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Journal of Environmental Management

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Research article

On the feasibility of small communities wellhead RO treatment for nitrateremoval and salinity reductionJin Yong Choia, Tae Leea, Abdullah B. Aleidana, Anditya Rahardiantoa,b, Madelyn Glickfeldb,Maria E. Kennedyd, Yian Chena, Peter Haasec, Carina Chenc, Yoram Cohena,b,∗aWater Technology Research Center, Chemical and Biomolecular Engineering Department, Henry Samueli School of Engineering and Applied Science, 5531 Boelter Hall,University of California, Los Angeles, CA, 90095-1592, USAb Institute of the Environment and Sustainability, 300 LaKretz Hall, University of California, Los Angeles, Los Angeles, CA, 90095-1496, USAc Sherwood Design Engineers, 2548 Mission St., San Francisco, CA, 94110, USAd Kennedy Communications, 9042 Camellia Court, Rancho Cucamonga, CA, 91737, USA

A R T I C L E I N F O

Keywords:Small water systemWellhead RO water treatmentNitrate removalSalinity reductionRO treatmentSeptic system

A B S T R A C T

The feasibility of wellhead water treatment in small communities for nitrate removal and salinity reduction via aflexible high recovery RO system was evaluated through analysis of treatment options, laboratory and onsitefield tests. In small remote communities that rely on septic systems for residential wastewater treatment, dis-charge of the RO residual stream (containing nitrate) to the community septic tank is shown to be a feasibleoption. It is demonstrated that RO treatment with a system that employs partial concentrate recycle, integratedwith a pressure intensifier, enabled the use of a relatively low-pressure feed pump while allowing high recoveryoperation. The approach of integrating RO treatment into existing community small water systems is demon-strated to be suitable for providing effective nitrate removal and salinity reduction over wide range of nitrate andsalinity levels, while meeting community water demand and regulatory water quality requirements.

1. Introduction

1.1. Nitrate and salinity impaired small community local water supplies

Groundwater nitrate contamination and excessive water salinity aresevere in many remote small communities, particularly adjacent toagricultural areas. These communities lack centralized potable watersupply and wastewater treatment and are dependent on groundwater fortheir drinking water supply. Nitrate contamination of community potablegroundwater sources has been reported in various areas throughout theU.S. (Burow et al., 2010; Lindsey and Rupert, 2012). In California, forexample, more than two hundred community water systems have beenreported to exceed the maximum contaminant level (MCL) of 10mg/L asN which has been set by both the USEPA National Primary DrinkingWater Regulations (USEPA, 2009) and the California Department ofPublic Health (SWRCB, 2017). It has been reported that about 254,000people in California's Tulare Lake Basin and Salinas Valley are at risk dueto nitrate contamination of their drinking water (Harter et al., 2012). It isalso noted that elevated salinity of small and remote communitiesgroundwater (> 1000mg/L total dissolved solids (TDS) (Hanak et al.,

2017) is above recommended limit of the secondary MCL (SMCL) of500mg/L TDS for drinking water (USEPA, 2009).

Nitrate contamination and excess groundwater salinity is particularlychallenging to mitigate in small disadvantaged communities (DACs) whooperate their own wells and distribution systems and are removed fromcentralized water supply and waste treatment infrastructure. InCalifornia, water systems with fewer than 15 connections are known asState Small Water systems while those with a greater number of con-nections are classified as Community Water Systems (CHSC, 1995a).Many of the DACs are in rural agricultural areas with low financial andhuman resources to mitigate their impaired water sources (Salvestrin andHagare, 2009; Schoeman and Steyn, 2003). Because permanent solutionsrequire significant time to plan, permit and construct, California hasprovided funds for interim measure to DACs in California. These includevarious emergency subsidies for temporary replacement drinking water(e.g., bottled and trucked drinking water) (Harter et al., 2012; Jensenet al., 2012; SWRCB, 2016), funding for water systems improvements,consolidation, cleanup and operation (AB, 2014; SB, 2019).

The choices for permanent safe drinking solutions for small remotecommunities are to either: (a) search for a new clean (i.e., uncontaminated)

https://doi.org/10.1016/j.jenvman.2019.109487Received 13 December 2018; Received in revised form 21 August 2019; Accepted 27 August 2019

∗ Corresponding author. Institute of the Environment and Sustainability, 300 LaKretz Hall, University of California, Los Angeles, Los Angeles, CA, 90095-1496,USA.

E-mail address: yoram@ucla.edu (Y. Cohen).

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water source (e.g., drilling a new well), or (b) physical consolidation andannexation of the community water system with a neighboring communitywater system that has a suitable source of drinking water or onsite treat-ment (in compliance with Human Right to Water law, AB 685, Chapter524) (SWRCB, 2013, 2015). Drilling a new well is costly and one cannot beassured, a priori, that the new well water will be of the desired quality overthe long term. Physical consolidation in many cases may be highly costlyand/or infeasible option if the affected community is far from a willingneighboring community with sufficient safe drinking water resource ca-pacity. It is noted that the California Legislature has given the State WaterResources Board the authority to mandate the consolidation of an affectedcommunity water systems with a suitable adjacent system (SB, 2014).However, while there are successful examples of consolidation, a realtivelyrecent study (Lai, 2017) concluded that consolidation projects for smallcommunities in California have been lagging. The above has been attrib-uted to the limited availability of county-level policy tools, lack of statesystem upgrade and operational/maintenance funding support and StateSystem funding support and prospects of increased water rate after con-solidation.

Treatment of small communities impaired well water can be at thepoint-of-use (POU; e.g., treatment at the immediate use location such asunder the sink), at the point-of-entry (POE) to residential units or at thewellhead or another location(s) along the distribution system prior to de-livery to the community residential units (USEPA, 2006). While POU andPOE water treatment options are technically feasible, deployment in mul-tiple community homes can be a challenge given the requirement for sig-nificant water quality monitoring in multiple treatment locations. In Cali-fornia, the State Water Board permits the use of POU treatment only if it isestablished that centralized treatment is not feasible, and for no more thanthree years or until funding for centralized treatment becomes available,whichever occurs first (CHSC, 1995b). Given the above, the alternative of acentralized single treatment system at the community wellhead (upstreamof the distribution system) could be an effective solution (as discussed inthe present work). Such a solution can be established in one location witheffective centralized monitoring, operation and maintenance.

1.2. Nitrate removal and salinity reduction technology options for smallwater systems

A viable approach to salinity reduction of source water in smallwater systems is via reverse osmosis (RO) desalination (Pérez-Gonzálezet al., 2012; Subramani and Jacangelo, 2014), although ion-exchange(IX) can be utilized if hardness is the major issue of concern (Cliffordet al., 2010). Nitrate removal (Jensen et al., 2012) from contaminatedwater sources can be achieved via a number of proposed methods suchas ion-exchange (IX) (Dziubek and Mackiewicz, 2009), RO treatment(Schoeman and Steyn, 2003), electrodialysis (Archna et al., 2012),biological treatment (Bidhendi et al., 2006), and chemical/electro-chemical denitrification (Bosko et al., 2014). The advantages and lim-itations of the above methods for mitigating nitrate levels in smallcommunities are summarized in Table S1 (Supplementary Material).

Biological (Bidhendi et al., 2006; Della Rocca et al., 2006; Wang andChu, 2016) and chemical denitrification (Guan et al., 2015; Siciliano,2015; Yang and Lee, 2005) technologies, which are potentially pro-mising approaches for large-scale deployment have been touted togenerate low volume of residuals. However, such approaches, whenused for drinking water treatment, require the use of suitable post fil-tration/treatment for removal of suspended and dissolved residuals/trace contaminants not removed via biological or chemical treatment.Also, in biological treatment, biofilm growth must be managed to en-sure dependable treatment, particularly when faced with seasonalvariations of water demand. Chemical nitrate reduction (e.g., usingmetal-based catalysts) requires precise process control (including pHadjustment (Ruangchainikom et al., 2006) and in certain cases alsoammonium removal (Guan et al., 2015). Given the operational com-plexity of both biological and chemical denitrification, that can demand

frequent operator's intervention, cost and limited field commercial ex-perience make such systems less attractive, at present, for small systemsdeployment in remote communities.

Ion-exchange and RO processes are simple and proven technologieswith significant field experience for selective and non-selective removal ofvarious organic and inorganic contaminants. Indeed, RO along with IXhave been the common technologies suggested by the USEPA (Haugenet al., 2002) and California State Code of Regulation §64447.2 (SWRCB,2016) as best available technologies (BAT) for treatment of small com-munities drinking water sources that are contaminated with nitrate. IX andRO/NF systems are commercially available and/or can be readily de-signed/customized for specific treatment target levels and water treatmentcapacity. Ion exchange is generally appropriate for nitrate removal insmall water systems given its simplicity, effectiveness, selectivity and highrecovery (Dziubek and Mackiewicz, 2009; Lito et al., 2012; Samatya et al.,2006). However, its application is typically limited to treatment of watersources in which the nitrate level is at most about twice the MCL of 10mg/L as N (Jensen et al., 2012). For high salinity water, use of IX for nitrateremoval may require prior water softening. It is emphasized that IX re-generation requires a large amount of salts that necessitates residualsmanagement (Samatya et al., 2006). Also, exhaustion of the IX exchangebed my result in nitrate leakage; thus, adequate monitoring of the treatedwater stream may be necessary to ensure robust operation.

In contrast with IX and biological/chemical nitrate removalmethods, RO membrane water treatment has the advantage of pro-viding an effective approach for salinity reduction and a broader levelof protection against multiple different contaminants. It is noted thatRO membranes for brackish water desalting at relatively high salt re-jection (99.0–99.8%) are commercially available. Also, nitrate rejectionby RO membranes has been reported to be at a level up to ~96%(Schoeman and Steyn, 2003; Richards et al., 2010; Hancock et al.,2013) which is suitable for reducing even high nitrate levels (≫ nitrateMCL). In this regard, membrane treatment may be more efficient andcost-effective relative to ion-exchange when confronted with high ni-trate concentration. RO treatment for nitrate removal can also be de-ployed for simultaneously achieving the target level of salinity reduc-tion. RO systems for nitrate and salinity reduction can be achieved viasingle or multiple stages or two-pass treatment. However, productwater recovery in RO treatment may be limited to various degreesdepending on the raw feed salinity and associated limitations of systemphysical components (e.g., with respect to maximum allowable oper-ating pressure and flow rates). Therefore, one must also deal with themanagement of the concentrated residual (i.e., brine) stream.

Irrespective of the level of RO recovery one would have to contendwith a residual stream of elevated nitrate concentration. Such a residualstream must be managed in a manner that complies with waste dischargeregulations (SWRCB, 2014). In certain cases, depending on the RO con-centrate nitrate level, this concentrate stream may be blended with a lownitrate water source for beneficial use (i.e., irrigation) or diverted to amunicipal sewer if within proximity and permitted by regulations. Whenthe residual RO volume is low, its management in drying beds/pondsmay be feasible, although the required footprint and need for complexenvironmental compliance may make this approach infeasible for DACs.Other concentrate management options, such as surface water or coastaldischarge, off-site hauling (e.g., via trucking or through pipe delivery),and deep well injection are unsuitable for small communities given thelikely high cost and potentially adverse environmental impacts (e.g., riskof contaminated underlying soil and groundwater, eutrophication)(Pérez-González et al., 2012; AWWA, 2004).

In the present work, the feasibility of small community source watertreatment using a novel flexible RO (FLERO) for nitrate removal andsalinity reduction was evaluated experimentally and via process ana-lysis. RO treatment system suitability was assessed with respect to theattainable product water recovery and RO concentrate management.Laboratory and field testing were carried out to assess the expectedlevel of water treatment performance with respect to nitrate removal

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and salinity reduction utilizing membranes of high nitrate and salt re-jection. Overall treatment system specifications, operational con-siderations and estimated treatment costs were then estimated for threespecific small communities considering source water quality data,community water consumption data and the available options for re-sidual management.

2. RO well water treatment for nitrate removal and salinityreduction in small remote communities

2.1. The RO treatment approach

Small and remote community water systems that rely on localgroundwater for their domestic supply of potable water and are notconnected to centralized sewer system utilize septic systems for hand-ling their domestic wastewater (Fig. 1a). Such communities whosewater supplies have been impaired due to nitrate contamination andoften also excessive salinity, could in principle utilize RO treatment oftheir well water to effectively upgrade their source water to producesafe drinking water (Fig. 1b). The RO product water typically undergoespost-treatment (i.e., limestone bed contactor) to condition the productwater (e.g., pH adjustment, alkalinity, and remineralization) which alsoserves to avoid potential corrosion of distribution pipes.

Water consumption is expected to vary temporally. Therefore, a pro-duct water storage tank can be used as a buffer from which water ispumped to the community pressure tank which would then feed thecommunity water distribution system. With such an arrangement(Fig. 1b), the RO system needs to operate continuously for only part of theday while meeting the daily product water demand even during days ofpeak water consumption (Section 4.2). The total volume of pumpedgroundwater to be treated by the RO system would have to be greater thanthe community level of water consumption (QW ) by a factor of Y1/ (i.e.,

=Q Q Y/W0 ) in which Y is the RO product water recovery (i.e.,=Y Q Q/P 0, whereQ0 andQP are the volumetric flow rates of the raw feed

and permeate water streams, respectively). RO treatment has to be ac-complished at a recovery level that is sufficiently high for both water useefficiency and to minimize the volume of generated RO concentrate. Toachieve water treatment at high recovery and with a small system foot-print, one can resort to RO system operation in which the RO concentrateis recycled back to the RO feed and where a pressure intensifier is utilizedto enable the use of a relatively low-pressure pump (Lee et al., 2019).

2.2. Water treatment/desalination with a flexible RO (FLERO) system

2.2.1. System configuration and operational considerationsWater treatment at the wellhead in small communities can be

achieved with a flexible RO (FLERO) system configuration that allowshigh recovery in a small footprint. In such a system configuration(Fig. 2) RO operation is with partial concentrate recycle and where apressure intensifier (acting also as an energy recovery device or ERD)serves to lower the required RO feed pump output pressure. A detaileddescription of the FLERO system, which was utilize in the preset study,can be found elsewhere (Lee et al., 2019). Briefly, the feed is pre-pressurized by the RO feed pump while the ERD recovers energy fromthe concentrate stream and depressurizes the concentrate effluent fromthe membrane module in order to intensify the RO module inlet pres-sure (i.e., from PFL to PFH) to the desired level needed to achieve thetarget water production capacity.

As illustrated in Fig. 3, the overall RO system product water re-covery ( =Y Q Q/P 0) can be set by the concentrate recycle ratio( =R Q Q/R 0), whereby = +Y R Y( 1) sp in which the single pass recoveryis =Y Q Q/SP P F , where QF is the RO module inlet flow rate, and whereY YSP. It is noted that while the single-pass recovery can remain fixed(i.e., given element specification and equipment design limits), theoverall system recovery can be varied by adjusting the raw feed (Q0)and recycle flow (QR) rates (and thus the recycle ratio).

It is important to recognize that the product water recovery is con-strained by the allowable concentrate volume that can be accommodated

Fig. 1. Water system in a small remote community without (a) and with (b) RO water treatment.

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by the community septic system (Section 2.2.2). In this regard, the abilityto adjust the feed flow rate and recycle ratio provides the flexibility ofadjusting the overall system recovery and thus the volume of generatedconcentrate. Analysis of the FLERO system operation, based on a steady-state salt balance around the overall RO system (Fig. 2), leads to thefollowing expression for the solute (e.g., salt or nitrate) concentration inthe product water, Cp,

=C C R YR Y Y R

(1 )(1 )1 (1 )P

S SP

S SP S0 (1)

in which C0 is the solute concentration in the raw feed water and RS is themembrane solute rejection (i.e., =R C C1 /S P F where CF is the soluteconcentration in the raw source water). The required RO module feedpressure, PFH, needed to attain the target recovery, can be determinedfrom the following expression (Lee et al., 2019):

= + +P P J L CP Y RR Y Y R

( ) / 1 (1 )1 (1 )FH P V exit P

SP S

S SP S0

(2)

in which PFH and PP are the RO module feed and permeate side pressures,respectively, (JV)exit and LP are the volumetric permeate flux at themembrane exit and membrane water permeability coefficient, respec-tively, CP is the concentration polarization modulus (i.e., =CP C C/m b

where Cm and Cb are the solute concentrations at the membrane surfaceand in the bulk solution, respectively) and 0 is the osmotic pressure ofthe raw feed source water. The calculations of (JV)exit and CP can beaccomplished following the approach described in (Lee et al., 2019) toobtain the required element feed pressure. For the present system con-figuration, the integration of an pressure intensifier (Fig. 2) reduces therequired RO feed pump outlet pressure (PFL) which can be determinedfrom an energy balance around the energy recovery device. Accordingly,for a system having an ERD of efficiency ERD, PFL is given as

=P P Y(1 (1 ))FL FH ERD SP (3)

2.2.2. RO residual stream and limit on system recoveryTreatment of the community source water via RO generates a re-

sidual concentrate stream (Fig. 2) of flow rate =Q Q Y(1 )D 0 . A viableoption for handling this stream is via discharge to the community septictank and/or its potential beneficial use with or without blending. It isnoted that, when the concentrate and product water streams are bothdischarged to the septic tank, on average the overall nitrate con-centration in the septic system (introduced via the well source water)should be essentially the same as in the feed water, unless a portion ofthe residual or product water streams are used for landscape irrigation.Salinity in the septic tank, however, may be above that which is in the

Fig. 2. Schematic of RO system configuration for high recovery water purification/desalination employing a pressure intensifier and concentrate recycle (Lee et al.,2019).

Fig. 3. Variation of FLERO volumetric flow rates, normalized with respect to the permeate flow rate (QP) and overall system recovery (Y), with respect to theconcentrate recycle ratio (R). Data were generated using the pilot FLERO system, for raw feed solution of salinity of 1000mg/L TDS, at a single pass recovery (YSP) of~9.3%. Note: the normalized RO element and raw-feed flow rates are given as = =Q Q Q Y/ 1/F F P SP and = =Q Q Q Y/ 1/P0 0 , respectively, where QP is the permeateflow rate, and = =Q Q Q Y Y/ 1/ 1/R R P SP .

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source water due to the added salinity introduced by domestic waste-water.

It has been reported that under anoxic conditions, which exist in theseptic tank, biologically-mediated denitrification of nitrate can occur(Andreoli et al., 1979; Gold et al., 1992; Lamb et al., 1990, 1991;Leverenz et al., 2002; Mbuligwe, 2005; Urynowicz et al., 2007). Theexisting body of evidence, albeit limited, indicates that nitrate in-troduced directly to the septic tank may undergo some degree of de-nitrification as documented by the body of evidence (SupplementaryMaterial, Section S1). Therefore, as suggested in the present study, it isof interest to assess, for small communities, the feasibility of ROtreatment of nitrate-laden water supplies with septic tank utilization forconcentrate handling.

In order to use of the community septic tank for handling the com-bined treated source water and RO concentrate, the following conditionsmust be met: (i) the septic tank volume should be of sufficient capacity tohandle the combined volume of the discharged domestic wastewater andRO concentrate stream, (ii) there needs to be sufficient hydraulic re-tention time ( = V Q/ST ST , in which VST and QST are the septic tank vo-lume and its influent flow rate, respectively) in the septic tank to allowfor its proper functioning (Otis et al., 2002; Tchobanoglous et al., 2003;WADOH, 2005), and (iii) nitrate input to the septic tank, originatingfrom the source water, should not lead to any or significant excess nitratein the septic tank effluent. A retention time of the order of one to threedays (Otis et al., 2002; Tchobanoglous et al., 2003; WADOH, 2005) istypically recommended for effective septic tank operation in order toallow for the physical (e.g., sedimentation) and microbial processes tooccur prior to septic tank effluent discharge to the leach field. Accord-ingly, the maximum feasible wastewater influent flow rate to the septictank (QST max), which includes the community water consumption rate(i.e., QP) and RO concentrate residual stream (QC) discharge flow rate, isdictated by the limit on the retention time, i.e., =Q V /F max ST min, , where

min is the minimum recommended retention time. Accordingly, theminimum required RO system product water recovery is given by

= =Y Q Q Q V/( ) ( / )min P P ST min0 max .

3. Methods

3.1. Overview

The feasibility of nitrate removal and salinity reduction via RO insmall community water systems was assessed based on the followingelements: (a) field determination of the adequacy of RO treatmentsystem for nitrate removal and salinity reduction at high recovery op-eration in selected remote and disadvantaged communities using aFLERO system in which a high permeability brackish water membranewas utilized, (b) laboratory tests of high nitrate rejecting membrane butof lower water permeability, (c) evaluation of the required RO treat-ment capacity, considering the salinity and nitrate ranges, for three

small communities ( 11 residential units) based on detailed quantifi-cation of the community water use patterns and other communitycharacteristics, and (d) overall treatment system specifications, opera-tional considerations, and water treatment costs.

3.2. Characteristics of study communities

The feasibility of deploying RO treatment in small remote commu-nities that utilize local groundwater for their potable water use wasevaluated for four typical small communities located in California. Thesecommunities (Table 1), located in the midst of agricultural fields in theSan Joaquin Valley, have 8–77 residential units (16–308 residents) withtheir household wastewater discharged to existing septic systems exceptat Community Site D. This latter community was excluded from theevaluation of water use patterns and final assessment of RO system de-ployment and associated cost since it did not qualify as per the State ofCalifornia regulatory definition of a small water system (SWRCB, 2016).

Water use patterns in communities A-C (Table 1) were evaluated viareal-time data obtained from wireless water meters (Metron-Farnier,Inc. Boulder, CO) installed at the community main water distributionpoint. These water meters collected water consumption rate (i.e., flowrate) every 5min and wirelessly transmitted the water use data (i.e., 5-min consumption along with calculated hourly total, daily total, cu-mulative consumption) to a remote data server. Source water quality(e.g., nitrate, turbidity, salinity, inorganic, organic contaminants) andgeneral water quality parameters (e.g., total dissolved solids, turbidity,coliform, pH) were determined via multiple samplings and analysis byCA State certified analytical laboratory.

3.3. Field testing RO system for nitrate removal and salinity reduction

Field and laboratory tests of nitrate and salinity removal were car-ried out using a small RO test system (Fig. 2) which is described else-where (Lee et al., 2019). Briefly, the RO unit contained a single 2.5-inchx 40-inch RO element in a pressure vessel rated up to 70 bar. Systempermeate production capacity was up to 1.5 m3/d (~400 gal/day) forbrackish water of salinity up to 5000mg/L TDS at up to 90% recovery.The single-pass water recovery was in the range of 9.3 ± 0.3%. Fieldtests of salinity reduction and nitrate removal were conducted with a2.5 inch×40 inch high permeability spiral-wound element (DOWFilmTec XLE-2540, hereafter M-1) having a manufacturer reportedmembrane area of 2.6 m2, water permeability of 4.6 L/m2·h·bar, and saltrejection of 99% (measured at 10 bar and 500 ppm NaCl solution).Laboratory tests demonstrated nitrate rejection of 90% (12.5mg/L ni-trate feed concentration as N) while field tests revealed nitrate rejectionrange of 83%–87%. An alternate high nitrate rejection membrane (CSMRE-2540 BE, Toray USA) was also subsequently laboratory tested. Thismembrane (hereinafter labeled M-2) had a manufacturer reportedmembrane area of 2.5m2, water permeability of 3.2 L/m2·h·bar, and

Table 1Study communities.

Study community Site A Site B Site C Site D (a)

Number of residential units 11 8 10 77Population 16 36 34 308(b)

Community area (m2) 70× 60 160×50 143×80 400×200Proximity to nearest centralized water delivery and sewer infrastructure (km) 2.2(c) 4.4(d) 4.1(d) 24(e)

Septic tank capacity (m3) 17.0 18.9 18.9 N/A(f)

Ave/Max water consumption(g) (m3/day) 3.38/10.8 9.63/15.1 5.28/12.5 173.7/361Average septic tank retention time(h) (day) without RO/with RO treatment (90% recovery) 5.0/4.5 2.0/1.8 3.6/3.2 N/A(f)

(a) Site D was evaluated for comparative analysis, but was excluded from the final analysis since it did not qualify as a Small Water System as defined in California(SWRCB, 2016); (b) Assuming an average of four persons per home; (c) Distance from Ecowater Water treatment plant, and (d) Distance from Soledad sewagetreatment plant; (e) Distance from wastewater treatment plant in Porterville CA; (f) domestic wastewater is discharged to a wastewater lagoon; (g) Based on two-yearwireless meters monitoring data for sites A-C and compiled historical water use data for site D; (h) Retention time in the range of 1–3 days is typically recommended(Otis et al., 2002; Tchobanoglous et al., 2003; WADOH, 2005).

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nitrate and salt rejections of 96% and 99.7% (determined at 15.5 barand 2000mg/L NaCl solution), respectively. Field tests with thismembrane at Community Site D demonstrated nitrate and salt rejectionof 97% and 99.4%, respectively.

RO system operation in the mode of partial concentrate recycle al-lowed high recovery of up to 90% at RO element crossflow velocity of0.23m/s (maintained for all field and laboratory tests) and permeateflux of 5.8 × 102 L/m2·d (within the recommended range for the ROelements). Given the use of a pressure intensifier (Fig. 2) the requiredRO feed pump output pressure was less than 5.5 bar, which was a factorof 3.1–5.0 below the required RO lead element feed pressure (Section4.1.2), over the range of encountered source water salinity (Section4.1.1). Flow rates and conductivity of the permeate, feed and con-centrate flow streams and system pressures (feed and exit) were mon-itored with in-line sensors as described elsewhere (Lee et al., 2019).

Nitrate concentrations in the feed and permeate streams weremonitored using an optical UV nitrate sensor (Model CAS51D,Endress + Hauser, Netherland) housed in a mobile platform. A slipstream from the RO system (feed or permeate), at a flow rate of100–300 mL/min, was first passed through a filtration unit (R series;cartridge filter of 20 and 5 μm cellulose pleated 2-1/2”×10″, PenairInc., USA) with the aid of auxiliary pump (medium duty peristalticpump, Hydrobuilder Inc. Chico, CA) and then directed to the opticalflow UV cell. The nitrate sensor was calibrated using synthetic nitratesolutions prepared by dissolving reagent grade sodium nitrate (NaNO3)and sodium chloride (NaCl) (Fisher Scientific, Pittsburgh, PA) in deio-nized water. The calibration was linear (R2 of 0.999) over a nitrateconcentration up to 45mg/L as N and essentially independent of sali-nity over the range of 50-3000mg/L TDS as NaCl. All laboratory testswere carried out using synthetic salt solutions prepared by dissolving,in deionized (DI) water, the predetermined amount of reagent-gradesodium chloride and sodium nitrate (Fisher Scientific Pittsburgh, PA).

For the field tests, the RO system and nitrate monitoring unit weretransported in a cargo van from their location at UCLA to the studycommunities located at a distance of about 300 miles away. At each ofthe community test sites the RO system was connected to the local powersupply and the source water was fed to the RO unit via a flexible hose.The permeate and concentrate were both subsequently combined fordischarge during the short-term tests. RO system operation was at a levelof 90% recovery at a permeate production rate of 1.0 L/min. Water grabsamples were collected from the community water well (i.e., raw sourcewater feed to the RO unit) and from residential water taps, followingState of California approved protocols, and delivered (on the same day)to a State Certified Laboratory for water quality analysis.

4. Results and discussion

4.1. Field testing of nitrate removal and salinity reduction via RO treatment

4.1.1. Source water qualityWater quality analysis (Table 2) revealed impaired local potable

water sources (i.e., groundwater wells) of the four study communities (A-D; Table 2), with respect to nitrate contamination, exceeding the nitrateMCL (10mg/L as N) by a factor of 1.03–4.06. The source water salinity ofcommunities A, B and D was about 80%–304% and in community Cabout 18% above the recommended level of 500mg/L TDS for drinkingwater (USEPA, 2009). In communities A-C there were no contaminants(organic or inorganic) above the MCL other than nitrate, while in com-munity D chromium (VI) was at a level of 12 μg/L which was above theMCL of 10 μg/L (SWRCB, 2016). Lead and copper concentrations in tapwater were below the action levels indicating the suitability of the waterdistribution system for delivering drinking water.

4.1.2. Product water recovery and required RO pump feed pressureThe minimum recovery limit at which the RO treatment must operate

is dictated by the community water consumption capacity and the septic

tank capability to accommodate the generated treatment residual volume(Section 2.2.2). Analysis of water use, over a period of two years, in thethree communities (A-C, Table 1) that are categorized as small watersystems (Section 1.1), revealed daily average water consumption in therange of 3.38–9.63m3/day (Table 1). Sporadic periods of peak waterusage ~56–221% higher than the average consumption were en-countered primarily during the months of July and August (Supple-mentary Material, Fig. S2). On average, the residual treatment volumewill decrease with increased product water recovery as shown in Fig. 4.Following the analysis presented in Section 2.2.2, it can be shown thatfor septic tank volumes (Table 1) and water consumption (Table 1) ofcommunities A-C, RO treatment would have to be carried out at aminimum recovery of Y=0.40, 0.9, and 0.56 for communities A, B and C,respectively, to provide a reasonable septic tank retention (~2 days).

RO treatment at the highest feasible recovery is critical in order toreduce the volume of the RO residual stream. For example, for RO well-head water treatment at 90% recovery (Fig. 4) the added excess volume(to the septic tank) of the residual RO concentrate would be 11.1%; thus,the septic tank retention time will be reduced by ~10% (Table 1) but stillwithin the recommended range (Section 2.2.2). It is noted that high re-covery operation would increase the RO element feed pressure require-ment (Lee et al., 2019). In the FLERO approach, the integration of apressure intensifier within the RO system along with concentrate recycling(Section 2.2.1, Fig. 2) allows for both high recovery operation and sig-nificant reduction of the RO feed pump pressure demand. Laboratory andfield tests for a water recovery range of 9%–90%, with membranes M-1and M-2 and feed water salinity of up to 38,900mg/L TDS, demonstratethat even when the required RO element feed pressure was up to 37.6 bara feed pump of output pressure delivery of only 7.2 bar was necessary(Supplementary Material, Fig. S3). For the study communities, the re-quired RO module feed pressure of 9.26–15.7 bar would require a corre-sponding RO feed pump output pressure of 3.56–4.33 bar. Here it is notedthat the use of a relatively low-pressure pump, afforded by the use of apressure intensifier (Fig. 2), should make it possible to potentially utilize arenewable energy source (e.g., solar or wind).

4.1.3. Experimental evaluation of nitrate removal and salinity reductionField testing of the feasibility of RO treatment was initially conducted

with membrane M-1 (Section 3.3) at recovery of 90%. Product waterquality was well below the MCL of nitrate by 23%, 41% and 75% for SiteA, B, C, and D, respectively (Fig. 5). Salinity reduction was also effectivewith the treated product water salinity being in the range of 46–300mg/L, well under the recommended upper limit of 500mg/L TDS for all sites.Nitrate rejection, however, was in the range of 83%–87% which couldprove to be insufficient to meet the MCL for nitrate removal from feedwater of nitrate level in excess of 17.3mg/L as N, and for salinity re-duction below the SMCL for feed water in excess of 2100mg/L TDS.

In order to accommodate the potential for upward temporal fluc-tuations in source water quality (particularly with respect to nitrate ingroundwater adjacent to agricultural areas (Amano et al., 2016) the useof higher nitrate and salt rejection membrane was considered. Ac-cordingly, a membrane having nitrate and salt rejection of 96% and99.7%, respectively was evaluated (i.e., membrane M-2, Section 3.3)for treatment of source water of nitrate concentration in the range of13–40mg/L as N and salinity of 1000mg/L TDS. Laboratory tests(Fig. 6; Supplementary Material, Fig. S4) demonstrated excellentagreements between the experimental data and predictions (Eq. (1)) ofthe product water nitrate concentration dependence on the treatmentsystem recovery. The above results indicate that RO water treatment, atrecovery of 94%–99%, of feed of nitrate concentration in the range of13–40mg/L (as N) can produce product water in which the nitrate levelis below the MCL. Moreover, predictions as per Eq. (1), indicate that forRO operation at 90% recovery with membrane M-2 one could treatsource water of up to a high nitrate level of 19mg/L as N and salinity ofup to 9400mg/L TDS and produce safe drinking water that is at least 1/3 of the MCL in nitrate and ~1/2 below the SMCL for salinity. Clearly,

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operation at higher recovery is feasible while meeting the MCL andSMCL limits for nitrate and salinity, respectively. However, given theneed to ensure a sufficient retention time in the septic tank, operation at90% recovery appears as the reasonable choice.

4.2. Water treatment specifications and operational considerations

Given the feasibility of high recovery RO treatment for nitrate re-moval and salinity reduction via RO system at high recovery operation(Sections 4.1), the required RO systems for the study communities canbe specified as indicated in Table 3. It is reasonable to limit the dailyoperational period of the RO system to 8 a.m.–6 p.m. to avoid nightoperation and thus the capacity requirements for the RO systems, withan added 10% overdesign capacity (Table 3). In order to achieve ROoperation at 90% recovery, the concentrate recycle ratio would be atthe range of 0.78–1.98. With the design treatment capacity and

installation of a product water storage tanks one can ensure (e.g., in theevent of a system shutdown repair and/or maintenance) the availabilityof safe drinking water for up to or greater than the maximum dailydemand (California, Title 22 CCR §64554, New and Existing SourceCapacity (SWRCB, 2016)).

RO process design calculations using available commercial mem-branes of high (96%) nitrate rejection (membrane M-2, Section 3.3) andoperating at the conditions listed in Table 3, indicate that safe drinkingwater can be produced, well below the MCL, over a wide range of ni-trate contamination level (Table 3; Supplementary Material, Fig. S5).RO treatment for the systems of the proposed design specifications(Table 3) would enable treatment of source water with a nitrate level afactor of 6.8–12 above the nitrate MCL while still delivering permeateproduct water that is a factor of 1.69–8.33 below the nitrate MCL(Table 3; Supplementary Material, Fig. S5). It is noted that the ROpermeate has to be post-treated by passing this stream through a

Table 2Summary of water quality analysis.

Sampling/Analytical Method Well Source Watera

Site A Site B Site C Site Dd

Turbidity (NTU) EPA180.1 0.15 0.15 0.82 0.1Total dissolved solids (mg/L) SM2540C 1126–1500 1091–2020 554–594 900–938Nitrate (mg/L as N) EPA 300.0 27.1–40.6 20.7–21.9 10.3–11.0 8.7–9.5pH SM4500-H + B 7.3 7.6 7.4 8.2Alkalinity (mg/L as CaCO3) SM2320B 348 244 112 130SICalciteb 4.33 4.76 0.84 2.41SIGypsumb 0.11 0.09 0.01 0.006

Sampling/Analytical Method Tap Waterc

Site A Site B Site C Site D

Lead (μg/L) EPA200.8 ND ND ND N/ACopper (μg/L) EPA200.8 75.1 624 21.8

a Total organic carbon (TOC) for all four sites was below≤1mg/L; N/A: not available.b The mineral salt saturation index is defined as SIi= IAP/Ksp,i, where IAP is the ion activity product and Ksp,i is the solubility product for the mineral salt i

(calculated based on detailed water quality analysis for each site using the OLI Stream Analyzer software (OLI System Inc, 2018).c kitchen tap water was collected based on the “Lead and Copper Rule” (SWRCB, 2018).d Site D was evaluated for comparative analysis, but was excluded from the final analysis since it did not qualify as a Small Water System as defined by the State of

California (SWRCB, 2016).

Fig. 4. Daily RO residual stream volume and septic tank retention time dependence on RO product water recovery (Section 2.2.2) for production of drinking water atthe required average daily consumption (Qw) for sites A-C (Table 1).

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limestone bed contactor (Lahav and Birnhack, 2007) to recondition theproduct water (e.g., pH adjustment, alkalization, remineralization) andpossibly avoid corrosion of metal pipes in the residential water dis-tribution system. Simulations of permeate limestone treatment(Letterman, 1995) for the community source water revealed that con-ditioned RO permeate would be of alkalinity in the range of

50.3–116mg/L (as CaCO3), calcium concentration of 17.3–34.5mg/Las Ca2+ and pH in the range of 7.35–7.95 (Table 3). The above are wellwithin the USA (USEPA, 2016) and WHO (WHO, 2007) guidelines.

In two of the communities (Sites A and B, Table 2) the level of cal-cium sulfate dihydrate (gypsum) in the source water was a factor of9.09–11.1 below the saturation limit (i.e., as quantified by the solubility

Fig. 5. Onsite wellhead RO treatment forthe study communities. The RO test system(capacity of ~1.5m3/day, see Section 3.3)was operated at 90% recovery. The dashedline denotes the nitrate MCL (10mg/L as N)and the recommended upper salinity level(500mg/L TDS) for drinking water. Theerror bars indicate the small uncertaintyrange of the nitrate and salinity measure-ments.

Fig. 6. Nitrate concentration in the RO productwater for treatment using a high nitrate rejectionmembrane (M-2) over a range of product water re-covery for initial feed nitrate concentration of13–40mg/L as N. The solid curves are prediction ofEq. (1) (Section 2.2.1). (Note: the maximum allow-able recovery must be set such that the product watermeets both the MCL and SMCL with respect to nitrateand salinity, respectively, and the feed pressure mustremain below the maximum allowed pressure for theRO elements).

Table 3Specification for RO systems for nitrate removal for the study communities.

RO System Specification(a)

Site A Site B Site C

Design Production(b) (m3/day) for RO operation at 90% recovery 7.44 20.9 11.6Number of membrane elements (4.0 inch) 2 4 3Recycle Ratio (Q Q/R 0, Fig. 2) 1.98 0.78 1.27Required RO element feed pressure (bar) 15.7 13.1 9.26Required RO pump feed pressure with use of pressure intensifier (bar) (c) 2.88 2.41 1.70Nitrate concentration (mg/L as N) in permeate water 5.9 1.7 1.2Total dissolved solids (mg/L TDS) in permeate water 114 49.8 38.9Alkalinity (mg/L as CaCO3) of permeate post limestone contactor treatment 116 67.9 50.3Calcium content (mg/L as Ca2+) in permeate post limestone contactor treatment 34.5 23.0 17.3pH of stabilized permeate water 7.35 7.72 7.95Maximum nitrate feed concentration (mg/L as N) that can be handled at design capacity while providing nitrate level in permeate at 1/3

the MCL(d) (3.3 mg/L as N) and at the MCL (in parentheses)22.6 (68.4) 40.6 (122) 25.8 (78.3)

(a) Design based on membrane M-2 (Section 3.3) for max encountered salinity and nitrate levels (Table 2); (b) System operates for a maximum period of up to ~10 h (c)

pressure intensifier (i.e., ERD) having efficiency of 90%; (d) Nitrate MCL=10mg/L as N (SWRCB, 2017). Note: average and maximum daily water demands are givenin Table 1.

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constant) for gypsum, but above the saturation limit (by a factor of4.33–4.76) for calcium carbonate (as calcite). At 90% RO recoverygypsum in the RO exit concentrate would be a factor of 1.61–1.69 abovesaturation for community Sites A, B, respectively, and below saturationfor site C; thus, the potential for membrane mineral scaling in RO well-head treatments in sites A and B. However, calcite concentration in theRO concentrate is expected to be significantly above saturation at theconcentrate pH of 7.37–7.50 such that the LSI for calcite (i.e., differencebetween the pH of the solution that at the condition of calcite saturation(USEPA, 2016),) would be at a value of 2.21, 1.89, and 1.41 for Site A, B,and C, respectively. Calcite solubility is pH dependent and can be sig-nificantly increased under acidic conditions; hence, acid pretreatment istypically implemented to mitigate calcite scaling. However, the aboveapproach may be counterproductive for water of high gypsum scalingpotential, which is also supersaturated with respect to calcium carbonate,but has low to medium bicarbonate content. For such a water source, thepositive effect of bicarbonate ions on retardation of gypsum precipitationmay diminish due to protonation of the bicarbonate ions at reduced pH(Rahardianto et al., 2008). Indeed, previous work has shown that ROdesalting of such water without acidification is preferred as there arekinetic effects whereby calcite and gypsum surface crystallization is re-tarded (Rahardianto et al., 2008). Mineral scaling can be suppressed toby using a suitable antiscalant (AS). This approach is typically re-commended for operation whereby the RO concentrate mineral satura-tion is such that SICaSO4 4 and LSI (calcium carbonate) 2.9(Hydranautics, 2013; McCool et al., 2010). As an alternative, effectivemembrane cleaning can be achieved by periodic permeate flush of theRO elements with the permeate (collected) prior to its pH adjustment viapost-treatment. The RO permeate is at acidic pH (i.e., pH=5.62, 5.86,and 5.70 for Site A, B, and C, respectively) at which the LSI for calciumcarbonate would be in the range of −3.7 to −4.7 for sites A-C; thus,permeate flush for dissolution of carbonate scale may be feasible if thetreatment is carried out at an optimal frequency.

4.3. Nitrate release to the environment

In order to meet the community water capcity demand, the volumetricflow rate of RO raw source water feed is a factor of Y1/ greater than thedaily water demand (Section 2.1). As a consequence, for example, ROsource water treatment at a recovery of 90% (i.e., =Y 0.9) would result in11.1% added daily nitrate loading to the community septic tank. Theabove estimate assumes that all of the treated sources water (i.e., thepotable product water and the RO concentrate) is ultimately discharged tothe septic tank. However, it is expected, as suggested by the existing lit-erature, that denitrification of nitrate (~27–50%) would take place underthe anoxic conditions in the septic tank (Supplementary Material, SectionS1). Denitrification in the septic tank would therefore result in a net re-moval of nitrate from the source water and thus a reduction in the overallnitrate loading to the environment (i.e., leach field). Here it is important toemphasize that it is the discharge of ammonia nitrogen from the septictank to the leach field and its subsequent nitrification, and not the sourcewater nitrate influent to the septic tank, which are responsible for theoften-stated association of septic systems with nitrate contamination ofproximal groundwater (WADOH, 2005).

4.4. The cost of RO treatment of community well water

A commercial RO treatment system of production capacity range forthe present three communities can be manufactured for a basic unit cost ofabout $53,330 - $55,707. The above system cost, arrived based on detailedsystem design and estimated cost by a system fabricator, includes a lime-stone contactor and chlorine dosing. The dimensions of the RO treatmentsystem are estimated to be about 160 cm (L) x 76 cm (W) x 122 cm (H)which is of a relatively low footprint. Additional site infrastructure includesstorage tanks for the feed, product water and RO concentrate (to allow forbeneficial use) at an estimated total cost of ~$10,000, pumps for water

delivery to the pressure tank, backflow preventers ($1000), bypass valves($2000), water meters for the different lines (~$2000), and infrastructureupgrade/retrofit (e.g., system enclosure shed, concrete slab and other in-frastructure improvements) estimated at ~$6000. These added costs of ~$21,000 would increase the overall water system capital (CAP) cost to$74,330-$76,707. System depreciation over the course of the system life-time (estimated at 15 years) and eventual system replacement would re-quire additional capital expense. The above CAP cost, which could also risesomewhat if site specific electrical or plumbing upgrades may be required,could be afforded by some of the more affluent communities, but likely tobe beyond the reach of DACs. Small disadvantaged communities, however,who currently do not have access to safe drinking water are in dire need ofimmediate solutions. Here is where government assistance to small dis-advantaged communities is essential in the form of grants, subsidies or lowinterest loans for infrastructure improvements.

Operations and maintenance (O&M) cost for brackish water ROtreatment is typically in the range of $0.34–0.79/m3 treated water(AMTA, 2007; Cohen et al., 2017). With remote monitoring and su-pervisory control of such systems, one should expect a measurabledecrease in the overall O&M cost. However, even at the higher costestimate, based on the water consumption data for the small dis-advantaged (DACs) study communities, the average monthly cost ofwater treatment for sites A, B and C is expected to be in the range of$35–80 ($3.60–8.20/apartment), $98–227 ($12.3–28.3/household),and $54–126 ($5.44–12.5/household), respectively. It is also noted thatfor the present water treatment system design (Section 4.2), the specificenergy consumption is estimated to be 0.17, 0.14 and 0.10 kwh/m3

permeate for communities A, B and C, respectively, which (at the es-timated residential electricity cost of about $0.16/kWh (ElectricityLocal, 2019; USEIA, 2019a,b) would represent about 1.2–8.2% of thetotal estimated O&M cost. The above O&M costs are within the ex-pected ranges for RO treatment of brackish water. However, there maybe additional costs associated with specific regulatory requirements forwater quality and system monitoring, which could be possibly reducedvia remote monitoring and self-adaptive system operation.

It is instructive to compare the O&M costs for water treatment with thecost of the alternative, albeit temporary solution, of providing replacementpotable water that would be trucked to the communities. Even if thecommunities would utilize such water only for drinking, cooking and per-sonal hygiene (except showering), it is estimated that, based on per personuse of 7.5–15 L per day (WHO, 2011), the total monthly volume of truckedwater would be 5.4, 12.1, and 11.5m3/month for communities, A, B and C,respectively. At the estimated cost of trucked water in the range of$13.2–31.7/m3, the total monthly cost of trucked water would be $121,$273, and $258 for communities A, B and C, respectively, or correspondingmonthly residential dwelling cost of $11, $34 and $26. It is stressed that theabove cost estimates are merely for drinking water and are in fact somewhathigher than the O&M cost of water treatment at the source.

In summary, Impaired well water sources of small disadvantagedcommunities can be upgraded via RO treatment. However, these com-munities may require funding assistance for both CAP expenditures andO&M costs. It is noted that in California, such assistance is availablethrough the California Drinking Water State Revolving Fund (DWRSF)program (AB, 2014) and is a resource to DACs that has been providingfunding at a level that would be sufficient for the needed capital im-provements. Also, the recently enacted California Senate Bill SB200allocates $130 million of state funds per year for cleanup of drinkingwater and operation of water systems of qualified disadvantagedcommunities (SB, 2019). Such assistance will clearly be critical for re-mote and disadvantaged communities that are not part of a centralizedinfrastructure of water supply and wastewater management.

5. Conclusions

The present study provides both field and laboratory evaluations,along with feasibility analysis, of wellhead treatment of impaired

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source water of small rural disadvantaged communities for nitrate re-moval and salinity reduction. It is shown that RO treatment via a novelflexible RO (FLERO) system that integrates partial concentrate recyclewith a pressure intensifier provides for high recovery operation in asmall footprint with a relatively low-pressure feed pump. Treatmentwith high nitrate and salt rejection membranes using FLERO can pro-vide high quality safe drinking water and with capability for handling areasonable level of small community growth. It is emphasized that thefeasibility of utilizing RO for water treatment, to produce safe drinkingwater, is predicated on the ability to discharge the residual RO con-centrate to either a sewer system for centralized treatment or to thecommunity septic tank. Therefore, in remote communities that rely onseptic systems high recovery RO treatment, which is afforded by theFLERO system design, it is critical to minimize the volume of the re-sidual stream (concentrate) generated by the treatment.

Acknowledgements

This work was supported in part, by the State of California WaterResources Control Board, Agreement 14-255-550 (C/A 367), theCalifornia Department of Water Resources (4600011630), U.S. Bureau ofReclamation (R17AC00149), Electric Power Research Institute (MA-10002732), and UCLA Water Technology Research (WaTeR) Center.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvman.2019.109487.

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Supplementary Material

On the Feasibility of Small Communities Wellhead RO Treatment

for Nitrate Removal and Salinity Reduction

Jin Yong Choi a, Tae Lee a, Abdullah B. Aleidan a, Anditya Rahardianto a,b, Madelyn Glickfeld b,

Maria Kennedy d, Yian Chen a, Peter Haase c, Carina Chen c, and Yoram Cohen a,b*

a Water Technology Research Center, Chemical and Biomolecular Engineering Department, Henry Samueli school of Engineering and Applied Science, 5531 Boelter Hall,

University of California, Los Angeles, CA 90095-1592, USA

b Institute of the Environment and Sustainability, 300 LaKretz Hall, University of California, Los Angeles,

Los Angeles, CA 90095-1496

c Sherwood Design Engineers, 2548 Mission St., San Francisco, CA 94110

d Kennedy Communications 9042 Camellia Court

Rancho Cucamonga, CA 91737

Number of Pages: 10

Number of Tables: 3

Number of Figures: 5

*Corresponding author; phone: +1 310 825 8766; e-mail: yoram@ucla.edu (Y. Cohen)

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Table 1. Comparison of Selected Nitrate Treatment Technologies (a)

Ion Exchange/ Adsorption

Low Pressure RO/ Nanofiltration

Electrodialysis (ED/EDR)

Biological Denitrification

Chemical Denitrification

Treatment Type Removal to waste stream Removal to waste stream Removal to waste stream

Biological reduction Chemical Reduction

Pretreatment Pre-filter, address scaling Pre-filter 4, mitigate mineral scaling

Pre-filter, address hardness

pH adjustment, nutrient/ substrate addition; anoxic conditions

pH adjustment

Post Treatment pH adjustment pH adjustment, remineralization

pH adjustment, remineralization

Filtration, disinfection, excess substrate removal

pH adjustment, iron/ammonia control

Residuals High Salinity Brine Concentrate Concentrate Sludge/Biosolids Media/Sludge Start/Stop Fast/Fast Fast/Fast Fast/Fast Slow initially Fast/Fast

Water Recovery 97%-99.9% 75-95% Up to 95% Nearly 100% Limited field experience

Barrier Protection No Yes No No No

Advantages

Selective (e.g. nitrate, arsenic); co-removal of some contaminants; high recovery (~100%); low residual volume; low complexity

Multiple contaminants removal; salinity reduction; recovery well above 90% in some cases; low to moderate complexity

Multiple contaminants removal; salinity reduction; less prone to silica scaling; high recovery

Low residuals volume; co-removal of some contaminants

Low residuals volume, co-removal of some contaminants

Disadvantages

High chemical use (salt); fouling; high salinity brine waste; potential nitrate peaking; potential DBP formation from resin residuals; resin disposal; complex resin regeneration

Moderate energy demand; fouling; concentrate waste disposal.

Moderate energy demand; fouling; concentrate disposal; high operational complexity and maintenance

Substrate/ nutrient addition; complex, sensitivity to env. conditions; potential nitrite formation; significant post-treatment

Inconsistent nitrate removal; potential nitrite and ammonia formation; pH and temp. dependence; possible need for iron removal

(a) Adapted, in part, from information in (Jensen et al., 2012; WADOH, 2005).

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S1. Septic tank system

A septic tank is generally sized such that: (a) its hydraulic retention time is about 1-3 days to

allow for effective sedimentation of solids and biological degradation to take place under the

anoxic conditions in the tank (WADOH, 2005; Otis et al., 2002; Tchobanoglous et al., 2003), and

(b) it has the capacity to accommodate the volume of generated wastewater and minimize the

necessary cleaning frequency (WADOH, 2005; Otis et al., 2002). It is important to note that in

typical domestic wastewater (Table S2), which enters the septic tank, nitrogen is present primarily

in the form of ammonia (NH3), ammonium ion (NH4+) and organic nitrogen. Once the septic tank

effluent enters the soil leach field, ammonia nitrogen undergoes biological nitrification whereby it

is converted sequentially to nitrite (NO2-) and then nitrate (NO3

-) (WADOH, 2005; Otis et al., 2002;

Tchobanoglous et al., 2003), and (b) it has the capacity to accommodate the volume of generated

wastewater and minimize the necessary cleaning frequency (WADOH, 2005; Otis et al., 2002). It

is important to note that in typical domestic wastewater (Table S2), which enters the septic tank,

nitrogen is present primarily in the form of ammonia (NH3), ammonium ion (NH4+) and organic

nitrogen. Once the septic tank effluent enters the soil leach field, ammonia nitrogen undergoes

biological nitrification whereby it is converted sequentially to nitrite (NO2-) and then nitrate (NO3

-)

(WADOH, 2005; Beal et al., 2005).

Table S2. Summary of typical domestic wastewater characteristics(a) Unit Concentration

Biological oxygen demand (BOD) mg/L 155 - 286

Total suspended solids (TSS) mg/L 155 - 330 Total Nitrogen mg/L as N 26 - 75

Ammonia mg/L as N 4 - 13 Nitrite + Nitrate mg/L as N < 1

Total Phosphorus mg/L as P 6 - 12 (a) Adapted from (WADOH, 2005; Tchobanoglous et al., 2003).

S4

Studies on various approaches for nitrogen removal from septic tank effluent have shown that,

following nitrification of ammonia nitrogen (under aerobic conditions), denitrification of high

nitrate effluent (18.7 to 145 mg/L as N) can be attained in an anoxic bioreactor (Mbuligwe, 2005)

or by recirculating the nitrified stream to the septic tank itself (WADOH, 2005; Costa et al., 2002).

It has been reported that under the anoxic conditions that exist in the septic tank biologically-

mediated denitrification of nitrate can occur (Mbuligwe, 2005). In the septic tank itself, the nitrate

level is generally very low (i.e., < 1 mg/L as N); thus, it is not surprising that data on nitrate

denitrification in septic tanks is scarce. A few studies, however, reported that up to ~50% nitrate

denitrification can take place in conventional anaerobic septic tanks that receive domestic

wastewater (Mbuligwe, 2005; Costa et al., 2002; Andreoli et al., 1979). Based on the available

literature, it is reasonable to expect that that nitrate which enters a septic tank, would be denitrified

to some extent (~27-50%, (Mbuligwe, 2005; Costa et al., 2002; Andreoli et al., 1979)). Thus, the

nitrate containing residual stream (concentrate) from RO treatment of small communities impaired

source water could be discharged to the community septic tank, provided that has sufficient volume

capacity.

S2. Nitrogen compounds in a septic tank system

The fate and transport of nitrogen from domestic wastewater effluent, which is discharged to

septic systems and subsequently to a leach field, involves both nitrification of ammonia nitrogen

under aerobic conditions and denitrification under anaerobic conditions as governed by the various

processes summarized in Table S3. In the septic tank’s anaerobic environment, microorganisms

break down organic nitrogen compounds to form ammonia (i.e., ammonification) which is the

primary nitrogen form in the septic tank effluent (Beal et al., 2005; Canter and Know, 1985;

McCray et al., 2005; Wilhelm et al., 1994). When the septic tank effluent enters the soil leach field,

S5

ammonia nitrogen undergoes biological nitrification such that it is sequentially converted to nitrite

(NO2-) and then to nitrate (NO3

-) in the undersaturated, aerobic zone (i.e., vadose zone of the

aerated soil layer) (WADOH, 2005; Beal et al., 2005). A number of studies have argued that

microbial denitrification (accounting up to ~20% of nitrogen removal in the leach field) can also

occur under anaerobic conditions that may exist in vadose zone microsites zone (Siegrist and Boyle,

1987), and also to some extent in the deeper soil under carbon limited conditions (Wilhelm et al.,

1994).

Figure S1. The fate of nitrogen in a septic system interfaced with an effluent media (sand) filter with recycle.

Reducing the overall nitrogen discharge to the leach field can be generally achieved by

nitrifying (under aerobic conditions) the nitrogen in the effluent form the septic tank, followed by

denitrification under anoxic conditions. For example, in the approach illustrated in Figure S1

nitrogen removal is achieved by first converting nitrogen that is in the ammonia form to nitrate

under aerobic conditions in a media (e.g., sand) filter. The nitrate-laden filtrate is then recycled

back to the carbon-bearing septic tank where denitrification occurs by heterotrophic bacteria

(under anoxic condition) where process effectiveness is governed by the recycle ratio (Sack et al.,

1988). The range of nitrogen removal achieved by the above approach was reported to be in the

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range of ~ 40-90% (Otis et al., 2002; Costa et al., 2002; Sack et al., 1988). Denitrification of

domestic graywater under anoxic conditions was also reported, demonstrating denitrification of

29-54% under anoxic conditions (Brooks, 1996; Gold et al., 1989). It has also been shown that

bioreactors, under anaerobic conditions with supplemental carbon source (e.g., acetic acid, waste

activated sludge, woodchips), can treat domestic wastewater with up to 65 to 91% denitrification

over a 6 to 24 hours period (Guo et al., 2017; Lepine et al., 2016; Sakuma, 2005). The rate and

efficiency of denitrification in the above approaches depend on site-specific conditions, including

the types and concentrations of biodegradable carbon source, pH, dissolved oxygen (DO) and

temperature (Tchobanoglous et al., 2003; Gold et al., 1992; Lamb et al., 1991; Lamb et al., 1990;

Urynowicz et al., 2007).

Table S3. Fate and attenuation mechanism of nitrogen in a septic tank system consisting of a septic tank and soil leach field. Process/description Reference

Septic tank (single or multi- chambered system)

Ammonification consisting of microbial conversion of organic nitrogen into ammonia

(McCray et al., 2005)

Sedimentation of organic particles containing nitrogen (WADOH, 2005)

Nitrogen assimilation involving the biological conversion of ammonia products back to organic nitrogen.

(WADOH, 2005)

Biological denitrification of ammonia nitrogen in the anoxic condition in the septic tank

(Mbuligwe, 2005)

Soil leach field (within and below subsurface soil absorption layer)

Nitrification of ammonia to form nitrate under aerobic condition in the leach field

(Otis et al., 2002)

Ammonia adsorption whereby NH4+ adsorbed onto soils

of high cation exchange capacity (e.g., montmorillonitic soils). (Harrison et al., 2000)

Ammonia volatilization can occur in alkaline soils, typically at pH >8

(Katz et al., 2010)

Biological denitrification can occur in the unsaturated zone if there are anaerobic microsites

(Wilhelm et al., 1994)

Chemical denitrification can occur in soils that contain electron donor (e.g., pyrite) where nitrate is the electron acceptor

(Postma et al., 1991)

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Figure S2. Water use patterns in study communities A-C (Table 1).

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Figure S3 RO feed pump outlet pressure needed to achieve the required RO module feed pressure with a FLERO RO test unit (Lee et al., 2019) desalting feed of salinity in the range of 1,000-39,000 mg/L TDS over a recovery range of 9%-90%. The data are compilation from both the current laboratory tests and from the data provided in (Lee et al., 2019). The solid line represent a best linear equation fit to the data ( ( ) ( )0.12 2.45FL FHbar P rP ba= + PFL(bar); R2 = 0.986) and the vertical dashed line represents the manufacturer upper operational limit (41 bar) for the RO elements used in the present study (CSM, 2010).

Figure S4. Variation of permeate nitrate concentration (CP), normalized with respect to the raw feed nitrate concentration (C0), in laboratory testing using a high nitrate rejection membrane (M-2) and field tests using membrane M-1 over a range of product water recovery. The solid curve represents the fit of Eq. 1 (Section 2.2.1) to the data. Note that site D (Table 3) is not included in the above dataset since the source water nitrate concentration was below the MCL.

S9

Figure S5. Nitrate concentration in the RO product water (permeate) for treatment of feed water of different nitrate contamination levels. Note: nitrate removal is not required when the feed nitrate level is below the MCL. Simulation were based on the specifications in Table 4.

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