Apparatus for gas bubble flotation

An apparatus for gas bubble flotation comprises an angular flotation tank (1a, 1b), at least one liquid inlet (2a, 2b) for a liquid containing suspended matter and a liquid admixed with gas bubbles, in which apparatus at least one liquid inlet (2a, 2b) is arranged in the vicinity of at least one flotation tank corner. Preferably, the flotation tank has a square cross section. This permits optimal utilization of the available space, and results in a continuously decreasing flow velocity and thus an improved flotation effect.

The invention relates to an apparatus for gas bubble flotation for 
separating solids from liquids. 
DISCUSSION OF THE PRIOR ART 
Gas bubble flotation is employed in the field of separating solids and 
liquids and especially in the sector of water treatment and wastewater 
treatment. This primarily involves applying the so-called dissolved-gas 
flotation, in which, in addition to a liquid containing solids or 
suspended matter, a liquid saturated with gas under pressure is added and 
is decompressed, so that as a result of the decompression gas bubbles are 
formed which attach themselves to the suspended matter to be separated and 
convey this to the surface in the flotation tank, so that it can be 
skimmed off there. The clear liquid freed of the suspended matter is 
discharged in the flotation tank bottom zone. 
Previous apparatus for gas bubble flotation essentially comprise an 
air-dissolving apparatus in which, under elevated pressure, gas is 
dissolved in the liquid medium. After a certain retention time, the 
gas-liquid mixture is decompressed to a lower pressure, gas 
supersaturation of the liquid medium occurring as a result and the gas, no 
longer dissolved, being released in the form of fine microbubbles. The 
solids-containing liquid is preferably admixed, prior to being passed into 
the flotation tank, with a precipitation or flocculation aid, to enable 
conversion of the substances to be separated into floccular form. 
The flotation tanks are either of round or of rectangular shape. In the 
rectangular flotation tanks, one side of the rectangle is usually made 
distinctly longer than the other side, and the inflow of the liquid 
containing suspended matter always takes place frontally, distributed over 
the shorter side of the rectangle. Flow takes place longitudinally through 
the system, the clear water being drawn off at the second short side 
situated opposite the inflow side. 
The flow velocity in the previously known rectangular flotation systems 
through which flow takes place in the longitudinal direction is uniform 
and virtually constant. In order to achieve lower flow velocities by means 
of a larger volume, the flotation tanks are constructed so as to be 
relatively deep. A significant drawback of this known system is that a 
larger volume, compared to round flotation tanks, is required while at the 
same time the specific load-handling capacity of the system is lower. 
While system configurations are known in which it was possible to increase 
the capacity of such rectangular systems by fitting oblique lamellae, this 
results in the drawback of a higher sensitivity to fouling. 
SUMMARY OF THE INVENTION 
Therefore, the object of the invention is to specify an apparatus for gas 
bubble flotation in accordance with the preamble of claim 1, which is 
distinguished by a small overall size, while at the same time a high 
separation efficiency is to be achieved. Moreover, the apparatus according 
to the invention is to utilize the available space as efficiently as 
possible. 
According to the invention, this object is achieved by an apparatus for gas 
bubble flotation which comprises an angular flotation tank, at least one 
liquid inlet for a liquid containing suspended matter and a liquid admixed 
with gas bubbles, wherein at least one liquid inlet is arranged in the 
vicinity of at least one flotation tank corner. Advantageous improvements 
of the inventive idea of the invention will become apparent from the 
subordinate claims. 
The essential advantages of the invention consist in optimal utilization of 
the available space being possible as a consequence of the angular, 
preferably right-angled triangular or square type of construction of the 
flotation tank. Compared to a round tank this results in an increase in 
the flotation tank cross-sectional area, given the fact that the space 
available for system is usually rectangular. Causing the liquid containing 
suspended matter to flow in the corner zone advantageously gives rise to a 
continuously decreasing flow velocity, which results in an improved 
flotation effect compared to the conventional rectangular tanks. In this 
respect, the invention combines the advantages of the two previous 
concepts of the round flotation tank with central inflow on the one hand 
and the rectangular flotation tank with inflow along a side wall on the 
other hand. 
The invention is particularly suitable for separating solid and liquid 
phases, especially for separating solids and lipophilic substances from a 
liquid phase such as a suspension. 
According to an advantageous embodiment of the invention, the flotation 
tank is designed to be essentially square or diamond-shaped, in each of 
two opposite corners at least one liquid inlet being provided. At the same 
time, preferably skimming or sweeping apparatus are provided for skimming 
the flotate in the flotation tank, which are aligned in the diagonals 
defined by the two corners not provided with liquid inlets. This design 
brings about efficient flotation combined with the best possible 
utilization of the available space. Although the square design of the 
flotation tank is preferred on practical grounds, a diamond-shaped design 
can also be employed if required. 
According to an alternative improvement of the invention, the flotation 
tank has an essentially isosceles triangular or right-angled triangular 
horizontal projection. This triangular shape makes it possible to 
introduce the liquid containing suspended matter at the apex corner, the 
decelerating flow occurring in the direction of the opposite side wall, 
where skimming of the flotate can be carried out. This improvement has the 
further advantage that two or more triangular flotation tanks according to 
the invention can be put together, to achieve efficient utilization of the 
space available for system. 
If two triangular flotation tanks are arranged in such a way that the 
liquid inlets are situated opposite one another, shared offtake devices 
for the flotate and the clarified liquid can advantageously be provided. 
Preferably, the sweeping apparatus are designed as two counter-rotating 
multiple-paddle sweepers parallel to one another, between which a flotate 
discharge channel is arranged. In this arrangement it is expedient for a 
clear-liquid offtake line to be arranged underneath the discharge channel. 
According to a further embodiment of the invention there is provided, 
underneath the skimming channel, a partition which divides the flotation 
tank into two triangular flotation subtanks. Thus it is possible, with a 
minor additional effort, to set up two separate flotation apparatus which 
are suitable for the flotation of different liquids or of liquids having 
different proportions or compositions of suspended matter. The offtake of 
the flotate or of the liquid free of suspended matter can take place, as 
required, via separate or shared offtake devices. 
According to a yet further improvement of the invention there is provided, 
at the bottom of the flotation tank, at least one sediment scraper. In 
this arrangement there is preferably formed, at the tank bottom, a sludge 
offtake channel into which the at least one pivotable sediment scraper 
conveys sediment sludge. Advantageously, two sediment scrapers pivotable 
about vertical axes are arranged in association with each liquid inlet, 
the pivots of which scrapers are situated in the vicinity of the other 
container corners and whose swiveling ranges extend up to the vicinity of 
the liquid inlets. 
In the present invention, there is preferably provided, in the tank 
corners, a liquid inlet for the liquid containing suspended matter and 
therebeneath a separate inlet for liquid containing gas bubbles, the inlet 
for liquid containing gas bubbles being designed as a depressurization 
valve. Owing to this direct decompression of pressurized gas-saturated 
liquid directly into the flotation tank, agglomeration of the gas bubbles 
formed can advantageous be avoided. Thus, optimal utilization of the gas 
bubbles is achieved, since it has been found that the smaller the gas 
bubbles are the better the flotation effect is. 
By an improvement of the arrangement of a horizontally pivotable baffle 
above the liquid inlet, the ability to direct the liquid flow uniformly 
into all directions can be achieved.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The course, in principle, of the flotation process is explained below with 
reference to the FIGS. 1 to 5, referring to the preferred application of 
the invention for wastewater treatment. The invention is not restricted to 
this application, however, and is suitable for all applications in which a 
liquid and a non-liquid and non-gaseous phase such as suspended matter, 
lipophilic substances or solids are to be separated. In particular, the 
invention can be used in the brewing sector. 
The flotation apparatus shown in FIG. 1 essentially comprises a square 
flotation tank 1a which, in the vicinity of two opposite corners, has one 
liquid inlet 2a and 2b each, which are each fed with liquid containing 
suspended matter via feeder pipes 3. 
In the vicinity of the two other corners, two supports 5a, 5b are arranged, 
which support sweeping devices 6a, 6b in the form of paddle sweepers 
which, situated parallel next to one another, diagonally span the 
flotation tank 1a. The two paddle sweepers 6a, 6b each comprise a 
plurality of approximately radially projecting transfer lips, distributed 
about the circumference of the longitudinal axis, which dip into the 
flotate layer 10 forming at the surface of the tank contents and skim the 
flotate into a flotate discharge channel 7 arranged between the paddle 
sweepers 6a, 6b. The paddle sweepers 6a, 6b are individually set in 
rotation by means of drive motors 8a, 8b. 
As can be seen from FIG. 2, the flotate discharge channel 7 for the flotate 
has two channel-like baffles 9a, 9b, which dip into the flotate layer 10 
and via which the flotate is conveyed by the paddle sweepers 6a, 6b into 
the channel 7 proper. 
In FIG. 3 it can be seen that the bottom of the flotate discharge channel 7 
runs obliquely downward, in order to ensure discharge of the viscous 
flotate 11. The flotate discharge channel 7 is in turn connected to a 
flotate offtake line 27. 
As depicted in the FIGS. 2 and 3, there is situated underneath the flotate 
discharge channel 7 a clear-water outflow pipe 12 which along its 
extension in the flotation tank 1a is provided with a multiplicity of 
passages 13 via which the clear water can enter in order to be removed 
from the flotation tank 1a. 
According to an embodiment of the invention, the flotation tank la is 
divided into two separate subtanks by a diagonal partition 14 shown in 
FIG. 3, the partition 14 extending from below the flotate discharge 
channel 7 as far as the tank bottom. In this arrangement it is also 
possible to provide two separate clear-water outflow pipes in each 
subtank, or, as shown in FIG. 3, a shared clear-water outflow pipe 12 is 
used. 
The water containing suspended matter is fed in, in the preferred 
embodiment shown of the invention, via the liquid inlets 2a and 2b in the 
tank corners. As shown in particular in FIGS. 4a and 4b, an inlet 15 for 
the liquid containing suspended matter is provided above an inlet 16 for 
gas-saturated pressurized liquid, the inlet 16 comprising a 
depressurization element 17 or itself being designed as such. The flows of 
liquid into unit 16 are guided by suitable baffles 18 and 19, in order to 
achieve as favorable a distribution as possible in the horizontal 
direction and mixing of the two fluid flows. Above the inlet 15 for the 
liquid containing suspended matter there is arranged a preferably 
pivotable inflow screen 20, so as to be able to optimally set the course 
of the flow as a function of throughput, pollution level or other 
parameters. 
At the bottom of the flotation tank 1a, a plurality of sediment scrapers 21 
are provided in the preferred embodiment, which are arranged, in the 
vicinity of the tank corners not provided with liquid inlets 2a, 2b, so as 
to be pivotable about vertical axes. By way of example, FIG. 3 depicts a 
drive unit 22 which, via a shaft 23, swivels the associated sediment 
scraper 21 along the swiveling range designated by the reference symbol 24 
in FIG. 1. The sediment scraper(s) serve to push sediments deposited on 
the tank bottom into a sediment hopper 25 recessed in the tank bottom 
below the clear-water outflow pipe 12. As shown in FIG. 3, the sediment 
hopper 25 preferably has a V-shaped sectional profile and debouches into a 
sediment offtake line 26. Insofar as a partition 14 (FIG. 3) is provided 
in the flotation tank 1a, it preferably ends in the sediment hopper 25, 
with the possibility of sedimented material being pushed in by the 
sediment scrapers 21 from both sides of the partition 14. 
FIG. 5 depicts an alternative embodiment of the invention, which 
essentially corresponds to the embodiment depicted in FIGS. 1 to 4, so 
that a detailed description is not required. Identical reference symbols 
as in the other figures refer to identical parts. 
The essential difference of this embodiment from that described above 
consists in the flotation tank 1b having a triangular horizontal 
projection, and only one paddle sweeper 6 being provided. 
From FIG. 6 it can be seen that the embodiment, shown in FIG. 5, of the 
flotation tank 1b permits many and diverse combination options of a 
plurality of flotation tanks, which provides for efficient adaptation to 
confined system areas in combination with the largest possible tank 
surface area. 
The flotation process using the apparatus according to the invention is 
described below in detail. 
The raw water from which the solids are to be separated is pumped by means 
of a pump (not shown) to the flotation tank 1a or 1b, respectively, in 
which the solid and liquid phase are separated, so that two branch streams 
are formed, namely the liquid, virtually solid-free clear phase 30 and the 
solid phase which can also be designated as the flotate and which contains 
the major proportion of the solids. The flotate in turn collects in the 
flotate layer 10. In the process, prior to the introduction into the 
flotation tank 1a or 1b, respectively, suitable chemicals can be added for 
the purpose of precipitation and/or flocculation, which serves to convert 
dissolved or partially dissolved, colloidal and free solids into floccular 
form. 
In order to form gas bubbles, a suitable gas, for example air, is dissolved 
in an apparatus (not shown) under a pressure which preferably is 
distinctly above ambient pressure. The liquid phase containing the 
dissolved gas is introduced by means of the inlets 15, the high pressure 
being maintained, into the flotation tank 1a or beforehand decompressed 
into the inflow pipe, as a result of which fine microbubbles are formed. 
The decompression of the gas bubble mixture, and the flow path are designed 
in such a way that there is formed, in the flotation tank 1a or 1b, 
respectively, a preferably horizontal two-layer flow. This comprises a gas 
bubble flow in the lower zone and a flow of the raw water in the upper 
zone of the flotation tank 1, containing the solids in flocculate form. 
Alternatively, the branch streams may also be mixed prior to entry into 
the flotation tank. 
The gas bubbles 31 formed can adhere to the solids flocculae to be 
separated and thus cause separation by flotation of the flocculated solid 
composites. 
The inflow in the case of tank 1a takes place, according to FIG. 1, from 
each of two opposite corners toward the center, the inflow being designed 
in such a way that a horizontal inflow in the lower zone of the flotation 
tank 1a or 1b, respectively, can be accomplished which is aligned toward 
the center. 
Above the main inflow pipe there is situated the movably mounted inflow 
screen 20. This inflow screen 20 is designed in such a way that uniform 
inflow over an angle of 90.degree. is possible. Since the inflow takes 
place from a corner of the basic square shape, the flow velocity 
immediately declines drastically, which creates ideal conditions for the 
flotation process. Owing to the low flow velocity, the gas bubbles can 
adhere to the solids/flocculae composites and cause these to float up. 
In the center between the two opposite inflow points there are arranged, 
diagonally with respect to the square, both the clear-water outflow pipe 
12 for the virtually solids-free clear phase and the sweeping apparatus 6 
for the flotate sludge produced. The flotate/sludge phase moving toward 
the center is picked up with the aid of the two counter-rotating paddle 
sweepers 6a, 6b. As a result of the rotation of said paddle sweepers 6a, 
6b, the flotate layer 10 is conveyed along the diagonal into the 
unilaterally or bilaterally inclined flotate discharge channel 7, whence 
the sludge 11 flows out and can be drawn off via the flotate offtake line 
26. 
The discharge of the clear phase is effected via the horizontal clear-phase 
outflow pipe 12 arranged diagonally in the bottom third of the flotation 
tank 1a, 1b and provided with passages 13, whose orifice cross sections 
preferably vary, so that uniform discharge over the entire length is 
possible. 
Non-floating heavy dirt or sediments collect at the tank bottom. In order 
to remove these sediments from the flotation tank 1a or 1b, respectively, 
they are pushed, with the aid of the sediment scrapers 21, into the 
likewise diagonally arranged sediment hopper 25 at the tank bottom. Each 
half of the square, diagonally divided flotation tank 1a has two sediment 
scrapers 21 which rotate about a vertical axis. The axes are disposed on 
the tank outer wall near those corners of the square at which no inflow 
takes place. The bearing arrangement of the rotatable shafts 23 is 
attached to the tank outer wall. The sediment scrapers 21 themselves are 
attached, at one end, to the vertical scraper arbor and are designed in 
such a way that a rubber lip pushes the sediments collected at the bottom 
into the sediment hopper 25. The scraping process takes place at intervals 
as required by the application. The scraper arbor is driven by a geared 
motor 22. The scraping process takes place in such a way that, after 
reaching the sediment hopper 25, the sediment scraper 21 immediately moves 
back into its starting position. 
The sediment scrapers 21 opposite one another in one tank half in this 
arrangement are operated alternately at intervals, so that any contact of 
the two sediment scrapers 21 is precluded. 
Depending on requirements, the inflow into the system can be accomplished 
in such a way that raw water and gas bubble stream separately enter the 
flotation tank 1a or 1b, respectively. The feed of the gas bubble stream 
should in principle take place underneath the raw water stream, with the 
same or a greater velocity. Decompression of the pressurized saturated 
liquid-gas mixture can take place both before inflow into the flotation 
tank 1a or 1b, respectively, and in the tank itself.