Advances in Reverse Osmosis Pretreatment Techniques

Robert Huehmer, P.E.* and Lisa Henthorne, P.E.** *Senior Technologist, CH2M HILL, 13921 Park Center Rd, Herndon, VA 20171 USA.

(robert.huehmer@ch2m.com) **Vice-President, Global Director of Desalination, CH2M HILL International, Al Thuraya

Tower, Ste 606, Dubai Internet City, P.O. Box 500554, Dubai, UAE. (lisa.henthorne@ch2m.com)

Abstract

The economics of implementing seawater desalination using reverse osmosis technology reaches a ‘scale of economy’ at larger production rates resulting in the lowest total water cost (TWC). The chemical properties of constituents found in seawater, particularly with respect to surface chemistry and colloidal interactions, have been shown by several researchers to vary greatly from those of non-brackish (fresh) surface water. Although the increased ionic strength of seawater compresses the stern layer and theoretically decreases particle stability, which should result in enhanced coagulation, organic materials present on particulates and colloids, along with reduced reaction rates due to low activity, frequently yield difficult to coagulate water, particularly in estuarial waters. As a result, an engineer’s knowledge of coagulation, flocculation, sedimentation and filtration processes garnered from non-brackish surface water sources is not typically applicable to seawater pretreatment. Experience indicates that with a limited database of the performance of these processes treating seawater, bench and pilot scale testing is appropriate for any large scale desalination facility under consideration. The recent movement to the use of low-pressure membrane filtration (MF/UF) prior to reverse osmosis units is not without unique challenges. Low-pressure membrane fouling may also be of concern, as both extra-cellular secretions from marine microorganisms and naturally occurring humic acids appear to have an affinity to membrane materials typically utilized. Alternatively, dissolved air flotation followed by media filtration is of growing interest, particularly in pre-treating waters prone to algal blooms or oil contamination. This paper provides a review of published theoretical and applied research into pretreatment technologies for open water intakes, as well as case studies documenting some of the state-of-the-art pretreatment technologies being utilized globally.

Keywords seawater; pretreatment; coagulation; dual media filtration; microfiltration; ultrafiltration

Introduction

Pretreatment of seawater prior to reverse osmosis is important to ensure reliable performance of desalination facilities. Pretreatment generally consists of the intake and screening systems, removal of particulate matter, control of biological growth and chemical conditioning of the water. Historically, pretreatment has typically consisted of coagulation and media filtration using a variety of filtration media. Both single- and two-stage filters have been utilized. The use of more challenging raw water supplies, such as estuarial waters, has resulted in greater amounts of particulate and organic foulants in raw seawater. In several instances, the use of

direct filtration has not been able to adequately pretreat the raw water prior to the desalination process, resulting in operational and maintenance issues at the facilities.

Recently, new pretreatment technologies have been piloted and utilized at full-scale for the pretreatment of raw water collected using a submerged open water intake. Several of these new technologies, such as dissolved air flotation and low-pressure membrane filtration (ultrafiltration and microfiltration) show great promise at providing superior pretreatment when compared to direct filtration. This paper reports on several case studies utilizing these new pretreatment technologies.

Impacts of fouling in reverse osmosis system performance

Five distinct types of membrane fouling are typically identified in most reverse osmosis literature, consisting of:

• particulate fouling • scaling • biofouling • organic fouling • colloidal fouling

Particulate fouling consists of physical occlusion of the feed water spacer by particulate and colloidal matter. This type of fouling is usually easily mitigated by improving pretreatment to the system. The mechanism of fouling is well understood. Scaling consists of crystallization and deposition of sparingly soluble salts within the feed spacer of reverse osmosis membranes under conditions where the solubility of the salts is exceeded and kinetics are favorable for crystal formation. Significant basic science has been performed examining crystal formation. Although there is still room for significant basic research in crystal formation kinetics, seeding and formation, there is a significant body of applied research into mitigation of scaling using techniques such as improved pretreatment, pH adjustment, dispersants, and crystallization inhibitors. Scaling of reverse osmosis membranes is usually easily mitigated once the predominant salts are identified, through one or a combination of the above techniques.

In biofouling, the reverse osmosis system becomes seeded with a micro-organism, which then reproduces and spreads as a biofilm within the reverse osmosis system. Although these micro-organisms typically do not result in negative health impacts of the produced water, they are capable of substantially reducing the output and efficiency of reverse osmosis systems. With the use of cellulose acetate membranes, some micro-organisms are able to use the membrane as a food substrate, ultimately destroying the membrane. Biogrowth of some species of bacteria can result in localized. Microorganisms colonized within a reverse osmosis system secrete compounds commonly referred to as EPS. Accumulation of colonies and associated EPS can result in decreased turbulence at the membrane surface, resulting in increased concentration polarization – and both decreased flux and increased occurrence of membrane scaling. Control of biofouling typically uses a multifaceted approach, including one or a combination of techniques such as improved system hygiene, prophylactic membrane cleaning, enhanced pretreatment and chemical dosing of biocides and/or dispersants. Once established, however, mitigation of biofouling is difficult to achieve. Biofouling is often more severe when substantial organic fouling is present, as the assimiable portion of organic fouling is a food substrate for micro-organisms.

Organic fouling results from reactions and interactions of large organic molecules found in the feed water with the membrane surface. Organics adhere to the membrane surface,

resulting in reduced mass transfer, and lower flux. Additionally, the assimiable fraction of the carbon provides a substrate source for bacteria, resulting in enhanced biofouling.

Colloidal fouling can be of a microbial nature, or be composed of organic or inorganic material. Colloidal fouling is challenging to prevent in that the small size of colloids (less than 1 micron by definition) are difficult to coagulate and filter.

Impact of Ineffective Pretreatment

Pretreatment has historically been the Achilles’ Heel of SWRO plants utilizing open or direct intakes. These intake systems deliver raw seawater to the facility having experienced a range of screening, from 0.5 mm (in the case of once-through cooling power plants) to 25 mm (in the case of sedimentation/flocculation basins). Fluctuating and often high turbidities combined with frequently high levels of organic, microbial and colloidal constituents are the root cause of the ailments of these SWRO pretreatment systems. The impact of inadequate or ineffective pretreatment can be any or a combination of the following:

� accelerated increase in net driving pressure � accelerated reduction in normalized permeate flow � accelerated increase in pressure drop across the vessel � increase in the RO cleaning frequency � reduced RO membrane life as a result of increased RO cleaning � reduced plant availability as a result of increased RO cleaning

The result of these operational impacts is a direct increase in the operational costs of the facility. To demonstrate the impact on the operation cost for an example 100,000 m3/day plant is shown in Table 1. The assumptions for this demonstration are:

� an increase in the feed pressure from 55.2 bar (800 psi) to 58.6 bar (850 psi), an increase in HP pumping of 6.3% (Assumptions: power cost of $0.05/kwhr)

� an increase in the cleaning frequency from 4 (an acceptable average) to 8 (Assumptions: Cleaning cost $18/element/cleaning cycle)

� a reduction in RO membrane life of 0.5% per year (RO element cost of $550/element) � a reduction in plant availability (lost revenue) of 0.5% (Assumption: All produced

water can be sold for $0.60/m3)

Table 1: Impact of ineffective pretreatment on operating costs Parameter Increase in Plant Operating Cost

($/year) Increase in feed pressure $830,000 Increase in cleaning frequency $580,000 Reduction in RO membrane life $ 20,000 Reduction in plant availability $120,000 Total $1,550,000

This represents an 8-11% increase in plant operating costs per year, accumulating to $46.5 million over the 30-year life of the plant. Keep in mind that this represents a mildly ineffective pretreatment system as some SWRO plants must clean 12-15 times per year.

Advancements in seawater reverse osmosis pretreatment

For removal of particulates and colloids, the focus of this paper, the following pretreatment systems have been previously utilized in seawater reverse osmosis systems.

� Coagulation and media filtration � Coagulation, flocculation and media filtration � Coagulation, flocculation, sedimentation and media filtration � Coagulation, flotation, media filtration � Coagulation, microfiltration/ultrafiltration � Coagulation, media filtration, precoat filtration

The media filtration can consist of sand filtration; sand and anthracite; sand, anthracite and garnet media; or porous media. These filters are generally configured in single or two-stage systems, and generally are capable of meeting the pretreatment system requirements.

Coagulation and Flocculation. The largest challenge is the difficulty in coagulating particulates in seawater. Once coagulated, removal via various filtration techniques is relatively straightforward. Metals salts are generally used in the coagulation process, with ferric salts being the most commonly used at levels of 2-7 mg/L, as iron. In sea water however, the zeta-potential of colloids is already very low due to the presence of high concentrations of ions, including many bivalent ions (the higher the charge of an ion, the stronger its effect on the zeta-potential), such as calcium and magnesium (Liss et al, 1975). In sea water colloids are frequently stabilised sterically by organic matter. In this case adding metal salts may have little effect. In seawater, flocculation occurs when particles are brought together mechanically, either by flow conditions causing particle collisions (Eisma, 1986; Kranck and Milligan, 1980) or by organisms that collect particles in faeces or pseudo-faeces (Eisma, 1986). As a general rule, flocculation in seawater requires particulate bridging with non-ionic molecules to form stable floc.

Dissolved Air Flotation. There exists a limited body of published literature on the efficacy of DAF in seawater pretreatment applications. Due to the high ionic strength of seawater relative to surface water, compression of the Stern layer can substantially impact not only coagulation chemistry due to increased repulsive forces, but also the over-all strength of floc. Other beneficial effects may occur to membrane performance. Research literature published on treatment of aquaria indicates that an air-water interface preferentially absorbs proteins in high ionic strength water. This phenomena has resulted in the development of commercially available ‘protein skimmers’ to enhance organics removal in aquaria. The use of DAF as pretreatment in seawater desalination may possess additional advantages over conventional coagulation/flocculation/ sedimentation/filtration by preferentially reducing proteins associated (in fresh water applications) with membrane fouling.

Studies performed in fresh water indicate that DAF can enhance the performance of downstream unit processes. Cleveland et al. (2002) studied the use of DAF as UF pretreatment for an algal laden surface water. They found that the UF flux could be increased 70% following DAF pretreatment, resulting in substantial reductions in capital costs. The results of research performed by Braghetta (1997) similarly showed enhanced UF membrane performance following DAF. Recently, DAF has been extensively piloted in seawater applications. Extensive piloting conducted in El Coloso, Chile indicated that 3-stage flocculation, DAF and 2-stage filtration was able to produce RO feedwater with SDI less than 4 (typically less than 3) over a wide-range of operating parameters when treating seawater possessing high-concentrations of algae and zoo-plankton with maximum turbidity of 2 NTU (Sanz et al, 2005). Additional pilot work was conducted by Degremont (Bonnelye, 2005). Rovel (2003) reports successful use of DAF in pilot testing for the Taweelah “C” SWRO plant in Abu Dhabi, conducted in 2002/2003. In this application, DAF was placed upstream of each of three filtration processes: 2-stage dual media filtration, submerged ultrafiltration (UF) and pressured UF. The DAF was shown to achieve on average a 20-35% removal of organics

as measured by UV absorbance. It was also expected to be useful during rough sea conditions to reduce turbidity, during oil spills to remove micronic oil droplets, and during algae and planktonic blooms. The Taweelah “C” full-scale plant as presently envisioned will use DAF before 2-stage media filters. When coupled with optimized coagulation chemistry and media filtration, DAF is capable of producing a desalination system feed-water possessing SDI of less than 4, with values of less than 3 frequently achieved.

Full-scale facilities using DAF as pretreatment for seawater desalination exist in El Coloso, Chile, Tarragona, Spain, Atacama, Chile, Mejillones, Chile and Tuas, Singapore (Sanz et al, 2005; Ding et al, 2005). The largest constructed to date, the Tuas Desalination Plant in Singapore utilizes a DAFF process to provide pretreatment to SWRO. The Singapore Tuas desalination plant also utilizes DAF but in this instance, the single-stage filtration is housed within the DAF, renamed a DAFF (Dissolved Air Flotation and Filtration) as reported by Huijbregsen et al (2005). This system has met with good success during piloting prior to plant construction but has been less successful in the full-scale facility. The Tuas DAFF is designed with filter loading of 8 m3/m2/hr using 1100 mm of medium sand with a recycle rate of up to 15% and utilizing pre-flocculation.

DAF is also being demonstrated preceding 2-stage filtration at the Layyah site in Sharjah, UAE for the 1,000 m3/day GrahamTek RO Demonstration Plant, and will also be used in the 23,000 m3/day SWRO facility presently under construction at this site using conventional RO desalination. Early results from this demonstration indicate the pretreatment system consistently provides SDI values less than 3. The DAF system performance will be thoroughly evaluated as part of this demonstration study being accomplished by CH2M HILL.

Precoat Filtration. Precoat filtration utilizing diatomaceous earth is used as pretreatment for SWRO in plants built by Pridesa in Spain and also the reconstructed Tampa Bay Water desalination plant in Florida, USA. Little technical information is available regarding the performance of these systems in the English literature. In the Tampa Bay Water application, the precoat filtration will be added downstream of the existing upflow continuously backwashing sand filtration units.

Microfiltration and Ultrafiltration. Although the application of low-pressure membrane technology has been documented to provide superior pretreatment (Van Hoof, 2001) to reverse osmosis seawater desalination systems, the increased capital costs and operating costs associated with these technologies have limited their application in full-scale facilities in the past. The use of MF/UF technology has been shown to provide very low SDI water, frequently superior to that of conventional filtration (Rovel, 2003; Allam et al, 2003). Comparative pilot studies (Rovel, 2003) between UF pretreatment and high-rate flotation and two-stage filtration were performed for the Taweelah Project of Abu Dhabi Water & Electricity. Pilot studies performed in Saudi Arabia (Allam et al, 2003) found that use of ferric chloride coagulation and acidification prior to UF produced filtrate with an average SDI of 2.2; as compared to over 4 for the direct filtration train. The UF unit operated with a sustainable flux of 95 mh (56 gfd), at a trans-membrane pressure of 0.15-0.20 bar (2.2-2.9 psi). The pretreatment system recovery was 94%. When challenged with marine algal blooms, the UF system consistently produced an SDI of greater than 3. During the pilot, no RO CIP was required during the 6 month duration of the study. Use of immersed ultrafiltration as pretreatment to seawater RO was studied by Cote et al (2001). The system treated water with a raw water turbidity range of 1- to 5-NTU at a temperature of 10 C. During the first phase of the study, the flux was 25 Lmh and TMP was stable at 0.25 bar (3.6 psi). At a flux of 30 Lmh, a slow increase in TMP can be observed, which would lead to a cleaning frequency estimate of 4 to 6 times per year. Tuelera et al (1999) studied the use of a pressurized hollow-fibre UF

system (Aquasource). They found that the seawater salinity did not impact the specific flux of the RO system. Vial et al (2003) studied the use Asahi Microza membranes to provide RO pretreatment. Jew et al (2003) studied the use of MF pretreatment for seawater desalination. Huehmer et al (2005) studied the use of pressurized microfiltration at Port Hueneme, California. They found that SDIs of less than 2 were consistently produced by the system. Henthorne (2005) studied multiple pretreatment technologies at the San Patricio Water District facility near Corpus Christi, Texas. The study evaluated the performance of MF and UF membrane pretreatment compared to conventional pretreatment of seawater RO desalination, in terms of improved pretreated water quality and impact on RO performance; and determined the subsequent cost benefits of membrane pretreatment compared to a range of conventional pretreatment options, through establishment of an objective life-cycle cost comparison. The UF unit was able to achieve flux rates in the range of 68-85 Lmh (40 to 50 gfd), with experimental flux rates as high as 102 Lmh (60 gfd) achievable. SDIs were consistently less than 3.

Case Studies

CH2M HILL has been actively involved in demonstrating high performance and innovative SWRO pretreatment technologies over the last three years. These studies include evaluation of the pretreatment in-line with the dedicated RO systems, in order to be able to evaluate not only water quality differences between various SWRO pretreatment technologies but also monitor downstream RO performance impacts. These studies have included evaluations in the California Pacific Ocean, the Asian Pacific, the Indian Ocean and the Arabian Gulf. Technologies demonstrated include the following:

� Media filtration including sand, anthracite and garnet with coagulation � Media filtration including sand and anthracite with coagulation � Pressurized ultrafiltration (UF) membrane pretreatment with and without coagulation � Immersed ultrafiltration (UF) membrane pretreatment � Dissolved air flotation

Case study results from some of the representative projects are presented below.

Case Study 1: UF versus Media Filtration on Asian Pacific Seawater

A one-year pilot study was conducted during 2005 comparing performance of three pretreatment technologies: immersed UF, pressurized UF, and 2-stage media filtration. The media filtration utilized anthracite in Stage 1 with a hydraulic loading rate of 24 m/hr and sand and garnet in Stage 2 at a loading rate of 10 m/hr (note garnet was not present during early testing). The performance results from the immersed and pressurized membrane pretreatment units indicate they consistently provided lower SDI results, Figures 1 and 2 respectively, compared to media filtration, shown in Figure 3. These figures represent the SDI readings for the pilot testing shown as a percentile at the respective SDI level. For instance, in Figure 1, approximately 70% of the SDI results had values less than 3. These results are tabulated and shown in Table 2.

0.0

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6.0

7.0

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Percentile (less than)

SD

I

Figure 1: SDI results from immersed membrane filtration pretreatment for SWRO

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00Percentile (less than)

SD

I

Figure 2: SDI results from pressurized membrane filtration pretreatment for SWRO

0.0

1.0

2.0

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5.0

6.0

7.0

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00Percentile (less than)

SD

I

Figure 3: SDI results from media filtration pretreatment for SWRO

Table 2 SDI Results from Three Pretreatment Methods on Asian Pacific Ocean Seawater Descriptor SDI – Immersed

Membrane Filtration

SDI – Pressurized Membrane Filtration

SDI – Media Filtration

Average 2.58 2.36 4.01 Minimum 1.48 1.50 1.78 Maximum 6.13 3.59 6.25 5% of time less than 1.60 1.67 2.48 10% of time less than 1.67 1.75 2.85 25% of time less than 2.05 1.94 3.29 75% of time less than 3.01 2.58 4.72 90% of time less than 3.43 3.14 5.03 95% of time less than 3.56 3.32 5.88

0.2

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1.2

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0 20 40 60 80 100 120 140 160Operating Days

Per

mea

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0.2

0.7

1.2

1.7

2.2

2.7

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)

Permeability - RO Train 2-A Permeability - RO Train 2-B Permeability - RO Train 2-C

Diff. Pressure - RO Train 2-A Diff. Pressure - RO Train 2-B Diff. Pressure - RO Train 2-C

Figure 4: RO permeability and differential pressure results using pressurized membrane filtration pretreatment

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Permeability - RO Train 3-A Permeability - RO Train 3-B Permeability - RO Train 3-C

Diff. Pressure - RO Train 3-A Diff. Pressure - RO Train 3-B Diff. Pressure - RO Train 3-C

Figure 5: RO permeability and differential pressure results using media filtration pretreatment

The SDI data collected in this study, demonstrate what has been proven in earlier SWRO pretreatment studies; that low-pressure membrane filtration pretreatment provides a higher quality pretreated water with regard to particulate fouling. The effect on the downstream RO is provided in Figures 4 and 5 for the side-by-side systems using pressurized membrane filtration pretreatment and media filtration pretreatment, respectively.

These figures indicate that membrane filtration pretreatment results in lower differential pressure drops across the membrane but RO membrane permeability is not significantly influenced. This study is ongoing and final results discussing differences in the RO cleaning frequencies, cartridge filter replacement, pretreatment system operating issues, chemical dosing rates, and organics removal by the pretreatment systems are presently being compiled and will be presented during 2007.

Case Study 2: UF versus Media Filtration on California Pacific Seawater

A 1-year operation pilot study was undertaken to evaluate impact of various pretreatments and difference in performance of RO membranes from different suppliers. Two side-by-side pretreatment/RO systems were constructed: (1) using a flocculator and clarifier followed by 2-stage filtration; and (2) using immersed membrane filtration technology from two different suppliers (one MF and one UF).

The media filtration system utilized anthracite in Stage 1 with a loading rate of approximately 10 m/hr followed by sand in Stage 2 with a similar loading rate. The results of the RO operation are shown in Figures 6 through 9.

The figures indicate NPF was marginally higher for membrane filtration pretreatment compared to media filtration, depending on the particular RO manufacturer. RO system differential pressures were significantly lower for membrane filtration pretreatment compared to media filtration, for all RO manufacturers. Though not shown, salt rejection was stable for both systems but slightly lower for the RO system utilizing membrane filtration pretreatment.

Figure 6: Normalized Permeate Flow of RO elements using submerged membrane filtration pretreatment

Figure 7: Normalized Permeate Flow of RO elements using media filtration pretreatment

Figure 8: Differential pressure results of RO system using immersed membrane filtration pretreatment

Figure 9: Differential pressure results of RO system using media filtration pretreatment

Case Study 3: DAF prior to Media Filtration on Arabian Gulf Seawater

CH2M HILL is presently commissioning a pilot DAF prior to 2-stage media filtration for application in the Arabian Gulf. The flotation system is designed to operate at a surface loading rate of 10.5 m/h and a recycle rate of approximately 15% of the feed flow. The float

will be hydraulically decanted from the system. The DAF is designed so that it can be operated intermittently and be by-passed if desired. The pilot DAF is part of a demonstration plant to demonstrate GrahamTek large diameter RO technology and is located on a site in which CH2M HILL is constructing a full-scale SWRO plant which will utilize this same pre-treatment scheme. In the full-scale plant, it is likely to be desirable to operate the DAF only during warm temperatures or during periods when there are high levels or organics or oil in the raw water.

The results from the first 6 weeks of DAF operation in conjunction with 2-stage media filtration will be presented in the IDS conference presentation.

Conclusions

Results of pilot and demonstration testing indicate that pre-treatment techniques (such as membrane filtration) exists today, which can provide a higher quality pre-treated water quality regarding particulate and potentially colloidal RO fouling. Still, there are questions regarding these techniques to noticeably impact organic and/or bio-fouling and the comparative performance of media filtration in these regards. Studies which CH2M HILL presently has underway will assist the industry understand the effect of these advanced pre-treatment techniques on these other fouling mechanisms. Our experience indicates that advanced pre-treatment technologies such as membrane filtration may not always provide the best solution for a particular site, but that piloting is a critical component to ensure pre-treatment success.

Pricing for the more advanced techniques such as membrane filtration has also been a past concern of engineers. Weighing the relative value of membrane filtration compared to media filters versus the additional capital cost of membrane filtration has been a challenge. CH2M HILL has tracked the comparative pricing of the pre-treatment options over the last few years and has seen a recent change in comparative pricing. Membrane filters have continued to drop in price, particular as some vendors have developed product lines specifically for SWRO applications with optimized engineered systems. Conversely, media filtration pricing has increased for small to medium plants due to increased filter housing costs from materials escalation over the last 12 months. Cumulatively, these changes in market pricing have resulted in membrane filtration with lower capital costs than media filtration for these plants. For instance, the 14,000 m3/day SWRO plant in Kalba, UAE being engineered and constructed by CH2M HILL will utilize UF pre-treatment due to pricing advantages offered to the client complimented by the superior pre-treatment performance being demonstrated by ongoing on-site pilot testing.

Overall, as the SWRO industry continues to grow and mature at its current rapid rate, advanced pre-treatment technologies and adequate and robust pre-treatment system designs are being developed and implemented which will reduce the frequency of poor performing pre-treatment systems that plague the industry’s history. Advancements in pre-treatment techniques should always be adequately piloted or demonstrated for any given feed-water due to the unpredictable nature of seawater.

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