Origin of
Specific Resistance to Filtration and Relationship between SRF and CST
Introduction
The sludge dewatering process could be seen as removing
liquid to convert the sludge into solid (Yukseler
et al., 2007). In practice, the dewatering process in a
wastewater treatment plant would use some special equipment. After dewatering,
the sludge become much easier to manage and transport. Because the volume and weight
of a sediment cake would be smaller than it was wet sludge. The detailed process
of dewatering the sludge sometimes depends on the specific condition of sludge.
There are two test parameters named CST and SRF to help to understand the
sludge property and dewater-ability.
Definition of SRF
and its origin.
The SRF is short for specific resistance to filtration. The
SRF test is used for estimating sludge dewater-ability (Sawalha, 2010). SRF is a measure how much the sediment cake resists the liquid being forced
out by the dewatering process (Kavanagh,
1980). The theoretical basis firstly appeared in 1933 (Carman, 1933). In 1938, Carman accept Darcy’s law and gave out two
assumptions of the cake form and filter media. Then the numerical model
appeared to describe the ratio of volume in time that slurry transporting solid
onto sediment cake (Smollen, 1986).
Then the common used model of SRF was governed (Christensen et al., 1993).
Here list the terms, their meanings and units in the model
as below.
|
Equation Term Table
|
||
|
Term
|
Meaning
|
Unit
|
|
t
|
time
in the filtration process
|
s
|
|
V
|
filtrate
volume
|
m3
|
|
μ
|
filtrate
viscosity
|
Pa·s
|
|
SRF
|
average
specific resistance to filtration
|
m kg-1
|
|
C
|
mass
of dry cake deposited per unit volume of filtrate
|
kg·m-3
|
|
A
|
area
of the filter medium
|
m2
|
|
P
|
applied
pressure
|
Pa
|
|
Rm
|
media
resistance
|
m-1
|
Christensen
et al., 1993
In the early period when SRF test was practiced, the SRF
results could be influenced by individual differences. To eliminate the
phenomena of individual differences happening, the standard test procedure was
set. The equipment of the standard SRF test is a Buchner funnel apparatus associated
with a vacuum
port and paper filter.
Definition of
CST and its origin.
The CST is short for capillary suction time. The standard
CST test was first developed by Gale and Baskerville in 1968 (Baskerville & Gale, 1968). It also
has a standard test process.
Sawalha,
2010. Figure 1.1.1 Diagram of capillary suction time test apparatus
The CST test showed advantages in four aspects, including
its short time use, high reliability, easy operation and low cost. There is no
special requirement in the CST test. At the meantime, the aims of both CST and
SRF are the same. Hence, to estimate SRF value with CST value would bring
convenience to the SRF.
Relationship
Generally, when testing on the
same sludge, the SRF test result is correlating to the CST result. It has been
found that sludge samples showing high CST would show high SRF values at the
same time (Baskerville & Gale, 1968).
This is more like a qualitative relationship.
Researchers also developed models
to investigate the quantitative relationship between SRF and CST. There might
be two types of quantitative models to describe the relationship, including the
mechanic model and the empirical model.
The mechanic model has been constructed (Lee & Hsu, 1993) as below.
αav--average
specific resistance to filtration (m/kg)
Pcd--capillary suction pressure, assuming a
diffusion-like process, in which the paper is unsaturated;
So--liquid saturation under the inner cylinder;
A--cross section area (m2);
Co--solid concentration (kg/m3);
t--time (s);
μ--liquid viscosity (Pa·s);
V--liquid invasion volume.
The empirical model is still being developed by the
researchers. There is an equation based on the practical experience as below (Sawalha, 2010).
loge
SRF = 46.128 – 1.346 T + 0.035 T2 + 13.760 F/TSS
SRF--the specific resistance to filtration (m/kg);
T--temperature (oC);
F--the filterability (loge s/m2)
TSS--the total suspended solids concentration (g/l).
Conclusion
The qualitative relationship between SRF and CST could be
found in the early research. This means SRF and CST are correlated. But the
quantitative relationship model between SRF and CST is still developing.
Reference
Yukseler, H., Tosun, I., and Yetis, U. (2007). A new
approach in assessing slurry filterability. Journal of Membrane Science, 303,
72-79.
Sawalha, O. (2010). Capillary Suction Time (CST) Test: Developments
in testing methodology and reliability of results. The University of Edinburgh.
Available at: https://www.era.lib.ed.ac.uk/bitstream/handle/1842/4887/Sawalha2011.pdf?sequence=1 [Accessed date: May 4th,
2016]
Kavanagh, B. (1980). The Dewatering of Activated Sludge:
Measurement of Specific Resistance to Filtration and Capillary Suction Time.
Wat. Pollut. Control. 388.
Carman, P. (1933). A Study of the Mechanism of Filtration. Part1.
Jour. Soc. Chem. Ind. 52:280.
Smollen, M. (1986). Dewaterability of municipal sludges 1: A
comparative study of specific resistance to filtration and capillary suction
time as dewaterability parameters. Water SA. Vol. 12. No. 3. Available at: http://www.wrc.org.za/Knowledge%20Hub%20Documents/Water%20SA%20Journals/Manuscripts/1986/WaterSA_1986_12_0401.PDF [Accessed date: May 4th,
2016]
Baskerville, R. and Gale, R. (1968). A simple automatic
instrument for determining the filterability of sewage sludges. Water pollution
control, 67, 233-241.
Lee, D., and Hsu, Y. (1993). Cake formation in capillary
suction apparatus. Industrial and Engineering Chemistry Research, 32,
1180-1185.
