AWWA WQTC64165

Metal salts are the most widely used coagulants for removing natural organic matter (NOM). The addition ofthese metal salts acts to reduce the negative charge of the organic matrix, destabilizingthe suspension and promoting aggregation of the small particles that form in to flocs. Coagulation of NOM is a charge related process, therefore anappropriate measurement of the charge in the suspension is by using zeta potential.This measures character of the charge on the colloids and particles. Heterogeneoussystems such as NOM are composed of a mixture of different organic compounds thatare inherently difficult to characterize, so it is important to note that zeta potentialmeasurements relate to the colloids in the system but cannot directly measure solublecompounds. Only once the soluble components have reacted with the coagulant intoan insoluble form can they be detected. However, these measurements still provide areproducible relationship which can be used to understand and optimize thecoagulation process. This has been shown in previous work where zetapotential and low dissolved organic carbon (DOC) residuals and turbidity are low and robust when the zetapotential is kept between -10 mV and +3 mV. Most research in NOM coagulationhas concentrated on removal performance with little emphasis on the quality andoperational properties of the flocs that form.When the carbon load on to a water treatment works (WTW) is high, NOM removalcan be compromised. There is also some evidence that floc properties are adverselyaffected with operational experience suggesting problems on water treatmentprocesses such as floc breakage during solid-liquid separation in the clarifiers and alsoduring filtration. The increased particle load on to the filters can lead tosignificantly reduced filter run times. The physical properties of flocs are thereforeconsidered 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 thesurface charge of the floc aggregate, leading to partial restabilization of the colloidalsuspension. Furthermore, floc breakage and the formation of smaller particles mayalso result in lower removal efficiencies. The strength of the floc is therefore a very important operational parameter. Flocstrength is derived from a combination of interactions between the primary particlesof the floc, including electrostatic forces and steric/bridging mechanisms. Whiledirect measurement of floc strength is difficult, many workers have attempted to dothis by considering the energy required to break flocs under tension, compression orshear. The following empirical relationship has generally held true for most flocsystems, where d is the floc diameter, C is the floc strength constant, G is the average velocitygradient 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 inthe floc matrix and also the influence of zeta potential, in order to identify the relativeimportance of surface charge and carbon content in relation to the physical propertiesof NOM flocs. Includes 19 references, table, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2006

Number of Pages:

13

File Size:

1 file , 840 KB