Novel technologies for reverse osmosis concentrate treatment: A review

Novel technologies for reverse osmosis concentrate treatment: A review

Journal of Environmental Management 150 (2015) 322e335 Contents lists available at ScienceDirect Journal of Environmental Management journal homepag...

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Journal of Environmental Management 150 (2015) 322e335

Contents lists available at ScienceDirect

Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman

Review

Novel technologies for reverse osmosis concentrate treatment: A review Sung Hee Joo a, *, Berrin Tansel b a

Department of Civil, Architectural, and Environmental Engineering, University of Miami, 1251 Memorial Dr. McArthur Engineering Building, Coral Gables, FL 33146-0630, USA b Department of Civil and Environmental Engineering, Florida International University, 10555 W. Flagler St., Miami, FL 33174, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2014 Received in revised form 16 October 2014 Accepted 28 October 2014 Available online

Global water shortages due to droughts and population growth have created increasing interest in water reuse and recycling and, concomitantly, development of effective water treatment processes. Pressured membrane processes, in particular reverse osmosis, have been adopted in water treatment industries and utilities despite the relatively high operational cost and energy consumption. However, emerging contaminants are present in reverse osmosis concentrate in higher concentrations than in the feed water, and have created challenges for treatment of the concentrate. Further, standards and guidelines for assessment and treatment of newly identified contaminants are currently lacking. Research is needed regarding the treatment and disposal of emerging contaminants of concern in reverse osmosis concentrate, in order to develop cost-effective methods for minimizing potential impacts on public health and the environment. This paper reviews treatment options for concentrate from membrane processes. Barriers to emerging treatment options are discussed and novel treatment processes are evaluated based on a literature review. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Emerging contaminants Reverse osmosis concentrate Concentrate management Advanced oxidation Pressure driven membranes Integrated water treatment

1. Introduction Membrane processes including reverse osmosis (RO) have been widely adopted for water treatment and reuse. The global market for RO continues to grow and is predicted to reach $8.1 billion by 2018 (Cumming, 2014) as RO is increasingly deployed in water treatment plants, quickly replacing conventional softening processes that use lime and soda ash. In particular, RO uses pressured membranes for treatment and desalination of brackish water, producing high-quality water. However, as with other membrane processes (i.e., nanofiltration, ultrafiltration, microfiltration), the challenge of RO is management of the concentrate generated from the filtration processes. Untreated or improperly managed concentrate can result in adverse environmental effects, due to high salinity, nutrients (phosphorus, nitrogen), organic contaminants including emerging contaminants, and trace amounts of inorganics. Cost-effective treatment and management strategies for concentrate are still in their infancy. While review articles have

focused on traditional RO treatment methods or recent advances in rez-Gonza lez et al., 2012; Malaeb and Ayoub, RO technology (Pe 2011; Fujioka et al., 2012), there are few studies addressing the characteristics of contaminants or treatment options for minimizing or removing contaminants of concern. Yet contaminants in concentrate can impact ecosystems and water quality in areas where the concentrate is discharged. Given that the characteristics of emerging organic contaminants significantly affect the efficiency of water treatment methods, and given the potent toxicity and persistence of such contaminants, innovative and cost-effective treatment technologies are needed. This review aims to provide (i) an overview of emerging contaminants, (ii) strategies for minimizing and treating concentrate from RO processes with the use of integrated water treatment processes, (iii) description of treatment technologies specifically targeting emerging contaminants of concern in concentrate, (iv) proposals for treatment options based on literature reviewed, and (v) discussion of future research needs. 2. Emerging contaminants in concentrate

* Corresponding author. Tel.: þ1 305 284 3489; fax: þ1 305 284 3492. E-mail address: [email protected] (S.H. Joo). http://dx.doi.org/10.1016/j.jenvman.2014.10.027 0301-4797/© 2014 Elsevier Ltd. All rights reserved.

The most significant drawbacks of using pressurized membrane systems for water treatment are membrane fouling and concentrate management. While there has been intensive research on

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membrane fouling, little information is available concerning the treatment of concentrates, a long-standing problem. Methods of treatment and volume reduction of concentrate must be based on the characteristics of the concentrate as well as the operational and design characteristics of membrane systems. The composition of raw water, operating parameters of the membrane process, and overall system elements (i.e., pre-treatment, cleaning chemicals used) can influence both quality and quantity of the concentrate generated. Development of effective treatment methods for the concentrate entails evaluating significant parameters, such as volume generated, concentration, characteristics of the feed water, and operational conditions, and using well-verified analytical methods to detect trace amounts of contaminants, including emerging contaminants. Emerging contaminants can be classified as persistent organic contaminants (e.g., pesticides), endocrine-disrupting chemicals (EDCs) (e.g., estrogens), pharmaceuticals and personal care products (e.g., drugs, sunscreens, cosmetics), and nanomaterials (e.g., nano-scale titania). Contaminants that are listed as “emerging” include industrials, pharmaceuticals, detergents, personal care products, disinfectants, and life-style compounds (i.e., caffeine, nicotine) (Meffe and De Bustamante, 2014). Recently, the U.S. EPA has described the newly identified emerging contaminants perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA). RO with other treatment processes (i.e., activated carbon) has been effective in reducing contaminant levels (e.g., perfluorochemicals, PFCs) to the drinking water quality standard (below 0.2 mg/L) (U.S. EPA, 2012). However, contaminants in the concentrate produced from RO have been difficult to destroy to undetectable trace levels, and recent studies show that wastes containing emerging contaminants such as PFOS and PFOA from RO concentrate still require incineration (Hartten, 2014; MDH, 2014; Vectis et al., 2009). Emerging contaminants detected in the environment typically originate from hospitals, pharmaceutical manufacturing plants, agricultural practices, and wastewater treatment facilities (Daughton and Ternes, 1999; Halling-Sorensen et al.,1998; Kasprzyk pezHordern et al., 2008; Kummerer, 2009; Lapworth et al., 2012; Lo Serna et al., 2013; Postigo et al., 2010). Reuse of municipal wastewater has been a particular concern due to N-nitrosodimethylamine (NDMA), a potently carcinogenic water disinfection byproduct formed in high concentrations (Mitch et al., 2003). 2.1. Characteristics of emerging contaminants 2.1.1. Environmental persistence Residue of emerging contaminants including bisphenol A (BPA), PPCPs, and PFCs, can be found in most environmental media (Murray et al., 2010; Arukwe et al., 2012). The most persistent emerging organic contaminants detected in sediments and sludge are reported to be, in order of persistence, polycyclic aromatic hydrocarbons (PAHs) > polychlorinated naphthalenes > polychlorinated dibenzo-p-dioxin/polychlorinated dibenzofuran (PCDDs/Fs) > polychlorinated biphenyl (PCBs) > polybrominated diphenylether (PBDEs) (Eljarrat and , 2003). Compounds with high sorption affinity, Barcelo log Kow > 4 (Pan et al., 2009), remain persistent in environmental media, and are consequently difficult to remove. For instance, carbamazepine has been identified as one of the most persistent € ffler et al., 2005; Williams et al., environmental contaminants (Lo ndez et al., 2014). Among emerging organic 2009; Martínez-Herna contaminants, most pesticides are persistent and often comprise the highest concentration detected in water, followed by industrial contaminants and pharmaceuticals (Meffe and De Bustamante, 2014). Hydrophilic and highly water-soluble chemicals may persist in both surface and ground water.

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2.1.2. Human and ecosystem effects Persistent emerging contaminants, detected in municipal wastewater, surface water, and drinking water, are usually toxic to humans and do impact ecosystems. Some emerging organic contaminants are hydrophobic, bioaccumulative, semi-volatile and toxic, including pesticides, polychlorinated biphenyls, and poly, 2003). Potential chlorinated naphthalenes (Eljarrat and Barcelo risks for humans and ecosystems are defined using acceptable daily intake (ADI) and the reference dose (RfD) (Dorne et al., 2007). Contaminants in the industrials category include bisphenols, alkylphenols, perfluorates, antioxidants, phthalates, PBDEs, and examples of PPCPs comprised of synthetic hormones and polycyclic musks (Murray et al., 2010). Among the emerging organic contaminants, the most frequently detected environmentally, and posing risks to human health, include industrials (PFOA, PFOS), pesticides (diazinon), and PPCPs (E1) (Schriks et al., 2010). Among the emerging contaminants, PBDEs have a potential to adversely affect human endocrine systems and bioaccumulate (Hooper and McDonald, 2000; Meerts et al., 2001; Rahman et al., 2001), while relatively limited information is available on the toxicity of PFOS and PFOA (Hekster et al., 2003). Lack of analytical methods for detecting trace amounts of such contaminants in the environment is a major limitation (Stewart et al., 2014). Among pharmaceuticals, amoxicillin is one of the top priorities for treatment. Amoxicillin in medical wastewater not only exhibits significant ecotoxicity (Escher et al., 2011), but is also detected in high concentrations, and is linked to the formation of antibioticresistant species. As previously noted, another commonly used emerging organic contaminant is carbamazepine. When residue contaminated with carbamazepine is metabolized by organisms, their growth and characteristics are adversely affected due to oxidative stress on critical organs (Nassef et al., 2010). Similarly, carbamazepine likely presents high risks to humans due to its impact on human embryonic cells (Murray et al., 2010; Pomati et al., 2006). Table 1 summarizes relevant properties of emerging contaminants of concern in concentrate by use category (industrials, pharmaceuticals, pesticides, and disinfection byproducts). According to hazard indexes, some endocrine-disrupting chemicals such as sulfamethoxazole and caffeine have been reported to constitute the majority of the total hazard quotient (Yan et al., 2014). Due to lack of data, it has been difficult to evaluate or predict adverse effects of emerging contaminants at concentrations detected in the environment. Table 1 lists a group of emerging contaminants defined as persistent based on toxicity and octanolewater coefficient (log Kow), which together provide basic data for risk assessment and development of treatment strategies. For quantifying the risks to humans and ecosystems, some of the ADI values (mg kg1 day1) are 1.5  103 (PFOA), 1.5  104 (PFOS), 5.0  102 (BPA), 1.3  105 (estrone, E1) and 9.0  105 (diazinon) (Fromme et al., 2009; Kolpin et al., 2002, 2004; Loos et al., 2009; Schriks et al., 2010; Snyder et al., 2008; Teuschler et al., 1999; U.S. EPA, 2010).

2.2. Factors affecting concentrations of emerging contaminants in concentrate Environmental conditions (i.e., temperature, pH, and ionic strength) can affect the levels of emerging contaminants present in concentrates. For instance, among N-nitrosamines (disinfection byproduct), feed solution temperature significantly influenced the rejection of NDMA, and a 10-fold increase of ionic strength resulted

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Table 1 Physicochemical properties of emerging contaminants of concern in concentrate. log Kow

MW (g mol1)

C8HF17O3S

Not measurabled

500

PFOA

C8HF15O2

Not measurabled

414

PBDEsn

C12H5Br5O

4.89e6.8e

480e959

PCBs

C12H10xClx

4.53 (PCB 1)f

395 (PCB180)

Anthracene (PAHs)

C14H10

4.68g

178

Bisphenol A

C15H16O2

3.18b

228

Pharmaceuticals Diclofenac

C14H11Cl2NO2

4.5e4.8h

296

Carbamazepine

C15H12N2O

1.51a

236

Amoxicillin

C16H19N3O5S

0.87i

365

Estrone (E1)

C18H22O2

2.95c

270

Pesticides Diazinon

C12H21N2O3PS

3.3e3.8j

304

Lindane

C6H6Cl6

3.7e4.1k

291

Dieldrin

C12H8Cl6O

3.7e6.2l

381

Disinfection byproducts NDMA

C2H6N2O

0.57m

74

Compound

Formula

Industrials PFOS

a b c d e f g h i j k l m n

Kow reference: Scheytt et al. (2005). Kow reference: Shao et al. (2007). Kow reference: De Mes et al. (2005). Kow reference: U.S. EPA (2012). Kow reference: Scheytt et al. (2005). Kow reference: WHO (2003). Kow reference: ECHA (2008). Kow reference: Virkutyte et al. (2010). Kow reference: Baghapour et al. (2014). Kow reference: Hornsby et al. (1996). Kow reference: Hornsby (1996). Kow reference: UNEP (2007). Kow reference: Health Canada (2011). pentabromodiphenyl ether.

Structure

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in noticeable decrease in the removal of NDMA (Fujioka et al., 2012). Concentrate quality also depends on membrane pore size. For example, suspended solids and colloidal particles are present in MF and UF concentrates, while small organics and high ion concentrations are found in NF and RO concentrates (Bruggen et al., 2003). Concentrate quality is further influenced by variations in the types and quantity of chemicals used during periodic membrane cleaning and/or scaling/biofouling prevention procedures, or in chemicals used for pH €isa €nen et al., 2002; Wallberg et al., 2001; Liikanen et al., control (Va 2002). 2.3. Rejection of emerging contaminants in concentrate by membrane processes The quantity of concentrates from membrane systems (i.e., MF, UF, NF, RO) can be significant. In relation to feed quantity, concentrate quantities are about 1e10% for MF and UF, 15e30% for NF, and 15e60% for RO (Bruggen et al., 2003). Recent research indicates that RO retentate contains the largest percentage of emerging contaminants, in particular pharmaceuticals (Dolar et al., 2012). In MR and UF systems, cleaning water and concentrate could be recycled to feed water, but concentrates from NF and RO are not suitable for reuse or are difficult to treat because of the high levels of organic contaminants and salinity. Because emerging contaminants are recalcitrant to conventional treatment, advanced treatments including membrane technologies are commonly employed. The rejection of emerging contaminants by several pressured membranes has been studied according to the types of membrane materials, chemical properties of contaminants of concern, or removal mechanisms of membranes. For instance, while antibiotics were completely removed by RO (type: XLE) and NF (tight NF-90) membranes, incomplete rejection of up to 65% of smaller molecules such as sulphonamide was reported with the loose type of NF (i.e. HL) (Dolar et al., 2009), and other smaller antibiotic molecules were also less effectively rejected by the loose NF membrane (Kosuti c et al., 2007). Different types of membranes appear to produce different rejection rates for emerging contaminants. Among several spiral membrane elements (i.e. TFC-HR, NF-90, NF-200, TFC-SR2, and XLE), ionic pharmaceutical residues and pesticides were rejected more than 95% with NF-90, XLE, and TFC-HR, and above 89% using NF-200 (Xu et al., 2005). The study also indicated that while partial rejection of hydrophobic nonionic contaminants was observed with the conventional TFC-HR type of RO membrane, tight-type NF and ultra-low-pressure RO membranes enhanced the rejection of such contaminants. It was reported that compared to cellulose acetate, polyamide membrane better rejects selected emerging contaminants, although it showed incomplete rejection, ranging from 57 to 91% (Kimura et al., 2004). The study also indicated that salt rejection or molecular weight cut-off (MWCO) did not correlate with the extent of rejection of such contaminants by NF/RO membranes, implying that membrane polymer materials would reveal a different retention rate, mainly due to different physicalechemical properties of the contaminants. Other factors including properties of emerging contaminants also affect rejection even by the same types of membrane. Low molecular weight organic contaminants including disinfection byproducts had low rejection rates, ranging from 52 to 97% for RO and tight NF membranes, and from 25 to 51% for the loose NF membrane (Kosuti c et al., 2007; Xu et al., 2005). The extent of rejections was correlated with solute molecular size (Kosuti c et al., 2007). While the presence of fouling and effluent organic matter enhances the rejection of ionic organics by tight NF and RO membranes (Xu et al., 2005), the presence of higher concentrations of

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divalent ions contributed to lower rejection of endocrine disrupting compounds (DEC) and pharmaceuticals, and personal care products (PPCPs) (Comerton et al., 2008). The UF membrane typically retains hydrophobic EDC/PPCPs, primarily through an adsorption mechanism, whereas a correlation between hydrophobicity and porosity of membranes appear to be critical in rejection of EDC/PPCP in the NF membrane (Yoon et al., 2007). Thus size exclusion is likely to be the dominant mechanism in NF, with pore size having a greater effect on removal compared to UF. Comparing between ‘loose’ and ‘tight’ NF membranes, more hydrophobic compounds had a higher rejection rate in the ‘tight’ NF membranes (Comerton et al., 2008). 3. Integrated treatment technologies for concentrate Optimizing membrane processes is one of the strategies for improving the quality and minimizing the quantity of concentrate. Integrated water treatment systems could be implemented in plants for effectively managing concentrate along with strategies for minimizing the quantity of concentrate. Such combined systems could involve hybrid processes and advanced oxidation processes (AOPs) such as MBR/RO, O3/UF(/MF)/RO, MF/RO/AOP, pretreatment/ UF(/MF)/RO, and pretreatment(/AOP)/RO. In this section, the most effective integrated treatment processes are reviewed. 3.1. MBReRO The integrated membrane bioreactor (MBR) and RO systems are becoming more widely used for removal of emerging contaminants from municipal wastewater (Dolar et al., 2012; Malamis et al., 2012; Joss et al., 2011; Tam et al., 2007; Qi et al., 2011) (Table 2, processes 1e5). Dolar et al. (2012) showed that pharmaceuticals, which can be recalcitrant to degradation by conventional wastewater treatment, are removed up to 95% in the MBR (e.g., metronidazole, hydrocodone, codeine, ranitidine), and further treatment by RO resulted in a removal rate of higher than 99% through size exclusion, steric hindrance, electrostatic interaction, and hydrophobic interaction between the contaminants and the membrane (Table 2, process 1) (Dolar et al., 2012; Bellona et al., 2004). While the system produces high quality water (permeate), concerns arise regarding emerging contaminants that remain present in concentrate (normally of high salinity and containing inorganic contaminants), and these concerns need to be addressed, along with the energy consumption rate for any viable technology. Findings of a recent study of the treatment of concentrate assessed in an MBReRO system indicate that in a fixed bed column packed with zeolite and connected to the system there was partial removal (17.5e28.4%) of organic matter, while effective removal of heavy metals (e.g., Cu, Pb, Zn) was observed (Joss et al., 2011). As part of efforts to control potential membrane fouling and minimize concentrate, disinfectants and UV are applied prior to RO membrane treatment, and concentrate is returned as influent to the RO (Table 2) (Comerton et al., 2005). However, disinfection byproducts (e.g., trihalomethanes (THM), haloacetic acids) formed during the process and subsequent RO treatment did not meet the standards for water reuse (Table 2, process 2). In the RO treatment system, the MS2 coliphage was completely removed physically, regardless of pre-RO disinfectant or membrane type (Comerton et al., 2005). Even after RO treatment, total trihalomethanes (TTHMs) remained at levels 40.2 ± 19.9 mg/L regardless of the preRO treatment (either by monochloramine or UV) or RO membrane. In this system, retentate is fed back as RO influent and controlled with disinfection. It is suggested that instead of applying disinfection, retentate can be returned as influent to RO in the system, applying post-advanced oxidation processes (AOP) after MBReRO

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Table 2 Comparison of the integrated treatment systems for reducing concentrate quantity. No

Description: benefits and limitations

Ref.

1

Process schematic

Integrated MBR þ RO process: Excellent removal of pharmaceuticals (>99%), EOCs are not addressed.

(Dolar et al., 2012)

2

MBR coupled with chlorine RO: THMs formed due to incomplete removal, concentrate returned as influent to the RO, enhanced recovery rate.

(Comerton et al., 2005)

3

Conditioning MBR effluent by lowering the pH in the RO feed water with CO2, and recycling concentrate back to the biological unit, and ozonation of concentrate: Significant reduction of concentrate quantity, adequate treatment with/without ozonation.

(Joss et al., 2011)

4

Countercurrent two-stage PAC adsorption and MF: concentrate from UF, PAC þ MF, secondary concentrate disposal with a high overall recovery (91%) of RO, efficient removal of dissolved organics in petrochemical concentrate.

(Zhao et al., 2013)

5

Combined MBReRO-fixed bed column system: concentrate treated by a fixed bed column packed with zeolite, partial removal of organic matter in concentrate, complete removal of Cu, Fe, Ni, Cr, and Pb (from RO); >90% metal removal (from MBReRO).

(Malamis et al., 2012)

6

Simultaneous MF, adsorption, and ultrasonic bath system: >99% removal of ECs (selected pharmaceuticals), lower frequency ultrasound irradiation provided better performance.

(Secondes et al., 2014)

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327

Table 2 (continued ) No

Process schematic

Description: benefits and limitations

Ref.

7

Combination of ozoneebiological aerated filter (BAF) with an upflow BAF as membrane pretreatment after RO with UF as pretreatment: Integrated unit showed higher removal rates of COD and color than the separate units, concentrate met discharge standards for treatment of textile wastewater.

(Qi et al., 2011)

8

Integrated ozoneeUFeRO process: Pre-ozonation of digital textile wastewater treatment, followed by UF and RO achieved 63% COD and 81% color removal; concentrates from UF and RO are recycled and treated in ozone contactor.

(Chang et al., 2009)

9

Combined ozoneebiological activated carbon (BAC)ecapacitive deionization (CDI) system: In the ozoneeBACeCDI process, a lower feed TOC after ozoneeBAC pretreatment prior to CDI resulted in higher water quality than RO, high potential for RO brine treatment and recovery; over 80% anion and cation removal efficiencies from the CDI process.

(Lee et al., 2009)

10

Integrated ROesoil filter system: MFeRO-fixed bed column filled with sand/iron-coated activated carbon, complete removal of DOC within 1 h.

(Joo, 2014a)

11

Integrated system with MFeRO-DynaSand filled with waste iron: Removal rate: 40% COD, 80% total phosphorus, and 20% total nitrogen removal.

(Joo, 2014a)

12

Treatment of RO concentrate with sequential adsorption using PAC and iron-activated persulfate oxidation.

(Joo, 2014b)

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to remove the remaining disinfection byproducts from the RO permeate as well as retentate. The MBR process produces high nitrate concentrations, and although RO treatment achieves high removal rates (over 90%), it was reported that RO permeate still contains nitrates as high as 3.6 mg N/L, not affected by pre-RO disinfection, regardless of membrane type (Comerton et al., 2005). More studies are needed on the efficiency of removing emerging contaminants in concentrate from the MBReRO systems, as well as on methods to minimize the issues related to operational conditions and chemicals used during membrane filtration. In an MBReRO system, scaling issues can be controlled by addition of CO2 while lowering pH in the feed to RO and recycling concentrate back to the biological unit (Table 2, process 3). This system demonstrated that with concentrate returned to the MBR, the quantity of concentrate is significantly reduced (Joss et al., 2011). Using a combined MBReRO treatment scheme with ozonation of concentrate, no nitrosamines were detected except NDMA, producing quality water suitable for most reuse purposes. 3.2. O3eUF/MFeRO Ozonation as pre-treatment is one of the treatment options for integrated systems such as UF/MFeRO for water reuse. For instance, in textile industries applying ozone as pretreatment, followed by a biological aerated filter (BAF) with RO, proved to be highly effective in removing chemical oxygen demand (COD) and color, satisfying the discharge standard for concentrate (Table 2, process 7) (Qi et al., 2011). A hybrid system (i.e., ozoneeUFeRO) developed by Chang et al. (2009) effectively treated digital textile printing wastewater (Table 2, process 8). The process, consisting of ozonation, UF, and RO, removed 63% of COD and 81% of color; without RO there was partial removal of color and organics (Table 2, process 8). Concentrates generated from UF and RO were returned to the ozone contactor. However, ozonation is most likely to form potentially carcinogenic byproducts (i.e., bromate) that pose a health risk to humans (von Gunten, 2003a,b). In addition, ozonation generated increasing particle size through aggregation of colloidal and small particles, which decreased cake resistance (Rc) as shown by the following equation, known as the Carman-Kozeny equation (Secondes et al., 2014):

Rc ¼

180ð1  3 ÞM rp $d2p $3 3 Am

(1)

where M ¼ mass of cake (kg), Am ¼ membrane area (m2), 3 3 ¼ porosity of cake, rp ¼ density of particles (kg/m ), dp ¼ particle diameter (m). In this system the final wastewater effluent treated through ozoneeUFeRO is said to have met the standard for direct discharge or reuse. However, the system may pose concerns, especially in treating recalcitrant organics, because increasing ozone dosages produces only limited degradation effects, and high ozone concentrations can even kill useful microbes in integrated biological processes. As the pretreatment, ozone can be used with other advanced oxidation processes in order to further enhance removal efficiency. Fig. 1 illustrates the efficiency of organic removal based on either a single oxidation process or combined advanced oxidation processes. As shown in Fig. 1a, the addition of ozone to TiO2 and UVA significantly enhanced the degradation of organics, compared to combined UVA and TiO2 (removal rate: 14.5%) (Fig. 1a). Interestingly, when ozone alone was used, treatment efficiency was low (21.7% removal) compared with other combined oxidation treatment processes (removal rates: US/H2O2/O3 (31.4%), UVA/H2O2/O3

(25.6%), US/O3 (27.8%), UVA/O3 (28.4%)). These results suggest that organics present in the concentrate are likely to be resistant to degradation by O3.

3.3. Pretreatment/AOPeMF/RO A simultaneous treatment hybrid system, consisting of activated carbon adsorption and ultrasound (US) irradiation in a UF system, has been applied for treating emerging contaminants, especially selected pharmaceuticals (diclofenac, carbamazepine, amoxicillin) (Table 2, process 6) (Secondes et al., 2014). The system removed over 99% of the emerging contaminants primarily through adsorption, depending upon the hydrophobicity of the compounds. The efficacy of this system suggests that, compared to individual processes, combined systems offer improved contaminant treatment efficiency. Further, examination of the effects of various ultrasonic irradiation frequencies suggested that low frequency provided vigorous cavitation bubble collapse, leading to better performance in removing contaminants (Mason and Peters, 2002). This system offers synergistic effects through accelerated adsorption of emerging contaminants on powdered activated carbon (PAC) supported by ultrasound. Pretreatment of organics by advanced treatment such as combined adsorption and oxidation prior to RO could be effective for significantly reducing emerging contaminants in concentrate. A prospective biological activated carbon process coupled with UVC/H2O2 has been reported to be effective in removing organic matter in highly saline concentrate from municipal wastewater reclamation. The combined system reduced 60% of dissolved organic carbon (DOC) and 50% of COD, showing a synergistic effect with no toxicity detected (Lu et al., 2013). The UVC/H2O2 system served a complementary function, overcoming limits in the biological process for treating bio-persistent organics and high salinity (Bagastyo et al., 2011). However, the combined system was not effective for nutrient removal, removing 24% total nitrogen and 17% total phosphorus, and without biological activated carbon, removal effectiveness was only 12% total nitrogen and 7% total phosphorus. Biological activated carbon treatment significantly reduced all regions shown in fluorescence excitationeemission matrix spectra (i.e., aromatic proteins: regions I and II; fulvic acid-like substances: region III; soluble microbial products such as protein and polysaccharides: region IV and V) (Chen et al., 2003), and treatment by UVC/H2O2 reduced humic compounds (Lu et al., 2013). After the combined UVC/H2O2/BAC treatment, most organics were reduced, with minimal remaining fluorescence (Lu et al., 2013). This result indicates that pre-oxidation prior to biological activated carbon facilitates the degradation of organics. However, the reduction of nutrients was low, and the combined treatment system, including individual processes, was not optimized, based on the composition of concentrate. Instead of biological activated carbon, coagulation coupled with UVC/H2O2 has also been reported to be effective in removing two high-salinity concentrate samples of comparable organic and inorganic content (Umar et al., 2014). As a coagulant, alum (1.5 mM Al3þ) was applied as pretreatment to UVC/H2O2, and the coupled process led to an improvement in biodegradability. The pretreatment resulted in an additional 10e12% reduction of DOC, and further biological treatment provided overall DOC decreases of 55e62% as well as reduction in energy consumption after pretreatment and biological post-treatment (Umar et al., 2014). To improve recovery rate and reduce discharge into the environment, there are other pretreatment options available for adsorption or oxidation or combined treatment, prior to further

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membrane filtration and/or recycling of the treatment effluent to RO feed water. As an example, ozone (ranging from 3 to 10 mg O3/L) coupled with biological activated carbon as a pre-treatment process was effective when used prior to the capacitive deionization process, generating a final effluent with more than 80% removal of ions and a lower total organic carbon (TOC) concentration (removal rate of 69.8 ± 8.1%), which has a higher water quality than RO permeate (Table 2, process 9) (Lee et al., 2009). However, concentrate generated from the capacitive deionization process still required treatment, and could be returned to either an ozone contact chamber or a biological activated carbon column. Without ozonation, TOC removal efficiency was three times lower. This system required several steps prior to capacitive deionization and longer ozone contact time for complete decolorization (e.g., 20 min), which could be costly and has not been evaluated in terms of viability as a cost-effective treatment. As an alternative to ozone, other advanced oxidation processes (e.g., photocatalytic oxidation, sonolysis, H2O2 oxidation) were

329

compared with and without coagulation for removal of concentrate (Zhou et al., 2011a). While AOP may assist in degrading contaminants, it may not be enough to completely mineralize contaminants, because organics present in concentrate are highly biorefractory, consisting of both natural organic matter and most categories of emerging organic contaminants (Ternes and Joss, 2006; Radjenovi c et al., 2008; Watkinson et al., 2007). Thus, coupling AOPs with other treatment methods could be an option for treatment of concentrate. As can be seen in Fig. 1, without coagulation AOPs (even coupled with oxidation treatment) were not sufficiently effective in treating concentrate, though UVC/TiO2/ O3 showed the highest removal rate of 52.2% through continuous in-situ generation of $OH. However, the removal rate is enhanced when coagulation is combined with the AOPs. The removal mechanism of combined AOPs along with removal rate with and without coagulation pretreatment is illustrated in Table 3. As shown in Table 3, the combined AOPs produce more hydroxyl radicals, contributing to the enhanced removal rate. In all cases, without O3 the removal rate of DOC is relatively low (<20%) even

Fig. 1. Comparison of efficiencies of AOPs for treating concentrate without (A) and with (B) coagulation pretreatment (Zhou et al., 2011a).

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Table 3 AOP treatment mechanisms and removal rates with and without coagulation (Zhou et al., 2011a). Hybrid AOPs Mechanisma

Removal rate (%) With coagulation Without pretreatment coagulation pretreatment

UV/H2O2 US/H2O2 UV/O3 UV/H2O2/O3

H2O2 þ hv / 2$OH 2.3 ± H2O2 þ hs / 2$OH 2.8 ± O3 þ H2O þ hv / 2$OH þ O2 28.4 ± O3 þ H2O2 þ H2O þ hv / 25.6 ± 4$OH þ O2

US/O3

O3 þ hs / O2 þ O O þ H2O / 2$OH

27.8 ± 4.8

17.4 ± 4.4

US/H2O2/O3

O3 þ H2O2 þ H2O þ hs / 4$OH þ O2

31.4 ± 5.9

29.2 ± 7.4

UV/TiO2/O3

TiO2 þ hv / e þ hþ e þ O3 /· O 3 þ · O 3 þ H / OH þ O2

52.2 ± 11.9

44.2 ± 4.7

a

2.8 1.5 2.8 5.8

17.3 16.0 13.5 38.3

± ± ± ±

7.4 4.0 6.9 4.8

hv and hs represent ultraviolet (UV) and ultrasonic (US) irradiation, respectively.

with coagulation, which suggests that oxidation by O3, which is a selective oxidant, performs better than non-specific $OH oxidation (Zhou et al., 2011a). There could be various factors influencing the removal rate such as scavenging effects of OH radicals present in concentrate (i.e., chloride, bicarbonate). However, no case in this study demonstrated further mineralization and/or complete treatment of concentrate, which could be attributed to contaminants resistant to O3. While the study indicated that biodegradability is enhanced using AOPs coupled with coagulation pretreatment, and degradation byproducts are less toxic than the parent compounds, some O3 based oxidation systems still exhibited toxic organic residues, as indicated by the high ecotoxicity of the concentrate. Organics that are partially degraded by the multiple processes can be returned to those processes prior to RO for further treatment. Removal of recalcitrant organics from concentrate, especially in the presence of high salinity, is difficult by conventional methods. However, the electro-Fenton process using graphite-felt as a cathode was reported to be highly efficient in degrading organics present in concentrate (Zhou et al., 2012). Unlike the standard Fenton process, this process was shown to be effective over wide pH ranges. However, process treatment performance is influenced by the available Fe3þ concentration as catalyst and cathodic potential. Under optimum conditions (i.e., 0.72 V of cathodic potential, 0.2 mM Fe3þ, initial pH 3), more than 62% of COD was removed. Applying solar energy to the Fenton process can enhance oxidation (Ioannou et al., 2013). Thus introducing electro-Fenton process as either pretreatment prior to the membrane systems or posttreatment of concentrate could be applied in minimizing or completely treating contaminants of concern in concentrate.

3.4. PretreatmenteUF/MFeRO Coupling of powdered activated carbon (PAC) adsorption with both coagulation pre-treatments prior to UF and UFeGAC adsorption post-treatment has been studied for the removal of emerging contaminants (Acero et al., 2012). Results indicate that a low dose of PAC (10e50 mg/L) was adequate to produce improved water quality while reducing membrane fouling and removing most emerging organic contaminants. On the other hand, coagulation pretreatment itself did not provide an increase in the permeate flux; however, when coagulation pretreatment was combined with UF, the permeate quality was improved.

Furthermore, compared with PAC/UF, coupled Fe3þ/PAC/UF achieved better performance in removing COD, UV254, and total phosphorus (Acero et al., 2012). Post-treatment by granular activated carbon (GAC) after the UF process showed significant rejection of emerging contaminants (Verliefde et al., 2007). However, when competing compounds such as natural organic matter were present, efficacy was significantly decreased due to pore blockage inside the GAC structure (Verliefde et al., 2007; Snyder et al., 2007). Therefore, UF filtration would ideally be placed prior to GAC for rejection of macromolecular organic matter. The use of PAC in the combined process is particularly effective for removing emerging contaminants, since PAC better adsorbs low molecular weight substances which are not removed effectively by UF. With the pretreatment process removing as large a fraction of emerging contaminants as possible from wastewater effluent prior to membrane filtration processes, concentrates generated from MF and RO could be minimized by recycling as influent back to the filtration systems. However, backwash chemicals used in the membrane operation, as well as concentrates, still need to be further treated, since trace amounts of emerging contaminants may still remain. A new hybrid process consisting of a countercurrent two-stage adsorptionemicrofiltration demonstrated efficiency in treating dissolved biorefactory organics from concentrate with a high overall recovery rate of 91% (Table 2, process 4) (Zhao et al., 2013). While dissolved organics in concentrate generated from petrochemical plants were seen to be removed by PACeMF (Zhao et al., 2012), the hybrid process resulted in removing 72.7% of DOC and 60.8% of COD with a high overall RO recovery rate of 91%. The countercurrent two-stage adsorption system saved significant amounts of adsorbent and was able to remove 70% of the contaminants from concentrate, which can increase recovery rate to 91e94% through extra RO processes (Table 2, process 4). In general, organics present in concentrate are resistant to biodegradation. Especially problematic to treat are the trace amounts of emerging contaminants from retentate, as retentate contains higher concentrations of the pollutants than the RO influent. Thus, beyond the combined PACeMF, an additional process such as an RO technology is needed to improve water quality. Applying GAC would be less effective than using PAC, since PAC has been shown to control irreversible membrane fouling, thereby reducing the frequency of chemical cleaning (Campinas and Rosa, 2010), and larger GAC results in lower adsorption capacities (Corwin and Summers, 2010). This effect was demonstrated in a study by Zhao et al. (2012) (Zhao et al., 2012), showing removal rate of 60% by PACeUF, while just 36% for GACeUF. In multi-stage NF/RO, intermediate treatment of concentrate for removal of scale-causing minerals and/or organics prior to sending to the next stage of NF/RO would increase recovery rates. For instance, according to a report by Comstock et al. (2011), DOC removal by lime softening, ferric sulfate coagulation, and anion exchange (using MIEX-Cl resin) was in the order of anion exchange z coagulation > lime softening. However, the study showed that several factors (alkalinity, sulfate, TDS) may inhibit treatment efficiency, such as lime softening and coagulation affected by alkalinity, and anion exchange affected by sulfate and TDS. 3.5. UFeROepellet reactoreelectrodialysis While electrodialysis has been commonly used for treating concentrate, a coupled pellet reactor and electrodialysis system can be used for reducing scaling potential prior to electrodialysis to overcome the drawbacks of electrodialysis. A study by Tran et al. (2012) found that effluent treated by the integrated pellet reactoreelectrodialysis system was able to return to the combined system, UFeRO. Fig. 2 presents the changes in wastewater

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331

Fig. 2. Changes in wastewater composition through the hybrid process (pellet reactoreED) (Tran et al., 2012).

composition after the hybrid system (pellet reactoreelectrodialysis) is applied, compared to pellet reactor only. As can be seen in Fig. 2, compared to pellet reactor only, the coupled pellet reactoreelectrodialysis system achieved ion removal in the  2þ order of Ca2þ ð90%Þ > SO2 ð60%Þ > 4 ð73%Þ > Cl ð65:5%Þ > Mg  þ þ NO3 ð50%Þ > K ð47%Þ > Na ð42%Þ, indicating a viable option for increasing recovery in RO by reducing the scaling potential before electrodialysis treatment of concentrate and before returning the effluent to the UFeRO system. However, electrodialysis treatment has a low removal rate of organics and requires a high energy usage. If renewable energy can be used as a power source for electrodialysis, both treatment costs and CO2 emission could be reduced. Electrodialysis could be jointly applied prior to either single or combined AOPs, with treated effluents being returned to a primary treatment process, minimizing the total volume of concentrate. In an integrated system consisting of electrodialysis and ozonation, 22.5% of water is returned to the biological treatment unit and 2.5% of water is electrodialysis concentrate, which is discharged to a canal (Zhang et al., 2012). In the system, concentrate is mixed with the UF backwash water and UF concentrate before being discharged to the canal. In order to enhance the recovery rate of the RO system, the electrodialysis-treated effluent is further ozonated before being recycled to the biological treatment unit. The cost of the electrodialysis process used in the system is estimated to be 2.4 times lower compared to the UFeRO treatment cost and 3.7 times lower than the cost of wastewater treatment (Cheng et al., 2008). This estimation was solely based on a single process which did not include ozonation, maintenance required for cleaning, and other costs. However, integrating electrodialysis and mixing with UF concentrate to dilute electrodialysis concentrate prior to discharge to the canal could be a viable option in terms of enhancing water quality and overall water recovery. Several factors need to be considered prior to integrating electrodialysis into the overall system, including, for example, potential hazardous gas emissions (i.e., continuous generation H2 from the reaction at the cathode) when electrodialysis is integrated in a large water treatment system (Zhang et al., 2012). Some parameters to consider include utilizing gas emissions from the stack (O2 from the

anode and H2 from the cathode), optimizing conditions for removal of salt with different feed flow rates and currents, deploying methods to reduce potential scaling issues, and utilizing renewable energy to minimize energy consumption and CO2 emissions. 3.6. MF/RO-nanocomposites in a continuous flow mode Recently nanocomposites doped on sand have been shown to be capable of treating concentrate in continuous mode (Table 2, processes 10e11) (Joo, 2014a). A modified DynaSand system, filled with waste iron, is a potential application of nanocomposites for concentrate treatment (Table 2, process 11). It was observed that as opposed to the potential effect of scavengers in concentrate (i.e., bicarbonate, carbonate, salinity), inhibitors present in concentrate were the primary factors affecting slow removal kinetics (Fig. 3) (Joo, 2014a). 3.7. UF/RO-sequential activated carbon and iron-activated persulfate oxidation A sequential process involving adsorption and iron-activated persulfate oxidation has been found to treat concentrate generated from membrane systems consisting of submerged UF and RO (Table 2, process 12) (Joo, 2014b). The sequential process comprised of PAC adsorption followed by treating with iron-activated persulfate for enhanced oxidation was demonstrated to be highly effective in removing both organics and nutrient salts. However, the optimum treatment system to address emerging contaminants of concern needs to be further developed, including systems which are applicable regardless of pollutant loads. 4. Proposed treatment options Removal of emerging contaminants by either NF or RO has been shown to be effective except for low molecular weight uncharged compounds (Dolar et al., 2012; Snyder et al., 2007). Some emerging contaminants, especially low molecular weight trace organic chemicals (e.g., NDMA) are treated with RO followed by either UV or combined UV and H2O2 in water reuse systems (Fujioka et al.,

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Fig. 3. Possible reaction of contaminants in concentrate with iron oxide/nFe(0) mixed in soil (Joo, 2014a).

2012). Concentrates from RO could be minimized, and recovery rate increased, by adding a second stage of RO for further treatment. For example, partially removed contaminants can be reclaimed through further RO processes. Considering cost, treatment efficiency, and ease of use, the following integrated treatment systems are suggested:

hydrophilic contaminants and those with low molecular weight. Solar energy applied to induce reactive oxygen species in the presence of metals as a catalyst could be integrated in AOPs alone or in a combined AOP and adsorption process, either for pretreatment or post-treatment of concentrate.

(1) Combined MF and RO system: Hybrid pretreatment (AOP þ adsorption) prior to membrane process, with concentrate returned to the main treatment unit for minimizing volume and enhancing recovery rate. Several stages of RO process could be also introduced in this system. Prior to RO treatment, it is critical to select suitable pretreatment options based on raw wastewater influent composition, operation parameters, selection of chemicals for cleaning membranes, and membrane types in order to minimize concentrate quantity and enhance recovery rate. There could be several treatment options based on the composition of influent and the characteristics of concentrate, and treatment choice may be influenced by operational parameters and cleaning chemicals used in the system. (2) Post AOP treatment after RO process: There is a case where disinfectant is injected prior to RO to prevent and minimize toxic byproducts which may contribute to a buildup of fouling layers on the RO membrane. However, disinfection byproducts are found to be resistant to RO treatment and treatment is ineffective once those are formed. As an alternative to this, AOPs could be introduced as a post-RO treatment. Concentrate generated is mixed with any other concentrates left from membrane processes and treated with the standard AOP process. The treated effluents then return to the main treatment unit process. (3) Renewable energy integrated electrodialysis and adsorption prior to RO system: Renewable energy supplied for electrodialysis and use of adsorbents (i.e. activated carbon) prior to RO could be an option to overcome drawbacks such as high cost due to high energy consumption and low treatment efficiency in removing organics. (4) Waste-iron-filled sand filter system as a post RO process: The modified sand filter system proposes to remove contaminants by a combined adsorptioneoxidationecomplexation mechanism. Since concentrate is more concentrated than raw water, though the composition would be similar to feed water, the post RO process needs to cycle through the hybrid treatment system prior to recycling back to the main treatment unit. (5) Integration of solar energy either as pretreatment or post RO process: As reviewed in Section 2, emerging organic contaminants remain very resistant to degradation. Membrane systems are found to be especially ineffective in treating

Regardless of the technology used, the treatment goal is to enhance the recovery of reusable water and to minimize quantities of contaminants disposed to the environment. This could be accomplished by integrated systems utilizing energy-efficient motors and pumps in their operation. Introducing solar energy to RO systems, and development of both energy- and treatmentefficient membranes are needed together in integrated treatment processes. 5. Conclusions Concentrations of emerging contaminants in the effluent from a typical wastewater treatment plant range from ng/L to mg/L (Ternes and Joss, 2006), and these contaminants are a concern as RO rez-Gonza lez et al., 2012), leavmembranes highly reject them (Pe ing residues remaining in concentrate. Compared to other pressure-driven membrane systems (i.e., MF, UF, NF), concentrate from RO systems is primarily discharged to surface water and concentrate in particular is considered difficult to treat and dispose of, due to high concentrations of organics and salinity. Integrated systems offer treatment improvements while enhancing overall recovery rate. In this paper, several hybrid systems have been introduced and reviewed in terms of their treatment efficiency and drawbacks, with possible solutions offered to overcome those drawbacks. Among these, a hybrid system using simultaneous MF, adsorption and oxidation with an ultrasound system removed more than 99% of selected pharmaceuticals, and nanocomposites packed in a sand filter completely removed organics in a continuous flow mode. Combined oxidation and adsorption was effective in treating RO concentrate, though reaction kinetics reach a plateau over time with slow oxidation, possibly due to a lack of available surface site for reactions. Applying ozone to biological aerated filters or UFeRO efficiently removed contaminants, but it has been found that there are contaminants resistant to ozone, leading to incomplete contaminant removal. Among the AOPs especially aimed at removing emerging organic contaminants present in concentrate, coagulation pretreatment followed by combined UVA/TiO2/O3 showed the most effective treatment with a removal rate over 68% (Fig. 1). The efficacy of combined AOPs is also illustrated with fluorescence excitationeemission matrix spectra, which showed significant reduction of aromatic proteins, fulvic acid-like substances, humic compounds, and soluble microbial products after treatment

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by UVC/H2O2-BAC. While there is correlation between relative intensity of excitationeemission matrix signals (i.e., visible humiclike, UV humic-like, protein-like) and concentrations of $OH (i.e., decreased signals when concentrations of $OH are increased), the presence of high concentrations of humic materials in concentrate resulted in slow oxidation over time. A pellet reactor coupled with electrodialysis was indeed effective in removing most ions, which allowed recycling of the treated effluent to a UFeRO process without scaling issues. If renewable energy can be utilized as the power source for electrodialysis, combining electrodialysis with other AOPs would significantly improve the efficacy of RO concentrate treatment.

6. Future research directions Careful consideration should be given to concentrates generated from RO, which are not only problematic for disposal, but also present potential risks to the environment and human health, especially given the presence of emerging contaminants in those concentrates. Recycling concentrate to main treatment units is not the only option, and further research on integrated treatment systems needs to be performed. Combinations of treatment systems or hybrid processes would enhance treatment efficiency, moving toward near-zero discharge. More compact and efficient treatment technologies should be further developed. While development of life-long and energy-saving membranes has contributed to decreased operational expense, the overall costs of treatment are likely to increase due to increased concentrate treatment costs. Future research directions, particularly in treating and/or minimizing concentrate are recommended as follows. 1) Treatment of unregulated emerging organic contaminants in concentrate: There has been a paucity of research on unregulated emerging organic contaminants (halogenated DBPs, Nnitrosamines). It has been reported that removal efficiencies are low for compounds that are relatively stable and have low molecular weights, and most data available for the removal of these types of contaminants indicate variable treatment performance (Sedlak and Kavanaugh, 2006). For instance, research on treatment efficacy for organic halogenated disinfection has revealed that emerging organic contaminants remain in concentrate post treatment. Even with treatment developments, formation of byproducts during degradation reaction is a concern, as intermediate compounds can be more toxic than the parent compounds. 2) Development of new methods to replace chemicals used in backwashing: In addition to the development of treatment processes for unregulated emerging contaminants, chemicals used for cleaning may also affect contaminant-removal efficiency. This concern requires replacement of cleaning chemicals and antifouling agents in membrane processes with environmentally benign methods. As an example, foulants on membranes could be removed by utilizing microwave-assisted or ultrasonic methods during the backwashing to replace or reduce the use of chemicals. The design of membrane systems must take into account the need to minimize generation of concentrate. 3) The correlation between the types of membranes (i.e., hydrophobic vs. hydrophilic membrane) and treatment performance for emerging organic contaminants of concern: Types of membrane modules may affect removal efficiency, which subsequently influences concentrates. For instance, small pore size and hydrophobic membranes exhibit higher fouling rates due to adsorption within pores (Gray et al., 2007).

333

Acknowledgments This work was supported by the research start-up grant at the University of Miami.

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Further reading €rkka €, H., Sillanpa €€ Zhou, M.H., Sa a, M., 2011b. A comparative experimental study on methyl orange degradation by electrochemical oxidation on BDD and MMO electrodes. Sep. Purif. Technol. 78, 290e297.