Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004.
*Corresponding author.
Reverse osmosis on open intake seawater:pre-treatment strategy
Véronique Bonnelye*, Miguel Angel Sanz, Jean-Pierre Durand, Ludovic Plasse,Frédéric Gueguen, Pierre Mazounie
Received 4 December 2003; accepted 15 January 2004
Abstract
Pre-treatment of seawater feeding reverse osmosis (RO) membranes is a key step in designing desalinationplants. The pre-treatment process must be adapted to the seawater quality to be treated (wells, open intake, etc.),especially when treating surface seawater with highly variable quality. After a general presentation of different pre-treatment options in relation to the seawater quality, this paper is focussing on two case studies, two open intakeseawater pre-treatment upstream reverse osmosis desalination. The first site is located in the Gulf of Oman (IndianOcean), the second in the Persian Gulf. The pre-treatment uses different technology strategies, conventional pre-treatment (coagulation and direct filtration on dual media filters) and innovative technologies (high rate dissolvedair flotation, ultrafiltration and microfiltration) according to the water quality. The parameters taken into accountfor the water quality characterisation are the suspended solids, turbidity, fouling tendency, organic matters andalgae content. This paper presents the pre-treated water quality achieved by the two types of pre-treatment anddiscusses potential impacts on RO hydraulic performances.
Keywords: Seawater reverse osmosis; Open intake; Pre-treatment
1. Pretreatment strategy
Reverse osmosis membranes are very sensitiveto foulants such as colloids, inorganic scale and
biofilm development (biofouling). The silt densityindex (SDI according to ASTM) is a useful toolfor particle evaluation and by extension, mem-brane fouling. Many SWRO systems are fed usingbeach wells with low suspended solids water. In
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such cases, it is possible to achieve SDI15 (15 min.SDI) below 3 [1] using sand filtration withoutcoagulant, or even simple 5 µm cartridge filters.Due to increasing plant size and the limitedpermeability of local soils implying a need fornumerous wells, onshore beach wells are usedwith less and less frequency and SWRO plantsare operated with direct feed from open seawaterintakes [2].
On open seawater intake, reverse osmosismembranes are sensitive to different types ofpollution: particles, precipitated metals, organicmatters, hydrocarbons, etc. An efficient pre-treatment must control the flux of each pollutant.The pre-treatment must be designed to face theworst water quality, providing a constant and goodRO feed water quality.
Direct filtration, using mono or dual media, isthe most common technology used for the filtra-tion of seawater upstream RO desalination plant.This technique must be optimised and improvedon variable quality surface seawater: the coagula-tion can be improved by a better understandingof the phenomenon, the use of different chemicals.Selected media and filters characteristics can alsoimprove the performances of this clarificationtreatment. Direct filtration treatment is evaluatedin terms of filtered water quality, cycle duration,maturation and risk of breakthrough. This treat-ment technology can be improved using a doublestep filtration.
On open intake surface water treatment, well-optimised direct filtration can be replaced bymembrane clarification, such as ultrafiltration ormicrofiltration [6]. Furthermore, the impact of alower cutoff pre-treatment could lead to anincrease of the RO reliability. The design permeateflux rate could be higher than the generallyrecommended design average flux rate in seawaterRO systems operating on surface seawater, whichis within the 7–8 gfd range (11.9–13.6 L/h.m²)[3],and recovery (40–45%)[2,3].
Finally, facing high turbid water, risk of algaebloom and/or hydrocarbon pollution, the main
clarification step (direct clarification or ultra-filtration/microfiltration membranes) can beprotected by complementary pre-treatment, suchas sand removal, settling and/or flotation. Thelimits of each treatment must be evaluated in termsof design, water quality and adaptation to apollution event.
This paper presents the results of two pilotstudies performed on open seawater intakes. Theobjectives were to assess the limit of eachtechnology and to explore the potential oftechnology associations to increase the reliabilityof the whole open intake seawater pre-treatmentupstream reverse osmosis desalination.
2. Material and method
Both pilot tests were conducted on an openseawater intake. Sand removal was evaluated asa pre-treatment step, mainly on the Persian Gulf.For high suspended solids content, a dissolved airflotation unit was tested for turbidity, oil andgrease removal on the same site. The high rateflotation unit includes a pressurized water generatorand a 1 m² separation cell.
The direct filtration was then studied in one ortwo stages, depending on the feed water quality,using pilot units with 4 filtering columns operatedin parallel or in series. Transparent plastic columnswere used to optimise the backwash sequences.
The filtering media tested were from severalorigins, anthracite, pumice, sand and garnet,allowing an evaluation in term of effective size,shape and density.
Two fully automated ultrafiltration units wereused. These pilot units included the filtrationsystem and automatic backwashes controlled bya PLC.
The reagents tested were usually chemicalsused in water clarification: sulphuric acid for pHcorrection, Fe or Al salt as coagulants (FeCl3,WAC HB, Kemira PIX 123), organic coagulantsas filter aid (Kemazur 4527, Nalco 8103 and 8105,RO Floc) and flocculent aids (AQnionic polymer
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ASP25, trach). Powdered activated carbon wasalso evaluated for organic matter removal im-provement, and chlorine for disinfection shocksor continuous preoxidation, and membrane back-wash.
Several on-line equipments were used for theprocess control: flowmeter (ABB) and flow con-trollers (Burkert), pressure sensors for the head-loss follow-up (Jumo).
The suspended solids were evaluated in termsof turbidity and particles count. Turbidity wasanalysed both on grab samples (Hach 2100P) andin-line, using both Seres turbilight and Hach1720C turbidimeters on raw and filtered water.
UV absorbance (at 254 nm) and TOC wereanalysed to characterise the organic matters. Theseanalyses were completed by pyrolysis — GCMSanalysis to evaluate the organic matters com-position.
Phytoplankton was followed during the wholestudy.
SDI was measured using manual apparatus andan automatic Chemetek FPA-3300 Filter plugginganalyzer. For filtered water and permeates theSDI15 was determined according to ASTM D4189-95 (15 min filtration in the 75% foulingrange). For raw water, the SDI yielding 75%membrane fouling was recorded (SDI5 or SDI3).Particle count analysis was performed with a Met
One WGS267 portable particle counter and in-line Met One PCT.
3. Results
3.1.The Gulf of Oman
The first site evaluated was good surfaceseawater, using only one pre-treatment stagebefore RO. An 18-month pilot study was per-formed on the pre-treatment to study and optimisethe direct coagulation in term of chemicals andfilter media characteristics. Two periods werestudied in 2002 and 2003 to take into account thewater seasonal variability. This paper presents thepilot study and the fisrt results of an industrialplant constructed based on the pilot results.
The water quality main characteristics arepresented in Table 1: water was taken 4 m belowthe sea surface. During the test period, thefollowing parameters remained almost constant:pH, turbidity, conductivity, Fe, mineralisation,algae, UV, and hydrocarbons. The water qualitywas generally very good with turbidity around 0.2NTU and 70% of the SDI5 below 6 (Fig. 1): norelation seems to exist between those two param-eters. 75% of the particles were in 1–2 µm range.The temperature was high, but normal for this partof the world.
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According to the UV absorbance (<1 m–1 mostof the time), TOC results (<2 mg/l), andhydrocarbons (<5 µg/l), the raw water organicmatters concentration is very low.
The algae counts were always low. Macro-algae development has been reported on someequipment of the pilot plant, but this problem isnormal with open or transparent equipment with-out chlorination.
The media tested on the single stage directcoagulation are listed in Table 2 (Fig. 2). Morethan 150 filtration cycles were performed duringthis study.
The main conclusions of the tests were:• a direct relation between the effective size of
the filter bed bottom layer, last in contact withthe filtered water: the lower the ES, the betterthe SDI value,
• an impact of the media high on the initial headloss of the filter bed: the values vary from a
Table 2Filter medias tested (effective size ES in mm)
Fig. 2. Direct filtration using ferric chloride as coagulant,and two types of media in parallel, dual media pumice/sand and mono-layer sand filter.
Top media Bottom media Anthracite (1.5) Garnet (0.3) Anthracite (0.9) Sand (0.5) Anthracite (0.9) Sand (0.3) Sand (0.5) — Pumice (1.6) Sand (0.5) Pumice (1.6) Sand (0.3)
few cmWC/m to more than 1 mWC/m ofmedia. This is a direct limitation of the lowermedia depth.
• a better retention capacity in the larger sizemedia, on top of the filter.
All the chemicals: WAC HB, Kemira PIX 123,Kemazur 4527, Nalco 8105, RO Floc, ASP25,starch polymer and continuous chlorination — didnot improve or deteriorate the treatment efficiency.
FeCl3, coagulant generally used for this appli-cation, gave surprisingly very bad results in termsof clogging rate of the filtering bed: a head lossprofile clearly showed the clogging of the top layerof the first dual media filter tested. After the choiceof the best media configuration (larger effectivesize on top), the results improved leading to anoptimum-dosing rate of 3 g/m3 as FeCl3.
The pH of coagulation impacts also theclogging velocity, allowing a cycle lengthoptimisation.
When the raw water SDI was higher than 10,a flocculant aid was necessary to lower the cloggingvelocity and to improve the filtered water SDI.
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Chlorine was used from time to time only tostop the biological development in the pipes andequipment. It does not improve the coagulationefficiency on removing SDI in the filtered water.
The filter backwash has to be optimised inaccordance with the media chosen, in terms of airand water velocities, the latest necessary to reachthe best media expansion/classification. Afterevery backwash, the filtered water quality has tobe evaluated to determine the maturation timeneeded (production duration with a filtered waterquality unacceptable for the reverse osmosis feedleading to filtered water rejection, filtration towaste). During the pilot test, this duration couldbe reduced down to 15 min.
With the media configuration chosen and afterthe chemicals optimisation described, the filtrationcycles were between 17 to 24 h with a filtrationvelocity of 10 m/h.
With the optimised treatment, the filtered waterquality was steady and in a good range for all theparameter checked:• SDI, the most important parameter for the
characterisation of RO feed water (Table 3),was efficiently removed using the lowesteffective size media (garnet or sand).
• Turbidity was always below 0.1 NTU in thefiltered water (average 0.07 NTU), with amaturation time to get this value below 15 min.While the filter was clogging, the turbidityremained below 0.1 NTU, demonstrating norisk of breakthrough (breakthrough time muchhigher than clogging time). Particles wereremoved at 98%, with less than 10 particles/mL larger than 2 µm.
• Iron concentration was below 0.01 mg/L andremained at this low value in the filtered water:all the ferric chloride used for coagulationeffectively precipitate in the filtering bed.
• In terms of mineralisation, chloride and sul-phate increased according to the treatment rateof sulphuric acid and ferric chloride, with acorresponding reduction of the alkalinity.
• Organic matter removal was mainly evaluated
using UV absorbtion at 254 nm: a 25% removalefficiency on this parameter was observed,with an average filtered water value around0.6/m. Hydrocarbons concentration remainedat very low levels (<3 µg/L) including whenhydrocarbons pellets could be seen floating ina rather large amount (note that this intake islocated only 3–4 miles from the “rail” goingout of the Arabian Gulf with tankers perm-anently queuing out from the Ormuz strait).
• The phytoplankton was very low, with algaeconcentration always below 10 cell/mL.
From the pilot plant trial, it was concluded thata single step gravity dual media filtration wassufficient to obtain adequate pre-treated waterquality subject to the following conditions:• pH adjustment between 6.5 and 7.2 depending
on raw seawater SDI,• Coagulant dosage ranging from 2 to 5 mg/L
pure FeCl3,• Under worst seawater quality conditions, the
dosage of a polymer to reduce the SDI and toimprove significantly the filterability of theflocs formed thus increasing the filtering runtime.
The first results obtained during the full in-dustrial plant commissioning were similar to theones obtained during the 9-month pilot test: Thefiltered water was very steady, and SDI was belowthe expected value (Fig. 3).
3.2. The Persian Gulf
The second site evaluated using pilot testingwas the Persian Gulf. The characteristics of theseawater in term of SDI value, concentrations of
Table 3SDI in the raw and filtered water
SDI, % Raw water Filtered water 95 <15 <3.3 90 <9 <3 50 <5 <2.6
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organic matters and suspended solids determinedthe choice of pre-treatment before reverseosmosis, and the number of stage of the pre-treatment. A four-month pilot study was conductedon a variable seawater quality. The objective ofthis study was to assess and compare conventional
Fig. 3. Full scale pre-treatment plant results in terms of SDI.
Intakepump + pipe
Sand/grit removal tank
FlotationAquaDAF™
Dual-media Filtration
Dual-media Filtration
Ultrafiltration 1
Ultrafiltration 2
ULTRAFILTRATION
Fig. 4. Pilot pre-treatment line.
pre-treatment and ultrafiltration (UF) pre-treatment efficiencies prior to RO for desaltingseawater with high-fouling tendency. A four-stageconventional pre-treatment process and two UFprocesses were operated in parallel on open intakeseawater (Fig. 4).
The main characteristics of the raw seawater,pumped 10 m below the sea surface (in a 15 mwater deep area) are presented in Table 4. Withan average turbidity around 0.7 NTU and algae
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counts below 100 cell/mL, this surface seawaterdoes not seem very difficult to treat, exceptregarding the SDI values, which are bothpermanently very high, and with wide fluctuations(Fig. 5).
The preliminary treatment, a dissolved airflotation, allows a good removal of turbidity and
Table 4Raw water analysis
Fig. 5. Raw water characteristics during the test period (temperature, pH, turbidity and UV absorbance).
Parameters Average Min. Max. Temperature, °C 32 32 35 pH 8.18 8.08 8.26 Conductivity, mS/cm 62.7 62.2 63.4 Turbidity, NTU 0.74 0.48 1.13 SDI, %/min 21.7 10 45 Total alkalinity,
suspended solids, highly variable in this veryshallow seawater intake (mainly during stormevents). UV absorbance removal was in 20–30%range (Fig. 6). Hydrocarbons were also wellremoved when present in the suspension form,protecting the aim of the pre-treatment: the directfiltration.
The direct filtration step included a doublefiltration with two coagulant injections: thistreatment line was selected to take into accountthe worst water quality expected in this surfaceseawater. Media were chosen to improve bothfiltration length and filtered water quality.
The results obtained on the filtration pilotduring the 4-month period of operation yieldingrather good SDI (1.8–2.9 with incoming sea waterSDI 10–45 %/min) (Table 5). Here also, thematuration was very short, and the risk ofbreakthrough very low, due to the sticky natureof the floc formed with the FeCl3-floculant aidapplied (Figs. 7 and 8). Filtering runs remainedlonger than 24 h.
Ultrafiltration pre-treatment was evaluated in
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Fig. 6. Floated water quality (coagulation using FeCl3).
direct filtration and after coagulation–flotation.The SDI measured in the permeate of the twomembranes tested was similar for both applica-tions (direct filtration and after dissolved airflotation), and in the same range as the secondstage filtered water. The SDI seems to be relatedto the membrane porosity, the higher the cut offthe higher the SDI value (in 1–4 range value).
Table 5SDI results
SDI after pre-treatment Time, % <3 100 <2.7 85 <2.5 65 <2.3 50
Fig. 7. Direct filtration optimisation —head losses and SDI follow up during afiltering run using pumice and sand me-dia, coagulation with ferric chlorine asso-ciated with a coagulant aid.
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Fig. 8. SDI value in the raw water in each of the dual media filtered water.
High filtration flux was obtained on thepressurized in-out Aquasource membranes, what-ever the feed water quality (120–140 L/h/m² indead-end filtration).
4. Discussion
The two seawaters tested can be characterisedby a low turbidity and low organic content: themain differences observed during the pilot studiesare the SDI value range and variability (Fig. 9).The Persian Gulf surface water has a high andunstable SDI. Some very high turbidity peaks, andsome hydrocarbon pollutions are also reported.The pre-treatment must be adapted to the surfaceseawater characteristics, and must be able to facethe degraded periods.
A single stage direct filtration using dual mediais well adapted to the good surface seawater en-countered in the Gulf of Oman. In this case, thefiltration media must be selected to optimise bothfiltered water quality (using a low effective size sandor garnet) and filtration duration (pumice or an-
thracite as top layer media). The coagulation strategyoptimised during the pilot test includes a doublecoagulant injection during the worst period (SDIhigher than 8) and adapted coagulation pH to thefouling tendency of the ferric hydroxide formed.
In the case of more degraded water, with a highwater quality variability expected, and to facepollution events (in terms of turbidity, algae bloomor hydrocarbons), the association of dissolved airflotation with double direct filtration gives a veryefficient and reliable pre-treatment upstreamreverse osmosis: the treatment line is more robustand able to handle very bad water quality. In thiscase again, filter’s media and coagulation chemicalsand conditions must be optimised to reach lowand steady SDI in the filtered water. Alternativeclarification treatment using membranes (ultra-filtration or microfiltration) gives similar treatedwater quality. The remaining question being theSDI adaptation to characterise the fouling potentialof a permeated water.
These very good values on both conventionaland membrane treatments should result in both
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Fig. 9. Comparison of the Gulf of Oman and the Persian Gulf water qualities.
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low cartridge consumption and long time betweencleanings, then long expected life of the reverseosmosis membranes.
5. Conclusion
The pilot testing during 9 months on the Gulfof Oman, which was paramount to optimise thepre-treatment stage and the chemical regime inrelation to the seawater quality improves theoptimisation of the process. The results were usedfor the commissioning of the full industrial plant,a 37.5 MGD open seawater intake desalinationplant located in Fujairah. Preliminary full-scaleplant results demonstrate that the foreseen per-formance and water quality are being achieved.
In the Persian Gulf, on a rather bad surfaceseawater intake, the pre-treatment includingdissolved air flotation, direct filtration or ultra-filtration, gives good results in terms of turbidity,algae and hydrocarbon removal, leading to areliable SDI far bellow 3 value.
These pilot tests improve the knowledge ofsurface seawater treatment upstream RO and
demonstrate the reliability and robustness of suchtreatment facing variable surface seawater.
References[1] M.A. Galloway and J.G. Minnery, Ultrafiltration as pre-
treatment to seawater reverse osmosis, Proc. 2001AWWA Membrane Conference, San Antonio, TX,2001, 10 p.
[2] N. Wade and K. Callister, Desalination: the state ofart, J. CIWEM, 11 April (1997) 87–97.
[3] M. Wilf and M.K. Schierach, Improved performanceand cost reduction of RO sweater systems using UFpretreatment, Desalination, 135 (2001) 61–68.
[4] S.C.J.M. van Hoof, A. Hashim and A.J. Kordes, Theeffect of ultrafiltration as pretreatment to reverseosmosis in wastewater reuse and seawater desalinationapplications, Desalination, 124 (1999) 231–242.
[5] A. Teuler, K. Glucina and J.M. Laîné, Assessment ofUF pretreatment prior RO membranes for seawaterdesalination, Desalination, 125 (1999) 89–96.
[6] V. Bonnélye, A. Brehant, M.A. Sanz and M. Perez,Surface seawater pre-treatment upstream reverseosmosis: long term test using ultrafiltration membranes,Proc. IDA World Congress, Nassau, Bahamas, Sept.2003, 15 p.
