A new technique for monitoring alum sludge conditioning

Wat. Res. Vol. 22, No. 1, pp. 85-90, 1988 Printed in Great Britain. All rights reserved

0043-1354/88 $3.00+ 0.00 Copyright © 1988 PergamonJournals Ltd

A NEW TECHNIQUE FOR MONITORING ALUM

SLUDGE CONDITIONING TAIWO O. KAYODE and JOHN GREGORY Department of Civil and Municipal Engineering, University College London, Gower Street, London WCIE 6BT, England (Received December 1986)

Abstract--A technique based on measurements of fluctuations in the intensity of light transmitted through a flowing sludge suspension was used to evaluate polymer performance in alum sludge conditioning. The ratio of the r.m.s, of the fluctuations to the average transmitted light intensity exhibits a well-defined response, usually a substantial increase, to sludge particle aggregation. This aggregation reflects the improvement in sludge filterability as also indicated by concurrent CST measurements. Five polymers, cationic, anionic and nonionic were used in conditioning the sludge. Tests were conducted using a mixer set at 700 rpm with a corresponding velocity gradient of 1900 s-~. The technique was shown to give a very rapid convenient indication of the conditioning effect of polymeric flocculants. It could be very easily adapted to online applications. Key words--alum, monitoring, sludge conditioning, polyelectrolytes, filterability

INTRODUCTION

BASIS OF THE TECHNIQUE

Sludge dewatering is often necessary to reduce the cost of sludge handling and transportation. It is usually preceeded by chemical conditioning to flocculate the sludge and hence enhance the dewatering process. Chemical conditioning involves coagulation-flocculation of sludge solids with hydrolysing metal salts or with synthetic organic polymers to form an open aggregated structure. In this research only synthetic organic polymeric flocculants were used with alum sludge from a waterworks. The optimum polymer dosage for an unchanging sludge can easily be determined. However changing intake conditions, as well as the intermittent use of chemicals, such as activated carbon, can significantly alter sludge properties. An online method for controlling the dosage of conditioning agents could be very valuable. The most common laboratory procedures for quantifying sludge dewaterability are capillary suction time (CST) (Baskerville and Gale, 1968) and specific resistance test (Swanwick and Davidson, 1961; Coackley and Jones, 1956). Both have a similar fundamental basis, but the advantage that CST has over specific resistance is the great reduction in time required to perform the tests. However they have a serious limitation, in that they are unsuitable for online applications. The new technique was developed by Gregory and Nelson (1984), for monitoring the state of aggregation of dispersions and can be applied to sludge conditioning. A flocculation monitor, based on the same principle, has been shown to be effective in water treatment applications (Brown et al., 1985).

The basis of the technique lies in the fundamental nonuniformity of suspensions when examined on a sufficiently small scale. When a flowing suspension is illuminated by a narrow beam of light, the number of particles in the light beam is continuously changing because of local variations in composition, these variations cause fluctuations in the intensity of the transmitted light (see Fig. 1). The light intensity is monitored by a suitable detector, the output of which is converted to a voltage proportional to the intensity. The output signal has a large d.c. component corresponding to the average transmitted light intensity and a much smaller fluctuating (a.c.) component due to random variations in particle number. The technique used is to derive the r.m.s, value of the fluctuating signal, which has been shown to be a sensitive indicator of the state of aggregation of the suspension (Gregory and Nelson, 1984). It has been shown that the r.m.s, value of the fluctuations in transmitted light intensity depends on the square root of the particle concentration, the light scattering cross-section of the particles and the dimensions of the sample volume (Gregory, 1985). For a uniform suspension, the ratio of the r.m.s, value, Vr.m.s. to the average value of the output of voltage, V, is given by:

85

Vr..... / V = ( N L / A ) ° ' s C

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to predict, not the least of the problems being the need to define the form of the aggregate. The simplest assumption is that the particles comprising the aggregates coalesce to form a sphere of the same volume. However this is an unrealistic assumption and "real" aggregates must be considered. For fairly small spherical particles, it can be shown (Lips et al., 1971) that "real" doublets scatter less light than the equivalent coalesced sphere, but the opposite effect is expected for larger particles (Latimer and Wamble, 1982). In the analyser, where aggregates flow through a capillary tube, it is probable that chain-like aggregates would tend to align with the flow and hence block more light than when randomly oriented. Even though a quantitative interpretation is not usually possible, the measured ratio value (Vr..... /V) provides a very convenient empirical index of the state of aggregation.

For nonuniform dispersions, the ratio is given by:

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where N i is the number of particles with scattering cross-section (7,. etc. The nature of the sum is such that the ratio is weighted heavily by the largest particles. When the polymer is injected and mixed into the sludge, aggregation occurs, the number of particles decreases and the scattering cross-section increases. These changes have opposing effects on the Vr...... but it has been shown that in virtually all cases of practical interest, the net effect is a substantial increase in Vr. . . . . and in the ratio value (Vr . . . . . /V) (Gregory, 1985). The interpretation of changes in the ratio output in terms of aggregation or disaggregation can, strictly, only be made if the sludge concentration remains constant. But concentration changes have a much smaller effect than changes in the state of aggregation. Equation (1) indicates that the ratio value should increase indefinitely as the square root of the particle concentration. In fact departures from the Lambert-Beer law cause the ratio value to level off at high concentrations and it may even pass through a maximum and decrease slightly (Gregory, 1985). The precise behaviour of the ratio value depends very much on the nature of the particles. Light scattering by the aggregates is very difficult

MATERIALS AND METHODS The alum sludge was obtained from the North Surrey Water Company, Staines. The source being the River Thames. It was collected via sludge concentration cones in the upflow clarifiers, usually in a 30-1. vessel. Table 1 shows the typical sludge characterization. All tests were carried out at controlled room temperature of 20°C. The polymers used (supplied by Allied Colloids Ltd) are listed in Table 2. The anionic polymers were based on hydrolysed polyacrylamide and the cationic polymers were based on copolymers of polyacrylamide and dimethyl aminoethyl acrylate (see Table 2). However there were notable differences in terms of charge density, and tool. wt. These properties are thought to be significant in determining the effectiveness of polymeric flocculants in sludge conditioning and an effective monitoring technique should be able to reflect these.

Table 1. Typical alum sludge properties Total solids (w/v)= 0.539% Specific resistance = 2.5 x 10~3m kg ~ Kinematic viscosity = 1.39 x 102 cm2s ~

Capillary suction time = 94.2s pH = 6.62 SVI = 111.3

Sludge conditioning monitor Table 2. Polymer data Intrinsic

Polymer E24

Type Anionic

Charge (%) 10

P155 PI40 P326 P351

Anionic Cationic Cationic Nonionic

30 10 30 0

viscosity 12.0 14.0 6.0 2.0 14.0

Stock solutions of 1% (w/v) were usually prepared. Dilutions were then made on a weight basis because the extremely high polymer viscosities made volumetric measurements diffficult. The polymers were dosed as 0.1 and 0.4% solutions, which were used from 2 h of preparation to a maximum of 48 h, after which fresh solution were prepared.

Equipment A variable speed stirrer with a 12-blade paddle was used for mixing. The speed was set at 700 r.p.m, with a corresponding velocity gradient of 1900s -~ as obtained from torque measurements with the aid of a strobe light. The mixing reactor was essentially a 500 mi beaker with baffles placed at the walls and an outlet at the base to enable flow-through measurements with the analyser (see Fig. 2). The specific resistance test was performed primarily to characterize the base sludge. Attempts to utilize it as a monitoring tool were deemed unsatisfactory considering that results were needed usually within 30-60 s. Whenever used, an initial time lag of 60-120 s was incorporated to nullify the effect of medium resistance. A Triton type 165 capillary suction time apparatus was used with the standard Triton CST filter papers (Triton Electronics Ltd). The 1.0 cm dia cylinder was always used with a sludge volume of 4 ml. The analyser. A photometric dispersion analyser, PDA 2000 (Rank Bros Ltd, Bottisham, Cambridge) was used for the measurements. This is based on the technique developed by Gregory and Nelson (1984). The flow cell accepts standard tubing of either 1 or 3 mm dia, transfer between these being accomplished by simply changing the spacer between the fibre optic probes. The optical fibres carry the incident light from the light source and the transmitted light

87

to the detector. The detector output goes to a pre-amplifier. The d.c. output from the detector pre-amplifier is a measure of the average transmitted light intensity. The magnitude is affected by the d.c. gain control, and the output signal may be smoothed using the filter switch. The r.m.s, output gives the r.m.s, value of the amplified a.c. signal and is affected by d.c. gain and r.m.s, gain controls. The ratio output is the ratio of the amplified r.m.s, and d.c. values, multiplied by 10. The scope output provides an a.c. waveform for display on an oscilloscope. This gives a very useful visual indication of the state of the flowing suspension. Limiter circuitry is also provided to reduce the effects of occasional nonrepresentative large particles or air bubbles. The instrument was allowed to warm up for 5-10min after switching on. Leads from the output socket were connected to a chart recorder for continuous d.c. and ratio readings. The 3 mm dia plastic tube was selected because results of preliminary investigation showed rather large aggregates were formed on injecting the flocculants. Samples for the CST were withdrawn via a tap provided just after the analyser (see Fig. 2). The sludge was recycled at a rate of 80 ml rain- I. The ratio output was used for monitoring application as it responds to changes in the state of aggregation in much the same way as the r.m.s, output. The main advantage being that it is almost unaffected by contamination of the tube walls in the flow cell or by drift in electronic components. These effects cause the same proportional changes in the d.c. and r.m.s, values, so that there is no change in the ratio output. With clean water flowing through the tube and the d.c. gain control was adjusted to give an incident voltage of 10 V, As would be expected, the r.m.s, and ratio readings were very low reflecting the virtual absence of particles. The sludge (400 ml) was then flowed through the analyser giving a d.c. reading of about 3.5 V (indicating 35% transmission). The r.m.s, gain was then adjusted to bring the ratio output to a suitable baseline ( < 1) to avoid overload conditioris as flocculation occurs. The polymer was then injected just prior to the initiation of mixing. The chart recorder provided continuous monitoring of the ratio and d.c. outputs, and hence of changes in sludge properties. CST measurements were made with samples taken from the tap at intervals of 30, 120, 300 and 600 s from start without interupting the flow,

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The monitor was used concurrently with the CST apparatus to enable a valid assessment of its effectiveness. A typical performance curve using a high mol. wt 30% anionic polymer is illustrated in Fig. 3. When the unconditioned sludge is fowed through the analyser, there is a slight increase in the ratio value [Fig. 3(a)], which might be due to a small degree of flocculation occurring in the tube, leading to the flow-cell. The filterability of the sludge rapidly worsens under continuous shearing, emphasizing the fragility of the unconditioned floc structure. With polymer doses of 5 and 10 mg 1-] [Fig. 3(b, c)], there is a substantial increase in the ratio value, during the initial 60 s, due to sludge aggregation. Although the floc structure is still fragile, it exhibits an increasing resistance to the high shear level, as indicated by the slower decrease in ratio values for Fig. 3(c) compared to Fig. 3(b). There is a substantial improvement in sludge filterability, (CST = 20 s, at t = 60s). The inherent weak floc structure is again reflected in the CST curves although in Fig. 3(c), there is a longer stable period (t = 300 s). For polymer doses of 20 mg 1-] and greater [Fig. 3(d,e,f)], the performance curves show similar characteristics-a very high ratio value (~-4.0), reflecting stable floc structure and a correspondingly low CST value (-~ 15 s). In fact Fig. 3(e, f) indicates overdose conditions, reflected in the worsening sludge filterability. It might well be that for medium stress conditioning systems, such as vacuum filtration, a polymer dose of 5 mg 1- ] would be deemed optimum (at t = 6 0 s , Gt=114,000, C S T = I 8 s ) . While for high stress conditioning systems, such as high-speed centrifugation, 20 mg 1-I could represent optimum

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polymer dose (at t = 600 s, Gt = 1,140,000, CST = 16 s). The six-fold increase over the unconditioned sludge ratio value (0.8-5.0), implies that the aggregates formed during conditioning comprise at least 50 individual sludge particles, depending on the assumed shape of the aggregates (Gregory, 1985). Gregory's treatment of particle aggregation is, of course for monodisperse spherical particles and hence can not strictly be applied, but it does provide an indication of the possible aggregate size from the rise in ratio value. The peak of each ratio curve has an optimum mixing time of around 60 s [Fig. 3(b-f)]. These peak ratio values were used with the corresponding CST measurements in subsequent analysis to enable direct comparison of individual polymer performance (Figs 4-6). The performance of the low mol. wt cationic polymer, P326, is clearly shown in Fig. 5. There is a gradual decrease in CST values with increasing poly16

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Fig. 5. Evaluation of polymer effectiveness using PDA' 2000. mer dose and a corresponding increase in ratio value. The ratio results reflect the inefficiency of this polymer in terms of its inability to coagulate the sludge, hence produce stable aggregates. The remaining four polymers exhibited a rapid decrease in CST which corresponded with increases in ratio values (Figs 4-6). The more effective polymers, such as P155 were also able to maintain a ratio value > 4.0 at a polymer dose of 20 mg 1- ~and higher (Fig. 5). This implies the formation of much bigger and stronger aggregates, capable of resisting the high shear levels for at least the duration of the experiments (Gt = 1,140,000). It must be noted that the cationic P140 produced a rather disappointing performance compared to the nonionic P351 and the anionic P155, but this can possibly be attributed to the differences in mol. wt. P140 has about a quarter the mol. wt of the others. A general survey of the performance of the polymers indicate that the high mol. wt polymers (P140, P155, P351, E24) were more effective than the low moi. wt (P326) (Figs 4-6). This underlines the importance of high mol. wt. Such polymers have very long chains which extend well into solution hence adsorb onto the particles to form aggregates by a bridging process. This bridging flocculation is of course acknowledged as one of the most important mechanisms by which polymeric flocculants operate (Ruehrwein and Ward, 1952; La Mer and Healey, 1963; Gregory, 1986). The type of charge was not as

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89

critical as might be expected, especiaUy as P351, a nonionic high mol. wt polymer produced a creditable result (Fig. 6). The reason why similar polymers with different charge densities (Fig. 5) gave comparable performance can probably be attributed to the fact that the sludge particles have very low charge. Apparently a substitution of 10% produces adequate extension of polymer chain for effective bridging flocculation to occur. This was also found applicable to cationic polymers (Kayode, 1986). It was not possible to derive quantitative information on, for instance, aggregate size distribution from the ratio values, because the light scattering properties of real aggregates are not known. More especially so as the unconditioned sludge is already flocculated, giving a range of aggregates sizes. On conditioning, the larger aggregates have a predominant influence on the ratio value, which makes the response even more sensitive than the calculations based on uniform suspensions suggest. CONCLUSIONS

The technique of utilizing fluctuating transmitted light intensities through flowing suspensions to monitor the state of aggregation as implemented in the photometric dispersion analyser proved to be an effective method of monitoring and assessing polymeric flocculants in sludge conditioning. The ratio value (Vr..... / V) which shows a marked increase as the sludge is conditioned, gives a very useful empirical indication of the state of aggregation and enables optimum dosages of the polymeric flocculants to be quickly established. The flow-through nature of the technique should make it suitable for the automated sludge conditioning test procedure, as it is applicable over wide ranges of suspension concentration and particle size. The rapid response of the monitor to changes in floc size means that a reading could be obtained only a few seconds after polymer dosing, allowing convenient feedback control of the dosing pump. REFERENCES

Baskerville R. C. and Gale R. S. (1968) A simple automatic instrument for determining the filterability of sewage sludges. Wat. Pollut. Control 67, 233-247. Brown G. M., Gregory J., Jackson P. J., Nelson D. W. and Tomlinson E. J. (1985) An on-line monitor for flocculation control. Proc. 4th 1.4 WPRC Workshop lnstrumn Control Wat. Wastewat. Treatment and Transport Systems, Houston, Tex.

Coackley P. S. and Jones B. R. S. (1956) Vacuum sludge filtration I: interpretation of results by concept of specific resistance. Sewage ind. Wastes 28, 963-976. Gregory J. (1985) Turbidity fluctuations in flowing suspensions. J. Colloid Interface ScL 105, 357-371. Gregory J. (1986) The action of polymeric flocculants. In Flocculation, Sedimentation and Consolidation (Edited by Moudgil B. and Somasundaran P.), pp. 125-137, Engineering Foundation, New York. Gregory J. and Nelson D. W. (1984) A new optical method for flocculation monitoring. In Solid-Liquid Separation

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TAIWO O. KAYODEand JOHN GREGORY

(Edited by Gregory J.), pp. 172-182. Ellis Horwood, Chichester. Kayode T. O. (1986) Polymeric flocculants in sludge conditioning. Internal Report, Department of Civil Engincering University College, London. La Mer V. K. and Healey T. W. (1963) Adsorptionflocculation reactions of macromolecules at the solid-liquid interface. Rev. pure appl. Chem. 13, 112. Latimer P. and Wamble F. (1982) Light scattering by aggregates of large colloidal particles. Appl. Optics 21, 2447-2455.

Lips A., Smart C. and Willis E. (1971) Light scattering studies on a coagulating polystyrene latex. Trans. Faraday Soc. 67, 2979-2988. Ruehrwein R. A. and Ward D. W. 0952) Mechanism of clay aggregation by polyelectrolytes. Soil Sci. 73, 485-492. Swanwick J. D. and Davidson M. F. (1961) Determination of specific resistance to filtration. Wat. Wastewat. Treatmt J. 8, 386-389.