THE USE OF POLYMER AS FLOTATION AID IN WATER TREATMENT

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1 THE USE OF POLYMER AS FLOTATION AID IN WATER TREATMENT 59 THE USE OF POLYMER AS FLOTATION AID IN WATER TREATMENT Marco A. P. Reali Luci Sartori Department of Hydraulics and Sanitation, São Carlos School of Engineering, University of São Paulo (EESC -USP), Av. Dr. Carlos Botelho, 465, CEP , São Carlos, SP, Brazil, Abstract This article presents results of studies carried out using two flotation units (batch lab scale and continuos flow) applied in the treatment of coloured and low turbidity water coagulated with aluminum sulfate aided by small synthetic polymer dosages. Different dosages of three types of polymer (cationic, anionic and non ionic) were applied aiming to decrease the required dosage of aluminum sulfate, and consequently, the mass of produced sludge. The application of 0.25 mg.l of non ionic polymer reduced the aluminum sulfate dosage from 40 mg.l to 20 mg.l, with 8% color removal by flotation (residual of 20 uc) and 79% turbidity removal (residual of.5 NTU). On the other hand, the application of 0.25 mg.l of cationic polymer reduced the aluminum sulfate dosage to only 2 mg.l keeping quite the same level of efficiency lowering significantly the sludge production and the concentration of the Al 3+ residual in the water. Key words: dissolved air flotation, flotation, polymer flocculation, polymer flotation water treatment. Introduction Throughout the history man has used natural polymers as raw material for products such as clothes, shelters and tools. But only in the beginning of twentieth century important discoveries about the modification of natural polymers took place, giving birth to the science of synthetic polymers (Young & Lovell, 99). Polymers play an important role in the modern way of life, even in water treatment one find applications. The polymers used in water treatment can be natural or synthetic, also known as polyelectrolytes, and present high capacity to adsorb on the surface of suspended particles in water (Letterman & Pero, 990). In 930 Baylis and Graf and Schworm (cited by Hilson & Richard, 980) showed that it was possible to obtain a significant improval in water treatment adding trivalent cation (primary coagulant) to the water with a small quantity of acid treated sodium silicate. The silicic acid formed through the neutralization of sodium silicate was, thus, the first polymer used as coagulant aid in water treatment. After this, several kinds of natural and synthetic polymers began to be used for the same purpose. Polymers can be classified regarding its charge as: i) anionic, which present negative sites when in aqueous solution; ii) cationic, that present positive sites; and iii) non ionic, that has low charged sites or low tendency to develop them in aqueous solution (Vorchheimer, 980). The Brazilian experience in the use of the synthetic polymers as flocculation aid in water treatment began in the early 70 s as described by Campos (972), who performed tests with commercial polymers available in Brazil, concluding that they improved the water treatment efficiency when used as flocculation and sedimentation aid. According to Bolto (995), some advantages of using polymers as flocculation aid are: reduction of sludge production due to lower coagulant dosages, increase of the filtration units operation, increase of desinfection efficiency due to reduction of the suspended solids concentration in the filtered water. In the present work it was investigated the influence of applying synthetic polymer as flocculation aid in the flotation efficiency of high rate DAF unit treating coloured and low turbidity water. Methods The experimental study was divided into two steps. Firstly, a batch laboratory scale flotation unit (Flotatest) was used to determine the most suitable aluminum sulfate (primary coagulant) and polymer (as flotation aid) dosages to treat a high color and low turbidity water by flotation. Further, tests were performed using a continuous flow high rate dissolved air flotation plant with the same raw Minerva, 4(2): 59-64

2 60 REALI & SARTORI water used in the first step. The raw water was prepared adding 4 mg.l of humic acid (Aldrich, ref. H/66752) and Kaolin (Fluka, ref ) to a deep well water to obtain water presenting 0 uc true color and 6.0 NTU turbidity values, as indicated by Reali & Marchetto (996). The mentioned author determined that the most suitable dosage of aluminum sulfate (only) for the flotation of this raw water was 40 mg.l. Figure shows a scheme of the Flotatest equipment, with four independent flotation columns connected to one saturation chamber. Each column is equipped with a high/ low speed mixer to promote rapid mix of the coagulant (aluminum sulfate) and slow mix for flocculation. This operation is performed in the same column where subsequently the flotation takes place, after withdrawing the mixers and introducing the air saturated water into the columns. Considering the optimal aluminum sulfate dosage obtained by Reali & Marchetto (996) (40 mg.l at ph 6.3) and that lower dosage of coagulant are aimed, two lower dosages were fixed at 2 mg.l and 20 mg.l, representing 30% and 50% of the optimal value, respectively. Following parameters were also fixed: i) gauge pressure into the saturation chamber around 450 kpa; ii) coagulant rapid mix period (Trm): 20 s; iii) mean velocity gradient during rapid mix (Grm): 00 s ; iv) flocculation time (Tf): 24 min; v) mean velocity gradient during flocculation (Gf): 60 s ; vi) dissolved air concentration applied for flotation (S*): 8.0 g of air/ m 3 raw water; vii) time interval between the beginning of flocculation and the polymers application: 5 min. The effect of four synthetic polymers was studied different dosages of anionic Adesol W 302, non ionic Adesol W 303, cationic Adesol W 34 and cationic Nalco Optimer 728 were added to the water after the above mentioned 5 min time log. Dosages of 0.05, 0.25, 0.50, 0.75 and.00 mg.l were tested. An air pulse was given opening and choosing quickly the needle valve at the bottom of the columns. Three water samples were collected at different times for each test, at a distance of 35 cm from the bottom, allowing to fix three flotation velocities (6.7 cm.min, 3.3 cm.min and 3.6 cm.min ). The best association of polymer and coagulant were tested in the second phase, using a high rate horizontal flow dissolved air flotation plant (HRDAF). The HRDAF plant contains a rapid mix unit, three flocculation tanks, followed by a high rate flotation unit. This units contains inclined (60 o with the horizontal plane) parallel plates in its interior promoting low Reynolds number flows This equipment was described in details by Reali & Santos (999) and Reali & Marchetto (996). For the present research an affluent flow of 432 L.h was applied to the pilot plant, resulting in a horizontal velocity between the plates (Vh) around 23 cm.min and Reynolds number around 2. Figure 2 shows a scheme of the HRDAF pilot plant. The flocculation conditions were kept constant during the experiments with the HRDAF unit Tf around 20 min and Gf values of 70 s, 50 s and 30 s, in the first, second and third flocculation compartments, respectively. All of the determinations were based on the recommendations of the Standard Methods for the Examination of Water and Wastewater (995). Results and Discussion Preliminary step using Batch Flotation Unit (Flotatest) Figure 3 shows efficiency values for apparent colour removal (for 3 values of flotation velocity) concerning the dosage of the 4 polymers associated with aluminum sulfate at a concentration of 2 mg.l (Figures 3a and 3d) and 20 mg.l (Figures 3e and 3h). For the essays with 2 mg.l Al 2 (SO 4 ) 3, the best results were obtained using the following dosages for each type of polymer: 0.05 mg.l of the W302 anionic polymer; 0.38 mg.l of the W303 (non ionic); 0.05 mg.l of the W34 (cationic) and 0.25 mg.l of the 728 (cationic). On the other hand, for the tests applying 20 mg.l of aluminum sulfate, the best dosages were: 0.3 mg.l (W302); 0.25 mg.l (W303); 0.05 mg.l (W34) and 0.05 mg.l (Optimer 728). Table shows the results of efficiency in colour, turbidity and TSS removal obtained for the mentioned best dosages (results referring to three flocculation velocity values of, 6.7, 3.3 and 3.6 cm.min ). Figure 3d shows that the best efficiencies for colour and turbidity removals for the three flotation velocities (Vf) tested and the concentration of 2 mg.l of aluminum sulfate were obtained with 0.25 mg.l of the Optimer 728 cationic polymer. For 3.3 cm.min of Vf, for example, this association resulted in 89% of colour and turbidity removal (residuals of 2 uc and 0.9 NTU). These results are quite close to that obtained applying 40 mg.l of aluminum sulfate only, without polymers (see Table ). For the dosage of aluminum sulfate of 20 mg.l, the best performance was obtained applying 0.25 mg.l of the W303 non ionic polymer. This association conduced to 95% of colour removal (residual of 5.0 uc) and 93% of turbidity removal (residual of 0.5 NTU) for 3.3 cm.min of Vf. This efficiency was practically the same to that obtained applying 40 mg.l of aluminum sulfate (95% colour removal and 94% turbidity removal). Minerva, 4(2): 59-64

3 THE USE OF POLYMER AS FLOTATION AID IN WATER TREATMENT 6 Eletric motor Mixer Flotation vessel (60 mm diameter plexiglass) Manometer Security valve W.L m Saturation chamber (75 mm plexiglass) Voltage regulator Waste : Needle valve Porous plate Tap water Compressed air Compressor Figure Schematic diagram of the lab-scale dissolved-air flotation (LSDAF) unit adapted from Reali (99). Coagulant Soda ash Deep weel 2 Raw water reservoir 3 Rapid mix unit 4 Flocculation unit 5 High rate daf unit (with parallel plates) 6 Saturation chamber 7 Compressed air Figure 2 Scheme of the high rate dissolved air flotation unit scheme with horizontal drainage HRDAF (adapted from Reali, 986). Table Abstract of the best results obtained during essays with the Flotatest for different dosages of polymers and of aluminum sulfate. Polymer type Better Efficiency apparent color Turbidity removal (%)/ polymer Alum removal (%)/ residual uc residual NTU dosage dosage See figure Flotation velocity (cm.min ) Flotation velocity (cm.min ) (mg.l (mg.l ) ) W 302 anionic a 73/26 80/22 82/20 69/2.6 8/.6 80/.7 (Adesol) b 78/20 89/ 9/8.4 8/.7 9/0.8 92/0.7 W303 non c 55/46 78/22 8/20 52/3.3 83/.2 86/0.9 ionic (Adesol) d 87/3 95/5.0 96/4.4 85/.0 93/0.5 93/0.5 W34 cationic e 60/43 74/24 76/22 59/2.7 8/.3 85/.0 (Adesol) f 83/6 93/6.8 9/7.8 83/.3 94/0.4 92/0.6 Optimer g 89/8 89/2 90/9 83/.4 89/0.9 90/0.8 cationic (Nalco) h 85/6 92/9.0 92/9.0 86/.0 94/0.4 93/0.5 * * 40 89/2 95/6.0 96/5.0 86/.0 94/0.5 94/0.5 * Without polymer (only alum). Minerva, 4(2): 59-64

4 62 REALI & SARTORI Continuous flow HRDAF unit results The best results obtained using the Flotatest were applied in the HRDAF pilot plant operating with an affluent of 432 L.h (Vh of 23 cm.min and Reynolds number around 2). First tests were performed applying only aluminum sulfate (40 mg.l ). Further, were conducted three tests applying 20 mg.l of aluminum sulfate and 0.25 mg.l of non ionic polymer (W303), followed by another three tests applying 2 mg.l of aluminum sulfate and 0.25 mg.l of cationic polymer (Optimer 728). In each situation three different values of air supplied for flotation (S*, in g of air/m 3 of raw water) was tested. Table 2 shows the efficiency results of colour, turbidity and TSS regarding these tests. When only aluminum sulfate (40 mg.l ) was applied, it is observed that the most suitable value of S* was 5.0 g of air/m 3 of water. In this situation it were obtained average efficiencies of 84% of colour removal (residual of 4 uc), 86% of turbidity removal (residual of 0.8 NTU) and total suspended solids (TSS) residual of 5.6 mg.l. In this case (named case ) the sludge production was of 26.0 g of sludge/m 3 of treated water. For the tests with 20 mg.l of aluminum sulfate and 0.25 mg.l of non ionic polymer, the best flotation conditions were obtained applying 3.2 g of air/m 3 of raw water, with 8% of colour removal efficiency (residual of 20 uc), 80% of turbidity removal (residual of.4 NTU) and TSS residual of 3.0 mg.l. In this situation (named case 2) the sludge production fell significantly to.0 g of sludge/m 3 of treated water. Finally, for the tests with 2 mg.l of aluminum sulfate and 0.25 mg.l of cationic polymer, a higher S* value was required (6.4 g/m 3 ) to obtain a better flotation performance (79% of colour removal, 78% of turbidity removal and residual of only.9 mg.l of TSS). In this case (named case 3), the sludge production fell drastically to 4.3 g of sludge/m 3 of treated water. The aluminum residual was 3 mg Al 3+.L, which is lower than values obtained previously (0.8 mg.l ). Cases 2 and 3 present very similar efficiencies, and lower than those presented in case. Concerning the required quantity of air (S*) and the production of sludge parameters, it was verified that case 3 was more advantageous for sludge production and less advantageous regarding S*. A decision about the association to use in practical situations involves other parameters such as costs regarding final disposal of sludge and dissolved air generation systems. Such parameters may vary from case to case. Conclusions The results obtained throughout this study lead to following conclusions: i) The use of synthetic polymers as flotation aid for the treatment of high coloured water can be a very appealing alternative, being capable of promoting aluminum sulfate economy, significant decrease in the production of sludge and lower concentration of aluminum residuals. Additionally, applying the correctly chosen and dosed polymers, it may be possible to use of higher rates in the flotation units associated with good colour, turbidity and TSS removal efficiencies. ii) For the water used in this research with apparent colour around 0 uc and turbidity at approximately 6.0 NTU and applying 0.25 mg.l of non ionic polymer as flocculation/flotation aid in the flotation pilot plant it was possible to reduce the aluminum sulfate dosage to 50% (from 40 mg.l to 20 mg.l ) without losing significant efficiency in the colour (residual increased from 4 to 20 uc), turbidity (residual increased from 0.8 to.5 NTU) an TSS (residual decreased from 5.6 to 3.0 mg.l ) removals. In this case the production of sludge fell from 26.0 to.0 g of sludge/m 3 of treated water. iii) Applying 0.25 mg.l of cationic polymer, it was possible to reduce to aluminum sulfate dosage in 70% (from 40 to 2 mg.l ), producing a good quality effluent (residuals: TSS of.9 mg.l, color of 20 uc and turbidity of.2 NTU). In this situation the sludge production fell drastically from 26.0 to 4.3 g of sludge/m 3 of treated water, and the concentration of aluminum residual fell from 0.8 to 0.3 mg.l. iv)the most suitable situation concerning the required quantity of air for flotation occurred applying 0.25 mg.l of non ionic polymer associated with 20 mg.l of aluminum sulfate, when only 3.2 g of air/m 3 raw water was enough. Therefore, situations in which the flotation units are surcharged the application of polymers can be a good alternative. Acknowledgement The authors gratefully acknowledge the financial support for this work by Brazilian agencies: FAPESP Fundação de Amparo a Pesquisa and CNPq Conselho Nacional de Pesquisa. Minerva, 4(2): 59-64

5 THE USE OF POLYMER AS FLOTATION AID IN WATER TREATMENT 63 0 W 302 (anionic) alum: 2 mg.l 3a VF = 3,6 cm/min. VF2 = 3,3 cm/min. VF3 = 6,7 cm.min. 0 W 302 (anionic) alum: 20 mg.l 3e 0 W 303 (non ionic) alum: 2 mg.l 0 W 303 (non ionic) alum: 20 mg.l 3b 3f 0 W 34 (cationic) alum: 2 mg.l 0 W 34 (cationic) alum: 20 mg.l 3c 3g 0 Optimer 728 (cationic) alum: 2 mg.l 0 Optimer 728 (cationic) alum: 20 mg.l 3d 3h Figure 3 Remaining fraction (0x C/Co) of apparent colour versus polymer dosage obtained in the batch essays (Flotatest) performed applying 2 mg.l (3a to 3d) and 20 mg.l of Al 2 (SO 4 ) 3 (3e to 3h) for three polymers (cationic, anionic and non ionic). Minerva, 4(2): 59-64

6 64 REALI & SARTORI Applied S* (g.m 3 ) Table 2 Results obtained during the operation of the pilot flotation unit (HRDAF) operating with a velocity between plates around 23 cm.min Reynolds number around 2. Conditions of essays Alum dosage (mg.l ) at ph = 6.3 Polymer type Polymer dosage (mg.l ) color removal (%)/ residual uc turbidity removal (%)/ residual ut Results residual TSS (mg.l ) TSS of flocculated water (mg.l ) Sludge production (g/m 3 water) Residual Al 3+ (mg of Al 3+ /L) /7 84/ (case) /4 86/ /7 85/ (case2) Non ionic Non ionic Non ionic /20 77/ /22 80/ /20 80/ (case3) Cationic /8 79/ Cationic /9 78/ cationic /35 6/ References APHA. Standard Methods for the Examination of Water and Wastewater. 9 th ed. American Public Health Association/American Water Works Association/ Water Environment Federation, Washington DC, USA BOLTO, B. A. Soluble polymer in water purification. Progress in Polymer Science, v. 20, p , 995. CAMPOS, J. R. Ensaios sobre a aplicação de polieletrólitos na floculação de águas de abastecimento Dissertation (Master) São Carlos School of Engineering of the University of São Paulo, SP, Brazil. (In Portuguese). EL-SHALL, H.; MOUDGIL, B. M.; FORBES, D. Solidgrade polymers for clarification and flocculation processes. Mineral and Metallurgical Processing, v. 94, n., p. 6-64, 996. HILSON, M. A.; RICHARDS,W. N. Polymeric flocculants. In: LEWIS, W. M. Developments in water treatment. London: Applied Science,, 980. p LETTERMAN, R. D.; PERO, R. W. Contaminants in polyelectrolytes used in water treatment. Jounal AWWA Research and Technology, v., p , 990. REALI, M. A. P. Conception and evaluation of compact system for water treatment using dissolved-air flotation process and declining rate filtration. 99. Thesis (PhD) São Carlos School of Engineering of the University of São Paulo, SP, Brazil. (In Portuguese). REALI, M. A. P.; SANTOS, S. P. high rate flotation unit with vertical interplates flow apllied to algae removal. Engenharia e Arquitetura Caderno de Engenharia Sanitária e Ambiental, v., n., p , 999. REALI, M. A. P.; MARCHETTO, M. Unidade de flotação por ar dissolvido com escoamento horizontal entre placas aplicada ao tratamento de água. In: XXV Congresso Interamericano de Ingenieria Sanitaria Y Ambiental AIDS 96. nov. 996, Mexico City, Mexico. VORCHHEIMER, N. Synthetic polyelectrolytes. In: Polyeletrolytes for water and wastewater treatment. Boca Raton, FL: CRC Press, 98. p YOUNG, R. J.; LOVELL, P. A. Introduction to polymers. 2 nd ed. Chapmam and Hall, 98. p Minerva, 4(2): 59-64