Liquid purification or separation – With means to add treating material – Plural distinct separators
Reexamination Certificate
2000-07-13
2001-11-06
Lithgow, Thomas M. (Department: 1724)
Liquid purification or separation
With means to add treating material
Plural distinct separators
C210S221200, C210S259000, C210S256000, C210S257100, C210S266000, C210S275000, C210S283000, C210S293000, C210S295000
Reexamination Certificate
active
06312592
ABSTRACT:
TECHNICAL FIELD
The invention relates to a clarifier for liquids laden with suspended matter, employing two liquid/solid separation techniques: dissolved air flotation and filtration on a filter medium (sand, anthracite or some other backwashable filter medium). The clarifier according to the invention can be used for separating suspended matter, hereinafter termed SM, contained in a liquid, and to do so in two stages:
first of all, by flotation—natural flotation (if its density is lower than that of the liquid), or in combination with the dissolved air flotation technique;
and then by filtration on a backwashable filter medium.
The clarifier according to the invention is particularly intended for clarifying a liquid effluent laden with SM.
In both these instances (natural flotation or dissolved air flotation) the construction and principle of operation of the clarifier according to the invention remain the same.
PRIOR ART
The fundamental techniques employed in the clarifier according to the invention are known per se.
Natural flotation and more particularly dissolved air flotation which uses the propensity of microbubbles of a gas to adhere to the particles in suspension (the SM) and cause them to rise to the surface of the liquid. This technique is often combined with various chemical treatments that cause the SM to clump together.
Filtration on a filter medium (sand or the like).
Backwashing of the filter medium.
The combined use of these techniques in a circular apparatus is also known. The first clarifier of this type was installed in the United States in 1980. Since then, this kind of clarifier has known various different designs, sold under the trade names of SANDFLOAT (SAF), SEDISANDFLOAT (SASF) and SANDFLOAT BP (SAF BP).
In summary, the points that these known clarifiers have in common are as follows. The clarifier consists of a circular tank in which the liquid that is to be clarified is introduced. The first stage of clarification is performed by dissolved air flotation which allows most of the SM to be moved away up to the surface of the liquid. >From the surface of the liquid, the floated matter is recovered by a spiral scoop and removed from the apparatus. The partially flotation clarified liquid drops to the bottom of the tank where it is filtered on a bed of sand or the like (the filter medium) arranged near the bottom of the tank of the apparatus. A device for recovering the filtered liquid placed under the filter medium allows this liquid to be removed to a space located under the bottom of the tank or to an external collector. The filter medium is placed in several independent sectors separated by vertical walls. The filter medium is backwashed sector by sector using liquid filtered by the other sectors. A washing hood suspended from the rotary bridge allows the washing liquid to be collected and recycled to the inlet of the apparatus so that the washing sludge is mixed with the incoming liquid that is to be treated and refloated. This clarifier is described, for example, in document U.S. Pat. No. 4,377,485.
The differences between these known clarifiers lie in the way the techniques used are performed and, more particularly, in
the way in which the clarifier is fed with the liquid that is to be treated (radial feed or central feed);
the way in which the filtered liquid is recovered (towards a space that is common to all the sectors or separately from each sector to an external collector);
the devices used to collect the filtered liquid (through meshes, suction strainers or using slotted tubular collectors);
whether or not a device is fitted for recovering the first filtrate;
the way in which the filter medium is backwashed (by suction up through the washing hood or under pressure from a central tank using a pump, or by pipework outside the tank feeding into each sector through a series of valves).
Each of these known clarifiers has its advantages and its disadvantages. Hence, the first of the clarifiers mentioned (the SAF) uses a radial method of feeding in liquid that is to be clarified. In this configuration, the entire flow of liquid to be clarified is introduced along a radius. Feed is by means of a device fitted on the rotary bridge and which is fed with liquid to be clarified via a rotary seal located along the axis at the centre of the apparatus. Distribution is according to the <<zero-velocity>> principle: the liquid emerges from the radial distribution device at the same speed as the speed at which the said device (and therefore the rotary bridge) retreats. What is more, this feed device is supplemented by a flocculator fitted on the bridge. This system for introducing the liquid that is to be clarified proves to be too unwieldy, too expensive and finally ill-suited to the function aimed at allowing flotation clarification prior to filtration. The problem is that at every moment, the entire flow of liquid for clarification is introduced along a radius in a rotary manner as in a circular channel, whereas filtration on the filter medium takes place uniformly and evenly across the entire area of the filter. It is obvious that those parts of the filter (the sectors) located in close proximity to the point at which the liquid that is to be clarified is introduced thus receive quantities of liquid which are practically unclarified by the flotation technique (or at least have not been well-clarified) because turbulence at the time of introduction is great and flotation is not very effective. In consequence, the filter clogs too quickly, having a prohibitive detrimental effect on the operation of a clarifier of this kind.
The second apparatus in this family, the SASF, makes it possible to overcome this drawback because it is fed centrally from a central tank, so that the flow of liquid to be clarified is spread uniformly over the entire filter area. By contrast, the backwashing of the filter sectors is, in this instance, performed by sucking up through the washing hood filtered liquid in a space located under the said sectors and common to all the sectors. In effect, backflow washing by suction through the washing hood poses problems of washing efficiency because the washing pump is “choked” on the suction side and its characteristic is readily moved off the operating curve. In consequence, the greater the extent to which the filter medium is clogged, the more the washing pump is “choked” on the suction side and the more the washing flow rate becomes unstable and tends to drop. The filter medium therefore becomes increasingly clogged and the pressure drops across the filter increase in proportion until the filter medium is completely clogged.
The last of the clarifiers mentioned (the SAF BP) does not present this problem because the backwashing of the filter medium is done under pressure. The major disadvantage of this apparatus is essentially of an economical order. This is because the external pipework, the high number of automatic shut-off valves, the devices for controlling these valves, the feedback control and the relatively sophisticated automation equipment required to operate it make the entire plant rather expensive.
BRIEF DESCRIPTION OF THE INVENTION
The device according to the invention makes it possible to employ a more simple and less expensive design of this type of combined clarifier, by using the aforementioned two separation techniques, namely flotation and filtration on filter medium under conditions best suited to each of them, while at the same time avoiding the drawbacks recalled hereinabove. Specifically, it avoids the risks of malfunction of the prior art devices of this family by combining a central feed mode with a pressurized means of washing the filter medium without exhibiting the significant economical drawbacks entailed by external collectors for separately recovering the clarified liquid from each sector and the collectors for feeding the sectors with liquid for backwashing.
To this end, the clarifier according to the invention comprises a compartment for introducing the liquid that is to be clarified, this c
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