Evaluation and performance of a tannin-based polymer as a coagulant in water treatment

Abstract

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 3 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 4 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.