Coagulation Process for Removal of Humic Substances from Drinking

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24 Coagulation Process for Removal of Humic Substances from Drinking Water Eilen Arctander Vik Aquateam, Norwegian Water Technology Centre A/S, P.O. Box 6326, Etterstad, 0604 Oslo 6, Norway Bjørnar Eikebrokk Norwegian Hydrotechnical Laboratory, SINTEF, 7034 Trondheim-NTH, Norway

Humic substances adversely affect the quality of drinking water in many ways. For instance, they impart color, serve as precursors to the formation of chlorinated compounds, possess ion-exchange and complexing properties that include association with toxic elements and micropollutants, and precipitate in distribution systems. This chapter reviews the coagulation process, in which aluminum sulfate is the most commonly used coagulant aid, along with ferric salts and some organic polyelectrolytes. Aluminum chemistry and coagulation mechanisms are reviewed in detail. Factors that are critical to the design of processes, such as the importance of the rapid-mixing process, are discussed. The results of several recent studies on conventional and directfiltrationtreatment are given. These results illustrate, from an operational perspective, the chemical theory that is presented. In spite of the development of new separation techniques for water treatment, the coagulation process will continue to be the most important water-treatment process worldwide for removal of humic substances. Further research is needed to improve the coagulation process, optimize coagulant dose, minimize residual aluminum, and integrate separation, disinfection, taste-, odor-, and corrosion-control treatment into the water-treatment program. 0065-2393/89/0219-0385$07.00/0 © 1989 American Chemical Society

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

386

AQUATIC H U M I C SUBSTANCES

HUMIC SUBSTANCES M I PART A YELLOW OR BROWN COLOR to waters wi


high organic content. These substances are undesirable in a potential water supply for a number of reasons, ranging from aesthetics to the fact that they are precursors of potentially carcinogenic compounds (trihalomethanes). The health effects of high concentrations of humic substances are unknown. Sev eral factors make aquatic humic substances an important constituent of nat ural water systems: • Humic substances, 40-60% of the dissolved organic carbon (DOC) in natural waters, are the largest fraction of natural organic matter in water. They are normally present in con centrations >1 mg of C / L , although identifiable organic com pounds can be present at concentrations that are orders of magnitude lower (I, 2). • Humic substances possess ion-exchange and complexing prop erties that are associated with most constituents of water, in cluding toxic elements and organic micropollutants (1-4). • Humic substances act as a vehicle for transport of toxic, waterinsoluble elements and organic micropollutants (1-4). • Chlorine combines with aquatic humic substances to form chlorinated organic compounds, such as chloroform (5-8) and complex chlorinated compounds (9-13), that may have nega tive effects on health (9-14). • Humic substances precipitate in the distribution system, where they lead to deterioration of tap water quality and i n crease the need for interior cleaning of the pipes. Concentrations of D O C in natural waters vary over a wide range. The world-stream-volume weighted mean, according to Meybeck (15), is 5.7 mg of C / L as D O C . Particulate organic carbon, including planktonic organisms, generally accounts for about 10% of the total organic matter (3, 16). D O C consists predominantly of humic substances. The lowest concentrations vary from 0.05 to 0.60 mg of C / L . Streams, rivers, and lakes contain from 0.50 to 4.0 mg of C / L . Colored rivers and lakes have much larger concentrations of humic substances, 10-30 mg of C / L (2).

Characteristics of Humic Substances The yellowish-brown color of natural surface water comes from humic sub stances leached from plant and soil organic matter. This color is seen in bogs and wetlands along streams. Organic acids dissolve into the stream. This



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Removal of HS from Drinking Water

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gives the yellow color and contributes protons to the soil weathering process (2). D O C , according to an operational definition, is organic carbon smaller than 0.45 μπι in diameter (Figure 1). At the molecular level, most of the dissolved organic carbon comes from polymeric organic acids called humic substances. These yellow organic acids (1000-2000 M W) are polyelectrolytes of carboxylic, hydroxyl, and phenolic functional groups. They compose 50-75% of the dissolved organic carbon and are the major class of organic compounds in natural waters. Figure 1 shows the continuum of organic carbon in natural waters. Dissolved molecules of fulvic acid are approximately 2 nm in diameter and at least 60 nm apart. There might be five inorganic ions (calcium, sodium, bicarbonate, chloride, and sulfate) between each pair of these organic mol ecules. There is also some colloidal organic matter in the water. These colloids are large aggregates of humic acids, 2-50 nm in diameter. They are com monly associated with clay minerals or oxides of iron or aluminum. In most natural waters the colloidal organic matter is approximately 10% of the D O C . Colloidal organic carbon is the humic acid fraction of humic substances. This fraction is larger in molecular weight (2000-100,000) (Figure 1) and contains fewer carboxylic and hydroxyl functional groups than the fulvic acid fraction. Humic acid adsorbs and chemically binds to the inorganic colloids, modifying their surface. Dissolved humic substances compose 5-10% of all anions in streams and rivers. This anionic character gives humic substances their aqueous solubility, binding sites for metals, buffer capacity, and other characteristics. The anionic character comes from the dissociation of carboxylic acid func tional groups: R—COOH ^=± R—COO" + H

+

(1)

Carboxylic functional groups occur on aquatic humic substances with a frequency of 5-10 per molecule. At the p H of most natural waters, 6-8, all carboxylic groups are anionic or dissociated. These charged groups repulse one another and spread out the molecule. The counterions balancing the charge of the negative groups are mostly calcium and sodium. However, trace metals may be bound to some carboxylic groups that have favorable steric location. That is, a carboxylic group in association with a phenolic group may form a chelate or a ring structure and bind metal ions. Because of the complex nature of humic substances, they have tradi tionally been classified according to the operational procedures needed to separate them. Oden's classification (17) of these substances has been used since its introduction in 1919. Several methods have been used for deter mining the molecular size of humus; results have varied according to method. Newer separation techniques have improved the results.



Où 00

S!

C/5

cCD

a

g

a

ο Χ

a

> Ο

oo

Figure 1. Continuum of particulate and dissolved organic carbon in natural waters. (Reproduced with permission from ref. 2. Copyright 1985 Kluwer Academic Publishers.)



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389

Thurman (2), in a broad discussion of literature on the molecular weight of humic substances, concluded that aquatic fulvic acid is generally less than 2000 M W . This low molecular weight means that most aquatic humic sub stances are dissolved, rather than colloidal. Because humic matter in water is associated with various metal ions, clays, and amorphous oxides of iron and aluminum, they may have larger molecular weight than determined with the purified free acid. Humic acid is 2000-5000 M W or greater, and is therefore considered to be colloidal. Thurman (2) also discussed the possible structures and structural units that may be present in aquatic humic substances. The separation of humic acid from fulvic acid by precipitation at p H 1 is important. Humic acid is less soluble than fulvic acid. The lower amount of carboxylic acid lowers the aqueous solubility of humic acid and is the main reason that most natural waters contain 5-25 times more fulvic acid than humic acid. The second factor is the size of the humic acid, 2-10 times larger than fulvic acid. This large size lowers the aqueous solubility of humic acid. Also, the phenolic content of the humic acid is somewhat greater than that of the fulvic acid, and there are more color centers on the humic acid molecule (18). Humic acid does contain longer-chain fatty-acid products than fulvic acid (19). This finding suggests that humic acid is more hydrophobic because of the longerchain fatty acids ( C - C ) . According to Stevenson (20), the biochemistry of the formation of humic substances is one of the least understood and most intriguing aspects of humus chemistry. Pathways suggested for the formation of humus in soil are the two lignin-degradation models, the polyphenol theory, and the sugar-amine condensation theory. Because aquatic humic substances are different from soil humic substances, those originating in water may have other mechanisms of formation (21). New analytical techniques have been developed within the last few years. Nuclear magnetic resonance (NMR) spectroscopy, and especially C N M R spectroscopy, has become important in elucidation of the chemical structures of humic materials (22). New an alytical methods are also giving a new perspective to a detailed understanding of the coagulation process for humic-substances removal. 1 2

1 8

1 3

The Coagulation Process C o a g u l a t i o n i n W a t e r T r e a t m e n t . Coagulation with aluminum sulfate, a standard procedure for removing suspended colloids, is also an effective method for removing aquatic humus. Coagulation is one of the most important processes used in treatment of surface waters (23). The conventional water-treatment process (Figure 2) includes coagu lation, flocculation, sedimentation, and final filtration. Direct filtration (Fig ure 3) is an important option in conventional water treatment. A status report (24) discussed results of a worldwide survey of direct filtration plants. Color



C O

Coagulation

Raw water

Coagulant

Sedimentation

Filtration

Figure 2. The conventional water-treatment plant.

Flocculation

+ Treated water

WÊÊÊËÊÊÊÊr

1


•ζ a m

I

CO

C

ζΛ

Ω

S c s

ο

> ο c

24.


VIK & EIKEBROKK

391

Coagulant

•

Raw water


Coagulation

I

Filtration

•Treated water

Figure 3. The directfiltrationplant.

exceeding 30-40 Hazen units and continuing turbidity greater than 15 F T U were pointed out as problem situations. Polyelectrolytes were proposed as a short-term substitute for all or part of the required primary coagulant; thus the amount of sludge was reduced. Full-scale experiments (25) indicate that this process is suitable for high concentrations of humic substances when turbidity is low. Further improvement may be obtained if the coagulation process is carried out at lower p H values (—4.7) and low alum concentrations (=0.3mgof Al/L). Coagulants i n Use. Aluminum is the most important coagulant i n use. Ferric compounds are applied to some extent. Alum and ferric salts are, however, not the only reactants used in coagulation. Magnesium car bonate, hydrolyzed with lime, has been shown to be as effective as alum (26, 27). Various organic polyelectrolytes have been used for organic substance removal (28-31). A l l of them show good humus removal, especially at lowto-moderate D O C concentrations. Most of the studies using polymers are related to the use of direct filtration (30-35). Synthetic organic cationic polyelectolytes are widely used as coagulants in direct filtration. Their use as primary coagulants in direct filtration of humic matter has also been studied (30-32, 36). Although cationic polymers showed good removal ef ficiency, they are not as effective as alum (aluminum sulfate). For waters containing a relatively high concentration of humic substances, direct filtra tion with cationic polyelectrolytes produced less head loss development and longer filter runs when a flocculation period was included (31). Many studies have been conducted to evaluate the performance of coagulation with aluminum and ferric salts (38). U p to 90% removal of the humic acid fraction has been achieved with both Al(III) and Fe(III) (39-41). Alum reacts differently in water than the chloride, nitrate, or perchlorate salts of aluminum (42). Polyaluminum chloride (PAC1), a coagulant that con sists of partially hydrolyzed aluminum chloride, is produced by the addition of base to concentrated A1C1 solution. Dempsey and co-workers (42) found PAC1 to be a promising alternative to alum as a coagulant for removing humic substances from water supplies. 3


392


The removal of soluble organic contaminants by lime softening has been studied (37). The precipitate in this case is mainly C a C 0 , but if magnesium is present and the p H is sufficiently high, M g ( O H ) will also be formed. The process effectively removes several humic substances, including a fulvic acid. Removal efficiency increased with increasing p H , increasing amount of pre cipitate, and decreasing concentrations of total organic carbon (TOC). Re moval efficiency was significantly enhanced by the presence of magnesium or phosphate, especially when the amount of precipitate formed was small. 3


2

A L U M I N U M CHEMISTRY. The aqueous chemistry of aluminum is com plex and diverse because of the numerous hydrolysis intermediates formed before precipitation of aluminum hydroxide, Al(OH) (s). The aluminum ion, A l , behaves very much like F e in solution, except that it has a greater tendency to form polynuclear species. Several authors have reported (16, 43, 44) a stepwise conversion of the positive aluminum hydrated ion to the negative aluminum ion. When aluminum salts are dissolved in water, the metal ion A l hydrates, coordinates six water molecules, and forms an aquometal ion, A l ( H 0 ) . For simplicity, the H 0 ligands attached to the A l ions are omitted (e.g., A l ( H 0 ) O H is written as A l O H ) . Several hydrolysis species are possible: 3

3 +

3 +

3 +

3 +

2

6

2

2 +

2

Al

3 +

-* AlOH

2 +

-» Al(OH)

+ 2

2 +

5

- » Al(OH) (aq) 3

2

3

A l ( O H ) - - » Al(OH) ~ 4

Al(OH) "

5

(2)

6

These species can form polymers with several of the hydrolysis products. The following polymers have been suggested: A l ( O H ) ; Al (OH) ; A1 (OH) ; Al (OH) ; Al (OH) . For more than 20 years, scientists have argued about the predominance and existence of aluminum polymers. Baes and Mesmer (45) indicated that aqueous aluminum equilibrium chemistry can be explained accurately (but not uniquely) by considering three polymeric species [ A l ( O H ) , A l ( O H ) , and A l 0 ( O H ) ] , five monomers [ A l , A l O H , A l ( O H ) , A l ( O H ) , and A l ( O H ) ] , and a solid precipitate [Al(OH) (s)]. Amirtharajah and Mills (46) emphasized that two important deductions follow from the existence of hydrolysis Al(III) species: 5 +

1 3

4+

8

20

3 +

6

4 +

3 4

7

1 7

2

2

4 +

1 5

2

2

4 +

5 +

3

7 +

4

1 3

4

3 +

2 +

2 4

+

2

3

4

1. Hydroxy metal complexes readily adsorb on surfaces, and the charges they carry may cause charge reversals of the surfaces on which they adsorb (44, 47, 48). The hydrolysis products of aluminum in aqueous solution are adsorbed more readily than the free A l ion. Matijevic (49) showed that the hydrolyzed aluminum species reversed the charge of the originally neg ative hydrolyzed halide ions, whereas the simple hydrated Al did not. The greater the degree of hydrolysis, the more extensive is the adsorption (49). 3 +

3 +


3

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393

+

2. The sequential hydrolysis reactions release H ions, which lower the p H of the solution i n which they are formed. A d ditionally, the concentration of the various hydrolysis species will be controlled by the final concentration of H ions, (i.e., by p H value) (46).


+

In addition to the hydroxy compounds, aluminum forms other com plexes such as the fluorides and sulfates. Although fluoride is a minor con stituent of most natural waters, the complexing action is strong enough to have considerable influence on the form of dissolved aluminum, even when very little fluoride is present (SO, 52). REACTION RATES FOR ALUMINUM COMPOUNDS.

A few attempts have

been made to establish the rate of the reactions between Al(III) and colloidal suspensions. Bratby (52) summarized data on reaction rates and calculated the time needed for adjusting the structure of the double layer at around 10" s and for a Brownian collision (diffusion) around 10~ -10~ s. According to Amirtharajah and Mills (46), sweep coagulation, which involves formation of Al(OH) (s) and entrapment of the colloid amid the precipitate, is a slower process that occurs in the range of 1-7 s. In contrast, the adsorption destabilization process occurs in approximately 10" s. Formation of polymer ized aluminum hydrolysis products is a slower process, on the order of 1 s. Reaction between aluminum and fulvic acid is faster than the reaction between aluminum and inorganic particles (53). Hahn and Stumm (54) found that the rate of coagulation of silica particles by hydrolyzed aluminum was limited by collision frequency and efficiency, rather than by the hydrolysis of aluminum. 8

7

3

3

4

Coagulation and Flocculation. Humic-substance colloids retain a dispersed or dissolved (stable) state in surface water. In the p H of most natural waters, humic materials are negatively charged macromolecules in the colloidal size range. The coagulation process is used to overcome the factors promoting the stability of humic-substance molecules in water. Alum is frequently used to destabilize humic molecules, and is included in this chapter's focus on the coagulation process. Destabilization is the conversion of the stable state of a given dispersion or solution to an unstable state. Such a process could alter the surface properties of particulate material and thereby increase the adsorptivity of the particles to a given filter medium or generate a tendency for small particles to aggregate into larger units. Alternatively, destabilization could precipitate dissolved material and thereby create particulate material for which separation by sedimentation or filtration is feasible. Coagulation achieves destabilization of a given suspension or solution. That is, the function of coagulation is to overcome the factors promoting the stability of a given system. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

394

AQUATIC HUMIC SUBSTANCES


Ffaccufation is the process whereby destabilized particles, or particles formed as a result of destabilization, are induced to come together, make contact, and thereby form larger agglomerates. Coagulation and floeeulation involve aggregation of particles into larger, more readily removable, aggregates. Aggregation of colloidal particles can be considered to involve two separate and distinct steps: particle transport to achieve interparticle contact (fluid and particle mechanisms) and particle destabilization to permit attachment when contact occurs (colloid and surface chemistry). COAGULATION MECHANISMS. There are four distinct mechanisms for destabilization of colloids: compression of the diffuse layer; adsorption to produce charge neutralization; enmeshment in the precipitate; and adsorp tion to permit interparticle bridging. O'Melia (47) discussed the four mech anisms as follows: 1. Compression of the diffuse layer involves purely electrostatic interactions. The phenomenon is described by the theoretical Verwey-Overbeek model and the empirical Schulze-Hardy rule. The coagulants, called indifferent electrolytes, are of limited interest in water and wastewater treatment processes. The concentrations of Na , C a , and A l required to de stabilize a negatively charged colloid vary approximately in the ratio of 1 to 10" -10" . Ionic strength is of major im portance, but the salt concentration required for effective destabilization is too high for practical water-treatment applications. Restabilization is not possible because of a chem ical overdose. +

2

2 +

3 +

3

Adsorption and charge neutralization produce destabilization at low dosages; overdosing may lead to restabilization and charge reversal. If coulombic interaction were the only driving force for destabilization, it would not be possible to adsorb excess counterions to produce charge reversal and restabili zation. Aluminium salts used in coagulation have been shown to follow this mechanism at low dosages. Enmeshment in a precipitate (also called sweep coagulation) can occur at high dosages of metal salts. The concentration of A 1 ( S 0 ) must be sufficiently high to cause rapid precipitation of Al(OH) (s) before the colloid particle can be enmeshed in these precipitates. To the first approximation, the rate of pre cipitation of a metal hydroxide is dependent upon the extent to which the solution is oversaturated. The extent of oversaturation is described by ( A l ) - ( O H " ) / K , where K is the solubility constant for the Al(OH) (s) product. For very rapid 2

4

3

3

3+

3

s

s

3


24.

VIK & EIKEBROKK


395


precipitation, the ratio must be 100 or even larger. In neutral and acid water the rate of precipitation is also increased by the presence of anions in solution, particularly sulfate ions. The colloidal particles serve as nuclei for the formation of the precipitate, so the rate of precipitation increases with increas ing concentration of the colloidal particles to be removed. The greater the concentration of colloidal material, the lower is the amount of metal coagulant required. 4. Adsorption and interparticle bridging usually occur when polymers are used as destabilizing agents in the treatment of water and wastewater. This mechanism will not be further discussed here.

Bratby (52) illustrated the three first mechanisms in Figures 4 and 5. Amirtharajah and Mills (46) did an extensive literature review, summarizing important coagulation studies. Figure 6 summarizes in schematic form the proposed predominant mechanism of coagulation with alum. Figure 7 pre sents a design and operational diagram for alum coagulation, including spe cific areas where coagulation would occur and the major mechanisms causing coagulation. The skeletal forms of the diagram were previously presented by other workers. Minor changes in the restabilization zone are expected, depending on the type of colloid. The area for optimum sweep coagulation

T3 3 3 T3

I

ccc Coagulant Concentration Figure 4. Destabilization characteristics where an electrical double-hyer repression mechanism is predominant. Increasing the coaguhnt concentration beyond the critical coagulation concentration (CCC) has no substantial effect.



396


CSC

1

4

Coagulant Concentration

Figure 5. Destabilization characteristics that involve adsorption of coagulant species to colloidal particles. CCC and CSC signify the concentration, C , necessary to destabilize and restabilize the dispersion, respectively. A further critical coagulation concentration, CCC , indicates double-layer repression or enmeshment mechanisms of higher coagulant dosages. t

t

2

(i.e., the area for best settling floes) is specifically defined by an alum dose of 20-50 m g / L , with a final p H of 6.8-8.2. RAPID-MIXING PROCESS. Several researchers have discussed the im portance of rapid-mix parameters. The major consideration in rapid mixing has been uniform distribution of the coagulant in the raw water in order to avoid over- and undertreatment of water. Amirtharajah and Mills (46) dis cussed the two modes of destabilization in relation to the influence of the rapid mix, based on the reaction rates previously discussed (Figure 6). For adsorption-destabilization, Amirtharajah and Mills (46) emphasized that the coagulants have to be uniformly distributed in the raw water stream as rapidly as possible (

Coagulation Process for Removal of Humic Substances from Drinking

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