CRREL REPORT 96-15

A Device for Mechanical

Freeze–Thaw Conditioning

of Alum Sludge

C. James Martel, Rosa T. Affleck and Melinda L. Yushak December 1996
Abstract: This report contains the results of a study to

develop a mechanical device for dewatering alum

sludge by freeze–thaw. This proposed device is a combination of two conventional unit operations: a vacuum

filter and a blast freezer. Bench-scale studies were conducted to evaluate this concept and develop preliminary

design criteria. The results of filter leaf tests indicate that

a suitable sludge layer could be collected on a cloth

medium at a vacuum level of only 100 mm of Hg and a

5.0-minute filtration time. The volume of sludge was

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reduced by 67%. The freezing tests indicated that low

freezing rate and a high initial solids content had a

tendency to produce large alum sludge particles.

However, fast freezing rates could be achieved without

reducing the effective grain size below that of a fine

sand. Curing time had no effect on grain size. The

electrical cost of freezing sludge with this device was

estimated to be $0.004/m3, which is not expensive in

relation to the total cost of water treatment which is

approximately $0.25 to $0.50/m3.

Cover: Cross-polarized photo of thin section of frozen alum sludge showing separation of solid particles and ice

crystals.
CRREL Report 96-15

A Device for Mechanical

Freeze–Thaw Conditioning

of Alum Sludge

C. James Martel, Rosa T. Affleck and Melinda L. Yushak December 1996

Prepared for

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared by C. James Martel, Environmental Engineer, Rosa T. Affleck,

Engineering Technician, and Melinda L. Yushak, Engineering Aid, Civil and Geotechnical

Engineering Research Division, Research and Engineering Directorate, U.S. Army Cold

Regions Research and Engineering Laboratory. This study was funded by Project

4A762784AT42, Cold Regions Engineering Technology, Work Package 211, Installation Management in Cold Regions, Work Unit CO-M20, Innovative Methods for Wastewater Treatment

and Sludge.

Dr. P. Aarne Vesilind of Duke University and James R. DeWolfe of Gannett Fleming

Engineers and Planners conducted a thorough technical review of this report.

Several people worked on this project over the last few years. Donna Thrasher and Beth

Nadeau conducted most of the filter leaf tests. John Bayer developed a method to concentrate the sludge without using polymers or drying the sludge. Rosa Affleck conducted the

grain size analysis and developed improved techniques for freezing, thawing and draining the sludge layers to enhance the reproducibility of the analysis. Melinda Yushak conducted most of the freezing tests and recorded the data.

The contents of this report are not to be used for advertising or promotional purposes.

Citation of brand names does not constitute an official endorsement or approval of the use

of such commercial products.

ii
iii

CONTENTS

Preface ..................................................................................................................................... ii

Introduction ............................................................................................................................ 1

Background ........................................................................................................................ 1

Objectives ........................................................................................................................... 2

Scope ................................................................................................................................... 2

History of mechanical freeze–thaw conditioning ............................................................. 2

Literature review ............................................................................................................... 2

Conclusions from literature review ................................................................................ 4

Proposed freeze separator concept ..................................................................................... 5

Materials and methods ......................................................................................................... 5

Filter leaf tests .................................................................................................................... 6

Freezing tests ..................................................................................................................... 6

Results ..................................................................................................................................... 9

Filter leaf tests .................................................................................................................... 9

Freezing test ....................................................................................................................... 11

Discussion ............................................................................................................................... 13

Conclusions ............................................................................................................................ 14

Literature cited ....................................................................................................................... 14

Appendix A: Filter leaf teat data for Lebanon, New Hampshire, water plant

sludge ................................................................................................................................. 17

Appendix B: Determination of convection coefficient for coldroom 165 ...................... 19

Appendix C: Freezing test data for six-millimeter layer of alum sludge ...................... 21

Abstract ................................................................................................................................... 23

ILLUSTRATIONS

Figure

1. Sample of freeze–thaw-conditioned alum sludge on a 1.0-mm square grid...... 1

2. Conceptual sketches of three sludge freezing devices .......................................... 2

3. Conceptual sketch of freeze separator ..................................................................... 5

4. Filter leaf test apparatus............................................................................................. 6

5. Size distribution curve for grains produced by freeze–thaw-conditioned alum

containing 6% total solids ................................................................................ 7

6. Nest of sieves used to concentrate alum sludge to 6% and 12% total solids ..... 7

7. Placing a tray containing alum sludge in a coldroom ........................................... 8

8. Tray of frozen slum sludge ........................................................................................ 9

9. Effect of vacuum level on the volume of filtrate produced by cloth 515 after

a 2.0-minute filtration time .............................................................................. 10

10. Effect of filtration time on the volume of filtrate produced at 100-mm Hg

vacuum ............................................................................................................... 10

11. Effect of filtration time on the weight of the sludge layer accumulated on

cloth 2019 at 100 mm Hg vacuum in alum sludge containing 1.85%

total solids .......................................................................................................... 11

12. Cloth 2019 after removal of sludge layer................................................................. 11
iv

Figure

13. Effect of repeated use of cloth 2019 at 100-mm Hg vacuum and a 5.0-minute

filtration time in alum sludge containing 1.0% solids ................................. 12

14. Effect of freezing rate of effective grain size of freeze–thaw-conditioned

alum sludge ........................................................................................................ 12

15. Effect of freezing rate on uniformity coefficient ..................................................... 12

TABLES

Table

1. Average turbidity of filtrate from selected filter cloths ......................................... 9

2. Data on effect of curing time on effective grain size and uniformity

coefficient of freeze–thaw contaminated alum sludge containing

2–3% solids ......................................................................................................... 13
INTRODUCTION

Background

Disposal of alum sludge can be a major concern for water treatment plants, including those

at military installations. The traditional practice

of discharging the sludge directly into a nearby

stream is becoming less acceptable because these

discharges can violate state stream standards.

Discharging the sludge into a sewer line is often

used as an alternative. However, many plants are

unable to use this alternative because they are not

located near a sewer line or the treatment plant is

unable to accept their discharge.

Because of the restrictions to stream and sewer

discharges, many water treatment plants have

turned to landfilling or land application as disposal methods. Most landfill regulations require

a minimum of 20% solids and no evidence of free

flowing liquid. Volume reduction prior to land

application is often necessary to reduce transportation costs. Thickened alum sludge produced at

the plant typically contains only 2% solids (98%

water). Therefore, some method of dewatering is

needed before alum sludge can be placed in a

landfill or land applied.

Alum sludge is widely recognized as one of

the most difficult sludges to dewater. About 40%

of the water is chemically bound to the particles,

which is difficult to remove by conventional

methods such as vacuum filters, belt presses or

centrifuges. With the aid of expensive polymers,

these processes may be able to increase the total

solids content up to 20%. However, this is not

usually the case, and often the sludge is not dry

enough to meet landfill requirements.

In contrast to conventional dewatering methods, freeze–thaw conditioning followed by gravity filtration can easily dewater this sludge without polymers. The process of freeze–thaw conditioning changes the sludge from a suspension of

A Device for Mechanical Freeze–Thaw Conditioning of Alum Sludge

C. JAMES MARTEL, ROSA T. AFFLECK AND MELINDA L. YUSHAK

Figure 1. Sample of freeze–thaw-conditioned alum sludge on a 1.0-

mm-square grid.
small particles to a granular material that resembles coffee grounds (see Fig. 1). These grains do not

dissolve or suspend upon rewetting, even with

vigorous agitation. This granular material can be

easily handled and transported with conventional

earth moving equipment. No other sludge conditioning or dewatering process can produce this

dramatic transformation.

Because freeze–thaw conditioned sludge is in a

granular form, land application becomes much

more practical and economical. In areas where

landfill costs are high, the land application option

offers a significant savings in disposal costs. New

Jersey American Water Company estimated that it

could avoid a $110/ton tipping fee if land application was possible (Brown et al. 1993). However, as

pointed out by DeWolfe and Dempsey (1992), care

must be taken to control a potential phosphorus

deficiency in soils, which may impact plant

growth.

The mechanism responsible for converting

alum sludge into granular particles is the ice crystal formation process. It is well known that ice

crystals form from water molecules only. All other

substances including alum floc particles are

pushed away from the growing ice crystal. This

forces the floc particles to become consolidated at

the boundaries between ice crystals. After freezing

is complete, the sludge is no longer a suspension of

fine floc particles but a matrix of ice crystals and

consolidated floc particles or grains. When the ice

crystals thaw, the grains remain consolidated and

do not redissolve. These grains are large enough to

easily settle by gravity in the clear meltwater. The

sludge is dewatered by decanting or draining the

meltwater.

In cold climates, freeze–thaw conditioning is

easily accomplished during the winter months in

outdoor freezing beds (Martel and Diener 1991).

However, this technology is not usable at water

treatment plants located in warm climates or at

plants without the available land area. For these situations, a mechanical freezing device is needed. This

report is the result of several years of study and

experimentation in developing such a device.

Objectives

The objectives of this study are:

1. To develop a conceptual mechanical freeze–

thaw conditioning device based on lessons

learned from previous attempts.

2. To develop preliminary design criteria for the

new device.

2

Scope

Objective 1 was accomplished by thoroughly

reviewing the literature and evaluating past

attempts at building a device. The main part of this

study was devoted to developing the preliminary

design criteria (Objective 2). These criteria were

developed by bench scale studies conducted at

CRREL in Hanover, New Hampshire.

HISTORY OF MECHANICAL

FREEZE–THAW CONDITIONING

Literature review

Generally, mechanical sludge freezing devices

can be categorized into bulk freezers, freeze crystallizers and layer freezers. A conceptual sketch of

each freezer is shown in Figure 2.

Bulk freezers

The bulk freezer was the first mechanical device

used to freeze sludge. It consists of a large container

with refrigeration coils. Sludge is pumped into the

tank where it remains until completely frozen.

Thawing is also accomplished in the same container by reversing the refrigeration cycle. The meltwater is drained out of the container, leaving the granular sludge particles at the bottom. The container is

then drained and emptied and the process is repeated. This method was used for several years at

the Stocks Filtration Plant in Yorkshire, England

(Benn and Doe 1969). It worked very well except

that the tanks had a tendency to rupture after relatively few cycles due to the expansion force of ice

(Farrell 1971).

A recent report by AWWA (1990) indicates that

there are a few operational bulk freeze–thaw plants

in Germany. The Wuppertal Municipal Works in

West Germany reportedly uses a bulk freezing device similar to that described in Doe et al. (1969).

Alum sludge containing approximately 2% solids

is pumped into one of two sequentially operated

freezing tanks. However, to avoid damaging the

tank by the expansion of ice, only 40% of the tank

contents are frozen.

Kawamura and Trussell (1991) report the use of

freeze–thaw conditioning at the Kashiwai Water

Purification Plant near Tokyo, Japan. Freeze–thaw

is used because sludge from this plant is difficult to

dewater by conventional methods. The sludge is

gravity thickened and centrifuged prior to freezing.

Twenty-five tanks are used to freeze and thaw the

sludge. After thawing is complete, it is filtered by

vertical belt presses to achieve 40–45% solids.
Crystallizers

Instead of freezing sludge into a solid mass,

sludge can be frozen into a slurry of concentrated

solids and pure ice crystals. This is done by mixing the sludge at a temperature below the freezing point. The slurry of ice crystals and solids is

then sent to a separator where the ice crystals are

washed with a portion of the melted ice crystals.

This process is called freeze crystallization or

freeze concentration. It has been used commercially for several years in concentrating fruit

juices, milk, beer and other liquids for human

consumption. It is also a proposed method for

desalting seawater.

Randall et al. (1975) and Randall (1978) proposed freezing sludge by mixing it in a crystallizer with a refrigerant such as butane. They claim

that this method results in better conditioning

and supernatant quality than solid freezing. Also,

they conclude that the cost of this process is more

competitive with conventional dewatering processes because direct contact freezing is more

energy efficient than bulk freezing. However, this

process has yet to receive widespread application

because of explosion hazards and the difficulty in

recovering all the butane.

Egan and Davis (1982) constructed a pilotscale freeze crystallizer and tested its effectiveness on black liquor from the kraft pulp-manufacturing process. They choose this process over

evaporation because it is less corrosive, produces

less volatile compounds, and is potentially more

energy efficient. Their tests demonstrated that a

15% black liquor can be concentrated to approximately 30% solids. In spite of this successful demonstration they were not able to find a commercially made, large freeze crystallizer. Because of

this limitation, they concluded that the pulp and

paper industry should not proceed any further,

but instead keep abreast of new developments of

this technology.

Knocke and Trahern (1989) investigated both

bulk freezing and crystallization on conditioning

chemical and biological sludges. They found that

3

Refrigerator

Sludge

a. Bulk Freezer

Figure 2. Conceptual sketches of three sludge freezing devices.

b. Crystallizer

Refrigerator

Sludge

Condenser

Gaseous Refrigerant

(bubbles)

Gas

Liquid

c. Layer Freezer

Refrigerator

Sludge
the bulk freezing method was superior to freeze

crystallization because it consistently improved

the dewaterability of each sludge tested. In some

cases, freeze crystallization actually worsened

sludge dewaterability.

Layer freezers

As the name implies, a layer freezing device

freezes sludge in thin layers or sheets. Freezing is

usually accomplished on a refrigerated plate or

belt. After freezing is complete, the layer is removed and broken into small pieces that are then

thawed and drained. The layer freezing method

is similar to bulk freezing in that sludge is frozen

indirectly by a refrigerated surface. However, it

avoids the mechanical stresses due to freezing by

allowing expansion in the opposite direction

from the freezing front. Also, it can freeze more

sludge per unit energy input because freezing in

thin layers is more efficient. The main disadvantage is the large surface area needed, which can

translate into a large floor space requirement.

The first layer freezer for sludge was patented

by Downes and Komline (no. 2,174,873) in 1939.

Essentially it consists of a flat conveyor belt,

a freezing chamber, and a thawing chamber.

Digested sludge from a wastewater treatment

plant is applied in a thin layer to the conveyor

belt. The belt then transports the sludge into the

freezing chamber where it is frozen. The conveyor belt continues through a heated section

where the sludge is thawed and drained. The energy required for both freezing and thawing is

derived from the anaerobic digestion process. No

information was found in the literature on

whether this device was ever tried full scale.

The Milwaukee Sewerage Commission (1971)

evaluated two layer freezing methods for conditioning and dewatering waste activated sludge.

The first method was a serpentine conveyor

mounted in a cold air blast freezing unit. A series

of pans were attached to the conveyor. Sludge

was deposited in each pan before it entered the

freezing unit. The pans entered at the top of the

unit and traveled vertically downward in a serpentine fashion. This concept was not extensively

evaluated because the pan conveyor was not

commercially available and a need was anticipated for frequent defrosting. The second method involved the use of an endless steel belt that

traveled over a brine cooled freezing section. This

concept was judged to be more feasible because

the belt freezing system was commercially available, and it had been used in other similar applications. Sludge containing 4% solids would be

placed on top of the belt and frozen in 0.5-in. (1.3-

cm) sheets. Studies indicated that it took 100 minutes to freeze the 1.3 cm of sludge. The Milwaukee Sewerage Commission’s analysis concluded

that energy costs and floor space requirements

were appreciably higher for this dewatering

method than a conventional vacuum filter. As a

result, no further testing was conducted.

In 1973 and again in 1975, Carrier Corporation

obtained patents (no. 3,745,782 and 3,880,756) on

a “falling film” freezing device. This device consists of a pair of shell and tube heat exchangers

which alternately serve as refrigerant evaporators and refrigerant condensers. The sludge is

pumped to the top of the heat exchangers where

it is distributed and allowed to flow down the interior of the heat exchange tubes. As it flows

downward, the sludge is frozen in the form of

hollow cylinders by the refrigerant evaporated

on the exterior of the tubes. Sludge that is not frozen by the time it reaches the bottom of the tubes

is collected and recirculated back to the top until

the desired quantity of sludge is frozen. The

refrigeration cycle is then reversed and the

sludge is melted. The inventors claim that this

process provides improved refrigeration cycle efficiency and avoids the structural failure problems associated with bulk freezing devices.

In 1976, the United Kingdom Atomic Energy

Authority filed a patent (no. 1,459,175) for a drum

freezer whereby sludge is sprayed on a refrigerated drum and allowed to freeze. Laboratory

tests indicated that a 2.0-mm layer of alum sludge

can be frozen within 20 seconds on a stainless

steel drum cooled to –10°C. The frozen layer is

removed by a rotating scraper and transferred to

a thaw and settling tank. Supernatant is recirculated through a vapor condenser in the refrigeration circuit and the heated return flow is sprayed

over the frozen sludge in the thaw and settling

tank.

The Electric Power Research Institute (EPRI)

conducted tests on a layer freezing device consisting of a vertical freezing plate. Sludge was

sprayed on the top of the plate and allowed to

flow downward by gravity. Any sludge that was

not frozen was collected and recycled back to the

top of the plate. Brown et al. (1993) found that this

technique was only partially successful. The rejection of solids by the advancing freezing front

caused a buildup of solids in the liquid phase.

Eventually, the pump was incapacitated because

it was unable to deliver the thickened sludge to

4
the freezing plate. Also, the sludge was too thick

to flow down the freezing plate.

Conclusions from literature review

The layer freezing method appears to offer the

best chance for commercializing the freeze–thaw

conditioning process. It avoids the structural failure problem common to bulk freezers and the ice

crystal/solid particle separation problem common to freeze crystallizers. The large surface area

required by the layer freezing method is the main

obstacle to overcome.

In spite of the high cost of freezing, we believe

there is a niche for this process in conditioning

difficult sludges like alum and other hydroxide

sludges. Freeze–thaw conditioning is the only

process that can transform these sludges from a

thin pudding-like liquid into a granular material.

Dewatering this granular material is a simple

matter of decanting or filtering the meltwater. No

polymers or further mechanical processing are

needed. The granular nature of the final product

has an added benefit in that it can greatly facilitate handling for both disposal and beneficial reuse. As a result, the overall cost of this method

may be less than that for conventional methods

when disposal costs are included.

PROPOSED FREEZE SEPARATOR

CONCEPT

The proposed freeze-separator combines a

vacuum drum filter with a horizontal belt freezer.

A sketch of the proposed freeze-separator device

is shown in Figure 3. This device is patented (no.

5,202,039).

The purpose of the drum filter is to remove

most of the free water from the sludge before

freezing. This will reduce the amount of sludge to

be frozen as well as the surface area requirement.

Another important function of the drum filter is

to attach the sludge to a flat surface in a uniform

thin layer. A uniform layer is needed in order to

ensure that the sludge is completely frozen by the

time it exits the freezer.

Operation of the device begins at the vacuum

drum filter. A rotating drum, immersed in a constantly replenished vat of sludge, filters free

water through an attached cloth or metal belt. As

the drum rotates, a layer of sludge builds up on

the belt. The thickness of the sludge layer will

depend on the filtration characteristics of the

media, and the amount of vacuum applied. The

filtrate is either returned to the head of the plant

or used in the washing section. After the belt

emerges from the vat, it enters the freezing chamber. The speed of the belt is controlled so that the

sludge layer is completely frozen by the time it

exits the chamber. The frozen sludge layer is then

separated from the belt and discharged into a

collection hopper. A heated roller may be needed

to break the bond between the sludge and the

belt. The frozen sludge layer is then thawed using

the heat removed from the freezing chamber.

Meltwater produced during this thawing operation is collected and mixed with the filtrate from

the vacuum section. The remaining granular

solids are then transported to a storage area,

landfill, or land application site. Meanwhile the

belt continues on through a washing section

where any residual sludge particles are removed.

The belt then reenters the vat and the cycle is

repeated.

As mentioned earlier, the main advantage of

this design over other freezing devices is that

much of the water is removed before freezing. As

a result, the energy required will be significantly

reduced. Another advantage is that it is a contin5



































  













 

 

 











 





 



 













 Cold Air Return

Freezing Coil Sludge Cake Fan

 Insulated Freezing Chamber

Frozen


Chunks

Washing Station

Thawing


Coils

Meltwater

Sludge Collection


Hopper

Granular Solids

Coarse Screen

Vacuum


Filter

Sludge

Filter Cloth

Heated Roller



Figure 3. Conceptual sketch of freeze separator.
uous process. Both freezing and thawing can be

conducted simultaneously. Also, the freezing and

thawing process can be designed so that the heat

removed during freezing can be used for thawing.

Therefore, the cost of thawing the sludge should be

negligible.

MATERIALS AND METHODS

Studies were conducted on the two main components of the freeze separator: the vacuum filter

and the freezing chamber. The vacuum filter was

evaluated using the filter leaf test. The freezing

chamber was simulated by freezing layers of

sludge in a coldroom.

The alum sludge used in this study was obtained from the water treatment plant in Lebanon,

New Hampshire. This plant has a design flow of

15,000 m3/day. Treatment processes include rapid

mixing, flocculation, sedimentation and sand filtration. Chemical additives include potassium permanganate, alum, powdered activated carbon and

sodium bicarbonate. At present the sludge is discharged to a lagoon where the solids are left to accumulate and the water is allowed to percolate into

the ground.

For these experiments, we took the settled

sludge from the sedimentation basins. We tried to

collect the thickest portions in order to reduce the

number of containers. The total solids content of

this sludge was typically 1 to 2%. The sludge was

then transported to CRREL and stored in a coldroom at 0°C ±2°C.

The characteristics of alum sludge from the Lebanon Water Treatment Plant were examined in a previous study (Martel 1988). This study found that

the Lebanon sludge had a specific gravity of 1.005

and a volatile solids content of 52% (of total solids).

This relatively high volatile solids content is due to

colloidal humic particles typically present in New

England waters. The specific resistance to filtration before and after freeze–thaw was 4.8 × 109 and

6.0 × 108 s2/g at 38.1 cm of Hg, respectively. This

reduction in specific resistance indicates an improvement in filterability due to freeze–thaw. Capillary section time (CST) tests showed similar results.

CSTs before and after freeze–thaw were 32 and 6

seconds, respectively.

Filter leaf tests

The purpose of this test is to determine the optimum design criteria for the vacuum filter component of the freeze separator. These criteria are the

type of filter cloth, vacuum level and filtration

time. The filter leaf test apparatus was purchased

from Komline Sanderson Engineering Corporation of Peapack, New Jersey. A filter leaf is essentially a 0.1-ft2 (0.0093-m2) compartment of a vacuum filter made of polypropylene. It resembles a

flattened funnel onto which a sample of the filter

cloth is attached. The filter leaf is submerged in a

container of sludge while a vacuum is applied at

the other end. A schematic of a typical leaf test apparatus is shown in Figure 4.

Each filter leaf test proceeded as follows. We selected a filter cloth and attached it to the filter leaf

test apparatus. The filter leaf was immersed in a 6-L

container of alum sludge, which had a total solids

content ranging from 0.34% to 2.08%. The desired

vacuum was applied to the leaf and maintained

throughout the filtration period. To simulate the

circular motion of a filter cloth through a vat of

sludge, the filter leaf was gently agitated by manually moving it up and down. Upon completion of

the filtration period, the filter leaf was removed

from the container and the sludge layer was allowed to “form-up” by maintaining vacuum for

another 30 seconds. The thickness of the layer was

measured with a machinist’s ruler, and the cloth was

removed from the filter leaf. The cloth and sludge

were then weighed to obtain the weight of wet cake.

The volume of filtrate was measured with a graduated cylinder and its turbidity was measured with a

turbidimeter.

To observe the effect of freezing, the cloth and

attached sludge layer were laid in an aluminum

6

Vacuum

Gauge

Pinch

Clamp

6 Liter

Sample

1 Liter

Graduate

Vacuum

Reservoir

Vacuum

Pump Filter

Leaf

Filter

Cloth

Figure 4. Filter leaf test apparatus.
pie plate, and placed in the coldroom at –20°C.

When freezing was complete, the sludge layer

was separated from the filter cloth by heating the

underside of the cloth for a few seconds with a

warm laboratory hot plate. The sludge layer was

then thawed at room temperature. After thawing

was complete, the meltwater was drained off and

the remaining solids were weighed to determine

a drained solids content. These solids were then

dried in an 105°C oven for one hour to obtain the

weight of dry cake. This completed a test. Altogether, 107 filter leaf tests were conducted in this

study. Data obtained from these tests can be

found in Appendix A.

Freezing tests

The purpose of the freezing test is to evaluate

the effect of freezing rate, curing time and initial

solids content on the quality of the product in

terms of the effective grain size and uniformity

coefficient. The freezing rate was varied by freezing the same quantity of sludge at four temperatures: –5°, –14°, –23°, and –30°C. The effect of curing time was determined by leaving the sludge in

a coldroom for 1 and 24 hours after freezing was

complete. The effect of initial solids content was

tested by freezing sludge at three different total

solids contents: 2%, 6%, and 12%. A 2% solids

content was chosen because it represents a typical

alum sludge after gravity thickening. A 6% solids

content was chosen as representative of a typical

alum sludge after vacuum filtration. A 12% solids

content was chosen to simulate a typical alum

sludge after dewatering with a conventional belt

press.

The effective grain size (D10) is defined as the

particle size corresponding to the 10% passing

size from the grain size curve that is determined

by a sieve analysis (see Fig. 5). It is well known

that the permeability of soil or in this case, a granular sludge, will be controlled by this grain size.

Since the meltwater must be drained after thawing, it is important that the final product have an

effective grain size that is highly permeable. For

practical purposes, this means that the effective

grain size should be in the sand size range or

higher. If the D10 value is in the silt or clay size

range, the permeability of the product will be significantly reduced. According to the Unified Soil

Classification System, the dividing line between

sand and silt size particles is a D10 of 0.074 mm.

This D10 value was used to determine practical

limits on freezing rate, curing time and initial solids content.

The uniformity coefficient (Cu) is another common soil property that we applied to this study. It

is defined as the ratio between the 60% passing

size and the 10% passing size (D60/D10). This ratio provides a comparative indication of the

range of particle sizes. A granular material is considered to have a uniform grain size distribution

if the Cu is less than 5.

Prior to conducting the freezing tests, we prepared three batches of sludge containing 2%, 6%,

and 12% total solids. The 2% batch was obtained

by allowing the sludge to settle for several days

7

6" 4" 2" 1" 10 40 200 3/8"

U.S. Standard Sieve Numbers Hydrometer

100

80

60

40

20

0 100

80

60

40

20

0

Percent Finer by Weight

Percent Coarser by Weight

100 10 1 0.1 0.01 0.001

Grain Size (mm)

Cobbles

Gravel

Coarse Fine C'rse Medium Fine

Sand

Silt or Clay

D60 = 0.654 mm

D10 = 0.194 mm

Cu = D60/D10 = 3.4

Figure 5. Size distribution curve for grains produced by freeze–thaw-conditioned

alum sludge containing 6% total solids. The freezing temperature was –5°C.
and decanting the supernatant. To produce the 6

and 12% batches, we had to filter the sludge

through a series of large sieves (see Fig. 6). These

sieves rested on a rectangular container that collected the filtrate as well as any particles passing

the sieves. After pouring sludge onto the sieves,

we covered them with plastic wrap, a board, and

various weights. The purpose of the plastic wrap

was to prevent drying, and the purpose of the

board and weights was to increase pressure on

the sludge. We monitored the filtration process

by periodically analyzing the total solids content.

After the desired total solids content was

achieved, the sludge was removed from each

sieve, mixed together, and stored in a container.

The particles in the filtrate were also added to the

container after they were separated from the water by a series of steps, including filtration

through no. 200 and 400 sieves, decanting, and

evaporation (without allowing the sludge particles to become dry). As a result none of the solids

were lost and the batch contained the full spectrum of sludge particle sizes.

To simulate layer freezing, the sludge was frozen in shallow trays made from clear plastic

acrylic. This material was chosen because it

allowed a visual observation of the underside of

the sludge layer. Each tray is designed to freeze a

layer 360 mm in diameter and 6 mm thick. A

thickness of 6 mm was chosen based on the results

of the filter leaf tests, which demonstrated that a

sludge layer of this thickness was possible. Three

trays were used in each test.

For thin layers, the freezing time can be predicted from the equation (Vesilind 1990)

 

t L

hT T f

f

c f af

= −

ρ ε

( ) (1)

where tf = the freezing time (hours)

ρf = the density of ice (917 kg/m3)

L = the latent heat of fusion (93 W-h/kg)

ε = the thickness of the sludge layer (0.006

m)

Tf = the freezing point of sludge (0°C)

Taf = the average air temperature in the

coldroom (°C)

 

hc = the average convection coefficient

(W/m2-°C).

This equation assumes that the sludge is already

at the freezing point before entering the coldroom

and that freezing occurs from the top surface

downward. We attempted to comply with these

assumptions by precooling the sludge in a 0°C

coldroom and by insulating the bottom of the

trays. The only unknown in eq 1 is the value of

the convection coefficient, 

hc . Based on a separate

study (see App. B), the average convection coefficient in the coldroom was calculated to be 23.1

W/m2-°C.

The actual freezing time was determined by

observing the bottom of the sludge layer through

the transparent freezing tray. We knew that a

sludge layer was completely frozen when it became transparent due to presence of clear ice

crystals.

The difference between the actual and predicted

freezing times was less than one minute on average. However, the standard deviation was ±32.3

minutes and differences ranged from –48.3 to 88.7

minutes (see App. C). The reason for these large

differences were due, at least in part, to slight differences in mass content. Under the same conditions, a tray containing a greater mass of sludge

took longer to freeze than one with less. When

adjusted for mass, differences in freezing rates

were reduced but not completely eliminated. For

the purpose of this experiment, an accurate prediction was not necessary. We used the actual

freezing times to calculate all freezing rates.

The procedure used to freeze sludge in the

trays is as follows:

1. The three trays and sludge were placed in a

8

Figure 6. Nest of sieves (no. 30, 50 and 100) used to

concentrate alum sludge to 6% and 12% total solids.
0°C coldroom until they reached ambient

temperature. The purpose of this step is to

start each test at the same initial temperature condition.

2. The trays and the sludge were transferred to

a coldroom, which was set at the desired

freezing temperature. The trays were placed

on a level, insulated table (see Fig. 7). Sludge

was poured into each tray and spread to a

uniform 6-mm depth with a plastic scraper.

3. For the freezing rate and initial solids content tests, the trays were removed as soon as

the sludge layer was frozen (see Fig. 8). For

the curing time test, only one of the trays

was removed and the remaining two trays

were allowed to cure. After removing the

trays from the coldroom, we carried them to

a laboratory where they were weighed and

allowed to thaw at room temperature.

Drainage was accomplished by placing each

tray at a slight angle so that meltwater could

drain away without dislocating the solids.

The trays were left in the laboratory overnight in order to allow the solids to dry out.

4. In the morning, each tray was placed in an

oven at 104°C for one hour to complete the

drying process. The trays were then removed, allowed to cool for 15-minutes, and

weighed to obtain the dry weight.

A grain size analysis was performed on the solids from each tray with U. S. Standard 8-in.-diam.

sieves 10, 20, 40, 60, 100, and 200. The sieves were

shaken for 10 minutes using a machine shaker.

The amount of solids retained on each tray were

then weighed and recorded. Grain size distribution curves were then plotted to determine D10

and Cu. Data from the layer freezing tests are

shown in Appendix C.

RESULTS

Filter leaf tests

Cloth selection

Because of the large number of filter cloths

available, we decided to base our initial selection

on the ones most likely to produce a clear filtrate.

9

Figure 7. Placing a tray containing alum sludge in a

coldroom.

Figure 8. Tray of frozen alum sludge.

Table 1. Average turbidity of filtrate from selected filter cloths.

Avg. filtrate

Cloth Porosity turbidity

number* Material Weave ft3/min (m3/min) (NTU)

515 Nylon Sateen 2–3 (0.06–0.08) 8.9

210 Dacron Crowfoot 5–2 (0.06–0.14) 14.7

2019 Polypropylene Twill 1–2 (0.03–0.06) 17.3

208 Dacron Crowfoot 14 (0.40) 21.3

201 Dacron Crowfoot 3 × 1 37 (1.05) 21.7

2015 Polypropylene Twill 2 × 2 18 (0.51) 26.0

525 Nylon Sateen 5 (0.14) 31.0

507 Nylon Sateen 25 (0.71) 40.0

2038 Polypropylene Twill 8 (0.23) 51.0

2016 Polypropylene Twill 3 (0.08) 62.3

2025 Polypropylene Twill 2 × 2 25 (0.71) 66.7

*According to Komline Sanderson Engineering Corp., Peapack, New Jersey.
100

80

60

40

20

0 200 400 600

Vacuum (mm Hg)

Filtrate Volume (mL)

10

Number 201

210

208

515

2015

2019

300

200

100

0 4 8 12

Filtration Time (min)

Filtrate Volume (mL)

Figure 9. Effect of vacuum level on the

volume of filtrate produced by cloth 515

after a 2.0-minute filtration time.

Figure 10. Effect of filtration time on the

volume of filtrate produced at 100-mm

Hg vacuum.

30

20

10

0 2 6 8 10 4

Filtration Time (min)

Sludge Layer Weight (g)

Figure 11. Effect of filtration time on

the weight of the sludge layer accumulated on cloth 2019 at 100 mm of

Hg vacuum in alum sludge containing 1.85% total solids.
11

A clear filtrate is important because it indicates a

good solids capture by the filter cloth and a low

solids loading in the filtrate return flow to the

plant. The clarity of the filtrate was measured with

a turbidimeter. We selected eleven cloths based on

their porosity values. Those with a low porosity

were assumed to have a greater potential for producing a clearer filtrate.

Each filter cloth was tested at a vacuum pressure of 100 mm of Hg, and filtration times of 2, 5,

and 10 minutes. As shown in Table 1, the clearest

filtrate was produced by cloth 515, although cloths

201, 208, 210, 2015 and 2019 all had turbidities less

than 30 NTU, which is reasonably clear. As expected, the clearer filtrates were produced by the

cloths with lower porosities. The type of material

or the weave of the cloth did not seem to make a

difference.

Vacuum level

To conserve energy, the vacuum level applied

should only be high enough to form a sludge layer

on the filter cloth while significantly reducing the

volume of sludge to be frozen. To determine this

optimum level, filter leaf tests were conducted

with cloth 515 at 100, 200, 300, 400, and 500 mm of

Hg. The total solids content in the alum sludge

was 1.14%, and the filtration time was set at 2.0

minutes. The results of these tests indicate that increasing the vacuum level did not significantly increase the volume of filtrate (see Fig. 9). This behavior is indicative of a compressible sludge layer

where particles are drawn into a more compact arrangement by the increased vacuum level. The

only benefit from a higher vacuum level appears

to be a more rapid rate of solids accumulation on

the media surface.

Filtration time

The filtration time is important because it determines the drum speed at a fixed submergence level. For practical purposes, it should be long

enough to coat the filter cloth with a thick layer of

sludge while continuously removing filtrate. Six

different filter cloths were tested (see Fig. 10) at

three filtration times: 2, 5, and 10 minutes. Five of

the filter cloths produced similar results in that the

volume of filtrate increased as the filtration time

increased. The only exception was cloth 2015,

which showed a decrease in filtrate volume after 5

minutes, presumably because of a more rapid solids buildup due to a higher initial solids content.

For the same reason we suspect that cloth 2019

produced low filtrate volumes because the sludge Figure 12. Cloth 2019 after removal of sludge layer.

Note clean separation of sludge layer from cloth.

used in these tests had almost twice the initial solids content.

In addition to removing water in the form of filtrate, the vacuum filter must also accumulate and

retain a uniform layer of sludge on the filter cloth

for subsequent freezing. Tests conducted with

cloth 2019 (see Fig. 11) indicate that the weight of

the sludge layer increased as the filtration time

was increased from 0 to 4 minutes. Longer filtration times caused a decrease in the sludge layer.

The reason for this decrease is not clear. Perhaps

some of the attached solids were released as the

layer compressed due to prolonged filtration.

Characteristics of sludge layer

The thickness of the sludge layer on the filter

cloth ranged from 1 to 7 mm. The total solids content in these layers ranged from 1.0% to 6.9%. The

main determining factor for layer thickness seems

to be the solids content in the initial sludge. A high

solids content in the sludge translated into a layer

of greater thickness and higher solids content.

After the sludge layer was frozen, it was removed from the filter cloth as described earlier. The

layer was mostly opaque except in the areas where

transparent ice crystals penetrated the sludge layer from top to bottom. As each sludge cake

thawed, the clear meltwater quickly drained

away. When thawing and drainage were complete, all that remained was a small deposit of

granular solids. The average solids content of this

deposit was 11.8%. If left to dry overnight, the solids content reached 70% or more.

One of the most important operational concerns when using vacuum filtration is the condition of the filter cloth after several cycles. Based on

visual observations, the filter cloth separated very

cleanly from the frozen sludge layer as long as the

layer was completely frozen (see Fig. 12). To evalu-
0 246

Cycle Number

60

40

20

0

100

80

60

40

20

Filtrate Volume (mL)

Sludge Layer Weight (g)

12

Figure 13. Effect of repeated use of

cloth 2019 at 100 mm of Hg vacuum

and a 5.0-minute filtration time in

alum sludge containing 1.0% solids.

0.250

0.200

0.150

0.100

0.050

0 12 34 567 89

Sand

Silt and Clay

Freezing Rate (kg/h-m ) 2

D10, Effective Grain Size (mm)

y = – 0.0064x + 0.2005

r = 0.8458 2

r = 0.627 2

y = – 0.0079x + 0.1964

r = 0.8111 2

y = – 0.0104x + 0.142

2-3 % Solids

6-7 %

12-13 % Figure 14. Effect of freezing rate of

effective grain size of freeze–thawconditioned alum sludge.

5

4

3

2

0 2468

Freezing Rate (kg/h-m ) 2

Uniformity Coefficient

2-3 % Solids

6-7 %

12-13 %

1

13 579

Figure 15. Effect of freezing rate

on uniformity coefficient.
13

ate this observation, a series of five leaf tests were

conducted with cloth 2019 at 100 mm of vacuum

and a 5-minute filtration time. The sludge layer

was completely frozen and the filter cloth was not

washed between cycles. Results of this test indicate that the filtrate volume dropped from 85 to

60 mL after the second cycle but then stabilized at

a constant 60 mL for the remaining three cycles

(see Fig. 13). Concomitantly, the sludge layer

weight gradually decreased during the first four

cycles and then increased slightly during the fifth

cycle. These results suggest that the filter cloth

had reached an operational steady state after the

first four cycles so the wash cycle could be eliminated.

Freezing test

The results of the freezing tests show that both

the rate of freezing and the initial solids content

have a significant effect on particle size. As

shown in Figure 14, the effective grain sizes (D10)

for all three sludges were in the sand size range.

However, D10 decreased as the freezing rate increased. Also, the sludges that contained a higher

initial solids content produced larger grains. The

largest grains were produced by the sludge containing 12–13% solids. The grains produced by

the sludge containing 2–3% solids were significantly smaller.

Based on the data shown in Figure 14, the

effective grain size (D10) of frozen 2–3% sludge

entered the silt and clay particle size range at a

freezing rate of 6.6 kg/h-m2. For the 6–7% and the

12–13% sludges, the crossover points are 15.5 and

19.8 kg/h-m2 respectively.

Freezing rate appeared to have little effect on

the uniformity coefficient (see Fig. 15). However,

the plot suggests that the uniformity coefficient

decreases with increasing initial solids content.

The average uniformity coefficients are 4.0, 3.3,

and 3.0 respectively for the 2–3%, 6–7%, and 12–

13% solids sludges, respectively. A statistical analysis of this data indicates that these differences are

significant at the 95% level. This means that a more

uniform grain size can be expected from freeze–

thaw conditioned sludges containing a greater initial solids content. The reason for this phenomenon is not clear but perhaps there is less opportunity for particle movement away from the freezing

front in sludges with higher initial solids because

of the close intraparticle proximity. As a result

there would be fewer opportunities to form various particle size combinations.

The curing time tests were conducted with

alum sludge containing 2–3% solids. After freezing was complete, sludge samples were cured at

–4°C for periods of 1 and 24 hours. As shown in

Table 2, curing times of 1 and 24 hours appeared to

have no effect on the grain size or uniformity coefficient of freeze–thaw conditioned alum sludge

containing 2–3% solids. Neither the 6–7% or the

12–13% solids sludges were tested because of the

apparent lack of any effect on the sludge with the

2–3% solids.

DISCUSSION

The results of the filter leaf tests show that vacuum filtration can remove most of the water from

the sludge and, at the same time, produce a uniform thin layer for freezing. The optimum operational conditions for the vacuum filter are a vacuum level of 100 mm of Hg and a 5.0-minute filtration time. Generally, vacuum filtration increased

the total solids content of alum sludge from 2% to

approximately 6%. This represents a 67% reduction in the volume of sludge to be frozen. After

freezing, thawing, and draining of meltwater, the

remaining granular material contained approximately 12% solids. Although 12% solids is relatively low, further dewatering will rapidly occur by air

drying. Our tests show that the sand size particles

dried to a solids content of 70% or more within a

few hours.

Several filter cloths were able to produce a clear

filtrate even after repeated use. Therefore, selection of the right filter cloth for this application may

depend more on its durability to withstand several

freeze–thaw cycles than its ability to produce a

Table 2. Data on effect of curing

time on effective grain size (D10)

and uniformity coefficient (Cu) of

freeze–thaw conditioned alum sludge

containing 2–3% solids.

Effective

Curing grain size Uniformity

Test time (D10) coefficient

number (hr) (mm) (Cu)

A3 24 0.13 3.9

B1 0 0.13 3.5

B2 1 0.13 4.0

B3 24 0.14 3.9

C1 0 0.13 4.0

C2 1 0.14 3.2

C3 24 0.12 4.3

D1 0 0.13 3.4

D2 1 0.11 3.7

D3 24 0.13 3.5
14

clear filtrate. Also, we were surprised by the relative cleanliness of the filter cloths after each use.

This observation and the tests we conducted suggest that washing requirements would be minimal, but a longer term study is needed to confirm

this finding.

The freezing tests indicate that the initial solids

content is an important parameter in determining

the effective grain size. Sludges that contained a

greater solids content had larger effective grain

sizes. For example, at a freezing rate of 6 kg/h-m2

the effective grain sizes were 0.16, 0.15, and 0.08

mm for the sludges containing the 12–13%, 6–7%,

and 2–3% solids, respectively. The reason for the

larger grain size in the thicker freeze–thaw conditioned sludges may be the availability of more solids

to fill the space between crystal boundaries. An important conclusion from this finding is that filtering alum sludge before freezing not only reduces

the amount of sludge to be frozen, but it increases

the grain size of the product. Therefore it may be

more advantageous to use a belt press rather than

a vacuum filter prior to freezing. As mentioned

previously, a vacuum filter will dewater alum

sludge to 6% solids, while a belt presses can typically dewater alum sludge to 12% solids.

The estimated size of the surface area needed

for the horizontal belt freezer serving a 3,785 m3/

day (1,000,000 gal./day) plant is 10.2 m2. This estimate is based on the following assumptions: 1) the

sludge production rate is 1% of the water processed (Sanks 1978) and the specific gravity is

approximately equal to water, i.e. 1.0, 2) the vacuum filter reduces the volume of sludge by 67%,

3) the freezing rate is 15.5 kg/h-m2, and 4) the

operational period is 8 hours/day. If the belt is 1.2

m wide, the length of the freezing chamber would

be 8.5 m. This is about the same size as the blast

freezers used by the food industry. A greater capacity could be achieved by operating for longer periods. For example, the unit could handle the sludge

produced by a 11,355 m3/day plant if it were operated 24 hours/day.

If a belt press were used instead of a vacuum

filter, the surface area of the horizontal belt freezer

could be reduced from 10.2 m2 to 4.0 m2. This reduction is based on the assumption that the belt

press will reduce the sludge volume by 83% and

the freezing rate can be increased to 19.8 kg/

h-m2, as indicated by our tests. This configuration

would be particularly beneficial to large plants

where floor space is limited.

One of the major concerns about the use of

freeze–thaw conditioning is the energy required

by the phase change operation. Theoretically it

takes 93 watt-hours to convert 1 kilogram of

water (or sludge) to ice. Also, it takes 1.16 W-h/C°

to cool the liquid sludge to the freezing point and

0.58 W-h/C° to cool the frozen sludge below

the freezing point. If we assume an initial temperature of 10° C and a final temperature of –2°C, the

energy required to freeze one metric ton (1,000 kg)

of sludge is 105.8 kW-h. Assuming 50% losses due

to air infiltration, the estimated energy required is

158.6 kW-h/metric ton. If the electrical cost is assumed to be $0.07/kilowatt-hour, the cost of freezing one metric ton of sludge is $11.10. In terms of

water used, the cost is $0.004/m3 since it takes approximately 3000 m3 of treated water to produce

one metric ton of sludge. This is a relatively small

cost compared to the total cost of water treatment

which typically ranges from $0.25 to $0.50/m3.

CONCLUSIONS

The bench-scale tests conducted in this study

indicate that the proposed freeze separator concept is technically and economically feasible. The

filter leaf tests show that the vacuum filter should

be able to reduce the sludge volume by at least

one-third. The optimum operational vacuum was

100 mm of Hg and the filtration time was 5.0 minutes. Any low porosity cloth appears to be suitable

from a filtrate quality point of view. However, the

durability of these filter cloths after many freeze–

thaw cycles is still unknown.

The results of the freezing studies show that

both freezing rate and initial solids content will affect the grain size of the freeze–thaw conditioned

sludge. In general, faster freezing rates will cause a

decrease in grain size. Conversely, sludges containing a greater initial solids content will produce

larger grains. The maximum freezing rate for the

proposed freeze separator is projected to be 15.5

kg/h-m2. Faster freezing will reduce the effective

grain size below that of fine sand. This could result

in a decease in the hydraulic conductivity of the

granular material, which is important for removing meltwater.

Based on the excellent results obtained from

freezing alum sludge containing 12–13% solids,

we concluded that it may be more effective to use a

belt filter rather than a vacuum filter prior to freezing. A belt press would remove up to 50% more

water from the sludge and thus reduce the cost of

freezing. Also, the freezing rate could be increased

to 19.8 kg/h-m2.

Curing time had no significant effect on grain
15

size. Therefore, there is no need to keep the

sludge frozen beyond the freezing time, so that

the thawing process can begin as soon as the

sludge exits the freezer.

LITERATURE CITED

American Water Works Research Foundation

(1990) Slib, schlam, sludge. Cooperative Research

Report with Keuringsinstituut voor Waterleidingartikelen, American Water Works Association,

Denver, Colorado.

Benn, D. and P.W. Doe (1969) The disposal of

sludge by the freezing and thawing process.

Filtration & Separation. July/August, p. 383–389.

Brown, G.N., K.B. Kirwan, G.J. Englehardt, and

J.R. DeWolfe (1993) Mechanical freeze/thaw conditioning of water works residuals pilot study,

Project Report to Municipal Water & Wastewater

Treatment Electrotechnology Committee, Electric

Power Research Institute, Miami, Florida.

DeWolfe, J.R. and B.A. Dempsey (1992) The effects

of land applied water treatment residuals on soil

phosphorus. In Proceedings of the ASCE Water Forum, Baltimore, Maryland.

Egan, C.J. and H. E. Davis (1982) Concentration of

black liquor by freeze-crystallization. Proceedings

of Technical Association of the Pulp and Paper Industry, San Francisco, 13–16 September 1982. Book III,

p. 471–479.

Farrell, J.B. (1971) A status report on the utilization of freezing in dewatering of sludges. In Proceedings of the XIIth International Congress of Refrigeration. Washington D.C., vol. 1, p. 739–743.

Kawamura, S. and R.R. Trussell (1991) Main features of large water treatment plants in Japan. Journal of the American Water Works Association, June, p.

56–62.

Knocke W.R. and P. Trahern (1989) Freeze–thaw

conditioning of chemical and biological sludges.

Water Research, 23(1): 35–42.

Martel, C.J. (1988) Development and design of

sludge freezing beds. USA Cold Regions Research

and Engineering Laboratory, CRREL Report 88-20.

Martel, C.J. and C.J. Diener (1991) A pilot-scale

study of alum sludge dewatering in a freezing

bed. Journal of the American Water Works Association, 83(12): 51–55.

Randall, C.W. (1978) Butane is nearly ‘ideal’ for

direct slurry freezing. Water and Waste Engineering, 15(3): 43–49.

Randall, C.W., M.Z. Ali Khan, and N.T. Stephens

(1975) Waste activated sludge conditioning by

direct slurry freezing. Water Research, 9: 917–925.

Sanks, R.L. (1978) Water Treatment Plant Design.

Ann Arbor, Michigan: Ann Arbor Science.

Sewerage Commission of the City of Milwaukee

(1971) Evaluation of conditioning and dewatering sewage sludge by freezing. NTIS PB-213-122,

U.S. Department of Commerce, Springfield, Virginia 22151.

Vesilind, P. A. (1990) Sludge freezing in shallow

layers. ASCE Journal of Environmental Engineering,

116(3): 646–650.
17

Initial Filtration Vacuum Filtrate Filtrate Cake Weight Wet cake Drained

Test Media solids time pressure volume clarity thickness Wet cake Dry cake solids solids

number number (%) (min) (mm Hg) (mL) (NTU) (mm) (g) (g) (%) (%)

1 2013

2 2019

3 2006 1.5 3

4 2016 1.5 190 3

5 2038 1.5 375 3

6 525 1.5 1

7 525 1.5 75 1

8 525 2.0 90 1

9 515 2.0 60 1

10 515 5.0 135 3

11 2019 5.0 130 3

12 210 2.0 90 1

13 210 5.0 135 3

14 531 2.0 100 1

15 531 5.0 165 1

16 515 1.14 2.0 500 70 1 19.554 1.111 5.7

17 515 1.14 5.0 500 100 1 29.349 1.299 4.4

18 515 1.14 7.0 500 185 1 22.707 1.226 5.4

19 515 1.14 7.0 500 230 1 8.812 0.558 6.3

20 525 1.14 2.0 500 80 3 22.655 1.027 4.5

21 515 1.14 7.0 500 110 3 32.203 1.475 4.6

22 525 1.14 5.0 500 115 3 19.693 1.081 5.5

23 525 1.14 7.0 500 140 3 12.678 0.875 6.9

24 515 1.14 2.0 100 75 5.5 1

25 515 1.14 2.0 200 80 5.4 1

26 515 1.14 2.0 300 95 15.0 1

27 515 1.14 2.0 400 85 12.0 1

28 515 1.14 2.0 400 100 8.0 1

29 515 1.14 2.0 500 70 8.0 1

30 515 1.14 5.0 100 130 2.5 1

31 507 1.14 2.0 100 115 55.0 1

32 507 1.14 2.0 200 135 55.0 1

33 507 1.14 2.0 300 130 55.0 1

34 507 1.14 2.0 400 125 40.0 1

35 507 1.14 2.0 500 125 35.0 1

36 210 0.41 2.0 100 95 25.0 1 5.055 0.232 4.6

37 210 0.41 5.0 100 160 15.0 1 12.282 0.420 3.4

38 210 0.41 10.0 100 230 4.0 1 13.532 0.413 3.1

39 208 0.41 2.0 100 95 35.0 1 10.043 0.359 3.6

40 208 0.41 5.0 100 155 25.0 1 13.710 0.454 3.3

41 208 0.41 10.0 100 205 4.0 1 10.476 0.368 3.5

42 201 0.34 2.0 100 135 35.0 1 5.012 0.209 4.2

43 201 0.34 5.0 100 210 15.0 1 4.652 0.201 4.3

44 201 0.34 10.0 100 270 15.0 1 5.024 0.251 5.0

45 515 0.34 2.0 100 105 10.0 1 5.026 0.229 4.6

46 515 0.34 5.0 100 155 8.5 1 10.428 0.403 3.9

47 515 0.34 10.0 100 195 8.2 1 8.348 0.371 4.4

48 525 0.47 2.0 100 120 50.0 1 6.551 0.306 4.7

49 525 0.47 5.0 100 215 25.0 3 16.383 0.530 3.2

50 525 0.47 10.0 100 215 18.0 3 20.562 0.628 3.1

51 507 0.47 2.0 100 90 55.0 1 9.048 0.367 4.1

52 507 0.47 5.0 100 155 30.0 1 10.286 0.419 4.1

53 507 0.47 10.0 100 185 35.0 1 11.900 0.478 4.0

54 2019 0.70 2.0 100 65 20.0 1 9.580 0.289 3.0

55 2019 0.70 5.0 100 105 15.0 3 21.458 0.584 2.7

56 2019 0.70 10.0 100 135 17.0 3 24.543 0.630 2.6

57 2016 0.70 2.0 100 75 65.0 3 15.745 0.416 2.6

58 2016 0.70 5.0 100 120 60.0 1 16.190 0.341 2.1

59 2016 0.70 10.0 100 63.0 1 12.523 0.379 3.0

APPENDIX A: FILTER LEAF TEST DATA FOR

LEBANON, NEW HAMPSHIRE, WATER PLANT SLUDGE.
18

60 2038 0.82 2.0 100 45 65.0 1 8.717 0.203 2.3

61 2038 0.82 5.0 100 165 35.0 3 18.179 0.404 2.2

62 2038 0.82 10.0 100 65 53.0 1 9.899 0.168 1.7

63 2015 0.82 2.0 100 85 33.0 3 32.315 0.687 2.1

64 2015 0.82 5.0 100 145 25.0 1 24.276 0.602 2.5

65 2015 0.82 10.0 100 130 20.0 1 21.738 0.629 2.9

66 2025 0.82 2.0 100 85 50.0 1 15.199 0.052 0.3

67 2025 0.82 5.0 100 105 85.0 1 21.906 0.653 3.0

68 2025 0.82 10.0 100 130 65.0 1 21.225 0.387 2.1

69 2019 0.98 5.0 100 85 15.0 3 33.807 1.042 3.1

70 2019 0.98 5.0 100 60 13.0 3 31.595 0.958 3.0

71 2019 0.98 5.0 100 60 18.0 1 24.436 0.804 3.3

72 2019 0.98 5.0 100 60 20.0 1 21.762 0.833 3.8

73 2019 0.98 5.0 100 60 18.0 1 22.315 0.768 3.4

74 2015 1.10 5.0 100 85 20.0 3 41.366 1.177 2.8

75 2015 1.10 5.0 100 85 17.0 3 38.858 1.117 2.9

76 2015 1.10 5.0 100 85 10.0 3 44.343 1.126 2.5

77 2015 1.10 5.0 100 80 22.0 3 34.715 1.030 3.0

78 2015 0.89 5.0 100 150 5.0 3 31.473 0.797 2.5

79 2015 0.89 5.0 100 140 5.3 3 31.822 0.891 2.8

80 2015 0.89 5.0 100 130 9.5 3 35.365 0.953 2.7

81 2015 0.89 5.0 100 130 9.5 3 37.791 0.993 2.6

82 2015 0.60 5.0 100 170 3.5 1 23.424 0.235 1.0

83 2015 0.60 5.0 100 200 5.0 1 12.919 0.335 2.6

84 2015 0.60 5.0 100 175 15.0 1 12.717 0.202 1.6

85 515 0.60 5.0 100 125 6.5 1 14.713 0.565 3.8

86 515 0.60 5.0 100 105 7.0 1 11.626 0.437 3.8

87 2019 1.85 1.0 100 25 25.0 3 24.848 0.887 3.6

88 2019 1.85 2.0 100 30 35.0 3 26.822 1.293 4.8

89 2019 1.85 4.0 100 35 20.0 4 31.493 1.055 3.4 11.4

90 2019 1.85 6.0 100 50 40.0 4 22.191 0.945 4.3 12.3

91 2019 1.85 10.0 100 50 30.0 3 15.830 0.732 4.6 11.7

92 210 2.08 1.0 100 15 30.0 2 13.152 0.545 4.1 11.0

93 210 2.08 2.0 100 25 30.0 2 13.879 0.634 4.6 10.6

94 210 2.08 4.0 100 30 45.0 3 18.204 0.752 4.1 11.9

95 210 2.08 4.0 300 40 45.0 5 28.444 1.386 4.9 13.8

96 210 2.08 1.0 300 15 75.0 2 14.428 0.503 3.5 9.3

97 210 2.08 2.0 300 25 45.0 2 16.382 0.663 4.0 12.2

98 210 2.08 1.0 500 15 40.0 2 16.881 0.698 4.1 13.6

99 210 2.08 2.0 500 20 55.0 1 7.654 0.422 5.5 10.3

100 210 2.08 4.0 500 40 35.0 3 20.657 0.365 5.3 14.0

101 210 2.08 2.0 500 25 42.0 2 13.495 0.817 6.0 14.9

102 2019 1.85 1.0 300 30 15.0 3 22.555 0.816 3.6 12.7

103 2019 1.85 2.0 300 45 20.0 7 39.809 1.407 3.5 10.8

104 2019 1.85 4.0 300 65 9.0 3 19.677 0.913 4.7 12.0

105 2019 1.85 1.0 500 40 15.0 4 28.398 0.978 3.4 11.2

106 2019 1.85 2.0 500 55 15.0 5 31.885 1.081 3.4 10.6

107 2019 1.85 4.0 500 75 10.0 8 56.411 1.746 3.1 10.7

Initial Filtration Vacuum Filtrate Filtrate Cake Weight Wet cake Drained

 Test Media solids time pressure volume clarity thickness Wet cake Dry cake solids solids

number number (%) (min) (mm Hg) (mL) (NTU) (mm) (g) (g) (%) (%)
19

According to Vesilind (1990), the rate of freezing (dy/dt) for a thin layer of sludge can be predicted by the equation

 

dy

dt

hT T

L = c ( ) f − af

ρf

(B1)

where 

hc = the average convection coefficient

(W/m2-°C )

ρf = the density of ice (917 kg/m3)

L = the latent heat of fusion (93 W-h/kg)

Tf = the freezing point of sludge (0°C)

Taf = the average air temperature in the

coldroom (°C).

By separating variables eq B1 becomes

 

ρf cf Ldy h T T dt = − ( )f . (B2)

At t = 0, y = 0, and when t = the freezing time

tf, y = the depth of frozen sludge, ε. Integrating

eq B2 between these limits yields

 

ρ ε f c f af L hT T t = − f 



 



 . (B3)

By rearranging terms and solving for 

hc , the

above equation becomes

 

h L

tT T c f

f f af

=

( ) −

ρ ε . (B4)

Substituting the values for each constant and

changing ii to minutes and ε to millimeters, eq 4

becomes:

 

h

t T c

f af

=

( ) −

5118ε . (B5)

To calculate 

hc we had to determine Taf, tf, and

ε. Taf was determined by setting coldroom 165 at

–14°C. The values for tf, and ε were determined

by freezing a tray of distilled water. Both the tray

and the distilled water were precooled to 0°C.

The tray was removed at various time intervals

and the depth of the ice layer was measured with

calipers. The data from this analysis are shown in

the table below. Based on these data the average

convection coefficient was calculated to be 23.1

W/m2-°C. This was the value used in eq B1 to

predict the freezing times.

APPENDIX B: DETERMINATION OF CONVECTION COEFFICIENT

FOR COLDROOM 165

Table B1. Laboratory data

on freezing distilled water

at –14°C.

Freezing Ice Conv.

time depth (ε) coefficient

(min) (mm) (W/m2-°C)

15 1.01 24.6

20 1.13 20.6

30 1.95 23.8

40 2.63 24.0

45 2.50 20.3

60 4.43 27.0

60 2.98 18.2

80 5.72 26.1

Avg. 23.1

s.d. 3.1
21

Predicted Actual Weight of Actual Effective

Solids freezing freezing frozen freezing grain size Uniformity

Test content Temperature time time sludge rate (D10) coefficient

no. (%) (°C) (min) (min) (g) (kg/h-m2) (mm) Cu

T1 2–3 –5 270.7 238 551 1.389 0.144 3.7

T2 2–3 –5 270.7 255 587 1.381 0.123 4.1

T3 2–3 –5 270.7 275 690 1.506 0.130 4.2

T4 2–3 –14 96.7 145 549 2.273 0.112 3.9

T5 2–3 –14 96.7 145 614 2.542 0.102 4.3

T6 2–3 –14 96.7 154 676 2.632 0.121 3.3

T10 2–3 –23 58.9 76 627 4.948 0.101 3.8

T11 2–3 –23 58.9 76 575 4.536 0.098 3.8

T12 2–3 –23 58.9 76 550 4.340 0.083 4.4

T13 2–3 –30 45.1 61 719 7.072 0.077 4.1

T14 2–3 –30 45.1 56 525 5.623 0.078 3.9

T15 2–3 –30 45.1 55 469 5.121 0.084 4.1

6%A 6–7 –23 58.9 70 578 4.955 0.150 3.7

6%B 6–7 –23 58.9 70 613 5.250 0.177 3.4

6%C 6–7 –23 58.9 70 530 4.539 0.182 3.4

6%D 6–7 –14 96.7 125 482 2.314 0.174 3.2

6%E 6–7 –14 96.7 125 591 2.836 0.182 3.3

6%F 6–7 –14 96.7 125 447 2.145 0.162 3.3

6%G 6–7 –5 270.7 240 602 1.505 0.194 3.4

6%H 6–7 –5 270.7 240 511 1.279 0.185 3.1

6%I 6–7 –5 270.7 240 405 1.012 0.184 3.0

6%J 6–7 –30 45.1 45 480 6.400 0.137 3.6

6%K 6–7 –30 45.1 60 671 6.714 0.138 3.0

6%L 6–7 –30 45.1 60 563 5.627 0.140 3.3

12%A 12–13 –30 45.1 38 502 7.918 0.151 2.75

12%B 12–13 –30 45.1 38 490 7.738 0.152 2.89

12%C 12–13 –30 45.1 48 658 8.226 0.148 2.87

12%D 12–13 –23 58.9 57 565 5.946 0.154 2.85

12%E 12–13 –23 58.9 52 474 5.472 0.153 2.91

12%F 12–13 –23 58.9 62 701 6.783 0.168 2.91

12%G 12–13 –14 96.7 88 548 3.736 0.171 3.14

12%H 12–13 –14 96.7 80 545 4.091 0.174 3.10

12%I 12–13 –14 96.7 100 642 3.853 0.183 3.16

12%J 12–13 –5 270.7 192 483 1.509 0.198 3.14

12%K 12–13 –5 270.7 182 463 1.526 0.185 3.04

12%L 12–13 –5 270.7 199 582 1.754 0.192 3.18

APPENDIX C: FREEZING TEST DATA FOR SIX-MILLIMETER LAYER OF ALUM SLUDGE
1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

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 AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

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13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS 15. NUMBER OF PAGES

16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

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24

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

December 1996

A Device for Mechanical Freeze–Thaw Conditioning of Alum Sludge PR: 4A762784AT42

WP: 211

WU: CO-M20

C. James Martel, Rosa T. Affleck and Melinda L. Yushak

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 96-15

Hanover, New Hampshire 03755-1290

Office of the Chief of Engineers

Washington, DC 20314

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

This report contains the results of a study to develop a mechanical device for dewatering alum sludge by

freeze–thaw. This proposed device is a combination of two conventional unit operations: a vacuum filter and a

blast freezer. Bench-scale studies were conducted to evaluate this concept and develop preliminary design

criteria. The results of filter leaf tests indicate that a suitable sludge layer could be collected on a cloth medium

at a vacuum level of only 100 mm of Hg and a 5.0-minute filtration time. The volume of sludge was reduced by

67%. The freezing tests indicated that low freezing rate and a high initial solids content had a tendency to

produce large alum sludge particles. However, fast freezing rates could be achieved without reducing the

effective grain size below that of a fine sand. Curing time had no effect on grain size. The electrical cost of

freezing sludge with this device was estimated to be $0.004/m3, which is not expensive in relation to the total

cost of water treatment which is approximately $0.25 to $0.50/m3.

Alum Water purification

Freeze–thaw conditioning Water treatment plants

Sludge disposal

 For conversion of SI units to non-SI units of measurement consult Standard Practice for Use of the

International System of Units (SI), ASTM Standard E380-93, published by the American Society for Testing and Materials,

1916 Race St., Philadelphia, Pa. 19103.