~

Typical sludge feed and dried product from reducing system.

After mechanical dewutering, radiant dryer further reduces sludge volume.

he need to reduce the volume of metal hydroxide sludge disposed from finishing operations is increasing as greater burdens are placed on the industry regarding the economics of disposal,

legislation mandating a reduction of hazardous wastes, and potential liabilities for landfill cleanup As metal finishers continue to install wastewater treatment systems to comply with regulations on effluent quality, additional requirements are being imposed to assure safe disposal of the resulting sludge

Gravity settling and thickening of metal hydroxide sludge is the first and most cost-effective technique for solids concentration This step produces a sludge with, typically, 1 to 2 percent by wt solids The dilute sludge then can be concentrated through mechanical dewatering devices with varying effectiveness Table 1 lists typical volume reductions

achievable with these devices After mechanical dewatering, the sludge will still consist Of

70 to 90 percent water While further reduction of the sludge volume can be achieved by drying, improvements in the dryness of the sludge cake through better mechanical means should al?vays be considered, as well The value of such improvements is sometimes overlooked when attention IS focused on the parameter "percent solids " Viewed In

terms of volume of water per pound of sludge solids however, the value of small increases in "percent solids"0f wet sludge becomes more apparent

As shown in Fig 1, a sludge containing 10 percent by fl solids, typical of lower-performance mechanical dewatering equipment, contains 9 Ib of water per Ib of solids Higher. performance mechanical dewatering can produce sludge cakes with 25 percent by wt solids, an increase Of l 5

Table 1 Effectiveness of Mechanical Dewatering Devices

On Metal Hydroxide Sludges

Device Sludge volume reduction Typical Oh solids Sludge bags 5: 1 5-8 Centrifuges 8: 1 8-1 4 Vacuum filters 12:l 12-20 Filter presses 20: 1 ' 20-40

Iercentage 11 :ontains on11 If 6 Ib of wai

The otherr 2xamining tP Joints in dryri emoval of c ndicate thai Iercent soli(( he lower thii he dried slui

Eff I cien t m

Y

$ 4

a 2

n z

0

I

ig 1-Simple? itaph shows ctually means

7 2-Hydro, -

;till consist of 3f the sludge nents in the mechanical

The value of ien attention . I ’ Viewed in Jdge solids, mt solids” of

ercent by wl 11 dewatering lids. Higher- juce sludge rease of 15

S

,ercentage points But the sludge at 25 percent by wt solids onlains only 3 Ib of water per Ib of sludge solids, a reduction

,f 6 Ib of water by mechanical means The other end Of the curve in Fig 1 is of interest when

)xaminlng thermal reduction An increase of 15 percentage ,olnts in dryness, from 80 to 95 percent solids, results in the emoval of Only 0 2 Ib of water per Ib of solids This would Idlcate that complete drying of the sludge to near 100 ,ercent sollds may not be of significant benefit because of l e lower thermal efficiency and dusting characteristics of l e dried sludge Efficient mechanical dewatering IS therefore a warranted

8 I 6 Ibs water / Ib solids

\

i \ 2.15 Ibs wo’tt.r/ Ib solids

Ib solids

0 20 10 60 80 100 1

PERCENl SOLIDS BY WElGWl

9 1-Simple relationship of sludge water content to percent solids raph shows that a small increase in percent solids of wet sludge rually means a substantial volume of water has been removed

step in the overall process of reducing sludge volume. However, in dealing with metal finishing hydroxide sludges, efficient m ec ha n i ca I dewater i ng pres en t s a sign if ican t challenge to the subsequent process of thermal reduction. The challenge results from the physical characteristics of well-dewatered hydroxide sludge cake.

Hydroxide sludge cakes discharged from filter presses (Fig. 2.) may consist of perhaps 65 to 80’ percent water yet still exhibit substantial structural strength. Usually, the dis- charged cake will break into pieces of various sizes-from granules to large slabs. The majority of the water is mechanically bound inside the sludge.

The small volume of surface moisture can be dried easily by any of a variety of methods. However, once the surface moisture is removed, bound water from within the cake must diffuse to the surface of the particles in order to be removed, Diffusion of bound water through slabs of filter-press cake is slow and inefficient. Unless sufficient water is removed, the sludge will retain its structural integrity, and little volume reduction will result.

Breaking the cake slabs into small pieces increases the wetted surface area, but the nature of the hydroxide cake defeats simple attempts to do this. The reason: As shearing is begun, the thixotropic sludge cake “liquifies” into a heavy, highly viscous paste. Continuous.dividing of the paste is required to expose wet surfaces. The division must be done uniformly throughout the mass of the sludge cake; otherwise, a portion will become paste-like and a portion unchanged.

Direct Vs. Indirect Dryers Methods of drying can be reduced to two alternatives: (1 ) direct drying, where heat is applied directly to the sludge by radiation or convection, and (2) indirect drying, where the

9.2-Hydroxide sludge cake being discharged from filter press.

IL 1985 21

1 2

WAVELENGTH(pm)

g. 3-Water absorbs long-wave infrared very well. Absorption axima are approximately 3 and 6 pm.

3 4-Distribution of radiant energy vs wavelength

ontainer holding the sludge is heated and transfers heat to l e sludge by conduction The medium for an indirect dryer is typically an externally

eated fluid, which is circulated through a jacket around the ontainer Very high wall temperatures can be achieved in lis manner while moderate exhaust vapor temperatures are till maintained Indirect dryers rely on intimate contact etween the sludge cake and the heat-transferring container all The paste-like nature of hydroxide cakes can lead to a nl ing of heat-transfer surfaces and a related reduction in fficiency In addition, the capital cost of I)roI)erlv desianed

indirect dryers is often prohibitive for the reduction of waslc sludge.

In a convective, direct dryer, air is heated and introduce: into the container and is exhausted along with releasee water vapor. A heat-transfer jacket and the possibility c; fouling are eliminated, but are traded for higher exhaust gas temperatures and the need to capture entrained dust as the sludge becomes dry.

A radiant direct dryer eliminates problems associated w ! y heat-transfer jackets and particle entrainment. In a radiar, dryer, infrared energy is directed at the surface moisturear the sludge paste. The heat source can be "tuned" to e m ' infrared energy in the spectral band best absorbed by th surface moisture. Figure 3 shows that water absorption r ' infrared energy has peak values at wavelengths of approx- imately 3 and 6 pm. The energy distribution from a rac'ia:. heat source under ideal, black-body conditions is shown I -

Fig. 4. It can be seen, then, that a radiant source temperatuit between 1200 and 1400" F will provide peak radiatior matching the peak absorption of water.

Radiant heat is also highly efficient in terms of speed. Thi heat-transfer rates of indirect, conduction dryers vary direcli, with temperature difference-Le., doubling the temperatuft of the heating medium doubles the rate of heat transfer. The heat output of a radiant source, however,'varies as the fourY power of the absolute temperature so that the heat outpL' can be doubled by increasing the heater temperature by or'. 19 percent. Consideration of capital costs, simplicity :' equipment, and ease of operation led to the development of 2

direct, radiant dryer.

Dryer Design The direct, radiant dryer* was developed specifically ' mechanically dewatered hydroxide sludges The unit (Fig 2

COnslsts Of a horizontal drying chamber fitted with I N counter-rotating agitator shafts The heat source is a bankc' infrared heaters contained in the chamber lid

In Operation, sludge cake is loaded into the chamber anc COntinUOUSly sheared during the drying cycle The agitatc' overcomes the limitations of more conventional designs ,. that sludge IS not pressed against the container wall in or& to effect shearing Rather, the rotating shafts, which coni? plowshare-like mixing ~OOIS, create a zone of interacli?- between agitators such that the velocity gradient through'' sludge mass is radially inverted The radial inversion crealc' a new flow path for a portion of the sludge, accelerati previous low-velocity particles and decelerating hi@ velocity particles The result is a uniformly applied shear' action and continuous exposure of surface moisture to '- heat source

The use of a radiant source provides a number of bene' for this application Radiant energy is absorbed by the SIUC-

mass and not by surrounding air, allowing for highly efflci. drying As the sludge dries, it breaks into a mass of srd individual granules that are continuously thrown by agitators into the free space within the dryer Radiant hi reaches these individual particles as they move through' free space A very large surface area therefore is expose: drying

Design aspects for materials handling have been 9'4 substantial attention in the development of the reducer unit is designed for installation at any convenient localiL

'J-Mate'" JWI Inc Holland Mi

PLATING AND SURFACE FIN'' 22

-

: 5 Drying chi

- and introduc 1 with relea5 re posslbilit, ier exhaust c: ned dust as 1.

associated ihi I

?nt In a radld Ice moisturr: 'tuned" to I

bsorbed by t t

'r absorptior) gths of apprc, 1 from a radld. sns IS shown ce tempera! J r ,

peak railiatlL1

s of speed Tb t ers vary direLt , he temperature. ,at transfer The ies as the fourt. :he heat outp,,; serature by on1 s, simplicity of 3velopment of a

specifically for The unit (Fig 5 fitted with tM

irce IS a bank d e chamber an le The agitatrr onal designs ier wall in ordt ,, which conta,' ? of interactlor lent through tht version create\ e, acceleratinc. erating high[ pplied shearir moisture to 1t

nber of benetit> ?d by the sludgt ' highly efficie, ' mass of sma

thrown, by thi 'r Radiant hCd' ive through lht re is exposed 1~

ive been give' ie reducer. T h mient location

SURFACE FINIS"

1 +Drying chamber wlth counter-rotating shafts

r

5 1

I I I I I I 4 5 6 7 8 9 I O

DRYING T I M E IN HOURS

. 1 6-Influence of sludge solids on drying time.

l e transport of dewatered sludge cake can present tech- -a1 and economic problems if not properly addressed 'uvatered sludge cannot be transported by screw con- gors and, especially with filter-press cake, the sludge is too r~ to pump economically While belt conveyors can be

\ r d there often is not sufficient floorspace adjacent to the vdtering equipment tor the addition of a conveyor and the

: u m 1 iypical installations the filter press discharges into

'?e l 55-gal drums or a dumpster Either container IS then wed toa shipping area, where the dumpsters are emptied '0 bulk material containers or where drums are sealed The *'a system however features automatic d u m m q mecha-

nismsfor barrels or carts The operator merely transports the containers to the reducer, which automatically dumps the contents of the containers into the drying hopper Processing time, which will vary with the initial moisture level of the sludge, is set by a timer At the completion of the timed cycle, the unit unloads the dry granular powder from the reducer into a receiving container

Perfomance Notes Extensive tests have been run with the reducer to determine the drying characteristics of a variety of metal hydroxide sludges. The test samples (in 55-gal drums) included sludges from a number of metal finishing processes using either caustic or lime for pH control. Test sludges were produced from sludge bags, centrifuges and filter presses.

Solids concentration of the feed samples varied not only by the type of mechanical dewatering process but by the operating efficiency of individual waste-treatment systems. Sludges from systems ernijloying lime showed a higher concentration of feed solids, of course, than did comparable sludges from caustic-controlled systems. Overall sludge solids ranged from 9 percent (sludge bag) to 41 percent (filter press) using a lime-neutralized system.

Drying efficiency, in terms of Ib of water removed per hr, is fairly independent of either pH control techniques (lime or caustic), type of metal hydroxide or sludge feed solids. Tests showed that drying time (Fig. 6) is primarily a function of the volume of water to be removed and not the physical characteristics of the sludge. That is, the plowshare-like mixing tools provide sufficient shearing and division of sludge to keep surface moisture exposed to the heaters. Volume reductions varied from 3 to 1 to almost 7 to 1 for the very wet sludge. Assuming an energy cost of 7 cents/ kw-hr, the tests showed that a 55-gal drum of filter-press cake at about 25 percent solids can be dried for a cost of about $5. An example of typical sludge feed and dried product from the reducer system is shown in the introductory photo to this article.

. '985 23

175 \ - - - 1 y . Payout - 1 y, h Y 0 " t

I I I I I ' 1 2 I 1 3 4

SLUDGI DISPOSAL CUBIC YARDS/ DAV

g. 7-Payout analysis for sludge reducer.

e Table 2 : ' Payout Analysis for Sludge Reducer

=est Ancrlpsis 'he justification for adding supplementary equipment for ludge reduction should include a consideration of regula- sry, legal and financial matters Recent RCRA legislation ?quires that generators of hazardous waste take steps to Pinimize waste disposal Reducing the volume of sludge ent to landfill also can be of benefit in reducing the potential ability associated with cleanup However, even without uch external motivating factors for sludge reduction, the sducer system can be justified strictly on an economic asis One method of economic analysis consists of calculating

i e time over which operating savings and tax credits will qual the required investment-that is, the payout period As n example, consider a metal finisher with the following ituation

Dewatering method: Filter press Cake solids: 20 percent Sludge generation: 1 yd'/day Operation: 250 days/year Current disposal cost: $1 800/1 6-yd3 rollaway

dumpster

he current annual disposal cost, which includes trans- ortation and taxes, is:

1 yd x 250 days x $1 aoo = $28,125 day vear 16 yd' ---

Now assume that this sludge can be reduced to one-fourth its current volume at a cost of $20/yd3 and that the capital cost of the thermal reduction unit is $50,000. The net operating saving is:

(% x $28,125) - ($20/yd' x 250 yd3/year) = $1 6,094

The first-year investment is equal to the equipment cost, less depreciation and investment tax credit The investment tax credit is 10 percent of the equipment price, or, in this case, $5000 However, as a tax 'credit" (as opposed to a tax deduction), it represents a direct reduction in taxes The equivalent value of the investment tax credit, viewed as a deduction, is

$5ooo = $9259 (assuming a 46% tax bracket) 1-46%

In other words, it would take $9259 of pre-tax income to produce the same after-tax benefit as the $5000 investment tax credit. The equipment price is reduced by half of the investment tax credit for purposes of calculating depre- ciation. First-year depreciation is 15 percent of the modified price, or 0.15 x 0.95 x $50,000 = $7125. Depreciation percentage the second year is 22 percent. The analysis is summarized in Table 2.

The payout period, the time at which net cash flow iszero, is 17 months. An approximation of payout times for various conditions of disposal costs and volumes of sludge to be disposed is shown in Fig. 7. A specific analysis should be performed for each application, of course.

To summarize, the sludge reducer provides an eco- nomical tool for further reduction in the generation of hazardous metal hydroxide wastes. It is a process that can be added to existing waste-treatment operations with minimal impact on the facility or on labor requirernents. 0

Picciotti Lucas

About the Authors Gene Picciotti is vice president, corporate development, for JWI, Inc., 2155 112th Ave., Holland, MI 49423. He is responsible for new product acquisitions and development Prior to joining the company, he accumulated 12 years Of

experience in managing product lines for water resourCeS and pollution control. Mr. Picciotti earned an MS degree In

mechanical engineering and is a registered professional engineer.

David Lucas is vice president, marketing, at JWI. He has held positions the past 14 years in sales and marketing management for equipment for water and wastewater treatment. He holds a BS degree in mechanical engineering from the University of Michigan. Mr. Lucas is a member ofthe AES Grand Rapids Branch and serves on the Environmental Committee of the Metal Finishina Sumliers' Association.

PLATING AND SURFACE FINISH^^

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24

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