ate this observation, a series of five leaf tests were

Freezing rate appeared to have little effect on

conducted with cloth 2019 at 100 mm of vacuum

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

and a 5-minute filtration time. The sludge layer

the plot suggests that the uniformity coefficient

was completely frozen and the filter cloth was not

decreases with increasing initial solids content.

washed between cycles. Results of this test indi-

The average uniformity coefficients are 4.0, 3.3,

cate that the filtrate volume dropped from 85 to

and 3.0 respectively for the 23%, 67%, and 12

60 mL after the second cycle but then stabilized at

13% solids sludges, respectively. A statistical anal-

a constant 60 mL for the remaining three cycles

ysis of this data indicates that these differences are

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

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

weight gradually decreased during the first four

uniform grain size can be expected from freeze

cycles and then increased slightly during the fifth

thaw conditioned sludges containing a greater ini-

cycle. These results suggest that the filter cloth

tial solids content. The reason for this phenome-

non is not clear but perhaps there is less opportu-

had reached an operational steady state after the

nity for particle movement away from the freezing

first four cycles so the wash cycle could be elimi-

front in sludges with higher initial solids because

nated.

of the close intraparticle proximity. As a result

there would be fewer opportunities to form vari-

Freezing test