Device for aerating sewage or sewage-sludges

The invention relates to an aerating device for sewage and comprises a drum-like rotor which rotates in a trough and is divided, by axially-parallel partitions, into a plurality of outwardly-open radial chambers, the said partitions being equipped with peripheral outer surfaces having apertures so that during rotation of the drum a volume of air is trapped in each chamber. Internal baffels in said chambers act to direct the trapped air initially to the lagging portion of the chamber and then, past bottom dead center, to the leading portion of the chamber, thereby efficiently aerating the sewage.

The invention relates to a device for aerating sewage or sewage-sludges, 
for the purpose of converting foreign, especially harmful, substances 
contained therein into harmless substances, the said device comprising a 
basin, for example a reaction or sludge-aerating basin, and a hollow 
element which is arranged to rotate therein about its axis, a portion of 
said hollow element protruding above the level of sewage when the basin is 
full. The hollow element is provided with a plurality of partitions 
directed parallel to and outwardly from the axis of rotation forming 
outwardly-open reaction chambers. Each partition is provided with front 
and rear surfaces adjacent its outer portion. The front surface extends 
downwardly at the point where the partition dips beneath the surface of 
the sewage, and the rear surface extends downwardly at the point where the 
portion emerges from beneath the surface of the sewage. 
For the purpose of converting harmful substances contained in sewage or 
sewage-sludges, it is known to undertake aeration in order to oxidize the 
said harmful substances. When sewage is aerated, a mostly flaky sludge is 
formed, and this can be separated from the water in a subsequent settling 
basin. If necessary, the water and sludge may also be denitrated. 
Sewage may be aerated in various ways. According to one particularly 
effective method, use is made of periodically immersed hollow elements 
equipped with apertures admitting water and air. These hollow elements 
allow air to be forced below the surface of the water and then to escape 
slowly. As the elements emerge, they lift the water and sludge with them 
allowing the water and sludge to trickle out gradually. This produces 
intensive aeration of the sludge with high conversion efficiency. 
It is known, according to German OS No. 26 38 665, to arrange a plurality 
of hollow tubular elements equidistantly around the periphery of rotatable 
rim-gears and parallel with the surface of the water. As the rim-gears 
rotate, the hollow elements dip periodically in and out, thus carrying air 
into the water and lifting water and sludge out. 
However, this arrangement of hollow elements is structurally complex. 
Furthermore, the volume of air carried below the surface of the water by 
the said elements is relatively small. More satisfactory in this respect 
is a design, also disclosed in the said prior publication, in which the 
hollow element is in the form of a drum suspended rotatably in the 
reaction basin with its axis above the surface of the water. The said 
element contains a plurality of partitions directed substantially radially 
outwardly and parallel with the axis of the element, the said partitions 
dividing the interior of the hollow element into chambers of equal size. 
Each of these partitions is provided with a front outer surface adjacent 
its outer edge, said surface extending downwardly at the point where the 
partition dips beneath the surface of the sewage. Each front outer surface 
is provided with an opening extending to the outer edge of the subsequent 
partition as viewed in the direction of rotation. As the downwardly 
extending outer surface enters the sewage, it combines with the partition 
to form a scoop which carries air below the surface. This air quickly 
escapes thereby aerating sewage outside the hollow element. As the scoop 
emerges from beneath the surface of the sewage, it brings with it water 
which, upon further rotation, trickles out of the said openings. 
The structural simplicity of this design of hollow element, and the larger 
volume of air it handles, is obtained at the expense of the relatively 
large amount of power required to drive the hollow element. Furthermore, 
the conversion efficiency is unsatisfactory, since the volume of air 
carried into the water escapes relatively quickly from the chambers and 
reaches the surface just as quickly. The amount of oxygen absorbed into 
the sewage is small, corresponding to the brief contact period. This is 
particularly apparent if--as suggested--discs are arranged in the reaction 
chambers to provide surfaces to which biological growths, consisting of 
micro-organisms, may adhere. These growths would be insufficiently aerated 
by the rapidly escaping air. 
In the case of a device of the type described at the beginning hereof, it 
is the purpose of the invention to design the hollow element in such a 
manner as to reduce the power required to drive it and to improve the 
conversion efficiency. 
According to the invention, this purpose is achieved by providing rear 
outer surfaces of the partitions, as viewed in the direction of rotation, 
which face downwardly at the point where the partition emerges from 
beneath the sewage surface. This design of the rear surfaces of the 
partitions prevents the air carried below the surface of the water from 
escaping out of the hollow element while the latter is underwater. Whereas 
the front outer surface of each partition, which may also be a part of the 
outer wall of the hollow element, faces downwardly after the manner of a 
hollow scoop, thus carrying an air bubble below the surface of the water, 
the rear outer surface of each partition forms a kind of retaining scoop 
for the air bubble which passes, as the front outer surface of the one 
partition rotates, to the rear outer surface of the opposite partition in 
the chamber. This air bubble thus remains relatively longer in the 
reaction chamber than is the case in known designs, which means that 
considerably more oxygen is dissolved in the sewage and that the 
biological growths in the reaction chambers are thoroughly aerated. This 
is encouraged by the air being compressed underwater resulting in improved 
solubility of the oxygen in the water. All of this provides a substantial 
increase in reaction efficiency. For a given efficiency, therefore, the 
rpm of the hollow element, and the power required to drive it, may be 
reduced. 
The air bubble near bottom dead centre also aids in driving the hollow 
element in its direction of rotation, thus compensating for the buoyancy 
of the air bubble formed upon immersion and persisting to bottom dead 
centre. This is not the case in existing designs since the air escapes 
from the reaction chamber before it can act in the direction of rotation. 
The power required to overcome the buoyancy against the direction of 
rotation is thus no longer required. 
It is not essential for the front and rear outer surfaces of the partitions 
to face exactly downwardly upon immersion and emersion. Instead, the said 
partitions, and the outer parts thereof, may face obliquely downwardly. 
All that is essential is that the intended scoop-action shall carry an air 
bubble below the water upon immersion, and that this air bubble be at 
least substantially prevented from escaping from the hollow element. 
According to the invention, the hollow element is equipped with partitions 
directed radially outwardly, the outer ends of which continue, in the form 
of covering webs, at least approximately in the peripheral direction, 
leaving slotted apertures. This design of the hollow element is 
structurally simple and also carries a large volume of air below the 
surface of the water. In this connection it may be desirable to shorten 
one of the covering webs forming the slotted apertures, preferably the 
front web of each reaction chamber as viewed in the direction of rotation. 
A particularly simple design for the hollow element may be obtained by 
making the covering webs flat and arranging them so that the said hollow 
element forms a regular polygon. One satisfactory arrangement is to divide 
the hollow element into six reaction chambers, thus forming a dodecagon. 
The said hollow element may be made by welding together single sheets. 
According to another characteristic of the invention, the reaction chambers 
contain baffle-plates running radially and parallel with the axis of 
rotation of the element, preferably arranged substantially upon the 
bisectors of the angles between the partitions constituting the reaction 
chambers. These baffle-plates guide the air and prevent air bubbles which 
form at the rear of the reaction chambers, upon immersion, from passing 
prematurely to the front. The said baffle-plates thus provide uniform 
aeration of the individual parts of the reaction chambers. Baffle-plates 
measuring between one half and three quarters of the length of the 
partitions have been found satisfactory. 
In conjunction with the baffle-plates, the invention also provides for the 
axis of rotation to be in the form of a hollow shaft having openings in 
the area between the baffle plate and the front partition in each reaction 
chamber, as viewed in the direction of rotation. In this way the volume 
enclosed in the hollow shaft is also used to convert the sewage. The said 
openings serve to allow not only the sewage, but also a part of the air 
carried below the surface of the water, to enter the hollow shaft. This 
air can then escape through the openings which happen to be at the top of 
the hollow shaft, thus aerating the residual water still remaining in the 
emerging chambers. In this connection it has been found sufficient for the 
dimensions of the said openings to be such as to allow about 10 to 20% of 
the air enclosed in the reaction chambers to escape into the hollow shaft 
as it rotates. 
According to still another characteristic of the invention, the basin is a 
good fit around the hollow element and is preferably in the form of a 
trough having a semi-circular bottom. With this arrangement, practically 
the entire volume of the basin is located in the reaction chambers and is 
thus aerated. This also makes it possible to provide the largest possible 
surfaces for biological growth in the smallest possible area. According to 
the invention, this is achieved by providing the reaction chambers with 
growth-discs which run radially and at right angles to the axis of 
rotation. A biological growth consisting of micro-organisms then forms 
upon these discs, and this contributes substantially to the elimination of 
harmful substances. The surfaces of the discs may be increased by making 
them corrugated. 
Another way of reducing the power required to drive the hollow element is 
to arrange it in such a manner that, when the basin is full, between two 
thirds and three quarters of the said element is immersed therein. In this 
way, less sewage and sludge is lifted above the surface of the water, and 
this reduces the power required for lifting. 
Finally, provision is made, according to the invention, for the hollow 
element to be divided, by separating discs running radially of the axis of 
rotation, into at least two sections, the partitions in the sections being 
displaced angularly in relation to each other. This arrangement is 
particularly desirable if the volume of the reaction chambers is 
relatively large, since otherwise dissimilarities and imbalances occur. 
The aerating efficiency of the device may be improved still further by 
arranging outwardly-open additional chambers externally of the covering 
webs and between the slotted apertures. The said additional chambers have 
axially parallel walls provided with outer surfaces. The outer surface of 
the leading axially parallel wall, as viewed in the direction of rotation, 
faces downwardly upon emerging from beneath the sewage surface. The outer 
surface of the lagging axially parallel wall, as viewed in the direction 
of rotation, faces downwardly at the point of immersion into the sewage. 
These axially parallel walls preferably consist of wall sections directed 
substantially axially and adjoining outer wall sections directed 
substantially peripherally. The end-edges of the outer wall sections, 
directed substantially peripherally, of each additional chamber, define an 
axial slot. 
This arrangement means that additional chambers are arranged upon the 
surface of the rotating hollow element. Inasmuch as these are closed and 
are not taken up by the slotted apertures, the said additional reaction 
chambers act in a manner substantially similar to the main chambers, but 
have a generally lesser radial extension. 
Whereas the volumes of air carried along by the main reaction chambers, as 
they pass through the lower part of the trough, always tend to ascend 
towards the hollow shaft, the additional chambers have the advantage that 
air may be carried by them directly to the bottom of the trough, where 
some of it escapes, thus ensuring that even sewage at the very bottom of 
the trough is aerated. This improves the supply of oxygen as a whole. The 
said additional chambers also provide additional biological growth 
surfaces.

FIGS. 1 and 2 illustrate a trough-like reaction basin for a sewage plant, 
into which sewage may be passed after coarse clarification by mechanical 
means. Basin 1 is of rectangular outline with a semi-circular bottom 2 and 
is open at the top. It rests upon four pedestals, only two of which, 3,4, 
are shown in the figure. A drum-like, cross-sectionally dodecagonal hollow 
element 5 is located in reaction basin 1 in such a manner as to leave a 
relatively small space between it and the inner wall of the said basin. 
Hollow element 5 is mounted, by means of an axis of rotation in the form 
of a hollow shaft 6, in lateral walls 7,8 of the basin and is driven, in 
the direction of arrow A, by a motor, not shown. When the basin is full up 
to level 9, about one quarter to one third of the diameter of the hollow 
element 5 protrudes above the sewage level 9. 
As may be gathered from FIG. 2 in particular, hollow element 5 is divided 
into six reaction chambers 16-21 by partitions 10-15 running radially 
outwardly from, and parallel with, axis of rotation 6 and arranged at 
equally angular distances from each other. These chambers are enclosed by 
lateral covering discs 22, 23. 
Each reaction chamber 16-21 has a slotted aperture 24-29 extending over the 
whole length of hollow element 5 and allowing an exchange of sewage and 
air. These slotted apertures are formed in covering webs 30-41 attached to 
and extending approximately peripherally in both directions from 
partitions 10-15. Since the front covering webs 36-41, as viewed in the 
direction of rotation, in each reaction chamber are shorter than the other 
covering webs 30-35, slotted apertures 24-29 are slightly offset, in the 
direction of rotation, from the bisectors of each reaction chamber 16-21. 
Arranged in the said reaction chambers, upon the bisectors thereof, are 
baffle-plates 42-47 running from hollow shaft 6. The said baffle-plates 
are used to guide the air carried along as each reaction chamber is 
immersed. Between baffle-plates 42-47 and front partition 10-15, as viewed 
in the direction of rotation, hollow shaft 6 is equipped with openings 
48-53 which allow sewage and, at specific locations also air, to enter 
hollow shaft 6. 
Arranged in reaction chambers 16-21 closely together and at right angles to 
the axis of rotation are settling discs 54 having corrugated surfaces. 
Deposited onto these discs are biological growths for the biochemical 
conversion of harmful substances contained in the sewage. As a result of 
the drum-like design of the hollow element, and the small distance between 
it and the inner wall of the basin, very large surfaces are available for 
biological growth even though the outside dimensions of the device are 
small. 
During one revolution of hollow element 5, a relatively large air-bubble is 
carried below the surface of the water in the submerging reaction chamber, 
since the front sides of partitions 10-15, as seen in the direction of 
rotation, and adjoining covering webs 30-35, form a kind of hollow scoop 
which retains air-bubble 55. As the hollow element continues to rotate, 
baffle plates 42-47 prevent air-bubble 55 from passing into the front 
part, as viewed in the direction of rotation, of each reaction chamber, 
thus ensuring adequate aeration of the rear part. 
Air-bubble 55 divides only in the vicinity of the bottom dead centre, part 
of the said bubble passing into the front part of the relevant reaction 
chamber. From this part, about 10 to 20% of the air can reach the cavity 
enclosed in hollow shaft 6, thus ensuring that any sewage therein is also 
aerated. This air can then escape, through any of openings 48-53 which 
happen to be at the top, into the reaction chambers that are not immersed, 
where it aerates any residual water. 
In the vicinity of bottom dead centre, air-bubble 55 develops buoyancy 
acting in the direction of rotation, and this compensates almost 
completely for the buoyancy in the first half acting in the opposite 
direction. Thus, the power hitherto required to carry the air under the 
surface of the water is no longer necessary with the design according to 
the invention. This saves a considerable amount of power. 
As the hollow element continues to rotate, all of air-bubble 55 passes to 
the front part of the relevant reaction chamber, from which it is 
prevented from escaping, until the chamber emerges again, by front 
covering web 36-41, as seen in the direction of rotation, in each reaction 
chamber. In this way, the assisting buoyancy effect is maintained to the 
end. In addition to this, the front part of each reaction chamber is 
thoroughly aerated. This aeration is also assisted by the fact that the 
air is compressed all the time it is under water, which increases its 
solubility in water. 
Viewed as a whole, hollow element 5 is characterized by the special design 
of its partitions 10-15 and by the fact that they continue on to form 
covering webs 30-41. The reaction efficiency is thus particularly high, 
since optimal aeration of the sewage and biological growths are assured, 
together with the largest possible surface to which the said growths may 
adhere. For a given reaction efficiency, the power required is 
substantially less than in hitherto known designs. The buoyancy forces of 
the air-bubbles carried along are mutually compensated for throughout the 
immersion phase by retaining the said bubbles in the reaction chambers, 
which also results in a substantial decrease in the power required. 
The design of the device according to the invention is not restricted to 
its use as a reaction basin. It may obviously also be used as a 
sludge-aerating basin, for example to convert the sludge arising during 
clarification into a product which can safely be disposed of. 
FIG. 3 illustrates another referred example of embodiment which differs 
from the device illustrated in FIG. 2. 
In this case, additional chambers 56 are arranged upon a wide covering web 
30-35 and a narrow covering web 36-41 in the main chambers, so that only 
slotted apertures 24-29 of the main chambers remain free. Each additional 
chamber consists of an axially-parallel wall 57 which leads, as viewed in 
the direction of rotation A of the hollow element, of an axially-parallel 
wall 58 which lags, and of lateral end-walls not shown in the drawing. 
Covering webs 30-41 form the bottom surfaces of the additional chambers. 
Each leading wall 57 consists of a substantially radial wall-section 59 
and of a wall-section 61 adjoining radial section 59 in a direction 
opposite to the direction of rotation and directed substantially 
peripherally. Lagging wall 58 consists, accordingly, of a substantially 
radial wall-section 60 and of a wall-section 62 adjoining this section in 
the direction of rotation and directed substantially peripherally. 
Wall-sections 61,62, directed substantially peripherally, define between 
them an axial slot 63 which operates substantially in the same way as 
slotted apertures 24-29 in the main chambers. 
In this special embodiment, axial slot 63 is arranged approximately in the 
vicinity of the transition between a narrow covering web 36-41 and a wide 
covering web 30-35. Wall-sections 61,62 directed substantially 
peripherally, are made of flat sheet-metal and run substantially parallel 
with the covering webs in the main chambers. 
Upon immersion into the liquid and as the element rotates, lagging walls 58 
carry along a volume of air which remains trapped in the lagging part of 
the relevant additional chamber 56 almost until bottom dead centre is 
reached. As bottom dead centre is passed, the air carried along passes 
slowly into the leading part of the additional chamber, but some of it 
escapes through axial slot 63 into the surrounding sewage. Air trapped 
during immersion of the additional chambers behind leading wall 57 also 
produces buoyancy in the additional chambers which prevents any undue 
increase in the drive-power. Upon emerging from the liquid, lagging wall 
58 carries along a volume of water which, upon reaching top dead centre, 
escapes in part from the additional chamber and is thus brought into 
contact with the ambient air. Part of this air, however, also passes to 
the leading part of the additional chamber and thus also assists in 
rotating the hollow element. The discharge of air from additional chamber 
56, near bottom dead centre, and the discharge of water from the 
additional chamber, near top dead centre, is aided by the fact that the 
lateral end-walls comprise, in the vicinity of the axial slots, an opening 
which may extend as far as the bottom of the additional chamber. As a 
result of this, more air is released, near bottom dead centre, into the 
sewage located in the bottom part of the trough, whereas, near top dead 
centre, a larger amount of water trickles back through the ambient air 
into the trough. Air or water may be deliberately allowed to escape from 
the additional chambers, through the said lateral openings in the 
end-faces of the additional chambers, through perforations arranged 
elsewhere in the defining walls of the additional chambers. The 
arrangement and magnitude of such openings depends upon how much air or 
water is to escape from the additional chambers, and at what point during 
the rotation, in order to achieve an optimal aerating effect. The most 
satisfactory arrangement is also dependent upon the level of the liquid in 
the trough and may easily be determined by one skilled in the matter. 
The additional chambers arranged at the periphery of the hollow element may 
also differ from the design shown in FIG. 3, as long as they fulfill the 
purpose of the invention, namely to introduce additional air mainly into 
the sewage at the bottom of the trough, and also to raise the sewage up in 
order to bring it into contact with the ambient air.