Study On Micro-flocsrestructuring For Enhancing Removal Of Humic Acids In Low Turbidity Water

Most of the sources of drinking water treatment plants are low turbidity water. In general, raw water with low turbidity can be difficult to treat, even using a higher coagulant dosage, due to the low content of particulate matters, lower probability of collision and aggregation, and lack of colloidal particles and other attachments for humic acid（HA） molecules. The treatment efficiency via simply adding coagulant is reduced; besides, the flocs produced with less compact structure have poor settlement. These problems have caused widespread concern. Some studies show that floc fragments have the potential of restructure and reuse, in addition, reuse of water treatment residuals is imperative. So study on micro-flocs restructure for improving removal of humic acids in low turbidity water is significant, in order to provide a viable new method of enhancing coagulation process for water treatment plants.Furthermore, the effects of different flocculation conditions（e.g. broken strength and additional coagulant）, a variety of factors（e.g. p H of raw water, initial concentration of HA, initial turbidity and temperature） and different aqueous environment background on HA removal efficiency were researched; we also explored the interface mechanisms of micro-floc restructuring for removal of humic acid in low turbidity water and analyzed amount of chemical agent to carry out a preliminary economic evaluation.The optimum parameters of micro-floc restructuring combined with PACl were conducted: PACl dosage was 6 mg/L, content of micro-flocs recycling was about 40±2 mg/L（corresponding to the blended water turbidity: approximately 30±5 NTU）. This process not only exhibits a better removal efficiency of HA than process of adding flocculent alone and kaolin powder directly under the same coagulant dosage, but also has a higher growth rate of floc（30 μm/min）. Various chemical agents（PACl, Al₂（SO₄）₃, Fe Cl₃ and KMnO₄） are all suitable for micro-flocs restructure process, especially, the macromolecule polymer-PACl get the best effect. Moreover, micro-flocs combined with flocculent has a good recyclability, and the change regularity of absorbance of HA through several reuse can be calculated by piecewise functions, which has good fitting relationships with the measured values.The broken flocs could not re-growth to the original particle size fully in Different hydraulic shear forces, and HA molecules adsorbed in flocs also have different degrees of release. Different hydraulic shear forces kinetic energy on floc produced broken are not fully restored to the original particle size, and adsorption in the floc humic acid molecules also have varying degrees of release. The micro-flocs broken under the shear force of 200 rpm/min will promote the removal efficiency of humic acid on the restructure process, but higher shear forces reduce the ability of humic acid removal. In addition, a small amount of additional flocculent（2,4 mg/L） after breakage can significantly improve floc strength factor and recovery factor and the removal efficiency of humic acid.Restructure flocs are more benefit to the removal of humic acid on condition of p H<=5, mainly because a large number of H+ could increase zeta potential, so adsorption and charge neutralizationcan functional better between micro-flocs and humic acid molecules; micro-flocs restructure process get higher removal efficiency of humic acid at the concentration range of 10¹5 mg/L, reaching more than 87%. However, the removal ability of restructure micro-floc for higher concentration of humic acid is limited. Micro-flocs restructuring process is more suitable for low turbidity raw water（especially <5 NTU）; the concentration of microcystis aeruginosa less than 35×10⁴ cell/m L has little effect on removal humic acids, but higher concentration of algae cell will block the adsorption of HA on micro-flocs, and the removal efficiency decreased to about 70%. In addition, for actual Songhua River purification, micro-flocs restructuring process gets better removal efficiency of UV₂₅₄ than that of conventional coagulation process only adding PACl.The micro-interface mechanisms of floc restructuring for enhancing removal of HA are mainly divided into two aspects:（1） The destabilized kaolin flocs contains a tetrahedral SiO₂ silica face, an alumina face carrying Al-OH groups, and edges containing both Si-OH and Al-OH sites, the humic acids molecules are adsorbed in flocs via van der Waals force, hydrogen bond, electrostatic adsorption, cation bond bridge;（2） flocs can restructure and be reused for several times after under-dosing flocculent, Subsequently, with continuous loading of HA molecules, the outer layer plays a major role in enhancing sorption capacity, because the flocs adsorbing HA have highly variable carbon functional group compositions such as-OH,-C=O and that lead to interaction with HA molecules in aqueous solution via hydrogen bond（R-O…HOOC-R） or hydrophobic interaction. The preliminary economic evaluation illustrates that micro-flocs recycling and restructuring can reduce coagulant dosage save water purification costs.