Eﬀects of polyelectrolytes on reduction of model compounds via coagulation

E.-E. Chang

, Pen-Chi Chiang

^b,*

, Wei-Yan Tang

, Su-Hei Chao

, Hao-Jan Hsing

aDepartment of Biochemistry, Taipei Medical University, Taipei, Taiwan, ROC

bGraduate Institute of Environmental Engineering, National Taiwan University, 71, Chou-Shan Road, Taipei, Taiwan 106, Taiwan, ROC

Received 17 March 2004

Abstract

The objective of this research work was to evaluate the performance of enhanced coagulation by alum and polymer.

Synthetic source waters containing high molecular weight humic acids, medium molecular weight tannic acids and low molecular weight p-hydroxybenzoic acid were formulated by adjusting the concentration of turbidity and pH; and jar tests were used to study the eﬀect of various types and dosages of polymer on reducing the above model compounds.

At a speciﬁc pH condition, the applied alum dosage would eﬃciently decrease the turbidity to 2 NTU follows the order: humic > tannic > p-hydroxybenzoic acid. Adjustment of pH inﬂuenced the performance of alum obviously but not of p-DADMAC. High p-DADMAC dosage overwhelming the eﬀects of alum is less aﬀected by pH adjustment.

The results of this investigation reveal that enhanced coagulation with p-DADMAC was founded to be very eﬀective for removing high-molecular-weight THM precursors, i.e., humic acid and tannic acid, and markedly reduced the alum dosages required for turbidity removal. The other two polymers, i.e., cationic PAM and non-ionic PAM, which had higher molecular weight but lower charge density than p-DADMAC, were not capable of removing organic precursors.

It was thus concluded that enhanced coagulation with polymer, p-DADMAC, could be considered as a promising tech-nique for removal of NOMs with hydrophobic and higher-molar-mass (>1 K) in water treatment plants.

Keywords: Enhance coagulation; Polymer; Polyacryamide; p-diallyldimethyl ammonium chloride; p-hydroxybenzoic acid; Tannic acid;

Humic acid; Trichloromethane (THM) formation

1. Introduction

The coagulation process is optimized primarily for the removal of turbidity. Although, nature organic matter

(NOM) is also removed by coagulation, the removal eﬃ-ciency varied with the physical and chemical characteris-tics of the water as well as the operating conditions of the coagulation process (Ratnaweera et al., 1999). Unless the raw water has a low total organic carbon (DOC) concen-tration, coagulant dosages are determined by the content of NOM in raw water rather than by turbidity (OMelia et al., 1999). Generally, the higher-molar-mass fraction of organic matter (OM) is readily removed by 0045-6535/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.chemosphere.2004.08.008

* Corresponding author. Tel.: +886 2 2362 2510; fax: +886 2 2366 1642.

E-mail address:[email protected](P.-C. Chiang).

Chemosphere 58 (2005) 1141–1150

www.elsevier.com/locate/chemosphere

coagulation. The type of OM in raw water is also a factor aﬀecting its removal by coagulation. Functional groups of OM inﬂuence the solubility of organic compounds;

hydrophobic OM is easier to be removed than hydrophilic OM (Collins et al., 1986; White et al., 1997).Owen et al.

(1993)indicated that a large percentage of disinfection by products (DBPs) was formed from the non-humic frac-tion of NOM. This fracfrac-tion is generally more hydrophilic than humic substances and thus more diﬃcult to remove by coagulation.

Polymers have particular advantages over inorganic coagulants for NOM removal. The performance is less pH dependent and there is a lower level of dissolved ions in the product water.Mallevialle et al. (1984)found that chlorination of polyacryamide (PAM) and acryamide monomers shows low reactivity, and generated a small amount of total organic halides (TOX) and trihalometh-ane (THM).Chang et al. (1999)found that the polydial-lyldimethyl ammonium chloride (p-DADMAC) not only eﬀectively removed the turbidity but also reduced the formation of THM. In evaluating cationic polyelec-trolytes for the removal of UV absorbers, addition of alum followed by cationic polymethacrylate, p-DAD-MAC or cationic PAM (CPAM) was found to be eﬀec-tive. A polymer with higher charge density (CD) is more eﬀective in reducing UV absorbers than that with low CD. p-DADMAC is not considered to be toxic and ac-cepted for use in treatment of municipal water supplies by USEPA. However, the USEPA acceptance is only by the speciﬁc name of the suppliers and not by generic type, the maximum dose for p-DADMAC is 10 mg l ¹ (AWWA, 1987). PAM is a high molecular weight organ-ic polymer and solute in water easily, and can resist the attack from microbial (Seybold, 1994). Chronic environ-mental studies indicated that no adverse eﬀects were dis-covered in workers exposed to PAM dust over a period of 5 years. It has also been known to be non-toxic to human animals, and ﬁsh (Anonymous, 1991).

A thorough understanding of the reaction mecha-nism is a necessary step in determining the proper type of polymer to be used for the coagulation process. The reaction of polymers with other chemicals such as disin-fectants in the form of chlorine may adversely aﬀect the success of the coagulation process. Therefore, the objec-tives of this paper were intended to investigate the eﬀects of three organics acids i.e., humic acid (HA), tannic acid (TA) and p-hydroxybenzoic acid (PHBA) on coagula-tion performance and THM formacoagula-tion potential as well as to determine the most suitable polymer as a coagu-lant-aid in the coagulation process.

ulate some of the wide range of organics found in NOM (Exell and Vanloon, 2000). HA represents fairly hydro-phobic, high-molar-mass (molecular weight (MW) = 10 to 100 thousands) natural compounds and is a nega-tively charged polyelectrolyte due to the dominance of carboxylic acid groups. A number of previous studies have utilized this material; it represents a good model humic substance (Chang et al., 2001; Mustafa and Walker, 2001). TA represents relatively hydrophilic com-pounds of medium molar mass (MW = 1700), and PHBA (MW = 138) represents small organic molecules found in nature. All of the model compounds containing carboxylic and phenolic groups. Jar tests with rapid mix-ing, followed by settling were conducted to evaluate the eﬃciencies of the coagulant and coagulant-aid in remov-ing these compounds and reducremov-ing turbidity, as well as THM formation potential under various pH conditions.

2.1. Polymers

Two types of polyacryamide (SNF Co.) including non-ionic PAM of high MW ranging from 5 to 15 mil-lion, and CPAM of positively-charged, with CD < 15%

containing very high molecular weight (3 to 15 million) were used in this study as coagulant aid. Another cat-ionic polymer, p-DADMAC, which has a high CD (100%) and varying MW was also used in this study.

2.2. Synthetic water

Synthetic water was made up to resemble the alkalin-ity, turbidalkalin-ity, and OM (HA, TA, and PHBA) levels of natural water. In 1 l of distilled water, sodium bicarbonate was added to produce on alkalinity of 100 ± 10 mg l ¹as CaCO₃, and 0.662 mg bentonite was added to obtain an approximate turbidity of 200 NTU. The DOC of the synthetic water prepared above was near 7 mg l ¹as C. This solution was mixed on a stir plate for 1 h before being transferred to 21 l. The water was then left in a closed container overnight (>18 h) and the pH was adjusted before it was used in jar tests.

2.3. Jar tests and analyses

All three coagulant-aids and each type of organic com-pounds were used to compare the eﬀectiveness of each coagulant-aid in removing various types of OM and tur-bidity under diﬀerent pH condition. The alum used as a coagulant that chemical formula was Al(SO₄)₃Æ18 H₂O (Kento Chemical). The solutions with coagulant were rapid-mixed at 100 rpm for 3 min, slow-mixed at 30 rpm 1142 E.-E. Chang et al. / Chemosphere 58 (2005) 1141–1150

the treated water samples. The QA/QC programs set forth in Standard Methods (APHA, 1995) were followed for all sample analyses. Water samples for DOC and UV254 analyses were ﬁltered through 0.45 lm ﬁlters and determined by a TOC instrument (model 700, O.I.

Corp.), and UV spectrophotometer (Hitachi U-2000) respectively. The chlorine concentration were adjusted to about 3 to 40 mg l ¹, which were depending on the chlorination period and would provide a free residual chlorine of at least 0.2 to 5 mg l ¹at the end of the incu-bation period (APHA, 1995). The analysis of residual chlorine was performed by using the DPD (N,N-diethyl-p-phenylene-diamine) ferrous titration method.

3. Results and disscussion

3.1. Eﬀects of polymers on coagulation enhancement at neutral (pH 7) condition

Fig. 1presents the results of jar test for water samples containing HA, TA and PHBA with alum coagulant at

pH 7. Under the neutralized condition, the concentra-tion of ﬂocs formed by Al(OH)3was low and, therefore, ﬂocs could not sweep the particles in water. Under acidic conditions, corrosion rate was accelerated that was not feasible in water treatment. They contribute to the con-centration of suspension and resulted in high turbidity in alum-treated water (Adin et al., 1998). The dissolved or-ganic carbon (DOC) in raw water was converted to a non-settling particulate form at low alum dosage and contributed to turbidity resulting in so-called ‘‘negative eﬀect’’ phenomenon (White et al., 1997; Singer and Bilyk, 2002).Manahan (1994)found that the humic sub-stances could bind the metal ions such as aluminum and iron. This binding can occur as chelation between a car-boxyl group and a phenolic hydroxyl group. It was evi-dently shown inFig. 1a that it required over 140 mg l ¹ of alum to render the residual turbidity lower than 2 NTU. Compared with the HA water, it requires less dos-age of alum for treating the TA and PHBA water. At a speciﬁc alum dosage (<140 mg l ¹), the lower residual is associated with decreasing organic MW. The more complex structure and functional groups, the higher

Fig. 1. Results of jar tests of (a) turbidity and (b) DOC for water samples by introducing alum dose at pH 7. (Raw water:

DOC = 7.0 ± 0.7 mg l ¹, Turbidity = 200 ± 10 NTU, Alkalinity = 100 ± 10 mg l ¹as CaCO3.)

E.-E. Chang et al. / Chemosphere 58 (2005) 1141–1150 1143

chemical dosage is needed to destabilize the system (Divakaran and Pillai, 2001).

The requirement of DOC removal for enhanced coag-ulation suggested in the USEPA D/DBP Rules, provides an operational procedure to establish a point of dimin-ishing returns (PODR) which is deﬁned as the alum dos-age beyond which <0.3 mg l ¹ DOC is removed per 10 mg l ¹addition of alum in various jar tests. 35% of DOC removal eﬃciency was set as an evaluation crite-rion in this research. It could be seen fromFig. 1b that the slope of DOC/alum became steeper over 100 mg l ¹ of alum, therefore, 100 mg l ¹was the threshold dosage for HA water. It was obvious that TA was relatively easy to be removed by coagulation than HA. Alum has very little eﬀects on PHBA removal at lower alum dosage, although it could remove the turbidity quite successfully.

Since great amounts of coagulants were needed to achieve DOC and turbidity removal requirements, vari-ous polymers are chosen as coagulant aids to enhance coagulation and reduce alum consumption. While treat-ing HA water, the addition of CPAM could only reduce turbidity slightly. Non-ionic PAM had better eﬃciency in removing turbidity than CPAM (Table 1). It is be-cause CPAM could neither adsorb positively charged ﬂocs nor neutralize the charge of particles due to its low CD. Contrarily, using p-DADMAC as coagulant-aid, the residual turbidity could be reduced to a lower level, even less than 1 NTU at higher dosages. For exam-ple, an alum dosage of 20 mg l ¹, 80% of turbidity was removed with 8 mg l ¹ of p-DADMAC for both TA and HA. As alum dose increased, p-DADMAC addition could reduce turbidity signiﬁcantly.

While treating PHBA water, both PAMs had better eﬀects on enhancing the turbidity removal than treating

HA or TA water. Non-ionic PAM was still better than CPAM in turbidity removal, but even at the highest chemical dosage, 60 mg l ¹ of alum and 10 mg l ¹ of non-ionic PAM, the residual turbidity of treated water (41 NTU), was extremely higher than the Drinking Water Quality Standard in Taiwan. About 5 mg l ¹ of p-DADMAC does reduce the turbidity of treated water to lower than 2 NTU, regardless of the amount of alum dosage. It was evident that the addition of p-DADMAC had signiﬁcant improvement on turbidity removal.

Among three types of polymers, p-DADMAC exhib-its the most eﬃcient performance for turbidity removal.

Comparing the properties of polymers, both PAMs had higher molecular weight and lower CD than p-DAD-MAC. The diﬀerence in coagulation performances exhibited by the various type of polymer suggests that the CD of a polymer should be more inﬂuential than the molecular weight. In this investigation, it was found that the organic composition in water would aﬀect the eﬃciency of turbidity removal. The organic compounds with complex structures and functional groups required higher chemical dosages to produce suﬃcient positive charged ﬂocs for turbidity removal by charge neutrali-zation and adsorption. It was thus concluded that the se-quence of the amounts of chemical needed for turbidity removal be: HA > TA > PHBA.

Fig. 2show DOC removal eﬃciencies for HA treated with polymers. While treating HA water, p-DADMAC is the only one to enhance the coagulation eﬃciency over the threshold of enhanced coagulation requirement over 35% of DOC removal. Both cationic and non-ionic PAM had little eﬀects on DOC removal. Moreover, higher PAM dosage would remain in treated water and result in higher residual of DOC concentration.

Table 1

Residual turbidity in three types of NOMs water samples treated by alum plus polymer coagulation process^a

Polymer Residual turbidity—NTU

Type Dose level

(mg l ¹)

Alum 20 mg l ¹ Alum 40 mg l ¹ Alum 60 mg l ¹

HA TA PHBA HA TA PHBA HA TA PHBA

200 162 186 236 130 167 262 117 113

p-DADMAC 2 175 140 37.6 181 105 30.8 137 22.4 10.5

p-DADMAC 5 103 98.0 0.9 38.3 7.6 1.8 11.2 1.2 1.7

1144 E.-E. Chang et al. / Chemosphere 58 (2005) 1141–1150

p-DADMAC could strengthen the linkage between par-ticles and ﬂocs, which enlarge the size of ﬂocs and make them easier to settle. Furthermore, the organic matter might be adsorbed on to the ﬂocs and be removed along with the precipitates. However, insuﬃcient dosage of p-DADMAC would increase DOC concentration in trea-ted water, and it was even much signiﬁcant than the over-dosage of cationic and non-ionic PAM. While dos-ing 20 mg l ¹of alum, over 8 mg l ¹of p-DADMAC was needed to achieve the 35% DOC removal requirement.

Similar patterns were observed that over 5 mg l ¹ p-DADMAC was needed when 40 mg l ¹ of alum was added and 2 mg l ¹ of p-DADMAC with 60 mg l ¹ of alum. Thus, it can be concluded that the higher the alum dosage, the lower dosing p-DADMAC is needed for

DOC removal. Consequently, lower dose of p-DAD-MAC could easily link the particles and ﬂocs together due to its high positive CD and results in the formation of polymer–ﬂoc complexes.

In general, results regarding DOC removals from the treatment of water containing TA were similar to those of HA water. Dosing 60 mg l ¹ of alum could reduce TOC concentration eﬀectively, polymers might not be necessary unless higher removal requirement is needed.

Both PAMs not only had little eﬀects on DOC removal, but also impeded the coagulation performances regard-less of changes in alum dosage. When the dosage of alum was 20 mg l ¹, more than 6 mg l ¹of p-DADMAC was needed to achieve the enhanced coagulation requirement.

TOC removal (%)TOC removal (%)TOC removal (%) DOC (mg l-1)DOC (mg l-1)DOC (mg l-1)

Fig. 2. Results of jar tests (DOC) for introducing alum as (a) Alum 20, (b) Alum 40, (c) Alum 60 plus polymers to humic acid water samples. (Raw water: DOC = 7.0 ± 0.7 mg l ¹as HA, Turbidity = 200 ± 10 NTU, Alkalinity = 100 ± 10 mg l ¹as CaCO3.)

E.-E. Chang et al. / Chemosphere 58 (2005) 1141–1150 1145

Since alum alone was not capable of removing PHBA, diﬀerent polymers were dosed to improve the coagulation performance. However, none of the poly-mers used in this research could enhance coagulation performance. It is evident that the hydrophilic property and smaller molecular weight of PHBA could impede the co-precipitation and adsorption of organic carbon resulted in lowering reduction of DOC.

3.2. Eﬀect of pH adjustment on turbidity and DOC removal for p-DADMAC

The pH adjustment with metal salt coagulants is an important operating parameter for the coagulation pro-cess. Adjusting the pH to the range between 4 and 5 are generally believed to enhance the coagulation perfor-mance with alum. The pH of synthetic water was ad-justed to 5, 6, and 7 prior to coagulant addition. While treating the HA water, p-DADMAC could help to re-move most of the turbidity in water. At low alum dos-age, e.g., 20 mg l ¹, the coagulation eﬀects mainly were contributed by p-DADMAC, however, pH eﬀect is not

signiﬁcant shown in Fig. 3a. When the alum dosage was increased to 60 mg l ¹, pH eﬀects became obvious.

As shown inFig. 3b, the percent turbidity removal at pH 5 is higher than that at pH 6 or 7. The addition of p-DADMAC became useless at pH 5 due to the suﬃ-cient alum and dosage. The role of p-DADMAC on treating TA water is similar to that on treating HA water. In general, the eﬀects of pH became obvious with increasing alum dosage.

It was concluded that pH would aﬀect the perfor-mance of alum in removing turbidity but not of p-DADMAC. Therefore, low p-DADMAC dosage in cooperation with high alum dosage would be aﬀected by pH adjustment. Since a slight reverse of turbidity re-moval was observed at high polymer dosage and low pH for treating the above organic precursors, the dosage of polymer must be controlled well in low pH conditions.

InFig. 3c and d, it was observed that DOC removal increased with decreasing pH value at 20 mg l ¹of alum dosage. It took about 5 mg l ¹ of p-DADMAC to achieve the same percent DOC removal requirement at pH 5, while higher dosage was needed at higher pH.

1146 E.-E. Chang et al. / Chemosphere 58 (2005) 1141–1150

However, when dosing 60 mg l ¹of alum at pH 5, higher dosage of p-DADMAC decreased DOC removal. High concentration of aluminum hydroxide species and hydrogen ions neutralized the negative charges on sus-pended particles during rapid mixing; part of the p-DADMAC added was utilized to bridge the particles.

Therefore, excessive p-DADMAC dose would remain in water sample and contribute to DOC concentration.

While treating the most irresponsive-to-coagulation or-ganic matter, PHBA, pH adjustment combining poly-mer addition is ineﬀective, and there is no obvious relationship between chemical dosage and DOC removal eﬃciency.

Comparing the UV254variation as shown inFig. 4a, it is obvious that the concentration of organic matter with aromatic structure (HA in this case) decreased with increasing p-DADMAC dosage. In many studies, UV absorbance was used as a surrogate indicator for deter-mining organic precursors (OMelia et al., 1999; Singer and Bilyk, 2002). In this research, UV254was also used as a supplementary index to determine the composition of organics in water. In order to determine the composi-tion of organics in water treated by high polymer dose, UV₂₅₄is again used as an index, as shown inFig. 4b.

It can be observed that the reverse shows up when the concentration of DOC is already low in water treated by alum without polymer addition. It implies that if DOC is reduced to a low level, the addition of p-DAD-MAC must be controlled carefully for treating the water containing low level of DOC, otherwise, it might be use-less and harmful.

3.3. Reduction of THM formation potential (THMFP) Among three types of organic precursors, HA had the highest THM yield (110 lg THM/mg DOC), PHBA the second highest (60 lg THM/mg DOC) and TA the lowest at about 50 lg THM/mg DOC. HA contains many activating functional groups such as hydroxyl, carbonyl, and acryloxy etc. which will react with chlo-rine to form THM.

Chlorine demand and THMFP are both related to DOC concentration. Enhanced coagulation by alum plus p-DADMAC could reduce the chlorine demand.

The THMFP and chlorine demand of raw, alum-, and

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