Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1997-05-30
2003-12-30
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S661000, C210S665000, C210S668000, C210S676000, C210S679000, C210S692000, C210S694000, C210S695000
Reexamination Certificate
active
06669849
ABSTRACT:
The present invention relates to water treatment, in particular to a process for the removal of dissolved organic carbon from water.
The processes used in water treatment are largely a function of raw water quality. Potable water supplies often contain unacceptably high levels of organic compounds dissolved, dispersed or suspended in raw water. These organic compounds are referred to herein as dissolved organic carbon (DOC). Other terms used to describe DOC include total organic carbon, organic colour, colour and natural organic matter. DOC often includes compounds such as humic and fulvic acids. Humic and fulvic acids are not discrete organic compounds but mixtures of organic compounds formed by the degradation of plant residues.
The removal of DOC from water is necessary in order to provide high quality water suitable for distribution and consumption. A majority of the compounds and materials which constitute DOC are soluble and not readily separable from the water. The DOC present in raw water renders conventional treatment difficult and expensive.
The provision of a safe potable water supply often requires treatment of water to make it aesthetically acceptable. The removal of suspended matter and colour is an important aspect of this treatment. Two approaches are commonly used for the removal of suspended matter and colour. One involves coagulation and the other membrane filtration.
In the process involving coagulation, a coagulant is applied to destabilise suspended matter and colour so that they coalesce and form a floc, which can then be physically removed by methods such as floating, settling, filtration or a combination thereof. Coagulants such as alum (aluminium sulphate), various iron salts and synthetic polymers are commonly used in processes for water treatment. However, many raw water sources have high levels of DOC present, which is the main cause of the colour, and the DOC reacts with the coagulant requiring a higher coagulant dose than would be required for removal of suspended matter alone. The bulk of the floc formed may then be removed by sedimentation or flotation and the water containing the remainder of the floc passed through a filter for final clarification. However, even after such treatment the treated water may contain as much as 30-70% of the initial DOC.
In the membrane filtration process the water is filtered through a membrane system. However, where the water contains high levels of DOC the membranes tend to be fouled by the DOC, thereby reducing the flux across the membrane, reducing the life of the membranes and increasing operating costs. Membrane systems designed to handle water containing high levels of DOC have much higher capital and operating costs than conventional membrane systems used for the production of potable water.
Ion-exchange resins have been used in water treatment processes for the removal of DOC by passing water treated to remove turbidity and other suspended particles through ion-exchange resin packed in columns or the like. Passing untreated water through a packed resin can cause the packed resin to become clogged and ineffective, problems similar to those faced in membrane filtration.
The present invention provides a process for the reduction or elimination of DOC from water using ion-exchange resins which can be conveniently separated from the water prior to subsequent treatment and its distribution and consumption. Accordingly, we provide a process for the removal of dissolved organic carbon from water, which process includes the following steps:
a. adding an ion-exchange resin to water containing dissolved organic carbon;
b. dispersing the resin in the water to enable adsorption of the dissolved organic carbon onto the resin; and
c. separating the resin loaded with the dissolved organic carbon from the water.
The ion-exchange resin is dispersed in the water so as to provide the maximum surface area of resin to adsorb the DOC. Dispersal of the ion-exchange resin may be achieved by any convenient means. Typically the resin may be dispersed by mechanical agitation such as stirrers and the like, mixing pumps immersed in the water or air agitation where a gas is bubbled through the water. Sufficient shear needs to be imparted on the water to achieve dispersal of the resin.
In some small scale operations for the ion exchange resin may be dispersed in a semi-fluidized bed provided pumping costs are not economically unfeasable. The use of a semi-fluidized bed is not only a convenient means for dispersal of the ion exchange resin but provides for the ready separation of the loaded resin from the water once DOC is adsorbed onto the ion exchange resin.
Separating the resin loaded with DOC from the water may be achieved by settling or screening or a combination thereof. Screening of the loaded resin from the water may be achieved by any convenient means. The screens may be selected with consideration for the size of resin particles to be removed from the water. The configuration of the screens may be such that clogging of the screens is reduced.
In a preferred embodiment, the ion-exchange resin may be more dense than the water and tend to settle to the bottom of the tank. This settling facilitates the convenient separation of the loaded resin from the water. Settling may be facilitated by the use of tube settlers and the like. The resin may then be collected by various means including vacuum collection, filtration and the like. It is preferable that the separation and collection means do not cause mechanical wear which may lead to attrition of the resin.
When a continuous fully suspended system is used, the resin may conveniently be separated from treated water by gravity settling. Based on resin characteristics, very effective (>99% solids removal) gravitational settling is achieved in high-rate settling modules with retention times less than 20 minutes.
In a preferred process for separating the ion-exchange resin from the water the bulk of resin particles settle out in the first quarter of the separating basin length which is devoid of settler modules (“free-flowing” settling). Further removal of resin particles (“enhanced” settling) from treated water is performed in the settler compartment filled with modules which may be either, tilted plates or tubular modules. The bottom of the settler is designed for collection of resin particles in cylindrical, conical or pyramidal hoppers from which the resin particles are pumped back to the front of the process. In this preferred process some mixing of the settled resin in the hoppers may be required to keep it in a fluid condition and to ensure uniform resin concentration of resin in the recycle system.
The ion-exchange resins suitable for use in the process of the present invention have cationic functional groups. The cationic functional groups provide suitable sites for the adsorption of the DOC.
It is preferred that the ion-exchange resins have a diameter less than 100 &mgr;M, preferably in the range of from 25 &mgr;M to 75 &mgr;M. This size range provides an ion-exchange resin which can be readily dispersed in the water and one which is suitable for subsequent separation from the water. The size of the resins affects the kinetics of adsorption of DOC and the effectiveness of separation. The optimal size range for a particular application may be readily determined by simple experimentation.
It is preferred that the ion-exchange resin is macroporous. This provides the resins with a substantially large surface area onto which the DOC can be adsorbed.
Water treatment processes involve the movement of water by stirring, pumping and other operations which can deleteriously effect the ion-exchange resin. It is preferred that the resin is manufactured from tough polymers with polystyrene crosslinkage. The resin may be selected to give the optimum balance between toughness and capacity.
In the process of the present invention the amount of ion-exchange resin necessary to remove DOC from water is dependent on a number of factors including the level of DOC initially present in the water
Bursill Donald Bruce
Drikas Mary
Morran James Young
Nguyen Hung Van
Pearce Veronica Laurel
Cintins Ivars
Orica Australia Pty Ltd
Pillsbury & Winthrop LLP
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