Technological characterization of sewage sludge

Waste Management & Research (1985) 3, 389-398

TECHNOLOGICAL CHARACTERIZATION OF SEWAGE SLUDGE L . Spinosa (Received 14 December 1984)

Laboratory tests for the technological characterization of sewage sludge are catalogued and reviewed . Procedures presently being tested and evaluated by IRSA (Water Research Institute, C .N .R ., Italy) are also discussed. Major attention is addressed to the evaluation of sludge dewaterability, since this operation is both expensive and widely utilized in sludge management . In particular, a new procedure for estimating the full-scale filterpress cake concentration and the floc-strength method for evaluating sludge centrifugability are described . Further studies are, however, required for a better correlation with industrial-scale operations . Other methodologies concern thickening, stabilization, final treatment and disposal : acceptable and simple tests are generally not available and in many cases it is difficult to interpret the data that are generated . The discussion gives evidence of the laboratory tests now used for sludge characterization being specific to the method of treatment ; tests yielding basic information about sludge are thus needed . Key Words-Sewage sludge, technological characterization, laboratory tests, conditioning procedure, evaluation of sludge processability . 1 . Introduction Wastewater treatment almost always leads to the production of sludges whose treatment and disposal accounts for about 50% of the total wastewater treatment cost . The selection of the best process sequence for sludge treatment requires preliminary investigations in order to define the sludge characteristics . Unfortunately, laboratory tests alone are not always capable of providing a description of the behaviour of sludges in industrial scale processes (Aveni et al. 1975), and the results obtained in different laboratories are seldom comparable because the tests are conducted differently in individual laboratories . The sequence of sludge treatment processes is reported in Fig . 1 ; the processes can be subdivided into the following four groups (Haugan & Mininni 1981) : thickening, stabilization, dewatering and final treatment and disposal . Each group includes general characterization parameters (pH, alkalinity, total, suspended and volatile solids, sludge and dry residue density, particle size distribution, etc .) and others which can be defined as specific to the treatment process . A vast standardization activity is currently in progress in Europe : the E .E .C . has initiated, within the scope of the Concerted Action on Sludges (COST 68), a joint project aimed at the eventual standardization of sludge testing methodologies . The objective of this review is to catalogue some of the tests for the technological characterization of sludges . Laboratory procedures presently being tested and evaluated by IRSA (Water Research Institute, C .N .R ., Italy) are also discussed . C .N .R . Istituto di Ricerca Sulle Acque, via F. De Blasio 5, 70123 Bari, Italy . 0734-242X185/040389 + 10 $03 .00/0

© 1985 ISWA



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PROCESS STAGE

PRELIMINARY TREATMENT

UNTREATED SLUDGE Surplus activated Humus

Primary Mixed

Chemical

Conditioning

Degritting

I PRIMARY THICKENING

Centrifuge

Gravity

Flotation

1

LIQUID STABILIZATION

Anaerobic Digestion Cold

Aerobic Digestion

Hot

Cold

Chemical

Hol

I SECONDARY THICKENING

Centrifuge

Gravity

Elutrialion

Chemical

Bag

Bell

Plate

Thermal

Vacuum Centrifuge

Bed

Lagoon

I FINAL TREATMENT

Incineration

Drying

STORAGE

Ash

Compost

Cake

TRANSPORTATION

FINAL SITE

Road

Landfill

Retail

Composting Chemical

Tank

Road

Sea

Pipeline

Agriculture

Fig . 1 . Sludge processing options (Haugan and Mininni 1981) .

Sea



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2. Thickening Thickening is defined as the capacity of sludge to increase in solid concentration by gravitational or centrifugal acceleration . Sludge thickenability is generally evaluated in laboratory by allowing it to settle in a graduate cylinder . However, sludge thickening behaviour thus measured can be affected by equipment geometry and operative conditions used (Aveni et al. 1975). The diameter of the settling column, for example, can have a marked effect on the settling velocity . Many techniques have been proposed to overcome these problems . A settling test published in Standard Methods for the Examination of Water and Wastewater (APHAAWWA-WPCF 1980) incorporates a 100 mm diameter column, at least 1000 mm high, with a slowly rotating stirrer used to reduce bridging . A commercially available column (100 mm diameter, 500 mm height) also has a low-speed stirrer (White 1975) . Another technique for estimating the thickening behaviour is based on the use of a low-speed stroboscopic centrifuge which allows the thickening performance, regarding both solids flow and final concentration, to be estimated ; the test is short and requires only a small amount of sludge (Lockyear & White 1979) . Although not originally intended for evaluating sludge thickening, it seems a reasonable laboratory test for that purpose. 3. Stability Possibly the most difficult sludge characteristic to measure is sludge stability, which can be defined as the set of characteristics making the disposal of sludge acceptable . A stable sludge is, therefore, one which can be used or disposed of without undue damage to the environment or to public health . For example, a stable sludge for disposal on farm land would have an acceptable odour and an acceptably low level of toxics and pathogens . Since the definition of stability is linked to a specific location and disposal method, and is thus difficult to standardize, stability is generally associated with sludge putrescibility or the tendency of organic matter to biodegrade . Thus, measurement of stability normally involves the measurements of either the organic substrate concentration, such as volatile suspended solids, COD, BOD S and organic carbon, or the organism concentration and microbial activity, such as oxygen demand, ATP and enzymatic activity . Unfortunately, none of these parameters by themselves seems to be sufficient ; Colin (1979a) proposed the adoption of a "Biological Evolution Index" which incorporates ATP, BOD S , volatile solids and enzymatic activity into one equation . However, the determination of this index involves the use of methodologies that are not easy to apply for continuous and rapid monitoring as well as of equipment that is not readily available . Probably, the most useful measure of sludge stability is its odour, although its measurement is notoriously difficult . Eikum (1984) has used the "Threshold Odour Value", but this technique requires a panel of people detecting progressive dilutions of the odour and is cumbersome, expensive and cannot be conducted in the field . A better technique may be to utilize a dynamic olfactometer which can be calibrated to individual odour sensitivity (Vesilind 1983) . 4. Dewatering The evaluation of sludge dewaterability is of great importance, as this operation is both



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I J 1000 ± 10 rpm

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0 .6 mm 0 .69 mm E E V

E E Ln N

E E

V

Ip

////

~

15+0 .6

T

0 .70±0 .5mm

r

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Fig . 2 . Standard stirrer .

expensive and widely utilized in sludge management . Dewatering can be accomplished by filtration or centrifugation, usually preceded by chemical conditioning in order to enhance the dewatering characteristics of sludge . The laboratory determinations are intended to (1) select the type of conditioner and its dosage and (2) estimate the efficiency of dewatering which would be attained by full-scale equipment . Due to the importance of chemical conditioning in determining sludge dewaterability, such laboratory procedures should be standardized . The procedure proposed by IRSA utilizes the Standard Stirrer shown in Fig . 2 and consists of adding the conditioning solution to the sludge in 10 s while stirring at 300-500 rpm, followed by additional mixing for 20 s . The volume of the conditioning agent must always be 10% of the sludge volume . The required dosage determined in the laboratory is generally slightly lower than that required in practice as it is difficult to reproduce in the laboratory the actual shear stresses which sludges are subjected to in the full-scale machines . Establishing fast and reliable methodologies for the continuous and automatic control of conditioning



Characterization

of sewage sludge

393

efficiency of the full-scale operations thus becomes very useful . Campbell & Crescuolo (1982) have suggested the use of rheological behaviour as an index of chemical conditioning . Sludge dewaterability can be evaluated by general parameters such as specific resistance to filtration, compressibility, Capillary Suction Time (CST), etc . and specific tests, the latter allowing the evaluation of the sludge behaviour when subjected to a specific dewatering technique . Because dewaterability by filtration is fundamentally different from centrifugal dewaterability, the tests used to evaluate the amenability of such dewatering must also be specific to the type of dewatering device considered .

4.1 . Filtration The classical parameter used to evaluate sludge filtrability is the Specific Resistance to Filtration (r) which represents the resistance offered to filtration by a cake deposited on the filter medium having a unit dry solids weight (Vesilind 1974) . Methods for determining this parameter are well known, but the test conditions are often not completely defined . In particular, the resistance must be attributable to the solids alone and not the filter medium ; IRSA standards suggest that the initial portion of filtrate collected can be ignored, and that the filtrate volume after 10% of that of the sludge to be filtered (100-200 cm 3 ) can be used for calculations . Complementary to r is the Compressibility Coefficient (S), obtained by measuring r at different pressures (Vesilind 1974) . This coefficient provides information about the most suitable operating pressure level ; values > 1 . 0 indicate a more than proportional increase of r with pressure and, thus, the advisability of operating at low pressures . The CST is also a simple, useful and rapid way to evaluate filtrability, especially for comparative tests . Gale (1972) showed that, for a certain sludge, the CST to suspended solids content ratio correlates with r with an error ranging between 50 and 100%, which can be considered acceptable if CST is utilized only for obtaining indications as to the order of magnitude of r . A new multi-probe CST apparatus enables a direct estimate of r to be made with a good correlation with values determined from the conventional buchner funnel apparatus . Specific resistance to filtration can also be utilized for predicting the performance of full-scale equipment . Gale (1972) showed that r can be related to vacuum filter performance with an expected error of ± 30% . Jones (1956) related filterpress performance to specific resistance to filtration, although this technique underestimates the capacity by about 40% (Mininni et al. 1982) . A new procedure developed by Mininni et al . (1984) allows full-scale filterpress cake concentration to be estimated as a function of filtering time and operating pressure once the following sludge characteristics are known : initial solids concentration, specific resistance to filtration, viscosity and dry residue density . Specific laboratory tests to evaluate vacuum filter and beltpress performance are also available . The filter-leaf test allows a vacuum filter cycle to be reproduced in laboratory and is the most applicable test for the design of vacuum filters (Vesilind 1974) . Methods for predicting beltpress performance have been proposed by Baskerville et al. (1978), Heide et al. (1982) and Spinosa et al . (1984) . The first consists in a series of drainage tests, thus estimating filter yield, followed by a pressure test using a piston press . The second method uses a Modified Filtration Test (MFT) to give an indication of the final dry solids content and of the rate of dewatering . The third consists of CST measurements and gravity drainage followed by filtration under vacuum . Owing to the generally high separation efficiency (< 95%) of filtration processes, the



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filtrate suspended solids content is not an important parameter in filtrability characterization . 4 .2 . Centrifugation Centrifugability is defined as the aptitude of the sludge to be dewatered under the action of the centrifugal force . Moreover, in industrial centrifuges, the consistency of the centrifuged sludge must be such that it can be easily conveyed by the screw ; the sludge characteristics affecting centrifugability are, therefore, settleability, scrollability and floc strength (Spinosa & Mininni 1984) . Unfortunately, a parameter for assessing sludge centrifugability has not yet been defined because it has not been possible to reproduce on a laboratory-scale the conditions occurring in a full-scale centrifuge . The method proposed by Vesilind (1971) allows both settling and scrolling properties to be evaluated . It consists of centrifugation of sludge by a laboratory centrifuge under various conditions of centrifugal force and centrifugation time . Settling properties are measured by determining the suspended solids concentration in the centrate while scrolling ones are determined by a penetration test . In many cases this method has provided unsatisfactory results with activated sludge because of the difficulty involved in evaluating the sludge consistency as the penetrability is almost equal to 100% . Comparative tests carried out by Aveni & Lamarca (1974) on both laboratory and full-scale equipment with cellulose production sludges, proved that such a methodology allows the efficiency of a full-scale machine, measured as percentage solids recovery (Vesilind 1974), to be predicted with errors of ± 10% (Fig . 3) . Alternatively, Vesilind & Zhang (1983) have shown that sludge can be characterized by plotting the final compacted solids concentration versus log z' . I x t, where z is the number of gravities and t the time of centrifugation . A typical series of tests is shown in Fig . 4 .

40

e

80

Efficiency (full-scale centrifuge)

Fig . 3 . IRB tests for comparison of full-scale and laboratory centrifuge efficiency for dewatering cellulose production sludge. O, Primary sludge from neutralizing treatment ; O, activated sludge ; 0, 70% primary sludge from neutralizing treatment plus 30% 7, sludge from coagulation treatment with lime of biological plant effluent .



Characterization of sewage sludge

395

180

160 oO

0 140 C

N U C

0

120

N

o 100 N rn

a

0

80

N U O Q

E 60 o U

10

4

10

5

Compact ivity, 2

10

6

1.5 X t

Fig . 4 . Typical correlations between compacted sludge solids concentration and compactivity . O, No conditioning; A, 1 . 5% cationic polymer ; 1, 20% quicklime .

TABLE 1 IRSA checking tests for the optimal conditioner dosage Laboratory optimal dosage (g m -3 )

Actual dosage (g m -3 )

Solids recovery (%)

120-170 200-250 250-300

100-150 150-250 250-350

96-98 89-98 88-98

f

100-150 175 100-150 150-175 150-200 150-200

110 128-192 150-160 173-184 208-231 159-173

99 99 99 98 91-99 98

f f f

100 100 250 250 100-125 150

100 115 255 245 100 147-155

90-99 85-88 87-95 90 92-99 93-99

Sludge type and test A (Slaughter house mixed sludge) A1 A2 A3 B BI (Municipal aerobically stabilized dephosphatation) B2 (as BI with simultaneous dephosphatation by Al e (S0 4 ) 3 ) B3 (as B1 with simultaneous dephosphatation by FeSO 4 ) C C1 (Municipal oxidized with pure oxygen) C2 (as Cl) C3 (Municipal oxidized with air)

Another method is based on floc strength measurements; it allows the mechanical resistance of the sludge flocs to be evaluated and thus their capacity to resist the stresses they are subjected to in full-scale machines (Spinosa & Mininni 1984) . The test involves stirring the sludge at 1000 rpm for different times, using a standard apparatus, and



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measuring the CST of the stirred sludge . Good industrial centrifuging results can be expected when (1) the pattern of CST v . stirring time is linear, with a slight slope, between 10 and 100 s, and (2) the CST value at 10 s stirring is around 10-12 s (with 18 mm diameter reservoir) . In Table I the polyelectrolyte dosages determined in the laboratory and the actual dosages adopted in centrifuging different kinds of sludges are compared (Spinosa and Mininni 1984) . Actual dosages used were found in some cases to be slightly higher, probably due to the less efficient sludge/conditioner mixing conditions realized in full-scale tests . A similar principle is applied in the method based on rheological measurements . The sludge is stirred using a properly modified rotational rheometer (Colin et al. 1976) and the torque variation plotted v . time . However the test requires the use of a device with which laboratories are not always equipped .

5 . Final treatment and disposal The most important technological aspects to be evaluated in these operations are those related to sludge transport and/or handling, which mainly depend on the physical state and rheological properties of the sludge . As regards the physical state, Colin (I 979h) has suggested that sludge be characterized into four basic states : liquid, plastic, solid with shrinkage and solid without shrinkage . Simple empirical laboratory tests allow the sludge state to be determined and the optimal operative disposal procedure to be chosen . From a rheological point of view sludges are non-Newtonian fluids and generally exhibit either plastic (initial yield stress followed by a shear stress proportional to shear rate) or pseudo-plastic properties (slope of the shear stress v . shear rate curve decreasing with increasing shear rate) . A typical rheogram is shown in Fig . 5 ; the hysteresis loop is a measure of the degree of thixotropy of the sludge . Sludge properties (high particle size, tendency to separate into two phases, low viscosity, etc .) make rheological determinations difficult, thus limiting the applicability of these parameters . To date, rheological parameters have generally been used for sizing pumps, pipes, mixers, etc ., but other potential applications can be envisaged considering

250

0 C)

50

Shear rate (s I )

Fig . 5 . Typical sludge rheogram .

Characterization of sewage sludge

397

that rheological parameter variations can follow biological modification of the sludge (Dick & Ewing 1967) . Recently, Campbell & Crescuolo (1984) showed the possibility of using rheological measurements as control parameters for sludge conditioning . A methodology for the on-line measurement of sludge rheology from a continuous flow, pilot-scale chemical conditioning system, was also developed . Frost (1984) gave evidence of the existence of intimate associations between certain aspects of rheological and solids/liquids separation behaviour (settleability, thickenability and filtrability) .

6 . Conclusions This review of the main technological sludge characterization parameters indicates that the methodologies now available, particularly those related to dewaterability, make it possible to compare different sludges, to select type and dosage of chemical conditions and to monitor full-scale plant operation . In many cases they also allow useful indications to be obtained for choosing the optimal operating conditions and predicting the performance of full-scale machines . For some operations, e .g . centrifugation, further studies are necessary in order to extend the application of laboratory methodologies to all types of sludges and to correlate them better to industrial-scale operations . For other treatments no simple and reliable methods are available . Several parameters can, for example, be utilized for the evaluation of stability, but none of them are sufficient in themselves to provide complete and correct evaluations . The above discussion also gives evidence of the laboratory tests used for characterization being specific to the method of treatment . Therefore, what is needed in the field of sludge characterization are tests which will yield basic, and not specific, information about sludges .

Acknowledgements Thanks are due to Professor P . Aarne Vesilind for useful suggestions and revision of this paper . References APHA-AWWA-WPCF (1980), Standard Methods for the Examination of Water and Wastewater, 15th Edn. Washington D .C ., U .S .A . Aveni, A . & Lamarca, V . (1974), Cooperative program on the characterization of the sludges : centrifugability-activity of the Instituto di Ricerca Breda, E .E.C . COST 68 Meeting, Dubendorf, Switzerland, 4-5 April 1974. Aveni, A ., Lamarca, V . & Songa, T . (1975), Caratterizzazione dei fanghi in laboratorio (Sludge laboratory characterization), Inquinamento, 17, 12-22 . Baskerville, R . C ., Bruce, A . M . & Day, M . C . (1978), Laboratory techniques for predicting and evaluating the performance of a filterbelt press, Filtration & Separation, (5 Reprinted by Uplands Press Ltd, 1 Katharine Street, Croydon, U .K .) Campbell, H . W . & Crescuolo, P . J . (1982), The use of rheology for sludge characterization, Water Science and Technology, 14, 475-489 . Campbell, H . W . & Crescuolo, P . J . (1984), Assessment of sludge conditionability using rheological properties . In Proceedings of the E.E.C. Workshop Methods of Characterization of Sewage Sludge, Dublin, Eire, 6 July 1983, pp . 31-46 . D. Reidel, Dordrecht, The Netherlands .



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Colin, F ., Cornier, J . C ., Daniel, J . L ., Jacquart, J . C ., Lefort, D ., Mathian, R . & Braunstein, J . P . (1976), Characterization des boues residuaires (Sewage sludge characterization), Techniques and Sciences Municipales, 1-76, Suppl . Colin, F . (1979a), Methodes d'evaluation de la stabilite biologique des boues residuaires (Methods for evaluating sludge biological stability), E.E.C .-COST 68 bis, Working Party 1 paper, WPSP 28 . Institut de Recherches Hydrologiques, Nancy, France . Colin, F . (1979b), Study of the characterization of the physical state of sludge, E .E.C .-COST 68 bis, Working Party 1 Meeting, Institut de Recherches Hydrologiques, Nancy, France, 25 September 1979 . Dick, R . I . & Ewing, B . B . (1967), The theology of activated sludge, Journal of the Water Pollution Control Federation, 39, 543-560 . Eikum, A . (1984), Study of processes in view of preventing odours . In Proceedings of the Third European Symposium Processing and Use of Sewage Sludge, Brighton, U .K ., 27-29 September 1983, pp . 61-66 . D . Reidel, Dordrecht, The Netherlands . Frost, R . C . (1984), Inter-relation between sludge characteristics . Proceedings of the E .E.C . Workshop Methods of Characterization of Sewage Sludge, Dublin, Eire, 6 July 1983, pp . 106-121 . D . Reidel, Dordrecht, The Netherlands . Gale, R . S . (1972), Research in filtration of sewage sludges, Filtration & Separation, 4, 431-436 . Haugan, B . E . & Mininni, G . (1981), Characterization of sewage sludges . In Proceedings of the Second European Symposium Characterization, Treatment and Use of Sewage Sludge, Vienna, Austria, 21-23 October 1980, pp . 13-26 . D . Reidel, Dordrecht, The Netherlands . Heide, B . A ., Kampf, R . & Visser, M . A . (1982), Manual for the selection and use of polyelectrolytes in dewatering sludge with beltpresses, TNO Report A 124E. Delft, The Netherlands . Jones, B . R . S . (1956), Vacuum sludge filtration, Sewage and Industrial Wastes, 28, 963-976 . Lockyear, C . F . & White, M . J . D . (1979), The WRC thickenability test using a low speed centrifuge, WRC Technical Report TR 118. Stevenage, U .K . Mininni, G ., Santon, M . & Spinosa, L . (1982), Tecnologie e problemi della disidratazione dei fanghi (Technologies and problems of sludge dewatering), L'Agricoltura Italiana, 119, 130146 . Mininni, G ., Spinosa, L . & Misiti, A . (1984), Evaluation of filter press performance for sludge dewatering, Journal of the Water Pollution Control Federation, 56, 331--336 . Spinosa, L. & Mininni, G . (1984), Assessment of sludge centrifugability . In Proceedings of the E.E.C . Workshop Methods of Characterization of Sewage Sludge, Dublin, Eire, 6 July 1983, pp . 16-29 . D . Reidel, Dordrecht, The Netherlands . Spinosa, L ., Mininni, G ., Barile, G . & Lore', F . (1984), Study of belt-press operation for sludge dewatering . In Proceedings of the Conference Solids/Liquids Separation practice and the Influence of New Techniques, Leeds, U .K ., 3-6 April 1984, pp . 85-96 . Vesilind, P . A . (1971), Estimation of sludge centrifuge performance (discussion by R . I . Dick) . Journal of the Sanitary Engineering Division, ASCE, 97 . SA2, 234-238 . Vesilind, P. A . (1974), Treatment and Disposal of Wastewater Sludges . Ann Arbor Science, Ann Arbor, U .S .A . Vesilind, P .A . (1983), Sludge odour measurements using a dynamic olfactometer . Personal communication . Vesilind, P . A . & Zhang, G . (1983), Technique for estimating sludge compactability in centrifugal dewatering . Personal communication . White, M . J. D . (1975), Settling of activated sludge, WRC Technical Report TR 11 . Stevenage, U .K .