nanofiltration/reverse osmosis for treatment of coproduced waters

Nanofiltration/Reverse Osmosisfor Treatment of CoproducedWatersSubrata Mondal, Ching-lun Hsiao, and S. Ranil WickramasingheDepartment of Chemical and Biological Engineering, Colorado State University, Fort Collins,CO 80523-1370; [email protected] (for correspondence)

Published online 15 April 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10271

Current high oil and gas prices have lead torenewed interest in exploration of nonconventionalenergy sources such as coal bed methane, tar sand,and oil shale. However oil and gas production fromthese nonconventional sources has lead to the copro-duction of large quantities of produced water. Whileproduced water is a waste product from oil and gasexploration it is a very valuable natural resource inthe arid Western United States. Thus treated producedwater could be a valuable new source of water.

Commercially available nanofiltration and low pres-sure reverse osmosis membranes have been used totreat three produced waters. The results obtained hereindicate that the permeate could be put to beneficialuses such as crop and livestock watering. Howeverminimizing membrane fouling will be essential for thedevelopment of a practical process. Field EmissionScanning Electron Microscopy imaging may be used toobserve membrane fouling. � 2008 American Institute ofChemical Engineers Environ Prog, 27: 173–179, 2008Keywords: coal bed methane, membrane filtration,

oily-waters, produced water

INTRODUCTION

Produced water (PW) refer to water that is copro-duced during oil and gas exploration. Traditional oiland gas reservoirs have a natural water layer (forma-tion water) that lies under the less dense hydrocar-bons [1]. Often, water is injected into the reservoir toforce the oil to the surface. As the oil field isdepleted, the volume of PW increases as the reservoirfills with water.

In this work we focus on PW from nontraditionaloil and gas sources in the Western United States.Unlike conventional oil and gas wells, production ofoil and gas from nontraditional sources is usuallyaccompanied by coproduction of large quantities ofwater. For example, the Wellington oil field in Well-ington CO produces about 98% water and 2% oil [2].In Wyoming, from 1987 to 2004, 2.9 billion barrels ofwater were produced while recovering 1.5 trillioncubic feet of coal bed methane (CBM). In 2003 alone,577 million barrels of water were produced. The totalvolume of PW in Wyoming, if all known reserves ofrecoverable CBM were extracted, is estimated to be55.5 billion barrels [3].

Management of PWs has become a major factor inthe feasibility of gas field development [4]. Todaymore than 60% of the PWs are reinjected into wellsthat are geologically isolated from underground sour-ces of drinking water. Reinjection costs vary from$0.40 to $1.75 per barrel, while installation costs varyfrom $400,000 to $3,000,000 per well [5]. Surface dis-charge of large volumes of PWs has already hadmany adverse environmental affects such as streambank erosion, change in natural vegetation, salt depo-sition, etc [6].

Development of economical treatment processesfor PWs is vital for two reasons:� Regions where CBM and other nonconventionaloil and gas exploration are occurring in the west-ern USA lack drinking and irrigation water. Thissituation will become much more severe as thepopulation in the western states increases.

� Economical and environmentally friendly methodsof disposal of PWs are vital in order to prevent� 2008 American Institute of Chemical Engineers

Environmental Progress (Vol.27, No.2) DOI 10.1002/ep July 2008 173

serious environmental damage and allow develop-ment of new energy resources.Currently most PW is treated as waste; yet, as the

demand for fresh water (especially in the westernstates) surpasses available supplies, treatment of PWcould provide a source of new water for beneficialuse. The composition of PWs varies widely since theyoriginate from different geological formations. Table1 gives some typical species that may be present.

Conventional treatment of PWs has included grav-ity separation and skimming, dissolved air flotation,deemulsification, coagulation and flocculation [7–10].However, there are numerous disadvantages associ-ated with these unit operations. For example, gravityseparation may not produce effluents that meet dis-charge limits; use of chemical emulsion breakersrequires customization for each site to determine thetypes and quantities of chemicals needed; large vol-umes of sludge are often produced; and operationcosts can be high.

The use of membrane filtration processes such asnanofiltration and reverse osmosis offer many advan-tages.� The technology is more widely applicable across arange of industries.

� The membrane is a positive barrier to rejectedcomponents, thus variation in feed water qualitywill have a minimal impact on permeate quality.

� No addition of chemicals is required.� Membranes can be used in process to allow recy-cling of selected waste streams.Both polymeric and ceramic membranes have

been used for PW treatment [11]. In general, poly-meric membranes are cheaper, but they must beoperated at lower temperatures and are less tolerantof harsh cleaning conditions. Here we have studied

the use of commercially available polymeric nanofil-tration and low-pressure reverse osmosis membranes.The membranes have different surface roughness andrejection behavior for salts. These membranes areused for brackish water treatment and may be wellsuited for treatment of PWs [4, 12]. The change inpermeate flux as a function of time at constant pres-sure driving force has been determined for three dif-ferent PWs. A reduction in permeate flux may berelated to SEM images that indicate deposition of fou-lants on the membrane surface.

EXPERIMENTAL

PW was obtained from three sources in Colorado.The PWs used in these experiments were not kept inan oxygen free environment. The two PWs, labeledCBM1 and CBM2, were from CBM manufacturingfacilities in Walsenburg, southern Colorado. The thirdPW, labeled ODG, was obtained from Wellington innorthern Colorado and is associated with oil produc-tion. The PWs were characterized using InductivelyCoupled Plasma Emission Spectroscopy (ICP AES). Inaddition, total organic carbon (TOC) and total dis-solved solids (TDS) were also determined.

Three membranes, two nanofiltration and one lowpressure reverse osmosis, were donated by FilmTecCorporation, Dow Chemical, Midland, MI. TheseFilmtec membranes consist of three layers: a polyes-ter support web, a microporous polysulfone inter-layer and an ultra thin polyamide barrier layer on thetop surface (See Table 2).

NF270 is a piperazine-based semi-aromatic poly-amide thin film composite membrane while NF90and BW30 are fully aromatic polyamide thin filmcomposite membranes. Tang et al. [14] have meas-ured the following root mean square roughness for

Table 1. Common components in PW.

Organic compounds Inorganic components Production chemicals

Aliphatic, aromatic,polar compounds,e.g. fatty acids, oil,grease, benzene, phenol

Na1, K1, Ca21, Mg21,Cl2, SO4

22, CO322, silicates

(H4SiO2), borates (H3BO3)

Emulsion breakers toimprove separability of oiland water, corrosion inhibitors

Table 2. Properties of membranes tested at 30 L m22 h-1, 2000-ppm solute, 258C, pH 7–8 and 10% recovery[13].

Membrane

Feedpressure(psi) Rejection (%) Comments

NF270 50 NaCl 80CaCl2 50MgSO4 99.3

MWCO between 200-400: NF270 hashigher MWCO and NF90 is the tighter pore size

NF90 70 NaCl 90–96BW30 150 NaCl 98.6 Brackish water treatment membrane operates

at lower pressure than salt watertreatment membranes

CaCl2 98.8MgSO4 99.7

174 July 2008 Environmental Progress (Vol.27, No.2) DOI 10.1002/ep

the NF270, NF90, and BW30 membranes 9.0 6 4.2,129.5 6 23.4, and 68.3 6 12.5 nm, respectively. Inaddition they measured the zeta potential of the vir-gin membranes. At pH 7.0 the zeta potential of theNF270, NF90, and BW30 membranes are 232.6,226.5, and 25.2 mV, respectively. The negativecharge on the membrane is most likely due to thedeprotonation of the carboxylic and amine groups athigher pH [15–17].

Prior to use, the membranes were not precom-pacted. However the membranes were washed witha 1:10:9 (volume basis) mixture of sulfuric acid/etha-nol/DI water [18]. Dead end filtration experimentswere conducted using a YT30 142 HW, MilliporeCorp, Bedford, MA filtration cell. The membrane di-ameter was 140 mm. The feed was pressurized usinga nitrogen cylinder attached to the feed reservoir. Thefeed volume was 500 mL. All experiments were con-ducted at room temperature.

In the first series of experiments, the feed pres-sures varied from 20–100 psi. Once the systemreached the required pressure, the pressure was heldfor 2 min to ensure equilibrium was reached. Nextthe permeate was collected over a 5-min interval.Then the pressure was set to the next value and theprocedure repeated. In the second series of experi-ments the variation of the apparent salt rejection (asmeasured by conductivity of the permeate) with timeat a constant pressure of 80 psi was determined. Thepermeate was collected for 20-min intervals and theTDS and conductivity determined using a handheldconductivity/TDS/temperature Meter, Oakton Instru-ments, Vernon Hills, IL. Apparent salt rejection wascalculated by using the following equation.

Apparent salt rejection ¼ ðCf1 � Cf2Þ=Cf1 3 100% (1)

where, Cf1 is the original conductivity of the feedwater and Cf2 is the conductivity of the permeatesample.

Clean and fouled membranes were inspected usingField Emission Scanning Electron Microscopy (FESEM).To prevent collapse of the pores during sample prepa-ration, all membranes were immersed in ethanol for 4h. The ethanol was then replaced by dry ice for about1 h after which the system was heated to critical pointof dry ice. Then, all membranes were coated with 10nm of gold. FESEM images were taken using JSM-6500F FESEM equipped with an in-lens thermal fieldemission electron gun. All membranes were imaged ata voltage of 15 kV and a magnification of 10,000times.

RESULTS

Table 3 gives the water quality results. The ODGPW has a much higher TDS, TOC and salinity thanthe CBM PWs from southern Colorado. Since the pHof the PWs tested is around 7–8, the all three mem-branes are negatively charged.

Figures 1–3 give the variation of permeate fluxwith pressure. Figure 1 gives results for CBM 1, Fig-ure 2 for CBM2 and Figure 3 for ODG PW. As Ta

ble

3.PW

analysis.

Sample

Ca

Mg

Na

KP

Al

Fe

Mn

Mo

Cd

Cr

Sr

BBa

Pb

Si

VTOC

TDS

pH

CBM

11.7

0.01

314

1.2

0.01

<0.01

0.01

<0.01

<0.01

<0.005

<0.01

0.06

0.26

<0.01

<0.005

7.4

<0.01

68.8

675

8.52

CBM

22.4

0.01

250

1.3

0.08

0.75

0.05

0.01

<0.01

<0.005

<0.01

0.08

0.21

0.01

<0.005

10.1

<0.01

47.7

547

8.41

ODG

11.0

1.30

782

10.5

<0.01

2.20

0.07

0.02

<0.01

<0.005

0.01

1.00

2.90

10.1

0.008

14.4

<0.01

136.4

1940

8.70

Allvaluesarein

mg/m

L.


expected the permeate flux increases with increasingpressure. As can be seen the looser (larger pore size)NF270 membrane gives the highest flux for all threePWs. The permeate flux at the same pressure is low-est for the ODG water for all three membranes.Though similar the permeate flux for all three mem-branes is higher for CBM 2 compared with CBM 1PW.

Figure 4 gives the variation of apparent salt rejec-tion, as measured by the conductivity of the perme-ate, with time. As expected, as the contents of thefeed reservoir are concentrated, the apparent saltrejection decreases since Eq. 1 uses the initial con-ductivity of the feed. In general, the salt rejection at

any time is greater the smaller the pore size of themembrane. The highest salt rejection at any time isobtained for the ODG PW using the BW30 membranefollowed by the NF90 membrane. Since rejection bythe NF270 membrane will be less than the NF90membrane, salt rejection data for the NF270 mem-brane are not included.

FESEM images are given in Figures 5–10. Figures 5and 6 give FESEM images for a clean and fouledNF270 membrane. Figures 7 and 8 give analogousresults for the NF90 membrane while Figures 9 and10 give results for the BW30 membrane. ComparingFigures 5, 7, and 9 with 6, 8, and 10 deposition ofsolute species present in CBM1 PW on the membranesurface is clearly visible. Further the NF270 mem-brane has the smoothest surface. Surface roughness

Figure 2. Variation of permeate flux with pressure forCBM 2 PW. The looser the membrane pore structure(larger pore) the higher the permeate flux. Diamonds,squares, and triangles represent results for NF270,NF90, and BW30 membranes.

Figure 3. Variation of permeate flux with pressure forODG water. The looser the membrane pore structure(larger pore) the higher the permeate flux. Thelowest permeate fluxes were obtained with the ODGwater. Diamonds, squares, and triangles representresults for NF270, NF90, and BW30 membranes.

Figure 1. Variation of permeate flux with pressure forCBM 1 PW. The looser the membrane pore structure(larger pores) the higher the permeate flux.Diamonds, squares, and triangles represent resultsfor NF270, NF90, and BW30 membranes.

Figure 4. Variation of salt rejection with time. Solidsymbols are for NF90, open symbols are for BW30membrane respectively. Diamonds, squares, and tri-angles represent CBM1, CBM2, and ODG PWs.


Figure 5. FESEM image of clean NF270 membrane.

Figure 6. FESEM image of fouled NF270 membrane.The membrane was used to treat CBM1 PW.

Figure 7. FESEM image of clean NF90 membrane.

Figure 8. FESEM image of fouled NF90 membrane.The membrane was used to treat CBM1 PW.

Figure 9. FESEM image of clean BW30 membrane.

Figure 10. FESEM image of fouled BW30 membrane.The membrane was used to treat CBM1 PW.


can have a large effect on the degree of membranefouling [19].

DISCUSSION

As the PWs used in these experiments were notkept in an oxygen free environment the pH of theoriginal PW may be different. In these experiments itwas decided not to store the PW under a controlledenvironment (e.g. under high purity argon) as actualtreatment of PW will occur well after it is firstexposed to the atmosphere.

Table 3 indicates that there is great variability inthe quality of PW. TDS as high as 170,000 mg L21

have been reported [20]. The TDS of a PW dependsstrongly on the geological formation and the origin ofthe water. Thus treatment requirements will vary. Fur-ther the degree of treatment also depends on thebeneficial use for the PW. The recommended TDS forpotable water is 50 mg L21 and 1000–2000 mg L21

for other beneficial uses such as stock ponds or irri-gation.

Comparing Table 1 and Figures 1–3 it appears thatthe higher the TDS the lower the permeate flux. Fur-ther the permeate flux is much higher for the NF270membrane. This is expected given the NF270 mem-brane has a much more open pore structure than theNF90 membrane [13]. In fact the permeate fluxes forthe NF90 and BW30 membrane are similar for allthree PWs.

In Figure 4 apparent salt rejection is determinedby using Eq. 1. Since Eq. 1 uses the conductivity ofthe feed and permeate it gives the overall apparentrejection of ionic species not just NaCl. Howeverfrom Table 3 it can be seen that the majority of thecations present are Na1. Consequently Eq. 1 is usedto approximate the apparent salt rejection. Compar-ing Table 2 and Figure 4 it appears that the actuallevel of salt rejection obtained is less than specifiedby the manufacturer. However Figure 4 gives theapparent rejection coefficient based on the initialfeed concentration while the manufacturer gives theactual rejection coefficient based on the actual feedconcentration. At small run times the apparent andactual rejection coefficients approach each other. Fig-ure 4 indicates that the apparent rejection coefficientapproaches the manufacturer’s stated value at smallrun times. Further the test conditions used by themanufacturer are quite different to the real PWstested here. In particular the presence of othercharged species will affect the observed rejection of

salt [21]. It can also be seen form Figure 4 that eventhough ODG PW has a higher Na1 concentration theapparent salt rejection is higher than for CBM 1 andCBM 2 PWs. This is probably due to the much higherTDS of the ODG PW. In an earlier study Wickram-singhe et al. [22] noted that a high TDS improved therejection of arsenic bound to ferric salts during micro-filtration. Using the average value of the apparentrejection coefficient at 20 and 140 min, the salt con-centration in the bulk permeate was estimated and isgiven in Table 4. The results suggest that the perme-ate could be used for beneficial uses such as live-stock watering.

Figures 5–10 indicate that FESEM images may beused to observe membrane fouling.

Our results indicate that the PWs tested here maybe treated with the BW30 or NF90 resulting in a per-meate that that may be used for beneficial uses suchas livestock and crop watering, the feasibility of usingmembranes depends on the degree of membranefouling and consequently the frequency of cleaning.FESEM imaging of the membranes could provide aquick way to observe the degree of membrane foul-ing. In this work we have used dead end filtration toevaluate the performance of three commerciallymembranes. While dead end filtration is a quick wayto assess membrane performance, tangential flow fil-tration data is needed prior to building a commercialfacility.

CONCLUSIONS

Three commercially available nanofiltration andlow pressure reverse osmosis membranes have beenused to treat three PWs. Two of the membranes,BW30 and NF90 produce a permeate that could beused for beneficial uses such as livestock and cropwatering. However the feasibility of using membranesto treat PWs depends on the degree of membranefouling. FESEM imaging of the clean and used mem-branes may be a rapid method to observe the degreeof membrane fouling.

ACKNOWLEDGMENTS

Funding for this work was provided by theNational Science Foundation IIP 0637664 and CBET0651646. Dr William Mickols, Dow-FilmTec, Edina,MN provided the membranes.

Table 4. Salt in bulk permeate estimated using the average apparent rejection coefficient.

Water Membrane

Permeateflux

(L m22 h21)

Permeateflow rate

(mL min21)

Rejectionafter

20 min (%)

Rejectionafter

140 min (%)

Salt in bulkpermeate(mg/L)

ODG BW30 5 1.3 70 50 315ODG NF90 5 1.3 70 30 390CBM1, CBM2, BW30 10 2.6 70 20 175, 140CBM1, CBM2 NF90 10 2.6 60 20 190, 150


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nanofiltration/reverse osmosis for treatment of coproduced waters

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