[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Trihalomethane Precursor and Total Organic Carbon Removal by Conventional Treatment and Carbon

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  • 34 Trihalomethane Precursor and Total Organic Carbon Removal by Conventional Treatment and Carbon

    Benjamin W. Lykins, Jr., and Robert M . Clark

    Drinking Water Research Division, Risk Reduction Engineering Laboratory, U.S. Environmental Protection Agency, Cincinnati, O H 45268

    Data from four water-treatment plants were used to describe the performance of conventional treatment and granular activated carbon for removing trihalomethane precursors to meet various treatment goals. Also presented are data for total organic carbon removal, which has been suggested as an organic surrogate for measuring the effectiveness of water treatment. Conventional treatment, as used in the four water-treatment plants evaluated, substantially reduced total organic carbon and trihalomethane precursor concentrations. Granular activated carbon may be a treatment alternative to consider for meeting trihalomethane standards as low as 50 g/L.

    DlSINFECTION BYPRODUCTS ARE BEING CONSIDERED FOR REGULATION under the Safe Drinking Water Act Amendments of 1986 (J). One of the most significant disinfection byproducts for utilities that use chlorine is total trihalomethanes (TTHMs). Pressure is growing to reconsider the existing T T H M standard of 0.1 mg/L (100 g/L) and to lower it to some as yet unspecified level. Trihalomethane levels as low as 10-50 g / L may be considered. Utilities may be forced to investigate disinfectants other than chlorine and to evaluate treatment modifications. New options might range from improved conventional treatment to granular-activated-carbon (GAC) adsorption.

    0065-2393/89/0219-0597$07.25/0 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.

  • 598 AQUATIC HUMIC SUBSTANCES

    Most water utilities are able to meet a T T H M level of 0.10 m g / L (100 g/L) by using properly operated conventional treatment. However, if the standard is reduced substantially, adding G A C to conventional treatment may be an acceptable option. The length of time during which G A C can remove T H M s to meet a 10-, 25-, 50-, or 1 0 0 ^ g / L standard wil l determine its efficacy as a viable treatment option.

    The U.S. Environmental Protection Agency's Drinking Water Research Division has collected extensive treatment data for removal of organic substances, including T T H M , their precursors, total organic carbon (TOC), and total organic halide (TOX) at several water utilities under actual operating conditions. In these studies G A C was used at some sitesincluding Cin cinnati, Ohio; Jefferson Parish, Louisiana; Manchester, New Hampshire; and Evansville, Indianato determine its ability to remove those organic compounds present after conventional treatment.

    Literature Survey Conventional Treatment. T T H M precursors can be reduced by

    proper conventional treatment (coagulation, flocculation, sedimentation, and filtration). The extent of reduction can depend on several factors, such as type of coagulant, p H , and temperature. The effects of pretreatment processes for removal of humic substances are site-specific because of raw water quality variables, treatment-plant operating conditions, and treatment-plant design (2, 3). The literature shows some diversity of findings that make it difficult to understand the THM-precursor removal process during coagulation.

    Reckhow and Singer (4) reported that alum coagulation of aquatic fulvic acid removed T O C and T H M formation potential proportionately. Jodellah and Weber (5) observed that high levels of T O C removal may yield no selective removal of T H M precursors. Just as there were differences in the findings of investigators during bench studies, water-treatment plants also showed varying removals for T O C and T H M precursors (6). Under slightly acidic p H conditions, Edzwald and co-workers (2) reported that similar T O C and THM-precursor removals were achieved despite differences in raw water quality.

    GAC Treatment. The specific coagulation process influences both the amount and the T H M reactivity of the residual organic matter remaining after treatment prior to chlorination (7). Higher-molecular-weight organic compounds were most effectively removed during pretreatment, and lower-molecular-weight organic materials were effectively reduced by G A C (7, 8). Jodellah and Weber (5) indicated that increased T O C removal by activated-carbon treatment resulted in decreased T H M formation in treated water.

    Proper pretreatment appears to benefit activated-carbon adsorption. Randtke and Jepsen (9) reported significant increases in the adsorption ca-

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

  • 34. LYKINS & CLARK Removal by Conventional Treatment and Carbon 599

    parity of organic substances after alum coagulation. Lee and co-workers (10) showed that alum coagulation enhanced both carbon adsorption capacity and the rate of uptake. Semmens and co-workers (II) observed improved G A C performance with greater levels of pretreatment. Weber and Jodellah (3) noted that alum coagulation improved overall adsorbability of T O C .

    Treatment at Research Locations

    Various conventional treatment methods were used at the research sites to remove or reduce the mix of compounds present in the source water. The type of treatment (conventional and GAC) used at these utilities is as follows.

    Cincinnati, Ohio. The primary source water for the Cincinnati Water Works is the Ohio River. To aid settling, 17 mg/L of alum was added to the raw water. Prior to flocculation and clarification, 17 mg/L of lime, ferric sulfate (8.6 mg /L for high turbidity and 3.4 mg/L for low turbidity), and chlorine (plant effluent concentration 1.8 mg/L of free chlorine) were added. Postfiltration adsorption was evaluated by deep-bed G A C contactors with an ultimate empty-bed contact time (EBCT) of 15.2 min.

    Jefferson Parish, Louisiana. The Mississippi River provides source water to the Jefferson Parish treatment plant. Potassium permanganate (0.5-1.0 mg/L) was added for taste and odor control. A cationic polyelec-trolyte (diallyldimethyl diammonium chloride; 0.5-8.0 mg/L) was added as the primary coagulant, with lime (7-10 mg/L) fed for p H adjustment to 8.0-8.3. Chlorine and ammonia (3:1 ratio) were added for chloramine disinfection (1.4-1.7 mg /L residual after filtration). A sand filter was converted to a postfilter G A C adsorber with about 20 min E B C T . In addition, four G A C pilot columns were operated in series, providing 11.6, 23.2, 34.7, and 46.3 min E B C T .

    Manchester, New Hampshire. The principal water source for the Manchester Water Works is Lake Massabesic. Alum and sodium aluminate were added for coagulation, p H adjustment, and alkalinity control at dosage levels averaging about 12 and 8 mg/L, respectively. Chlorine was added prior to sand filtration at an average dose of 1 mg/L. At the clearwell, chlorine was again added in the range of 2-3 mg/L to produce an average-distribution free chlorine residual of 0.5 mg/L. A G A C filter normally used for taste and odor control was used for postfiltration adsorption with 23 min E B C T .

    Evansville, Indiana. The Evansville Water Works uses Ohio River water as its source. Chlorine and alum were added before primary settling, with average concentrations of 6 and 28 mg/L, respectively. A free chlorine residual of 1.5-2.0 mg /L was maintained after sand filtration. Approximately

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

  • 600 AQUATIC HUMIC SUBSTANCES

    12 m g / L of lime was added after primary settling for p H control to 8.0. A pilot plant operating parallel with the full-scale plant used chlorine dioxide for disinfection. Average alum and polymer (anionic high molecular weight) dosages of 12 and 0.8 mg/L, respectively, were added to the raw water of the pilot plant. An average lime dose of about 6 m g / L was used for p H control to 8.0. Post-pilot-plant G A C contactors had an E B C T of 9.6 min.

    Results

    T O C removal has been suggested as a means of measuring treatment performance. Although T O C is relatively easy to analyze and incorporates all organic compounds, it does not relate to any specific regulatory requirements. In the following evaluation, however, T O C was used as a general surrogate parameter to determine the performance of conventiona