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Wat. Sci. Tech. Vol. 36, No. II,pp. 107-114, 1997.C 1997 IAWQ. Published by Elsevier Science Lid

Printed in Oreat Britain.0273-1223/97 $17'00 + 0'00

ACTIVATED SLUDGE FLOC STRUCTUREDURING AEROBIC DIGESTION

K. Barbusinski and H. Koscielniak

Department ofEnvironmental Engineering, Silesian Technical University,44-/01 Gliwice, Poland

ABSTRACT

Additional observations of the physical characteristics have been introduced to current monitoring of aerobicdigestion of sludges coming from two wastewater treatment plants. New parameters have includeddetermination of floc size and sludge specific surface. The mternal floc structure was also examined with theuse of microtome technique for slicing flocs into very thin sections. During aeration, considerable changes infloc dispersion occurred. Analysis of floc size distribution enables observation of regroupment of differentsludge fractions and successive increase in frequency occurrence of the finest fractions, which usually hinderdewatering process. The decrease of average floc size has been accompanied by the increase in sludgespecific surface. The time needed for "biologIcal stabilization" has not occurred simultaneously with the timewhen physical parameters have been shaped in the best possible manner for funher sludge treatment. Thejoint analysis of biochemical and physical propenies including above mentioned parameters has enabled theassignment of aerobic digestion tIme more precisely. It has special meaning in pre-designing research as wellas during the start·up of the aerobic digestion systems. © 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS

Activated sludge properties; aerobic digestion; floc size; floc structure; microtome slicing; specific surface.

INTRODUCTION

The most widely spread methods of sludge digestion are biological processes consisting of degradation oforganic matter included in excess sludges, by microorganisms in anaerobic or aerobic conditions. The choiceof biological sludge digestion method depends on the wastewater treatment plant dimension and costs of theprocess. Due to the low capital costs and high energy-consumption, it can be presumed that the aerobicdigestion is more beneficial for small wastewater treatment plants. In practice, this process is also often usedin medium-size treatment plants. The advantage of aerobic digestion is the fact that low content of organicpollutions is observed in the supernatant phase, and the supernatant liquids which tum back to the treatmentsystem have not any influence on the wastewater treatment process.

The final result of the aerobic digestion process should be obtaining of mineralized, well-settling, easilythickening and dewatering sludge. Meanwhile in many treatment plants, occur problems connected withmechanical treatment of aerobic stabilized sludge. The reason can be found in unsuitable estimation of theoptimum sludge retention time in a digester. In spite of many research works conducted, no uniformparameter has been found to determine univocally degree of stabilization and digestion time. In practice, so•called technical digestion time is defined on the basis of estimation of many often various parameters, Total

107

108 K. BARBUSINSKI and H. KOSCIELNIAK

suspended solids (TSS), volatile suspended solids (VSS) and at times biodegradable volatile suspendedsolids (BVSS) are usually used for this purpose (Benedek et al., 1972; Adams et al., 1974; D'Antonio 1983).Recently, the measurements. of the sludge activity (Matsch et aI., 1977; Ganczarczyk et aI., 1980; Droste etaI., 1983) as well as the redox potential (Peddie et al., 1990; Wareham et al., 1994) have been popularmethods for optimization of aerobic digestion.

During the aerobic digestion considerable changes occur also in the structure and these physical features offlocs which are directly connected with the settling properties and dewatering capacity. Aoe size andparticle size distribution are considered two of the most important factors in the dewatering of sludge (Karret al., 1978; OlbOter et al., 1993). It is well known that especially the amount of small particles in a sludge isresponsible for poor dewatering. Lawler (1986) concluded that the dewaterability decreases with decreasingparticle size, measured as specific surface area. Also Novak et aI. (1988) showed that small particles tend toblind the sludge cake during filtration.

In this research, the authors have introduced observations of the physical characteristics to currentmonitoring of aerobic stabilization of sludges coming from two different wastewater treatment plants. Thenew parameters included determination of floc size and sludge specific surface. The internal floe structurewas also examined with the use of microtome technique for slicing floes into very thin sections.

METHODS

The aerobic digestion was performed in two series. Waste activated sludge was obtained from two full-scalewastewater treatment plants. First of these is a municipal plant in the vicinity of Katowice, while second oneis an activated sludge plant treating industrial sewage from a chemical works located in the district of Opole.The collected slUdges were thickened to obtain initial total solids concentration of 8500 mgl- I and werebrought to laboratory batch reactor which had initial sludge volume of 11.5 I. In order to aerate and mix thecontent, air was being introduced in the bottom part of reactor through a porous stone diffuser. The airsupply was regulated so that the dissolved oxygen concentration in the reactor was always >2 mg/0t!, andaeration was sufficient to keep the solids in suspension. Evaporation losses were made up each day withdistilled water prior to sampling. The aerobic digestion was continued for 40 days.

The measurements of soluble chemical oxygen demand (COD), total and volatile suspended solids (TSS,VSS), dissolved oxygen (DO), sludge volume index (SVI) and settleability as well as pH were performed tomonitor the progress of aerobic digestion process. Determination of the microbial actiVity comprisedmeasurements of oxygen uptake rate (OUR) and dehydrogenase activity (DHA). All above analyticalprocedures (except DHA) were measured in accordance with Standard Methods (APHA, 1992). UndilutedSVI detenrunations were performed in unstirred I litre graduated cylinder (6.8 em in diameter). The DHAwas determined with the help of TIC test according to Miksch (1985).

Additional experimental procedures employed in this work comprised analysis of physical properties of theactivated sludge, such as average size of floes and their size distribution, sludge specific surface as well asinternal floc structure. In order to determine the average floe size (AFS), direct, microscopic examinationswere performed using an eyepiece micrometer. This technique has been described in details by Barbusinskiet al. (1995). The specific surface area of flocs was determined by a dye adsorption technique. Thenaphthalene based dye, known as Lissamine Scarlet 4R, was used according to Smith el aI. (1983). Theprocedure for preparation of the activated sludge floes to analyze their internal structure comprised physicalstabilization, embedding in paraffin and then slicing them with a microtome into sections 7 m thick,according to the method described by Barbusinski et al. (1997). This method was similar to that describedby Ganczarczyk et al. (1992). Embedding in paraffin was carried out in order to retain the fragile structureof floes during microtome sectioning. The microtome sections were stained with hematoxilyn eosin and thenexamined by light microscopy.

ActivalCd sludge floc structure

RESULTS AND DISCUSSION

109

Because of observed close similarities for both types of sludge as well as by reason of limited volume of thispaper the presented detailCd results of aerobic digestion refer to one panicular sludge only. During theaerobic digestion of waste activated sludge collected from the industrial wastewater treatment plant changesof TSS and VSS stabilized finally after 22 days of aeration (Fig. 1). At that moment the decrease in TSSvalues reached 41.2% of its primary value and VSS content within the sludge came down from 71.2 to42.4%.

I~TS8 ~V88 -coDI

.,1 11100

Qo II,' 1310

~ Qo

~1,1 1100 t....

1 3,4 110 0

8! 1,7 1100

o 310o 2 4 8 12 18 20 24 28 33 40

Digestion time (days)

Fiaure 1. Changes in TSS, VSS and soluble COD during aerobic digestion.

The decline in OUR and DHA activity occurred slightly earlier - after 20 and 18 days respectively (Fig. 2).Considering the above results total digestion time needed for "biological stabilization" should be regarded as20-22 days' period. Since then DO more biochemical activity of the sludge was recorded and the rate ofdecrease in TSS and VSS concentration dropped to minimum.

28

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~ ...o

,,.,

~J~U·.U

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Figure 2. Chanles in OUR and DHA IICtivilY during aerobic digestion.

The sludge, however, was characterized by high values of SVI and by weak settleability (Figs 3 and 4).Moreover, some COD fluctuations were observed whal indicates sorption and desorption processes of thebiomass destruction products (Fig. 1). Therefore, for the research purposes, funher digestion process wascontinued.

110 K. BARBUSINSKI and H. KOSCIELNIAK

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250 ,-----------------,. 200

200 HIO

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l ~100 80

50 40

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Figure 3. Changes in SVt and average nee size during aerobic digeslion.

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Figure 4. Varialion of sellling characleristics during aerobic digeslion.

Additional observations of physical characteristics of the sludge during aerobic digestion enableddetennination of changes in the floc structure and also helped to evaluate the digestion time more precisely.1be initial digestion period was characterized by considerable reduction of the floe size and the sludgespecific surface (Figs 3 and S). During first 4 days of the performance average diameter of flocs decreasedsignificantly. approximately to half of its primary value (from 12S to 6S 11m) and next increased to 90-98 J.lm(day 6-8). From that time. successive reduction of the floc size occurred reaching values of S6 11m on 22ndday and 22 11m on 40th day of aerobic digestion. In spite of strong floe dispersion during the initial period.sludge specific surface decrea<;ed - most likely due to changes in internal structure and porosity of the floesas a response to the changes of environmental conditions. This period was followed by significant growth ofthe floc specific surface up to 128 - 135 m2g- l .

Activated sludge floe structure

1<40 r-------------;;;~ 5

...... 125 +--------~""nrCF_~----l~4,5Ct

~ 110 4 "';'"-- 95 3.5--

I : :.. j1: ~.5 w

20 1o 2 4 8 12 18 20 24 28 33 40

Digestion time (days)

Figure S. Variations of sludge specific surface and elongation of floes during aerobic digestion.

III

With developing digestion process the amount of fine sized floes was growing. This phenomenon is depictedby floe size distributions made at the beginning of the process as well as after 24 and 40 days of performance(Fig. 6). Aerobic digestion caused gradual increase in the amount of the finest range floes and also of floesbelow the range of measurement. Prior to digestion process, floes between 75 and 100~m prevailed, after 24days the dominating floe size was 15-35 ~m and after 40 days the most common floc size was I0-15 ~m.

Analysis of the floc size distributions make possible observation regroupment of dominating floe sizestowards finest fractions which usually hinder the efficiency of dewatering process. The floc size distributionon the 40th day responds to the floe breakup (pin-point floc) showing dramatically low SVI values andenhanced turbidity of supernatant liquid. Continuing floc dispersion was accompanied by changes theirelongation coefficient (floc length to breadth ratio). This coefficient showed increasing values up to 20th dayof digestion. Afterwards the floes became more spherical in shape (Fig. 5).

Periodic examination of internal floe structure made with the use of microscopic analysis of microtomesections revealed weaker cohesion and bigger internal water gaps observed during initial period of digestionprocess. However, developing dispersion of f10cs with the time flow made the observations difficult tocontinue as this method is reponed to be well applicable for analysing larger sludge f10cs (Li et al., 1990;Barbusinski et al., 1997).

During the proeess of aerobic digestion significant changes of SVI and the slUdge settleability oecurred (Figs3 and 4). SVI values dropped primarily due to the reduction of floe size and then grew significantly reachingtheir maximum on the 22nd day (the end of "biological stabilization" period). On the 24 day SVI valuesdropped suddenly to 80 mIg-I. At that time the sludge settleability and the thickening characteristicssignificantly improved. It was characteristic that. beginning with 2Q-22nd day of the proeess. average floesize as well as TSS and VSS values remained almost stable. Specific surface values of the sludge showedsimilar tendency. The ascenained changes in physical parameters resulted in permanent improvement insludge settleability and SVI. "Biological stabilization" period did not occur simultaneously with the timewhen physical parameters of sludge are optimum for funher treatment performance. Thus, with respect toappropriate settling propenies and dewatering effectiveness, the advantageous digestion time should be 24days. On the other hand, too long digestion time (longer than 28 days) caused rapid growth of finest sludgefractions which hinder seriously filtration and dewatering of sludges (Karr et al., 1978; Novak et al., 1988;0IMteretal.• 1993).

112 K BARBUSINSKI and H. KOSCIELNIAK

60

50

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10

10 15 35 50 75 100 125 150 175 200 225 250 275 300

60

50

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60

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ii 40

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Figure 6. Influence of aerobic digestion time on noc sile distribution; lOp· at the beginning. middle - day 24.bonom - day 40.

Aerobic digestion of waste activated sludge obtained from the municipal wastewater treatment plantproceeded slightly faster. however the shift between "biological stabilization" period (13 days) and the timewhen physical parameters were optimum for further treatment (16 days). was also observed. The differencebetween adequate digestion times recorded for both industrial and municipal sludges may have resulted fromthe fact that the industrial plant received the wastewater highly resistant to biological degradation.Moreover. the initial VSS content within the "industrial" sludge amounted 71.2% and within the "municipal"

Activated sludge floc structure 113

sludge 66.8%. The "municipal" sludge showed similar relations between particular indices of digestionprocess as it was in the case of "industrial" sludge however the floc breakup within the "municipal" sludgeoccurred earlier - on 30th d~y of digestion.

Joint analysis of biochemical and physical parameters of sludge stabilization degree enables more precisedetermination of aerobic digestion time which is of significant importance for the research precedingengineering design or accompanying technological tnals on wastewater sludge treatment installations. Itshould be noted that although batch aerobic digestion results cannot be directly applied for semi-continuousor continuous process conditions (Ganczarczyk et al., 1980) however, the authors suggest that analysis ofboth biochemical and physical parameters for appropriate digestion process monitoring can be useful inevery conditions.

CONCLUSIONS

During aerobic digestion of the sludges significant changes in floc structure occurred. The size, shape anddispersion degree of the flocs as well as sludge specific surface have changed, that is, these physical featureswhich are directly connected with the settling properties and dewatering efficiency of the sludge.

The period characterized by optimum values of physical parameters for further sludges treatmentperformance. was longer than the period needed for "biological stabilization" in case of both "industrial" (2days' shift) and "municipal" (3 days' shift) type of sludge.

Extension of aerobic digestion performance to the time when physical parameters of sludge were optimumfor further treatment, resulted in rapid and permanent decrease in SVI values and significant improvement ofsettleability and thickening efficiency of the sludges.

Investigations on physical properties of the sludges such as floc dispersion degree, sludge specific surfaceand the presence of undesired finest floc fractions enable optimizing of the aerobic digestion process.Therefore, such investigations may be useful for the research preceding engineering design, formathematical modelling of the process and during technological trials on sludge treatment installations.

REFERENCES

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Washington, D. C.Barbusinslci, K. and Koscielniak, H. (1995). Influence of substrate loading intensity on floc size in activated sludge process. Wat.

Res.,29,1703-171O.Barbusinslci, K. and Koscielniak, H. (1997). Preparation of activated sludge floes to study theIr internal structure (in Polish).

Biotechn%gia 1 (36), 37-44.Benedek, P., Farkas, P. and Literary. P. (1972). Kinetics of aerobic sludge stabilization. Wal. Res.• 6, 91-97.D'Antonio, G. (1983). Aerobic digestion of thickened activated sludge. Wal. Res., 17, 1525-1531.Droste. R. L. and Sanchez, W. A. (1983). MicrobIal activity in aerobic sludge digestion. Wal. Res.• 17,975-983.Ganczarczyk. J., Hamoda, M. F. and Hong-Lit Wong (1980). Performance of aerobIC digestion at different sludge solid levels and

operation pauerns. Wal. Res.• 14, 627-633.Ganczarczyk, J. J., Zahid, W. M. and Li, D. H. (1992). Physical stabilization and embedding of microbial aggregates for light

microscopy sludies. Wat. Res., 26. 1695-1699.Karr. P. R. and Keinath. T. M. (1978). Influence of panicle size on sludge dewalerability. J. Wal. Pol/ut. Control F~d., 50,1911-

1930.Lawler. D. F. (1986). Removing particles in water and wastewater. Envir. Sci. Tuhnol., 20. 856-861.Li. D. H. and Ganczarczyk, J. J. (1990). Structure of activated sludge floes. Biot~chnol. Bio~ng.. 35, 57·65.Matsch, L. C. and Drnevich. R. F. (1977). Autothermal digestion. J. Wal. Pollul. Conrrol Fed., 49. 296-310.Miksch, K. (1985). The choice of the optimal methodology for the determining the activity of activated sludge by means of TTC•

test (in German). Yom Wasser, 64. 187-198.Novak. J. T .• Goodman, G. L., Pariroo. A. and Huang, loCh. (1988). The blinding of sludges during IiItrallon. J. Wal. Pollul.

Conlro/ F~d.. 60. 206-214.

114 K. BARBUSINSKI and H. KOSCIELNIAK

OlbOter, L. and Vogelpohl. A. (1993). Influence of particle size distribution on the dewatering of organic sludges. War. Sci. Tuh.,28(1),149-157.

Peddie. C. C., Mavlnic. D. S. and Jenkins. C. J. (1990). Use of ORP for monitoring and control of aerobic sludge digestion. J.mv;r Ellgllg. Am. Soc. civ. Ellgrs. 116,461-471.

Smith, P. G. and Coackley, P. (1983). Method for detennining specific surface area of activated sludge by dye adsorption. War.Rn, 17,595-598.

Wareham. D. G., Mavimc, D. S. and Hall. K. J. (1994). Sludge digestion using ORP-regulated aerobic-anoxic cycles. War. Rts.,28. 373-384.