Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2002-04-26
2004-03-23
Cecil, Terry K. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S702000, C210S805000, C210S806000, C210S791000, C210S196000, C210S321600, C210S321650, C210S206000, C210S097000, C210S258000, C210S332000
Reexamination Certificate
active
06709599
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to processing of wastes by filtration. In particular, the present invention relates to the processing of radioactive waste streams at nuclear facilities such as nuclear power plants to separate the contaminants from an aqueous effluent.
BACKGROUND OF THE INVENTION
In a nuclear power plant and at other manufacturing and processing facilities where nuclear materials are handled, the waste streams from the facility operations must be processed prior to discharge or reuse in order to remove radioactive contaminants for disposal. These waste streams come from a variety of sources, such as a spent fuel pool, floor drains, and resin tank drains. Because of increasing disposal costs, the separation of the contaminants from the water that carries them has become more and more important. The goal of this separation is to (1) remove sufficient contamination from the aqueous waste stream so that the resulting effluent can be reused or released to the environment and (2) reduce the volume of the waste that must be disposed of.
A number of techniques are used to separate wastes from waste streams, including filtration and ion exchange. Because the wastes may be in the form of particulate of varying sizes and in the form of dissolved ions, these two techniques are commonly used in a particular sequence. Filtration is essentially the removal of particulate from water by passing the water through a porous structure leaving the particles, whose passage is blocked by the structure, behind. Sometimes more than one type of filter is used. By using ever finer filters, including mechanical filters, microfilters, ultra filters, and nano-filters, a very high percentage of particulate can be removed. These various filtration devices are used in a specific sequence; the coarser filters being used first to remove the larger particulate. Then finer filters are used to remove the smaller particulate.
This approach makes good sense. If a fine filter is used first, the amount of particulate it would remove would be so great that the filter, blinded with particulate both fine and coarse, would stop the flow altogether soon after being placed in service. Using filters in sequence from coarse to fine assures that the throughput of each filter is as high as possible. Furthermore, because filters with smaller pore size are generally more expensive, it makes economic sense to use fine filters only for filtering the smallest particles and not also particles that could be filtered with less expensive filters.
From the finest filters used, the waste stream is routed to an ion exchange bed and/or reverse osmosis membranes where dissolved contaminants can be removed. The resulting effluent is nearly free of both particulate and dissolved solids.
Referring to
FIG. 1
, which illustrates the current practice in a nuclear power plant for handling aqueous waste streams, the aqueous wastes are collected in a waste holdup tank
10
. Heavier particulate will settle in the tank, the waste containing suspended particulate is directed through an ultrafiltration membrane
20
. Ultrafiltration membrane
20
generally has the capability of ejecting particles of about 0.005 microns or larger at an operating pressure of 3-10 bar. Low molecular weight substances such as sugars and salts pass through it. The effluent from ultrafiltration membrane
20
is next directed through a reverse osmosis membrane
30
or an ion exchange bed
40
. Essentially, only water passes through reverse osmosis membrane
30
at its normal operating pressure range of 20-60 bar. The effluent from reverse osmosis membrane
30
or ion exchange bed
40
will be highly purified.
The wastes from each of these steps: the settled particulate from waste tank
10
, the rejects from ultrafiltration membrane
20
and reverse osmosis membrane
30
and, eventually, the resins from ion exchange bed
40
will be subjected to further processing
60
to stabilize them for disposal in various ways, including drying and/or solidifying them in a cementitious medium.
The process just described works well. It produces a very clean effluent and the waste itself can be disposed of safely. However, it focuses solely on obtaining a clean effluent. As disposal prices have continued to climb, there has been a growing need to reduce the volume of wastes being disposed of. Thus, there is a need for a way to process wastes that results in less volume and easier handling but does not compromise the quality of the effluent.
SUMMARY OF THE INVENTION
According to its major aspects and briefly recited, the present invention is a method and apparatus for processing aqueous waste streams, especially waste streams from nuclear power plants. The present process reduces the volume of waste to be disposed of compared to the prior art process without affecting the purity of the effluent. It also simplifies handling of the wastes.
In order to achieve this reduction in volume of waste, a portion of the reject from the ultrafiltration step described above is processed in an additional filtration step. The effluent from this additional step is returned upstream of the ultrafiltration membrane and “recycled”; that is, it again repeatedly directed to the ultrafiltration membrane and then through the additional filtration step. The concentrate from this additional step may then be disposed of directly after de-watering.
The additional step includes passing a stream of concentrate collected by the ultrafiltration membrane through a microfilter at a low flux. The velocity through the filter is maintained very low to maximize the solids loading of the microfilter. As solids are collected on the microfilter, the pressure drop will eventually increase. The filter will be replaced when either the maximum allowable pressure drop across it, or, in the case of filtration of radioactive contaminants, the maximum allowable radiation dose is reached.
The microfilter removes substantially all the particulate, including particles smaller than its pore size, because, for all practical purposes, it has a nonzero particle removal efficiency over the range of particle sizes in the waste stream. As long as the particle removal efficiency is greater than zero, the filter will eventually, after repeated passes, remove substantially all particles. By recycling the microfilter effluent to the ultrafiltration membrane, the concentration of the particles in the recycle loop will build up to a level where the rate of particle removal in one pass through the microfilter is equal to the rate of particles being introduced with the feed into the ultrafiltration membrane, establishing an equilibrium.
One contributing factor as to why the microfilter will remove particles smaller than its pore size is due to particle agglomeration; however, agglomeration is just one of the reasons that particles are removed by the microfilter (in fact, agglomeration is not necessary for the microfilter to work as long as other particle removal mechanisms produce a particle removal efficiency that is greater than zero). Agglomeration is a tendency of the particles to form clusters that interlock. The interlocking clusters define narrow, twisting passages through a cake-like matrix that allow water to flow. These passages are irregular, that is, they change direction and cross section, resulting in a filtering action that will trap particulate including particulate that would otherwise pass through the microfilter.
Other forces, such as adsorption, contribute to the removal of small particles. The formation of the cake of particulate against the upstream side of the filtration medium also contributes to the removal of small particles. The cake in effect becomes part of the filter. Cake formation reduces filter pore size and increases the depth of the filter, increasing the likelihood that a small particle will become trapped or adsorbed by the filter. The low flux helps to are that the forces acting on particles from fluid flow do not disrupt the adsorption forces or break up particulate aggrega
Raymount, Jr. John M
Rosenberger Stefan
Underwood Randall B.
Whitehead Howard
Wilson James H
Cecil Terry K.
Hardaway, III John B
Mann Michael A
Nexsen Pruet Jacobs & Pollard LLC
Rwe Nukem Corporation
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