Freezing tests

U.S. Standard Sieve Numbers

Hydrometer

6" 4"

Figure 5. Size distribution curve for grains produced by freezethaw-conditioned

alum sludge containing 6% total solids. The freezing temperature was 5C.

pie plate, and placed in the coldroom at 20C.

sludge after dewatering with a conventional belt

press.

When freezing was complete, the sludge layer

The effective grain size (D10) is defined as the

was separated from the filter cloth by heating the

particle size corresponding to the 10% passing

underside of the cloth for a few seconds with a

size from the grain size curve that is determined

warm laboratory hot plate. The sludge layer was

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

then thawed at room temperature. After thawing

that the permeability of soil or in this case, a gran-

was complete, the meltwater was drained off and

ular sludge, will be controlled by this grain size.

the remaining solids were weighed to determine

Since the meltwater must be drained after thaw-

a drained solids content. These solids were then

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

ing, it is important that the final product have an

effective grain size that is highly permeable. For

weight of dry cake. This completed a test. Alto-

practical purposes, this means that the effective

gether, 107 filter leaf tests were conducted in this

grain size should be in the sand size range or

study. Data obtained from these tests can be

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

found in Appendix A.

range, the permeability of the product will be sig-

nificantly reduced. According to the Unified Soil

Freezing tests

Classification System, the dividing line between

The purpose of the freezing test is to evaluate

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

the effect of freezing rate, curing time and initial

This D10 value was used to determine practical

solids content on the quality of the product in

limits on freezing rate, curing time and initial sol-

terms of the effective grain size and uniformity

ids content.

coefficient. The freezing rate was varied by freez-

The uniformity coefficient (Cu) is another com-

ing the same quantity of sludge at four tempera-

tures: 5, 14, 23, and 30C. The effect of cur-

mon soil property that we applied to this study. It

is defined as the ratio between the 60% passing

ing time was determined by leaving the sludge in

size and the 10% passing size (D60/D10). This ra-

a coldroom for 1 and 24 hours after freezing was

tio provides a comparative indication of the

complete. The effect of initial solids content was

range of particle sizes. A granular material is con-

tested by freezing sludge at three different total

sidered to have a uniform grain size distribution

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

if the Cu is less than 5.

content was chosen because it represents a typical

Prior to conducting the freezing tests, we pre-

alum sludge after gravity thickening. A 6% solids

pared three batches of sludge containing 2%, 6%,

content was chosen as representative of a typical

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

alum sludge after vacuum filtration. A 12% solids

by allowing the sludge to settle for several days

content was chosen to simulate a typical alum

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