6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

COAGULATION- PROCESSES IN WATER/WASTEWATER TREATMENT: THE APPLICATION OF NEW GENERATION OF CHEMICAL REAGENTS

N. D. TZOUPANOS AND A. I. ZOUBOULIS*

Division of Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124, GREECE * email: [email protected], tel: +302310997794

Abstract: The aim of this paper is to provide an overall description of coagulation-flocculation process and its applications in water and wastewater treatment. The significance of coagulation – flocculation in the area of water and wastewater treatment is reviewed and evaluated, emphasizing on the series of applications employed, including destabilization of , removal of inorganic and organic matter (particulate and/or dissolved), removal of metals and anions (arsenic, phosphate etc), as well as removal of pathogen microorganisms. Furthermore, the latest developments in the coagulation field, regarding the evolution of the coagulation reagents, are also under investigation. The development of simple pre-polymerized coagulants (i.e. polyaluminium chloride,) seems no longer to be sufficient enough. The need for more effective coagulants has lead to the development of new coagulant categories, via the introduction of various additives in the structure of pre-polymerized coagulants. The first effort was reported 15 years ago, suggesting the use of silica in the form of polysilicates for such a purpose. Nowadays, the range of additives has expanded, including organic compounds, such as anionic, cationic or non-ionic , leading to new composite coagulants. Overall, it is evident that the tendency in the coagulation field nowadays is the production of modified composite coagulants, which they are becoming more and more complicated, regarding their composition, but also more effective, when compared with the traditionally applied reagents.

Key-Words: Coagulation; flocculation; composite coagulants; inorganic additives; organic additives.

1. Introduction stage seems to be a key factor for the improvement The need for drinking water of high quality is of the overall treatment efficiency. increasing, as the non-polluted water sources are The whole treatment process of coagulation – continuously decreasing. Additionally, the flocculation can be divided into two distinct discharge criteria of wastewaters are becoming procedures, which should be applied consecutively. stricter, according to the new legislation in force, in The first one termed coagulation, is the process order to prevent environmental pollution and/or whereby destabilization of a given colloidal infection of drinking water sources. It is evident or solution is taking place. The function that more effective water and wastewater of coagulation is to overcome the factors that treatments are needed. promote the stability of a given system. It is A very important step in water and in achieved with the use of appropriate chemicals, wastewater treatment is the coagulation- usually aluminium or iron salts, the so-called flocculation process, which is widely used, due to coagulant agents. The second sub-process, termed its simplicity and cost-effectiveness. Regardless of flocculation, refers to the induction of destabilized the nature of the treated sample (e.g. various types in order to come together, to make contact of water or wastewater) and the overall applied and thereby, to form large agglomerates, which can treatment scheme, coagulation-flocculation is be separated easier usually through gravity settling usually included, either as pre-, or as post-treatment [1]. Coagulation usually completes in a very short step. The efficiency of coagulation-flocculation period of time (e.g. about 10 s), whereas strongly affects the overall treatment performance; flocculation occurs usually over a period of 20 to hence, the increase of the efficiency of coagulation 45 min. It is a common practice, especially in Greece, the enhancement of particles aggregation

ISSN: 1790-5095 309 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

Table 1. Classification of sizes [1]. Total surface Time required to settle Particle size Classification Examples area 100 mm, if specific (mm) (m2/cm3) gravity = 2.65 10 Coarse Gravel, coarse sand, mineral 6 x 10-4 0.1 s 1 (visible to naked eye) substances, precipitated and 6 x 10-3 1 s -1 flocculated particles, silt, -2 10 macroplankton 6 x 10 13 s -2 Mineral substances, precipitated 10 Fine particulate 0.6 11 min -3 and flocculated particles, silt, 10 dispersion (visible 6 20 hours bacteria, plankton, and other -4 under microscope) 10 organisms 60 80 days 10-5 Mineral substances, hydrolysis 600 2 years Colloidal dispersion and precipitated products, 10-6 (submicroscopic) macromolecules, biopolymers, 6000 20 years viruses Inorganic simple and complex ions, molecules and polymeric <10-6 Solution species, polyelectrolytes, organic molecules, undissociated solutes during the 2nd step by the addition of organic electrostatic repulsive forces constrain the particles polyelectrolytes, the so-called flocculant aids, or from approaching each other and the suspension is simply flocculants. characterized as stable; therefore, long time period is required for settling. 2. Coagulation – flocculation in water In order to accelerate the settling time, destabilization is required, denoting the importance and wastewater treatment of coagulation. The destabilization can be achieved

with one, or a combination of two or more of the The suspended material in waters and in following mechanisms, after the addition of a wastewaters mostly arise from land erosion, the coagulant agent [2, 3]: dissolution of minerals, the decay of vegetation and 1) Compression of the electrical double layer. from several domestic and industrial waste 2) Adsorption and charge neutralization. discharges. For a given water or wastewater, such 3) Adsorption and interparticle bridging. material may comprise suspended, dissolved 4) Enmeshment in precipitate (by using of excess organic and/or inorganic matter, as well as several coagulant dose, “sweep flocculation”). biological organisms, such as bacteria, algae or After the destabilization, flocculation promotes viruses. This material has to be removed, as it the aggregation and flocs formation, usually after causes deterioration of water quality by reducing the addition of an appropriate flocculant agent. the clarity (e.g. causing turbidity or colour), Two general types of flocculation can be identified: causing infection, and eventually carrying toxic micro-flocculation (or perikinetic flocculation), in compounds, adsorbed on their surfaces. In addition, which particles aggregation is brought about by the organic matter is the main precursor to the thermal motion of fluid molecules (Brownian formation of disinfection by-products, when motion) and macro-flocculation (orthokinetic chlorine is applied as disinfection agent. flocculation), in which particle aggregation is Table 1 [1] classifies according to the size the brought about by inducing velocity gradients and common materials present in waters or mixing in the suspension [3]. wastewaters. Particles smaller than around 10-5 mm Coagulation-flocculation is used widely during may be referred as colloids and particles smaller water or wastewater treatment. It is an integral than 10-6 mm as solutions. It can be noticed that treatment step in the surface or underground waters with decreasing size, the time required for settling treatment, indented for human consumption. increases, up to several years for certain solutions Typical applications are the removal/separation of ingredients. Due to the very small size, the only colloids and suspended particles, of natural organic way for settling and the subsequent separation is to matter, or of metal ions. In wastewater treatment, come closer, to make contacts and to form larger additional applications include the removal of toxic particles, which can be settle easier. This procedure metals, anions (i.e. phosphates), color, odor etc. however, is hindered due to the homonymous negative charge this material carries. The

ISSN: 1790-5095 310 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

or polyaluminium chloro-sulfates (PACS), with 3. Coagulation reagents variable degrees of polymerization. These products and especially the first one (PACl) are used 3.1 Metal coagulants extensively worldwide during the last decades, with The commonly used metal coagulants fall into an ever increasing demand. Their properties were two general categories: those based on aluminium intensively examined and have proved to be more and those based on iron. This paper deals with the efficient in lower dosages, in wider pH, aluminium based coagulants, hence these will be temperature and colloids concentration ranges, than described further. the conventional simpler ones, leading to cost and The most widely used metal coagulant is operative more effective treatment [3, 4]. probably the aluminium sulfate (“alum”), which Their superiority is related to the different has been used for during the past aluminium species distribution in the polymerized, decades. It is commonly manufactured from the as compared with the non-polymerized solutions. Aluminium exists in water solutions and at pH reaction of bauxite ores with sulfuric acid. 3+ Evaporation of water in the process results in a dry values < 3 as six-coordinated Al ion. With product, having the approximate formula increase of pH value, the aluminium ion begins to hydrolyze, and various products can be formed. Al2(SO4)3.14-18H2O, and with an Al content ranging from 7.4 to 9.5%. It is supplied as powder The hydrolyzation of Al ion is extremely or in liquid form and its applications include water complicated, involving hydrolysis and or wastewater treatment. polymerization reactions, thus leading apart from A product prepared from alum and sulfuric acid various monomeric, also to several polymeric is acidified aluminium sulfate (acid alum). It is species formation [5]. With conventional supplied as a liquid and is used mainly in paper- coagulants, these reactions are entirely making industry. Other Al coagulants are the uncontrolled, and the coagulation efficiency is aluminium chloride (AlCl .6H O) and the sodium based more to Al(OH)3 formation (around the pH 3 2 6.5-7) and less to charge neutralization, as the aluminate (NaAlO2). The first one is supplied as liquid (containing 10.5% Al) and is used widely for hydrolysis reactions are proceeding fast and hence, sludge conditioning. The latter is usually supplied the concentration of positively charged Al ions is as a viscous, strongly alkaline and corrosive liquid restricted. In the case of PACl, the pre- (containing 13% Al). It is rarely used alone, but polymerization aims to control up to a certain generally with alum to obtain some specific results, extent the aforementioned reactions. i.e. in the treatment of highly colored waters [1, 3]. With the partial neutralization of Al solution by The application of simple metal coagulants adding a base solution and then by aging, these (conventional) is widespread, especially due to the reactions may take place at some extent and various Al species can be formed. The most important relatively low cost and the simpler application - route. However, they exhibit several disadvantages, reaction is supposed to be that leading to Al(OH)4 such as the need for pH adjustment before or after formation. This aluminium anionic form is claimed to be the precursor for the polymeric Al13 creation treatment, the sensitivity to temperature changes, 6+,7+ the need for higher dosages because the charge [6]. Al13 (AlO4Al12(OH)24(H2O)12 ) with ε- neutralization is not usually sufficient, the Keggin structure is one of many possible PACl polymeric compounds, like dimmers (e.g. sensitivity to sample specific characteristics and 4+ 5+ composition, as well as the excessive sludge Al2(OH)2 ), trimers (e.g. Al3(OH)4 ) among production. several others, even larger than Al13 i.e. with more than 13 aluminium atoms (e.g. 18+ 3.2 Pre-polymerized coagulants Al30O8(OH)56(H2O)24 ), which transform continuously from one form to another [7]. Several research efforts have been devoted However, Al is claimed to be the most stable to improve the efficiency of coagulation- 13 aluminium specie in a partially neutralized flocculation process. After studying the aquatic aluminium solution [8] and the improved chemistry and the behavior of simple Al salts, the coagulation properties of PACl are thought to be way for improvement seemed to be their (partial) due to its existence, i.e. increased charge polymerization. The result of these efforts was the neutralization capability (coagulation) and production of a range of pre-polymerized increased molecular size and aggregation ability aluminium solutions, referred as polyaluminium (flocculation). Further on, the decrease of chlorides (PACl), polyaluminium sulfates (PAS), monomeric Al in favor of Al13 and of the other

ISSN: 1790-5095 311 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

polymeric compounds eliminates the hydrolysis silica. Hasegawa et al. [10] noticed that by reactions and therefore, results in a minor impact of introducing metal ions into polymerized silicic acid pH value after treatment. solution, the molecular weight of the product The production of pre-polymerized coagulants increased and the respective stability and involves the partial neutralization of an Al solution coagulation performance were improved. In this with a base solution. Many parameters can affect case, the product was rather an inorganic metal- the properties of the final product, i.e. the base polysilicate flocculant, where silica was the main addition rate, the stirring speed, the synthesis component, than a pre-polymerized inorganic temperature, the presence of other anions (e.g. Cl-, coagulant. More recently, research has focused in - 2- SO4 , CO3 ), the aging time etc. The most the incorporation of silica within the pre- important parameter however, is claimed to be the polymerized metal solutions, aiming to products molar ratio of bound hydroxide to the concentration with larger molecular weight by the application of of metal cations (i.e. the OH/Al molar ratio). It is two techniques; i.e. either by introducing referred as basicity and is also used to describe polymerized silica in the pre-polymerized metal conveniently the polymerization degree of solution, or by introducing polymerized silica in the coagulant agents. metal solution, followed by polymerization. The first method is referred as composite polymerization, whereas the latter one as co- 4. New generation coagulation polymerization. reagents Polymerized silica can be obtained when the concentration of monosilicic acid (Si(OH) ) in In spite of their improved properties, the pre- 4 aqueous solution exceeds the solubility limit of polymerized coagulants performance still remains amorphous silica, or otherwise by bringing the inferior, when compared to the performance of solution temporarily into the pH-concentration organic polyelectrolytes, e.g. when used in water range of oversaturation with respect to amorphous treatment [9]. In recent years, the relevant research silica [11, 12]. Excess monomers in the solution in the coagulation-flocculation field has focused gradually disappear to form polymers. The mainly in understanding the behaviour and aquatic polymers, or polysilicic acid, consist of silica chemistry of pre-polymerized coagulants, such as tetrahedrons that are linked via silicon-oxygen- polyaluminium chloride, and to the further silicon bonds (e.g. ≡Si-OH + HO-Si≡ → ≡Si-O-Si≡ improvement of their properties. The main reason + H2O) [12]. Various species can be formed, e.g. for the higher efficiency of organic polymers is 2- 3- Si O (OH) , Si O (OH) etc., and exact nature their higher molecular weight (MW), which implies 2 3 4 3 5 5 and the extent of their formation is affected by the better flocculation properties. Thus, the increase of temperature, the concentration of silica, the molecular weight and size of the pre-polymerized presence of other ions, the pH value and the aging coagulants is thought to be the way for further time. Soluble, internally hydrated, polymeric silicic improvement. acids of colloidal dimensions, or discrete particles The general concept followed is, the of colloidal size, can be present. The most introduction of various additives in the structure of important parameter that affects the kinetics of a pre-polymerized coagulant, in order to produce a silica polymerization is the pH value. Particularly, homogenous, stable product with higher MW and the polymerization is thought to be catalyzed by improved coagulation-flocculation performance, hydroxyl anions and is very fast at neutral or than the initial reagent. The challenge to confront is slightly alkaline pH range. Over pH value 9, the the desirable combination of higher efficiency and reaction rate decreases. Furthermore, by aging the cost-effectiveness, which are the basic prerequisites polymerization can lead to the formation of bigger for the development of new products. Various aggregates and finally, to formation additives were examined, which can be classified (amorphous (SiO) ). In order to avoid gelation and into two main categories; inorganic and organic. n to preserve the polysilicates at soluble form, the These compounds and the respective products solution has to be brought outside the insolubility derived, based on aluminium coagulants, will be range. This can be achieved by acidifying the described in the following. solution to pH values 2–3. Particularly, at pH

around 2 the polymerization rate is retarded enough 4.1 Inorganic additives and the polysilicates are stable in soluble form for A suitable additive for the improvement of several days [11-13]. flocculation properties is thought to be polymerized

ISSN: 1790-5095 312 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

Based on these observations and the use of a synthesis can be manipulated to produce polymers suitable silicic acid source, i.e. the sodium silicate of varying size (MW), charge groups, number of solution or water glass, several efforts have been charge groups per polymer chain (charge density) made for the introduction of acidified polysilicates and varying structure (linear or branched). Many into the structure of pre-polymerized coagulants. synthetic polyelectrolytes are based on The mixed product in the case of aluminium is polyacrylamide and its copolymers with called polyaluminium silicate chloride, though it polyacrylic acid. Typical examples are the should be rather referred as a category of hydrolyzed polyacrylamides (anionic, MW 104-107 coagulation reagents, because with different g/mole), polydiallyldimethyl ammonium chloride combinations of OH/Al or Al/Si molar ratios, (poly-DADMAC, cationic, MW 104-107 g/mole) various reagents can be produced. Polyaluminium and various polyacrylamides (nonionic, MW 105- silicate chloride has shown superior coagulation 107 g/mole) [18]. performance, than the conventional or simple pre- Organic polymers are not generally used as polymerized coagulants in water or wastewater primary coagulants. Only in certain cases cationic treatment [9, 14, 15]. The better performance can polyelectrolytes will be applied for the be attributed to the alteration in a specific degree of destabilization of colloids (coagulation) and the coagulants composition. Specifically, it has usually, in conjunction with inorganic metal salts. been proved that the introduction of silica chains The primary use of polymers is as flocculant aids, into the structure of PACl increases the molecular whereas non-ionic or anionic polyelectrolytes are weight of the coagulants components with the added after the addition of an inorganic coagulant formation of alumino-silicate complexes, thus [19]. Despite their efficiency, their use is leading in enhanced aggregating power and bigger- accompanied with significant drawbacks, such as denser flocs formation [9, 14-17]. the increased treatment cost. Considering that apart from the product cost itself, which is higher than 4.2 Organic additives inorganic salts, polymers are delivered usually in Organic polyelectrolytes are typical solid form and hence, a separate dissolution set-up hydrophilic colloids and contain certain functional is needed, which further increases the treatment groups, which may be ionizable. When these cost. Furthermore, the existence of remaining groups dissociate, the polymer molecules become unreacted monomers results in residual toxicity in charged, either positively or negatively, and are the treated sample. thus referred to as cationic or anionic The combination of an inorganic salt polyelectrolytes, respectively. If no ionizable (coagulant) and a suitable polyelectolyte groups are present, the polymers are termed as non- (flocculant) into one reagent is thought to improve, ionic [1]. apart from the treatment efficiency of the inorganic They can be classified according to their origin salt, the overall cost-effectiveness. No addition of a into two main categories, the natural and or flocculant aid will be needed any more, whereas synthetic polymers. Natural polymers the introduction of the into the (biopolymers) have long been used as flocculants structure of the metal salt will also significantly (bioflocculants). Typical examples are starch reduce the eventual residual toxicity. derivatives that are pregelatinized and water- These suggestions lead to the production of soluble. These can be starches, anionic oxidized modified inorganic coagulants with organic starches, or amine-treated cationic starches. Other additives, a field that is under investigation classes include polysaccharides, such as guar gums, nowadays. All the aforementioned charged or not tannins, chitosan and the alginates [3, 18]. charged polyelectrolytes can be used for this Although natural polyelectrolytes have the purpose, with different impact in the nature of the advantage of being toxic-free, the use of synthetic new product. Particularly, by the introduction of a polyelectrolytes is more widespread. They are in cationic polyelectrolyte into the structure of a pre- general more effective as flocculants, principally polymerized metal coagulant, the charge due to the possibility of controlling properties, such neutralization capability is mainly expected to be as the number and type of charged units and the improved, and in a lower extent, the aggregating molecular weight. Moreover, they are much effect as well. Moreover, the usage of an anionic cheaper, than those made from natural sources. polyelectrolyte with higher molecular weight (as Synthetic organic polymers are made either by compared to cationic) would further improve the homo-polymerization of the monomer, or by co- aggregating ability. In this case, however, the polymerization of two monomers. Polymer anionic polymer should have weaker anionic

ISSN: 1790-5095 313 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

charge, in order not to deteriorate extensively the biologically pre-treated landfill leachates with charge neutralization strength of the combined coagulation-flocculation [24]. This kind of coagulant agent. A non-ionic polyelectrolyte would wastewater is difficult to be further treated, mainly have a minor effect in the coagulants charge due to its bio-refractory organic content (humic density (possible negative), but would increase substances). The performance of PASiC (OH/Al = significantly the flocculation ability. Key factors 1, Al/Si = 15) is evaluated and compared with the affecting strongly the properties of the combined, respective performance of a conventional coagulant modified coagulant are the charge, MW and (alum, OH/Al = 0), as well as with two simple pre- content of the polyelectrolyte. polymerized coagulants, a widely used commercial Some efforts have been reported, regarding the product with slightly higher polymerization degree combination of PACl with various anionic, cationic (PACl-18, OH/Al = 1.2) and a laboratory prepared and non ionic organic polymers [20] and the PACl solution with the same polymerization degree combination of PACl with poly-DADMAC [21, (PACllab, OH/Al = 1). Turbidity, UV absorbance at 22]. Although the research in this field is in an 254 nm (corresponding to the presence of Natural initial stage, promising results were obtained from Organic Matter / NOM) and chemical oxygen the aforementioned, publications regarding the demand (COD) were among the monitored coagulation performance. parameters during these experiments. The introduction of the polyelectrolyte can be From table 1 it can be seen that the least accomplished by two procedures, as in the case of effective coagulant in almost all cases is alum, silica addition, i.e. with the co-polymerization, or whereas the most effective is PASiC. The highest the composite polymerization. The content of the removal efficiencies for all the examined polyelectrolyte (PE) in the final product is usually parameters are achieved with the new composite expressed as the Al/PE ratio (mg/L), can take coagulant for the highest dosage added (300 mg/L). values between 5 and 20 and together with the Worthnoting is the fact that with the addition of -1 aforementioned parameters in the case of pre- alum or PACllab over 200 mg L the quality of polymerized coagulants, designates the properties treated sample was deteriorated by means of of the final product. turbidity (denoted as 0% removal). Therefore, the further increase of concentration leads to 4.3 Applications of the new generation overdosing, reversibility of coagulation phenomena coagulants and re-stabilization of colloids. However, in the In the 3rd IASME/WSEAS Conference case of silica-based coagulants, the addition of even -1 (Crete, 2007) [23], the authors presented the 300 mg L results in the further improvement of superior performance of polyaluminium silicate treated wastewater (leachate) characteristics, thus chloride, as compared to the performance of with these coagulants the application of higher conventional or simple pre-polymerized coagulants dosages is possible in order to obtain better in water treatment. Particularly, it was shown that treatment results. the incorporation of polymerized silica enhances Nevertheless, it should be mentioned that in the aggregation ability, as the duration of flocs order to prepare a superior novel silica-based growth period decreases and the size of generated coagulation reagent, specific attention has to be flocs increases. This enhancement compensates given to the polymerization degree, the silica efficiently the decrease in coagulants charge content, as well as to the preparation method, neutralization capability due to silica addition, as because from these experiments it was revealed the coagulation performance of PASiC was clearly that composite coagulants with different OH/Al and better, especially regarding residual Al Al/Si molar ratios or preparation methods can be concentration. The advantages of PASiC in water most efficient in different cases [14, 24]. As an treatment can be summarized in the more efficient example, fig. 1 displays the percent (%) removals treatment, using lower dosages of coagulants and in of organic matter (by measuring the UV a wider pH range, whereas the Al concentration absorbance at 254 nm), after treating a synthetic that remains in the treated sample is significantly wastewater sample (simulated domestic lower. wastewater) with several polyaluminium silicates A matter that has not been considered yet in coagulants, containing different silica content detail is the application of polyaluminium silicate (Al/Si 5-15, Al/Si=0 corresponds to PACl). It can chloride in wastewater treatment. Table 2 be noticed that not all composite coagulants are demonstrates the results of the post-treatment of more efficient than PACl, but only those with Al/Si = 10 or 15.

ISSN: 1790-5095 314 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

Regarding the new composite coagulants with organic additives, the evaluation of their Table 2. Comparison of coagulation* performance for several Al-based coagulants when applied in waste water treatment. Turbidity removal Absorbance at 254 nm COD removal Coagulant (%) removal (%) (%) Coagulant dosage (mg/L) Coagulant dosage (mg/L) Coagulant dosage (mg/L) 50 100 300 50 100 300 50 100 300 Alum 65 72 0 16 29 39 30 51 61 PACl-18 83 88 77 18 31 70 31 46 73 PACllab 84 88 0 20 32 71 35 49 63 PASiC 86 90 94 19 33 74 32 51 77 * Initial turbidity 4.5 NTU, initial absorbance at 254nm 2.740, pH of treated sample 8.6 Table 3. Comparison of coagulation* performance for polymers-based coagulants, when applied in water treatment. Absorbance at 254 nm Residual Al concentration Turbidity removal (%) Coagulant removal (%) (mg/L) Coagulant dosage (mg/L) Coagulant dosage (mg/L) Coagulant dosage (mg/L) 2 4 6 2 4 6 2 4 6

PACllab 94 97 97 70 73 79 150 172 178 PACPE 95 99 99 83 86 87 155 150 158 * Initial turbidity 16.2 NTU, initial absorbance at 254nm 0.120, pH of treated sample 7.78 coagulation performance in water treatment has flocculant aid in the case of PACl (in started recently. concentrations equal to 1/10 of PACl concentrations), whereas in the case of PACPE no flocculant aid was added. Both examined 70 coagulants are efficient in turbidity removal. Significant differences can be observed in absorbance removal, where PACPE exhibits better 65 performance. It can be noticed that even with relatively low dosage of coagulants (2 mg/L), the

60 absorbance removal is over 80% with PACPE, whereas with PACl the respective removal is 70%. Moreover, the residual Al concentration is 55 Abs removal (%) much lower in the case of PACPE, especially for the dosages of 4 and 6 mg/L. The most important

50 advantage however, is related with the amount of 0 5 10 15 20 PE used in both cases. Particularly, it was noticed Al/Si molar ratio that with 30% less PE, the performance of PACPE Fig. 1. Absorbance removal after treating a was better than the respective performance of synthetic wastewater sample with PACl and several PACl. Not only the two treatment steps polyaluminium silicate chlorides reagents. (coagulation-flocculation) were combined in one, Concentration of coagulants 50 mg/L, initial but better treatment was achieved with lower absorbance at 254 nm 1.38, pH 7.45. amounts of polyelectrolyte, pointing out not only the superior coagulation performance, but also the Table 3 illustrates the comparative results of cost effectiveness related with the use of the new kaolin-humic acid model suspension (simulated composite coagulation agents. surface water) treatment with laboratory prepared The improved properties of the composite PACl (OH/Al= 2) and a composite coagulant coagulants with polyelectrolytes can be devoted to prepared with the incorporation of a non-ionic the better flocculation properties, compared to the polyacrylamide (Magnaflock LT-20) in PACl conventional coagulants, similarly with the case of (OH/Al = 2). This specific polymer is used in water PASiC. Figure 2 illustrates the results of the treatment facilities in Greece, whereas the examination of coagulation kinetics for the respective Al/Si ratio (mg/L) in the final product aforementioned in the coagulation experiments pre- (PACPE) was 15. The coagulation experiments polymerized and composite coagulants. The flocs were conducted with the use of the same PE as growth (presented as Ratio values), relatively to

ISSN: 1790-5095 315 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

time (sec) is presented. Two distinct effects can be determination of the specific properties that imply noticed, due to PE addition in the PACl reagent. superior coagulation performance. Coagulants with The first is related with the time needed for the different basicities and/or additives content may flocs to growth, the so-called lag phase. It is have different applications (water/wastewater evident, that in the case of the composite coagulant treatment). Moreover, suggestions for the the lag phase is significantly limited. The latter is preparation of new reagents appear in the literature related with the extent of flocs growth, and continually, i.e. different combinations of Al and specifically with the respective maximum Ratio Fe in one reagent and possible addition in a second values achieved with the two coagulants. Higher step of an inorganic or organic compound. Ratio values indicate greater flocs size [25], therefore it can be concluded that with the Acknowledgements composite coagulant bigger flocs are formed. Thanks are due to the Greek Ministry of 4.0 Development (General Secretariat for Research and 3.5 Technology) which supported this research through

3.0 the PENED program (25%) and to the European Union, which co-founded this program (75%). This 2.5 research is part of the Ph.D. Thesis of N.D. 2.0 Ratio

Tzoupanos.

1.5 References: 1.0 0.5 PACl 2 [1] Bratby, J. (ed), Coagulants, in Coagulation PACPE 0.0 and Flocculation in Water and Wastewater 0 200 400 600 800 1000 1200 1400 time (sec) Treatment, second ed., IWA Publishing, London, 2006, pp. 50-68. [2] Duan, J., Gregory, J., Coagulation by Figure 2. Kinetics of coagulation of the Al-based hydrolysing metal salts, Advances in & coagulants. Interface Science, Vol. 100-102, 2003, pp. 475-

502.

[3] J.C. Crittenden, R.R. Trussel, D.W. Hand, 5. Conclusions K.J. Howe, G. Tchobanoglous (eds), Coagulation, mixing and flocculation, in: Water Treatment: The application of the new generation of Principles and Design, second ed., John Wiley & coagulation reagents exhibits several advantages, Sons, New Jersey, 2005, pp. 643-779. compared to the conventional and simple pre- [4] S. Sinha, Y. Yoon, G. Amy, J. Yoon, polymerized coagulants. The incorporation of Determining the effectiveness of conventional and various additives, inorganic or organic, results in an alternative coagulants through effective increment of molecular weight and components characterization schemes, Chemosphere, Vol. 57, size, which compensates efficiently the decrease of 2004, pp. 1115-1122. charge neutralization capability in the new [5] Z. Jia, F. He, Z. Liu, Synthesis of coagulants. Overall they present better treatment polyaluminium chloride with a membrane reactor: performance, lower residual metal concentration operating parameter effects and reaction pathways, and wider effective pH range, which are the main Industrial and Engineering Chemistry Research, advantages of the composite coagulants. In the case Vol. 43, 2004, pp. 12-17. of composite coagulants with polyelectrolytes, the [6] Parker, D.R. and Bertsch, P.M., Formation economical benefits should also be taken into of the ‘Al13’ tridecameric polycation under diverse consideration. synthesis conditions, Environmental Science & However, despite the aforementioned Technology, Vol. 26, 2002, pp. 914-921. advantages of newly prepared coagulants, the [7] Bi, S., Wang, C., Cao, Q. and Zhang, C., research in the coagulation-flocculation field is Studies on the mechanism of hydrolysis and continuing. Optimalization of the preparation polymerization of aluminium salts in aqueous conditions is needed, especially in the case of solution: correlations between the Core-links model composite coagulants with organic additives, and

ISSN: 1790-5095 316 ISBN: 978-960-6766-97-8 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008

and Cage-like keggin-Al13 model, Coordination [18] Bache, D.B. and Gregory, R. (eds) Flocs in Chemistry Reviews, Vol. 248, 2004, pp. 441-455. Water Treatment, IWA Publishing, 2007, London, [8] Wang, M. and Muhammed, M., Novel UK. synthesis of Al13-cluster based alumina materials, [19] Kim, Y.H. (ed.), Coagulants and NanoStructured Materials, Vol. 11, 1999, pp. Flocculants; Theory and Practice, Tall Oaks 1219-1229. Publishing, 1995, Littleton, USA. [9] Gao, B.Y., Hahn, H.H., Hoffmann, E., [20] Tang, H., Shi, B., The characteristics of Evaluation of aluminum-silicate polymer composite flocculants synthesized with inorganic composite as a coagulant for water treatment, polyaluminum and organic polymers, in Hahn, H., Water Research, Vol. 36, 2002, pp. 3573-3581. Hoffmann, E., Odegaard, H. (eds) Chemical Water [10] Hasegawa, T., Hashimoto, K., Onitsuka, and Wastewater Treatment VII, Proc. of the 10th T., Goto, K., Tambo, N., Characteristics of metal- Gothenburg Symposium 2002, IWA Publishing, polysilicate coagulants, Water Science & Gothenburg, Sweden, 2002, pp. 17-28. Technology, Vol. 23, 1990, pp. 1713-1722. [21] Gao B-Y., Wang Y., Yue Q-Y., The [11] Chan, S.H., A review on solubility and chemical species distribution of aluminiumin polymerization of silica, Geothermics, Vol. 18, composite flocculants prepared from polyaluminum 1989, pp. 49-56. chloride (PAC) and Polydimethyldiallylammonium [12] Stumm, W., Huper, H., Champlin, L., chloride (PDMDAAC), Acta Hydrochimica et Formation of polysilicates as determined by Hydrobiologica, Vol. 33, No 4, 2005, pp. 365-371. coagulation effects, Environmental Science & [22] Gao, B.-Y., Wang, Y., Yue, Q.-Y., Wei, J.- Technology, Vol. 1, 1967, pp. 221-227. C., Li, Q., Color removal from simulated dye water [13] Dietzel, M., Dissolution of silicates and the and actual textile wastewater using a composite stability of polysilicic acid, Geochimica et coagulant prepared by polyferric chloride and poly- Cosmochimica Acta, Vol. 64, 2000, pp. 3275-3281. dimethyldiallylammonium chloride, Separation [14] Zouboulis A.I. and Tzoupanos, N.D., and Purification Technology, Vol. 54, 2007, pp. Polyaluminium Silicate Chloride – a systematic 157-163. study for the preparation and application of an [23] Zouboulis, A.I., Tzoupanos Ν.D., Moussas, efficient coagulant for water or wastewater P.A., Inorganic Pre-Polymerised Coagulants: treatment, Journal of Hazardous Materials, 2008, Current Status and Future Trends, 3rd in press. IASME/WSEAS International Conference, S. [15] Cheng, P.C., Chi, F.G., Yu, R.F., Shi, P.Z., Nikolaos, Greece, 24-26 July 2007, pp. 292-300. Evaluating the coagulants of polyaluminium [24] Tzoupanos, N.D., Zouboulis A.I., Zhao Y.- silicate chlorides on turbidity removal, Separation C., The application of novel coagulant reagent Science & Technology, Vol. 41, 2005, pp. 297-309. (polyaluminium silicate chloride) for the post- [16] B.Y. Gao, Q.Y. Yue, B.J. Wang, The treatment of landfill leachates. Chemosphere, 2008, chemical species distribution and transformation of in press. polyaluminium silicate chloride coagulant, [25] Zouboulis, A.I., G. Traskas, Comparable Chemosphere, Vol. 46, 2002, pp. 809-813. evaluation of various commercially available [17] Song, Y.H., Luan, Z.K., Tang, H.X., aluminum-based coagulants for the treatment of Preparation and characterisation of polyaluminium surface water and for the post-treatment of urban silicate chloride coagulant, Environmental wastewater, Journal of Chemical Technology & Technology, Vol. 24, 2003, pp. 319-327. Biotechnology, Vol. 80, 2005, pp. 1136-1147.

ISSN: 1790-5095 317 ISBN: 978-960-6766-97-8