In drinking water treatment, there is growing interest in the application
of natural cationic polymers that provide an alternate means to achieve enhanced
coagulation. A review of the relevant literature concerning the coagulation
mechanisms and action of polymer is presented with particular reference to the
polymer character, such as polymer type, charge density and molecular weight. In
addition, basic knowledge of a novel coagulant, a tannin-based modified polymer,
is described.
A full characterization of the tannin-based polymer (TBP) has been
undertaken to provide an unambiguous description of the polymer, or monomer,
structure. Some specialised newer analytical techniques in combination with
several old classical techniques for polymer examination have been used to
determine the chemical nature of the TBP, including its dissociation and
precipitation behaviour, molecular weight, charge density, charge variability with
pH, elemental content, functional group and chemical bonding, etc. The overall
assessment of TBP indicated that it can be classified as a medium molecular
weight polymer with a non-quaternized amine group and a charge density that
varies with pH and time.
The fundamental coagulation mechanisms and stoichiometry of
suspended solid/dissolved organic matter with TBP have been investigated
through laboratory experiments. Suspensions of kaolin clay and humic acid have
been flocculated in a Gator jar using TBP as a sole primary coagulant. Using
online analysis by Photometric Dispersion Analyzer (PDA), the relative floc size
was indicated by a Flocculation Index (FI) during the coagulation process and the
optimal concentration of coagulants was determined in overall terms by NPDOC,
turbidity, colour, UV/Vis absorbance and Floc volume. Under given conditions
the optimum dose of TBP corresponded to that required for maximum a
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Flocculation Index (FI). The optimum dose of TBP was found to depend on the
charge density of TBP and hence on the pH values of the solution. At neutral and
acid condition, quantitative evidence of a stoichiometric relationship between
TBP dosages with the concentrations of model impurity was illustrated.
Complicating effects were present at higher pH values. The coagulation
behaviour of TBP was generally in agreement with the coagulation mechanisms
widely observed, and typical, of cationic polymers. It was evident that the
coagulation performance and kinetics of TBP was also influenced by other factors,
such as the velocity gradient, ageing of polyelectrolyte and reactor design. For
comparative purposes, alum and a commonly used synthetic cationic polymer
(polyDADMAC) were also assessed in this study as coagulants.
The potential benefits to improving coagulation performance through the
combination of TBP with alum as a dual primary coagulant have been
investigated. Coagulation experiments using different model waters were carried
out under conditions designed to optimize the maximization of flocculation. A
full matrix of coagulation tests demonstrated that a unique optimal dosage of
combined alum and cationic TBP exists at a given pH and component
concentration. In this case, a significant reduction of alum with an improvement
of coagulation efficiency was achieved. An approach to minimize the residual
soluble TBP in treated waters and increase the floc settling by attaching the TBP
to an inert solid (fine sand) has been attempted. The coagulation performance
using this particle suspension (‘solid bound TBP’) as coagulant was found to be
inferior with a high shear rate in accordance with the floc strength interpretations
of TBP. Additional tests with raw waters were carried out to confirm the validity
of the findings from the model water experiments using TBP and the alum/TBP
combinations as primary coagulants.
In the light of these studies, the relative importance of TBP’s chemical
properties, especially molecular weight, charge density and solubility, as crucial
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parameters of coagulation mechanism is discussed. Furthermore, approaches to
improve flocculation performance with either partial replacement of inorganic coagulant or the combination with microsand are suggested and analysed.