Evaluation of Reverse Osmosis To Concentrate Organic Contaminants

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 18, 2016 | http://pubs.acs.org Publication Date: December 15, 1986 | doi: 10.1021/ba-1987-0214.ch021

Evaluation of Reverse Osmosis To Concentrate Organic Contaminants from Water Stephen C. Lynch and James K. Smith Gulf South Research Institute, New Orleans,LA70186 Reverse osmosis for concentrating trace organic contaminants in aqueous systems by using cellulose acetate and Film Tec FΤ-30 commercial membrane systems was evaluated for the recovery of 19 trace organics representing 10 chemical classes. Mass balance analysis required determination of solute rejection, ad sorption within the system, and leachates. The rejections with the cellulose acetate membrane ranged from a negative value to 97%, whereas the FT-30 membrane exhibited 46-99% rejection. Ad sorption was a major problem; some model solutes showed up to 70% losses. These losses can be minimized by the mode of operation in the field. Leachables were not a major problem. Actual recoveries are reported for a field trial in which 9500 L was concentrated to 190 L. IPuBLIC C O N C E R N over trace chemical contamination of drinking water continues to increase each year despite extensive efforts to define and rectify the problem. The major focus of water contamination has shifted from the microbiological problems of several decades ago to the trace organic and inorganic chemical problems of today. Solutions can be developed only as potential classes of problem compounds, such as trihalomethanes, are identified. For example, significant advances have been made toward correcting and reducing levels of trihalomethanes because the problem was studied and defined and solutions were brought forth. Many types of chemical contamination have not yet evolved to this point. In vivo and in vitro toxicity testing methods are used to assess potential adverse health effects of chemical contaminants. These methods have been used to confirm many suspected substances as toxic and carcinogenic. To date, only a small fraction of the organic makeup of most drinking waters has been elucidated and tested. Broad spectrum 0065-2393/87/0214/0437$08.00/0 ® 1987 American Chemical Society

Suffet and Malaiyandi; Organic Pollutants in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1986.


438

ORGANIC POLLUTANTS IN WATER

concentration techniques are now employed to help identify organic components and to prepare samples for toxicity testing. These techniques are necessary because of the very l o w levels (parts per billion) of the organics and the l o w sensitivity of the toxicity assays to these compounds. Reverse-osmosis (RO) concentration is one method used to prepare drinking water concentrates for toxicity testing. Other methods that have been evaluated for concentrating organics in drinking water include adsorbent resins (e.g., X A D s ) , vacuum distillation, supercritical fluid ( C 0 ) extraction, and parfait-distillation. The value of the R O concentration method lies primarily in the ability to concentrate a broad range of different chemical compounds, originally present at trace levels in natural waters, to levels that are of value in toxicological evaluations. The process has the advantage of l o w temperature, no extracting solvent, and no phase change. This method has not been developed as a low-detection-level analytical procedure. 2

Reverse Osmosis Concentration This concentration method uses a polymeric semipermeable membrane and principles of R O to effect separation of water f r o m the organics in drinking water samples. In this process, a water sample is recirculated past the semipermeable membrane while hydraulic pressures exceeding the osmotic pressure are maintained. Water is transported through the membrane under these conditions (permeation). The concentration of solutes continues to build as water is removed f r o m the system. M a x i m i z e d recovery of as many different organic compounds as possible is a primary goal of the R O concentration process. Potential losses of compounds can occur through (1) the permeate water stream, (2) volatile headspaces, (3) adsorption and absorption onto system components, and (4) binding or coprecipitation to or with other compounds such as humic acids or insoluble inorganic salts. Control of most if not all of these factors can be attained through the manipulation of process variables. M a x i m u m recoveries can be significantly affected b y permeation losses. Control of permeation losses must be achieved through the proper choice of membrane type. A membrane with high rejection of most organics must be selected. The membrane rejection (R) for a solute is defined in terms of the permeate solute concentration ( C ) and feed solute concentration ( C ) : p

F

R = [1 - ( C 7 C ) ] F

(1)

Recovery of organic solutes can be predicted by the following equation for R O concentrations operated in batch modes (I):

Mm f

= (CfVf)Z(CfVf) = (Vf/Vf)"-'


(2)

21.

LYNCH A N D SMITH

RO To Concentrate Organic Contaminants

where M and M , are the final and initial masses, respectively; Cf and C f are the final and initial concentrations, respectively; Vf and Vf are the final and initial volumes, respectively; and R is the membrane rejection. The exponential nature of the recovery response is represented graphically in Figure 1. The necessity for selecting membranes with high organic rejections is quite apparent. For example, in a 50-fold concentration, the recovery of a compound w i l l not exceed 70% unless membrane rejection of that solute exceeds 0.9 (90$ rejection). Asymmetric cellulose acetate membranes were developed in the early 1960s by Loeb and Sourirajan (2). For more than a decade, cellulose acetate and its blends were the only commercially available R O membranes. Improved membranes (with respect to operating p H , biodégradation, compaction, and organic compound rejection) were developed in the early 1970s (3). These membranes used aromatic f


439

1.0 ι

1

1

1

1

r

Membrane Rejection (R) Figure 1. Theoretical recovery versus rejection for 50X concentration.



440


polyamide polymer. Both cellulose acetate and the i m p r o v e d polyamide were used i n early R O concentrations (4). In 1977, the first thin-film composite membranes started to appear on the market (5). The com posite membranes offered potential for enhanced R O recovery of organics. Improved organic rejections result f r o m the chemical nature of the newer p o l y m e r i c materials i n the nylon and thin-film composite membranes. Data b y Chian and Fang (6) represented i n Figure 2 illustrate different membrane rejections for a variety of organic c o m -

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Figure 2. Rejection performance of five different RO membranes (6). A, methanol; B, aniline; C, formaldehyde; D, methyl acetate; E, acetic acid; F, urea; G, ethanol; H acetone; I, hydroquinone; J, isopropyl alcohol; K glycerol; L, sodium chlonde; Ai, ethyl ether; N, phenol. Conditions: pressure, 40.8 atm; temperature, 24 °C; feed, 0.30 gal/min. y


y


21.

LYNCH A N D SMITH


441

pounds and membrane types. Solute concentrations were maintained at the parts-per-million level for the majority of the compounds studied. Several points are of interest in Figure 2. First, sodium chloride salt rejection cannot be used as a guide for potential organic rejection. However, membranes with l o w salt rejection are unlikely to exhibit acceptable rejection of organics. The three cellulose acetate membranes demonstrated markedly lower overall rejection of organics than the newer nylon and composite membranes. Several compounds were even preferentially transported through the cellulose acetate membrane, as indicated b y negative rejection values. N e w e r membranes not based on cellulose acetate offer potential for i m p r o v e d recovery through their enhanced rejection of organic compounds. Proper control and design of process variables can improve recoveries significantly. Volatile chemicals can be better retained with suitable equipment closures and regulation of operating temperatures. Materials of construction and conditioning of the process equipment may w e l l compensate for compounds that can be lost because of adsorption-absorption. Proper operation of process variables such as p H and appropriate use of water softening (by a membrane process such as Donnan softening) can prevent inorganic chemical precipitation and consequent losses of organic compounds due to entrapment, coprecipitation, and degraded system performance (7). Factors as subtle as mode of operation can significantly affect the ultimate organic recovery (J). Figure 3 compares predicted recoveries for concentration processes operated in a batch mode (where the total volume to be reduced is processed in discrete sequential batches) and a less efficient continuous mode (where the total volume to be reduced is processed continuously b y maintaining a w o r k i n g volume with makeup rate equal to permeate rate). A 90% membrane rejection is assumed. Batch processing is the preferred procedure for large-volume samples and higher concentration levels at which the mode becomes critical. In R O concentration, as with all separation methods, complete recovery of all compounds is not possible. R O offers several distinct advantages for the concentration of organics in water for the analysis and assessment of health effects. These advantages are the following: 1. A sample can be concentrated without removing the solutes from the aqueous matrix. 2. N o phase changes are required. 3. It is a low-temperature process. 4. Large volumes can be processed. 5. H i g h concentration factors can be achieved. 6. The system has been demonstrated in the field.


442


I

I

1

1

1


Batch

-

Continuous

1 10

1 20

1 30

1 40

1 50

Concentration Factor (Vj / V f )

Figure 3. Theoretical recoveries for batch and continuous processing modes (membrane rejection = 90%).

7. A broad spectrum of organics is recovered. 8. Inorganic ions can be removed concurrently with organic concentration.

Factors Affecting Reverse-Osmosis Concentration T o provide a more comprehensive understanding of the use of R O as a method for concentrating organics from water supplies, several factors must be addressed: 1. What recoveries of specific organic compounds can be expected i n a R O concentration process? What recoveries are actually observed? 2. What are the membranes' inherent rejections of different classes of organic compounds that may be found in water at trace levels? C a n measurable improvements i n the rejection of organics be expected from newer membrane materials?


21.

LYNCH A N D SMITH


443


3. What are the magnitude and probable cause of loss of organic compounds during the concentration process? 4. What materials are added to concentrates through leaching of system components, and does flushing remove these compounds from the system? 5. What effect do disinfectants, such as chlorine, have on leachable organic compounds? This chapter provides some insight into factors affecting RO concentration of organics present at trace levels in water. The information was compiled primarily from data generated in a study comparing alternate methodologies for preparing organic drinking water concentrates (8), which was sponsored by the U.S. Environmental Protection Agency (USEPA) Health Effects Research Laboratory.

Experimental Model Compounds. A spectrum of organic compounds incorporating varied chemical functionalities (acidic, amine, aldehyde, hydroxyl, aromatic, etc.) and aqueous solubilities was selected to evaluate the RO process. Membrane-screening and concentration test series using two starting levels of model solutes were performed in this study. Table I lists specific model compounds, target concentrations, and methodologies developed for the analysis of the compounds. Higher levels (2500 and 250 ppb) were used in the early screening test, whereas the low levels (50 and 5 ppb) were starting solute concentrations in actual concentration experiments. All evaluation studies were performed by using trace quantities of the organic compounds spiked into a synthetic tap water matrix. The inorganic makeups of the synthetic tap waters used for the screening test and concentration test series are shown in Table II. Levels of inorganic salts used in the screening test, in which concentrations were not allowed to build up, were near the saturation point for the least soluble salt, calcium sulfate (i.e., 5 times the base levels). This condition simulates the worst case with respect to any potential organic salting-out effects that might exist. Analytical methodologies listed in Table I were optimized for the conditions present in samples generated during this study (i.e., low concentrations of model solutes, high inorganic salt level, high humic acid backgrounds, and presence of acetone carrier solvent). Calibration curves were constructed to establish response linearity and detection limits. Interferences were investigated and sensitivities were maximized to provide reliable analysis of the model compounds. Despite these efforts, limits of detection and low concentrations in the water matrix and permeated water samples resulted in inadequate analytical definition for some organic solutes. Membrane-Screening Test. Membrane-screening tests were performed to evaluate (1) the adequacy of flushing procedures, (2) the rejection of model compounds by the RO membrane, and (3) losses of model compounds to system components via adsorption, volatilization, solubility artifacts, or other phenomena. The apparatus shown in Figure 4 was used to complete the membrane-



2,6-(u-£err-butyl-4-methylphenol chloroform

trimesic acid stearic acid humic acid glycine furfural quinoline caffeine 5-chlorouracil glucose 2,4'-dichlorobiphenyl 2,2',5,5'tetrachlorobiphenyl bis(2-ethylhexyl) phthalate 1 -chlorododecane biphenyl isophorone anthraquinone methyl isobutyl ketone 2,4-dichlorophenol

Model Compound

"See text for description of these test series. H P L C denotes high-performance liquid chromatography. N T means not tested.

c

Trihalomethanes

Phenols

Ketones

Esters Hydrocarbons

Carbohydrates Chlorobiphenyls

Aldehydes Amines

Acids

Chemical Cfoss

C

2500 2500 2000 2500 2500 2500 2500 2500 NT 250 2500 2500 250 2500 2500 2500 2500 2500 2500 2500

0

Screening Test

2000 50 50 50 50 50 50 5 50 50 5 50 50 50 so 50 50 50

50

50

a

Concentration Test

Concentration (μg/L)

HPLC GC, GC-MS HPLC fluorescence HPLC GC, GC-MS GC, GC-MS HPLC liquid scintillation GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC, GC-MS GC

Method of Detection

Table I. Model Compounds and Conditions Used in Evaluation of the RO Concentration Method


21.

LYNCH A N D SMITH


445

Table II. Inorganic Makeup of Synthetic Tap Water Used in Laboratory Test Concentration (mg/L) Ion Na Ca HCO3" S0 ~ +


2+

4

cr

2

Total

Screening Test

Concentration Test

96 241 254 424 114 1130

19.2 48.1 50.8 84.7 22.7 226

0

°Salt levels at beginning of concentration test are given.

screening studies. Of particular interest is the equipment shown in the righthand portion of the illustration, the RO circulation loop. The system (55-gal stainless steel reservoir, high-pressure pumps, RO membrane, and associated control instrumentation) was operated on total recycle for the screening test. Permeate and reject flows from the RO membrane were returned to the reservoir during tests. The permeate would normally be discarded during an actual RO concentration. No solute buildup (either inorganic or organic) was expected when operating in this mode. Operation of the system on total recycle provided a convenient evaluation of compound losses (adsorptive or volatile), membrane rejections, and leachable substances. A routine screening test first involved a series of flush-discard cycles followed by a period (3-4 h) of total recycle with nonspiked synthetic tap water. Analysis of water at the end of the recirculation period provided an assessment of flush efficiencies and an estimate of leachable organic substances. Membrane rejection of model compounds and losses of organics were determined after spiking and mixing the model compounds into the synthetic tap water. The spiked water was sampled at the beginning and end of an extended period (4 h) of total recirculation to measure loss of solutes. Analysis of this data yielded estimates of combined losses to adsorption, volatilization, precipitation, or other unknown mechanisms. Analysis of a permeate sample collected at the end of the recirculation period provided an assessment of membrane rejection of solutes. The 4-h equilibration period was chosen because actual field use of the method incorporates 50-gal batch cycles requiring approximately 4-h process time. There are indications that low molecular weight unsaturated compounds may require longer equilibration periods before a steady state condition is established (9). Longer equilibrations were not examined in this study. Two different RO membrane types were evaluated in this study. The first was a standard cellulose acetate based asymmetric membrane. The second type, a proprietary cross-linked polyamine thin-film composite membrane supported on polysulfone backing, was selected to represent potentially improved (especially for organic rejection) membranes. Manufacturer specifications for these membranes are provided in Table III. Important considerations in the selection of both membranes were commercial availability, high rejection (sodium chloride), and purported tolerance for levels of chlorine typically found in drinking water supplies. Other membrane types having excellent potential for organic recovery were not evaluated either because they were not commercially


Suffet and Malaiyandi; Organic Pollutants in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1986. Strip Solution Reservoir

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Figure 4. Major components of an RO concentration-Donnan dialysis organic recovery system.

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21.

L Y N C H A N D SMITH Table III.

Membrane


Cellulose acetate

FT-30 polysulfone thin-film composite

RO To Concentrate

447

Organic Contaminants

Manufacturers, Suppliers, and Performance Specifications for Membranes Evaluated in Laboratory Tests Observed Performance

Specifications

Data manufacturer model salt rejection feed concentration pressure module flux temperature percent conversion

Osmonics, Inc. OSMO-112-97-CA 94-97$ 1000 ppm of NaCl 400 l b / i n . 0.38 L/min 25 ° C 102

manufacturer model salt rejection feed concentration pressure module flux temperature percent conversion

Film Tec Corporation BW30-20-26 982 10,000 ppm of NaCl 350 lb/in. 0.38 L/min 25 ° C 52

2

2

—

972 500 ppm (mixed salts) 400 l b / i n . 0.38 L/min 26 ° C 42 2

— —

99.22 500 ppm (mixed salts) 400 lb/in. 0.94 L/min 26 ° C 42 2

available or because they did not possess the required chemical resistance to chlorine. Membrane Concentration Test. Process potential was demonstrated by concentrating 500 L of synthetic tap water spiked with trace levels of the model compounds. A 50X volumetric concentration was achieved by reducing the sample volume from 500 to 10 L. The recovery of model compounds and membrane rejection of compounds were evaluated, and the location of system losses was approximated. The experimental design is presented schematically in Figure 5. Concentration experiments were performed by using the system shown in Figure 4. It was necessary to maintain the concentration of inorganics below precipitation levels. This condition was achieved by using a secondary membrane process, Donnan softening. The original 500-L sample was concentrated in three 167-L batches to an approximate concentration factor of 12.6X; then the sample was composited and further processed. During this step, 460 L of permeate water was discarded. At this point, the Donnan softening was performed. After a fixed Donnan softening period adequate to prevent precipitation, the intermediate concentrate was further processed to 10 L and evaluated for model compound recoveries. RO feed and permeate samples were collected at various volumetric concentration factor stages during the process. The RO feed samples were representative of the concentrate developing in the process reservoir.

Remits and Discussion Model compounds, selected by the Health Effects Research Laboratory at the USEPA, represent a broad spectrum of chemical classes and include several compounds of current environmental interest. Spike

American Chemical Society Library 1155 16th St, N.W.

Suffet and Malaiyandi; Organic Pollutants in Water Washington, DSociety: . L 20036 Advances in Chemistry; American Chemical Washington, DC, 1986.

448


5 0 0 Liters

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973> sodium chloride). Two types of membranes, cellulose acetate and FT-30, have shown short-term (-75 -73 -8.7 -47

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+9.5 -85 +19 +203 -22 0 -3.8 -50 -93 -31 -65 +29 -68

—

FT-30 (618-09) No Humics

CA (618-30) With Humics

CA (618-04) No Humics

-19 -75 -38

—

-84 -84 -83 0

— .__

-24 +4.7

—

_

_

-70

FT-30 (589-81) No Humics

-14.5 -70 -30

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b

-1.5 -70 to - 7 3 +12

Average Loss

-21 -1.2 -3.1 -48 to - 7 0 -80 to - 9 0 -50 to - 6 8 -77 -2.9 -65

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—

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FT-30 (618-34) With Humics

h

N O T E : Some original target compounds (see Table I) are not included because of inconsistent or incomplete data. Positive values suggest gains in solute levels during recirculation period. C A denotes cellulose acetate; — means insufficient data to calculate values. Data were too scattered to assess an average percent loss.

0

Trihalomethanes

Phenols

Ketones

Hydrocarbons

Chlorobiphenyls Esters

Aldehydes Amines

Acids

Chemical Class

CA (589-85) No Humics

0

Typical Losses (Percent)

Table VII. Percent Solute Adsorption Losses (Gains) Experienced During Screening Test



456


support, and other wetted surfaces) may play a primary role in these losses. A l l values are expressed as percentages of the initial solute mass. The adsorption losses (%) shown in Table V I I were used to calculate the amount of solute taken up b y a freshly flushed system. F i e l d application of the R O concentration method incorporated conditioning periods in w h i c h membranes and other system components were exposed to the sample (and its concentrates) to satisfy and minimize adsorptive solute loss. Tables V I I I , I X , and X present mass balance data for three concentration tests. T w o tests (one with each membrane type) were completed without humic acid, and the third was a repeat of the F T - 3 0 composite membrane test including humic acid. Referral to Figure 6 w i l l help clarify the first six columns in Tables V I I I - X . C o l u m n 1 ( M f ) and the sum of the components (column 6) were used to calculate mass accountability values (column 7). Although scattered, accountability of mass was good, considering the complexity of the system and the trace levels at which this work was performed. Final sample mass (Mf ) and initial concentrate mass, adjusted for expected adsorption losses (Mf), were used to calculate total mass recovery (see last column in Tables V I I I - X ) . These values reflect efforts to correct recoveries for adsorption losses. Ultimate recovery of specific organic compounds was calculated from the initial sample mass (Mf ) and final concentrate masses (Mf ) listed in columns 1 and 2 (Tables V I I I - X ) . Calculated recovery values are listed in Table X I . M a x i m u m potential recoveries are projected b y using eq 2 (and experimentally determined solute rejections). The sum of all organic compounds added to this system was

Evaluation of Reverse Osmosis To Concentrate Organic Contaminants

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