Influence of Colloidal Particles on Dewatering of Activated Sludge with Polyelectrolyte Kelvin Roberts” and Olof Olsson The Swedish Institute for Surface Chemistry, Drottning Kristinas vag 45, S-114 28 Stockholm, Sweden
Dewatering of activated sludge samples of solids content less than 1%has been studied using capillary suction time techniques and a cationic polyelectrolyte for flocculation. The polyelectrolyte dosage for optimal dewatering was independent of the original solids content of the sludge, and corresponded to the amount of polyelectrolyte required to obtain a zero charge on the anionic colloidal particles in the sludge. Further, it was shown that the negative charge on the colloidal particle could be partially neutralized using a metal hydroxide instead of a cationic polyelectrolyte, and that this reduced the polyelectrolyte requirement for dewatering of the total sludge correspondingly. These investigations showed that dewatering of low-solids activated sludges from domestic sewage treatment plants with cationic polyelectrolyte is determined largely by interactions between the cationic polyelectrolyte and the colloidal materials in the sludge, and that the mechanism of dewatering is probably a cross-linking of the added cationic polyelectrolyte with the natural anionic monomers and polymers in the sludge. In recent years, with the development of sewage treatment plants for municipal waste, the production of biological as well as chemical sludges has given rise to increasing problems in dewatering (1, 2). Under many operating conditions, the use of a filter press or a centrifuge is attractive from economic and space considerations ( 3 , 4 ) .The performance of such mechanical equipment is critically dependent on the foregoing sludge “conditioning,” which is normally carried out by addition of small quantities of highmolecular-weight straight chain polymers, usually derivatives of polyacrylamide (5, 6). This results in a build-up of aggregates which are both larger and stronger than these in the original sludge and therefore more susceptible to separation either by filtration or by centrifugation. Some of the more serious problems in obtaining a suitable polyelectrolyte and in controlling polyelectrolyte dosage under different operating conditions have occurred with sludges of biological origin, both activated and digested types. In view of the interest, both academic and economic, in the nature of the interactions responsible for conditioning of sludge with polyelectrolytes, concern with this problem has been great. Laboratory investigations into dewatering of a series of sludges with a series of polyelectrolytes showed that the relative effectivities of the polyelectrolytes used varied from sludge to sludge (7, 8). Variations occurred even between samples of the same sludge type removed from a given sewage plant a t different times. A group of sludge samples shows simultaneous variations in type and concentration of both suspended and dissolved materials. Direct comparison of the properties of such sludges does not therefore allow a ready elucidation of the factors responsible for efficient dewatering. In view of this, we have studied the filtrative dewatering, measured by Capillary Suction Time, of a series of separate sludges, all produced from one original sludge sample. This was effected by separating the “macroscopic” (larger
than lop) particles by centrifugation, leaving the colloidal particles suspended. These “macroscopic” particles were then mixed in a series of ratios with the suspended colloids to obtain a series of samples of constant concentration of colloidal components, but with varying concentration of “macroscopic” particles-i.e., a series of sludge samples with constant concentrations of colloidal material and varying solids contents. This has permitted an estimation of the relative contributions of colloidal and macroscopic particles to the dewatering of sewage sludge of the type here studied. In addition, elutriation of colloidal particles, as well as partial neutralization of particles charge by added aluminum hydroxide has been investigated as a method for reduction of polyelectrolyte requirement. The colloidal particles separated contributed a negligible proportion of the total solids in the sludge in all samples here studied. Experimental
Activated sludge samples were taken from a domestic cleaning plant a t Ronninge, Sweden. The samples were removed from a sludge-thickening ‘unit and all had solids content of about 1%.Zetag 92, a high-molecular-weight (15 x 106 by viscosity) cationic polyacrylamide derivative (about 40% charged chain units) supplied by Allied Colloids Ltd., Bradford, England, was used as flocculant. The flocculant was made up in an 0.1% solution and added to the samples with rapid mixing for 10 sec followed by slow mixing for 5 min to allow flocculation to occur. The degree of conditioning of the sludge was measured by its ease of filtration as determined by capillary suction time (CST) (9, IO), using an apparatus supplied by Triton Electronics Ltd., England. The sludge samples removed from the sewage plant were treated in the following way: Samples were centrifuged at 3000 rpm for 15 min. This time was determined experimentally as the time after which the major portion of the large (over colloidal size) particles had been settled, leaving the colloidal material dispersed. A confirmation of this was obtained from Coulter counter measurement. The sludge samples were thus separated into a “solid” and “supernatant” phase, the former containing the “macroscopic” particles, the latter containing colloidal material. These two components were than mixed in different ratios to give a series of sludge samples. The first sample was the original sludge of solids content of about 1%, the second contained the same concentration of colloidal particles, while the concentration of macroscopic particles had been reduced to one half of the original value-Le., the solids content of the sample was about 0.5%. Similar samples a t and of the original concentration of macroscopic particles were made with solids content of about 0.25 and 0.12%, respectively. These samples were then treated with a series of concentrations of Zetag 92 under mixing conditions previously described, and the CST was determined. Capillary suction times were determined for Zetag 92 concentrations (referred to as P.E.) of 1, 5 , 10, 20, 50, 100, 200, 500, and 1000 ppm (reckoned on water). Volume 9, Number IO, October 1975
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A series of samples was made up with constant concentration of macroscopic particles and varying nominal concentrations of colloidal particles. This was achieved by removing a portion of the supernatant after the initial centrifugation, and replacing it with distilled water. The sludge was redispersed, and allowed to equilibrate for 45 sec, 4 min, and 6 hr a t +4OC before experiments were carried out. One group of samples was treated with a series of concentrations of Zetag 92, and CST determined as before. Another group was centrifuged, and the supernatants were removed and treated with the same series of dosages of Zetag 92. The electrophoretic mobility of these latter suspensions was then determined using a Rank MK I1 electrophoresis apparatus. Estimates of the change in the number of colloidal particles for those samples allowed to stand for 6 hr were made by analyzing the supernatants for proteins, saccharides, and organic polyphosphates. One-half g/l. of alum [aluminum sulfate, Alz(SO&, 18 HzO] was added to bhe colloidal particles for mobility determinations and to the sludge, prior to polyelectrolyte for CST determination. All sludge samples were removed from the sewage treatment plant not more than 1 hr before experiments began. Whenever possible, experiments were carried out over a 24-hr period, and on occasion, samples were stored overnight. No drastic changes in the sludges were observed within a 30-hr period, the maximum attempted under these experiments, so long as the samples were stored a t +4OC. Protein was determined from the absorption at 530 nm of the complex formed with acetylacetone (11). Saccharide was determined as glucose after hydrolysis with sulfuric acid. Glucose was determined from the absorption of the yellow-green Anthron Compex at 625 nm (12). Organicplus polyphosphate was determined as the difference between orthophosphate and total phosphate in solution. All phosphorus was converted to the orthophosphate form by oxidative hydrolysis with a mixture of perchloric and nitric acids. Orthophosphate was determined from the absorption at 880 nm of the colored molybdate complex, reduced with ascorbic acid ( 1 3 ) .
+
Results Figure l ( a ) shows the CST values for an activated sludge sample which contained 0.82% solids initially, as a function of P.E. dosage at different dilutions of the “macroscopic” particles in the sludge (i.e., as a function of varying solids content). The original sludge, solids about 0.8% (CST 50 sec) shows an optimal dewatering, as indicated by minimal
CST of 15 sec, at 10 ppm of added polyelectrolyte. Reduced dewatering rate occurs above and below this amount of polyelectrolyte addition. Reduction in the amount of “macroscopic” particles in the sludge successively reduces the CST of the untreated sludge to 25, 12, and 10 sec, respective!y, on reduction of the content of macroscopic particles to M, l/4, and Y8, respectively, of the original value. The sludge in which the concentration of macroscopic particles is halved to about 0.4% also displays an optimal dewatering (minimum CST of 10 sec) at 10 ppm polyelectrolyte, though the minimum is less sharp. At lower solids content, little effect of polyelectrolyte is obtained on CST until 100 ppm, when higher CST values or 20-30 sec are obtained. In Figure l ( b ) are shown electrophoretic mobility data for the supernatant alone from the samples used in Figure l(a), when Zetag 92 is added. An initial negative mobility of -1.5 kcm/V sec increases to a value of 0 at about 12 ppm of a polyelectrolyte addition, near the optimum dosage for dewatering observed in Figure l(a). Addition of 100 ppm of polyelectrolyte results in a positively charged colloidal dispersion with the mobility of f1.2 WcmN sec. Figure 2 shows data from the CST measurements on a different sludge sample from the same sewage treatment plant (0.64% solids) in which the colloid-containing supernatants have been partially replaced with distilled water and allowed to stand for 6 hr before measurement. The CST value of 50 sec for the untreated sludge decreases to a minimum (optimal dewatering) a t 10 ppm of added polyelectrolyte, and then increases to a value of 28 sec when 100 ppm of polyelectrolyte is added. A minimal CST value is obtained at polyelectrolyte dosage of 10 ppm for the original sludge. The samples in which the supernatant has been diluted (“dilution” of colloidal particles) are all very similar to the curve for undiluted sludge, all showing an initial CST of between 50 and 70 sec, and an optimal CST of 15-18 sec a t 10 ppm of added polyelectrolyte. Figure 3 shows the effects of dilution of the colloidal material plus standing 6 hr on the concentration of various dissolved and colloidal materials in the supernatant after centrifugation as described. The initial organic- plus polyphosphate concentration is 18 ppm and apparently increases to 25 ppm on 4 times dilution, and to 27 ppm on 8 times dilution. In the same figure are also shown the effects of dilution of the colloidal supernatant on the concentration of carbohydrates and proteins, respectively. The carbohydrate concentration is initially 15 ppm and increases to 30 ppm on dilution 8 times. Similar data for protein concentrations show that the initial concentration is 500 ppm and increases to 1000 ppm on 2 times dilution. Further
4001 30
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io
io0
io00
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Flgure l ( a ) . Influence of dilution of “macroscopic” particles (i.e., variation of solids content)on the dewatering of sludge using cationic
Zetag 92 as flocculant 0 Original sludge, about 0.8% solids. 0 2X diluted “macroscopic” particles, about 0.4% solids. 4X diluted “macroscopic” particles, about 0.2% solids. A 8X diluted “macroscopic” particles,about 0.1 % solids
*
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(b) Effect of added Zetag 92 on electrophoretic mobility of colloidal particles removed from sludge sample described in Figure l(a) X Colloidal particles from original sludge
dilution increases the protein concentration successively to 1300 ppm at 8 times dilution. The effect of replacement of 50% of the colloidal supernatant on the dewatering after short equilibration times is shown in Figure 4 for a further sludge sample (solids 0.9%). The optimal CST of 60 sec at 100 ppm added P.E. obtained for the original sludge was reduced to 15 sec at a lower P.E. dosage of 75 ppm after 45-sec equilibration, but the minimum CST increased to 40 sec and the optimal P.E. dosage to 100 ppm after 4-min equilibration. In Figure 5(a) is shown the effect of addition of aluminum sulfate (0.5 g/l.) on the dewatering of a further sample (0.7% solids). In Figure 5(b) are shown the corresponding electrophoretic mobility data for the colloidal particles from the sludge. The optimal CST of 18 sec at 25 ppm P.E. remains essentially constant (20 sec ) but is obtained at a lower P.E. dosage of 10 ppm in the presence of 0.5 g/l. of alum. The p H remained constant a t 6.5 on alum addition. This alum dosage reduces the negative mobility of the sludge colloidal particles from -1.6 wcm/V sec to -0.8 pcm/Vsec, a reduction otherwise obtained by about 18 ppm of added P.E. The zero mobility value in the absence of added alum is obtained a t 28 ppm added P.E., similar to the dosage for optimal dewatering obtained in Figure 5(a).
Discussion All the sludges used in the experiments here reported have relatively low solids contents. This might be expected to influence the dewatering properties such that the colloidal particles in the sludges, being relatively larger in number, play a relatively important rule. The data shown in Figure l ( a ) and (b) support this in that the optimal polyelectrolyte dose for dewatering does not change when the solids content of the sludge changes. The data in Figure 2, on the other hand, suggest that a “nominal” dilution of the colloidal particles has little influence on the optimal CST for dewatering. An explanation for this apparent paradox might be found in the analytical data of Figure 3. Dilution of the colloid-containing supernatant by 2 times, followed by 6 hr of equilibration with the macroscopic particles in the sludge does not result in a reduction in concentration of either proteins, saccharides, ! o or organic- polyphosphates in the colloidal phase subsequently separated. This indicates that a desorption of soluble and colloidal species from the surface of the bacteria in the “macroscopic” fraction of the sludge occurs on dilution. Since the proteins are certainly in colloidal form and the saccharides and the organic phosphates probably in colloidal form, this suggests that a dilution plus equilibration of the supernatant does not necessarily reduce the concentration of colloidal particles present in the system. That desorption of organic materials from bacterial cell walls should occur on dilution of the suspending medium is hardly surprising when one considers the nature of the BET model for adsorption at the surface, in which a multilayer adsorption is postulated with an equilibrium between species in the outermost adsorbed layer and species in solution (14). The colloidal species present in sludge probably arise partly from the cilia of the bacterial cell, which are known to break off, and can thereby be liberated into solution (15). This process requires a certain equilibration time, however, as evidenced by the data in Figure 4,where a shorter equilibration time of 45 sec reduced both minimum CST and P.E. requirement. Though the P.E. requirement increased again after 4 min, dewatering was still more rapid (lower C S T ) than for the original sludge.
The correlation between optimal P.E. dosage for dewatering and zero electrophoretic mobility in Figures l ( a ) and (b) and 5(a) and 5(b) suggests that reaction of the cationic polyelectrolyte with the anionic colloidally dispersed materials is a major factor in dewatering sludges of this type. Polymer flocculation is believed to occur by adsorption of loops of the polymer chains to form molecular “bridges” between suspended particles, and has been analyzed both practically and theoretically (16, 17). The data here presented, however, suggest that interactions between the added high-molecular-weight cationic
Flgure 2. Influence of dilution of colloidal supernatant on dewatering of sludges using Zetag 92 as flocculant t3 Original sludge, about 0.6% solids. 0 2X diluted colloidal supernatant, about 0.6% solids. 4X diluted colloidal supernatant, about 0.6% solids. A 8X diluted colloidal supernatant, about 0.6% solids
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Figure 3. Influence of dilution of colloidal supernatant and 2 hr standing concentration of dissolved and colloidal materials in colloid phase centrifuged from activated sludge (same sludge sample as Figure 2) Carbohydrate (LH axis). 0 Organic phosphate (LH axis). A Protein (RH axis)
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Figure 4. Effect of equilibration time on dewatering with P.E. of sludge when the “supernatant” has been diluted 8X with water, and
then mixed 0 Original sludge.
* 45-sec equilibration time.
A 4-min equilibration time
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Figure 5(a). Dewatering of activated sludge (0.7% solids) with P.E.
pH = 6.5 0 P.E. alone. A (b)
0.5 g/l. alum
+ P.E.
Electrophoretic mobility of colloidal particles from sludge used in
5 a pH = 6.5 X With P.E. addition. A With 0.5 g/l. alum added.
polyelectrolyte and the anionic colloidal and dissolved materials present in the sludge are decisive for flocculation and dewatering of sludges of the type here studied. Such interactions are probably related to the well-known quantitative reaction of anionic with cationic surfactants to produce sediments (18) and the reactions of inorganic cationic polyhydroxyions with anionic high-molecular-weight polyacrylamide derivatives to produce polymer gels (19). Such gels are three-dimensional structures containing disordered polymer chains. In view of this, it is probable that the added cationic flocculant reacts with suspended colloidal materials to generate a gel-like aggregate, which encloses the suspended “macroscopic” particles and which is readily dewatered. I t is probable that the added P.E. also reacts with the negatively charged groups at the surface of the “macroscopic” particles, but, in view of the low solids contents of the sludges here studied, it is probable that the larger relative number of colloidal particles ensures that these dominate the system. The reactions of anionic with cationic polymeric materials are also employed in flocculation of colloidal suspensions. A low-molecular-weight polymer of one charge is first adsorbed at the surfaces of suspended particles, and followed by addition of a high-molecular-weight polymer of opposite charge to give a three-dimensional interparticle network, which sediments rapidly. Such systems have also been applied to sludge dewatering (patent). The anionic dissolved and colloidal materials in activated sludges of low-solids content may function in a similar fashion on ad-
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dition of high-molecular-weight cationic flocculants to aid formation of three-dimensional networks and dewatering. Aluminum hydroxide, of positive surface potential, at pH 6.5 would be expected to adsorb the collidal materials in these sludges, and this would explain the reduction the P.E. requirement for dewatering (Figure 5). An excess of alum would be expected to totally react with the colloidal materials and inhibit the network formation discussed above, so that a control mechanism for efficient dewatering is suggested. These investigations have shown that P.E. requirement for conditioning of activated sludges of low-solids content is determined by the colloidal particles present, and is independent of the solids content for a given sludge. The P.E. requirement can be reduced by elutriation of the colloidal particles with water, or by partial neutralization of the negative charge on the colloidal particles with positively charged aluminum hydroxide. In comparing different sludge samples, the concentration of colloidal material will vary depending on the type and amount of the “macroscopic” particles present. Acknowledgment
The authors are grateful to Prof. Stig Friberg, The Swedish Institute for Surface Chemistry, for valuable help and discussions, and to Dr. Lars Ulmgren, The Swedish Environmental Protection Board, for help and advice on sampling. Literature Cited (1) Dick, R. I., J . Water Pollut. Control Fed., 44,959 (1972). (2) Dick, R. I., ibid., 45,968 (1973). (3) Sleeth, R. E., J. Water Pollut. Control, 31 (1970). (4) Albertson, 0. E., Grudi, E. E. Jr., J . Water Pollut. Control Fed., 41,607 (1969). (5) Gale, R. S., Filtr. Sep., 8,531,(1971). (6) Coackley, P., Wilson, F., ibid., 61 (1971). (7) Baskerville, R. C., Gale, R. S., J. Znst. Water Pollut. Control, 2,3-11 (1968). (8) Swanwick, J. D., Davidson, M. F., Water Waste Treat., 8, 368-90 (1961). (9) Garwood, J., Effluent Water Treat. J., 7,380 (1967). (10) Gale, R. S.,Baskerville, R. C., Filtr. Sep., 1,37 (1970). (11) Keeler, R. F., Science, 129,1617 (1959). (12) Scott, T. A,, Melvin, E. H., Anal. Chem., 25,656 (1953). (13) Fiske, C. H., Subbarow, Y., J . Biol. Chem., 66,375 (1925). (14) Brunauer, S., Emmett, P. H., Teller, E., J . Am. Chem. SOC., 60,309-317 (1938). (15) Bull. A. T.. J . A D D BioenP.. ~. 22. 261-92 (1972). (16) La Mer, V. K.,. Healy, TT W . , ‘ R e u . Pure Appl. Chem., 13, 112-33 (1963). (17) Hesselink, F. Th., J. Phys. Chem., 75,65-70 (1971). (18) Tornell, B., to be published, 1975. (19) Roberts, K., Kowalewska, N., Friberg, S., J . Colloid Interface Sci., in press. (?) Hawkes, B. E., “Polyelectrolytes in Water Treatment,” New Eng. Water Works Assoc., 84,189 (1970). (?) Olgird, G., Swedish Pat. 1,920,590, 1969.
Received for review November 7, 1974. Accepted June 2, 1975. Work supported by The Swedish Board for Technical Development.
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