Download - [Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Effect of Ozonation and Chlorination on Humic Substances in Water

37 Effect of Ozonation and Chlorination on Humic Substances in Water

Joop C. Kruithof, Marten A. van der Gaag, and Dick van der Kooy

Kiwa Ltd., P.O. Box 1072, 3430 BB Nieuwegein, Netherlands

Ozonation converts humic substances, as can be seen by a small decrease in dissolved organic carbon (DOC) and a substantial decrease in UV extinction. This conversion is related to the formation of low-molecular-weight biodegradable compounds, which enhance regrowth of organic substances in water during distribution. Post-treatment by coagulation and filtration processes can remove these types of compounds. Chlorination of humic substances causes the production of trihalomethanes (THMs), high-molecular-weight organohalides, and mutagenicity. A partial THM-precursor removal by pretreatment does not reduce the THM content under practical conditions and causes a shift to production of more highly brominated THMs. Extensive pretreatment with ozonation and granular activated carbon (GAC) filtration lowers the adsorbable organohalogen (AOX) content and the mutagenic activity in the Ames test.

WATER SOURCES USED FOR DRINKING-WATER PREPARATION contain humic substances, which account for roughly 75% of the total dissolved organic carbon (DOC). These humic substances strongly interfere with water-treatment processes, especially in surface-water treatment. Therefore, the Netherlands Waterworks Testing and Research Institute Kiwa Ltd. and the Netherlands Waterworks have been carrying out investigations into the removal and conversion of bulk organic materials (mainly humic substances) by water-treatment processes.

0065-2393/89/0219-0663$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.

664 AQUATIC HUMIC SUBSTANCES

Many analytical methods are available for a general characterization of humic substances in water: determination of D O C content as a measure of the concentration of humic substances; spectrophotometric analysis (measurement of U V extinction and color) as a measure of the concentration and, in combination with the D O C content, as a first indication of the character of the humic substances; and gel permeation chromatography and X-ray scattering to determine the molecular-size distribution. Other methods are used to assess the interference of humic substances with treatment processes. Examples of these methods are determination of easily assimilable organic carbon (AOC) content as a measure of the regrowth potential of the organic materials, especially after oxidative treatment, and determination of trihalomethane (THM) precursors as a measure of the potential T H M formation upon chlorination.

A l l of these parameters have been used to characterize the concentration and nature of humic substances during Dutch drinking-water treatment. For this chapter we will concentrate on

• D O C content as a measure of the content of humic substances

• UV254 extinction at a wavelength of 254 nm as an indication of the changing character of the humic substances during oxidative treatment

• A O C content as a measure of the regrowth potential of the water

• THM-precursor content as a measure of potential T H M formation upon chlorination.

Treatment processes influencing the concentration of humic substances are coagulation and granular activated carbon (GAC) filtration. Standardized jar test equipment was developed (1) to study the effect of coagulation on the content of natural organic compounds. G A C filtration is applied at 12 full-scale plants, primarily to remove toxic compounds and to improve taste (2, 3). Removal of humic substances (removal of color, U V extinction, and dissolved organic carbon) is an important secondary aim.

This chapter will consider the interaction of oxidative processes with humic substances in water. The processes studied most extensively are ozonation and chlorination.

Data on oxidation of humic substances by ozonation are available from 12 pilot-scale and eight full-scale experiments (4). Ozonation of humic substances leads to the production of low-molecular-weight compounds, which increase the availability for bacteria of organic compounds in water. This phenomenon is responsible for the regrowth of bacteria during distribution (5). Removal of these biodegradable compounds by posttreatment of the ozonated water is necessary to restrict bacterial regrowth during distribution

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37. KRUITHOF ET AL. Effect of Ozonation and Chlonnation 665

(6). The effects of ozonation will be illustrated through D O C , U V extinction, and A O C measurements gathered from pilot-scale and full-scale experiments carried out at the treatment plants at Driemond (Municipal Waterworks of Amsterdam) and at Kralingen (Waterworks of Rotterdam).

In the years since the discovery of T H M production during drinking-water chlorination (7, 8), measures have been taken to reduce this side effect of chlorination as much as possible (9). One option was the removal of humic substances by G A C filtration before chlorination. Under Dutch chlorination conditions, partial removal of humic substances by G A C filtration did not give a substantial reduction of the T H M content for long GAC-filter running times. Moreover, chlorination of GAC-filter effluents caused a shift to the production of more highly brominated T H M s , especially at short GAC-filter running times (10-12).

In addition to a relatively high adsorbable organic halogen (AOX) content, mutagenic activity was found after postchlorination with a relatively low chlorine dosage of about 0.5 mg/L (13, 14). Both side effects of chlorination will be illustrated on the basis of T H M and THM-precursor measurements gathered from full-scale experiments carried out at the treatment plant at Zevenbergen (Waterworks of North West Brabant) and on the basis of A O X and mutagenic activity measurements carried out at the pilot plant at Nieuwegein (Kiwa Ltd).

Effects and Side Effects of Ozonation

Experimental Parameters. An impression of the character and the content of humic substances in the water was obtained by determination of the UV extinction and the DOC content. In samples of nonozonated and ozonated water, the UV extinction was determined by spectrophotometer (Perkin Elmer, Type 500S) at a wavelength of 254 nm. The DOC content was determined subsequently, after acidification to pH 2 with concentrated hydrochloric acid and membrane filtration, with an ultra-low-level organic analyzer system (Dohrmann DC-54).

To determine the concentration of AOC, 600 mL of water was heated in thoroughly cleaned Pyrex Erlenmeyer flasks at 60 °C for 30 min to inactivate the bacteria orginally present in the water. After cooling, pure cultures of selected bacteria were inoculated into the samples, which were incubated at 15 °C. Growth of these bacteria in the water samples was measured by periodic determinations of the number of viable organisms. The maximum colony count is considered a measure of the amount of AOC available for the organism in the water used in the growth experiment.

Two bacterial strains were used: Pseudomonas fluorescens strain P17, able to metabolize a great variety of organic compounds such as amino acids, carbohydrates, and aromatic acids: AOC (P17); and Spirillum species strain NOX, specialized in the use of carboxylic acids such as formic acid, glyoxylic acid, and oxalic acid: Δ AOC (NOX). Yield values of P17 for acetate and of NOX for oxalate have been determined for use in calculation of the AOC concentration. The total AOC (AOCT) is AOC T = AOC (P17) + Δ AOC (NOX). In this equation, AOC (P17) is expressed in micrograms of acetic acid carbon (Ac C) per liter and Δ AOC (NOX) is expressed in micrograms of oxalic acid carbon (Ox C) per liter. Detailed information about the determination has been published elsewhere (6, 15, 16).

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AQUATIC HUMIC SUBSTANCES

Application of Ozonation. In the Netherlands ozone is applied at eight full-scale treatment plants. The two largest plants are the Driemond plant of the Municipal Waterworks of Amsterdam with a production of 22 Χ 10 6 kL/year and the Kralingen plant of the Waterworks of Rotterdam with a production of 33 X 10 6 kL/year. The treatment systems of these plants are shown in Figures 1 and 2. The average ozone dosage at both plants is about 2.5-2.7 mg/L.

Pilot-scale ozonation experiments have been carried out with rapid filtrate from lake water at the Driemond plant and surface water after coagulation at the Kralingen plant. The effect of the ozonation on humic substances is expressed as the reduction of the U V extinction and D O C as a function of the ozone dosage (mg of 0 3 / m g of DOC) . The side effect of the ozonation on humic substances is expressed as the A O C T = A O C (P17) + Δ A O C (NOX) fraction of the D O C content as a function of the ozone dosage.

Full-scale experiments were carried out to investigate the variation in A O C (P17) from raw to finished water at both treatment plants. In addition, full-scale data were gathered about the formation of A O C T during the ozonation step.

Effect of Ozonation on Humic Substances. The reduction of the U V extinction as a function of the ozone dosage is presented in Figure 3. At a maximum ozone dosage of about 2.0 mg of 0 3 / m g of D O C , the U V

coagulation II

storage II

rapid f i l t r a t i o n II

ozonation II

coagulation II

rapid f i l t r a t i o n II

slow sand f i l t r a t i o n

Figure 1. Treatment system at Driemond, Municipal Waterworks of Amsterdam.

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37. KRUITHOF ET AL. Effect of Ozonation and Chlorination 667

storage

coagulation

ozonation

rapid f i l t r a t i o n

GAC f i l t r a t i o n

post chlorination

Figure 2. Treatment system at Kralingen, Waterworks of Rotterdam.

uv 0-uv UV0

100

80

Xioo

• = Kralingen ο = Driemond

1 1 1 1 ι • · -1 1.5 2

ozone dosage (mg 03/mg DOC) 2.5

Figure 3. Reduction of UV extinction by ozonation; pilot-scale experiments at Driemond and Kralingen.

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extinction was reduced by about 60-70% for both water types. About 40% of the initially present UV-absorbing compounds showed the greatest susceptibility for ozonation at ozone dosages up to 0.5 mg of 0 3 / m g of D O C . This conversion of UV-absorbing compounds is reached under practical conditions. The reduction of the D O C content is given in Figure 4. At maximum ozone dosage, the D O C at Kralingen was reduced by about 8%. At Driemond a maximum conversion of 18% could be reached. Under practical ozonation conditions, these percentages proved to be about 4% and 8%, respectively.

These experiments indicate that for the maximum ozone dosage the U V extinction is reduced 60-70% and the D O C removal 8-18%. This preferential decrease of the 254-nm chromophore indicates that ozonation causes a change in the character of the humic substances, without a complete oxidation of the organic compounds to carbon dioxide. This conversion of humic substances is the cause of the increased concentration of biodegradable compounds.

AOC Formation. For both water types, pilot-scale experiments showed an increase of A O C T as a function of the ozone dosage. At the maximum ozone dosage A O C T amounted to 550 μ g / L for ozonated rapid filtrate at Driemond and 800 μ g / L for ozonated coagulated water at Kralingen. The A O C x / D O C ratio as a function of the ozone dosage is presented in Figure 5. For both water types the A O C x / D O C ratio showed a gradual rise. The values for ozonated water at Kralingen were much higher than those at Driemond (0.3 and 0.09, respectively). These data indicate that the

100

υ ο Q !

<y ο ο

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ο Q

60

40 Η


Ozone dosage (mg 03/mg DOC)

Figure 4. Reduction of DOC by ozonation; pilot-scale experiments at Driemond and Kralingen.

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100·

80

8

υ ο ο

60 Η


Ozone dosage (mg 03/mg D O C )

Figure 5. Formation of AOC by ozonation; pilot-scale experiments at Driemond and Kralingen.

extent of A O C production by ozonation is strongly affected by the nature of the humic substances present in the water.

For both full-scale plants, the variation in A O C (P17) has been determined from raw water to finished water. At Driemond (Figure 6) the A O C (P17) content after storage of lake water is 46 μg /L . Rapid filtration gives a reduction to 14 μg /L . Ozonation causes a large increase to 120 μg /L . Finally, coagulation, rapid filtration, and slow sand filtration lower the A O C (P17) content to 18 μg /L . Roughly the same pattern is obtained for the treatment of surface water after storage at Kralingen (Figure 7). The A O C (PI7) content after storage is lowered by coagulation from 27 to 12 μg /L . Ozonation once again causes a large rise to about 100 μg /L . Rapid filtration and especially G A C filtration remove assimilable compounds to a concentration of 12 μg /L . A final application of chlorine gives a small rise to 22 μg /L . Ful l -scale data for A O C T have been gathered for the effect of ozonation (see Table I). At Driemond, with an ozone dose of 2.7 mg/L, 4.3% of the total organic carbon (TOC) content of 6.5 mg /L is converted into assimilable compounds. At Kralingen, with an ozone dose of 2.5 mg/L, 12.6% of the T O C content of 2.6 mg/L is converted into A O C .

Discussion. The data presented here clearly show that ozonation changes the character of humic substances (change in U V / D O C ratio) and causes an increase in the concentration of biodegradable compounds. The U V extinction was reduced up to 60-70% for both surface (reservoir) water and lake water. The effect of ozone decreased as ozone dosage increased.

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AOC (P17) 1 2 θ Ί

(/jg/ï AcC) 100-

80-

Figure 6. Variations in AOC (PIT) content from raw to finished water; full-scale data at Driemond.

AOC (P17) 1 2 0

(jjg/l) AcC 100

GAC pCI

Figure 7. Variations in AOC (P17) content from raw to finished water; full-scale data at Kralingen.

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Table I. Formation of Biodegradable Compounds

AOC(P17) MOC(NOX) AOCT A 0 C * x 100

Pfont (u,g/L Ac C) (μβ/L Ox C) ML) DOC Driemond" 97 175 272 4.3 Kralingen0 95 220 315 12.6 eFull-scale data.

This correlation indicates a growing resistance of humic substances toward ozone. D O C was reduced by 18% for lake water at Driemond and by 8% for surface water at Kralingen. Hydrogen carbonate concentration and p H of both water types are in the same order of magnitude, so different oxidation mechanisms may be excluded. Therefore the percentage of complete oxidation to C O 2 is dependent on the type of humic substances. Both reduction percentages show clearly that the effect of ozonation is dependent on the type of humic substances present in the water and that ozonation causes a change in the character of the humic substances.

This change in the composition of the humic substances is once more established by an increase in the concentration of biodegradable compounds expressed by an increase in A O C T . This rise in A O C T concentration explains the pronounced bacterial regrowth in the distribution systems of waterworks supplying ozonated water without any further posttreatment. Oxidation of humic substances as present in lake water at Driemond produced a large D O C reduction (18%) and a relatively small A O C formation ( A O C T / D O C = 9%). On the other hand, oxidation of humic substances as present in surface water at Kralingen showed a small D O C reduction (8%) and a relatively extensive A O C formation ( A O C T / D O C = 31%). Excluding other reaction mechanisms, this result again demonstrates dependence on the type of humic substances present in the water. The biodegradable organic compounds can be removed by coagulation and filtration processes such as rapid filtration, slow sand filtration, and G A C filtration. Up to 70% of the A O C can be removed by coagulation and rapid filtration. This posttreatment of ozonated water is needed to prevent an extensive regrowth in the distribution system. However, final A O C concentrations in the water can be higher than they were before ozonation. Extensive treatment with G A C filtration, slow sand filtration, or even postdisinfection with chlorine may be necessary to prevent regrowth.

Effects and Side Effects of Chlorination Experimental Parameters. The total trihalomethane (TTHM) concen

tration in aqueous samples was measured with a headspace technique that uses gas chromatography with a capillary column and an electron-capture detection system.

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Samples for the T H M formation potential (THMFP) were prepared by using a chlorine dose of 35 mg/L and a reaction time of 48 h.

The AOX content of water samples was determined after they are purged with nitrogen to remove volatile compounds. Then the water was mixed with carbon, sodium nitrate, and nitric acid. The sample was filtered over a polycarbonate filter. Carbon and filter were washed with a sodium nitrate solution and used for AOX measurements. This isolation procedure enabled the determination of the nonvolatile, high-molecular-weight organohalide fraction only. AOX was measured with a microcoulometric titration system after pyrolysis of the sample at 850-1000 °C in an oxygen atmosphere. The AOX samples were manually introduced into the oven tube through a boat inlet system.

The determination of mutagenicity in the Ames test with strains TA98 and TA 100 was carried out in styrene-divinylbenzene resin (XAD-4) isolates according to Maron and Ames (17), with slight modifications (18). After removal of the volatile organic material by a nitrogen purge, the nonvolatile organic material was adsorbed sequentially on two XAD-4 columns at pH 7 and 2. Each fraction was eluted and concentrated in ethanol (19). The XAD samples were tested at six dose levels (10-140 μL of ethanol per plate), depending on the type of water tested. In all samples, organic substances from 1 L of water were concentrated in 25 μL of ethanol. The liver homogenate (S9) was prepared from Aroclor-induced Sprague-Dawley rats. (Arochlor is a series of polychlorinated polyphenyls.) In the assays with S9 mix, 0.5 mL of S9 mix containing 0.075 mL of S9 was added to the top agar.

Application of Chlorination. In the Netherlands chlorination is applied at 12 full-scale treatment plants. One of these plants, the surface-water plant at Zevenbergen of the Waterworks of North West Brabant, has an annual production of 2.5 Χ 10 6 k L . The treatment system of this plant, shown in Figure 8, includes breakpoint chlorination and postchlorination. The average dosage for breakpoint chlorination amounts to 1.7 mg/L; postchlorination is applied with a chlorine dosage of 0.5 mg/L. The application of chlorine and the side effects of chlorination are investigated extensively in pilot-plant studies. One of the pilot plants used for this type of investigation is the Kiwa pilot plant at Nieuwegein, with a capacity of 2.5 k L / h . The treatment system of this pilot plant is represented in Figure 9. The rapid filtrate, the ozonated water, and both G A C filtrates are postchlorinated according to a criterion of 0.2 mg of free chlorine per liter after a 20-min contact time.

With dual-media filtrate and G A C filtrate at the Zevenbergen plant, pilot-scale chlorination experiments have been carried out with chlorine dosage corresponding to the dose used in practice for postchlorination (0.5 mg/L). With these experiments the side effects of the chlorination on humic substances are expressed as the T H M formation, measured as a function of the GAC-filter running time. Extensive attention has been paid to T H M production in relation to T H M F P reduction.

With dual-media filtrate and both G A C filtrates, chlorination experiments have been carried out at the Nieuwegein pilot plant with a chlorine dosage of 0.2 mg of free chlorine per liter after a 20-min contact time. These experiments led to the characterization of the side effects of chlorination,

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storage

coagulation

super chlorination

dual media f i l t r a t i o n

GAC f i l t r a t i o n

post chlorination

Figure 8. Treatment system at Zevenbergen, Waterworks of North West Brabant.

r a w w a t e r

a

s e t t l i n g

c o a g u l a t i o n

d u a l m e d i a f i l t r a t i o n

ozonation GAC f i l t r a t i o n

GAC f i l t r a t i o n phlorination chlorination

chlorination

chlorination

Figure 9. Treatment system of Kiwa Ltd. pilot plant at Nieuwegein.

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with A O X formation and the formation of mutagenicity in the Ames test as a function of the GAC-filter running time.

Fo rma t ion of T H M s . For dual-media filtrate (the GAC-filter influent) and G A C filtrate at Zevenbergen, the T H M F P was determined as a function of the GAC-filter running time (Figure 10). The T H M F P of dual-media filtrate varied from 150 to 240 μg /L . T H M F P in G A C filtrate increased with increasing carbon life from 20 to 160 μg /L . Marked T H M F P reduction occurred only during very short filter runs. A removal of about 80% could be achieved for no more than 3 weeks. Precursor removal rapidly decreased and amounted to only 35% after a 26-week filter run. The T H M content in chlorinated dual-media filtrate amounted to 13-25 μg /L , with C H C 1 3 in the highest concentrations, followed by C H B r C l 2 , C H B r 2 C l , and C H B r 3 (Figure 11a). The T H M content in chlorinated G A C filtrate increased from 5 to 30 μg /L . By far the highest T H M levels were C H B r 3 (up to 20 μg/L) , followed by C H B r 2 C l , C H B r C l 2 , and C H C 1 3 (Figure l i b ) . Thus, in comparison with the chlorinated dual-media filtrate, no significant reduction of the T H M content was achieved. Moreover, more highly brominated T H M s are formed in G A C filtrates, especially for short carbon filter runs of about 10 weeks.

Thus, although carbon filtration leads to a significant T H M F P reduction, this is not accompanied by a reduction of the T H M content under practical conditions. It leads to formation of more highly brominated T H M s , especially at relatively short filter runs. This phenomenon is illustrated in Figure 12. The T H M F P increased as a function of D O C . The T H M content rose to a

THMFP 2 5 0

200-

150-1

100 Η

50 Η

0 I ι 1 I 1 ι • ι — · — ι — · — ι — 1 — ι — 1 — ι — 1 — ι — 1 — ι — 1 — ι — 1 — ι — 1 — I 0 2 4- 6 θ 10 12 14 16 18 20 22 24 26

running time (weeks)

Figure 10. THMFP in chlorinated rapid filtrate and GAC filtrate as a function of the GAC-filter running time; full-scale data of Zevenbergen.

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40

CHBrCC

CHCI3

I 1 1 1 I 1 I " I 1 I 1 I 1 I 1 I 1 I 1 I 1 I - ι · ι · ι ι

t 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35


40

CHBr3

CHBr2CI

CHBrCG

CHCI3

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 running time (weeks)

Figure 11. THM content in chlorinated rapidfiltrate as a function of the GAC-filter running time; full-scale data at Zevenbergen.

D O C level of about 1.2 mg/L, then remained constant. The C H B r 3 content rose to a somewhat lower D O C level of 1.0 mg/L, then decreased. Only THM-precursor removal to a T H M F P lower than 0.5 μιηοΙ/L (60 μg/L) led to a reduction of the T H M content under practical conditions. This coincided with a D O C reduction to 1.2 mg/L. Maximum formation of C H B r 3 took place at about the same D O C concentration.

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THM

DOC(mg/l)

Figure 12. TEMFF, THM, and CHBr3 content as a function of the DOC content; full-scale data at Zevenbergen.

Formation of AOX and Mutagenicity in the Ames Test. For chlorinated dual-media filtrate and both chlorinated GAC-filter effluents at Nieuwegein, the A O X and the mutagenicity in the Ames test have been expressed as a function of G A C filter run. The A O X content for all chlorinated effluents is shown in Figure 13. High concentrations of A O X were formed upon chlorination of dual-media filtrate. The A O X content varied from 22 to 104 μg /L , with an average value of 65 μg /L . Formation of A O X in G A C filtrates started after more than 6 weeks of filtration and increased gradually for both carbon filters. In the G A C effluent fed with dual-media filtrate, the A O X content reached values of about 20 μ g / L after 6 months and about 65 μ g / L at the end of the 18-month filter run. Much better results were obtained by a combination of ozonation and G A C filtration. Chlorination of this GAC-filter effluent produced only 15 μ g / L of A O X at the end of the GAC-filter running time of 18 months.

The mutagenicity in the Ames test for chlorinated dual-media filtrate and ozonate is shown in Table II for strain TA100-S9 mix. Mutagenicity was about 10 times higher than before chlorination. Part of the mutagenicity was inactivated by liver homogenate. Ozonation produced substantial removal of the mutagenic effect. Chlorination of both G A C effluents caused a gradual increase of the mutagenicity as a function of the filter run (Figure 14). Mutagenicity of the chlorinated G A C effluent (influent dual-media filtrate) for TA100-S9 mix was first observed after 5 weeks. The breakthrough was initially higher in the p H 2 fraction, but after about 40 weeks the mutagenic

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ν I 1 I 1 I 1 I 1 I 1 ι 1 ι — • — ι — ' — ' ' ' 0 10 20 30 40 50 60 70 80 90


Figure 13. AOX content in chlorinated effluents; pilot-plant data at Kiwa.

Table II. Mean Mutagenicity of Chlorinated Rapid Filtrate and Ozonate Effluent TA98-S9 TA98 + S9 TA100-S9 TA100 + S9 Chlorinated rapid filtrate

pH 7 550 790 1240 m pH 2 190 100 880 210

Chlorinated ozonate pH 7 160 60 190 110 pH 2 35 10 170 70

activity of the p H 7 fraction reached the same level. Mutagenic effects following chlorination of the G A C effluent (influent ozonate) started appearing only after 10 weeks in the p H 2 fraction and after 40 weeks in the p H 7 fraction. In both fractions, this mutagenic activity leveled off at 150-200 induced revertants per 1.6 L equivalents, where it remained for the rest of the filter run. This level was the same as in the chlorinated ozonate, which means that there is no additional effect of the G A C filtration after that filter running time.

Discussion. From the data presented in the second section of this chapter, it can be concluded that

• partial T H M F P reduction did not lead to a decrease in T H M concentration under practical chlorination conditions. In ad-

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Figure 14. Mutagenicity in chlorinated effluents; pdot-pfont data at Kiwa.

dition, a shift took place toward the formation of more highly brominated T H M s .

• besides conversion into T H M s , humic substances are converted by chlorination into adsorbable organohalides and thereby cause a strong rise in the mutagenic activity in the Ames test. In view of the applied isolation technique, both organohalide content and mutagenic activity represent nonvolatile, high-molecular-weight organic substances.

The T H M F P is reduced by G A C filtration as a function of the filter run. Initially 80% of all T H M precursors are adsorbed; after a filter run of 5 weeks the removal is about 70%. By the end of the running time, removal of T H M precursors has dropped to 35%. The T H M concentration under customary chlorination conditions in the Netherlands (a chlorine dosage of about 0.5 mg/L) is reduced only for a T H M F P reduction of at least 70%. Therefore, at G A C filter runs longer than 5 weeks, T H M F P removal does not lead to a decrease in the T H M concentration under practical conditions, in combination with a shift toward the formation of more highly brominated T H M .

In chlorinated dual-media filtrate the average A O X content amounted to 65 μg /L . G A C filtration initially caused a complete reduction of the A O C content upon chlorination. Formation of A O X after chlorination started after short filter runs. After a complete 18-month filter run, there was only a small difference compared to chlorinated dual-media filtrate. Much better results

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37. KRUITHOF ET AL. Effect of Ozonation and Chlonnation 679

were achieved by a combination of ozonation and G A C filtration. Even at the end of the filter run the A O X production amounted to only 15 μg /L , an improvement of about 80% over chlorinated dual-media filtrate. Roughly the same conclusions are valid for the formation of mutagenicity in dual-media filtrate and both GAC-filter effluents. From the results it can be concluded that the performance of the 0 3 / G A C system in controlling A O X formation and formation of mutagenic activity was superior to that of G A C filtration alone. The total difference between the two systems could be attributed to the effect of ozonation.

Evaluation The results of the studies presented in this chapter show that most humic substances present in the water are not converted in C 0 2 by oxidative processes such as ozonation and chlorination. Therefore, both processes suffer from side effects.

Ozonation changes the character of humic substances strongly. U V absorbance of humic substances is greatly lowered, but there is hardly any decrease of the D O C content. Therefore complete oxidation to C 0 2 is unlikely. Instead, easily assimilable low-molecular-weight organic compounds are formed, enhancing regrowth of bacteria in the distribution system. Post-treatment by coagulation and filtration processes is needed to remove these biodegradable compounds in order to prevent regrowth. Even further treatment of the filtrate (e.g., by disinfection) may be necessary to prevent regrowth. These negative side effects of ozonation must be considered in making decisions about the use of ozonation in a drinking-water treatment plant.

Chlorination of humic substances causes the formation of T H M s , nonvolatile high-molecular-weight organohalides, and mutagenicity. Partial removal of T H M precursors (humic substances) by pretreatment does not produce T H M reduction under practical postchlorination conditions, and it causes a shift to the formation of more highly brominated T H M s . Extensive pretreatment, especially with a combination of ozonation and G A C filtration, lowers the A O X content and the mutagenic activity. As long as the evaluation of the health effects of these halogenated compounds is not completed, the concentration of these compounds in water should be kept as low as possible. Therefore, postchlorination should be omitted when the biological quality (hygienic aspects, aftergrowth) is sufficient.

Based on these principles, the following disinfection philosophy is followed:

• No chemical disinfection is applied when sufficient physical, mechanical, and biological barriers are present.

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• When chemical disinfection is needed, alternatives to chlorine can be considered if their side effects have proven to be less important than those of chlorine.

• In the meantime, when chemical disinfection is needed, a l imited dose of chlorine may be applied.

References 1. Meijers, A. P.; de Moel, P. J.; van Paassen, J. A. M. Kiwa communication no.

70; Kruithof, J. C.; Kostense, Α., Eds.; KIWA Ltd.: Nieuwegein, Netherlands, July 1984.

2. Kruithof, J. C.; Hess, A. F.; Manwaring, J. F.; Beville, P. B. Aqua 1983, 2, 89-99.

3. Kruithof, J. C.; van der Leer, R. Chr. In Activated Carbon in Drinking Water Technology; Kruithof, J. C.; van der Leer, R. Chr., Eds.; AWWA RF: Denver, 1983; pp 57-118.

4. Meijers, A. P. Water Res. 1977, 11, 647-652. 5. Stalder, K.; Klosterkötter, W. Zentralbl. Bakteriol. Mikrobiol. Hyg. Abt. 1, Orig.

Β 1976, 161, 474-481. 6. van der Kooij, D.; Visser, D.; Hijnen, W. A. M. J. Am. Water Works Assoc.

1982, 74, 540-544. 7. Rook, J. J. Water Treat. Exam. 1974, 23, 234-245. 8. Bellar, Τ. Α.; Lichtenberg, J. J.; Kroner, R. C. J. Am. Water Works Assoc. 1974,

66, 703-706. 9. Sybrandi, J. C.; Meijers, A. P.; Graveland, Α.; Poels, C. L. M.; Rook, J. J.;

Piet, G. J. Kiwa communication no. 57; KIWA Ltd.: Rijswijk, Netherlands, 1978. 10. Kruithof, J. C.; Nuhn, P. Α. Ν. M.; van Paassen, J. A. M. H2O 1982, 15, 277-284. 11. Graveland, Α.; Kruithof, J. C.; Nuhn, P. Α. Ν. M. Abstracts of Papers, 181st

National Meeting of the American Chemical Society, Atlanta, GA; American Chemical Society: Washington, DC, 1981; ENVR 63.

12. Bassie, W.; Kruithof, J. C.; van Puffelen, J.; Smeenk, J. G. M. M. In Chlorination By-products; Production and Control; Kruithof, J. C., Ed.; AWWA RF: Denver, 1986; pp 129-156.

13. Kruithof, J. C.; Noordsij, Α.; Puijker, L. M.; van der Gaag, M. A. In Water Chlorination: Chemistry, Environmental Impact and Health Effects, Vol. 5; Jolley, R. L.; Bull, R. J.; Davis, W. P.; Katz, S.; Roberts, M . H., Jr.; Jacobs, V. Α., Eds.; Lewis Publishers: Chelsea, MI, 1985; pp 1137-1163.

14. van der Gaag, Μ. Α.; Kruithof, J. C.; Puijker, L. M. In Organic Micropollutants in Drinking Water and Health; de Kruijf, H. A. M.; Kool, H. J., Eds.; Elsevier Science Publishers: Amsterdam, 1985; pp 137-154.

15. van der Kooij, D.; Hijnen, W. A. M. Appl. Environ. Microbiol. 1984, 47, 551-559.

16. van der Kooij, D.; Visser, Α.; Oranje, J. P. Antonie van Leeuwenhoek 1982, 48, 229-243.

17. Maron, D. M . ; Ames, Β. N. Mutat. Res. 1983, 113, 173-215. 18. van der Gaag, Μ. Α.; Oranje, J. P. H2O 1984, 17, 257-261. 19. Noordsij, Α.; van Beveren, J.; Brandt, A. Int. J. Environ. Anal. Chem. 1983,

13, 205-217.

RECEIVED for review July 24, 1987. ACCEPTED for publication February 26, 1988.

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