Gravity Thickening of Water-Treatment-Plant SludgesAuthor(s): George Gebauer and Ingolf JanerusSource: Journal (American Water Works Association), Vol. 70, No. 1, Contaminants in Water(January 1978), pp. 47-48Published by: American Water Works AssociationStable URL: http://www.jstor.org/stable/41269315 .

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isotherm in predicting column capacity depends on factors, such as the hetero- geneity of the aqueous organics and the extent of biological growth in the column. Pilot columns will give the most accurate estimate of reactivation fre- quency. Based on laboratory work by the authors of this commentary, the CEs of carbons A and B were calculated using the first cost equation in the Sylvia, Bancroft, and Miller article. The CE for carbon A was 2.7 X 10" 3 m3 per dollar (0.096 cu ft per dollar), while that for carbon B was 2.0 X 10~3 m3 per dollar (0.072 cu ft per dollar). Based on the CE criterion, carbon A is slightly better and should be used in the plant. Carbon B has 8.3 times less capacity (volume basis) as indicated by the isotherm, however. Although carbon A is still superior, the wide difference between relative CE values and relative capacities indicates the possibility that a carbon may have a higher CE but a lower capacity than a comparable carbon. In addition to cost per unit volume and reactivation fre- quency, other factors important in deter- mining the relative cost of using partic- ular carbons are the loss of carbon on backwashing or regeneration and the loss of capacity during regeneration. Sylvia, Bancroft, and Miller suggest the use of an abrasion number to account for regeneration losses, but apparently pre- vious efforts to do this have not been

successful.8 An additional complication arises if

fluorescence mistakenly is used to mea- sure trace organics when important nonfluorescent compounds, such as the odorous 2-methylisoborneol (MIB), are present. One commercially available large pore carbon has a high capacity for one type of humic substance but a rela- tively low capacity for MIB; the reverse is true for a second carbon with smaller pores but with higher surface area. If both the humic acid and MIB are present, use of fluorescence to determine the best carbon by the Sylvia-Bancroft-Miller procedure would lead to selection of the large pore carbon. If the carbon subse- quently were used to remove odor and threshold-odor measurement were used to determine when reactivation or replacement was necessary, the smaller pore carbon likely would yield the lowest overall cost.

In summary, several aspects of the simple procedure presented by Sylvia, Bancroft, and Miller require further test- ing. Development of reliable small scale tests for determining the best carbon for a particular application is a worthy goal, however, and research on this topic should be continued.

References 1. Sontheimer, H. Considerations on the

Optimization of Activated Carbon Use in Waterworks, Translation of Reports on Special Problems of Water Technology , 9- Adsorption. EPA-600/9-76-030. EPA, Washington, D.C. (Dec. 1976). p. 208.

2. Federal Register (Dec. 24, 1975). Reprinted in Jour. AWWA, 68:2:57 (Feb. 1976).

3. Herzing, D. et al. Activated Carbon Adsorption of the Odorous Compounds 2-Methylisoborneol and Geosmin. Jour. AWWA, 69:4:223 (Apr. 1977).

4. Schnitzer, M. & Khan, S.U. Humic Substances in the Environment. Marcel Dekker, Inc., New York, N.Y. (1972).

5. Rook, J.J. Haloforms in Drinking Water. Jour. AWWA, 68:3:168 (Mar. 1976).

6. Culp, G.L. & Culp, R.L. New Concepts in Water Purification. Van Nostrand Rein- hold Co., New York, N.Y. (1974).

7. Clark, R.M. et al. The Cost or Removing Chloroform and Other Trihalomethanes From Drinking Water Supplies, Interim Treatment Guide for the Control of Chlo- roform and Other Trihalomethanes. EPA, Munic. Envir. Res. Lab., Cincinnati, Ohio (Jun. 1976).

8. Process Design Manual for Carbon Ad- sorption. EPA 625/l-71-002a. EPA, Washington, D.C. (Oct. 1973).

A paper contributed to and selected by the JOURNAL, authored by W.E. Thacker (Stu- dent Member, AWWA) and V.L. Snoeyink (Active Member, AWWA) prof., dept. of civ. engrg., Univ. of Illinois, Urbana, III.

67674 4140

Gravity Thickening of Water-Treatment-

Plant Sludges

George Gebauer and Ingolf Janerus

The following comments are in response to the article "Gravity Thickening of Water-Treatment-Plant Sludges" by Peter Kos, which appeared in the May 1977 issue of the JOURNAL.

Peter Kos illustrated very clearly that for many sludges the classic solids-flux theory can not be used accurately to predict an underflow concentration from a real thickener in which a compression zone is established. Instead he intro- duced concepts from filtration and consolidation theories to describe the thickening of "non-ideal" sludges. He applied his developed model to a number of laboratory tests on alum sludge from a treatment plant and concluded, "the recommended operating conditions for water-treatment-plant alum sludge with powdered activated carbon are

Suspended solids loading: 15-24 kg/ m2/hr (3.0-5.0 lb/ft2/day); Depth of the tank: 2.4 m (8 ft) [1.5 m (5 ft) thickening zone]; Resulting concentration of thick-

ened sludge: 2.5-3.0 per cent." The trend is for water-treatment

sludges to be dewatered and land filled rather than returned to the source. Conventionally produced sludges are difficult to thicken and dewater, how- ever, and the capital and operating costs for that aspect of sludge processing are often higher than for the flocculat- ing-settling operation. For the type of sludge Kos investigated, other mech- anisms, if properly utilized during gravi- ty thickening, can yield not only three- to fivefold higher sludge concentrations but also corresponding increases in sludge- filtration rates and decreases in the amounts of sludge conditioner needed, all compared to the conventional tech- nique described by Kos.

Gravity Settling-Thickening Aluminum hydroxide as well as other

metal hydroxides are very voluminous in water suspensions. The "fluffy" floes consist of a multitude of unordered, mostly needle-shaped crystals which provide a vast surface area on which water is adsorbed. The adsorbed water gives the resulting particle a low specific gravity and prevents close packing. A decrease in surface area (e.g., by increased particle size) will decrease the amount of adsorbed water, resulting in a particle with higher specific gravity. A bed of such particles would of course have compression and filtration proper- ties quite other than those of conven- tional sludge. As sludge conditioning entails changing the surface properties of the particles (e.g., as measured by the change in zeta-potential), it is also obvious that a decrease in surface area will decrease the amount of conditioner needed.

JANUARY 1978 0003-1 50x / 78/ 01 00-0047$01 .00 G. GEBAUER & I. JANERUS 47 ©1 978 American Water Works Association

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Plate settler-thickener is said to produce better sludge concentration and faster filtra- tion rates.

A

THICKENER'SCRAPER DRlVe-^^^ FIXED BRIDGE^. 1 ~

*41 / ' 1 1 <^' ' o> INFLUENT / ' ( ' _L LFFLUI Nr

FLOCCULATION PLATE (OPTIONAL) J U ^P" D PACKS

PICKET-FENCE THICKENER-""^

^^^^^mDERr-LOW SLUDGE SCRAPERS SLUDGE

Modifying the structure of the metal- hydroxide floes to a form with less surface area would thus change the thickening and dewatering properties. Pilot tests as well as full-size installa- tions on surface-water treatment with ferric chloride as the coagulant have shown that thickening by a gravity set- tler-thickener* supplies a mechanism for such modification. It has not been deter- mined whether the surface area reduc- tion is caused by ordering and close packing of the needle-shaped crystals or by some other mode of particle-size growth.

The gravity settling-thickening unit combines in one tank an upper inclined- plate settler and a lower picket-fence sludge thickener. The inclined-plate settler-picket-fence thickener substitutes more than adequately for a conventional clarifier-thickener combination. The conditions necessary to provide the particle-modification mechanism are

1. An inclined-plate settler that pro- vides stagnant conditions below the plates. It also gives an almost ideal distri- bution of solids across the cross section of the thickener compartment, preclud- ing any density currents.

2. A picket-fence sludge thickener to work the sludge mechanically. The freshly precipitated and settled-out floes slide down the plates and are discharged

•Lamella Gravity Settler-Thickener (LGST), Parkson Corp., Fort Lauderdale, Fla.

48 NOTES AND COMMENTS

(the same amount from each plate) into the sludge compartment where they settle through stagnant water until they come to rest on top of the sludge bed. Thus the floes are not subject to the shear forces and turbulence of a transfer (by pump) from a conventional clarifier to a thickener. At the top of the sludge bed, the floes are worked on by the picket-fence mechanism. The floes are destroyed, and the work of the picket- fence thickener will, with time, change the floe structure towards a more ordered form with less surface area, resulting in higher sludge concentrations and improved filtration properties. No reflocculation will take place in the sludge bed since stagnant conditions prevail.

Experience with Alum Sludge As previous experience had been

almost exclusively with ferric chloride as a coagulant, a pilot test using alum was performed Oct. -Nov. 1976 on Missis- sippi River water. The pilot unit used is almost identical with the smallest com- mercial unit. The raw-water quality varied during the test. Kos used a sludge generated from settling a low-turbidity water dosed with 10.5 mg/1 powdered activated carbon and 36 mg/1 alum. The results reported here are therefore those generated under similar conditions. For two weeks the raw water contained approximately 10 mg/1 turbidity, with an alum dosage of between 35 and 45 mg/1.

The sludge concentration from the gravi- ty settler-thickener was 8-9 per cent versus the approximately 3 per cent by conventional thickening reported by Kos.

Given the same conditions, an even higher sludge concentration than 8-9 per cent is expected in a full-size unit, as the pilot unit never reached the limit of its capacity before the tests were discontin- ued. No dewatering tests were made with this sludge but earlier tests on ferric- hydroxide sludge (sludge concentrations of 8-20 per cent were achieved from the gravity settler-thickener) showed filtra- tion rates at least three times as high as for conventionally produced sludge, and the lime consumed for conditioning was one-third or less- e.g., a filtration rate of 45-50 l/m'/hr (approximately 1.2 gal/sq ft/hr) was obtained for a sludge of 18 per cent.)

Although the mechanism producing the improvements in thickening and dewatering properties is not fully under- stood, its result is considerable savings in the cost of dewatering alum and ferric sludges from water-treatment plants. The first commercial-size (2500 gpm) gravity settler-thickener using alum as coagulant will start up in 1978.

A paper contributed to and selected by the JOURNAL, authored by George Gebauer, vice pres., and Ingolf Janerus, tech. mgr., Parkson Corp., Fort Lauderdale, Fla. 53875 4201

JOURNAL AWWA

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