PhD Research Program
University of Strathclyde, Glasgow (UK)
Oct. 1995- Sep. 1998
On the Role of Aluminium
Hydroxide in the Conditioning of an Alum Sludge
Eleftherios N. Papavasilopoulos
Supervised by Dr D. H. Bache BSc PhD DSc
MCIWEM, Reader in the Dept. of Civil Engineering at the
University of Strathclyde.
Useful Background Reading
Alum sludges arise from the treatment of raw waters based on
aluminium sulphate as the primary coagulant. The resultant alum
sludge floc forms a three-dimensional network and is a two
\endash phase mixture of solids and water. The water content
varies between 95 to 99 %, this being typical of the levels found
in such sludges before and after thickening. The solids can be
thought as forming the skeleton o f the floc and arise from two
sources: (a) the entrapment of impurities in the water (e.g.
colour, turbidity, algae, hardness) and (b) the coagulant
There is a wealth of research work providing insight into the
mechanisms for colour and turbidity removal using alum as a
coagulant. In the case of colored \endash upland waters,
important mechanisms for colour removal are: (a) charge
neutralisation \endash precipitation, (b) adsorption and (c)
simultaneous precipitation. Often, waterworks operate within a pH
range of 6 - 8 in conjunction with dosage rates of alum providing
conditions for the formation of amorphous, Al(OH)3(s). In this
instance, colour removal occurs by adsorption of the colour
inducing substances (mainly humic and fulvic acids) onto the
gelatinous precipitate of Al(OH)3 (s) through bonds which can
include van der Waals interactions, H-bonding. The gelatinous
precipitate can represent up to 40 - 50 % of the dry floc mass
and is expected to have a major b earing on the overall sludge
Of the various sludge handling methods in association with
sludges from low in turbidity, coloured waters, dewatering by
means of plate filter presses is generally a favoured option.
Alum sludges are often regarded as "difficult to
dewater". Following the general pattern of water release in
a flocculated suspension, it is a characteristic feature of alum
sludge that even at very large pressures the final cake is
characterised by a 40 % moisture content. In the process of
dewatering, work needs to be done in order to overcome the water
binding forces (related to the chemical nature of the sludge
particles) and the friction within the sludge cake as water
percolates through the system (tied t o particle size). An added
problem in dewatering of alum sludges stems from their
From the above it appears that the composition of the floc,
its structure and the particle size distribution are all of major
concern in dewatering. Evidence in literature indicates that
difficulties in alum sludge dewatering are associated with the
behaviour of the gelatinous aluminium hydroxide but this aspect
has not been thoroughly explored. This suggests that dewatering
is closely linked to coagulation practices. With regard to
particle size there is a difficulty in defining what is meant by
the unit size for sludges which tend to be flocculent by nature
i.e. is it a fundamental particle size or is it tightly bound
aggregat e s within the floc. Generally, sludge containing a
large fraction of fine material dewaters poorly, this being
largely attributed to the blinding of sludge filter cakes from
the migration of fines. The remedy for this type of problem rests
with the control of fines, either removing them (i.e. elutriation
in wastewater sludges) or in the case of alum sludges, tying them
together with polymers to form larger subunits as an aid to water
back to top
sludge conditioning and the concept of optimum dose
Alum sludge is often dosed with synthetic organic polymers in
order to enhance dewatering. The process of conditioning has
become an essential component of sludge management and can
represent up to 50 % of the total costs. There appear to be a
variety of polymers available to the water industry, the main
difference between the various products being in their molecular
weight and ionicity. The most recent synthetic polymers have
molecular weights in the order of millions and are frequently
used as conditioning agents.
It is generally agreed that polymer dosing causes a shift into
larger size aggregates/groupings of flocs. The sludge attains a
more open and porous structure and the dewatering rate increases.
Despite the dramatic effect of polymers on the dewatering rates,
views expressed in literature suggest that polymer conditioning
is not matched by changes in the extent of water release. Full-
scale investigations generally indicate that conventional
conditioning and dewatering processes yield cakes, which are
usually, characterised by a 18 - 25 % wt DS.
Considering the costs of conditioning and the potential health
risks associated with overdosing (toxicity of residual monomers
discharged to waterways), it is important to establish the
appropriate dose (termed as optimum dose) during conditioning.
The theme of optimum polymer conditions has been a target of much
research over the last 25 years. However, from a n appraisal of
the available literature, it appears that there is no universal
criterion for determining the optimum polymer dosage. It is
widely accepted that each sludge behaves in a different manner in
terms of the particular polymer demand, this mainly arising from
differences in the nature of the sludge. Given the variety of
polymeric flocculants available to the water industry, the
identification of the optimum conditions depends on a series of
factors related to the sludge nature, the polymer propert ies,
costs and disposal route and the dewatering efficiency. Thus, it
is prudent to note that an optimal polymer dose actually depends
on the specific requirements of the treatment process.
Generally, the symptoms of polymer dosing can be gauged by an
a rray of tests. Standard tests such as the Capillary Suction
Time (CST) and the Specific Resistance to Filtration (SRF) have
featured for decades as means of selecting an optimum dose, the
optimum often interpreted at the minimum CST and SRF. These
standar d tests have been criticised over the years, limitations
have been identified and several modifications have been
suggested. New approaches include the investigation of the
rheological properties of conditioned sludge, the viscosity of
the liquid stream (centrate / filtrate) and the charge status of
the conditioned sludge and / or the liquid stream. Despite the
wealth of studies following the theme of optimum dosage
conditions, little is known on the precise factors that control
the optimum dosage as identified in the individual tests and how
these should be interpreted.
back to top
Aims of the study
The main aims of this study were:
(1) to provide insight into the factors that control the
optimum dosage of a nonionic polymer when dosed in an alum sludge
derived from treatment of coloured water - low in turbidity and
(2) to identify which of the tests (if any) provide the best
guide of the optimum dose in practice.
back to top
- It has been demonstrated that Size Exclusion
Chromatography can be used for the measurement of a
variety of polymers following alum sludge conditioning.
The detection limit for a nonionic polymer was 0.066 mg/l
in sludge supernatant and the technique's throughput was
at 12 samples per hour. Although the method appears to be
promising for waterworks sludges, it has been apparent
that SEC does not appear to be viable for measuring
polymer residuals in digested sewage sludge because of
strong interferences induced by substances in sludge of
similar molecular weight as the polymer.
- CST and SRF displayed optimal behaviour which was
controlled by the initial sludge solids concentration.
The corresponding dosages were equivalent to 1.3 and 1.6
kg/T respectively. Studies of the nonionic polymer
adsorption showed that the optimum dosages emanating from
the CST and SRF tests correspond to a 70 - 85 % of the
- The viscosity approach showed a clearly identifiable
minimum, which occurred at a dose, which was proportional
to the initial sludge solids concentration. From analysis
of the viscosity trends, the viscosity of the centrate
appears to be controlled by the sum of the viscosity
contributions of pure water, electrolytes, residual
polymer and turbidity. The turbidity and residual polymer
contributions appear to control the position of the
viscosity minimum. The viscosity of pure water and the
electrolytic contribution behave as constants for a given
- From nonionic polymer adsorption studies on freshly
prepared hydroxide precipitate and fresh alum sludge at
similar Al content, it was clearly demonstrated that the
adsorption behaviour of the sludge is dominated by the
adsorption on the precipitate.
- The magnitude of the SRF and its response to the nonionic
polymer dose appeared to be controlled by the dewatering
characteristics of the aluminium hydroxide.
- It was clearly shown that polymer dosing affects the rate
but not the extent of water release.
- From studies of the in situ polymer dosage at three
waterworks treating low turbidity - coloured waters, the
estimates of the dosage in practice were found to be
higher than the dosage corresponding to optimal behaviour
of CST and SRF and be in the domain of adsorption
- From the literature review it was noted that there was no
universally accepted criterion for identifying optimum
polymer dosage conditions. The current study emphasizes
the lack of correspondence between the estimates of the
optimum dose from an array of laboratory - based
techniques and indeed with the dosages used in practice.
Of the tests which were appraised we consider the
centrate viscosity approach as being the closest to
describing dosages in practice.
back to top
The contributions of the research
The main contributions of this research work can be listed as
- the development of an SEC methodology for measuring
polymer residuals following alum sludge conditioning can
provide a valuable pathway for understanding polymer
usage and its mechanisms;
- greater insight into the role of the hydroxide
precipitate in controlling polymer demand and dewatering
- insight into the mechanisms controlling the centrate
- insight into the efficacy of laboratory - based test
methods vis a vis the appropriate dosages used in
back to top
- Abu - Orf M M and Dentel S K. Laboratory studies of
liquid stream viscosity and streaming current for
characterisation and monitoring dewaterability, in
optimal dosing of Coagulants and Flocculants, IWSA - IAWQ
Workshop 1994, 10, pp. 65 - 80.
- Abu - Orf M M and Dentel S K. Polymer dose assessment
using the streaming current detector. Water Environment
Research, Vol 69, No 6, pp.1075 - 1085, Sep / Oct 1997.
- Abu-Orf M M and Dentel S K. Effect of mixing on the
Rheological characteristics of conditioned sludge: Full -
scale studies. Wat. Sci. Tech. Vol. 36, No. 11, pp. 51 -
- Bache D H and Hossain M D. Optimum coagulation conditions
for colored water in terms of floc properties. J. Water
SRT - Aqua, Vol 40, No 3, pp 170 -178, 1991.
- Bache D H, Johnson C, McGilligan J F and Rasool E. A
conceptual view of floc structure in the sweep floc
domain. Proc. 4th Int. Conference of the IAWQ - IWSA
Joint Specialist Group on Particle Separation.
Jerousalem, Oct. 1996, 25 - 32.
- Bishop M M, Cornwell D A, Rolan A T, Bailey T L.
Mechanical dewatering of alum solids and acidified
solids: An evaluation. J AWWA, Sep 1991, pp. 50 - 55.
- Christensen J R, Sorensen P B, Christensen G L and Hansen
J A. Mechanisms for Overdosing in sludge conditioning. J.
of Env. Engng, Vol 119, No. 1, January/February, 1993.
- Dentel S K and Abu - Orf M M. Laboratory and Full scale
studies of liquid stream viscosity and streaming current
for characterisation and monitoring of dewaterability.
Wat. Res. Vol 29, No. 12, pp. 2663 - 2672, 1995.
- Dentel S K and Abu-Orf M M. Application of the streaming
current detector in sludge conditioner selection and
control. Wat. Sci. Tech. Vol 28, No 1, pp. 169 - 179,
- Dentel S K and Gosset J M. Mechanisms of Coagulation with
aluminium salts. J. AWWA, Apr 1988, pp. 187 - 198.
- Dentel S K, Griskowitz N J, Chang L-L. WERF: Analysis of
Polymer Concentrations in Wastewater Process Streams.
69th WEFTEC, Dallas, Texas, 1996.
- Dentel S K, Wehnes K M and Abu-Orf M M. Use of streaming
current and other parameters for polymer dose control in
sludge conditioning. Proceedings of the 6th Gothenburg
Symposium 1994, June 20 -22, 1994, Gothenburg, Sweden,
Rudolf Klute & Hemann H. Hahn (Eds), Springer -
- Dentel S K. Evaluation and Role of Rheological Properties
in Sludge Management, Wat. Sci. Tech. Vol 36, No. 11, pp.
1 - 8, 1997.
- Edwards G A and Amirtharajah A. Removing color caused by
Humic acids. J. AWWA, March 1985, pp. 50 - 57.
- Edwards M. Predicting DOC removal during enhanced
coagulation. J. AWWA, Vol. 89, Issue 5, 78 - 89, 1997.
- Edzwald J K. Coagulation in Drinking water treatment:
Particles, Organics and Coagulants, Wat. Sci. Tech 27
(11), pp. 21-35, 1993.
- Gregory J. Effects of polymers on colloid stability In:
The Scientific Basis of Flocculation (edited by Kenneth J
Ives), Sijthoff & Noordhoff, 1978.
- Gregory J. Polymer adsorption and flocculation in sheared
suspensions. Colloids and Surfaces, 31(1988) 231 - 253.
- Herwijn A J M . Fundamental aspects of sludge
characterisation. PhD Thesis. Technical University
- Johnson C. Chemical factors in floc formation and
structure. PhD Thesis, University of Strathclyde, 1998.
- Knocke W R and Wakeland D L. Fundamental characteristics
of water treatment plant sluges. J. AWWA, pp. 516 - 523,
- Knocke W R and Zentkovich T L. Effects of mean cell
residence time and particle size distribution on
activated vacuum dewatering characteristics, Vol 58, No
12, pp. 1118 - 1123, Dec. 1986.
- Knocke W R, Dishman C M and Miller G F. Measurement of
chemical sludge floc density and implications related to
sludge dewatering. Water Environment Research, Vol 65,
No. 6, September/October 1993.
- Knocke W R, Ghosh M M and Novak J T. Vacuum Filtration of
Metal Hydroxide Sludges. J. of Env. Engng, Vol 106, No.
EE2, pp. 363 - 376, April 1980.
- Knocke W R, Hamon J R and Dulin B E. Effects of
coagulation on sludge thickening and dewatering. J. AWWA,
June 1987, pp. 89 - 98.
- La Heij E J. An analysis of sludge filtration and
expression. PhD Thesis. Technical University Eindhoven,
- La Mer V K and Healy T W. The role of Filtration in
Investigating Flocculation and Redispersion of Colloidal
Dispersions. J. Physic. Chem., 1963, 67, 2417.
- La Mer V K. Filtration of colloidal dispersions
flocculated by anionic and cationic polyelectrolytes.
Disc. Faraday Soc, 1966, 42, 248 - 254.
- Moody G M. Pre-treatment Chemicals. Filtration and
Separation, pp. 329 - 336, April 1995.
- Novak J T and Bandak N. Chemical conditioning and the
resistance of sludges to shear. J WPCF, Vol 61, No 3, pp.
327 - 332, Mar 1989.
- Novak J T and Bandak N. The effect of shear on the
dewatering of water treatment residuals. J. AWWA, Nov
1994, pp. 84 - 91.
- Novak J T and Calkins D C. Sludge dewatering and its
Physical Properties. J. AWWA, pp. 42 - 45, January 1975.
- Novak J T and Lynch D P. The effect of shear on
conditioning: Chemical requirements during mechanical
sludge dewatering. Wat Sci Tech. Vol 22, No 12, pp. 117 -
- Vesilind P A. Capillary suction time as a fundamental
measure of sludge dewaterability. J. WPCF, Vol. 60, No.
2, pp. 215 - 220, February 1988.
- Wakeman R J, Sabri M N and Tarleton E S. Factors
affecting the formation and properties of wet compacts.
Powder Technology, 65 (1991), 283 - 292.
- Warden J H. Thickening and Dewatering of Hydroxide
sludges. In: Solid Liquid Separation, Gregory J (Ed),
Ellis Horwood Ltd., 1984.
- Wen H J, Liu C I and Lee D J. Size and density of
flocculated sludge flocs. J. Environ. Sci. Health, A32
(4), 1125 -1137 (1997).
- Wu C C and Huang C. Effects of Recycling-Sludge operation
on the structure and moisture content of floc in Water
treatment plant. Separation Science and Technology,
32(17), pp. 2873 - 2882, 1997.
back to top