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

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 µg/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 μg/L) and to lower it to some as yet unspecified level. Trihalomethane levels as low as 10-50 μ 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 μg/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 sites—including Cin cinnati, Ohio; Jefferson Parish, Louisiana; Manchester, New Hampshire; and Evansville, Indiana—to 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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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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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 conventional treatment and G A C adsorption.

Removal of instantaneous trihalomethanes and their precursors to meet a T T H M standard was also evaluated by using the terminal trihalomethane (terminal T H M ) parameter. Because the utilities studied used various disinfectants that affected the trihalomethane concentrations, terminal T H M (instantaneous T H M plus T H M formation potential) allows a comparison among utilities by indicating the maximum trihalomethane in the distribution system at a given time. In this evaluation, ambient p H and temperature were maintained. Chlorine dosages were chosen to ensure a chlorine residual after a storage time that simulated the time from the treatment plant to the farthest point in the distribution system.

Conventional Treatment. The T O C raw water concentration at Evansville, Indiana, varied from 2.8 to 3.6 mg /L during one 85-day operational phase. Average raw water T O C concentration was 3.0 mg/L. Average T O C removal was 37% with full-scale conventional treatment and 40% for the pilot plant. Average sand filter T O C concentration was 1.9 and 1.8 m g / L for the full-scale and pilot plant, respectively. Evansville's 3-day raw water terminal T H M concentrations ranged from 95 to 178 pg/L, for an average of 140 μg /L . After conventional treatment the average concentration was 82 μ g / L for the full-scale plant (an average reduction of 41%) and 34 μ g / L for the pilot plant (a 76% reduction). More efficient T H M precusor removal in the pilot plant for this operational phase was attributed to the addition of a polymer (anionic high molecular weight) for effective turbidity removal.

The initial raw water T O C concentration at Manchester was 4.6 mg/L. It varied from 3.8 to 4.8 mg/L, with an average concentration of 4.5 m g / L for 130 days of operation. The raw water T O C concentration was reduced about 47% to an average of 2.4 mg/L. Three-day terminal T H M concentrations for Manchester's raw water at ambient temperature ranged from 104

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to 191 μ g / L (an average of 151 μg/L) . Precursor removal through conventional treatment reduced the 3-day terminal T H M to an average of 70 μg /L , a 54% reduction.

In Cincinnati, where ferric sulfate was used as the primary coagulant, a 41% reduction in the T O C concentration was seen through conventional treatment. Raw water T O C concentrations ranged from 1.9 to 5.9 mg/L, for an average of 3.4 mg/L. After conventional treatment the T O C concentrations ranged from 1.1 to 3.4 mg/L, for an average of 2.0 mg/L. The average reduction through conventional treatment was 41%. Three-day terminal T H M concentrations for the raw water ranged from 64 to 211 μg /L , for an average of 146 μg /L . After conventional treatment, the terminal T H M concentrations ranged from 39 to 181 μg /L , for an average of 89 μg /L , producing an average terminal T H M reduction of 39%.

At Jefferson Parish, polymers were used as the primary coagulant. The raw water (Mississippi River) T O C concentration ranged from 2.9 to 5.9 mg/L, with an average of 4.0 mg/L. After conventional treatment the T O C concentrations ranged from 2.3 to 3.8 mg/L, with an average of 2.9 mg/L, for a reduction of 27.5%. Five-day terminal T H M concentrations for the raw water ranged from 133 to 511 μg /L , with an average of 281 μg /L . After conventional treatment the range was 82 to 364 μg /L , for an average of 175 μg /L . Average 5-day terminal T H M reduction through conventional treatment was 37.7%.

Table I summarizes the removal of the T O C through conventional treatment. Table II shows removal of terminal trihalomethanes through various steps in the treatment process. In this case, terminal trihalomethanes are used because they represent the formation potential of T T H M in the dis-

Table I. Average Total Organic Carbon Removal During Conventional Treatment

Raw Water Sand Filter Percent Water Utility (mg/L) Effluent (mg/L) Removal Cincinnati, O H 3.4 2.0 41 Jefferson Parish, LA 4.0 2.9 28 Manchester, N H 4.5 2.4 47 Evansville, IN 3.0 1.9 37

Table II. Average Terminal Trihalomethane Removal During Conventional Treatment Terminal Raw Water Sand Filter Percent

Water Utility Day ^g /L) Effluent (μg/L) Removal Cincinnati, O H 3 146 89 39 Jefferson Parish, LA 5 281 175 38 Manchester, N H 3 151 70 54 Evansville, IN 3 140 82 41

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tribution system itself. The time to the most distant customer in the distribution system is represented by the terminal day.

As can be seen from Tables I and II, the utilities examined experienced variable performance in average percent removal of both T O C and terminal T H M . This variability may be due in part to source water quality. For example, Cincinnati, Jefferson Parish, and Evansville (with a river water source) had lower percent removal efficiency for terminal T H M than did Manchester (with a lake source).

Granular Activated Carbon Treatment. G A C performance for removing both T O C and terminal T H M also varied for the different utilities evaluated. For instance, at Evansville, Indiana, after conventional treatment, G A C further reduced the T O C concentration during about 30 days of operation, after which the G A C effluent tracked just below the filter effluent (Figure 1). The 3-day terminal T H M concentration of the G A C effluent was essentially the same as the filter effluent after about 30 days of operation (Figure 2).

At Manchester, New Hampshire, the T O C concentration of the G A C effluent was about 0.5 m g / L at the start of one evaluation. It increased in concentration until about 35 days of operation, before tracking just below the sand-filter effluent (Figure 3). The 3-day terminal T H M concentration was initially about 10 μg /L , increasing to about 45 μ g / L after 40 days of operation, and then tracking below the sand-filter effluent (Figure 4).

The T O C effluent G A C concentration at Cincinnati, Ohio, was about 0.2 m g / L at the start of one of the runs and increased to about 1.1 m g / L after approximately 100 days of operation. As with Manchester and Evansville, the T O C then tracked just below the sand-filter effluent (Figure 5). The 3-day terminal T H M concentration of the G A C effluent was about 3 μ g / L at the start of an adsorption study, and "breakthrough" occurred after about 50 days of operation. From about day 110, the 3-day terminal T H M effluent was approximately the same increment below the sand-filter effluent throughout the 320-day study (Figure 6).

The full-scale G A C adsorber at Jefferson Parish, Louisiana, seemed to remove the T O C concentration steadily for about 160 days. Initial concentration was 0.2 mg/L, increasing to about 2.0 mg/L (Figure 7). The 5-day terminal T H M G A C effluent concentration for the full-scale system at Jefferson Parish was about 15 μ g / L at the start of one run and, like the T O C , steadily increased for 140 days (Figure 8).

Effect of Empty-Bed Contact Time The length of G A C operation before replacement or reactivation depends on several factors, one of which is empty-bed contact time (EBCT). If a drinking-water utility is required to use existing filters, very little flexibility is available for selection of E B C T . In designing a new system, however, the

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utility has an opportunity to determine the best E B C T , relative to performance and cost.

A G A C exhaustion criteria of 1.0 mg /L of T O C has been suggested as a reasonable performance standard (12). By applying this criterion to T O C breakthrough curves for virgin G A C , one can see the effects of E B C T . The G A C contactor at Cincinnati, Ohio, had sampling ports located at 4.3 ft (1.3 m), 7.0 ft (2.1 m), and 15.0 ft (4.6 m), yielding EBCTs of 4.4, 7.2, and 15.2 min, respectively. Length of G A C operation to the T O C exhaustion criterion and the carbon-use rate as shown in Table III indicate that, for the Cincinnati evaluation, longer EBCTs provided more efficient use of the G A C .

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The terminal T H M removal through various EBCTs showed the same general trend as noted with T O C . For Cincinnati, Ohio, increased E B C T up to 15.2 min produced additional removals (Figure 9). For Jefferson Parish, Louisiana, the closeness of the terminal T H M concentrations for 23.2 min E B C T and higher indicates that relatively little, if any, advantage is gained at higher EBCTs (Figure 10).

Granular Activated Carbon for Trihalomethane Control

Some water utilities are able to maintain their T H M concentrations below the existing promulgated standard of 0.10 mg /L (100 μg/L) by proper eon-

Table III. Summary Data at TOC Exhaustion of 1.0 mg/L Hydraulic Carbon Organic

EBCT Loading TOC Exhaustion Use Rate Loading GAC Depth (min) (m/hr) (gpm/fi2) Time (days) (kglmL) (g/kg) 1.3 m (4.3 ft) 4.4 17.8 7.4 22 46 15 2.1 m (7.0 ft) 7.2 17.8 7.4 71 34 25 4 . 6 m (15.0 ft) 15.2 17.8 7.4 204 26 51

NOTE: Virgin bituminous coal 12 x 40 GAC.

Table IV. Summary Data at TOC Exhaustion of 1.0 mg/L Hydraulic Carbon Organic

EBCT Loading TOC Exhaustion Use Rate Loading GAC Depth (min) (m/hr) (gpm/ft2) Time (days) (kglmL) (g/kg) 0.9 m (3.0 ft) 11.6 4.9 2.02 42 73 32 1.8 m (6.0 ft) 23.2 4.9 2.02 105 30 78 2.7 m (9.0 ft) 34.7 4.9 2.02 140 22 107 3.7 m (12.0 ft) 46.3 4.9 2.02 159 20 126 NOTE: Virgin bituminous coal GAC.

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ventional treatment. If, however, the standard is reduced substantially, other treatment alternatives wil l be required. G A C may be an alternative worth evaluating. The length of time that G A C can remove trihalomethanes to meet a standard of 10, 25, 50, or 100 μ g / L wil l determine its efficacy as a viable treatment option.

Because terminal T H M values can simulate concentrations in the distribution system, one can estimate the length of G A C operation for meeting T H M goals. Table V gives an indication of how long G A C can remove various concentrations of T H M s .

As can be seen from Table V, establishment of a 1 0 ^ g / L trihalomethane standard will probably negate the use of G A C . In addition, use of G A C to meet a 2 5 ^ g / L standard may not be feasible. However, G A C may be more attractive at the 5 0 ^ g / L trihalomethane concentration. Carbon-use rates for the operational days at the 5 0 ^ g / L trihalomethane concentration were 0.216, 0.716, 0.690, and 0.459 lb/1000 gal for Cincinnati, Jefferson Parish, Manchester, and Evansville, respectively.

Data Normalization and Prediction of THM Concentrations

The data reported here have shown the performance of G A C in removal of terminal T H M over various days of operation and bed volumes through G A C adsorbers at different locations. Normalization of the data by using percent removal shows that the G A C adsorbers used at Cincinnati produced the overall highest removal rate for terminal T H M (Figure 11). Evansville had the lowest percent removal.

T O C has been suggested as a surrogate for prediction of T H M concentrations. If T O C is removed through G A C adsorption, wil l T H M precursors be selectively removed? A definite pattern of T O C with 3-day T H M formation potential (THMFP) and T O C with 7-day T H M F P was noted in C i n cinnati for the G A C effluent. This pattern indicates that T O C may be used as a predictive tool at that location (Figures 12 and 13). With Jefferson Parish, Louisiana (another plant using river water as its source), T O C and 5-day

Table V. Length of GAC Operation Before Exceeding Terminal T H M Levels

Location

10 u^/Le 25 \LglL* 5Q\LglLa 100 pg/La

Location Day Inflow ML) Day

Inflow (mIL) Day

Inflow ML)

Inflou Day (u /̂L;

Cincinnati, OH (3-day term, 15.2-min EBCT) 50 75 155 45 208 70 280 150

Jefferson Parish, LA (5-day term, 18.8-min EBCT) — — 20 80 63 170 103 220

Manchester, Ν H (3-day term, 23-min EBCT) 2 73. 16 70 98 65 — —

Evansville, IN (3-day term, 9.6-min EBCT) — _ _ 6 96 56 53 — —

T H M goals.

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T H M F P also seemed to follow a pattern. A close correlation was also seen for Manchester, New Hampshire, G A C effluent (lake water source) with T O C and 3-day T H M F P .

Removal of T O C by G A C may give an indication of T H M F P removal. In a comparison of percent T H M F P removal to percent T O C removal for the four utilities evaluated, Figure 14 shows a 45° line of equal percent removal. Regression of this data, however, indicates that removing T O C does not necessarily mean removing an equal percentage of T H M F P . The following equation describes the data from the four utilities:

T H M F P = 7.83 + 0.87 T O C (fi2 = 0.73) (1)

The instantaneous organic halide did not seem to be as good a predictor of T H M F P as T O C . However, the instantaneous organic halide might be used to predict G A C instantaneous T H M breakthrough (Figure 15).

Summary

Proper conventional treatment can reduce T O C and T H M precursors substantially. For the full-scale systems, average percent T O C removal was variable, possibly because of the type of coagulants used. Average percent terminal T H M removal, however, seemed to follow a pattern. The river water sources were about the same, with lower average terminal T H M percent removal through conventional treatment than for the lake water at Manchester. This result may be attributed to better T O C removal during conventional treatment and different T H M precursors in the lake water than in the river source water.

Although T H M precursors are reduced during conventional treatment, this reduction will probably not be enough to meet T H M concentrations much below 100 μ g / L if chlorine is used as the primary disinfectant. With G A C adsorption, however, additional precursors are removed. For the utilities evaluated, meeting a 5 0 ^ g / L T H M standard appears to be possible after G A C treatment.

Acknowledgments The authors thank Sue Campbell for her direction and coordination in producing the graphics. The authors also thank Sandi Dryer for typing the manuscript. This paper has been reviewed in accordance with the U.S . Environmental Protection Agency's peer and administrative review policies and approved for presentation and publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Environmental Protection Agency.

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References 1. "Safe Drinking Water Act" as amended by the "Safe Drinking Water Act Amend

ments" of 1986; Public Law 99-339, 1986. 2. Edzwald, J. K.; Becker, W. C.; Wattier, K. L. J. Am. Water Works Assoc. 1985,

77(4), 122-132. 3. Weber, W. J.; Jodellah, A. M. J. Am. Water Works Assoc. 1985, 77(4), 132-137. 4. Reckhow, D. Α.; Singer, P. C. Proc. Am. Water Works Assoc. Annu. Conf. Las

Vegas, NV; American Water Works Association: Denver, CO, 1983. 5. Jodellah, A. M.; Weber, W. J. J. Am. Water Works Assoc. 1985, 77(10), 95-100. 6. Ohio River Valley Water Sanitation Commission. Water Treatment Process Mod

ification for Trihalomethane Control and Organic Substances in the Ohio River; U.S. Environmental Protection Agency; National Technical Information Service: Springfield, VA, 1980; ΕPA-600/2-80-028.

7. Collins, M. R.; Amy, G. L.; King, P. H. J. Environ. Eng. (Ν.Y.) 1985, 111(6), 850-864.

8. Semmens, M. J.; Staples, A. B. J. Am. Water Works Assoc. 1986, 78(2), 76-81. 9. Randtke, S. J.; Jepsen, C. P. J. Am. Water Works Assoc. 1981, 73(8), 411-419.

10. Lee, M. C.; Snoeyink, V. L.; Crittenden, J. C. J. Am. Water Works Assoc. 1981, 73(8), 440-446.

11. Semmens, M. J.; Staples, A. B.; Hohenstein, G.; Norgaard, G. E. J. Am. Water Works Assoc. 1986, 78(8), 80-84.

12. Lykins, B. W., Jr.; Geldreich, Ε. E.; Adams, J. Q.; Ireland, J. C.; Clark, R. M. Granular Activated Carbon For Removing Nontrihalomethane Organics From Drinking Water; U.S. Environmental Protection Agency; National Technical Information Service: Springfield, VA, 1984; ΕPA-600/2-84-165.

RECEIVED for review July 24, 1987. A C C E P T E D for publication March 10, 1988.

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