AWWA WQTC64165

Metal salts are the most widely used coagulants for removing natural organic matter (NOM). The addition of these metal salts acts to reduce the negative charge of the organic matrix, destabilizing the suspension and promoting aggregation of the small particles that form in to flocs. Coagulation of NOM is a charge related process, therefore an appropriate measurement of the charge in the suspension is by using zeta potential. This measures character of the charge on the colloids and particles. Heterogeneous systems such as NOM are composed of a mixture of different organic compounds that are inherently difficult to characterize, so it is important to note that zeta potential measurements relate to the colloids in the system but cannot directly measure soluble compounds. Only once the soluble components have reacted with the coagulant into an insoluble form can they be detected. However, these measurements still provide a reproducible relationship which can be used to understand and optimize the coagulation process. This has been shown in previous work where zeta potential and low dissolved organic carbon (DOC) residuals and turbidity are low and robust when the zeta potential is kept between -10 mV and +3 mV. Most research in NOM coagulation has concentrated on removal performance with little emphasis on the quality and operational properties of the flocs that form. When the carbon load on to a water treatment works (WTW) is high, NOM removal can be compromised. There is also some evidence that floc properties are adversely affected with operational experience suggesting problems on water treatment processes such as floc breakage during solid-liquid separation in the clarifiers and also during filtration. The increased particle load on to the filters can lead to significantly reduced filter run times. The physical properties of flocs are therefore considered to be very important for the efficient operation of a WTW. For example, resistance to shear is important as newly exposed surfaces of aggregates may alter the surface charge of the floc aggregate, leading to partial restabilization of the colloidal suspension. Furthermore, floc breakage and the formation of smaller particles may also result in lower removal efficiencies. The strength of the floc is therefore a very important operational parameter. Floc strength is derived from a combination of interactions between the primary particles of the floc, including electrostatic forces and steric/bridging mechanisms. While direct measurement of floc strength is difficult, many workers have attempted to do this by considering the energy required to break flocs under tension, compression or shear. The following empirical relationship has generally held true for most floc systems, where d is the floc diameter, C is the floc strength constant, G is the average velocity gradient and ¿¿¿¿ is the stable floc size exponent. A modified version has also been used, with RPM in place of the velocity gradient G. The objective of this paper was to investigate both the impact of the ratio of organic in the floc matrix and also the influence of zeta potential, in order to identify the relative importance of surface charge and carbon content in relation to the physical properties of NOM flocs. Includes 19 references, table, figures.