Flotation method for purification of a liquid from solid and liquid impurities

Method for purifying a liquid from solid and liquid impurities by flotation-filtration in which the dirt flocks in the liquid to be purified are separated from the liquid to be purified after which they are led to the top of the liquid to form a sludge bed 16 from there to continue out to a sludge canal 17 and to a drain. The purified liquid flows further through a sand filter 18 to an isolated space 20 under the filter, through filter nozzles 19 and from there further through a pipe system 23, to, for example, a pure water reservoir. The filter is washed in an upstream direction by an opposite water flow. The sludge canal which also works as a rinsing water canal (17) or only as a rinsing water canal and the rinsing water canal (22) are placed on different sides of the filtration apparatus and only the canal that works as a rinsing water canal (22) is situated behind the separation canal (12) which means that the rinsing water has to flow over the separation canal (12).

BACKGROUND OF THE INVENTION 
The invention is concerned with a method for purifying liquid of solid and 
liquid impurities by flotation filtration through which the dirt flocks 
and liquid impurities 15 in the liquid are separated from the liquid to be 
purified after which they are led to the top of the liquid to form a 
sludge bed 16 and from there to continue out to a sludge canal 17 and to a 
drain, the purified liquid continuing through a sand filter 18 to an 
isolated space 20 under the filter, through filter nozzles 19 and from 
there onwards through pipe system 23, for example to a pure water 
reservoir, which filter is washed in an upstream direction by an opposite 
water flow. 
The best method for purification of drinking water, household water and 
industrial process water in Finland has proven to be the flotation method. 
It is equally appropriate for purification of surface water and ground 
water. It is also very appropriate for purification of both municipal and 
industrial waste water. The flotation method is very economical due to its 
small cubic volume requirement, to its many form possibilities, to its 
effectiveness, to its small power demand and due to the excellent 
purification results that can be achieved. It can also be very 
advantageously used to increase the power of old plants and to improve the 
purification results. 
Surface waters in Finland have a very big humus content. When chemical 
purification is used to remove humus and other impurities very light 
flocks are produced. Because of this the flotation method is very 
suitable, since it makes the dirt flocks that are on their way to the 
surface rise all the way to the surface and furthermore all the flocks 
rise to the surface. 
The process is also adequate for iron bearing ground water, because the 
fine bubble air used in this process divided in the right way catches the 
dirt flocks so well that there is no hindrance to their gathering at the 
surface. 
Based on experience it has been proven that no surface or ground draw water 
exists that cannot be purified by using the flotation process correctly, 
together with the right chemicals, with the result being good clean water. 
The aforementioned results of the flotation process are the same when waste 
water is being treated because solid particles can be decreased to a 
minimum. 
Two different ways for using the flotation method in liquid purification 
can be distinguished. One way is to use it as it is for clarification but 
when optimum purification results are desired, an additive sand filtration 
is needed. 
Although the flotation process is very fast, it has still been found 
possible to develop a method wherein the flotation process itself and the 
removal of the sludge which is produced by the process take place in the 
upper part and the sand filtration takes place in the lower part of the 
same basin. Because the sand filters will get dirty in due course and will 
have to be washed, that is, rinsed upstream, and because the flotation 
sludge bed has to be removed at times, which does not happen at the same 
time if the plant is being used economically, that will place obstacles in 
the way of the size of the flotation filters and in the way of the loss of 
the waste water quantity, particularly when big plants are used. When 
these are being used this can be a decisive factor. 
SUMMARY OF THE INVENTION 
An object of the invention is to utilize the aforementioned information 
such that the process will be as efficient as possible. 
To achieve this object the method of the invention is mainly characterized 
in that the canal which functions as a sludge canal functions also as a 
rinsing water or washwater canal or functions only as a rinsing water 
canal and that another rinsing water canal is placed on a different side 
of the filtration apparatus and that only the canal that works as a 
rinsing water canal is situated behind the separation canal which means 
that the rinsing water has to flow over the separation canal. 
The method represented in the invention eliminates the above-mentioned 
disadvantages and produces ideal circumstances for the flotation 
operation, for sludge removal, for the sand filter filtration, for 
backwashing of the sand filter, and for all shapes and cubic capacities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The flotation filtration apparatus in FIGS. 1 and 2 has been identified 
generally with the reference number 10. The plant 10 comprises a flotation 
filter 11 and apparatus that has been attached to it. Flotation 
clarification, removal of the separated sludge, sand filtration and 
backwashing of the filter all take place in this same basin. Apparatus 10 
also comprises an inflow opening 21 through which the liquid to be 
purified (wherein the impurities already have the shape of flocks) flows 
to the separation canal 12. In the preferred embodiment the filtration 
apparatus is of rectangular shape at its base and water enters at one of 
its long sides. To the liquid to be purified there is added a small amount 
of water which contains dissolved air through a distribution pipe 13. The 
dissolved air is made to spread across the width of the whole basin 11. 
This occurs because of the nozzles 14 in this distribution pipe, wherein 
the air has the shape of microscopically small air bubbles. These air 
bubbles catch the dirt particles 15 in the separation canal 12 and they 
catch, for example, the grease in the oil particles 15 which constitute 
liquid impurities in the liquid to be purified and they lift it to the 
surface of the liquid to be purified. These solid and liquid impurities 
spread over the whole basin and form sludge bed 16. The sludge bed is 
moved at intervals to canal 17 and from there on to the drain. 
In this manner the purified water flows downwards through the sand filter 
18 and onwards for example through the filter nozzles 19 on the bottom of 
the sand filters to the isolated space 20 under the filters and from there 
through the piping 23 to, for example, a pure water reservoir. 
In the event that there are some dirt particles left in the liquid to be 
purified, the sand filter will catch them. Due to this the filter will 
slowly get dirty and, when it gets sufficiently dirty, the filter has to 
be backwashed, that is, rinsed out. This is done in the following manner: 
Water is pumped through the piping 23 to the isolated space 20 under the 
filter. It will flow upstream through nozzles 19 to the sand layer 18 of 
the filter 18 and through this layer to the upper part of the filter and 
further to the sludge removal canal 17 and to the rinsing water canal 22. 
The rinsing water canal 22 is situated on the opposite side of the 
separation canal 12 as the sand filter, which means that the rinsing water 
or washwater has to flow over the separation canal. Accordingly the rising 
water or washwater in separation canal 12 spreads evenly in the filtration 
apparatus 10 on both sides and the filtration apparatus 10 can be made 
considerably wider than if there was only one canal on one side of the 
filtration apparatus. Thus the filtration apparatus 10 can be given an 
ideal shape wherein its increased width allows for greater throughput of 
the liquid to be purified. Also clarification, filtration, sludge removal 
and backwashing, and other process operations in the flotation filtration 
apparatus 10 can thus take place within an ideally shaped filtration 
apparatus. In a preferred embodiment of the invention there are mounted in 
the front wall of the input or separation canal 12 guiding disks or plates 
for the filters (not shown) to hinder the sand from coming to the 
separation canal 12 during the backwashing of the filter 18. 
If the rinsing water canal or the other canals were placed in a previously 
known way transversely in relation to the flotation filtration apparatus, 
the above-mentioned functions would deteriorate in their interactions with 
each other, that is, they could not all be made to function at the same 
time in an ideal way. 
The lack of the extra canal in the flotation filtration apparatus disclosed 
in the present invention has been a great disadvantage in its otherwise 
ideal shape ever since the flotation filtration apparatus was brought into 
use 20 years ago. This, among other things, gives the invention its 
importance. 
In the following there are presented the patent claims. The details of the 
invention can vary within the scope of the inventive ideal that is defined 
by the patent claims.