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Strength and regrowth properties of polyferric-polymer dual-coagulant flocs in surface water treatment
Journal of Hazardous Materials 175 (2010) 949–954
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Strength and regrowth properties of polyferric-polymer dual-coagulant flocs in surface water treatment J.C. Wei, B.Y. Gao ∗ , Q.Y. Yue, Y. Wang Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, 250100 Jinan, China
a r t i c l e
i n f o
Article history: Received 7 July 2009 Received in revised form 22 October 2009 Accepted 23 October 2009 Available online 30 October 2009 Keywords: Dual-coagulant Floc strength Floc regrowth Surface water
a b s t r a c t The floc strength and regrowth properties of three polyferric-cationic polymer dual-coagulants were comparatively evaluated using surface water sample. The first dual-coagulant PFC-PD was prepared by premixing of polyferric chloride (PFC) and polydiallyldimethylammonium (PDADMAC) before dosing. The other two, PFCF (PFC dosed firstly) and PDF (PDADMAC dosed firstly), were achieved by dosing PFC and PDADMAC in different order. Floc strength properties were measured in response to increasing shear levels in the long period and high shear level in the short or long period. For the given optimum dose (3.0 mg L−1 ) and water pH (6.5) condition, the order of floc strength was PFCF > PDF > PFC-PD. The dualcoagulant which gave stronger flocs also gave a lower absolute value of zeta potential. The floc regrowth properties of all three dual-coagulants after short and long period high shear level were also investigated. The floc recoverability was in the following order: PFCF > PFC-PD > PDF. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Coagulation is an essential process in drinking water treatment [1]. Although the optimum particulates and organic matter removal is an essential requirement and the major removal mechanisms of these pollutants have been well studied, little thought is given to the floc operational parameters, such as floc strength and flocs regrowth after being broken [2]. If small increases in shear during water works unit processes give rise to floc breakage, downstream systems will be challenged by smaller particles [3]. For example, reduced floc sizes give rise to lower sedimentation rates and reduce particle transport during removal in filtration [4,5]. In addition, newly exposed surfaces of aggregates may alter the surface charge of the floc aggregate, leading to partial re-stabilization [6]. However, high shear regions, such as areas around the impeller zone of flocculating tanks and transfer over weirs and ledges, are prevalent in unit processes at drinking water treatment plants [6]. Therefore, floc strength and recoverability should also be considered as an important parameter for overall process optimization. The coagulants applied should aim to not only initially have a low degradation rate on exposure to shear but also have a good recovery capacity. Recently, a few researches have focused on the breakage and regrowth nature of flocs coagulated with ferric sulfate, alum
∗ Corresponding author. Tel.: +86 531 88364832; fax: +86 531 88364513. E-mail addresses: [email protected] (J.C. Wei), baoyugao [email protected] (B.Y. Gao), [email protected] (Q.Y. Yue), [email protected] (Y. Wang). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.10.101
and polydiallyldimethylammonium (PDADMAC) [6–9], and it was found that the use of Fe coagulant gave more benefit than alum from a combined view of dissolved organic carbon (DOC) removal and floc structural character [2]. In addition, the alum salt may adversely affect human and animals [10]. Consequently, ferric coagulant was selected for investigation in this study. Beside metal coagulants, organic polymeric flocculants have been used in water purification for several decades [11]. Compared with metal coagulants, organic polymers work within a wider pH range, improve the finished water quality, increase floc settling rate, reduce sludge production and improve sludge dewatering characteristics [12,13]. Previous studies have shown that better removal performance can be achieved when a metal coagulant is used in combination with organic polymer [14–16]. In many drinking water treatment plants, small doses of polymer are often added some time after the aluminum or ferric-based coagulant with the aim of improving floc structure [17]. Jarvis et al. investigated the floc physical characteristics using PDADMAC as a coagulant aid for a high DOC, low alkalinity surface water [18]. However, very little is known about the strength and regrowth properties when coagulating with metal-polymer dual-coagulants flocs with different dosing method, such as metal coagulants and polymer were premixed before dosing or dosed in different order. As a result, an understanding of these properties may provide an insight into the fields of natural organic matter (NOM) coagulation with dual-coagulants. In this work, three dual-coagulants were comparatively investigated in terms of strength and regrowth properties in the coagulation of surface water. PFC and PDADMAC were chosen as the representative metal coagulant and organic polymer, respectively.
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The first dual-coagulant, PFC-PD, was prepared by premixing of PFC and PDADMAC before dosing. The other two were achieved by dosing PFC and PDMDAAC in reverse order. Floc strength and regrowth properties were measured in response to increasing shear levels through breakage and subsequent regrowth potential. The relationship between floc strength and coagulation mechanism was also discussed. 2. Experimental methods 2.1. Raw water The water sample was collected in July 2007 from the Wohushan Reservoir, an important drinking water source in Jinan, China. Its major characteristics were summarized as follows: pH (20 ◦ C) 8.17, Turbidity 4.9 NTU, Alkalinity 92 mg L−1 as CaCO3 , DOC 10.51 mg L−1 . The raw water was stored at 4 ◦ C for subsequent test. The pH of test water was adjusted by 0.1 mol L−1 HCl and 0.1 mol L−1 NaOH solutions. 2.2. Preparation of coagulants The PFC used in this study was prepared with FeCl3 ·6H2 O (A.R.) and Na2 CO3 (A.R.) in our laboratory. Firstly, FeCl3 solution with a Fe concentration of about 7%, w/w, was prepared by dissolving FeCl3 ·6H2 O (A.R.) in distilled water. Then, Na2 CO3 powder was gradually added to FeCl3 solutions by stirring at room temperature to reach the final (OH− )/(Fe) ratio 0.5, which had been proved the optimal value for turbidity and DOC removal when coagulating with PFC in this study. The mixture was stirred until foam disappeared. Lastly, Na2 HPO4 (A.R.) was added to the solution as a stabilizer ((Na2 HPO4 )/(Fe) = 0.08). The concentration of Fe was about 7%, w/w, in the target PFC solution. The cationic polymer PDADMAC (40%, w/w, aqueous solution, 100% charge density, intrinsic viscosity 1.02 dL/g) was provided by Bin Zhou Chemical Co., Shandong, China. The first dual-coagulant, denoted as PFC-PD, was prepared by premixing PFC and PDADMAC before dosing. A measured amount of PDADMAC was injected into the PFC solution and the mixture was stirred thoroughly until PDADMAC was absolutely mixed with PFC solution. In the following two cases, PFC and PDADMAC were dosed successively in different sequence. For the second dual-coagulant PFCF, PFC was added at the start of the rapid mixing, and then PDADMAC was added after 1 min. The third dual-coagulant, written as PDF for short, was similar with the second one except PFC and PDADMAC was dosed in reverse order. The mass ratio of Fe and PDADMAC is 1:1 in all three dual-coagulants in this investigation, which is based on comprehensive consideration of coagulants cost and coagulation performance. The dosages of all the three dual-coagulants were calculated as milligrams per liters of Fe for convenience. 2.3. Jar tests Coagulation and flocculation experiments were performed in a jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China) at a room temperature of 20 ± 1 ◦ C. Each sample (1.0 L) was rapidly mixed at 200 revolutions per minute (rpm) for 1.5 min, slowly mixed at 40 rpm for 15 min, and then settled (0 rpm) for 15 min. At the end of each jar test, the supernatant sample was withdrawn by syringe from about 3 cm below the water surface for analysis. An unfiltered sample was used for residual turbidity measurements using a 2100P turbidimeter (Hach, USA), and a filtered sample through a 0.45 m glass fiber membrane was tested for DOC and zeta potential. DOC was measured using 5000A TOC analyzer (Shimadzu, Japan). The zeta
potential was measured with a Zetasizer 3000HSa (Malvern Instruments, UK). 2.4. Floc breakage and regrowth Coagulation tests were conducted on a jar tester as before. However, after the slow stir phase the effect of increased shear was investigated by increasing the rpm on the jar tester for a further 15 min. Each experiment was repeated twice for rpm of: 40, 50, 75, 100, 150 and 200. The dynamic floc size was measured during the growth and breakage of the flocs using a laser diffraction instrument (Malvern Mastersizer 2000, Malvern, UK). The suspension was monitored by drawing water through the optical unit of the Mastersizer and back into the jar by a peristaltic pump on the 5 mm internal diameter return tube at a flow rate of 2.0 L/h. Size measurements were taken every 0.5 min for the duration of the jar test and logged onto a PC. The rate at which a floc size decays on exposure to shear is indicative of the floc strength. The empirical relationship between the applied shear and broken floc size has been used by many researchers to evaluate the floc strength [19–21]. log d = log C − log G
(1)
where d, C, G and are the floc diameter (m), floc strength, average velocity gradient (s−1 ) and stable floc size exponent, respectively. A modified version with the velocity gradient G being replaced by rpm has also been used by many researchers [3,7,18–21]. log d = log C − log rpm
(2)
Then, the broken floc size after 15 min shear was plotted against the rpm on a log–log scale, and the slope of this line ( ) gives an indication of degradation rate. A larger value is indicative of flocs that are more prone to break into smaller sizes with increasing shear force. In the floc regrowth tests, flocs were exposed to a shear force at 200 rpm after the slow stir phase was completed. Two separate breakage periods were investigated: a short breakage period of 30 s and a long breakage period of 15 min. After the breakage phase, the slow stir at 30 rpm was reintroduced for a further 15 min. Floc size was monitored as before. Floc strength and recovery factors, which have previously been used to compare the relative breakage and regrowth of flocs in different flocculated systems were calculated as follows [2,20,22]: strength factor =
d2 × 100 d1
(3)
recovery factor =
d3 − d2 × 100 d1 − d2
(4)
where d1 is the average floc size of the steady phase before breakage, d2 is the floc size after the floc breakage period, and d3 is the floc size after regrowth to the new steady phase. A larger value of strength factor indicates that flocs are stronger than the flocs with a lower factor. Likewise, the floc with a larger recovery factor shows better regrowth after high shear. 3. Results 3.1. Coagulation optimization and suspension characterization Initially, coagulation optimization tests were performed to ascertain the optimum dosage and water pH for organic and turbidity removal. For all three dual-coagulants used in this study, the results showed that a combined optimum for DOC and turbidity removal was obtained within a dosage between 3.0 and 4.0 mg L−1 at pH 6.5. The coagulant dosage and water pH was therefore chosen
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Table 1 The turbidity removal, DOC removal, zeta potential and flocs equivalent diameter of dual-coagulants at dose of 3.0 mg L−1 as Fe and water pH 6.5. Coagulants
Settled turbidity (NTU)
DOC removal (%)
Zeta potential (mV)
Floc equivalent diameter (m) d10
d50
d90
PFC-PD PFCF PDF
1.04 ± 0.08 1.33 ± 0.11 1.65 ± 0.10
70 ± 2 67 ± 1 65 ± 1
−3.6 ± 0.4 −0.6 ± 0.2 −0.8 ± 0.2
303 ± 9 224 ± 15 264 ± 18
812 ± 43 830 ± 64 821 ± 56
1427 ± 73 1462 ± 77 1452 ± 69
Fig. 1. The growth and breakage profile of dual-coagulant flocs with increasing rpm (, 40 RPM; , 50 RPM; , 75 RPM; , 100 RPM; ×, 150 RPM; , 200 RPM).
as 3.0 and 6.5 mg L−1 respectively for all subsequent floc breakage and regrowth experiments. The average floc size of the steady phase, coagulation effect and zeta potential under this condition were shown in Table 1. It can be seen that the DOC and turbidity removal was in the following order: PFC-PD > PFCF > PDF. The zeta potential was the greatest for PFCF (−0.6 mV) followed by PDF (−0.8 mV) with PFCPF having the lowest value (−3.6 mV). The size data is expressed as an equivalent volumetric diameter. It is also shown that the three dual-coagulants generated the similar 50 and 90 percentile floc size (d50 and d90 ) while an obvious difference can be seen for d10 . The d10 for PFC-PD flocs (303 m) was much larger than that for PFCF flocs and PDF flocs (224 m and 264 m, respectively), which suggested that there were more fine flocs in the suspension when coagulating with PFCF and PDF. It is known that fine flocs were more prone to suspend in the supernatant and thus resulted in lower removal efficiencies by setting. These may be the reasons why PFCF and PDF gave poorer turbidity removal although their charge neutralization was slightly stronger than PFC-PD.
dual-coagulants in the response of floc d50 to increased shear, the floc size after 15 min of elevated shear was plotted against RPM in Fig. 2. It can be seen that a straight line could be drawn through the data on a log-log scale. As mentioned above, the slope ( ) of this line gives an indication of floc size degradation rate. The smaller the gradient of the slope is, the stronger the floc is. As is shown in Fig. 2, PFCF gave a more gentle decrease in floc size than other two dual-coagulants with the increase of rpm, as reflected by the lowest value of (0.51). Thus PFCF flocs are far more resistant on exposure to increased shear. The next largest floc decrease in size was obtained by PDF ( = 0.62). PFC-PD gave the most rapid drop in floc size and the highest value (0.72). 3.3. Floc breakage and recovery The effect of short period (30 s) and long period (15 min) of high shear (200 rpm) on floc breakage and regrowth is shown in Fig. 3. In the case of 30 s shear period, it can be seen that only a limited extent of the floc breakage was observed for all the
3.2. Floc strength A comparison of the floc growth and breakage profiles for all three dual-coagulants was investigated in this section. The change of floc size versus coagulation time was shown in Fig. 1. The d50 is selected as the representative floc sizes and the similar trends are seen for d10 and d90 . From these profiles, it can be seen that the floc size was almost the same for the three dual-coagulants at the end of slow stir phase. After the flocs had reached steady-state size at the end of slow stir period, they were exposed to increasing shear with different shear rate. It is shown that the floc d50 decreases with increasing shear rate and the response of the floc d50 were similar for the three dual-coagulants. At low rpm (40 and 50), there was only a gradual decline in floc size. At an rpm of 75 and above, a significant drop in floc d50 can be seen immediately after the introduction of increased shear, and followed by a gradual decline. After 10 min exposure to the shear, no further significant degradation in floc size can be observed, and the floc aggregates approach a new steady-state floc size. In order to quantitatively compare the difference of the three
Fig. 2. Floc breakage rates of dual-coagulants.
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Fig. 3. Breakage and regrowth profile of dual-coagulant flocs (d50 ) after exposure to 30 s or 15 min shear period. Table 2 Strength and recovery factors of flocs (d50 ) after 30 s and 15 min of breakage followed by regrowth for 15 min. Strength factor
PFC-PD PFCF PDF
Recovery factor
30 s
15 min
30 s
15 min
43 ± 2 51 ± 3 46 ± 3
35 ± 2 44 ± 2 37 ± 1
94 ± 3 98 ± 4 90 ± 3
64 ± 3 67 ± 3 59 ± 2
dual-coagulants. PFCF flocs were better able to resist short period shear with floc size 410 ± 21 m after breakage. For PFC-PD and PDF, the corresponding floc sizes were only 354 ± 18 m and 375 ± 20 m, respectively. As the shear was reduced again, the flocs begin to regrow. After 10 min of regrowth, almost full reversibility of floc breakage was observed. Different from floc size before breakage, the floc size in the new steady stage was in the following order: PFCF > PFC-PD > PDF. In the case of 15 min shear period, the d50 size of the flocs after shear was about 360 ± 15 m for PFCF. In contrast, PFC-PD flocs and PDF flocs were broken into smaller floc sizes of around the 250 ± 14 m. Compared with the short period shear, the recoverability of dual-coagulant flocs decayed after long period shear. The floc size after regrowth followed the following order: PFCF (670 ± 29 m) > PFC-PD (580 ± 25 m) > PDF (550 ± 24 m). The strength and recovery factor for d50 were summarized in Table 2. The results showed that the order of floc strength factor and recovery factor for 30 s high shear were consistent with those for 15 min high shear. The capacity to withstand shear was in the same order with the results obtained in the “floc strength” section of this paper, that is: PFCF > PDF > PFC-PD, While the regrowth capacity was in the order: PFCF > PFC-PD > PDF. 4. Discussion 4.1. Floc strength The results from this investigation showed that the floc strength and recovery of PFC/PDADMAC dual-coagulants were influenced by dosing method of PFC and PDADMAC. Analysis of coagulation mechanism can give important interpreting of the difference in floc breakage and regrowth properties. It is known that the coagulation mechanisms of hydrolyzing coagulants for NOM removal are complex and pH dependent. It is generally accepted that the complexation of NOM with soluble metal species into insoluble precipitates is the main removal route at pH <6, while the removal of NOM is believed to be dominated by adsorption onto precipitated metal hydroxides at pH >6
[23–25]. In this study, the zeta potential was slightly influenced by coagulant dose when PFC was used alone (lower than −8 mV in this investigation), and thus entrapment and adsorption of NOM on to Fe(III) precipitation were the possible dominant coagulation mechanism of PFC. It also should be noted that the pH of raw water under investigation is 6.5, which further confirmed the above conclusion. For organic cationic polymer, the electrostatic patch, much like charge neutralization, was generally accepted as the dominant mechanisms when cationic polymers are applied to particles with negative surface charge [26]. When PDADMAC was used alone in this study, the zeta potential was significantly elevated and close to zero at its optimal dosage. As the coagulant dosage further increased, the charge was reversed, which indicated charge neutralization was its dominant mechanism. In addition, the zeta potential obtained by PDADMAC alone (−1.5 mV) was close to that of PFC-PD, PFCF and PDF (−3.6 mV, −0.6 mV and −0.8 mV, respectively). Consequently, PDADMAC was the principal positive charge contributor in the dual-coagulants. Previous work has shown that the floc strength is interrelated with the number and strength of the interior bonds of flocs [27]. Therefore, a floc will break if the stress applied on its surface is larger than the bonding strength within the floc [28]. When PFC was used combination with PDADMAC, the flocs formation and aggregation mechanism became complicated, and the flocs showed different conformation and strength with different addition sequences of PFC and PDADMAC. However, a simple relationship still could be found between zeta potential (Table 1) and floc strength for the three dual-coagulants. The coagulants which generated a zeta potential more close to zero also gave stronger flocs. This is consistent with the finding of Sharp et al. who observed that the response of NOM flocs to elevated shear seems to be strongly related to the zeta potential [7]. The current understanding of floc formation is depicted as follows: soon after the addition of coagulant, primary particles destabilize and come together to form small floccules which further aggregate to microflocs [29]. In the case of PFCF, PFC hydrolyzed instantaneously and the microflocs formed by adsorption of NOM onto Fe(III) precipitates. It is noted that these microflocs were still highly negatively charged. As mentioned above, electrostatic patch mechanisms generally occurred between the polymers and negative particles when cationic polymers were applied in coagulation. In this case, PDADMAC chains with high positive charge density nearly completely adsorbed onto the negatively charged microflocs after PDADMAC was dosed. The net residual charge of the PDADMAC patch on one microfloc surface can attach to the bare part of an oppositely charged microfloc, and the negative charges on microfloc surface were effectively neutralized. Therefore, the microflocs were tightly bound together with weak interior repulsion, which contributed to the formation of strong PFCF flocs. In the case of PDF, the negative surface charge on particles and colloids were significantly reduced by PDADMAC and the slightly negative microflocs formed. After PFC was dosed, the microflocs aggregated together through the combination of entrapment and adsorption mechanism by Fe (III) precipitates and bridging by spare polymer. Although the zeta potential obtained by PDF (−0.8 mV) was slightly lower than that of PFCF (−0.6 mV), the PFC flocs were much weaker than PFCF flocs. Thus, the difference in floc strength cannot be completely explained in terms of zeta potential. In this case, entrapment and adsorption of microflocs onto Fe(III) precipitates played leading role in the aggregation of microflocs. It has been reported that the flocs formed by entrapment and adsorption of NOM onto metal precipitates are considered weak and fragile [27], which attributed to a lack of bridging bonds holding these flocs together [2]. This is also the possible reason why PDF flocs were weaker in strength than PFC flocs. Among the three dual-coagulants, PFC-PD produced the weakest flocs and gave the lowest zeta potential (−3.6 mV),
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indicating that the surface charge of particles and colloids were not effectively neutralized here. The stronger internal electrostatic repulsive force may be the main reason why the flocs were more fragile than PFCF and PFC-PD under high shear. The low zeta potential also suggested the charge neutralization of the main positive charge contributor, PDADMAC, was weakened in this case. This can be explained in terms of the conformation of PDADMAC chains in the internal structure of PFC-PD flocs. After PFC-PD was dosed, PFC and PDADMAC reacted with the particles and colloids almost at the same time. The charge neutralization of PDADMAC and adsorption of Fe(III) precipitates are two competitive reactions between the coagulants and NOM. Steric crowding of Fe(III) precipitates and PDADMAC chains on the particle surface tend to favor an extended conformation of PDADMAC chains away from the particle surface. Different from PFCF flocs and PDF flocs, in which PDADMAC was adsorbed on the particle surface in a flat conformation, only one end or segment of PDADMAC chains attached to the particle surface in the PFC-PD flocs. As a consequence, the charge neutralization of the positive charge on PDADMAC chains was not exerted adequately. Although the charge neutralization ability was weakened, PDADMAC functioned as a combination of charge neutralization and bridge between particles. In addition, the internal structure of PFC-PD flocs was more homogenous than that of PFCF flocs and PDF flocs, as reflected by less residual fine flocs when using PFC-PD. These may be the reason why PFC-PD gave higher DOC and turbidity removal but weaker charge neutralization than PFCF and PDF. 4.2. Floc regrowth Analysis of the floc regrowth results of three dual-coagulants indicated that the same order of floc regrowth capacity was obtained after both short and long period shear: PFCF > PFCPD > PDF. Previous research has reported that recoverability of flocs gives some indication of the floc internal bonding structure [2]. The flocs formed by charge neutralization should give total recoverability [30], while sweep flocs show poor regrowth after breakage [4,31]. Yukselen and Gregory observed the flocs formed by cationic polyelectrolytes are more reversible after breakage in the coagulation of kaolin clay. As discussed above, the secondly dosed coagulant played an important role in the aggregation of microflocs when PFC and PDADMAC were dosed successively [22]. For PFCF, PDADMAC generated strong charge neutralization in the aggregation of microflocs. After the flocs were broken, the PDADMAC on newly exposed microfloc surface possibly also dominated the recovery of PFCF flocs. As a result, the electrostatic attraction and Van der Waals effectively bonded the floc fragments together, and PFCF flocs achieved good recoverability. Compared with PFCF, PDF gave similar final zeta potential but much poorer floc recovery, which suggested that the entrapment/adsorption of Fe(III) precipitate was significantly weakened in the floc regrowth process. Previous work has shown that the irreversible floc breakage for hydrolyzing coagulants is believed to be due to the chemical bonds being broken during floc disruption [22], and this is also the possible reason for the poor recoverability of PDF flocs. In the case of PFC-PD, particles were held together by the synergistic effect of PFC and PD, as reflected by a combination of charge neutralization and bridge. During the floc reaggregation process, the adsorption of Fe(III) precipitate was almost lost, the charge neutralization and bridging of PDADMAC became the dominant driving force. Therefore, PFC-PD gave an intermediate recoverability in three dual-coagulants. 5. Conclusions Floc strength and regrowth properties of three PFC/PDADMAC dual-coagulants with different dosing methods were compar-
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atively investigated in surface water treatment. For the given optimum dose (3.0 mg L−1 ) and water pH (6.5) condition, the order of floc strength was PFCF > PDF > PFC-PD. The floc recoverability after short or long period high shear level was in the following order: PFCF > PFC-PD > PDF. It was also found that the floc strength was negatively correlated with the absolute value of the zeta potential.
Acknowledgements This work is supported by the key projects in the national science & technology pillar program in the eleventh five-year plan period (2006BAJ08B05), the national natural science foundation of China (50808114). The kind suggestions from the anonymous reviewers are greatly acknowledged.
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[25] K.E. Dennett, A. Amirtharajah, T.F. Moran, J.P. Gould, Coagulation: its effect on organic matter, J. Am. Water Works Assoc. 84 (4) (1996) 129–142. [26] J. Bratby, Coagulation and Flocculation, Upland Press, Croydon, UK, 1980. [27] D.H. Bache, C. Johnson, J.F. McGilligan, E. Rasool, A conceptual view of floc structure in the sweep floc domain, Water Sci. Technol. 36 (1997) 49– 56. [28] M. Boller, S. Blaser, Particles under stress, Water Sci. Technol. 37 (1998) 9–29.
[29] B. Gorczyca, J. Ganczarczyk, Structure and porosity of alum coagulation flocs, Water Qual. Res. J. Can. 34 (4) (1999) 653–666. [30] V. Chaignon, B.S. Lartiges, A. El Samrani, C. Mustin, Evolution of size distribution and transfer of mineral particles between flocs in activated sludges: an insight into floc exchange dynamics, Water Res. 36 (3) (2002) 676–684. [31] P. Jarvis, B. Jefferson, S.A. Parsons, The duplicity of floc strength, in: Proceedings of the Nano and Micro Particles in Water and Wastewater Treatment Conference, International Water Association, Zurich, Switzerland, 2003.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Strength and regrowth properties of polyferric-polymer dual-coagulant flocs in surface water treatment J.C. Wei, B.Y. Gao ∗ , Q.Y. Yue, Y. Wang Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, 250100 Jinan, China
a r t i c l e
i n f o
Article history: Received 7 July 2009 Received in revised form 22 October 2009 Accepted 23 October 2009 Available online 30 October 2009 Keywords: Dual-coagulant Floc strength Floc regrowth Surface water
a b s t r a c t The floc strength and regrowth properties of three polyferric-cationic polymer dual-coagulants were comparatively evaluated using surface water sample. The first dual-coagulant PFC-PD was prepared by premixing of polyferric chloride (PFC) and polydiallyldimethylammonium (PDADMAC) before dosing. The other two, PFCF (PFC dosed firstly) and PDF (PDADMAC dosed firstly), were achieved by dosing PFC and PDADMAC in different order. Floc strength properties were measured in response to increasing shear levels in the long period and high shear level in the short or long period. For the given optimum dose (3.0 mg L−1 ) and water pH (6.5) condition, the order of floc strength was PFCF > PDF > PFC-PD. The dualcoagulant which gave stronger flocs also gave a lower absolute value of zeta potential. The floc regrowth properties of all three dual-coagulants after short and long period high shear level were also investigated. The floc recoverability was in the following order: PFCF > PFC-PD > PDF. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Coagulation is an essential process in drinking water treatment [1]. Although the optimum particulates and organic matter removal is an essential requirement and the major removal mechanisms of these pollutants have been well studied, little thought is given to the floc operational parameters, such as floc strength and flocs regrowth after being broken [2]. If small increases in shear during water works unit processes give rise to floc breakage, downstream systems will be challenged by smaller particles [3]. For example, reduced floc sizes give rise to lower sedimentation rates and reduce particle transport during removal in filtration [4,5]. In addition, newly exposed surfaces of aggregates may alter the surface charge of the floc aggregate, leading to partial re-stabilization [6]. However, high shear regions, such as areas around the impeller zone of flocculating tanks and transfer over weirs and ledges, are prevalent in unit processes at drinking water treatment plants [6]. Therefore, floc strength and recoverability should also be considered as an important parameter for overall process optimization. The coagulants applied should aim to not only initially have a low degradation rate on exposure to shear but also have a good recovery capacity. Recently, a few researches have focused on the breakage and regrowth nature of flocs coagulated with ferric sulfate, alum
∗ Corresponding author. Tel.: +86 531 88364832; fax: +86 531 88364513. E-mail addresses: [email protected] (J.C. Wei), baoyugao [email protected] (B.Y. Gao), [email protected] (Q.Y. Yue), [email protected] (Y. Wang). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.10.101
and polydiallyldimethylammonium (PDADMAC) [6–9], and it was found that the use of Fe coagulant gave more benefit than alum from a combined view of dissolved organic carbon (DOC) removal and floc structural character [2]. In addition, the alum salt may adversely affect human and animals [10]. Consequently, ferric coagulant was selected for investigation in this study. Beside metal coagulants, organic polymeric flocculants have been used in water purification for several decades [11]. Compared with metal coagulants, organic polymers work within a wider pH range, improve the finished water quality, increase floc settling rate, reduce sludge production and improve sludge dewatering characteristics [12,13]. Previous studies have shown that better removal performance can be achieved when a metal coagulant is used in combination with organic polymer [14–16]. In many drinking water treatment plants, small doses of polymer are often added some time after the aluminum or ferric-based coagulant with the aim of improving floc structure [17]. Jarvis et al. investigated the floc physical characteristics using PDADMAC as a coagulant aid for a high DOC, low alkalinity surface water [18]. However, very little is known about the strength and regrowth properties when coagulating with metal-polymer dual-coagulants flocs with different dosing method, such as metal coagulants and polymer were premixed before dosing or dosed in different order. As a result, an understanding of these properties may provide an insight into the fields of natural organic matter (NOM) coagulation with dual-coagulants. In this work, three dual-coagulants were comparatively investigated in terms of strength and regrowth properties in the coagulation of surface water. PFC and PDADMAC were chosen as the representative metal coagulant and organic polymer, respectively.
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The first dual-coagulant, PFC-PD, was prepared by premixing of PFC and PDADMAC before dosing. The other two were achieved by dosing PFC and PDMDAAC in reverse order. Floc strength and regrowth properties were measured in response to increasing shear levels through breakage and subsequent regrowth potential. The relationship between floc strength and coagulation mechanism was also discussed. 2. Experimental methods 2.1. Raw water The water sample was collected in July 2007 from the Wohushan Reservoir, an important drinking water source in Jinan, China. Its major characteristics were summarized as follows: pH (20 ◦ C) 8.17, Turbidity 4.9 NTU, Alkalinity 92 mg L−1 as CaCO3 , DOC 10.51 mg L−1 . The raw water was stored at 4 ◦ C for subsequent test. The pH of test water was adjusted by 0.1 mol L−1 HCl and 0.1 mol L−1 NaOH solutions. 2.2. Preparation of coagulants The PFC used in this study was prepared with FeCl3 ·6H2 O (A.R.) and Na2 CO3 (A.R.) in our laboratory. Firstly, FeCl3 solution with a Fe concentration of about 7%, w/w, was prepared by dissolving FeCl3 ·6H2 O (A.R.) in distilled water. Then, Na2 CO3 powder was gradually added to FeCl3 solutions by stirring at room temperature to reach the final (OH− )/(Fe) ratio 0.5, which had been proved the optimal value for turbidity and DOC removal when coagulating with PFC in this study. The mixture was stirred until foam disappeared. Lastly, Na2 HPO4 (A.R.) was added to the solution as a stabilizer ((Na2 HPO4 )/(Fe) = 0.08). The concentration of Fe was about 7%, w/w, in the target PFC solution. The cationic polymer PDADMAC (40%, w/w, aqueous solution, 100% charge density, intrinsic viscosity 1.02 dL/g) was provided by Bin Zhou Chemical Co., Shandong, China. The first dual-coagulant, denoted as PFC-PD, was prepared by premixing PFC and PDADMAC before dosing. A measured amount of PDADMAC was injected into the PFC solution and the mixture was stirred thoroughly until PDADMAC was absolutely mixed with PFC solution. In the following two cases, PFC and PDADMAC were dosed successively in different sequence. For the second dual-coagulant PFCF, PFC was added at the start of the rapid mixing, and then PDADMAC was added after 1 min. The third dual-coagulant, written as PDF for short, was similar with the second one except PFC and PDADMAC was dosed in reverse order. The mass ratio of Fe and PDADMAC is 1:1 in all three dual-coagulants in this investigation, which is based on comprehensive consideration of coagulants cost and coagulation performance. The dosages of all the three dual-coagulants were calculated as milligrams per liters of Fe for convenience. 2.3. Jar tests Coagulation and flocculation experiments were performed in a jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China) at a room temperature of 20 ± 1 ◦ C. Each sample (1.0 L) was rapidly mixed at 200 revolutions per minute (rpm) for 1.5 min, slowly mixed at 40 rpm for 15 min, and then settled (0 rpm) for 15 min. At the end of each jar test, the supernatant sample was withdrawn by syringe from about 3 cm below the water surface for analysis. An unfiltered sample was used for residual turbidity measurements using a 2100P turbidimeter (Hach, USA), and a filtered sample through a 0.45 m glass fiber membrane was tested for DOC and zeta potential. DOC was measured using 5000A TOC analyzer (Shimadzu, Japan). The zeta
potential was measured with a Zetasizer 3000HSa (Malvern Instruments, UK). 2.4. Floc breakage and regrowth Coagulation tests were conducted on a jar tester as before. However, after the slow stir phase the effect of increased shear was investigated by increasing the rpm on the jar tester for a further 15 min. Each experiment was repeated twice for rpm of: 40, 50, 75, 100, 150 and 200. The dynamic floc size was measured during the growth and breakage of the flocs using a laser diffraction instrument (Malvern Mastersizer 2000, Malvern, UK). The suspension was monitored by drawing water through the optical unit of the Mastersizer and back into the jar by a peristaltic pump on the 5 mm internal diameter return tube at a flow rate of 2.0 L/h. Size measurements were taken every 0.5 min for the duration of the jar test and logged onto a PC. The rate at which a floc size decays on exposure to shear is indicative of the floc strength. The empirical relationship between the applied shear and broken floc size has been used by many researchers to evaluate the floc strength [19–21]. log d = log C − log G
(1)
where d, C, G and are the floc diameter (m), floc strength, average velocity gradient (s−1 ) and stable floc size exponent, respectively. A modified version with the velocity gradient G being replaced by rpm has also been used by many researchers [3,7,18–21]. log d = log C − log rpm
(2)
Then, the broken floc size after 15 min shear was plotted against the rpm on a log–log scale, and the slope of this line ( ) gives an indication of degradation rate. A larger value is indicative of flocs that are more prone to break into smaller sizes with increasing shear force. In the floc regrowth tests, flocs were exposed to a shear force at 200 rpm after the slow stir phase was completed. Two separate breakage periods were investigated: a short breakage period of 30 s and a long breakage period of 15 min. After the breakage phase, the slow stir at 30 rpm was reintroduced for a further 15 min. Floc size was monitored as before. Floc strength and recovery factors, which have previously been used to compare the relative breakage and regrowth of flocs in different flocculated systems were calculated as follows [2,20,22]: strength factor =
d2 × 100 d1
(3)
recovery factor =
d3 − d2 × 100 d1 − d2
(4)
where d1 is the average floc size of the steady phase before breakage, d2 is the floc size after the floc breakage period, and d3 is the floc size after regrowth to the new steady phase. A larger value of strength factor indicates that flocs are stronger than the flocs with a lower factor. Likewise, the floc with a larger recovery factor shows better regrowth after high shear. 3. Results 3.1. Coagulation optimization and suspension characterization Initially, coagulation optimization tests were performed to ascertain the optimum dosage and water pH for organic and turbidity removal. For all three dual-coagulants used in this study, the results showed that a combined optimum for DOC and turbidity removal was obtained within a dosage between 3.0 and 4.0 mg L−1 at pH 6.5. The coagulant dosage and water pH was therefore chosen
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Table 1 The turbidity removal, DOC removal, zeta potential and flocs equivalent diameter of dual-coagulants at dose of 3.0 mg L−1 as Fe and water pH 6.5. Coagulants
Settled turbidity (NTU)
DOC removal (%)
Zeta potential (mV)
Floc equivalent diameter (m) d10
d50
d90
PFC-PD PFCF PDF
1.04 ± 0.08 1.33 ± 0.11 1.65 ± 0.10
70 ± 2 67 ± 1 65 ± 1
−3.6 ± 0.4 −0.6 ± 0.2 −0.8 ± 0.2
303 ± 9 224 ± 15 264 ± 18
812 ± 43 830 ± 64 821 ± 56
1427 ± 73 1462 ± 77 1452 ± 69
Fig. 1. The growth and breakage profile of dual-coagulant flocs with increasing rpm (, 40 RPM; , 50 RPM; , 75 RPM; , 100 RPM; ×, 150 RPM; , 200 RPM).
as 3.0 and 6.5 mg L−1 respectively for all subsequent floc breakage and regrowth experiments. The average floc size of the steady phase, coagulation effect and zeta potential under this condition were shown in Table 1. It can be seen that the DOC and turbidity removal was in the following order: PFC-PD > PFCF > PDF. The zeta potential was the greatest for PFCF (−0.6 mV) followed by PDF (−0.8 mV) with PFCPF having the lowest value (−3.6 mV). The size data is expressed as an equivalent volumetric diameter. It is also shown that the three dual-coagulants generated the similar 50 and 90 percentile floc size (d50 and d90 ) while an obvious difference can be seen for d10 . The d10 for PFC-PD flocs (303 m) was much larger than that for PFCF flocs and PDF flocs (224 m and 264 m, respectively), which suggested that there were more fine flocs in the suspension when coagulating with PFCF and PDF. It is known that fine flocs were more prone to suspend in the supernatant and thus resulted in lower removal efficiencies by setting. These may be the reasons why PFCF and PDF gave poorer turbidity removal although their charge neutralization was slightly stronger than PFC-PD.
dual-coagulants in the response of floc d50 to increased shear, the floc size after 15 min of elevated shear was plotted against RPM in Fig. 2. It can be seen that a straight line could be drawn through the data on a log-log scale. As mentioned above, the slope ( ) of this line gives an indication of floc size degradation rate. The smaller the gradient of the slope is, the stronger the floc is. As is shown in Fig. 2, PFCF gave a more gentle decrease in floc size than other two dual-coagulants with the increase of rpm, as reflected by the lowest value of (0.51). Thus PFCF flocs are far more resistant on exposure to increased shear. The next largest floc decrease in size was obtained by PDF ( = 0.62). PFC-PD gave the most rapid drop in floc size and the highest value (0.72). 3.3. Floc breakage and recovery The effect of short period (30 s) and long period (15 min) of high shear (200 rpm) on floc breakage and regrowth is shown in Fig. 3. In the case of 30 s shear period, it can be seen that only a limited extent of the floc breakage was observed for all the
3.2. Floc strength A comparison of the floc growth and breakage profiles for all three dual-coagulants was investigated in this section. The change of floc size versus coagulation time was shown in Fig. 1. The d50 is selected as the representative floc sizes and the similar trends are seen for d10 and d90 . From these profiles, it can be seen that the floc size was almost the same for the three dual-coagulants at the end of slow stir phase. After the flocs had reached steady-state size at the end of slow stir period, they were exposed to increasing shear with different shear rate. It is shown that the floc d50 decreases with increasing shear rate and the response of the floc d50 were similar for the three dual-coagulants. At low rpm (40 and 50), there was only a gradual decline in floc size. At an rpm of 75 and above, a significant drop in floc d50 can be seen immediately after the introduction of increased shear, and followed by a gradual decline. After 10 min exposure to the shear, no further significant degradation in floc size can be observed, and the floc aggregates approach a new steady-state floc size. In order to quantitatively compare the difference of the three
Fig. 2. Floc breakage rates of dual-coagulants.
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Fig. 3. Breakage and regrowth profile of dual-coagulant flocs (d50 ) after exposure to 30 s or 15 min shear period. Table 2 Strength and recovery factors of flocs (d50 ) after 30 s and 15 min of breakage followed by regrowth for 15 min. Strength factor
PFC-PD PFCF PDF
Recovery factor
30 s
15 min
30 s
15 min
43 ± 2 51 ± 3 46 ± 3
35 ± 2 44 ± 2 37 ± 1
94 ± 3 98 ± 4 90 ± 3
64 ± 3 67 ± 3 59 ± 2
dual-coagulants. PFCF flocs were better able to resist short period shear with floc size 410 ± 21 m after breakage. For PFC-PD and PDF, the corresponding floc sizes were only 354 ± 18 m and 375 ± 20 m, respectively. As the shear was reduced again, the flocs begin to regrow. After 10 min of regrowth, almost full reversibility of floc breakage was observed. Different from floc size before breakage, the floc size in the new steady stage was in the following order: PFCF > PFC-PD > PDF. In the case of 15 min shear period, the d50 size of the flocs after shear was about 360 ± 15 m for PFCF. In contrast, PFC-PD flocs and PDF flocs were broken into smaller floc sizes of around the 250 ± 14 m. Compared with the short period shear, the recoverability of dual-coagulant flocs decayed after long period shear. The floc size after regrowth followed the following order: PFCF (670 ± 29 m) > PFC-PD (580 ± 25 m) > PDF (550 ± 24 m). The strength and recovery factor for d50 were summarized in Table 2. The results showed that the order of floc strength factor and recovery factor for 30 s high shear were consistent with those for 15 min high shear. The capacity to withstand shear was in the same order with the results obtained in the “floc strength” section of this paper, that is: PFCF > PDF > PFC-PD, While the regrowth capacity was in the order: PFCF > PFC-PD > PDF. 4. Discussion 4.1. Floc strength The results from this investigation showed that the floc strength and recovery of PFC/PDADMAC dual-coagulants were influenced by dosing method of PFC and PDADMAC. Analysis of coagulation mechanism can give important interpreting of the difference in floc breakage and regrowth properties. It is known that the coagulation mechanisms of hydrolyzing coagulants for NOM removal are complex and pH dependent. It is generally accepted that the complexation of NOM with soluble metal species into insoluble precipitates is the main removal route at pH <6, while the removal of NOM is believed to be dominated by adsorption onto precipitated metal hydroxides at pH >6
[23–25]. In this study, the zeta potential was slightly influenced by coagulant dose when PFC was used alone (lower than −8 mV in this investigation), and thus entrapment and adsorption of NOM on to Fe(III) precipitation were the possible dominant coagulation mechanism of PFC. It also should be noted that the pH of raw water under investigation is 6.5, which further confirmed the above conclusion. For organic cationic polymer, the electrostatic patch, much like charge neutralization, was generally accepted as the dominant mechanisms when cationic polymers are applied to particles with negative surface charge [26]. When PDADMAC was used alone in this study, the zeta potential was significantly elevated and close to zero at its optimal dosage. As the coagulant dosage further increased, the charge was reversed, which indicated charge neutralization was its dominant mechanism. In addition, the zeta potential obtained by PDADMAC alone (−1.5 mV) was close to that of PFC-PD, PFCF and PDF (−3.6 mV, −0.6 mV and −0.8 mV, respectively). Consequently, PDADMAC was the principal positive charge contributor in the dual-coagulants. Previous work has shown that the floc strength is interrelated with the number and strength of the interior bonds of flocs [27]. Therefore, a floc will break if the stress applied on its surface is larger than the bonding strength within the floc [28]. When PFC was used combination with PDADMAC, the flocs formation and aggregation mechanism became complicated, and the flocs showed different conformation and strength with different addition sequences of PFC and PDADMAC. However, a simple relationship still could be found between zeta potential (Table 1) and floc strength for the three dual-coagulants. The coagulants which generated a zeta potential more close to zero also gave stronger flocs. This is consistent with the finding of Sharp et al. who observed that the response of NOM flocs to elevated shear seems to be strongly related to the zeta potential [7]. The current understanding of floc formation is depicted as follows: soon after the addition of coagulant, primary particles destabilize and come together to form small floccules which further aggregate to microflocs [29]. In the case of PFCF, PFC hydrolyzed instantaneously and the microflocs formed by adsorption of NOM onto Fe(III) precipitates. It is noted that these microflocs were still highly negatively charged. As mentioned above, electrostatic patch mechanisms generally occurred between the polymers and negative particles when cationic polymers were applied in coagulation. In this case, PDADMAC chains with high positive charge density nearly completely adsorbed onto the negatively charged microflocs after PDADMAC was dosed. The net residual charge of the PDADMAC patch on one microfloc surface can attach to the bare part of an oppositely charged microfloc, and the negative charges on microfloc surface were effectively neutralized. Therefore, the microflocs were tightly bound together with weak interior repulsion, which contributed to the formation of strong PFCF flocs. In the case of PDF, the negative surface charge on particles and colloids were significantly reduced by PDADMAC and the slightly negative microflocs formed. After PFC was dosed, the microflocs aggregated together through the combination of entrapment and adsorption mechanism by Fe (III) precipitates and bridging by spare polymer. Although the zeta potential obtained by PDF (−0.8 mV) was slightly lower than that of PFCF (−0.6 mV), the PFC flocs were much weaker than PFCF flocs. Thus, the difference in floc strength cannot be completely explained in terms of zeta potential. In this case, entrapment and adsorption of microflocs onto Fe(III) precipitates played leading role in the aggregation of microflocs. It has been reported that the flocs formed by entrapment and adsorption of NOM onto metal precipitates are considered weak and fragile [27], which attributed to a lack of bridging bonds holding these flocs together [2]. This is also the possible reason why PDF flocs were weaker in strength than PFC flocs. Among the three dual-coagulants, PFC-PD produced the weakest flocs and gave the lowest zeta potential (−3.6 mV),
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indicating that the surface charge of particles and colloids were not effectively neutralized here. The stronger internal electrostatic repulsive force may be the main reason why the flocs were more fragile than PFCF and PFC-PD under high shear. The low zeta potential also suggested the charge neutralization of the main positive charge contributor, PDADMAC, was weakened in this case. This can be explained in terms of the conformation of PDADMAC chains in the internal structure of PFC-PD flocs. After PFC-PD was dosed, PFC and PDADMAC reacted with the particles and colloids almost at the same time. The charge neutralization of PDADMAC and adsorption of Fe(III) precipitates are two competitive reactions between the coagulants and NOM. Steric crowding of Fe(III) precipitates and PDADMAC chains on the particle surface tend to favor an extended conformation of PDADMAC chains away from the particle surface. Different from PFCF flocs and PDF flocs, in which PDADMAC was adsorbed on the particle surface in a flat conformation, only one end or segment of PDADMAC chains attached to the particle surface in the PFC-PD flocs. As a consequence, the charge neutralization of the positive charge on PDADMAC chains was not exerted adequately. Although the charge neutralization ability was weakened, PDADMAC functioned as a combination of charge neutralization and bridge between particles. In addition, the internal structure of PFC-PD flocs was more homogenous than that of PFCF flocs and PDF flocs, as reflected by less residual fine flocs when using PFC-PD. These may be the reason why PFC-PD gave higher DOC and turbidity removal but weaker charge neutralization than PFCF and PDF. 4.2. Floc regrowth Analysis of the floc regrowth results of three dual-coagulants indicated that the same order of floc regrowth capacity was obtained after both short and long period shear: PFCF > PFCPD > PDF. Previous research has reported that recoverability of flocs gives some indication of the floc internal bonding structure [2]. The flocs formed by charge neutralization should give total recoverability [30], while sweep flocs show poor regrowth after breakage [4,31]. Yukselen and Gregory observed the flocs formed by cationic polyelectrolytes are more reversible after breakage in the coagulation of kaolin clay. As discussed above, the secondly dosed coagulant played an important role in the aggregation of microflocs when PFC and PDADMAC were dosed successively [22]. For PFCF, PDADMAC generated strong charge neutralization in the aggregation of microflocs. After the flocs were broken, the PDADMAC on newly exposed microfloc surface possibly also dominated the recovery of PFCF flocs. As a result, the electrostatic attraction and Van der Waals effectively bonded the floc fragments together, and PFCF flocs achieved good recoverability. Compared with PFCF, PDF gave similar final zeta potential but much poorer floc recovery, which suggested that the entrapment/adsorption of Fe(III) precipitate was significantly weakened in the floc regrowth process. Previous work has shown that the irreversible floc breakage for hydrolyzing coagulants is believed to be due to the chemical bonds being broken during floc disruption [22], and this is also the possible reason for the poor recoverability of PDF flocs. In the case of PFC-PD, particles were held together by the synergistic effect of PFC and PD, as reflected by a combination of charge neutralization and bridge. During the floc reaggregation process, the adsorption of Fe(III) precipitate was almost lost, the charge neutralization and bridging of PDADMAC became the dominant driving force. Therefore, PFC-PD gave an intermediate recoverability in three dual-coagulants. 5. Conclusions Floc strength and regrowth properties of three PFC/PDADMAC dual-coagulants with different dosing methods were compar-
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atively investigated in surface water treatment. For the given optimum dose (3.0 mg L−1 ) and water pH (6.5) condition, the order of floc strength was PFCF > PDF > PFC-PD. The floc recoverability after short or long period high shear level was in the following order: PFCF > PFC-PD > PDF. It was also found that the floc strength was negatively correlated with the absolute value of the zeta potential.
Acknowledgements This work is supported by the key projects in the national science & technology pillar program in the eleventh five-year plan period (2006BAJ08B05), the national natural science foundation of China (50808114). The kind suggestions from the anonymous reviewers are greatly acknowledged.
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