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Sonication induced changes of particle size and theireffects on activated sludge dewaterabilityT. Hall aa Department of Biology , University of York , Heslington, York, YO1 5DD, U.K.Published online: 17 Dec 2008.

To cite this article: T. Hall (1982) Sonication induced changes of particle size and their effects on activated sludgedewaterability, Environmental Technology Letters, 3:1-11, 79-88

To link to this article: http://dx.doi.org/10.1080/09593338209384102

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Environmental Technology Letters, Vol. 3, pp. 79-88© Science and Technology Letters, 1982

SONICATION INDUCED CHANGES OFPARTICLE SIZE AND THEIR EFFECTS

ON ACTIVATED SLUDGE DEWATERABILITYT. Hall

Department of Biology. University of York,Heslington, York, YO1 5DD, U.K.

(Received 29 September 1981 ; in final form 14 January 1982)

ABSTRACT

The breakdown of activated sludge flocs by sonication i s pa r t i a l l y irreversiblein terms of the effects on Capillary Suction Time (CST) and non-settleable suspendedsolids (SS) concentrations. Sonication can therefore be used to produce activatedsludge samples which are chemically very similar but differ greatly in pa r t i c l e sizedistribution. Examples are given of the use of sonication to compare the effects ofparti c l e size reduction on t e s t s of activated sludge dewaterability.

INTRODUCTION

Sonication i s a method of breaking down activated sludge floes for analysis oftheir constituent bacteria (1), and has also been proposed as a method of studyingfloe strength and reflocculation of activated sludges-(2). Very high levels of floeshear can be obtained rapidly, resulting in corresponding increases in CST and fine,non-settleable SS. These parameters are reduced by reflocculation after sonication.However, the reflocculation i s never sufficient to return the sludge to i t s originals t a t e . Furthermore, the reflocculation r e s u l t s in constant CST values within one houraf t e r sonication (see Table 1). There i s strong evidence to suggest that sonicationdoes not significantly a l t e r the chemical nature of the sludge (see Table 2). Hencesonication followed by reflocculation to constant CST can be used to produce sludgesamples which are chemically very similar but d i f f e r in part i c l e size distribution.By altering the proportion of sonicated and non-sonicated sludge in mixtures a widerange of pa r t i c l e sizes can be obtained. Such mixture can be subjected to dewateringt e s t s to compare the effects of pa r t i c l e size reduction on these t e s t s .

METHODS AND MATERIALS

Sludges were obtained from a 1.5m p i l o t plant operated a t the University ofYork t r e a t i n g campus domestic sewage, and from f u l l scale plants operated by Yorkshireand North-West Water Authorities, U.K.

Sonications were carried out using a Dawe model 7532A Soniprobe and the proce-dures described by Hall (2). Sonication times of up to 30 sec. were used as required,and the temperature r i s e in the samples during sonication was never more than 2°C.CST determinations were made using the 10 mm rese r v o i r and the same batch of CSTpapers throughout ( 2 ) . Apparent specific r e s i s t a n c e to f i l t r a t i o n was also measuredat 49 K Pa using Whatman No. 17 chromatography paper (4, 5). A constant level ofvacuum was obtained by means of a Millipore adjustable vacuum pump. Sludge samplesof 200 ml. were used in the t e s t s . The sludge used had a low SS concentration, and sothe apparent s p e c i f i c r e s i s t a n c e should not d i f f e r g reatly from the real averagevalue ( 4).

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Thickening properties were investigated using the Water Research Centre (WRC)designed low-speed centrifuge fitted with 14 mm i.d. tubes (3). The majority oftests were carried out at a centrifuge speed of 400 rev. min"l which produced anacceleration of 21 g at the mid-point between the top and bottom of the tubes. Thetests gave values for the mean ultimate concentrations achieved (calculated from thethickened sludge volume) and the thickening time (C« and a respectively). Thethickening time i s a coefficient which defines the shape of the thickening curve(mean SS concentration vs. time) and is roughly equal to the time required to reacha mean concentration half-way between the i n i t i a l and ultimate concentrations (3).The concentrations of supernatant SS in the tubes were never high enough to signi-ficantly influence the calculation of the mean thickened sludge concentrations.

Suspended solids values were obtained by fi l t e r i n g known volumes of samplethrough pre-washed and weighed Whatman GF/A f i l t e r s and drying overnight at 1O5°C.Concentrations of non-settleable solids were determined by settling 100 ml. of samplein a 100 ml. measuring cylinder and removing as much of the supernatant as possibleafter 30 min.

Polyelectrolyte dosing experiments were carried out by adding the required con-centrations of polyelectrolyte solution (5 ml.) to 100 ml. samples of sludge in250 ml. beakeTS being stirred at maximum speed (approximately 1,500 rev. min"l usinga Gallenkamp magnetic s t i r r e r with a 4.5 cm stirrer-bar. The high speed stirring wascontinued for 15 sec. and followed by low speed stirring (500 rev. min"l) for afurther 15 sec. The dosed samples were then allowed to stand for 1 min beforestarting CST tes t s . Blanks containing distilled water instead of polyelectrolytesolutions were given similar treatments. The polyelectrolytes examined were a l lcationic polyacrylamides supplied by Allied Colloids. Their characteristics wereas follows :

Zetag 51; low molecular weight, very high charge density.Zetag 57; high molecular weight, very high charge density.Zetag 63; high molecular weight, high charge density.

The results of the polyelectrolyte experiments were plotted as CST against polymerdose from which the dose required for any particular CST reduction could be obtained.The polyelectrolyte dose was expressed as percentage dry solids (%.D.S.) which canbe defined by:

%.D.S. = wt. of active polyelectrolyte added x 100%

wt. of sludge dry solids in sample

Mixing before CST, specific resistance and centrifuge tests was carried out bypouring the sample twice from one 250 ml. beaker to another and back again. In thisway a reproducible mixing was obtained which was efficient but avoided excessiveshearing forces.

The sonicated mixtures described were prepared by adding the required quantityof sonicated sludge to non-sonicated sludge immediately after sonication. Suchmixtures were allowed to reflocculate to constant CST before dewatering tests werecarried out.

Electrophoretic mobilities were measured using a Rank Mk.II particle micro-electrophoresis apparatus. Observations (at 25°C using both stationary levels in afl a t cell) were made on settled sludge supernatant particles, before and after soni-cation. Electrophoretic mobilities were calculated from the mean particle velocitiesand applied voltage (80V).

RESULTS

Table 1 shows the reflocculation of a 50% sonicated mixture to a constant levelof CST within a period of 20 min. after sonication and mixing.

Table 2 gives electrophoretic mobilities before and after sonication, indicatingthat sonication does not change the surface charge of the sludge supernatant particles.

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100

10

• o 40% sonicated• D non-sonicated— CST— CSTC

0 4000 8000 12000 16000

SS (mgr1 )

Fig. 1: SS concentration dependence of CST and CST .

Time after sonication (min.) 5 9 20 50 60 90

65.2 52.2 39.7 37.4 39.8 37.0CST (sec.)

(SS = 3782 mg 1 , non-sonicated CST = 13.7 s e c , sonication time = 30. sec.)

Table 1: reflocculation of 50% sonicated mixture to constant CST.

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1015.

en

enE

aiLJcÖ

—I/)a)

to

10 u

101 3

caic_Oa.OL

• CST° Specific Resistance• Time to Filter 50 ml.

1000

100

nCO

3re

to

10 -re

Jo0 100 200 300 400

Supernatant SS ( mg I"1 ) af ter 30min. settling

F i g . 2 ; : , « , r . ,-n filtration proporfi>s with increasing concentrations of fine' particles.

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Time sonicated(sec.)

0

10

30

Electrophoretic mobility(ym sec."-1- volt"! cm)

1.63 - 0.07

1.64 - 0.08

1.58 - 0.05

Supernatant SS(mg 1-1)

54

. 340

540

(Sludge SS = 2236 mg. l " )

Table 2: effect of sonication on electrophoretic mobility and supernatant SS.

32000

_ 24000

E

~ 16000co

gz:

8000

/ p• o/ /• o

//..-'

Ji/

,

• non-sonicatedo 3 3 % sonicated• 66% sonicated

4 8 12 16

Time ( min ) at 21g

20

Fig. 3: changes in rate of thickening with increasing proportions of sonicatedsludge.

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The sample pH was 7.40 before and after sonication. The current passing in theelectrophoresis cell did not change on sonication suggesting no significant changein ionic strength to be occurring.

To test the effect of sonicate addition on the suspended solids dependence ofCST a sample of sludge was thickened to approximately 15,000 mg. I"*- by settlingand diluted with supernatant to give a range of SS concentrations for which CSTvalues were obtained. A further sample of thickened sludge was sonicated for 30 sec.and mixed in the proportion 4:6 with non-sonicated thickened sludge (i.e. a 40%sonicated mixture). This mixed sample was allowed to reflocculate to constant CSTand was diluted with supernatant to give a range of concentrations for CST deter-mination. Each concentration should therefore have contained a similar particlesize distribution relative to the total suspended solids present. The results areplotted on Fig. 1 as log. CST against SS or log. CSTC against SS, where:

CSTC = CST (sample) - CST (water)

This correction is recommended by Kavanagh (5) to allow for the effects of CSTpaper resistance at low sludge CST values.

A comparison of the effects of sonicate addition on the CST and specificresistance tests is shown in Fig. 2. A range of samples with increasing proportionsof sonicate (0% to 80%) was prepared and each sample was allowed to reflocculate toconstant CST. The sludge used had a SS concentration of 2,850 mg. 1"1. Afterreflocculation the CST, specific resistance to filtration and concentration of non-settleable SS were measured for each sample. The results are plotted on Fig. 2 asapparent specific resistance, CST and time to obtain 50 ml. filtrate in the speci-fic resistance test against concentration of non-settleable SS. An increasing con-centration of fine particles appears to influence the tests in a different manner.In this case the CST values have not been corrected for the CST of water, but sucha correction would not have made a great difference to the plot, particularly in theregion where the results begin to show divergence.

Sonicate addition was found to reduce the rate of thickening obtained in thelow speed centrifuge test (Fig. 3). However, the ultimate concentration achievedwas not altered at a range of centrifugal accelerations of 21 g to 223 g. Similarresults were obtained for two other sludges. CST values for the sludges subjectedto centrifuge thickening with varying sonicate addition are plotted against thicken-ing time (a) on Fig. 4. There appears to be a very good linear correlation betweenIn. CST and thickening time for the three sludges used both before and after.soni-cate addition.

The effect of sonicate addition on polyelectrolyte conditioner demand was inves-tigated by determining the amount of polyelectrolyte required to reduce the CST ofreflocculated sonicated mixtures to 10 sec. The results are shown in Fig. 5 as theunconditioned CST (i.e. that of the blank with distilled water instead of polyelec-trolyte solution) against dose of Zetag 63 required. Good linear correlations canbe seen to exist for two sludges from different plants. The increases in demand forother polyelectrolytes after addition of sonicate are shown in Table 3. The sludgeused was Sludge A shown in Fig. 5. It can be seen from Table 3 that the increase indemand on sonicate addition is greatest for the lower molecular weight polyelectrolyte

Table 3: the effects of sonicate addition on dose requirements for different poly-electrolytes.

PolyelectrolyteType

Zetag 51Zetag 57Zetag 63

Dose (%.D.S.) toNon-sonicated

0.220.050.22

reduce CST to50%

10 sec.sonicated

1.000.550.60

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2000

1000

ucu(S)

100CO

10

in.CST = 0-46OC + 2 17r = 0-98

• 8940 mg l"1

o 11000 mg I"1

• 9700 mg l"1

2 4 6 8 10Thickening Time ( min ) at 21 g

12

Fig. 4: relationship between CST and a.

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1000

- ocuCO

| 10

r = 0-95

r = 1-00

• sludge Ao sludge A - sonicate added. sludge Bo sludge B - sonicate added

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DISCUSSION

It is likely that any significant chemical changes which occur on sonicationwould influence sludge surface charge, sample pH or total ionic strength of thesludge suspending medium (related to conductivity and therefore proportional to thecurrent passing in the electrophoresis cell under conditions of constant temperatureand applied voltage). As these parameters were not altered by sonication i t seemsreasonable to assume that no major change in the sludge chemical nature occurs-. Therelative effects of increasing concentrations of fine particles on the CST andspecific resistance to filtration tests can best be seen by comparing the CST andthe time to obtain 50 ml. f i l t r a t e in the specific resistance test. Both para-meters represent a time taken to remove a fixed (but different) volume of f i l t r a t efrom the sludge. The times are very similar i n i t i a l l y , but the time to f i l t e r 50 ml.increases much more rapidly than the CST with increasing fine particle concentration.This may result from the difference in filtration pressure. Also, the mobility ofthe fine particles is thought to be greater in the specific resistance test becauseof the higher f i l t r a t e flows (6). This leads to greater "blinding" of the cake andfi l t e r medium in the specific resistance test and could account for the resultsobtained. According to Kavanagh (5) plots of log. (specific resistance x weight ofsolids deposited per unit volume of fi l t r a t e ) against log. CSTC should be linearwith slope unity. For a single SS concentration with varying particle size d i s t r i -butions a plot of log. (specific resistance) against log. CSTC should be linear ifthe tests respond similarly to changes in particle size. This is obviously not thecase for the data shown on Fig. 2, probably as a result of differences in cakesolids concentrations resulting from differences in particle mobility during thetests. In situations where high and variable levels of floe shear occur, care shouldtherefore be taken when predictions of specific resistance are to be made from CSTdata.

The slope of the plot of log. CST against SS concentration appears to be relatedto the particle size, with sonicate addition increasing the slope significantly(Fig. 1), i.e. smaller particles having a greater effect on CST at higher SS concen-trations. This probably results from the channels and voids in the sludge which formduring the CST test being smaller at higher SS concentrations and therefore moresusceptible to blockage by fines.

The high correlation between log. CST and a shown on Fig. 4 suggests thatsimilar mechanisms may control both tests. Both parameters plotted are concentrationdependent, although the range of SS concentrations used was not great. The resultsobtained are of practical significance, as i t may be possible to estimate thickeningrates from CST in cases where centrifuge results are not available.

I t is evident from Fig. 5 that for chemically similar sludges the polyelectro-lyte requirement (using Zetag 63) for a reduction in CST to 10 sec. is linearlycorrelated with the log of the in i t i a l untreated CST. This suggests that both para-meteres are related to particle size distribution in a similar fashion. The factthat the low molecular weight polymer-demand shows the greatest increase after soni-cate addition suggests that in cases where high levels of floe shear occur thegreatest cost-effectiveness might be obtained using higher molecular weight poly-electrolytes.

No attempts were made in this study to measure particle size ranges in thesludge samples used, as the primary aim of the work was to compare the effects ofsevere floe shear on dewatering tests. However, sonication provides a nachod ofobtaining sludge samples with widely varying particle size and similar chemicalproperties. Particle size analysis on sonicated mixtures could be used to quantifythe effects of particle size distribution on activated sludge dewaterability.

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REFERENCES

1. C.J. Banks and I . Walker, J. Gen. Microbiol., 98, 363-368, (1977).2. T. Hal l , Environ. Technol. L e t t s . , (1981) ( i n p r e s s ) .3. C.F. Lockyear and M.J.D. White, Water Research Centre, Technical Report TR118,

(1979).4. R.S. Gale, Water P o l l u t . Contr., 66, ( 6 ) , 622-632, (1967).5. B.V. Kavanagh, Water P o l l u t . Contr., 79, ( 3 ) , 388-398, (1980).6. P.R. Karr and T.M. Keinath, F i l t r . Separ., 15, ( 6 ) , 543-544, (1978).

ACKNOWLEDGEMENTS

Thanks to Mrs. D. Pa t t e r s o n and Mr. C. Hastwell f o r h e l p with p r e p a r a t i o n ofthe manuscript. Thanks a l s o t o Dr. M. Davies (York U n i v e r s i t y ) , and Mr. A. Bruceand Dr. C.F. Lockyear (WRC, Stevenage) for h e l p f u l d i s c u s s i o n . The work was fundedby WRC, Stevenage.

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