Oxygenates in Gasoline - ACS Publications - American Chemical

Chapter 15

Evaluation of the Adsorption Process for the Removal of Methyl tert-Butyl Ether from Drinking Water 1

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Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0799.ch015

Tom C. Shih , Medhi Wangpaichitr , and Mel Suffet * 1

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Environmental Science and Engineering Program and Department of Environmental Health Sciences, School of Public Health, University of California at Los Angeles, 46-081 CHS, Box 951771, Los Angeles, CA 90095-1772

Methyltert-butylether (MTBE) is a fuel additive used as a replacement for lead and an octane booster (1). It was first used in United States in late 1970s and has since become the oxygenate of choice due to economic and supply considerations (2). By 1998, MTBE has become the fourth-highest produced organic chemical in the United States (3). The widespread use of MTBE, combined with its chemical and physical characteristics, has resulted in its detection in ground and surface waters in many urban regions throughout the country (4, 5). The Report of the Blue Ribbon Panel on Oxygenates in Gasoline (6) stated that between 5 and 10% of community drinking water supplies in high MTBE use areas show at least detectable concentrations of MTBE. California Department of Health Services (CAL-DHS) has set primary and secondary drinking water standards of 13 and 5μg/L,respectively. Removal of MTBE from affected drinking water sites can be achieved through several water treatment processes such as air stripping, advanced oxidation, membrane separation, and sorption (7). This paper will briefly review different water treatment options and their relationship to the use of adsorption

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© 2002 American Chemical Society

In Oxygenates in Gasoline; Diaz, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

209 processes. This paper will then develop cost data for adsorption processes, which can be used in a cost effective manner, alone, as a final cleanup or after another treatment such as advanced oxidation processes.


Air Stripping Air stripping can remove more than 99% of MTBE and trichloroethene from groundwater (2). However, air stripping of MTBE requires high air-to-water ratios because the compound is highly water soluble with a low Henry's law constant. Sevilla et al. (8) reported over six times higher air-to-water ratio is required for the treatment of MTBE contaminated waters compared to other petroleum hydrocarbons. Thus, the use of air stripping as a sole process for removing MTBE may not be cost effective. In addition, the process involves mass transfer from water to air phase, producing contaminated air stream that may require further treatment, such as sorption by activated carbon, depending on local air emissions regulations (2).

Advanced Oxidation Processes During advanced oxidation processes (AOPs), ozone, U V light, hydrogen peroxide (H 0 ), metal oxides (such as titanium dioxide, Ti0 ), Fenton's reagent (iron sulfate and H 0 in an aqueous solution at pH = 2.5), and ultrasonic cavitation, or in combination, to produce hydroxyl radical (OH*). MTBE could react with hydroxyl radical and form formaldehyde and /er/-butyl alcohol (TBA). Vel Leitner et al. (9) indicated that effective removal of MTBE of greater than 80% from water can be achieved using peroxone (ozone/peroxide) oxidation. Liang et al. (10, 11) completed an initial pilot plant study of ozonation and peroxone treatment of MTBE under optimum conditions for taste and odor control. The results for two MTBE concentrations (18 to 76 pg/L) indicated peroxone with 4 mg/L of ozone and 1.3 mg/L of hydrogen peroxide at pH 8.3 could achieve about 80% removal for waters from the State Project and Colorado River. However, bromate is a by-product of this process and exceeded the 10 pg/L MCL. Ozonation alone was also effective at longer detention times (12). Thus, AOPs are a viable option with polishing of any MTBE remaining by GAC, if the bromate is controlled. GAC polishing is needed to assure that any residual MTBE or its breakdown products are not released into the drinking water. In addition, GAC will remove biodegradable organic carbon produced by AOPs, which should be removed before drinking water is distributed to prevent bacterial growth in the distribution system. 2

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Membrane Processes Although nanofiltration (NF) and reverse osmosis (RO) have the potential to remove MTBE from water, they have not been as widely studied as aeration, AOPs, and adsorption. The use of membrane processes, especially RO, in general water treatment applications is not cost effective unless other treatment requirements are included (7). On the other hand, microporous hollow fiber membranes (HFM) have been successfully used to strip various compounds from water (2). HFM can improve the mass transfer rate of MTBEfromwater to air, as contaminated water is pumped through the lumen side of bundled microporous fibers while a vacuum is drawn counter-currently on the outside of the fibers. Compare to air stripping, HFM allows for more efficient transfer of volatile compoundsfromaqueous to gas phase. Studies have shown that mass transfer of volatile organics could be an order of magnitude greater than achievable by packed tower aeration when using HFM (2).

Adsorption Granular activated carbon (GAC) and powder activated carbon (PAC) have been widely used for control of taste and odor in drinking water (13, 14, 15). The application of PAC is more flexible and requires less capital costs than GAC. However, for long periods of activated carbon application, it may be more economical to use GAC. A well designed and maintained GAC column can be operated efficiently for several years to remove low to moderate concentrations of contaminants (16). Adsorption is a proven technology for treating water contaminated with many taste- and odor-causing organics and synthetic organic chemicals (2, 7 7). GAC could be used alone or after any type of AOP to remove residual MTBE, oxidation products of MTBE (e.g. TBA), and biodegradable organic carbon produced by AOPs (e.g. aldehydes).

Treatment Options Many treatment options are described above. However, if any of them are used in a cost effective manner, GAC treatment may be included - if not alone, then as afinalcleanup after another treatment such as AOPs or even applied as an adsorbent for the gaseous phase after air stripping. Therefore of all the treatments knowledge about the cost effectiveness of GAC is needed to evaluate any comparisons. The remainder of the paper will evaluate the cost effectiveness of the GAC adsorbents of choice in a column mode.


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Objectives The present study investigated the cost and effectiveness of the adsorption process, both as a sole process for removing MTBE from drinking water to taste and odor threshold levels ( 0.95). The coconut GAC was further investigated with MTBE, NOM, and TBA in groundwater from Santa Monica, California (total organic carbon [TOC] = 0.5 mg/L). The effects of NOM and TBA on the equilibrium capacity of coconut GAC are shown in Figure 2 (7). Significant reductions in equilibrium adsorption capacity were observed by the addition of NOM and/or TBA, with the competitive effect of NOM on MTBE adsorption greater than TBA since the equilibrium adsorption capacity with TBA in organic-free water is higher than in the presence of ΝΟΜ. The isotherms show the coconut GAC to be the most cost-effective adsorbent for use as a sole process and as a polishing process after advanced oxidation; however, competitive adsorption of TBA and NOM in the background competes with the adsorption of MTBE to coconut GAC (7). Consequently, further evaluation of the most cost-effective adsorbent, the coconut GAC was conducted using two candidate coconut GACs, the PCB (Calgon) and CC-602 (U.S. Filter) GAC in both organic-free water and in two groundwater sources and a surface water source. The water quality parameters 2



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-Φ-BituI Hfr-Bitu II H k - lignite -*-wood -*-peat -•-coconutl

10 100 Concentration in ug/L

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Figure 1. Isotherm of 6 GACs with 1,000 pg/L influent MTBE in organic-free water. Bestfitline is shown for clarity. Actual data points are not shown (Adapted with permission from reference 7. Copyright American Water Works Association).



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_ 4.5 C φ η ο 4 CO

ω m

η

2

3.5

ca

σ> c 3 h

0FW4

'•S

SMW

es

ο g) 2.5 ο

MTBE only MTBE + TBA

0.5

1 1.5 2 Log-Concentration (ug/L)

2.5

Figure 2. Isotherm ofcoconut GAC (GRC-22) in organic-free water (OFW in Santa Monica water (SMW) with MTBE (1,000 pg/L) or MTBE and tert alcohol (100 pg/L) (Reproduced with permission from reference 7. Cop American Water Works Association).


214 for the natural water sources are shown in Table I. Each of these water sources has different sources of NOM; Le. South Lake Tahoe Utility District [SLTUD] (pine forest), Arcadia Well Field [ARWF] (groundwater from desert flora and urban runoff), and Lake Penis [LP] (Colorado River Water stored in a lake environment which changes with the season and nutrient levels).


Table I. Analysis of Water Quality Parameters Water Source ARWF SLTUD LP

pH 7.8 7.9 8.5

Conductivity TOC (pmhos/cm)(ppm) 1130 1.0 77 0.2 640 3.2

UV Abs 0.008 0.004 0.068

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SUVA" MTBE 0.8 2.0 2.1

TBA

(m/L)JML.

Oxygenates in Gasoline - ACS Publications - American Chemical

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