[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

43 Dissolved Organic Components in Process Water at the Los Banos Desalting Facility

Julius Glater, Lawrence C. Wilson, and Johannes B. Neethling

School of Engineering and Applied Science, University of California, Los Angeles, CA 90024

Agricultural wastewater reclamation is under serious consideration in the California Central San Joaquin Valley. A reverse osmosis (RO) pilot plant at Los Banos was designed with an elaborate pretreatment system to protect membranes from fouling and degradation. This chapter reports a surveillance of dissolved organic materials in process water throughout this plant. Samples were analyzed by gas chro-matography-mass spectrometry (GC-MS) with standard extraction techniques. Untreated water contains a highly complex mixture of essentially nonvolatile organic substances. Following chlorination, the mixture increases in complexity and includes three trihalomethane (THM) fractions, CHCl2Br, CHClBr2, and CHBr3. Bromoform was present at the highest concentration and chloroform, if present, was below the GC detection limit. Dissolved organic concentrations were observed to decline progressively following clarification and filtration. RO membranes tested at Los Banos did not significantly reject THMs reported in this study.

(CONVENTIONAL PRETREATMENT for reverse osmosis membrane plants involves unit processes for removal of suspended solids, p H adjustment, control of scale, and suppression of biological fouling. These strategies are generally adequate for most brackish and sea-water applications, but other factors must be considered with feedwaters of greater chemical complexity.

0065-2393/89/0219-0783$06.00/0 © 1989 American Chemical Society

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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

784 AQUATIC HUMIC SUBSTANCES

Such a situation arises in the treatment of wastewaters containing high levels of dissolved organic compounds. The present study is concerned specifically with reclamation of agricultural drainage water in the San Joaquin Valley of California. Desalination of this type of water by reverse osmosis (RO) technology has been under serious study since 1971. Pilot plants operated at Firebaugh, California (I); Yuma, Arizona (2); and Los Banos, California (3) have provided useful data, but a variety of technical problems remain to be solved. One of the principal issues involves dissolved organic substances in feedwater and products resulting from the reaction of these compounds with the chlorine used to control biological fouling.

Studies at the Yuma Desalting Facility (2) suggest that cellulose acetate membrane performance may be adversely affected by the presence of various dissolved organic compounds in process water. Unusually high rates of product flux decline were observed during the 1980 proof testing, amounting to approximately 2-4% per 1000 h of operation. Continuation of this process was estimated to shorten membrane life by more than 50%.

According to Moody et al. (2), organic compounds causing flux decline were classified as purgeable and nonpurgeable. The first category designates low-molecular-weight halocarbons, and the second includes all other dissolved organic materials. Following those observations at Yuma, a laboratory study reported by Milstead and Riley (4) showed that both bromoform and chloroform cause flux decline in cellulose acetate membranes. These experiments were conducted in a synthetic Yuma feedwater at an organic concentration approximately 1000 times the ambient level.

This chapter wil l summarize our efforts in sampling and characterization of dissolved organic components at the Los Banos Desalting Facility. The fate of feedwater organic substances through the process flow train wil l be reported. Figure 1 shows the schematic process flowsheet of the Los Banos Desalting Facility. Pretreatment included chlorination, clarification, dual-media filtration, and ion exchange. Dechlorination, threshold treatment, and acid injection were also applied just prior to RO processing. The desalting unit consisted of three RO installations, representing a wide variety of membrane types and configurations.

Feedwater for the Los Banos Desalting Facility results primarily from subsurface tile drainage of the Westlands Irrigation District. Composite drainage is collected in the San Luis drain. Routine analyses of this water over several years show total dissolved solids (TDS) values averaging approximately 9600 m g / L (5). Dissolved organic carbon (DOC) levels have also been measured and show average values of approximately 7.75 mg/L. These levels are more than twice those reported at the Yuma Desalting Facility (6). A wide variety of dissolved organic compounds have also been identified at Yuma. The principal compounds are humic substances, which make up more than 25% of the D O C . The high D O C values reported at Los Banos suggest that this water may contain even greater concentrations

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BRIN

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of similar substances. The work presented here represents the first effort to characterize organic compounds and their chlorination products in San Joaquin Valley agricultural drainage water.

Analytical data derived from this study will be useful for planning RO product water management and brine disposal. It should also help to establish guidelines for future studies of membrane-chemical interaction.

Experimental Details All wastewater samples reported in this chapter were collected at Los Banos between May and August 1986. Samples were taken directly from the San Luis drain and at four additional points in the pretreatment train identified as A, B, C, and D on Figure 2. Limited sampling was also conducted on various fractions of RO permeate and brine.

Two types of samples were obtained at each location. The first was collected in 120-mL bottles at ambient pH. The bottles were fitted with poly(tetrafluoro-ethylene) (Teflon) airtight caps, and care was taken to exclude all air from the system. These samples were scheduled for isolation of volatile organic compounds via pentane extraction. The second type of sample was collected in 1-gal bottles containing 4.0 mL of concentrated sulfuric acid. The resulting diluted sample pH was less than 2.0. These samples would be extracted with methylene chloride and used to detect acidic and neutral organic species over the entire gas chromatographable range. The two types of samples will be referred to as volatile organic substances and acid-extractable organic substances, respectively.

Organic separations were conducted by gas chromatography (GC) with a gas chromatograph (Varian Vista 6000) using a fused silica column (30 m x 0.32 mm i.d.) packed with a polar stationary phase of polyethylene glycol. Organic substances

DRAIN WATER J INFLUENT

PRIMARY CHLORINATION

j - - A

CLARIFICATION

i · -B

DUAL MEDIA FILTRATION

I · C

CHLORINATION OF CLEARVELL

j~ · D

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I INFLUENT, Κ % C, 0 ARE SAMPLE LOCATIONS

Figure 2. Sampling locations in pretreatment train.

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43. G LATER E T AL. DOM in Process Water at a Desalting Facility 787

eluted from the column were split into parallel streams and detected with both flame ionization detectors and electron-capture detectors (FID and ECD, respectively). An integrator (Hewlett Packard 3392) recorded the detector signals. The FID is generally responsive to organic compounds, regardless of functionality. The ECD is selective to halogenated species over a wide molecular weight range. ECD sensitivity to halogenated organic substances is considerably greater than that of the FID detector.

Samples containing volatile organic materials were concentrated by pentane extraction in an airtight system as described by Glaze (7). Extract aliquots were injected into the GC, which was programmed as follows: held at 50 °C for 4 min; temperature increased at 10 °C per min up to 140 °C; held for 2 min.

Acid-extractable organic compounds were concentrated by methylene chloride extraction as described by Fam (8). Aliquots of the final extract were injected into the GC, which was programmed as follows: held at 65 °C for 4 min; temperature increased at 4 °C per min up to 250 °C; held for 30-35 min.

The acid extraction technique was used for the Los Banos sample because prior work (8, 9) indicated that agricultural and domestic wastewater contain relatively large concentrations of acidic organic compounds, such as phenols and carboxylic acids. These species and most of their chlorination products are completely proton -ated at pH 2.0 and readily extractable in methylene chloride.

The methylene chloride extraction technique is capable of isolating compounds over a wide molecular weight range. This range includes trihalomethanes (THMs) and other volatile organic substances. Certain disadvantages, however, must be considered when attempting to quantify volatile organic materials. First, some volatile compounds may be lost between sampling and extraction. Second, solvent interference is usually a problem in the volatile GC region. This situation results from high sensitivity of the ECD to chlorinated organic species. Impurities in the methylene chloride solvent are often detected by the ECD and thus interfere with volatile organic peaks.

As a result of these complications, we chose pentane extraction for analysis of volatile organic compounds. Our confidence in this technique, is greater because minimal sample losses occur in airtight containers. In addition, the ECD is unresponsive to the pentane extraction solvent.

Volatile sample fractions were examined by gas chromatography-mass spectrometry (GC-MS) in an attempt to characterize specific compounds present in these samples. The most prédominent compounds isolated corresponded to mixed chloro and bromo trihalomethanes (THMs). MS determinations were conducted with a GC-MS (Finnigan 4000) and data system (INCOS 2300).

THM concentrations were quantified by comparing ECD-generated areas of samples with areas obtained from known THM standards. ECD areas are essentially linear with respect to concentration for the range reported in this chapter. Standards were prepared in water to give fixed concentration levels according to procedures described by the U.S. Environmental Protection Agency (10). These standards were then pentane-extracted and analyzed in the same manner as our wastewater samples. This method was reproducible for replicate samples to approximately 5%.

Results and Discussion

San Luis drain water sampled at the Los Banos Desalting Facility shows average p H and TDS values of 8.2 and 9600 mg/L, respectively. This water is a sodium chloride type, but contains unusually high levels of calcium and sulfate ions. As a result, an elaborate ion-exchange system was used in the

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pretreatment train to prevent calcium sulfate scale deposition on RO membrane surfaces. The schematic process flowsheet is given in Figure 2. Points labeled A , B, C , and D are sample locations throughout the pretreatment train prior to ion exchange.

Figure 3 shows gas chromatograms of water samples collected at the labeled sample locations. Influent samples consist of untreated San Luis

Figure 3. Gas chromatograms of acid methylene chloride extracted water samples collected at locations shown in Figure 2.

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43. G LATER ET AL. DOM in Process Water at a Desalting Facility 789

Drain water. The horizontal axis on each trace represents a temperature profile increasing at the rate of 4 °C/min. This axis also indicates species retention time within the column. Peak heights and, more specifically, areas under peaks are related to relative concentrations of sample species. The two lines on each trace correspond to compounds detected by F I D and E C D detectors. Considerably more detail is shown in the E C D trace because of its greater sensitivity to the halogenated organic compounds that appear in large concentrations following chlorination. Chromatograms shown in Figure 3 were generated from acidified methylene chloride extracted samples and represent both volatile and nonvolatile organic compounds. A wide variety of organic species with molecular weights up to approximately 400 A M U are detected.

Figure 3 indicates that untreated influent drainwater samples contain few GC-detectable organic substances. This condition is especially true of T H M s , which show up clearly on traces A - D . Most GC-detectable organic compounds found in these samples are produced from primary chlorination of canal water. The greatest increase occurs in the T H M fraction. In addition, a wide variety of both chlorinated and unchlorinated organic products show up on these chromatograms. To date, we have not attempted to positively identify any of these species except for T H M s .

Most of the low-molecular-weight organic materials result from cleavage of humic substances present in agricultural drainage water. The humic substances, consisting primarily of fulvic acids, are composed of macromolecules containing a variety of aromatic ring structures. Most prominent among these are carboxylic acids, ketones, ethers, and phenols. The mechanism of T H M production has been studied by various investigators (11-13) and is believed to involve cleavage of phenol and resorcinol moieties in the presence of chlorine.

Morris and Baum showed (14) that methyl ketones are degraded to T H M s via the classical haloform reaction. This reaction is not the only mechanism; a variety of reaction schemes may describe oxidative degradation of humic substances following chlorination. In addition to T H M s , Christman (IS) has characterized approximately 50 chlorinated and unchlorinated low-and intermediate-molecular-weight organic compounds derived from fulvic acid chlorination. According to Christman, less than 15% of the organic products are detectable by gas chromatography.

When bromide ion is present in wastewaters, the resulting T H M s may contain bromine alone or consist of mixed chloro and bromo compounds. The following reaction sequence is involved.

C l 2 + H 2 0 HC1 + HOC1 (1)

HOC1 + B r " - » H O B r + C l " (2)

HOC1 + H O B r + fulvic acids - » chloro + bromo T H M s (3)

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The shift toward more highly brominated T H M s in Los Banos process water can be explained by the relatively high bromide ion concentration in the San Luis drain (approximately 0.3 mg/L).

In addition, reactions 1-3 are all known to be pH-dependent. For Los Banos water (average p H 8.2), however, reaction 2 is known to be considerably faster than reaction 3. This situation results in a higher concentration of H O B r available for reaction with humic substances.

Figure 4 represents a portion of a typical gas chromatogram for pentane-extracted volatile organic components. The sample was collected following primary chlorination at point A on Figure 2. The three major peaks (II, III, and IV) were scanned by MS and identified as mixed chloro-bromo T H M s . Sample identity and relative concentrations are given in Table I. A peak for chloroform (CHC1 3) does not show in Figure 4 because of extremely low concentration.

The total T H M concentration (Table I) is approximately 122 μg /L , which represents only a very small fraction of the D O C value of San Luis drain water. Evidently a large portion of the dissolved organic products do not react with chlorine or are converted to other types of compounds. According to the literature (15), these compounds may be both halogenated and un-

ECD

FID

ILL ISOTHERM 5 0 # C ICO'C !40*C

Figure 4. Gas chromatograms of trihalomethanes in water sample following primary chlorination.

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halogenated products of oxidative cleavage. We are now attempting to determine what portion of the organic components are halogenated and to identify other prominent compounds in the gas chromatographable region.

We have also determined the total acid-extractable organic concentrations at various locations in the Los Banos pretreatment train. Values of total ECD-integrated areas at sample locations A - D relative to the influent sample area are shown in Figure 5. A significant increase, greater than 400%, occurs

Table I. Comparison of Reverse Osmosis Influent THM Concentrations at Los Banos and Yuma Desalting Facilities

Los Banos Yuma* Concentration Percent Concentration Percent

THM ML) of Total ML) of Total CHC1 3

— 0 4 4 CHCl 2 Br 25 21 14 14 CHClBr 2 43 35 40 41 CHBr 3 54 44 40 41 Total 122 100 98 100 "Data are from ref. 6.

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in GC-detectable species following primary chlorination (point A). A significant loss in organic materials follows clarification (point B) and filtration (point C). We are unable to suggest a firm mechanism for this observed decline in organic substances, but it may involve volatility losses or some adsorptive process. This phenomenon needs to be further investigated. E C D areas in Figure 5 are not directly comparable to mass concentrations. This condition is primarily a result of the variable detector response to different types of organic compounds. These data may therefore be considered semiquantitative, but should reflect general changes in overall organic concentrations.

Increased concentrations of chlorinated organic substances in the clear well (point D) occur as a result of rechlorination here. At point A , feedwater was chlorinated at levels between 10 and 20 mg/L. Secondary chlorination (point D) was at 3-4 mg/L. A l l samples were collected at steady-state conditions, and no effort has been made to study the kinetics of chlorine-organic interaction. This increased concentration seems to indicate, however, that some of the precursors remain unreacted in process water.

Prior to shutdown of the Los Banos facility in late 1986, a single series of samples was collected from the RO units in August of that year. These units were in continuous operation for about a week prior to sampling. The R O systems shown in Figure 6 are brine staged and equipped with the membrane types indicated. Sampling locations for influent, brine, and product water are marked on this diagram as points 1, 2, and 3. In addition, some samples were collected following specific membrane modules to compare the selectivity of various membrane types.

Figure 7 illustrates G C traces for acid-extractable organic materials at the three locations indicated on Figure 6. These traces show that higher molecular weight organic substances are strongly rejected by the R O membranes in place. Volatile samples confirmed, however, that T H M s and other small organic substances easily penetrate these RO membranes. Such data is in good agreement with material balances for cellulose acetate membranes reported at Yuma (5).

Conclusions Agricultural drainage water from the San Luis drain in the Central San Joaquin Valley contains a highly complex mixture of gas chromatographable organic compounds. These chemical species are essentially nonvolatile. Following chlorination, the mixture increases in complexity, with a shift toward lower-molecular-weight organic compounds and the appearance of volatile species, including three T H M fractions. These fractions have been identified as C H C l 2 B r , C H C l B r 2 , and C H B r 3 . Bromoform was present at the highest concentration, although the chloroform concentration was below the G C

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ί THM 'REGION

200*C 250'C ISOTHERMAL 65 e C 100* C 150* C 10 20 30 40

RETENTION TIME, min. 50 60

Figure 7. Gas chromatograms of 1, influent; 2, brine; and 3, product samples from RO units (acid methylene chloride extracted samples).

detection limit. Concentrations of T H M species were in similar ratios to those reported at the Yuma Desalting Facility. Production of T H M s is a relatively small, but important, part of the overall chlorine-organic chemistry of agricultural drainage water. Identification of other prominent organic products is presently underway.

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Surveillance of organic compounds throughout the Los Banos flow train has revealed some interesting information. First, the concentration of acid-extractable organic compounds is diminished progressively following clarification and filtration. The exact mechanism of this removal is unclear. Second, the membranes tested at Los Banos did not significantly reject T H M s that carried over into product water. Third, RO membranes strongly reject most other gas chromatographable acid-extractable organic substances. Polyamide membranes appear to be more selective than cellulose acetate membranes.

We are concerned not only with the ability of RO membranes to pass or reject dissolved organic compounds, but also with potential interaction between membrane polymers and these organic substances. Such interactions may seriously affect the performance and life expectancy of RO membranes. We are currently engaged in such studies with model compounds and will report our results in a forthcoming paper.

Acknowledgment The authors acknowledge the support provided for this research by the California Department of Water Resources. Special thanks are due to Brian Smith of D W R for his cooperation in obtaining water samples and for the many useful discussions in his office. We also thank Michael Stenstrom and Kasi Gabbita, who have provided numerous stimuli during the preparation of this manuscript.

References 1. Antoniuk, D.; McCutchan, J. W. Desalting Irrigation Field Drainage Water by

Reverse Osmosis, Firebaugh, California; University of California—Los Angeles: Los Angeles, 1973; Engineering Report 7368.

2. Moody, C. D.; Kaakinen, J. W.; Lozier, J. C.; Laverty, P. E. Desalination 1983, 47, 239.

3. Smith, B. E.; Brice, D. B.; Kasper, D. R.; Everest, W. R. "Agricultural Waste-water Desalting in California DWR: Test Facility Description", Proc. 10th Annu. Conf. Water Supply Improvement Assoc. Honolulu, HI; Vol. II, July 1982.

4. Milstead, C. E.; Riley, R. L., UOP, Fluid Systems: San Diego, unpublished report.

5. California Department of Water Resources, San Luis Drain Water Quality Analyses, San Joaquin District: Sacramento, 1985.

6. Malcolm, R. L.; Wershaw, R. L.; Thurman, Ε. M.; Aiken, G. R.; Pickney, D. J.; Kaakinen, J. Reconnaissance Samplings and Characterization of Aquatic Humic Substances at the Yuma Desalting Test Facility, Arizona, USGS Water Resources Investigations, 1981; Report USGS/WRI 81-42.

7. Glaze, W. H.; Rawley, R.; Burleson, J. L.; Mapel, D.; Scott, D. R. In Advances in the Identification and Analysis of Organic Pollutants in Water, Vol. 1; Keith, L. H., Ed.; Ann Arbor: Ann Arbor, 1981; pp 267-280.

8. Fam, S. Ph.D. Thesis, University of California—Los Angeles, 1986.

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9. Miller, J. W.; Uden, P. C. Environ. Sci. Technol. 1983, 17, 150. 10. U.S. Environmental Protection Agency. Fed. Regist. 1984, 49, 209. 11. Rook, J. J. Environ. Sci. Technol. 1977, 11, 478. 12. Peters, C. J.; Young, R. J.; Perry, R. Environ. Sci. Technol. 1980, 14, 1391. 13. Hirose, Y; Oldtsu, T.; Kanno S. Chemosphere 1982, 11, 81. 14. Morris, J. C.; Baum, B. In Water Chlorination—Environmental Impact and

Health Effects, Vol. 2; Jolley, R. L., Ed.; Ann Arbor Science: Ann Arbor, 1978; pp 29-48.

15. Christman, R. F.; Johnson, J. D.; Norwood, D. L.; Liao, W. T.; Hass, J. R.; Pfaender, F. K.; Webb, M. R.; Bobenrieth, M. J. Chlorination of Aquatic Humic Substances, U.S. Environmental Protection Agency R&D Report EPA—600/32-81-016, 1981.

RECEIVED for review September 17, 1987. A C C E P T E D for publication February 12, 1988.

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[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

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