Flotation apparatus and process

Details Flotation apparatus and process Flotation apparatus and process

1998-06-19

2001-04-17

Lithgow, Thomas M. (Department: 1724)

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

C210S802000, C210S804000, C210S294000, C210S320000, C210S221200, C210S522000

Reexamination Certificate

active

06217777

ABSTRACT:

The present invention relates to the purification of polluted water, such as surface water, ground water as well as sewage water, and relates more specifically to an apparatus for such purification and to a process that can be performed in such apparatus.
In all purification of polluted water, such as surface water, ground water as well as sewage water, the separation processes are of utmost importance. For the manufacture of drinking water or for other purification of sewage water contaminants are removed in the form of particles. Said particles may occur naturally in the water entering the purification plant or they are formed in the biological or chemical processing. Normally, particles descend to the bottom of a vessel where they are separated as a bottom sludge. During history this has been the traditional manner of purifying water.
Another method for the purification of water is flotation which has been developed and refined since the 1950:ies. In flotation the particles are separated from the water by transportation up to the surface by means of very small air bubbles. These attach to the particles and form so called aggregates, the ascending rate of which is five to ten times that of the descending rate of corresponding particles.
In a traditional flotation plant for the treatment of water the water to be treated moves downward and leaves the tank from exit means near the bottom. A normal surface load for example in a process for potable water is 7-10 m/h. When this load is exceeded the downward velocity of the water increases to such an extent that air bubbles from the dispersion which normally rise to the surface leave the tank together with the outlet water. These air bubbles enter the subsequent filtration step where they behave like particles. The head loss of the filter increases dramatically and causes a more frequent backwash. The operational cost of the filtration process is thereby subject to increase. It seems as though the entrained air bubbles are not carrying any particles or very few particles since the turbidity on water samples containing air bubbles after the air bubbles have been removed increases only slightly.
The air bubble carry-over in the effluent is thus a limiting factor for the surface load of the process based on dissolved air flotation.
The main object of the present invention is to provide a flotation apparatus for the purification of polluted water, said apparatus enabling a significant increase of the surface load without significant entrainment of air bubbles in the discharged water.
Another object of the present invention is to provide a process for the purification of polluted water by flotation, in which the disadvantages associated with the prior art techniques will be substantially eliminated or at least significantly reduced.
Other objects and advantages of the present invention will be made clear in the following disclosure of specific embodiments.
The present invention is based on a new concept where the lamellae clarification technique is applied to flotation techniques in using an assembly of spaced elements defining passages for the flow of water through said elements where, instead of leaving the process from the top as in normal lamellae clarification the treated water is imparted a downward flow through the spaced elements to be discharged at the bottom. Air bubbles and particles attached thereto have a rising tendency but are forced by the flow of water at a high velocity downwards. During their downward transportation they will adhere to the back side of the spaced elements where they coalesce to form larger aggregates with other air bubbles until they have a rising force strong enough for leaving the spaced elements and rise to the top. Under this new concept all water to be purified has to pass from the top and down through the spaced elements to leave at the bottom.
Accordingly, the present invention provides for a lamellaeflotation apparatus for the purification of polluted water, comprising a flotation vessel having an inlet means for supplying polluted water to said vessel, an assembly of spaced elements defining passages for the flow of water through said elements, and outlet means for surface sludge and outlet means for treated water, characterized by flow directing means forcing the incoming polluted water to reach a level above said assembly and to flow downwardly through said assembly and towards said treated water outlet means.
In such flotation apparatus said water supply means is positioned at the bottom of said vessel at one end thereof.
The spaced elements constituting said assembly are preferably comprised by a set of spaced inclined parallel lamellae defining narrow passages for the flow of water, but may also be comprised by a set of spaced inclined parallel pipes forming such narrow passages.
Said water supply means is preferably constituted by inlet slot extending across substantially the whole width of the vessel.
It is also preferred that said treated water outlet means is positioned at the bottom of said vessel. In another embodiment according to the invention said outlet means is comprised by a flat box defining a bottom of said vessel, said box comprising a top wall and a bottom wall joined by side strips and being open at one end thereof and closed at the other end thereof, and said top wall being provided with a plurality of orifices communicating with said narrow passages for the flow of purified water toward said treated water outlet means defined by said one end of the box.
Said flat bottom box can also be comprised of squared pipes packed together being open at one end and closed in the other and with orifices communicating with said narrow passages for the flow and thus having the same function as the described flat bottom box.
In such embodiment said vessel is suitably provided with double side walls, the inside walls of which at the lower end thereof are provided with a plurality of openings communicating with said narrow passages defined by said lamellae, the purified water flowing via said openings upwardly between said sidewalls toward said treated water outlet means.
Furthermore, in the embodiment just described said water outlet means is preferably constituted by outlet weirs arranged at the top of said sidewalls, each weir being associated with a channel extending from one of said openings coextensively with and juxtaposed the corresponding narrow passage and positioned inside said sidewall.
In the flotation apparatus according to the invention it is preferred to include dispersion means positioned in association with said inlet means for the introduction of a gas for providing flotation.
The invention also provides for a process for the purification of polluted water by flotation, and said process involves the following steps:
a) introducing polluted water into a flotation zone at the top thereof while dispersing the water by the introduction of a gas therein;
b) generating a downward flow of the dispersed water through a zone of increased surface at such a flow rate that small gas bubbles are entrained with the water but coalesce on contact with said surface thereby rising to the top of the flotation zone;
c) removing skimmings from the top of the flotation zone and discharging separately purified water.
In the process according to this invention the surface load on the polluted water in the flotation zone preferably exceeds about 7 m/h and especially exceeds about 10 m/h.
In a preferred embodiment of the process according to the invention the polluted water is introduced at a level substantially equal to the bottom level of the flotation zone where it is dispersed with gas, and wherein the dispersed water is then imparted an upward flow outside of the flotation zone and is then directed to and distributed over said zone for downward flow therethrough.


REFERENCES:
patent: 3613889 (1971-10-01), Reed
patent: 3706384 (1972-12-01), Weijman-Hane
patent: 4160737 (1979-07-01), Plekerwood
patent: 4231867 (1980-11-01), Sutphin
patent: 4277347 (1981-07-01), Pielkenrood
patent: 4344845

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