Fluidized bed reactor for biological purification of effluent

The fluidized bed reactor for biological treatment of effluent essentially comprises a container (1) over the base 2 of which elongated gassing units having gas exit apertures (5) are arranged parallel to one another, in such a way that in each case two neighboring circulating currents (8, 9) in mirror-image symmetry are produced in the reactor. In addition, the reactor (1) is supplied with a retention system (16) arranged on the outside for the separation of carrier particles for biocatalysts suspended in the effluent.

BACKGROUND OF THE INVENTION 
The invention relates to a fluidised bed reactor for biological 
purification of effluent having carrier particles for biocatalysts, 
gassing units and having a retention system for the carrier particles. 
In the purification of effluent, carrier particles for the microorganisms 
are used to an increasing extent in order to increase activity. Reactors 
used are fixed bed reactors and fluidised bed reactors. In fixed bed 
reactors, only a part of the reactor space is used due to uneven fouling 
which is adjusted naturally. In addition, constant process conditions 
(such as, for example pH value) are not maintained in the fixed bed. When 
concentration varies in the feed, the buffer effect of submersed 
biological life is not fully utilised due to lack of thorough mixing. 
Well mixed fluidised bed reactors, in which the carriers for the 
microorganisms are suspended or are temporarily suspended, are therefore 
more suitable for the purification of effluent. Fluidisation is achieved 
by means of pumping liquid, by means of stirrers or by means of gassing 
the liquid. The disadvantage in pumping or stirring the liquid is that the 
effluent and the carrier particles can come into contact with the drive 
units. The carrier particles are thus subjected to intensive mechanical 
load which can lead in the long term to its destruction. To prevent this, 
additional screens must be installed to retain the solids. 
In anaerobic purification of effluent, there is the further problem that 
the carrier particles have a tendency to float as a result of the floating 
effect of small bubbles of biogas which adhere to the solid. The solid 
layer which then forms on the surface of the liquid, as a rule, can no 
longer be recirculated by pumping or by the conventional stirring systems. 
On the other hand, when gassing the liquid, the microbubbles which adhere 
to the solid cause coalescence with the larger gas bubbles produced on 
gassing, so that floating of the solids can be avoided. 
SUMMARY OF THE INVENTION 
The object of the invention is to effect the movement of the carrier bodies 
for the biocatalysts in a fluidised bed reactor in the most favorable 
manner in terms of energy. 
This object is achieved in accordance with the invention in a fluidised bed 
reactor by means of gassing units and a retention system for the carrier 
particles. In that pairs of elongated gassing units having gas outlet 
apertures are arranged parallel to one another over the base of the 
reactor, in such a way that in each case two neighbouring, circulating 
currents in mirror-image symmetry are produced. 
A gassing unit preferably comprises a number of gas outlet apertures 
arranged close to one another, sinter bodies or injectors arranged in a 
line or band. 
Futhermore, it has proved to be favorable if wedge-shaped inserts having a 
deflector plate are arranged between the gassing units. 
A further development is that a control device is provided to supply the 
gassing units alternately with gas. The result of alternately supplying 
two gassing units is that there is partial gassing in the reactor which 
leads to sequential layering of the solid. 
The retention system preferably comprises a sedimentation separator 
arranged outside of the reactor or built into the reactor, or comprises a 
bar screen. 
Optimum current conditions are reached when the filling height of the 
fluidised bed reactor is approximately the same as the maximum dimension 
of a circulating current. This means that the ratio of filling height to 
length of the tank should be approximately 0.5. For the case when longer 
tanks have to be used for the sake of looking at a problem, it is 
advisable to operate using more than two circulating currents. Then, 
several (&gt;2) gassing units, in each case two at a time, must be arranged 
one behind another in the longitudinal direction as a module in the 
fluidised bed reactor. 
If gassing is continuous, at least two circulating currents form which 
cause the solid particles to be fluidised at lower gas rates than when 
using single unit surface gassing. The decisive criterion is where the gas 
is introduced. The most favorable solution in terms of energy results, 
surprisingly, when the gas is introduced near to the contact plane of the 
two neighbouring current regions. Compared to surface gassing, 
approximately 75% lower gas rates are required in this case for 
fluidisation. 
Further improvement is possible by incorporating the above-mentioned 
current-inducing inserts between the gassing units. The gas rate could be 
reduced in this manner to only 5% of the gas rate required for surface 
gassing. 
If only partial gassing of the tank takes place, intensive movement results 
in the gassed region, such that the solid is fluidised there, whereas only 
slight movement takes place in the non-gassed region, such that the 
greater part of the solid rests there. After a while it can be detected 
that solid is gradually transported from the zone of intensive movement to 
the zone of slight movement, and is deposited there. As a result of this, 
the solid content in the gassed half of the tank is reduced such that the 
solid remaining here can be moved at an even lower gas rate. This effect 
is particularly pronounced if the deposition and retention of the solid in 
the non-gassed half of the tank is assisted by deflector plates. 
These facts give rise to a further possiblitiy for gassing which is 
efficient in terms of energy, and this is that only one gassing unit is 
supplied with gas and after a certain time gassing is switched to the 
other unit (alternating gassing). For alternating gassing, switch-over to 
the other gassing unit advantageously does not take place until the solid 
particles are completely fluidised in the gassed sections of the reactor. 
The switch-over time thus depends on the ratio of the gas capacity used 
and the solid mass to be moved. 
An embodiment of the invention is explained in more detail below using 
drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a schematic side view of the fluidised bed reactor with 
circulating separator, 
FIG. 2 shows a plan view of the reactor according to FIG. 1, and 
FIG. 3 shows a section A-B according to FIG. 2 (gassing pipe).

DETAILED DESCRIPTION OF THE INVENTION 
The reactor according to FIG. 1 comprises a guadrilateral container 1 
having a side 11 m long and being 6 m high with a flat base 2. Two gassing 
units comprising gas distributors are arranged just above the base 2 of 
the container. The gas distributors comprise elongated straight pipes 3, 4 
which are assembled at a distance of 230 cm from one another and 35 cm 
above the base 2. As shown in FIG. 3, the gas distributing pipes 3 and 4 
are provided with bores 5 of diameter 3 mm, 5 cm apart. These bore 5 are 
the gas outlet apertures. A wedge 6 having a base of 200 cm and a height 
of 170 cm is situated between the pipes 3 and 4. The tip of the wedge 6 
becomes a deflecting plate 7 which has a height of 130 cm. 
The distance d between the gassing units is preferably 0.1 to 0.25 times 
the reactor length D. 
The gassing units are arranged as mirror-images to the symmetrical plane S 
of the reactor. The gas bubbles emerging in the effluent from the gas 
distributing pipes 3 and 4, produce two neighboring circulating currents 8 
and 9 showing mirror-image symmetry. The formation of these opposing 
circulating currents is promoted by the inserts 6 and 7. 
The gas distributing pipes 3 and 4 are connected to the main gas pipe 10 
and can be supplied with gas separately via the valves 11 and 12. For this 
purpose, the valves 11 and 12 are connected to a control unit 13 which in 
particular permits switching over of the gas supply between the gassing 
units and with the aid thereof, alternating gassing of the two halves of 
the reactor can be achieved. 
In the reactor, there are 25 volume % of carbon-filled polyurethane foam 
particles which are covered with bacteria for purification of effluent. 
The average size of the particles is approximately 4 mm and their density 
when in water is 1040 kg/m.sup.3. The carrier particles can be kept in 
motion by an amount of gas of only 320 m.sup.3 /h which corresponds to a 
gas rate in an empty pipe of 2 m/h and a specific gas capacity of 5.5 
Watt/m.sup.3. 
When gassing is alternating, 80% of the solid is displaced from the gassed 
half of the tank to the non-gassed half at a cycle time of approximately 
40 minutes, such that a rest time of 40 minutes at the most is estimated 
for approximately 80% of the solid. By increasing the gas rate to, for 
example 4 m/h, this rest time can be reduced to approximately 8 minutes. 
By a further increase to 6 m/h, fluidisation and displacement of the solid 
finally takes place in less than 1 minute. By changing the gas rate, the 
state of movement of the catalysts can be adapted to the particular 
requirements of an effluent purification plant. 
The effluent stream which flows into the fluidised bed reactor 1 via the 
fitting 14, leaves the reactor via the outlet fitting 15 having the 
retention system 16 with outlet 17 connected to it. By way of example, a 
circulation separator having a built-in bar screen 18 can be used as the 
retention system. An intensive liquid current is produced at the bar 
screen 18 as a result of the tangential confluence 15 of the effluent 
stream, such that the required screen surface need only be approximately 
0.5 m.sup.2 at an effluent throughput of 125 m.sup.3 /h. Bar screens which 
are constructed such that the narrowest cross-section of the screen 
extends only over a small depth and the current channel widens following 
on from that, have proved to be particularly suitable for the retention of 
solid particles. 
The gassing described is equally suitable for fluidised bed reactors 
operating aerobically and anaerobically. For aerobic operation, air or an 
oxygen-air mixture is fed into the main gas pipe. On the other hand, for 
anaerobic operation, biogas is fed to the main gas pipe 10, the biogas 
being produced in the reactor 1 in an anaerobic biological degradation. 
The advantages of the invention are: 
a) the saving of stirring devices to fluidise the effluent having carrier 
particles suspended therein, 
b) optimised fluidisation with regard to energy and thorough mixing in the 
reactor, 
c) careful treatment of the carrier particles and biomass adhered thereto, 
in the sense of minimising abrasion and mechanical removal, and 
d) the avoidance of floating carriers.