The Mechanism of Organic Removal During Coagulation

Abstract

<p>Coagulation is a common water treatment step primarily designed to aggregate and thereby help remove true particulate (turbidity causing) matter. Organic compounds of natural origin (i.e. humic and fulvic acids) have also been observed to be removed by Coagulation. This research was designed primarily to identify the limiting mechanisms responsible for the removal of natural organics by the coagulation process. This identification is thought to be crucial in the optimization of this important water treatment step as it may help to maximize the benefits obtained from coagulants.</p> <p>The examination of the literature related to this topic suggests two possible removal routes:</p> <p>(i) a reaction whereby the fulvic acids form an original compound with the coagulating ion Al³⁺ or one of its hydroxy complexes.</p> <p>(ii) the adsorption of the fulvic acid molecules or jons onto the surface of a solid precipitating independently of these compounds.</p> <p>These two possibilities are examined in detail on theoretical grounds, and two following pieces of information are defined as being discriminatory with respect to the two removal routes:</p> <p>(i) The shape of the isotherm, as defined in typical adsorption studies.</p> <p>(ii) The ligand number or OH/Al ratio of the precipitate, accessible by processing the dissolved Aluminum data with respect to pH.</p> <p>These considerations lead to an experimental design allowing the convenient evaluation of these characteristics. An array of nine treatment dosages and four operating pH is applied on two raw waters, using four Aluminum based coagulants.</p> <p>The results suggest that under these experimental conditions (dosage between .1 and 1 mM Al/L pH between 5 and 8.5), the Fulvic acids are removed by Adsorption onto Al(OH)₃, regardless of the type of coagulant. Increasing pH and the presence of SuIfate in the coagulant were found detrimental to this adsorption. Increasing OH/Al ratio in the coagulant is detrimental at low pH, low dosage, and becomes beneficial at neutral to mildly alkaline pH conditions. A two stage treatment scheme was found efficient at neutral to mildly alkaline pH, using Alum.</p>