"August, 1978.""This paper will be submitted for publication in Tappi.

Dugal, Hardev S.

Joyce, T. W.

Webb, Arthur A.

Scholarly Materials And Research @ Georgia Tech

THE INSTITUTE OF PAPER CHEMISTRY, APPLETON, WISCONSINIPC TECHNICAL PAPER SERIESNUMBER 63DEWATERABILITY OF A MODEL PRIMARY SLUDGETHOMAS W. JOYCE, ARTHUR A. WEBB, ANDHARDEV S. DUGALAUGUST, 1978Dewaterability of a model primary sludgeThomas W. Joyce, Arthur A. Webb, and Hardev S. DugalDivision of Environmental Sciences, The Institute of Paper Chemistry,Appleton, Wis. 54911IntroductionSludge dewatering and disposal is a major environmental problem facing pulp andpaper mills. As environmental regulations are stiffened, the situation is expectedto worsen. At present, approximately one-half the cost of building and running awaste treatment plant is devoted to sludge handling.At this same time, more and more mills are searching for ways to recover fiberfrom their waste effluents. Some uses have been found for sludges and researchdevoted to sludge utilization continues. However, basic to any sludge managementstrategy is the ability to produce a sludge cake with low moisture content.It had previously been thought that only the sludge fiber size distributioncontrolled dewaterability. Based on work done at the Institute several years ago,it was believed that the chemical makeup of sludges may also influence dewaterability.Accordingly, a research program was undertaken to determine the relationship betweensludge chemical composition and dewaterability. We have found that the chemistryof the sludge, as opposed to simple fiber size distribution, may be the factorcontrolling dewaterability. Work is continuing in our laboratories on practicalmeans to achieve sludge cakes with low moisture contents.This paper will be submitted for publication in Tappi.Dewaterability of a model primary sludgeThomas W. Joyce, Arthur A. Webb, and Hardev S. DugalDivision of EnvironmentalSciences, The Institute of Paper Chemistry,Appleton, Wis. 54911AbstractSludge dewatering and disposal is a major environmental problemfacing pulp and paper mills. Little is known of the factors which makesome.primary sludges difficult to dewater. The-objective of this studywas to determine the relationship between sludge chemical compositionand dewaterability. Based on results obtained, it appears that chemicalcomposition plays a key role in determining sludge dewaterability. Itwas previously thought that dewaterability was primarily a function ofparticle size distribution.IntroductionAlthough pulp and paper mills have continually searched for means toincrease utilization of fiber resources, a significant amount is stilllost to the waste water stream (1). This fiber eventually is physicallyremoved from the waste water stream either by a sedimentation or flotationunit operation. Theconcentrated slurry, now known as-the primary sludge,is further processed to reduce the moisture content prior to final dis-position.Because the cost of sludge disposal continues to escalate (2,3) andbecause there is a growing concern over the proposed laws regulating theland application of sludges (4), many workers are investigating alterna-tive uses for primary sludges (5-7). Basic to the economics of any sludge-2-management strategy is the ability to produce a sludge cake of low mois-ture content.Little is known Of the mechanisms by which hydrous sludges are ableto bind large quantities of water. Atalla (8) hypothesized a relation-ship between not only the fiber size distribution but also the chemicalcomposition of the fibers and their ability to be dewatered. Brecht andStrittmatter (9-11) also found a relationship between the dewaterabilityof model sludges and both the fiber size distribution and the ash contentof the sludge.It is generally accepted that sludges containing large proportionsof long fiber are easier to dewater than sludges containing only fiberfines. However, the effect of fiber chemical composition has not beenfully elucidated. This study was undertaken in order to further investi-gate the relationship between the chemical composition of a model primarysludge and its dewaterability characteristics.The experimental approach taken was to pulp a softwood species usinga mild delignification procedure which minimized the loss of other woodcomponents. The fibers were then subjected to progressively strongersolvents in order to selectively remove the water and alkali extractablecomponents. After milling and size classification, each fiber size groupwas tested for dewaterability by a modified centrifugal water retentiontest method and by the standard Buchner funnel specific resistance tofiltration test method. By simultaneously varying both fiber size andchemical composition, it became possible to perform a regression analysiswhich indicated the relative importance of chemical composition andfiber size to dewaterability.-3-ExperimentalWafers, of dimensions 0.75" x 0.25" x 0.015", were prepared from afreshly harvested bolt of Southern loblolly pine (Pinus taeda L.).Holocellulose was isolated from the wafers by the following procedure.After room temperature extraction for 3 days in 1:1 chloroform/ethanol,approximately 100 g.a.d. wafers were placed in a 4-liter beaker and 3-liter distilled water and 45 g NaC102 (Olin, technical grade) were added.The pH was adjusted to 4.0 ± 0.2 with 4N acetic acid. At this point, avacuum was applied to ensure penetration of the chemicals into the wafers.The mixture was then gently stirred for one hour at 30 ± 2°C at whichtime the temperature was raised to 40 ± 2°C. Delignification was con-tinued until a test wafer fell apart when concentrated NaOH was addedto it and when the methoxyl content remained constant. When delignifi-cation was complete, usually in about 4 days, the pulping liquor was de-canted and the wafers washed in distilled water and neutralized. Duringwashing, the chips were defiberized; this material was then termed holo-cellulose.The prepared holocellulose was then extracted at 20°C for 24 hoursin distilled water. Excess pulping chemicals were the main component ofthe extract liquor. A sample of fiber was taken for physical testing.The fiber was then extracted at 75°C for 3 hours in distilled water.The main component extracted was pectin. A sample of the fiber was takenfor physical testing.The remaining fiber was extracted with Ba(OH) 2/NaOH, 1% NaOH, andfinally 15% NaOH according to the method of Beelik, et al. (12). This-4-procedure permits the selective extraction of hemicelluloses from holo-cellulose. Xylans are the main component of the Ba(OH) 2/NaOH extractliquor while galactoglucomannan is dissolved in the 1% NaOH extractliquor. Glucomannans are dissolved in the 15% NaOH extract liquor.After each extraction a sample of fiber was taken for physical testing.The extracted fiber samples were freeze dried and put through aWiley mill for size classification. Screen sizes used were 80, 100, 150,and 200 mesh.Carbohydrate analysis was done by the method of Borchardt (13).A modified Centrifugal Water Retention test method was adoptedfrom Thode, et al. (14).The Buchner funnel specific resistance to filtration test was doneaccording to standard techniques (15).Results and discussionThe carbohydrate composition of the fibers after each extraction isgiven in Table I. These values compare quite favorably with those givenby Beelik, et al. (12) for western hemlock and grand fir species. Aspredicted, the content of xylose dropped precipitously after the Ba(OH)2/NaOH extraction. After each extraction, carbohydrate composition was asexpected.Table II shows the results of the Centrifugal Water Retention CCWR)tests. This test was chosen because it gives reproducible results andrequires small samples. Since the holocellulose could be produced inonly small quantities, small sample size was a desirable test feature.-5-The CWR test consists, in essence, of placing a small amount of pulpslurry in a specially designed centrifuge tube and centrifuging at 2800 gfor 15 minutes. The cake is then carefully removed from the centrifugetube and the moisture content is determined. All free and most of theinterstitial water is removed during centrifugation and only the waterchemically associated with the solids remains.The results of the CWR tests should not be interpreted in absoluteterms. Rather, trends in the data are of more significance. As can beseen, as more wood components are extracted within a particular sizecategory, the solids content increases, indicating greater ease in de-waterability. Because these extraction steps selectively remove hemi-celluloses, the large difference between the solids content of the hotwater and Ba(OH)2/NaOH extraction steps in all mesh sizes suggests thatremoval of some hemicelluloses may be governing the dewatering process.The results of the Buchner funnel tests are given in Table III.It was hoped that this test would provide more easily interpretabledata since the test is used to design vacuum filtration equipment. Itwas disappointing to find that differences between the values are toosmall to draw any firm conclusions. However, looking at the data astrend indicators, reveals much the same as the CWR test. The only dif-ference is that there is an increase in resistance (indicating a moredifficult to dewater sludge) between the 1% and 15% NaOH extraction step.It is thought this may be due to swelling of the fibers after hemicellu-loses are removed. A major difference between the CWR and the Buchnerfunnel tests is the magnitude of the mechanical forces exerted on thefibers; much more force is used in the CWR test. This would allow more-6-of the interstitial water to be removed and permit small differences inchemically bound water to be observed.The correlation coefficient (r2) from the analysis of variance testsare shown in Table IV. A correlation coefficient greater than 0.90 wasconsidered very significant. As can be clearly seen, there were no cluesto the relationship between chemical composition and specific resistanceto filtration. Somewhat surprisingly, not even mesh size could be cor-related with dewaterability.Arabinose and xylose content in the fibers shows a high correlationwith CWR. Total hemicellulose also shows a high correlation; this couldbe an experimental artifact in that the total hemicellulose content forloblolly pine is dominated by xylose. The experimental procedure suffersfrom a limitation because it is not possible to change the order of hemi-celluloses extracted. Thus, it is not possible to determine if the highcorrelation of CWR to xylose is due to the presence of xylose on thefiber or to its order of removal from the fiber. Once again, mesh sizewas not shown to be statistically significant.These results may be used to suggest reasons for differences in de-waterability between mill primary sludges. For example, it is thoughtgroundwood sludges are difficult to dewater because of the large finescontent. These results indicate the problem may be due to the high hemi-cellulose content of this high yield process. Conversely, low yieldkraft primary sludges are usually quite easy to dewater; this could bedue to the low hemicellulose composition of the sludges.This investigation has suggested that the chemical composition ofprimary sludges may be more significant than fiber size distribution inrelation to dewaterability. One practical question which has not beenaddressed is whether all size fractions within a particular sludge havethe same chemical composition. As mills close their white water systems,this may have an adverse impact on treatment plant operation. Anotherquestion is whether mills who use more than one type of pulp may experi-ence problems in chemical dewatering aid addition because the sludgecharacteristics have changed due to differences in chemical compositionbetween furnishes. Work on these issues continues in our laboratories.AcknowledgmentsWe thank R. H. Atalla and N. S. Thompson, of The Institute of PaperChemistry for many useful discussions and suggestions.Literature cited1. Slatin, B. "Resource Recovery and Reuse," Tappi 61(4): 47 (1978).2. Smith, J., "Inventory of Energy Use in Wastewater Sludge Treatmentand Disposal," Industrial Water Eng. 14(4): 20 (1977).3. Burley, M., and Bayley, R., "Sludge Disposal Strategy: Processesand Costs," Water Pollution Control (British) 76(2): 205 (1977).4. A Laboratory Investigation of Heat Treatment for Pulp and PaperMill Sludge Conditioning, NCASI Technical Bulletin No. 293, May,1977.5. Marshall, D., "Options and Implications in the Utilization of Pulpand Paper Industry Sludges," Southern Pulp Paper Mfr. 40(2): 19(1977).6. Soderhjelm, L., "Possible Uses for Fibrous Sludges from the Pulp andPaper Industry," Paperi Puu 9:620 (1976).7. Anon., "Sludge Disposal by Earthworms," Japan Pulp Paper 13(4): 9(1975).8. The Relation Between the Constitutive and Dewatering Properties ofHydrous Sludges, NCASI Technical Bulletin No. 260, Oct., 1972.-8-9. Brecht, W., and Strittmatter, G., "Fundamental Studies Dealing withFiltration and Sedimentation Drainage of Model Sludges Typical ofThose Found in the Paper Industry," Papier 22(104): 799 (1968).(In German).10. Brecht, W., and Strittmatter, G., "New Information on the Draina-bility of Waste Water Sludges of the Paper Industry," Papier 23(7):369 (1969). (In German).11. Brecht, W., and Strittmatter, G., "New Information on the Drainabilityof Waste Water Sludges of the Paper Industry," Papier 23(9): 541(1969). (In German).12. Beelik, A., et al., "Selective Extraction of Hemicelluloses fromSoftwoods," Tappi 50(2): 78 (1967).13. Borchardt, L., and Piper, C., "A Gas Chromatographic Method forCarbohydrates as Alditol-Acetates," Tappi 53(2): 257 (1970).14. Thode, E., et al., "The Application of a Centrifugal Water-RetentionTest to Pulp Evaluation," Tappi 43(5): 505 (1960).15. Vesilind, P., "Treatment and Disposal of Wastewater Sludges," AnnArbor Sci., Ann Arbor, Michigan, 1974.I. Carbohydrate composition of fibers after indicated extractionWeight PercentageExtraction Arabinose Xylose Mannose Galactose GlucoseChloroform/ethanolCold waterHot waterBa(OH)2/NaOH1% NaOH15% NaOH1.10.80.80.30.20.05.07.06.92.82.21.012.213.413.514.915.413.22.22.01.91.61.01.043.462.062.770.767.577.1II. Solids content of cake (%) after centrifugalwater retention testExtractionMesh Cold water Hot water Ba(OH)2/NaOH 1% NaOH 15% NaOH80 37.5 38.9 47.8 50.6 54.0100 35.5 35.8 44.8 49.1 52.6150 34.3 35.0 44.2 46.8 52.3200 35.2 33.7 43.8 46.3 49.7III. Specific resistance of fibers to filtrationSpecific Resistance x 10- 8 (sec 2 /g)Mesh Cold water . Hot water Ba(OH)2/NaOH 1% NaOH 15% NaOH80 2.4 2.3 2.1 0.6 1.6100 4.5 2.1 2.2 1.0 1.6150 7.2 3.3 2.6 1.3 1.4200 4.7 3.1 0.7 1.6 1.3IV. Analysis of variance results2rSpecificVariable CWR resistanceArabinose 0.94 0.48Xylose 0.93 0.49Mannose 0.10 0.19Galactose 0.85 0.49Glucose 0.79 0.33Total hemicellulose 0.91 0.40Mesh size 0.04 0.02

Dewaterability of a model primary sludge

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Dewaterability of a model primary sludge

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