하천수 중 NOM과 하수방류수 중 인 처리를 위한 알루미늄계 응집제의 적용

Abstract

Using aluminium based coagulants for the removal of natural organic matter and turbidity from surface water and phosphorus from sewage effluent were investigated. Conventional Al salts, such as alum and AlCl3, and Polyaluminum Chlorides (PACl) with different r (r = [OH]/[Al]) values were prepared in laboratory. Based on the difference in reaction kinetics of Al species and ferron reagent, the hydrolytic Al species in aluminum based coagulants were classified as fast reacting monomeric Ala, slow reacting polymeric Alb, and very slow/ or non reacting colloidal/precipitate Alc. The results showed that the higher the r value of PACl, the lower the Ala and the higher the Alb and Alc obtained. The characteristics of raw waters were analysed before performing jar tests. Various fractions of dissolved organic matter (DOM) in The Nakdong River water (Busan, Korea), such as hydrophobic acid, hydrophilic acid, and transphilic organic were isolated and coagulation behavior of each fraction was investigated. Also, phosphorus in sewage effluent from Nambu Sewage Treatment Plants was classified as organic-, condensed-, and ortho-P. Preformed monomeric Ala is the most unstable species and quickly transformed to other Al species while preformed Alb and Alc were relatively stable during coagulation. Performance of the coagulants was controlled not only by preformed species but also by those forming in situ during coagulation. The presence of Al species formed in situ strongly depends on coagulant dose and coagulation pH. At neutral and basic pHs, PACls containing more stable preformed Alb were more efficient for turbidity and organic matter removal. At slightly acidic pH, predominant Ala coagulants were more efficient since more Alb was formed in situ at this condition. Experiments with isolated hydrophilic acids, humic acids, and fulvic acids also indicated Alb was effective for removal of hydrophilic acids, the organic fraction that is most difficult to coagulate. Alb (preformed and in situ formed), therefore was effective for removal of turbidity and NOM. However, at high Al doses and high pH, where Alc was the predominant species formed in situ, the superior performance of Alb in removing of turbidity disappeared. Due to its bulky structure which is effective in sweep flocs mechanism, Alc is the most effective species for turbidity removal. Experiments conducted with sewage effluent showed that phosphorus removal was correlated well with the content of Ala in coagulants. Positive effects of Ala for the removal of P might be due to high ability of monomeric Al in forming the Al(OH)x(PO43-)3-x precipitates and in adsorption of phosphates into aluminum hydroxide precipitates that are formed predominantly in hydrolysis process of Ala coagulants. Alb, although has been known as the most effective species in removal of organic matter, did not show any superior performance in P removal. Alc (preformed and in situ formed) might be favorable for adsorption of P and this species also showed a positive effect on removal of suspended P due to its superior performance in removal of turbidity from water. The residual P concentrations obtained were strongly depended on the coagulant type used, or the contents of Ala, Alb, and Alc in the coagulants. Results obtained from the study could be used for making an appropriate coagulant for a preferential aim of coagulation. Prehydrolyzed degree of PACl (r value) used should be determined according to a specific water characteristic and a target substance that need to be removed during coagulation.