is based on the principle of periodically reversing the feed flow direction within the pressure vessel (flow tangential to the membrane surface along the membrane element) to

reset the mineral salt “crystallization induction clock” and thus prevent mineral scaling [13,14]. Permeate flushing of the RO membranes is used when the feed water is supersaturated [18–20]. In PFRO, the RO concentrate is discharged once every few seconds in intensive short pulses with high shear forces [15]. The membrane cleaning effect comes from a combination of shearing velocity, vibrations and osmotic back-wash, driven by changing osmotic and gauge pressures [15,21].

CCRO is a patented process by Desalitech (DuPont Water Solutions) that utilizes standard RO components in a single-stage semi-batch design and a unique control methodology [22,23]. Conventional multi-stage RO configurations use membrane-in-series design to achieve high re-covery, subjecting the lead elements in the first stage to high flux and the tail elements in the last stage to low permeate flux and low crossflow conditions. This often leads to biofouling in the first stage and mineral precipitation (scaling) in the last stage [24,25]. Unlike the steady-state single-pass configuration of a multi-stage RO system, the single stage CCRO primarily operates in a closed-circuit mode, at 100% permeate production while recirculating the concentrate back to the feed in a semi-batch process [26]. The recirculation in closed circuit mode con-tinues for minutes to a few hours until a given set-point is reached (based on recovery, pressure and/or conductivity threshold) and then switches to plug flow mode during which the concentrate is flushed from the system. Unlike conventional RO where the recovery, crossflow velocity and permeate flux are correlated, in CCRO these set-points can be changed independently. Recovery is achieved in time, crossflow is adjusted using a circulation pump, and permeate flux is controlled by the speed of a high-pressure pump [16,27,28]. Under this unique configuration, CCRO systems can operate with a lower lead element flux and higher crossflow velocity, which provide better control of fouling and scaling than traditional multi-stage RO systems. Frequent feed flushing is thought to help limit or mitigate scaling, as the concentrate is flushed before the induction period for mineral precipitation [26].

CCRO technology has been piloted at multiple municipal reuse fa-cilities in California with positive results (Table 1). In January 2014, the first municipal reuse CCRO pilot was successfully commissioned by the Los Angeles County Sanitation Districts to treat tertiary effluent and provided a sustained RO system recovery of 93% [29]. The City of Los Angeles [30] and the Padre Dam Municipal Water District [31,32] subsequently commenced similar pilots in 2015 and 2016, respectively, that demonstrated more than 95% recovery was sustainable by incor-porating CCRO to treat tertiary effluent for potable reuse. A key differ-ence between the CCRO pilot in the present study compared to these previous municipal potable reuse pilots is they treated RO concentrate derived from micro/ultrafiltered nitrified/denitrified effluent at a pri-mary RO system recovery of 75 to 80%, while the CCRO pilot in the present study featured a more challenging feed water quality treating RO concentrate derived from microfiltered partially-nitrified effluent at a primary RO recovery of 85% (i.e., CCRO serving as a ‘fourth stage’ RO unit) (Table 1).

Despite the high recovery rate achieved in these studies, the CCRO technology has some limitations. Lee et al. [33] has found in a recent experimental study with a semi-batch RO system (same design as a CCRO system [17]) treating a gypsum model solution that even with a flushing period three times longer than the filtration time, complete removal of surface scale was not possible. Furthermore, they found that any fragments of crystals that remain on the membrane surface can serve as sites for further crystal growth in subsequent cycles [33]. In other words, although CCRO can reduce fouling and scaling, it cannot

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completely eliminate scaling. Operating at high recoveries will still result in progressive membrane fouling, especially in the case of organic and biofouling with simultaneous occurrence of mineral scaling [26,28,31,33,34]. Another challenge is that due to the semi-batch na-ture of the CCRO, data collection and normalization of the CCRO system is more complex compared with conventional RO systems; and there are concerns regarding the robustness of the system, including the potential for the rapid pressure change during closed-circuit and purge sequences to cause membrane integrity problems over long term operation.

In this study, CCRO treatment of GWRS RO feed water and RO concentrate was tested with a ReFlex™ Max CCRO pilot (Desalitech/ Dupont, Newton, MA). The goal of the pilot study was to determine the feasibility of recovering more potable water from RO concentrate using CCRO, recognizing that full-scale success would require a reasonable clean-in-place (CIP) interval (i.e., membrane cleanings ideally no more than monthly), suitable permeate water quality, and maximum recovery to promote economic feasibility. Three alternative feed sources were tested, Phase I: primary plant RO feed water (i.e., microfiltered partially- nitrified wastewater effluent) was used to fill both the pilot membrane pressure vessel (MPV) and an empty pressure vessel – called the “side conduit” (SC); Phase II: RO concentrate from primary three-stage RO plant (at a RO recovery of 85%) was used for both MPV and SC; and Phase III: RO concentrate was used to fill the MPV and microfiltration effluent (primary plant RO feed water) used to fill the SC. Three key parameters were examined in order to optimize the operation, i) RO permeate flux, ii) permeate volumetric recovery per cycle (until SC flushing), and iii) CIP interval. In order to cope with this challenging wastewater source, a novel adaptive control strategy was developed to operate CCRO in a variable recovery mode whereby the start of each cycle was triggered by key setpoints monitored in real time.

2. Materials and methods

2.1. OCWD advanced water purification facility (AWPF) for potable reuse

The study was conducted at OCWD’s GWRS AWPF RO facility in Fountain Valley, California. The GWRS is the world’s largest potable reuse facility and produces 100 MGD (4.4 m3/s) of potable water from a secondary-treated municipal wastewater effluent. The purified water is used for groundwater recharge to augment drinking water supply and for injection into a coastal seawater intrusion barrier. The plant treats a blend of 25% trickling filter and 75% activated sludge effluent from OC San by full advanced treatment consisting of microfiltration (MF), RO, and an ultraviolet-advanced oxidation process (UV-AOP) with hydrogen peroxide to create a high-quality product water (Fig. S2). The existing

three-stage, 85% recovery RO system generates 18 MGD (0.79 m3/s) of concentrate that is discharged via OC San’s 5-mile ocean outfall at no cost to OCWD. Upon completion of the final GWRS plant expansion in 2023, the full production capacity will increase from 100 to 130 MGD (5.7 m3/s) with disposal of 23 MGD (1.0 m3/s) of RO concentrate. To further increase GWRS purified water production, OCWD is investi-gating cost effective options to recover additional water from the RO concentrate to generate a new water supply for the region.

The purification process begins with the addition of sodium hypo-chlorite (12.5% NaOCl, Gallade Chemical, Santa Ana, CA) to react with residual ammonia in the wastewater to form chloramines to reduce membrane biofouling (Fig. S2). After MF, sulfuric acid (93%, Univar Solutions Inc., Downers Grove, IL) is added with a targeted pH of 6.9 in addition to 3.5 mg/L dose of antiscalant (AWC A-110, American Water Chemicals, Plant City, FL) to control mineral scaling on the RO mem-branes. In late March 2018, the antiscalant was switched to AWC A-108 at a dose of 2.5 mg/L. Thus, these same chemicals (sodium hypochlorite, sulfuric acid, and antiscalant) were in the RO feed water (or RO concentrate) that was the influent to CCRO pilot (depending on testing phase).

The full-scale RO plant provides the RO concentrate to CCRO pilot. The current RO recovery (85%) is limited by the traditional multi-stage membrane array along with the saturation of sparingly soluble salts in the RO concentrate, namely calcium carbonate, calcium phosphate and silica. For this study, a slipstream of RO concentrate from the AWPF process was plumbed to continuously supply the feed tank for the CCRO pilot unit. Typical water quality of the RO feed and concentrate streams of the AWPF RO plant are shown in Table 2.

2.2. Pilot description

OCWD acquired a CCRO pilot system in 2017 and commenced a pilot program to evaluate and demonstrate the system’s ability to enhance recovery at the GWRS Advanced Water Purification Facility (AWPF). The CCRO pilot comprises a high-pressure feed pump, single-stage MPV with three 8-in. × 40-in. spiral wound RO elements (Hydranautics ESPA2-LD, Oceanside, CA), a concentrate recirculation pump, and pro-cess control valves (photograph of pilot in Fig. S3). Cartridge filters (1- μm) (Fil-Trek Corporation KG-1-20-E5-BN, Cambridge, Ontario, Can-ada) were installed on the pilot feed and SC feed lines to remove any suspended solids.

CCRO systems alternate between two operational modes. In “closed- circuit” (CC) mode, concentrate produced by the CCRO system is recirculated and blended with the pressurized feed water such that the feed water salinity increases continuously (Fig. 1a). This continues until desired water production (volume of permeate) is reached based on one

Table 1 Summary of piloting of closed circuit reverse osmosis (CCRO) for potable reuse in North America.

Utility Facility name Pilot duration Description Effluent Primary RO pretreatment

Primary RO design

City of Los Angeles Tillman Advanced Water Purification Facility (AWPF) Pilot Plant

2015 to 2017 Pilot-testing CCRO as primary and secondary RO to provide overall RO system recovery of 92 to 95%.

Nitrified/denitrified tertiary effluent

Microfiltration 2-stage RO R = 75%

Los Angeles County Sanitation Districts (LACSD)

Valencia Water Reclamation Plant Advanced Water Treatment Facility Pilot

2014 (7 months pilot operation)

Pilot-testing completed by LACSD showed CCRO could increase facility recovery to 93% when operated as either the primary or secondary RO.

Nitrified/denitrified tertiary effluent

Microfiltration and ultrafiltration (parallel)

2-stage RO R = 74%

Padre Dam Municipal Water District

East County Advanced Water Purification Demonstration Facility

2016 to present

Side-by-side pilot to compare conventional RO versus CCRO for third stage concentrate recovery; CCRO sustained a recovery of 96%.

Nitrified/denitrified tertiary effluent Ultrafiltration

2-stage RO R = 75 to 80%

OCWD (this study)

Groundwater Replenishment System Advanced Water Purification Facility

2017 to 2019 Pilot testing as primary RO and secondary RO at 91% recovery with a CIP interval of >2 months.

75% (AS nitrified/ denitrified), 25% TF secondary effluent

Microfiltration 3-stage RO R = 85%

Note: R = recovery, AS = activated sludge, TF = trickling filter.

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of the following set-points: i) the maximum volume recovered (i.e., production volume of permeate), ii) concentrate conductivity attained, or iii) feed pressure attained. No concentrate is bled from the system during closed-circuit operation (i.e. 100% permeate recovery). In “plug flow” (PF) mode, the concentrate is purged from the system when the desired set point is reached and replaced with fresh feed water. The Desalitech ReFlex™ Max pilot used in this study utilizes a side conduit (SC) to complete the PF mode in two steps, which is a key difference compared to previously tested municipal potable reuse CCRO pilots [22]. The SC is a blank pressure vessel (i.e. no membrane elements installed) with a volume equal to the MPV. The pressurized CCRO concentrate is first purged into the SC while fresh feed water (from the last sequence) is simultaneously pushed to the front of the MPV (Fig. 1b). The concentrate replacement pump then refills the SC with fresh feed water while flushing the concentrate to waste without depressurizing the membranes in MPV (as shown in Fig. 1c).

This exchange and flushing sequence is executed without stopping the high-pressure feed pump, and the permeate production continues without stopping. The ReFlex™ Max pilot design also provides the flexibility to operate at shorter sequence times (as low as 90 s) compared to the ReFlex™ design which requires a minimum sequence time of 6 min. This allows the ReFlex™ Max to operate at recovery rates ranging from 35% to 98%, providing flexibility in both brackish and seawater applications [22,23,28].

2.3. Experimental plan

The operational goal of the CCRO pilot testing was to evaluate the performance of the ReFlex™ Max CCRO configuration for recovery of water from AWPF RO concentrate. The pilot testing was completed in three phases. In Phase I, CCRO was operated as a primary RO unit treating the AWPF RO feedwater (chloraminated MF filtrate chemically conditioned by acidification and antiscalant addition). Phase I evaluated CCRO as an alternative to conventional 3-stage RO for potable reuse. Phase I commenced with a 100-h break-in period of operation at a conservative recovery (85%), then the CCRO pilot was operated for two

consecutive sequences at five recovery set points between 85 and 92% and a permeate flux of 8.0 gallons/ft2/day (gfd) (which is equal to the 3rd stage flux in the AWPF primary RO) in a series of short-term fixed recovery tests. The pilot was subsequently operated for extended runs at the selected recovery for the remainder of the phase. This established the benchmark recovery for Phase II and III testing.

In Phase II, CCRO was operated as a ‘fourth’ RO stage treating AWPF RO concentrate with the MPV and SC both fed with AWPF RO concen-trate. The pilot was operated for an extended duration (~3 months) at the target recovery determined in the previous phase. In Phase III, CCRO was operated as a ‘hybrid’ fourth RO stage still treating AWPF RO concentrate but RO feed water (MF filtrate) was used to fill the SC such that this lower-salinity RO feed water entered the CCRO MPV at the start of each cycle.

During each phase, system performance was evaluated regarding the following: i) CCRO volumetric recovery, ii) specific flux (normalized against temperature), iii) CIP frequency, and iv) permeate water quality (determined by electrical conductivity). Additional water quality pa-rameters were measured including wastewater-derived organic con-taminants and reported by the authors for publication [35,36]. The operational metric for the extended testing was to demonstrate the CCRO system could operate for at least 30 days between CIP events while achieving the following goals: (1) <60% decline of normalized specific flux, (2) <30% increase of normalized salt rejection, and (3) <30% increase of normalized differential pressure.

2.4. CCRO recovery

Two terms were used to calculate recovery of the CCRO system, different from a conventional RO system due to the semi-batch or “cy-clic” nature of operation. The apparent volumetric recovery percentage (RCCRO) was defined as the ratio of the permeate volume to the total volume, where the total volume is the CCRO permeate volume plus the waste stream volume, or the permeate volume plus the SC feed flowrate:

RCCRO (%) =CCRO Permeate Volume

CCRO Permeate Volume + CCRO Concentrate Volume× 100

(1)

RCCRO will increase with increasing time associated with each CCRO cycle. Because the permeate produced by CCRO would add to the main RO facility permeate in a full-scale implementation, the GWRS overall recovery percentage (Roverall) was defined as the theoretical full-scale, overall recovery of the entire AWPF system downstream of the MF processes based on the additional water produced by CCRO (i.e., ac-counting for the additional recovery by CCRO), calculated as:

Roverall (%) =AWPF RO Permeate + CCRO Permeate

MF Effluent Flow× 100 (2)

2.5. Membrane clean-in-place (CIP) and autopsy

CIPs were performed when required due to a reduction in the normalized specific flux (more than 60% decline from initial specific flux), increase in feed pressure, and after each phase of testing. Each CIP was intended to restore any loss of specific flux and/or significant changes in salt passage and differential pressure. The CIP protocol initially adopted was similar to that used by OCWD for cleaning all stages of the GWRS RO system given that fouling and scaling in the CCRO was anticipated to be similar to that in the GWRS RO system, particularly in the third stage. A 2.0% (wt/vol) sodium tripolyphosphate (STPP) (Brenntag Pacific, Santa Fe Springs, CA) and 0.20% (wt/vol) sodium dodecylbenzenesulfonic acid (SDDBS) (Brenntag Pacific, Santa Fe Springs, CA) solution were made with RO permeate in a 120-gallon tank. The pH was raised to 11 with sodium hydroxide and maintained between 30 and 35 ◦C with a heating coil placed in the CIP tank. The solution was recirculated through the pressure vessel for 30 min (20 gpm

Table 2 Average water quality (October 2018–February 2019) of the feed and concen-trate streams of the reverse osmosis (RO) plant at the Orange County Water District (OCWD) Groundwater Replenishment System (GWRS) Advanced Water Purification Facility (AWPF) operated at ~85% water recovery.

Analyte Unit RO feed RO concentrate

Mean % St. Dev.a

Mean % St. Dev.a

Aluminum μg/L 3.3 26 23.5 38 Barium μg/L 35.2 5 229 4 Bicarbonate (as HCO3

− ) mg/L 227 4 1451c 4 Calcium mg/L 81 5 519 4 Chloride mg/L 340b – 2035b – Electrical Conductivity μS/

cm 1744 2 – –

Iron μg/L 108 11 708 10 Magnesium mg/L 26.5 6 175 6 Manganese μg/L 54.7 4 358 5 Orthophosphate (as

PO43− )

mg/L 1.42 62 7.54 44

pH 6.9 1 7.6 1 Potassium mg/L 19 5 124 4 Silica (as SiO2) mg/L 21.4 4 124 13 Sodium mg/L 235 4 1516 5 Sulfate mg/L 205 14 1334 15 TDS mg/L 1029 2 6570 3 Total organic carbon mg/L 7.6 2 – – Zinc μg/L 18.4 10 95.2 10

a % St. Dev. = standard deviation/mean × 100%. b Estimated based on charge balance. c Estimated based on feed concentration and water recovery data.

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Fig. 1. Schematic of a CCRO pilot (Desalitech ReFlex™ Max) in (a) closed circuit mode, (b) purge mode and (c) during side conduit refill (image courtesy of Desalitech/DuPont).

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[1.26 L/s] per pressure vessel), followed by a 1-h soak (repeated three times), and ended with a 45-min flush with RO permeate. A specialty RO cleaner (AWC C-227) was used during Phase II and III to improve CIP results (details are shown in Supplementary Text S1).

At the completion of Phase I and III, the tail (third) element from the CCRO pilot was removed for performance testing and autopsy at the AWC laboratory (Plant City, Florida). The tail element is subject to the lowest cross flow velocity, permeation, and highest concentration po-larization effect, especially at the concentrate exit region of the element, and thus has the highest scaling potential for various mineral salts. Biological and organic fouling (including antiscalant fouling) can also develop on the sites of mineral scaling [12]. The membrane surface was analyzed by scanning electron microscopy (SEM), x-ray spectroscopy (EDX), energy dispersive spectroscopy (EDS) with superimposed elemental imaging (SEI), and Fourier transform infrared (FTIR) spectroscopy.

2.6. Water quality analysis

If CCRO is used to treat RO concentrate as a ‘fourth stage’ RO unit in a full-scale potable reuse application, CCRO permeate would ideally be blended with permeate from the 3-stage (primary) RO system. OCWD GWRS permit requirements for RO permeate quality are limited to turbidity (shall not exceed 0.2 Nephelometric Turbidity Units [NTU] for more than 1.2 h in any one day period; and 0.5 NTU at any time) and percent ultraviolet transmittance at 254 nm (%UVT254) (>90%) [37]. The plant must also demonstrate 2-log removal of total organic carbon (TOC) by the RO system as a surrogate for pathogen (virus) removal which is measured by online RO feed and permeate TOC analyzers. OCWD also observes an internal RO permeate critical control point limit of less than 0.1 milligrams per liter (mg/L) TOC via the online permeate analyzer as an indicator of normal operating conditions. The GWRS finished water (after UV/AOP, post-treatment decarbonation, and lime addition) is subject to a TOC limit (<0.5 mg/L) and total nitrogen limit (<10 mg/L, <5 mg/L preferred) along with various other monitoring requirements for regulated drinking water quality parameters as well as organic constituents dictated by the California Recycled Water Policy [37,38]. At this time, California has not established specific permitting requirements for CCRO for potable reuse. Recognizing the need to achieve suitable water quality consistent with California potable reuse requirements, an objective of the pilot study was to characterize the quality of the CCRO permeate and how it varies during the closed-circuit sequence.

During Phase I, one sampling event was conducted to profile the quality of the CCRO feed, permeate, and concentrate at the beginning, middle, and end of the closed-circuit sequence. This event included analysis of inorganic compounds (e.g. metals, anions, TOC and total dissolved solid [TDS]) by standard methods at OCWD’s Philip L. An-thony Water Quality Laboratory. Table 3 summarizes the water quality sampling and analysis performed during a typical CCRO cycle at the end of Phase I. The present study focuses on CCRO operational performance and related key water quality indicators of performance; additional comprehensive water quality samplings were performed during all phases (e.g., organic contaminants) and will be reported in a follow-up publication [35].

Given the importance of TOC as a surrogate for pathogen removal at the GWRS, online TOC measurements were taken for the CCRO feed and permeate during select periods of the testing. The online TOC moni-toring was performed using Suez (formerly GE) Sievers 900 TOC online analyzers, programmed to report TOC data at 4-min intervals. This data served to characterize real-time changes in TOC during a typical closed- circuit sequence and to calculate surrogate pathogen log removal values (LRVs) by the CCRO system.

3. Results and discussion

Table 4 list the pilot test conditions (e.g. duration, recovery, and flux) for Phase I, II, and III. To manage mineral precipitation in the full- scale GWRS RO system, feed water (MF effluent) was dosed with AWC A- 110 antiscalant at 3.5 mg/L until late March 2018 (during Phase I and II), at which time the antiscalant was switched to AWC A-108 at a dose of 2.5 mg/L (partway through Phase III). The projected scaling potential for sparingly soluble salts in the concentrate based on GWRS RO system operation at 85% recovery and the use of both AWC A-110 and A-108 are presented in Fig. S4a and b, as modeled by the AWC Proton software, with the scaling potential at 92% recovery shown in Fig. S4c [39]. Silica and the antiscalant are the most likely scaling species in the feed (su-persaturated in concentrate).

Table 3 Water quality testing matrix over CCRO operating cycle at end of phase A.

Parameter Stream

CCRO FEED CCRO-CONC. (diluted) CCRO-PERM

CB CE CB CE CB CM CE

RO parameters X X X X X X X pH X X X X X X X Conductivity X X X X X X X Temperature X X X X X X X

Notes: Water quality sampling for the extended test run was performed once on February 6, 2018 during operation at 92% recovery. Operating conditions and results are further described in Section 4. CCRO-FEED = sampling port (S3) of system feed (before recirculation enters line). CCRO-CONC = sampling port (S4) of system with immediate dilution (100-mL sample diluted with 1 L of distilled water). CCRO-PERM = product water sample port (S5) collected from line (i.e., changing quality; not permeate tank). CB, CM, CE = sample collected at cycle beginning, cycle middle, or cycle end. X = one sample collected. RO parameters = various inorganic parameters generally used to assess or predict RO performance in term of containments removal: Ca, Mg, Na, K, Ba, Sr, NH4 (via NH3), HCO3, CO3 (via alkalinity), Cl, SO4, F, NO3, PO4, SiO2, TDS.

Table 4 Duration, recovery, and flux test conditions and feed water source for each CCRO pilot test phase.

Parameter Pilot study phases

Phase I Phase II Phase III

Feed water source (membrane pressure vessel)

Primary RO feed 3rd stage RO concentrate

Diluted 3rd stage RO concentrate

Feed water source (side conduit)

Primary RO feed 3rd stage RO concentrate

Primary RO feed

Permeate flux (gfd) 8.0 gfd (13.6 LMH)/6.4 (11.0 LMH)

6.4 (11.0 LMH) 6.4 (11.0 LMH)

Pilot period 09/14/2017–02/ 27/2018

3/2/2018–7/ 11/2018

7/12/2018–2/ 21/2019 2/21/2019–11/ 30/2019a

Pilot duration (weeks) 27 19 31, 40

Notes: gfd = gallon(s) per square foot per day. LMH = liter(s) per square meter per hour.

a Testing during this period was completed using DuPont Filmtec BW30XFRLE RO membrane elements in the CCRO pilot. All other testing shown was completed using Hydranautics ESPA2-LD RO membrane elements.

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3.1. CCRO treatment of RO feed water (Phase I)

3.1.1. Recovery optimization based on short and extended runs at fixed recovery

In Phase I, the CCRO ReFlex™ Max pilot was operated with AWPF RO feed water (i.e., microfiltration effluent) with the objective to determine the maximum recovery rate (RCCRO) while meeting the operational metrics. Operation as a primary RO resulted in the longest closed-circuit (CC) sequence times and the RO elements were exposed to the widest range of concentrate quality (i.e., TDS). Data collected during Phase I were used to generate the operational set-points for Phase II and Phase III testing. A 100-h break-in period at 85% recovery and 8 gfd or 13.6 L/min/h (LMH) permeate flux equal to the AWPF RO unit third stage permeate flux was completed to stabilize the RO elements on the AWPF feed water. The operational data was normalized and the baseline performance determined for specific flux, salt rejection, and trans-membrane pressure.

The fixed-recovery optimization tests included a series of dual- sequence (two consecutive sequences) runs and extended runs encom-passing multiple sequences. The sequence times (CC + PF) and operating conditions for both the dual-sequence and extended runs are presented in Table 5. The dual-sequence runs were completed at five recoveries: 85%, 87%, 89%, 90%, and 92%. For the first four targeted recovery setpoints, the normalized specific flux returned to the initial level. At 92% recovery, the feed pressure rapidly reached approximately 250 psi, which is relatively high and indicative of membrane scaling or fouling, precluding tests at higher recovery. Following these tests, the unit re-covery rate was temporarily reduced to 85% recovery, during which the maximum feed pressure at the end of an operating sequence/cycle declined and stabilized at approximately 180 psi.

A longer test comprising eight sequences was performed at 90% re-covery at 8.0 gfd (13.6 LMH) for approximately 2.5 days (Fig. S5). During this time, the feed pressure reached as high as 22.1 bar (330 psi) at the end of a cycle (Fig. S6). After a reduction in recovery to 85%, the run continued for another 5 days. At the reduced recovery, feed pressure again declined and generally stabilized, demonstrating sustainable operation. In another extended test, a recovery of 93% at 6.4 gfd (11.0 LMH) was attempted for 10 sequences, which resulted in a rapid and substantial decline in specific flux (Fig. S7). Recovery was subsequently reduced to 92%, after which stable performance was observed. Fast specific flux recovery was observed when the recovery point was deceased from 93% to 92% (Fig. S7) [40]. In summary, the short and extended runs shows that 90–92% is likely the maximum RCCRO range the CCRO pilot can stably operate when treating AWPF RO feed (at a permeate flux of 11.0 LMH).

3.1.2. Stabilization and optimization of CCRO performance During Phase I, several adjustments were made to the CCRO pilot

operation to optimize the operation. These included reducing permeate flux, increasing cross flow velocity, and optimizing the SC engagement volume required to adequately flush the CCRO unit during PF mode. Feed water dosing of an organic dispersant (AWC A-132) at 5 and 8 mg/ L was also attempted but yielded no noticeable changes in performance. Of these modifications, the most beneficial was operation of the pilot at

a reduced flux of 6.4 gfd (10.9 LMH) and increased recirculation flow rate of 52 gpm (3.28 L/s) to maintain the same crossflow velocity as operation at 8 gfd (13.6 LMH) flux. Under these conditions, a maximum recovery of 92% was sustained, limited by amorphous silica scaling and fouling associated with silts/clays when operated with AWPF RO feed water as the feed to the CCRO process. This established the benchmark operational recovery percentage for tests in Phase II and III.

The recovery and pressure profile of the CCRO pilot operated during Phase I at 92% recovery is shown in Fig. 2. As the salinity of the feed water increased due to recirculation of the CCRO concentrate, the pressure required to maintain specific flux increased. It is hypothesized that scaling was mitigated by operating the RO elements in CC-mode with periodic SC flushing and refill.

An alternate operational strategy was also tested by application of periodic low recovery purge sequences or “mini-cleaning” periods be-tween high recovery CC sequences. In this mode, the CCRO pilot oper-ated at an elevated recovery of 94% for several CC sequences (~1.9 h/ sequence) followed by low recovery (50%) operation for three CC se-quences (~0.05 h/sequence). This strategy appeared to achieve high recovery for a few CCRO cycles (Figs. S8 and S9), but on close exami-nation, the average recovery over extended operation only achieved approximately 89% (based on sequence timing).

Upon completion of Phase I, the tail element from the pilot unit was removed for performance testing and membrane autopsy. The autopsy identified the primary foulants as silts/clays and organics (Supplemen-tary Text S2). A CIP was performed on the remaining two elements using the OCWD CIP protocol and the third element was replaced with a new element before beginning Phase II (Table 7).

3.1.3. Permeate water quality during a closed circuit (CC) sequence At the end of Phase I, sampling during a typical CCRO closed-circuit

sequence during operation at 92% recovery was conducted. Feed, permeate, and concentrate samples were collected and analyzed at the beginning and end of a cycle, and permeate collected and analyzed in the middle of the cycle (Fig. 3). The feed quality remained consistent from beginning to end of the cycle (data not shown). Except for calcium, the results clearly illustrate how the concentration of each parameter increased during the closed-circuit cycle. The increase in the permeate solute concentration ranged from 1×–7× by the end of the cycle as compared to a concentration factor of 1×–12.5× for constituents in the CCRO feed based on operation at 92% recovery. The percent rejection for each parameter is shown at the beginning, middle, and end of the CCRO cycle in Fig. 3. An increase in permeate concentration was observed as the concentrations of constituents in the feed water increased during the cycle, although the amount of constituent increase in the permeate and recovery loss varied based on constituent. Rejection generally remains constant or slightly increase for most constituents and only decreased for ammonia nitrogen and silica. Greater change was observed for constituents having lower initial rejection (ammonia ni-trogen and boron,). Nonetheless, the total nitrogen was <5 mg/L as nitrogen (N) and TOC < 0.5 mg/L as carbon (C) in the CCRO permeate which complied with the GWRS finished water requirements of <10 mg/ L as N (with <5 mg/L as N preferred) and <0.5 mg/L as C.

The silica levels in the ‘end of cycle’ sample (279 mg/L, which

Table 5 Summary of sequence times for recovery optimization test period (Phase I).

Recovery (%) Part 1 – dual-sequence recovery testing Part 2 – multiple-sequence recovery testing

Number of sequences Sequence time (minutes) Run time (hours) Number of sequences Sequence time (minutes) Run time (hours)

85% 2 37 0.6 38 37 24 87% 2 60 1.0 N/A N/A N/A 89% 2 72 1.2 N/A N/A N/A 90% 2 81 1.4 64 81 87 92% 2 108 1.8 135 108 242 93% N/A N/A N/A 9 144 22

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corresponds to the CCRO concentrate) were slighter greater than the level assumed in the Proton antiscalant projection (274 mg/L). None-theless, the silica concentration did not result in silica scaling at 92% recovery, which was the primary consideration for the maximum RCCRO that could be sustainably achieved.

The online TOC data was collected over an approximately 7-h period during Phase I (Fig. S10). The feed TOC varied from 7.2 to 8.2 mg/L during this period, while permeate TOC exhibited cyclic levels that varied from 0.110 to 0.180 mg/L. The cyclical behavior was consistent with the results of permeate grab sampling and reflected the increase in TOC concentration in the CCRO feed water (via recirculation loop) over the course of a closed-circuit cycle. LRVs for TOC ranged from 1.68 to 1.87 (see Fig. S10). This is less than the 2-log removal required for the primary RO system at GWRS to be granted 2 LRV credit for pathogens by the State of California Division of Drinking Water (DDW) [37]. The primary RO system of GWRS consistently achieves 2-log or more of pathogen credit based on online TOC monitoring (LRVs range from 2.0–2.8 annually, with greater than 2.3 LRV 90% of the time) However, if CCRO was implemented at full scale (such as the case in Fig. S1), the relatively small flow of CCRO permeate (20 MGD) would be blended with the much larger permeate flow from the primary RO system (119.25 MGD), resulting in flow-weighted log removal in the range of 2.14 to 2.48. Thus, depending on the regulator-determined approach for crediting a CCRO-based ‘fourth’ stage treatment system, there is

potential for establishing 2-log credit using TOC (based on primary RO feed compared to blended RO permeate). Further, other pathogen sur-rogates with greater log removal could be utilized in place of TOC, including strontium which has demonstrated >3-log removal by CCRO in other studies [41].

3.2. CCRO treatment of RO concentrate with concentrate SC flush (Phase II)

In Phase II, the CCRO pilot unit was operated to treat and recover water from a portion of the concentrate from the AWPF RO system, with the objective of determining the CIP interval while achieving an Roverall of 92% that was demonstrated in Phase I. The only difference between operating as a primary RO compared to operating as a fourth stage RO was the amount of time the CCRO pilot was in CC mode to achieve the 45% target RCCRO. The pilot test conditions (e.g. duration, recovery, and flux) are listed in Table 4. A reduction in specific flux to 0.04 gfd/psi (0.998 LMH/bar) triggered a CIP.

Rapid specific flux (permeability) decline, 60% loss within 3 weeks, was observed during the first run shown in Fig. 4. The recirculation velocity was increased from 40 to 65 gpm in the second and third run by increasing the recirculation rate of the pilot. The higher velocity of water flow was thought to mitigate deposition of foulants and formation of scalants on the membrane surface through increased turbulence over the

Fig. 2. Phase I volumetric recovery and membrane feed/outlet array pressure operating at 92% recovery (data from 12/10/2017).

Fig. 3. Phase I CCRO permeate water quality at the beginning, middle, and end of a CCRO cycle operating at 92% recovery. The percent rejection is shown at the top of the bars (data from 2/6/2018).

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feed spacers. However, this change did not increase run time between CIPs. Finally, reducing the RCCRO from 45% to 40% (91% Roverall) then to 36.5% (90.5% Roverall) while increasing the crossflow rate (velocity) to 65 gpm in the third run was able to achieve a run time of 26 days (Fig. 4). An autopsy following completion of Phase II was not considered necessary given the sustainable operation of the CCRO unit.

3.3. CCRO treatment of RO concentrate with primary RO feed water side conduit (SC) flush (Phase III)

During Phase III the CCRO unit continued to operate as a ‘fourth stage’ treating AWPF RO concentrate but in this phase the SC was filled with AWPF RO feed (ROF) instead of RO concentrate. The CCRO pilot was operated to determine if the use of ROF to flush the CCRO pressure vessel would reduce the rate of specific flux decline during normal op-erations. The ROF flush had two potential benefits. First, the undersat-urated (in sparingly soluble salts) ROF water introduced to the CCRO feed at the beginning of the CC mode could potentially act as a “mini- clean” to dissolve any scale or crystals already formed on the membrane surface. Second, it increased the duration of the CC sequence, exposing the membrane to a pressure and salinity ‘swing’ that could help to reduce biological fouling [26]. However, if the SC is filled with ROF from the AWPF, this source water is no longer available for treatment in the primary RO system, which must be accounted for in the determi-nation of overall recovery of a plant as it reduces the calculated recovery of the primary RO.

Six different recovery scenarios using either the AWPF ROF or RO concentrate for the SC supply are listed in Table 6. The derivation based on mass balance around the CCRO and the AWPF (three-stage RO) +CCRO (as ‘fourth’ stage RO) is shown in the Supplementary Text S3. The data illustrate that higher RCCRO is required (~61%) to obtain 91% Roverall, when using the AWPF ROF as feed for the SC flush. A 40.0% RCCRO is required to obtain 91% overall recovery, when using the AWPF RO concentrate as feed for the SC flush. As noted in Section 2.4 (Eq. (2)), the Roverall is the hypothetical recovery for the entire system—AWPF and CCRO combined. It assumes all the AWPF RO concentrate is treated by the CCRO system. Therefore, Roverall would be less if all RO concentrate was not treated by the CCRO system.

Fluctuations in RO concentrate water quality created a major chal-lenge for maintaining RO membrane performance in Phase II. Diurnal variations in the electrical conductivity (EC) of the AWPF ROC water was mirrored in the CCRO recirculation loop (Fig. S11). The ROC EC varied from 8200 to 12,000 μs/cm with an estimated TOC between 40 and 54 mg/L that represented a significant load of organic fouling po-tential. In addition to diurnal variations, AWPF ROC conductivity was observed to be lower on Sundays and Mondays and vary hourly (sometimes dropped to below 2 mS due to intrusion of CIP rinse water from RO plant) (Fig. S11).

In order to mitigate the impacts of feed water quality changes on pilot performance, an adaptive control strategy was implemented at the beginning of Phase III. Instead of initiating the purge mode (PF) once a fixed maximum recovery was achieved (volumetric) for every sequence, the pilot was operated in a variable recovery mode where the command for entering the purge mode was triggered when one of the three thresholds was met. The first trigger for activating the purge mode was the CCRO concentrate EC measured prior to entering the circulation pump (see Fig. 1), which represents the point of highest EC at any given time during CC operation. The rate of membrane fouling was noted to increase as indicated by initial pressure at the start of a CC sequence when CCRO concentrate conductivity exceeded 16.5 mS (tests were conducted up to a conductivity of 19.5 mS). Using this trigger, the CCRO pilot adapted to the changes in the CCRO concentrate (and feed)

Fig. 4. Phase II CCRO pilot average specific flux, feed pressure, normalized pressure drop across MPV, and membrane salt rejection based on EC measurements where AWPF primary RO concentrate was the feed water to CCRO and side conduit. CIP trigger was the normalized specific flux of 0.04 gfd/psi (0.998 LMH/bar).

Table 6 Apparent CCRO recovery and GWRS overall recovery using AWPF RO feed and RO concentrate for CCRO side conduit (SC) feed.

Case AWPF primary RO recovery

Apparent CCRO recovery

GWRS overall recovery (RO system)

CCRO SC supply =AWPF RO concentrate

CCRO SC supply =AWPF RO feed

1 85.0% 40.0% 91.0% 81.6% 2 85.0% 45.0% 91.8% 84.5% 3 85.0% 57.5% 93.6% 90.0% 4 85.0% 60.5% 94.1% 91.1% 5 85.0% 61.0% 94.2% 91.2% 6 85.0% 66.0% 94.9% 92.8%

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conductivity in real time, thereby optimizing the overall pilot system recovery while avoiding the point of failure identified as a CCRO concentrate EC > 16.5 mS.

The second trigger to activate the purge mode was the apparent CCRO recovery. Recovery set points at 61.0, 61.5%, 62.0%, 62.5%, and 63.0% were tested with an EC set point of 16.5 mS/cm, where each point was tested for one to three days. The feed pressure and average specific flux of the membranes were closely monitored. If there was rapid initial feed pressure increase at the beginning of a CC cycle (>10 psi), the re-covery set point was too high. To recover the system, the recovery set point was lowered to 50%, so that the pressure could drop slowly over time by the flush mode. A RCCRO of 61.0% was determined to be most suitable given weekly episodes of low conductivity that exhibited higher fouling potential, as operating >16.5 mS in the concentrate on these ‘low conductivity’ days resulted in fouling of the membranes. Fig. 5 shows the evolution of feed and recirculation concentrate EC and RCCRO for several CCRO sequences controlled by the maximum recovery trigger in the case of treating feed water with a low EC (~9 mS). In Fig. 6, adaptive operation is shown in the case of high feed EC (~12 mS), the purge mode was triggered when the CCRO concentrate EC reach 17 mS (set based on scaling potential).

Lastly, a high feed pressure of 220 psi (15.2 bar) was the third trigger to enter purge mode. If the pilot operates in CC mode for too long, the pressure will build up quickly in the MPV due to the increase in feed water salt concentration. High feed pressure in RO MPV typically in-dicates severe membrane fouling or scaling and a CIP is required to restore performance. Although triggering a purge mode won’t replace a CIP, by limiting CCRO operation to this pressure, the filtration time will be shortened, and the degree of fouling/scaling (measured by specific flux decline) could be reduced by SC flushing before the application of a CIP. Fig. 7 illustrates the adaptive control strategies for the CCRO pilot over a 9-day period.

After switching the feed source to the side conduit from AWPF RO concentrate to ROF in Phase III, together with the programming changes to trigger the purge mode when one of three setpoints was reached, the CCRO was successfully operated for 55 days before a CIP was required. The specific flux and feed pressure for the first run are shown in Fig. 8. In the next run, also shown in Fig. 8, 62 days of operation was achieved before a CIP was required. These runs illustrated that the programming changes enabled optimum operation of the pilot unit. The RCCRO ranged from 59.0–61.0%, corresponding to a theoretical Roverall between 90.6 and 91.2%. Upon successful completion of the two extended runs, the

Hydranautics ESPA2-LD elements were removed from the CCRO and the tail element was tested for performance and an autopsy performed.

DuPont Filmtec BW30 XFRLE-400/34 RO elements were then installed in the CCRO unit and further runs were completed (see Fig. 8) due to some concern that the Hydranautics membranes had inadver-tently exceeded pressure recommendations (potential compaction) during the early multi-phase trials and to test a different manufacturer’s membrane. The DuPont Filmtec membrane was selected based on suc-cessful performance in the GWRS AWPF and ability to maintain stable salt rejection following repeated CIPs.

As shown in Fig. 8, the CCRO pilot loaded with DuPont elements was operated continuously using the same programming setpoints described above at 59.0–61.0% apparent CCRO recovery for 71 days prior to requiring a CIP. Specific flux was restored by the CIP after which a second run was completed for 73 days, demonstrating that CCRO operation is sustainable at ~60% recovery (Table 7). After the second CIP, a third run was initiated at 59–61% recovery. After 30 days, re-covery was increased to 66%, corresponding to a theoretical GWRS overall recovery of 92.8%, and the unit was operated for an additional 33 days (63 days total). At the higher recovery, the feed pressure increased and specific flux decreased at greater rates, indicating a greater rate of fouling similar to operation at 93% recovery observed in

Fig. 5. Illustration of adaptive operation of CCRO pilot under conditions of low feed EC. The CCRO purge mode was triggered when CCRO apparent recovery reached 61%. The lower dashed line indicates the initial (lowest) EC in the CCRO concentrate during purge mode of PV with GWRS ROF. Permeate flux: 6.4 gfd (11.0 LMH). CCRO sequence duration: 9.5–12 min. Data collected: 4/ 15/2019.

Fig. 6. Illustration of adaptive operation of CCRO pilot under conditions of high feed EC. The CCRO purge mode was triggered when the CCRO concentrate EC reached 17 mS. The dashed line indicates the initial (lowest) EC in the CCRO concentrate during purge of PV with GWRS ROF. Permeate flux: 6.4 gfd (11.0 LMH). CCRO sequence duration: 9.5–12 min. Data from 4/17/2019.

Fig. 7. Demonstration of Phase III adaptive control strategies of the CCRO pilot over a 9-day period. The CCRO pilot enters purge mode when one of three setpoints (recirculation EC, volumetric recovery, and feed pressure) was reached. The CCRO feed EC is shown for reference.

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Phase I. The four extended runs conducted with the Hydranautics and

DuPont membranes demonstrated that the theoretical Roverall can be sustained near 91% and that 92% could be achieved while meeting the 30-day minimum preferred CIP interval. A greater than 2-month CIP interval is more attractive from an operational perspective. All CIPs performed during Phase III used the modified CIP protocol (Supple-mentary Text S1 and Table 7). The pilot was able to maintain high average normalized salt rejection and normalized differential pressure for the two membranes tested (Fig. 9).

Although Phase III was successful in which primary RO feed water (microfiltration effluent) was used to fill the SC, the use of primary RO feed water for CCRO PV flush does have disadvantages. In a full-scale

implementation of CCRO as a ‘fourth’ stage RO, primary RO feed must be diverted to the installed CCRO system and the volume used for flushing CCRO is no longer available to feed the primary RO system. This must be accounted for to determine overall system recovery as it results in a reduction in the primary system volumetric production and an equivalent increase in the required CCRO capacity (or CCRO recovery) to achieve the same overall system recovery. In the case of an existing primary RO plant now oversized, the reduction in water produced by the primary RO can be addressed in one of two ways: 1) reduce the average flux of the existing RO units and, thus, their permeate production; 2) remove one or more trains from service and rotate trains in operation. Both options have the advantage of reduced fouling and CIPs for the existing RO units and the second option has the advantage of

Fig. 8. Phase III daily average of specific flux and feed pressure of the CCRO pilot illustrating completion of membrane CIPs whenever the feed pressure reaches trigger of 15.2 bar. CCRO R% = CCRO apparent recovery; overall R% = GWRS overall recovery.

Table 7 Summary of CIP events.

Phase Date Membrane CIP regime Average conditions over three CIP circuits

Pre-CIP Post-CIP

pHa Temperature (◦C)b

Cross flow (gpm)c

Feed pressure (psi)

Normalized specific flux (gfd/psi)

Feed pressure (psi)

Normalized specific flux (gfd/psi)

I 2/27/ 2018

ESPA2-LD 2% STPP/ 0.20% SDDBS

11.62 26.8 21.9 260 0.023 73 0.132

II 4/24/ 2018d

ESPA2-LD 2% STPP/ 0.20% SDDBS

11.40 26.3 23.0 244 0.043 172 0.076

II 5/7/ 2018

ESPA2-LD 2% AWC C- 227

12.04 23.3 30.0 207 0.057 117 0.110

II 5/30/ 2018

ESPA2-LD 2% AWC C- 227

12.29 31.4 46.7 256 0.036 126 0.100

II 6/28/ 2018

ESPA2-LD 2% AWC C- 227

12.21 31.0 50.0 236 0.040 95.3 0.086

III 11/8/ 2018

ESPA2-LD 2% AWC C- 227

11.97 27.7 52.0 213 0.055 85 0.112

III 11/20/ 2018

ESPA2-LD 2% AWC C- 227

12.24 31.0 52.0 206 0.054 103 0.090

III 5/22/ 2019

BW30- XFRLE

2% AWC C- 227

12.08 33.9 49.0 220 0.044 120 0.100

III 9/12/ 2019

BW30- XFRLE

2% AWC C- 227

11.91 38.0 48.0 225 0.045 120 0.120

a pH adjustment achieved with sodium hydroxide. Target pH was 11 for 2% STPP/0.20% SDDBS and 12 for 2% AWC C-227. b Target temperature was 30–35 ◦C for 2% STPP/0.20% SDDBS and 35–39 ◦C for 2% AWC C-227. c Target cross flow was 20 gpm for 2% STPP/0.20% SDDBS and 50 gpm for 2% AWC C-227. d CIP on 4/24/2018 with 2% STPP/0.20% SDDBS was ineffective and resulted in switch to 2% AWC C-227 for next CIP on 5/7/2018.

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maintaining current train operating conditions but will result in the need for train flushing when removed from service. The water displaced during flushing could be used as make-up to the CCRO system, although a means to capture, store, and transfer this water would be required.

3.3.1. Chemical water quality assessment for Phase III There was a noticeable difference in CCRO permeate water quality

between the start of a cycle and end of a cycle that was expected due to the recirculation of the concentrate through the CCRO process (see Figs. 3 and S8). The concentration of select water quality parameters and inorganic constituents in CCRO permeate are shown in Fig. S12 and compared to the primary RO system (GWRS AWPF) permeate for reference. The two permeates were comparable, notably considering the more challenging RO concentrate feed of the CCRO pilot. The CCRO permeate generally met permit limits for the OCWD AWPF finished water. The average CCRO permeate TOC was low at 0.17 ± 0.049 mg/L but slightly higher than the 6-month average GWRS AWPF RO permeate TOC of 0.11 ± 0.03 mg/L. The CCRO permeate was still below the AWPF finished water permit limit for TOC < 0.5 mg/L and in a theoretical full- scale application, could be reduced significantly through blending with the primary RO system permeate. The average TDS of the CCRO permeate was 50.5 ± 28.9 mg/L, higher than the 6-month average AWPF RO permeate TDS of 17.0 ± 8.0 mg/L, but well below the AWPF finished water permit requirement of <500 mg/L. All sulfate (SO4

2− ) concentrations were below the detection limit of 0.5 mg/L except for one sample of CCRO permeate (0.7 mg/L) on 2/21/19, which was well below the permit requirement of 100 mg/L. The average CCRO permeate chloride (Cl− ) concentration was 30.9 ± 35.7 mg/L that was higher than the 6-month average AWPF RO permeate Cl− concentration of 5.0 ± 0.98 mg/L. Finally, the nitrate in the CCRO permeate was low (9.1 ± 4.1 mg/L), though higher than AWPF RO permeate, but well below the GWRS-FPW permit requirement of 45 mg/L as NO3.

4. Conclusions

Two years of pilot testing demonstrated the sustainable use of CCRO for RO concentrate treatment. The testing focused on operational opti-mization and treatment of concentrate from the RO process of a full- scale potable reuse facility, the OCWD GWRS AWPF, to recover addi-tional potable quality water to increase the overall system recovery above current 85%. In the first phase of testing, a maximum pilot re-covery of 92% was achieved from an MF effluent feedwater to the CCRO process. Although considered the a priori limit to maximizing recovery, mineral scaling was not thought as the constraint but rather silt and

organic fouling. This is an important point as it differs the results of this testing from other CCRO pilots where scaling (silica or calcium phos-phate) was responsible for flux loss and what limit sustainable recovery. Deposition of silts and clays were not anticipated given the use of MF pretreatment (0.1 μm nominal pore size), however the age of the MF membranes most likely allowed for passage of such particulates into the AWPF RO feed and their concentration in the CCRO feed. In the second and third phases, the pilot was operated as a “fourth stage” RO treating RO concentrate directly, where the salinity is near supersaturation. The SC enables shorter filtration duration (filtration time as short as 1.5 min), and thus a greater recovery range (low to high) is feasible. An operation flux of 6.4 gfd (11.0 LMH) and a MPV crossflow velocity ≥ 60 gpm (0.23 m3/min) were demonstrated to best minimize the accumu-lation of silts/clays and dissolved organics at the membrane surface and minimize the decline in specific flux. Under these conditions, and sup-plying the side conduit of the pilot with AWPF RO feed water rather than RO concentrate, the pilot demonstrated sustainable operation at a re-covery range of 57–61% (RCCRO) over a long-term run (up to 66% re-covery in a short-term run). This corresponds to an overall theoretical GWRS recovery of 90–92% at full-scale (i.e., assuming all primary sys-tem RO concentrate is treated by CCRO and the CCRO permeate blended with primary RO permeate to increase production), while allowing for CCRO operation with a CIP frequency to between 63 and 73 days, two times greater than the 30-day minimum CIP interval recommended by plant managers. These operations were accomplished without increasing antiscalant or acid dosing to the AWPF RO feedwater for control of fouling/scaling.

In order to cope with fluctuating feed water quality, an adaptive control strategy was implemented as an alternative to operation at a preset maximum recovery, in which the pilot operated in a variable recovery mode where the CCRO cycle trigger was controlled by the maximum CCRO concentrate conductivity, maximum recovery setpoint and the maximum feed pressure. Additional information regarding CCRO permeate water quality and estimated theoretical full-scale implementation costs compared to FO-RO will be reported in a future follow-up publication.

Water recovery from concentrate is becoming more feasible as the cost of membrane treatment technologies has dropped and the value of water has increased. Piloting promising technologies using real facility source waters and operational preferences over longer timescales is critical to advancing technology adoption to improve water security. This comprehensive long-term pilot dataset collected over a two-year period has not been previously presented in the literature. With the current interest in increasing RO recoveries and minimizing discharge of

Fig. 9. Phase III daily average of normalized salt rejection and normalized differential pressure during CCRO pilot testing.

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produced concentrates, studies like this one are important for estab-lishing long-term feasibility of high-recovery membrane technologies.

CRediT authorship contribution statement

Han Gu: Investigation, Formal analysis, Visualization, Writing - Original Draft, Review & Editing. Megan H. Plumlee: Conceptualiza-tion, Supervision, Project administration, Writing - Review & Editing. Michael Boyd: Conceptualization, Resources, Visualization, Writing - Review & Editing. Michael Hwang: Conceptualization, Formal analysis, Writing - Original Draft. James C. Lozier: Conceptualization, Writing: draft preparation and review.

Declaration of competing interest

The authors declare the following financial interests/personal re-lationships which may be considered as potential competing interests: Han Gu, Megan H. Plumlee, Michael Hwang, and James C. Lozier declare that they have no conflict of interest. Michael Boyd is a current employee of DuPont Water Solutions.

Acknowledgements

This work was partially funded by the U.S. Department of the Interior - Bureau of Reclamation under agreement No. R17AC00151. The con-tributions of Derrick Mansell, Don Supernaw, and Chau Tran at OCWD in assisting with the pilot study, including maintenance and daily operation, are gratefully acknowledged. We would like to acknowledge Ran Nadav (Desalitech/DuPont) for pilot technical support and Mo Malki (American Water Chemicals) for contribution to discussion of membrane autopsy results. We would like to thank Jana Safarik and Ken Ishida at OCWD for assistance with the SEM-EDS analysis and proof-reading the manuscript, respectively. We also greatly appreciate the support from OCWD staff Mehul Patel, Patrick Lewis, Robert Phillips, Robert Raley, and Jeff Kirkwood.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.desal.2021.115300.

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