Aquatic Humic Substances - ACS Publications - American Chemical

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch029

Characteristics of Humic Substances and Their Removal Behavior in Water Treatment 2

J. S. K i m 1 , E . S. K. Chian, F. M. Saunders, E . Michael Perdue , and M . F. Giabbai 1

School of Civil Engineering, Georgia Institute of Technology, Atlanta, GA 30332 The characteristics of naturally occurring aquatic humic substances and their removal behavior in water treatment were investigated in a plant operation and in alum coagulation with conventional laboratory jar-test experiments. Specific characteristics of humic substances affected the performance of alum coagulation in removing both humic substances and turbidity. Up to 50% of the humic substances were removed in both the treatment plant and alum coagulation. High-molecular-weight humic substances were preferentially removed. This preference resulted in substantial decreases in color intensity and trihalomethane formation potential per unit mass of humic substances. The characteristics and removal behavior by alum coagulation of commercial humic acid weresignificantlydifferent from those of the source-water humic substances.

^REMOVAL OF HUMC I SUBSTANCES

has long been of concern in water treat ment because of their abundance in natural waters and their potential adverse effects on public health and esthetics. They impart color to water and are able to form complexes with other inorganic and organic species (1-4) that frequently prevent the removal of these pollutants in treatment processes. More importantly, humic substances are major precursors in the formation 1Current address: HazWaste Industries, Inc., 2264 Northwest Parkway, Suite F, Marietta, GA 30067 2Current address: School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, GA 30332 0065-2393/89/0219-0473$07.25/0 © 1989 American Chemical Society

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


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AQUATIC HUMIC SUBSTANCES

of trihalomethanes (THM) upon chlorination (5, 6). The ubiquity of T H M s in drinking water (7), together with their potential carcinogenic activity (8), prompted the U . S . Environmental Protection Agency (EPA) to promulgate the maximum contaminant level of 100 μ g / L of total T H M s in water systems serving more than 10,000 persons (9). Most traditional water-treatment plants have been designed and op erated to maximize removal of turbidity and pathogens through the use of coagulation-flocculation-sedimentation, filtration, and chlorination proc esses. Now that humic substances can be removed in water treatment, a considerable amount of effort has been focused on the optimization of existing treatment processes, especially coagulation, for effective removal of humic substances (10-17). However, there are still questions relating to the per formance of these processes in removing humic substances because the nature of humic substances is not fully understood. Because the nature of humic substances can be an important factor influencing the performance of the water-treatment processes and the formation of T H M s upon chlori nation, this study was initiated to investigate the specific characteristics of humic substances in natural water sources and their removal behavior in water treatment. Humic substances in natural waters are unresolvably complex mixtures of organic matter; their physical and chemical properties are difficult to characterize. They are generally described as yellow, acidic, chemically com plex, and polyelectrolytelike materials that range in molecular weight from a few hundred to several thousands (18). Recent studies (19-21) have in dicated that humic substances in natural waters from different sources are relatively similar to each other in nature, but are significantly different from the commercial model humic substances commonly used in laboratory studies. Most information on the removal ofhumic substances in water treatment and their impact on water quality has been obtained from studies with model humic substances (11, 13, 14). A clear distinction should be made between data acquired from model humic substances and those from natural water sources. In order to address this concern, the characteristics ofhumic substances and their removal behavior in water treatment were investigated at a fullscale operating plant and in alum coagulation with conventional laboratory jar tests. The Chattahoochee Water Treatment Plant (CWTP) of Atlanta, G A , was selected for this study. C W T P is a conventional plant that uses the Chattahoochee River as its source of raw water. The treatment sequences are coagulationflocculation-sedimentation, filtration, and chlorination. Humic substances isolated from the C W T P source water and the model Aldrich humic acid were used in alum coagulation. The specific objectives of this study were to characterize the humic substances isolated from the source water and from


29.

KIM ET AL.

Humic-Substance Removal Behavior in Water Treatment 475

the treated effluents of each subsequent treatment process of the C W T P ; to evaluate the removal behavior ofhumic substances by alum coagulation with conventional jar-test procedures; and to compare the nature of both types of humic substances and their removal behavior by alum coagulation.


Experimental Materials and Methods CWTP and Water Sampling. The CWTP is a conventional plant that employs coagulation-flocculation-sedimentation,filtration,and chlorination (Figure 1). Alum is used as a coagulant in coagulation-flocculation after pH adjustment with lime. Prechlorination is practiced in both plant-intake and rapid-mix units. The supernate from the sedimentation basins isfilteredthrough a dual-media filter (anthracite and sand); this process is followed by postchlorination. The sludge produced is chemically conditioned and dewatered with afilterpress. The resulting sludge cakes are disposed of in a sanitary landfill. The CWTP, designed to treat 230 Χ 10 L/day, was treating an average of 150 X 10 L/day. It uses the Chattahoochee River as a water source; general water characteristics are shown in Table I. The average color and turbidity values were 28 Pt-Co units and 27 nephelometric turbidity units (NTU), respectively. These values represent relatively low color and high turbidity, compared to many surface waters (22). The water-sampling points, selected for the isolation of humic substances and investigation of their characteristics and removal behavior in the CWTP, included the source water (SW), effluentfromcoagulation-flocculation-sedimentation (ACFS), filtration effluent (AF), and the clear well (CW). The water volumes and sampling dates taken for the isolation ofhumic substancesfromeach sampling point are shown in Table II, along with the amount of humic substances isolated. Because a large volume of waterfromeach sampling point was required to isolate sufficient quantities of humic substances for subsequent laboratory experiments, water samples were taken during several periods of time rather than at intervals chosen according to the hydraulic retention time of each unit process. Approximately 200-700 L of water samples per day were takenfroma sampling point. The source-water quality and the performance of the CWTP were considered to influence the removal behavior of humic substances in the treatment processes. Therefore, various water-quality parameters were measured at each sampling point according to the hydraulic retention time of each unit process during the watersampling period, as shown in Table II. Figure 2 illustrates the results of the water-quality parameters for the four sampling points at the CWTP. Instantaneous THMs were produced after prechlo rination and increased to 29.1 μg/L in CW. Trihalomethane formation potential (THMFP) is a measure of the maximum amount of THMs that can be formed by the reaction offreeresidual chlorine with humic substances. A comparison of SW THMFP (228.5 μg/L) with CW THMFP (95.2 μg/L) shows a 69% reduction of THM pre cursors. The nonvolatile TOC (NVTOC) was reduced by 50% (i.e.,from4.3 to 2.1 mg/L), whereas UV absorbance at 254 nm and color were removed by 71 and 100%, respectively, through the treatment processes. All water-quality parameters for humic substances decrease significantly dur ing the treatment processes (Figure 2), especially after alum coagulationflocculation-sedimentation. NVTOC decreased to a lesser extent than THMFP and humic substances (as measured by color). This preferential removal of THMFP and humic substances corroborates the results of Babcock and Singer (23), who observed 6

6



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Process hydraulic detention time

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mixing

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Sampling points

15 min

Filtration

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well

Clear

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Figure 1. F/ou; diagram and operational conditions of the Chattahoochee Water Treatment fiant, along with the four sampling points for the isolation of humic substances. Chemical doses and process detention time are approximate annual averages for a flow of approximately 150 x 10 L/day.

I*-

SW


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KIM ET AL.

Humic-Substance Removal Behavior in Water Treatment 477

Table I. General Characteristics of CWTP Source Water Concentrations Parameters Dissolved oxygen 8.6 28.0 Color (Pt-Co unit) 6.8 pH 12.0 Alkalinity (mg/L as CaC0 ) 10.8 Hardness (mg/L as CaC0 ) 27.0 Turbidity (NTU) 17.6 Suspended solids 37.5 Dissolved solids 0.1 Fluoride 5.4 Sulfate 0.2 Phosphate 0.04 Nitrate Aluminum 0.6 Iron 0.4 Calcium 2.5 0.9 Magnesium 2.2 Sodium 1.4 Potassium Aldrin NF NF Chlordane DDT NF Dieldrin NF Endrin NF Heptachlor NF Heptachlor epoxide NF NF Lindane NF Methoxychlor Toxaphene NF

0

3


3

fc

NOTE: Data were obtained from plant records. Values presented are averages of two determinations sampled on October 5 and November 5, 1984. 'In milligrams per liter. Not found. fc

Table H . Humic Substances in CWTP Water Sampling Water Vol. of Water Humic Sampling Sampled Substances Dates Points (L) Isolated (g) (1984) Nov. 4-Nov. 15 SW 3102 3.99 Oct. 25-Nov. 3 ACFS 3143 2.92 Oct. 1-Oct. 14 AF 6467 4.79 Oct. 15-Oct. 24 CW 7164 4.94

that alum coagulation selectively removed those portions of organic matter most responsible for T H M production. These observations allowed us to investigate more closely the types of humic substances removed by alum coagulation. The humic substances remaining after alum coagulation are ultimately chlorinated, and chlori nation results in the formation of THMs.


478

AQUATIC HUMIC SUBSTANCES

200 CO

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5000 (which is totally excluded by the column) and 1000 < M W < 5000 (which was totally included by the column). S W - H S showed the following A M W distribution: 30.3% in > X M 5 0 ( M W >50,000), 51.2% in U M 1 0 - X M 5 0 (10,000 < M W < 50,000) and 18.5% in < U M 1 0 ( M W X M 5 0 fraction; this distribution indicates that A L - H A are composed of higher-molecular-weight compounds than S W - H S . The U F was reasonably successful in fractionating humic substances on the basis of their molecular weights, as evidenced by the G P C chromato grams of S W - H S and their three A M W fractions (Figure 8). However, the M W values assigned for each fraction of S W - H S by the U F method are higher than those assigned by the G P C method. The > X M 5 0 ( M W >50,000), U M 1 0 - X M 5 0 (10,000 < M W < 50,000), and < U M 1 0 ( M W 5000, 2000 < M W < 5000, and 1000 < M W < 5000 fractions by the G P C method, respectively. Similar observations were reported by Thurman et al. (30), who concluded that the M W values determined by the U F method for humic substances are 2 to 10 times higher than those shown by the G P C method. Because the three A M W fractions of S W - H S obtained by U F were used in the jar-test experiments, they were also characterized, and the results are shown in Table III. Trends linking higher molecular weight with higher color and T H M F P and lower acidic functional groups were observed. No intuitively obvious reason exists for these correlations, and more studies are needed to explain these phenomena.



1.9 1.4 1.5

(0.5) (0.4) (0.4)

DOC {mg CIL) 2.1 (0.7)

{%f Elemental Composition Color" H Ν C Ο Ash 50 - L/mg - cm 56.4 6.0 1.0 36.6 1.9 0.051 0.117 0.029 0.008 56.2 6.1 1.0 36.7 3.7 0.029 56.4 6.1 0.8 36.7 5.1 0.029 56.9 6.1 0.8 36.2 3.1 0.015 51.5 4.8 0.8 43.0 9.4 0.345

1

67.8 79.9 72.9 58.2 57.0 51.4 50.2 133.4

THMFP (ug THM)/ (mgHS)

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Acidic Functional AMW Distribution Groups (meq/g) by UF (%y COOH Phenolic OH >XM50 UM10-XM50

Aquatic Humic Substances - ACS Publications - American Chemical

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