The use of selective and direct DAF for removal of particulate contaminants in drinking water treatment

~ Pergamon 0273-1223(95)00203-0

War. Sci. Tech. Vol. 31, No. 3-4, pp. 49-57,1995. Copyright e 1995 IAWQ Printed in Great Britain. All rights reserved. 0273-1223195 $9'50 + 0'00

THE USE OF SELECTIVE AND DIRECT DAF FOR REMOVAL OF PARTICULATE CONTAMINANTS IN DRINKING WATER TREATMENT James P. Malley, Jr Assistant Professor of Civil Engineering, ERG - The Environmental Research Group, 236 Kingsbury Hall, University of New Hampshire, Durham, NH 03824-3591, USA

ABSTRACT Cationic polymers were found to coat bubbles resulting in charge reversal and increasingly positive EPM. Batch bench-scale dissolved air flotation (OAF) studies show promising benefits from the addition of cationic or non-ionic polymers to the saturated recycle line of conventional DAF. In cases where low turbidity, low color waters are being treated, direct DAF compared well with conventional DAF. Polymer addition improved the percentage solids of the float. Anionic polymers were not found to be effective. Further research at the pilot and full-scale is needed to verify these findings.

KEYWORDS Cationic; anionic; non-ionic polymers; particle removals; humic acid; montmorillonite clay; natural waters; float solids. INTRODUCTION Dissolved Air Flotation (DAF) is being evaluated by numerous drinking water utilities and their consultants throughout the USA, including such major cities as New York and Boston, and several full-scale plants have been constructed. Recent research by Edzwald et at. (1990) and Malley and Edzwald (1991a.b) demonstrated that chemical pre-treatment was critical to DAF performance and lower flocculation times could be employed in DAF. Malley and Edzwald (1991a,b) adapted a particle deposition model, commonly used in filtration, to provide a rational basis for selection, design and operation of DAF. Upon further examination of this fundamental model for DAF, another important observation was made - that the bubble volume concentration (or number of bubbles) far exceeds the particle volume concentration (or number of particles) for low turbidity waters containing color and/or algae that have been coagulated with metal salts. This observation combined with Derjaguin and Dukhin (1960); Derjaguin et at. (1984) and numerous references in water treatment (Kitchener and Gochin, 1981; Klassen and Mokrousov, 1963; Vrablik, 1959) and in mining and mineral processing industries (Mangravite, Jr., 1972; Mangravite, Jr. et al., 1972, 1975) which suggest that bubble surface chemistry can be altered either by changing the gas type or adding hydrophobic surfactants leads to the research question - would modification of the bubble surface chemistry in addition to or rather than the particle chemistry improve DAF performance in drinking water treatment? In this research paper, bubble surface modification in addition to conventional particle 49

J. P. MALLEY, JR

50

coagulation/flocculation will be referred to as "Selective DAF" or SDAF and bubble surface modification alone without separate coagulation/flocculation will be referred to as "Direct DAF" of DDAF. RESEARCH APPROACH A bench-scale DAF system was purchased from Aztec Environmental Control Ltd (England) and used in this research. A complete description of the unit and its operation has been reported previously (Malley, 1988; Malley and Edzwald, 1991a,b). Three types of batch bench scale flotation studies were conducted in this research. Conventional DAF (CDAF) studies were conducted by adding predetermined alum dosages to rapid mix (0 = 70 sec-I) for 2 minutes. The water was then dosed with a determined amount of polymer and flocculated for 10 minutes (0 = 20 secI). After flocculation, flotation was initiated by activating the solenoid values for 15 seconds (allowing 8% recycle of volume). In all cases, previously treated (under the same conditions) water was used as the recycle water for the batch saturator, to simulate a real-plant scenario. Post flotation samples of the float (solids residual) and of the water were taken ten minutes after recycle had stopped. The CDAF studies were modified to test SDAF by moving the polymer addition to the recycle saturator tank. Experiments were performed using four different polymers donated by Calgon Corporation, Pittsburgh, PA. Cattloc TL and Cattloc LS were used as low and high molecular weight cationic, polydadmac type, polymers. Calgon CA233 was used as a non-ionic, polyacrylamide type, polymer. Calgon CA243 was used as a 25% charged anionic polymer. The catflocs were prepared from emulsions in Milli-Q'" Type I laboratory water to yield I mg/ml stocks. CA 233 and CA 243 were prepared in Milli-Ql" from dry polymer to yield I mg/ml stocks. All stocks were stored in low atinic glass at 4°C and replaced monthly. Polymers were dosed into the saturator tank and rapidly mixed for 2 minutes at 500 rpm using a T-Iine mixer. Polymer dosages were reported as the final effective concentration of polymer added to the test jar volume. These experiments will be referred to as selective DAF (SDAF) studies. The final set of experiments employed SDAF without CDAF. Water samples where solenoids were added to the flotation jars then the flotation solenoids were activated allowing the polymer containing recycle to enter the jars. No pretreatment (i.e., coagulation/flocculation) of any kind was performed. These studies will be referred to as Direct DAF (DDAF) experiments. DDAF experiments were statistically compared to CDAF experiments which only used polymer dosages (no alum) as the coagulant (i.e., polymer only CDAF). Waters Studied Preliminary studies were performed to determine the effects of polymer addition on bubble charge (or mobility) using Milli-Q'" water. Synthetic water studies were performed on samples containing Montmorillonite clay, Aldrich" humic acid (HA), and a mixture of the two. Montmorillonite clay suspensions were prepared in Milli-Q'" water to yield NPTOC values of 2 and 5 mg/1. 10 ppm clay suspensions and 2 mg/L HA solutions were mixed to yield a third type of sample. The water matrix of all samples was adjusted by spiking in 5 x 10-4M NaHC03 adjusting to pH 7.0 with I Nand 0.1 N HC I and fixing the ionic strength at 2.3 x 10-3 M using I N NaC I. Studies were also performed on water samples collected from a surface water in the spring and summer. Table I summarizes water quality and coagulant dosages used. Analyses Samples before and after treatment were analyzed for turbidity, particles, NPTOC and pH using Standard Methods (APHA et al., 1989). Practice analysis was performed using proposed method 2560B (unapproved draft) with a HiaclRoyco Model 4103 particle counter. Apertures were selected to measure between I and 30 11m size particles. The practical lower limit observed during the studies above background noise was 211m. All particle results are reported as percent reductions of volume concentration after correction for background. Extreme care was taken to degas samples by nitrogen sparging after DAF to avoid interferences

51

Selective and direct DAF

in particle counting from the bubbles. Electrophoretic mobilit y (EPM) measuremen ts were performed on bubbles during the prelimin ary experiments. Initial attempts to use Laser Z equipment proved unsuccessful due to bubble adhesion to surfaces and vertical migration. Conversati ons with Dr Francis J. Mangravi te of U.S. Water Inc. resulted in the use of a Komline-Sanderson Zeta-Reader. The meter allowed for observa tion of larger samples hence bubble adhesion and migration could be dealt with to some extent . The Zeta meter was calibrated with red blood cells to equate meter response with EPM in (um/s/ v /cm). The absolute numerical values of these studies have limited precisions (% RSD varied from 25 to 100%) nevertheless charge reversal was clearly observed. In experimental run, the float was removed 10 minutes after flotation had stopped using a wide bore pipette and the percent solids of the collected 25 ml sample was determ ined as per Standard Method s. RES ULTS Figure I demonstrates the effects of polymer addition on bubble charge and mobility. These data suggest that the cationic polymers can coat bubbles resulting in charge reversal and increasingly positive EPM values. Both Catfloc TL and Catfloc LS exhibited this behavior. The nonionic (CA 233) and the anionic (CA 243) polymers did not significantly affect the charge or the EPM of the bubbles. 1. 2 ~

1. 00

•

Cationic

Cat-floc LS

0 .7~

•

Nonionic

CA-233

c.se

•

An ionic

r~

~

E

0

<, III

<,

0.00

::::;; 0-0 . 2 ~

-J

lD lD

)

.:•

0.2~

:l.

w w

\/

j

E

~

/

CA-243

> <,

I r ~d,,--!j -1'-]/ )r

,1\.17"!"--./!\ J

-0 .~0

~

m

-0 . 7~

1

- 1.0 0

/

I"--j

1

-1. 2 ~

0

2

4

8

II

Polymer Dosage

'0

12

14

(mg/L)

Fig. I. The effe ct of polymer addition to bubble charge and EPM at 20-22°C. pH 7.0.

Convent ional jar tests (Malley, 1988) were performed to determine the alum dosage and the polymer dosage requ ired to ach ieve the largest removal of particles during flotation (Table I). In studies using HA or HNclay mixtures coagulant dosages were based both on total particl e removal and a minimum NPTOC removal of 50%. In these latter studies, alum controlled NPTOC removal as expected (Graham et al. , 1992) and only small amount" of polymer (Ta ble I) were needed to optimize particle removal.

52

J. P. MALLEY, JR

Table 1. Characteristics and Coagulant Dosages of Waters Used in this Study Mean Parameter

Clay Waters

NPTOC (HA) Waters

10 ppm 50 ppm 100 ppm

2 mg/L

5 mg/L

Mixture

Natural Water

2 mg/U10 ppm Spring Summer

UV (em")

NM·

NM

NM

0.124

0.359

0.201

0.040

0.065

True Color (Pt-Co)

NM

NM

NM

35

75

40

1.0

20

NPTOC (mg/L)

NM

NM

NM

2.1

5.2

2.2

1.8

2.7

Turbidity

1.30

6.10

12.1

0.20

0.15

1.41

1.90

0.20

1x10'

3xlO'

2x10'o

500

2000

2x20'

8x107

5x10'

1.4

3.5

5.4

2.2

4.1

2.3

1.5

0.5

Catfloc LS (mg/L)

0.3 (2.0)"·

0.5 (2.8)

0.9 (4.0)

0.3 (2.0)

1.0 (6.0)

1.0 (3.0)

0.8 (0.8)

0.3 (2.0)

Catfloc TL (mg/L)

0.3 (1.0)

0.6 (3.6)

0.9 (5.0)

0.3 (3.0)

1.0 (8.0)

1.0 (4.5)

0.8 (3.0)

0.3 (1.0)

CA233 (mg/L)

0.5

1.6

1.6

0.8

1.6

1.6

1.0

0.3

(NTU)

Particles (#IL) Alum Dose (mg/L as AlH

)

"NM - Not Mentioned "Dosage Applied in the DDAF Experiments

100

p = 0.000

--

Catfloc LS

90

~

u,

-c

0

80

(f)

>.

CD

"'0

70

>

0

E Q)

0::

60

Q)

....... 0

c

a,

10 ppm Clay 50 ppm Clay ... 100 ppm Clay • 2 mg/L NPTOC HA a 5 mg/L NPTOC HA 2/10 HA-Clay Mix

..

A

50

"'0 .... 0

to-

""0

• ""0

50

60

70

80

90

100

Total Particle Removal By CDAF (%) Fig. 2. Comparison of particle removals by conventional DAF (CDAF) versus Selective DAF (SDAF) for synthetic waters at 20-22°C, pH 7.0 using Catfloc LC.

Selective and direct OAF

53

10 0

Ca t flo c

0 .000

P

LS

00

E u,

-c

0

80

Ul >.

a= "0

70

>

0

E ClJ a:

80

ClJ

~

•

.

0

a.

.0

"0

Sp r ing S u m me r

-0 I' 0

JO Il..-_ - - ' - _- ' -_ JO ' 0

'0

_

'--_-'-~_'_~_:'_.........,."

80

70

10 0

80

Total Particle Rem ova l By CDAF (%) Fig. 3. Comparison of particle removals by conventional OAF (CDAF) versus Select ive OAF (SOAR for natural water samples at 20-22°C. pH 7.0 using Catfloc LS.

Figures 2 and 3 are a comparison of particle removal by CDAF versus SDAF using Catfloc LS. The se bivariate plots contain a 45 ' line representing the line of equal value . The P-values shown are the significance probability (the lowest a-val ue at which the hypothesis of eq ual effects can be rejected). In general a P-value greater than 0.05 indicates that the treatments (CDAF vs, SDAF) have equal effects at the 95% confidence level. The lower the P-value the more significant the difference. As shown in Figures 2 and 3, SDAF produced higher percent total particle removals than CDAF in all cases. The error bars represent standard deviations (± 10) from duplicate CDAF and SDAF experiments. Figure I suggests that the enhanced particle removals by SDAF are a result of coating the bubbles with polymers to enhance the particle bubble attachment efficiency. The enhanced removal s observed with SDAF may also be the result of enhanced polymer mixing and delivery achieved by addition with the bubbles via the recycle stream . Catfl oc TL was also used in the natural water studies and produced comparable results (data not shown ). p

~

~

0 .0'5

Non- ionic CA-243

70

~

.

'"

~

0.

;;

~

54

b 10 ppm Clay • 50 ppm Cla y .. 100 ppm Clay

• 2 milL KPTOe HA 5 mi lL NPTOC HA • 2 /10 }iA-Clay Mix c

'0

100

Tota l Pa rt ic le Remove l By COAF ( 51)

Fig. 4. Comparison of particle removals by conventional OAF (CDAF) versus Selecti ve DAF (SDAR for synthetic water s at 20-22°C. pH 7.0 using non-ionic CA- 233.

J. P. MALLEY, JR

54

Studies using the non-ionic polyacrylamide (CA 233) produced similar results to the Catfloc LS polymer as shown in Figures 4 and 5. Statistical analysis suggests that CA233 does not enhance removals to as large a degree as the two Catfloc polymers. Nevertheless, SDAF using CA233 is significantly better than CDAF using CA233 at the 95% confidence level (P value = 0.045 for synthetic waters; 0.009 fir natural water samples). ' 00

P

~

0 .009

Non-ionic

CA-233

'0

s... -c

.0

. '" 0

Vl

10

"0 >

0

E

" " ~

a::

'0

.•

0

a-

.0

20

.

~

Sp r i n g Su m m e r

Tolal Particle Rem oval By COM (%)

Fig. 5. Comparison of particle removals by conve ntional OAF (CDAP) versus Selective OAF (SOAP) for na tural water samples at 20-22°C, pH 7.0 using non-ionic CA-233.

Result>; of NPTOC meas urements for all applicable studies comparing CDAF to SDAF (Figure 6) show that there is no significant difference (P = 0.999) between the two treatme nts. This is consistent with coagulant dosage studies (Table I) showing that alum dosage controls NPTOC removals in the HA and HNclay systems. Further, particle distribution results suggest that SDAF improves removals of smaller particles (2 to 10 um) which are unlikely to contribute to significant NPTOC. '0'.---- _ -- - - - - - - - - " p

" .0

...

'0

Vl

'0

<§

~

0 .999

• 2 mg/L NPTOC HA o 5 mg/L NPTOC HA • 2/10 HA-Cl ay Mix e Sp ring • Summer

J; ~

50

o

~

40 ·

g

30

"o

0.. Z

'0

., oo~-:':"-'~O~"'-~40-'~O-.o-,-'co--='.o--,o-' ,oo NPTOC Ramoval By CDAF (ll)

Fig. 6. Effects of conventional OAF (COAP) versus Selective OAF (SOAP) on NPDOC removals at 20-22°C. pH 7.0.

55

Selective and direct OAF

Two studies were attempted with the anionic CA243 using clay and natural waters. In both cases, performance of both SDAF and CDAF were reduced over comparable studies using the other polymers (data not shown). CA243 was abandoned at that point. These results were not surprising given the EPM data shown in Figure I and previous research on polyelectrolyte coagulation (Edzwald, 1983; 1985a,b).

..

6

10

•

Calfloc LS 10 ppm

•

E.

... ~ '" en

o

Z mg/L NPTOC HA

z/iO HA-Clay lUx Spring

• Summer 0.595

70

P -

~

r

II

~o

,iF

E ~

'" u :e~ ~

'"

T

30

..

"0

~

20

50 ppm

... 100 ppm a 5 mg /L NPTOC HA P - 0 .000

•.~":',.,......,'~.--:30'o--:':"',......,'~.----=,.:-~,.--:,'::-.----='.:-::'00 Tal a l Partic le Removo l By CDAF (:Il)

Fig. 7. Comparison of Direct OAF (OOAFl with polymer only conventional OAF (CDAP) for synthetic clay waters and natural waters.

Synthetic and natural water samples were studied further to test the effectiveness of direct flotation (DDAF). In these studies, the cationic polymer Catfloc LS, dosage needed to produce the maximum particle removals was determined. This dosage was then used as the sole coagulant in the CDAF mode and compared to the case where the polymer was added to the recycle line only (DDAF). Figure 7 shows results for synthetic waters and the spring and summer natural water samples. These data suggest that CDAF is far more effective than DDAF when 50 ppm and 100 ppm clay and the 5 mg/l HA samples are considered (P = 0.000). However, statistical analysis of the low clay (10 ppm), 2110 HNclay and the natural water samples alone suggests that CDAF and DDAF were not significantly different (P = 0.595). This implies that in relatively clear, low turbidity, low color (NPTOC) waters DDAF may be an effective mode of operation. 1 .0 , -

---"

1 .0

E ,.,

C

30

~

:::!

"0

2.5

'" .,

•

0

•

0

10 ppm Spring

..

6

Summer

Z mg /L NPTOC HA 2/10 HA-Clay Mix

Percen t Solids in Float After CQAF (~)

Fig. 8. Percentage solids of the float for Conventional OAF (COAF) versus Selective OAF (SOAFl. JWST 3/4·[

56

J. P. MALLEY, JR

Percentage solids results for samples of the float taken after CDAF and SDAF are shown in Figure 8 for the cationic polymer (Catfloc LS) and the non-ionic polyacrylamide (CA 233). Statistical analysis of this data indicates that SDAP float had higher percentage solids in all cases. Of particular interest is the percentage solids in the float when CA233 was used. This data suggest that the non-ionic polyacrylamide raised the average percentage solids in the float from 3 percent to 5 percent when applied in the SDAF mode. CONCLUSIONS Comparison of conventional DAF (CDAF) to selective DAF (SDAF) and direct DAF (DDAP) has resulted in the following conclusions. I. EPM studies suggest that the cationinc polymers can coat bubbles and change both their mobility and more importantly their charge. 2. Addition of cationinc polymers Catfloc LS or Catfloc TL; or non-ionic polyacrylamide polymer CA233 to the DAF recycle line significantly improved particle removal. 3. Addition of non-ionic CA233 via SDAP raised the percentage solids of the float from 3 to 5% over CDAF. The Catfloc polymers also improved the float solids. 4. In low turbidity, low color synthetic waters containing clay and/or 2 mg/l NPTOC; and a low turbidity, low color natural water; direct DAF (DDAF) produced comparable particle removal to polymer only CDAF. 5. NPTOC removal was controlled by alum addition and larger (>10 11m) particle removals and not significantly improved by SDAP. DDAF would not be effective in highly colored (NPTOC) and/or highly turbid waters. 6. Studies with the anionic polymer, CA243, used in the CDAP and SDAF mode were not effective when compared to CDAF with alum plus the other polymers. RESEARCH NEEDS The results presented in this paper suggest interesting particle-polymer coated bubble interactions and promising practical applications. However, two critical areas warrant further research. First, these reported studies did not involve the integration of DAF and post filtration research. It is essential to determine if SDAF or DDAF will have significant impacts on the subsequent filter performance. Secondly, the studies reported here are batch, bench-scale efforts. The true effectiveness of SDAF and DDAF cannot be verified without pilot and eventually full-scale confirmation. One particular concern which must be addressed is how does polymer addition affect operation and maintenance of continuous flow recycle (e.g., saturator, nozzles, etc.) systems. As interest in DAF applications and installations increases across the US, researchers, consultants and/or utilities are urged to explore the potential for SDAF and/or DDAF during their pilot testing and full-scale plant shakedown regimes. ACKNOWLEDGEMENTS Much of the groundwork for this research was laid during the author's Ph.D. research. Dr. James K. Edzwald is gratefully acknowledged as the author's mentor as are the USEPA-RREL (CR-812639) and AWWA Abel Wolman Doctoral Fellowship for supporting that early work. Later efforts have been made possible by inkind contributions from Calgon Corporation and UNH-ERG. REFERENCES APHA, AWWA, WEF, (1989). Standard Methods. For the examination of water and wastewater, 17th Edition, American Public Health Association, Washington, DC 20005, USA.

Selective and direct OAF

57

Derjagum, B. Y.• Dukhin, S. S. and Landau. L. (1984) . Kinetic Theory of Flotation of Small Particles, Surface and Colloid Sci.• 13, 17-113 . Derjagu in, B. Y. and Dukhin, S. S. (1960). Theory of Flotation of Small and Medium-Sized Particles, Trans. Am lnst , Met.• 70. 221-246. Edzw ald, J. K. (l985a). Cationic Polyelectrolytes in Water Treatment Proceedings of the Engineering Foundation Conference on Flocculation Sedimentation and Consolidation. B. M. Moudgil and P. Somasumdaran, editors. Amer. lnst, Chem. Eng.• 171-180. Edzwald, J. K. (l985b). Conventional Water Treatment and Direct Filtration: Treatment and Removal of Total Organic Carbon and Tribolomethane Precursors. Organic Carcinogens in Drinking Water : Detection, Treatment and Risk Asses sment (Ram. N.M. et a . editors) John Wiley and Sons . New York. Edzwald, J. K. (1983). Coagulation-Sedimentation-Filtration Processes for Removing Organic Substances from Drinking Water. Control of Organic Substan ces in Water and Wastewater (Berger. B. B., editor). USEPA. EPA-600/8-83-01I, Washington, DC. Edzwald, J. K.• Malley. J. P.• Jr. and Yu, C. (]990). A Conceptual Model for Dissolved Air Flotation in Water Treatment, Water Supply. 8. Jonkoping, pp. 141-150. Graham, N. J . D.• Brandao, C. C. S. and Luckham, P. F. (1992). Evaluating the Removal of Color from Water Using Direct Filtration and Dual Coagulants, Journal AWWA, 84(5). 105-113. Kitchener, J. A. and Gochin, R. J. (1981). The Mechanism of Dissolved Air Flotation for Potable Water; Basic Analysis and a Proposal. Wat. Res .• 15. 585-590. Klassen . V. I. and Mokrousov, V. A. (1963). An Introduction to the Theory of Flotation. Butterworth. London, UK. Malley, J. P.• Jr. (1990). Removal of Organic Halide Precursors by Dissolved Air Flotation, Environmental Technology, 11, 11611168. Malley, J. P., Jr. (1982). A Fundamental Study of Dissolved Air Flotation for Treatment of Low Turbidity Waters Containing Natural Organic Matter, Pb.D . Dissertation, University of Massachusetts. Amherst, MA, USA . Malley, J. P., Jr . and Edzwald, J. K. (l99Ia). Concepts for Dissolved Air Flotation Treatment of Drinking Waters. AQUA . 40(1), 7-17 . Malley, J. P., Jr. and Edzwald, J. K. (199 I b). Compariosn of Dissolved Air Flotation to Conventional Water Treatment Journal American Water Works Association, 83(9), 56 -61. Mangravite, Jr., F. J.• Buzzell, T . M. , Cas sell, E. A., Matijevic. E. and Saxton. G . B. (1975 ). Removal of Humic Acid by Coagulation and Microflotation. JA WWA. 67.88-94. Mangravite , Jr .• F. J., Cassell, E. A. and Matijevic, E. (1972). Microflotalion of Colloidal Silica. Ph.D. Dissertation. Clarkson College of Technology. Potsdam NY. USA. Mang ravite, Jr .• F. J., et al. ( 1972 ). The M icroflotation of Silica. Jour. of Colloid and Interface Science, 39 (2), 357 -36 6. Vrabik , E. R. (1959). Fundamental Principles of Dissolved Air Flotation of Industrial Wastes, Proceedings ofthe J4th Purdu e Inti. Waste co«, 14.743-779.