the impact of digestion on sludge dewaterability

THE IMPACT OF DIGESTION ON SLUDGE

DEWATERABILITY

J. I. HOUGHTON, J. QUARMBY and T. STEPHENSON (MEMBER)

School of Water Sciences, Cran®eld University, Cran®eld, Bedfordshire, UK

Sludge disposal is a growing problem for all wastewater treatment companies, asintensifying wastewater processing to meet more stringent discharge limits leads toincreased sludge production. Anaerobic digestion can reduce the overall mass of the

waste sludge requiring disposal, but the end product is generally more dif®cult to dewater.Microbial extracellular polymer (ECP) has previously been shown to in¯uence sludgedewaterability. This investigation examined the relationship between the quantity andcomposition of ECP present in sludge samples, prior to and after the digestion process, at full-scale municipal sewage treatment works. Anaerobic digestion was shown to alter the quantityof ECP present, with digested sludge containing 25% less ECP on average than raw sludge.The level of ECP that may give maximum sludge dewaterability was calculated as 21 mg ECPg±1 SS for raw sludge and 10 mg ECP g±1 SS for digested sludge. The organic composition ofthe ECP extracted from digested sludge contained more protein with respect to carbohydratethan that extracted from pre-digested sludge. These changes in the sludge ECP were shown tocorrelate with changes in sludge dewaterability.

Keywords: capillary suction time; dewaterability; digestion; extracellular polymer; sludge.

INTRODUCTION

The EC Urban Wastewater Treatment Directive (91/271/EEC)1 is having a major impact on sludge production anddisposal in the UK. The introduction of minimum levels oftreatment for the majority of sewage waste and prohibitionof sludge disposal at sea has led to an estimated 60%increase in sludge production between 1992 and 20002

whilst limiting disposal options. One method of decreasingthe quantity of waste sludge produced is anaerobicdigestion3, a process that is used extensively in the UK.

During anaerobic digestion, raw sludge is stabilized usinga natural biological process that converts organic matter intomethane and carbon dioxide. The destruction of the organicmatter results in a breakdown of the solid material presentÐfor example, bacterial cells, paper, food debrisÐand a shiftin the particle size distribution of the sludge to a greaterproportion of ®ne material. The waste sludge remaining afterdigestion is mainly the non-biodegradable fraction of thein¯uent sewage, and is more homogenous in structure. Thesechanges in sludge structure in¯uence the overall properties ofthe sludgeÐfor example, its dewaterability4.

Dewatering of the waste sludge prior to disposal isessential, both to reduce the overall volume of waste andassist in the handling process. One factor that has beenshown to in¯uence the dewaterability of sludge is the levelof natural extracellular polymer (ECP) present5,6,7, withincreasing levels of ECP in activated sludge being reportedto make the sludge more dif®cult to dewater.

ECP is produced by bacteria present in the sewagetreatment process and is found either associated with thebacterial cell wall or in suspension, detached from thebacterial cell8. ECP is extremely hydrated, containing up to

98% water8, and prevents desiccation of the bacterialcell under natural environmental conditions. Anaerobicstorage of activated sludge causes a deterioration in thesludge dewaterability9, and changes in the composition ofthe sludge ECP10.

The aim of this work was to carry out a preliminary studyinto the impact of the digestion process on sludgedewaterability, utilizing samples from full-scale municipalsewage treatment works. The ECP content of the sludge wasexamined, and changes in ECP yield and composition wererelated to sludge dewaterability. The results obtained willform the basis for further investigation into the manipulationof the sludge ECP content, aiming to assist in the dewateringprocess and contribute towards the minimization of the totalvolume of waste sludge requiring disposal.

MATERIALS AND METHODS

Sludge Samples

Raw and digested sludge samples were collected from sixdifferent sewage treatment works (STW) in the Anglian andSouthern Water regions. Raw sludge samples were obtainedfrom the storage tank prior to the digester, and digestedsludge samples from the digester out¯ow. The type ofdigester employed at each works is detailed in Table 1. SiteB STW was sampled twice, once before (B-1) and once after(B-2) the commencement of chemical dosing (ferrouschloride) for phosphate removal. Site A was the onlyother STW sampled at which chemical dosing for phosphateremoval was occurring. Site F differed from the other sitesin that chemical polymer was added to the storage tanks. Allsamples were returned to the laboratory immediately after

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Trans IChemE, Vol 78, Part B, March 2000

collection and ECP extraction and solids analysis carriedout. The time lapse between sample collection and thecommencement of ECP extraction was always less than 3 h.Each sample was analysed for suspended solids (SS) andvolatile suspended solids (VSS) according to standardmethods11.

ECP Extraction Procedure

ECP extraction was carried out using a method similar tothat reported by Morgan et al12. Sludge samples (500 ml)were rinsed and resuspended in ˆ-strength Ringer’ssolution, heated at 80 8 C for 1 h, and allowed to cool. TheECP was separated from the sludge solids by centrifugationfor 20 min at 1800 g, followed by a further 20 min at 5000g.To extract the ECP from the sludge supernatant, one partsolvent (3:1 acetone:ethanol v/v) was added to one partsupernatant and the whole left to stand at 4 8 C for at least12 h. The precipitated polymer was harvested by centrifuga-tion (5000 g, 20 min), any remaining solvent being allowedto evaporate before the ®nal weight of the extracted polymerwas calculated and related to the SS content of the sludge.

Sludge Dewaterability

The rate of water release from each sludge sample wasdetermined by the capillary suction time (CST) test, using aTriton CST Filtrability Tester, model 200 (Triton Electro-nics Ltd, Essex, UK). All tests were carried out as per themanufacturer’s instructions, using CST ®lter paper suppliedby Triton Electronics and the 18 mm sludge reservoir. Acorrection for the different SS content of each sludge wasmade to all CST results obtained, as detailed in APHAmethod 2710G11. The CST for distilled water wasdetermined for each batch of ®lter papers used andsubtracted from the sludge CSTs.

Determination of Extracted ECP Organic and InorganicContent

The organic and inorganic fractions of the extracted ECPwere calculated gravimetrically. Samples of extractedpolymer were weighed into a dry, pre-weighed porcelaincrucible, placed in a furnace set at 5008 C for 5 h, and thecrucible re-weighed after cooling. The ash remaining wasclassed as the inorganic fraction, allowing determination ofthe organic and inorganic portions to be expressed as apercentage of the total ECP extracted.

The extracted ECP was analysed for carbohydrate andprotein. For analysis, samples of the extracted polymer weredissolved in distilled water (1 mg ml±1). Carbohydrate

analysis13 was carried out using glucose as the standard,and protein levels were determined14 using bovine serumalbumin as the standard.

Ion analysis of the inorganic fraction of the extracted ECPwas carried out using inductively coupled plasma (ICP)analysis (Atom Scan 25, Thermo Jarrell Ash Corporation),set at the following parameters: carrier gas Argon; torch gas¯ow high; auxiliary gas ¯ow 1.0 l min±1. Samples foranalysis were prepared by dissolving the inorganic ECP ashin concentrated nitric acid (1 mg ash to 1 ml acid), followedby a 10x dilution using distilled water to ensure enoughsample for analysis. A total element scan was initiallycarried out on one sample. Twelve elements were detected:Al; Ca; Co; Fe; K; Mg; Mn; Na; Ni; P; S and Zn. Proprietarystandards for each of these elements were obtained forcalibration purposes (Merck Ltd). Each sample was thenanalysed for each individual element and the resultsexpressed as a percentage, assuming that 100% of the ionspresent were detected.

RESULTS

Characterization of the sludge solids (Table 2) illustratedthat the chemical polymer added to the storage tanks at SiteF enabled a SS content of nearly 8% to be achieved for theraw sludge, compared to 1.4-4.6% at the other sites. Theaddition of chemical polymer will also affect the CSTobtained for samples from this site. The data obtained forSite F dewaterability is therefore not directly comparable tothat of the other sites.

For all of the sludge samples investigated, except SiteB-1, the digested sludge sample was more dif®cult todewater than the raw sludge sample (Figure 1). ExcludingB-1, the increase in CST between raw and digested sludgesamples varied from between 142% for Site A to >400% forSite E. The level of variation between the CSTs of the rawsludge samples was less (SD 1.64) than that present betweenthe digested sludge samples (SD 6.25).

154 HOUGHTON et al.


Table 1. Type of digester employed at each site.

Residence time, OperatingSite Type Mixing days temperature, 8 C

A Fixed-roof, sealed Gas 12 35 6 3B Fixed-roof, sealed Gas 14±15 35 6 3C Floating-roof Mechanical 15 33-35D Fixed-dome Gas 17 35 6 3E Floating-roof Gas 12 32-36F Fixed-roof, sealed Mechanical 14 35 6 3

Table 2. Characterization of sludge samples collected for analysis.

Raw sludge samples Digested sludge samples

STW SS, gl-1 VSS, gl-1 SS, gl-1 VSS, gl-1

Site A 46.0 33.1 28.2 17.7Site B-1 30.7 23.5 34.1 20.7Site B-2 40.7 28.5 25.6 16.5Site C 32.7 26.2 15.6 9.8Site D 14.0 10.9 22.2 15.6Site E 19.4 15.3 21.7 14.9Site F 77.5 59.7 45.9 30.3

The quantity of ECP extracted from each sludge samplewas related to the SS content of the sludge. For each STWinvestigated, except Site A, there was a decrease in the yieldof ECP (mg g±1 SS) obtained after the digestion process(Figure 2). This decrease in the level of ECP extractedranged from 15% in the case of Site B-2 to 43% for Site B-1,the average decrease being approximately 25%.

Relating the ECP yield to the CST of the sludge (Figure 3)indicated that there appears to be a level of ECP at which thedewaterability of the sludge is at an optimum, the sludgebecoming harder to dewater either side of this point.The relationship appears stronger for the raw sludge

(R2= 0.8408), with the data points showing less scatter.The level of ECP for maximum sludge dewaterability wascalculated as 21 mg ECP g±1 SS for raw sludge and 10 mgECP g±1 SS for digested sludge.

Ashing the extracted polymer indicated that the ECP wasmainly organic in composition (Table 3). Further analysisenabled the composition of between 47.7±61.4% of theorganic fraction to be determined (Table 4). The unknownportion of the organic fraction was assumed to be composedof varying amounts of lipids and nucleic acids in the form ofDNA and RNA, but these were not assayed for.

In all cases, except Site B-1, there were greater levels of

155IMPACT OF DIGESTION ON SLUDGE DEWATERABILITY


0

2

4

6

8

10

12

14

16

18

20

Site A Site B-1 Site B-2 Site C Site D Site E

Sewage Treatment W orks

CS

T (

s) S

S =

1 g

l-1

Raw sludgeDigested sludgeMean digested sludgeMean raw sludge

Figure 1. The effect of digestion on sludge dewaterability.

0

5

10

15

20

25

30

35

Site A Site B-1 Site B-2 Site C Site D Site E Site F


EC

P y

ield

(m

g g-1

SS

)

Raw sludgeDigested sludge

Figure 2. The impact of digestion on extracted ECP yield.

protein present in the ECP extracted from the digestedsludge samples than the raw sludge samples. The actualdifference in the level of protein present in the ECPextracted from the raw and digested sludges varied from-15% in the case of Site B-1, to +50% for Site A. The levelsof carbohydratepresent in the extracted ECP also varied, butnot in line with variations in the protein levels. For all of thesamples examined, the level of carbohydrate present in theextracted polymer was lower (4±28% decrease) in the ECPobtained from the digested sludge samples than thatobtained from the raw sludge samples (Table 4). Toillustrate the changes in the protein and carbohydrate

levels of the extracted ECP, with respect to each other,the protein to carbohydrate ratios were calculated (Figure 4).In all cases, the protein to carbohydrate ratio of the extractedECP is higher for the digested sludge samples than for theraw sludge samples.

Inorganic analysis of the extracted ECP did not show suchdistinctive trends as the organic analysis. In all of thesamples analysed, two elements were dominant, Ca and P(Table 5). Together, Ca and P accounted for 66.3±89.5% ofthe total inorganic material present. Where chemical dosingwith ferrous chloride was occurring for phosphate removal(Site A, Site B-2), increased levels of Fe were present withrespect to the other samples. For these samples, the Fe levelof the extracted ECP decreased sharply after digestion. Thethird most plentiful ion was Na. Levels of Na present variedfrom 2.68±13.1%, and the level of Na present in the ECPextracted from digested sludge was always higher than thatin ECP extracted from raw sludge at the same STW. Noother elements showed distinct differences between the rawand digested samples, either individually or when combined.

DISCUSSION

The results show that the digestion process has an impacton the sludge dewaterability, and that this may be related to

156 HOUGHTON et al.


y = 0.0216x2 - 0.9015x + 10.516R2 = 0.8408

y = 0.0402x2 - 0.8004x + 5.8811R2 = 0.5209

0

2

4

6

8

10

12

14

16

18

20

10 15 20 25 30 35

ECP yield (mg g-1 SS)

CS

T(s

) S

S =

1 g

l-1

Figure 3. The relationship between sludge dewaterability and extracted ECP yield for raw (e) and digested ( s ) sludge.

Table 3. Extracted ECP composition.

Raw sludge samples Digested sludge samples

% organic % inorganic % organic % inorganic

Sewagetreatmentworks

Site A 81 19 74 26Site B-1 66 34 65 35Site B-2 80 20 77 23Site C 76 24 76 23Site D 72 28 73 27Site E 71 29 76 24Site F 81 28 80 20

Table 4. Extracted ECP organic composition.

Raw sludge samples Digested sludge samplesOrganic composition, % Organic composition, %

Protein Carbohydrate Unknown Protein Carbohydrate Unknown

Sewagetreatmentworks

Site A 27.0 21.0 52.0 40.5 19.1 40.4Site B-1 36.1 21.8 42.1 30.8 16.9 52.3Site B-2 30.3 20.8 48.9 41.4 20.0 38.6Site C 32.7 17.1 50.1 38.3 16.4 45.3Site D 35.8 24.7 39.4 42.6 17.7 39.7Site E 30.0 18.0 52.0 38.0 16.7 45.3Site F 30.7 18.3 51.0 33.9 16.1 50.0

the quantity and nature of the microbial ECP present. It mustbe remembered that both the raw and the digested sludgesamples were collected at the same time from eachtreatment plant. Therefore, there may be some differencebetween the raw sludge sample collected for analysis andthat which formed the basis of the digested sludge samplecollected. This is in greatest evidence at Site D, where theSS concentration of the raw sludge is nearly 40% less thanthat of the digested sludge (Table 2). This was due toextreme weather conditions in the days before sampling wascarried out. Also, the in¯uent ¯ow and operating conditionspresent are variable between each treatment plant. As eachplant examined received large volumes of waste, it wasenvisaged that any changes in in¯uent sewage would beminimized during storage of the raw sludge prior todigestion. This may not always have occurred, owing todifferent sites consolidating one or more different types ofsludge in the storage tanks prior to digestion.

Generally, the digestion process made the sludge moredif®cult to dewater (Figure 1) and reduced the level of ECPpresent in the sludge (Figure 2), although exceptions did

occur. The reduction in the quantity of ECP present was notdirectly related to an increase in CST, as the greatestpercentage drop in digested sludge ECP yield did notcorrespond to the greatest increase in CST between the rawand digested samples from any one plant. This may be dueto the sample collection procedure, plant operating condi-tions, or the in¯uence of other factors that have been shownto affect sludge dewaterability, such as particle size4,15.Further experimentation under controlled laboratory condi-tions would allow some of this variation to be eliminated.

The level of variation present between the CSTs of thedigested samples was high compared to that of the rawsludge samples (Figure 1). This was also illustrated by thegreater level of variation between the data points (digestedsludge: R2= 0.5209; raw sludge: R2= 0.8408) when therelationship between the CST and the ECP yield was plottedgraphically (Figure 3). This relationship is poor for thedigested sludge. The consolidation of raw sludge in storagetanks prior to digestion is likely to produce similarconditions at each site, and hence less variation, whereasthe mode of operation and digester ef®ciency will vary



Site ASite B-1

Site B-2Site C

Site DSite E

Site F

Raw sludge

Digested sludge0

0.5

1

1.5

2

2.5

Pro

tein

to c

arbo

hydr

ate

ratio


Sludge type

Figure 4. Variation in the extracted ECP protein to carbohydrate ratio for raw and digested sludge samples.

Table 5. Extracted ECP inorganic composition.

Extracted ECP inorganic composition, %

Sewage treatment works Sample Al Mn K Na Mg Ca Fe Ni P Co S Zn

Site A Raw 3.56 0.13 3.19 6.05 1.71 44.7 14.3 0.03 21.6 0.0 4.52 0.26Digested 2.61 0.02 2.33 7.68 3.44 52.3 4.89 0.02 23.4 0.0 3.12 0.20

Site B-1 Raw 0.30 0.17 1.07 5.04 2.62 55.1 2.79 0.03 32.4 0.01 0.37 0.08Digested 0.15 0.04 1.30 8.00 7.13 51.0 1.19 0.01 30.0 0.0 1.17 0.06

Site B-2 Raw 0.55 0.10 1.26 6.29 2.04 46.6 14.5 0.02 25.3 0.0 2.43 0.22Digested 0.90 0.03 2.51 10.4 3.78 50.0 5.19 0.02 23.6 0.0 3.43 0.14

Site C Raw 0.60 0.08 1.01 2.68 4.91 51.9 0.60 0.04 37.6 0.0 0.50 0.06Digested 2.74 0.08 1.81 12.9 7.08 38.3 2.38 0.03 32.8 0.0 1.57 0.28

Site D Raw 0.33 0.06 0.83 5.60 2.33 57.7 0.56 0.02 30.5 0.0 1.93 0.11Digested 1.06 0.06 0.27 5.69 1.42 64.1 0.78 0.04 24.3 0.0 2.08 0.27

Site E Raw 0.37 0.06 1.13 3.10 5.71 54.9 0.39 0.03 33.6 0.0 0.65 0.08Digested 1.12 0.03 1.79 8.88 5.60 60.3 0.73 0.04 18.9 0.0 2.42 0.18

Site F Raw 0.57 0.12 0.74 3.30 2.95 52.4 8.31 0.03 31.0 0.0 0.50 0.07Digested 1.98 0.06 1.94 13.1 5.38 46.7 4.22 0.02 23.2 0.0 3.26 0.14

between sites. Examination of additional digested sludgesamples, preferably obtained from digesters operating undersimilar conditions, is required to con®rm the signi®cance ofthis result.

In the majority of the samples examined, the sludgebecame harder to dewater as the level of ECP increased.This corroborates results obtained previously for activatedsludge7,16. Whilst a number of workers have extracted ECPfrom anaerobic sludges12,17,18 and studied the extractedECP yield19 and composition20,21, the direct relationshipbetween the ECP yield and anaerobic sludge dewaterabilityhas not been reported. In anaerobic sludges, the presence ofhigh levels of ECP is thought to give better granulation andsludge structure19. Reducing the quantity of ®ne particlespresent within the system should aid the sludge dewater-ability15, but ECP itself is highly hydrated8. A balance cantherefore be expected where the level of ECP present aidsdewaterability by increasing the particle size of the solids,but does not itself hold too much water within the sludgematrix. From the results obtained here (Figure 3) thisconcentration appears to be 21 mg ECP g±1 SS for rawsludge and 10 mg ECP g±1 SS for digested sludge. Furtherwork, especially with digested sludge, will con®rm therelevance of these values.

Volatile suspended solids reduction during digestion, afactor determined by the ef®ciency of the digestion process,feed sludge characteristics and upstream processing3, wasfound to range from 3 to 63%, excluding Site D (calculatedfrom Table 2). Four of the sites, (A, B-2, C and F) had a VSSreduction >40%, the normal expected VSS reduction formesophilic anaerobic digestion. Where this did not occur, itis likely that the in¯uent conditions were very different atthe time the digested sludge was sampled (B-1, D and E), asindicated by the SS concentration.

In all of the samples examined, the extracted ECPcarbohydrate fraction decreased in the digested sludgesamples (Table 4) and the protein to carbohydrate ratioincreased (Figure 4). The dominance of protein in theextracted ECP of anaerobic sludges has been observedpreviously using the same extraction technique12. Thisapparent change in the ECP composition as the sludge isdigested may be due to the change in the bacterialpopulation present, with different products being excretedby the cell to form the ECP, or the fact that the carbohydratefraction, especially if it consists of low molecular weightmolecules, may be more easier to digest than protein.

Examination of the inorganic content of the extractedECP did not show any major differences between the rawand digested sludge samples. For both sludge types the mainelements present were Ca and P. Calcium has long beenknown to be essential for the ¯occulation of bacteria22 andthe stability of sludge ¯ocs23, so its predominance in theinorganic fraction of the extracted ECP was not unexpected,and a similar result has been reported previously using adifferent method of analysis12. The monovalent-to-divalent(M:D) cation ratio of activated sludge has been shownto in¯uence the sludge dewaterability24, the cationsassociating with the protein fraction of the ECP during the¯occulation process. Examination of the M:D cation ratiopresent in the extracted ECP samples did not highlight anytrend between the sludge dewaterability and ECP M:Dcation ratio. The other ion found in large quantities was P.Along with C and N, P is the main component of bacterial

cells. Any C or N present in the extracted ECP would havebeen removed during the ashing process, leaving the Pbehind. The high proportion of organic matter present in theextracted ECP (Table 3) accounts for the levels of Pdetected.

CONCLUSIONS

Anaerobic digestion generally impedes sludge dewater-ability. Examination of the quantity and nature of microbialECP present illustrates that the level of ECP and its organiccomposition appears to in¯uence the sludge dewaterability.Reducing the level of ECP present in the sludge to 21 mgECP g±1 SS for raw sludge and 10 mg ECP g±1 SS fordigested sludge, may produce a sludge that is easier todewater. A reduction in the protein to carbohydrate ratio ofthe sludge ECP may also assist the dewatering process. Theproduction of sludges that are easier to dewater may reduce,or even eliminate, the need to use chemical coagulants inmechanical dewatering processes. This would bene®t boththe waste treatment companies, by reducing operating costs,and the environment through reducing the overall volume ofwaste requiring disposal.

REFERENCES

1. Council of European Communities, 1991, Directive concerning urbanwastewater treatment (91/271/EEC), Of®cial Journal L135/40, 30 May1991.

2. Davis, R. D., 1996, The impact of EU and UK environmental pressureson the future of sludge treatment and disposal, J CIWEM, 10: 65±69.

3. Parkin, G. F. and Owen, W. F., 1986, Fundamentals of anaerobicdigestion of wastewater sludges, J Env Engineer, 112: 867±920.

4. Lawler, D. F., Chung, Y. J., Hwang, S.±J. and Hull, B. A., 1986,Anaerobic digestion: effects on particle size and dewaterability,Journal WPCF, 58: 1107±1117.

5. Ryssov-Nielson, H., 1975, The role of natural extracellular polymers inthe bio¯occulation and dewatering of sludge (literature survey), Vatten,31: 33±39.

6. Novak, T., Becker, H. and Zurow, A., 1977, Factors in¯uencingactivated sludge properties, J Env Engineer, 103: 815±828.

7. Kang, S.±M., Kishimoto, M., Shioya, S., Yoshida T., Suga, K.-I. andTaguchi, H., 1989, Dewatering characteristics of activated sludges andeffect of extracellular polymer, J Ferment and Bioeng, 68: 117±122.

8. Wilkinson, J. F., 1958, The extracellular polysaccharides of bacteria,Bacteriol Rev, 22: 46±73.

9. Day, M. and Fernandes, X., 1990, Control of anaerobic digestion forimproved consolidation and dewatering performance, WRc ReportNumber UM1071 (WRc Swindon, Wilts, UK).

10. Nielsen, P. H., Frùlund, B. and Keiding, K., 1996, Changes in thecomposition of extracellular polymeric substances in activated sludgeduring anaerobic storage, Appl Microbiol Biotechnol, 44: 823±830.

11. APHA, 1992, Standard Methods for the Examination of Water andWaste Water, 18th ed (American Public Health Association/AmericanWater Works Association/Water EnvironmentFederation, WashingtonDC, USA).

12. Morgan, J. W., Forster, C. F. and Evison L., 1990, A comparative studyof the nature of biopolymers extracted from anaerobic and activatedsludges, Wat Res, 24: 743±750.

13. Dubois, M., Gilles, K. A., Hamilton, J. K., Rebers, P. A. and Smith, F.,1956, Colorimetric method for determination of sugars and relatedsubstances, Anal Chem, 28: 350-356.

14. Lowry, O. H., Rosebrough, N. J., Farr, A. L. and Randall, R. J., 1951Protein measurement with the folin phenol reagent J Biol Chem, 193:265±275.

15. Karr, P. R. and Keinath, T. M., 1978, In¯uence of particle size onsludge dewaterability, J Wat Pollut Control Fed, 50: 1911±1930.

16. Shioyama, M. and Toriyama, A., 1985, Yields of materials extractedfrom activated sludge by sodium hydroxide solution, and its relation-ship to speci®c resistance to ®ltration, J Jap Sew Works Assoc, 22: 22±28.

158 HOUGHTON et al.


17. Fang, H. H. P. and Jia, X. S., 1996, Extraction of extracellular polymerfrom anaerobic sludges, Biotechnol Tech, 10: 803±808.

18. Karapanagiotis, N. K., Rudd, T., Sterritt, R. M. and Lester, L. N., 1989,Extraction and characterization of extracellular polymers in digestedsewage sludge, J Chem Tech Biotechnol, 44: 107±120.

19. Jia, X. S., Furumai, H. and Fang, H. H. P., 1996, Yield of biomass andextracellular polymers in four anaerobic sludges, Environ Tech, 17:283±291.

20. Morgan, J. W., Evison, L. M., and Forster, C. F., 1991, An examinationinto the composition of extracellular polymers extracted fromanaerobic sludges, Trans IChemE, Part B, Proc Safe Env Prot,69(B4): 231±236.

21. Jia, X. S., Fang, H. H. P., and Furumai, H., 1996, Surface charge andextracellular polymer of sludge in the anaerobic degradation process,Wat Sci Tech, 34 (5±6): 309±316.

22. Tezuka, Y., 1969, Cation-dependent ¯occulation in a Flavobacteriumspecies predominant in activated sludge, Appl Microbiol, 17: 222±226.

23. Bruus, J. H., Nielsen, P. H. and Keiding, K., 1992, On the stability ofactivated sludge ¯ocs with implications to dewatering, Wat Res, 26:1597±1604.

24. Higgins, M. J. and Novak, J. T., 1997. The effect of cations on the

settling and dewatering of activated sludges: laboratory results, WatEnviron Res, 69: 215±224.

ACKNOWLEDGEMENTS

JIH is funded by EPSRC Grant GR/L 87255 and sponsored by AnglianWater plc. JIH acknowledges the assistance of both Anglian Water plc andSouthern Water plc in providing site access and samples for analysis.

ADDRESS

Correspondence concerning this paper should be addressed to Mrs J. I.Houghton, School of Water Sciences, Cran®eld University, Cran®eld,Bedfordshire MK43 OAL, UK.

This paper was presented at IChemE’s Research 2000 Conference heldat the University of Bath, UK, 6±7 January 2000. The manuscript wasreceived 6 September 1999 and accepted for publication after revision 21December 1999.



the impact of digestion on sludge dewaterability

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