Sewage ventilating basin

A sewage ventilating basin of a sewage treatment plant comprises a sewage container which is sunk into the ground and into which project three vertical tubes which connect the surface water of the container with the deepest water thereof. By means of feed pumps, a downwardly-directed flow is produced in the tubes so that the sewage in the container circulates upwardly and recirculates in the tubes. Disposed in each tube in the upper part thereof are devices for the fine-bubble introduction of a ventilating gas. The flow velocity of the sewage in the tube and directly below this upper part is higher than the uplift velocity of the gas bubbles being introduced into the flow at the same point. Since the gas is introduced into the sewage not in the bottom zone of the container but instead in the upper part of the tube, the gas pressure is lower than in the case of the direct introduction of the gas into the bottom zone, and also the energy expenditure is smaller with a high effectiveness of the ventilation.

FIELD OF THE INVENTION 
The present invention relates to a sewage ventilating basin for use in a 
sewage treatment plant including a tube passing from the top to the bottom 
of the basin and a pump for causing water circulation in the basin, 
through the tube. Known plants of this kind have been described, for 
example, in the Lehr- und Handbuch der Abwassertechnik [Text- and Handbook 
of Waste Water Techniques], volumes I-III, 1975 (Abwassertechnische 
Vereinigung e.V. in Bonn; Verlag Wilhelm Ernst und Sohn, 
Berlin--Muenchen-- Duesseldorf) [Waste Water-Technical Association, a 
registered association, in Bonn; Wilhelm Ernst and Son, Publishers, 
Berlin--Munich--Duesseldorf]. 
BACKGROUND OF THE INVENTION 
Oxygen is needed for the biological decomposition of organic pollutants 
contained in waste waters. In customary basins of the biological 
activated-sludge sewage treatment plants, the introduction of oxygen into 
the waste water is effected either by blowing compressed air into a depth 
of a few meters below water level, or by a surface ventilation. In 
isolated cases, pure oxygen is used instead of air. The waste water 
absorbs the oxygen in the dissolved form. In connection with the 
introduction of air and/or oxygen below the water surface, the blow-in 
depth and the size of the gas bubbles are of significance for the 
effectiveness of the plant. A higher pressure combined with a greater 
blow-in depth enhances the gas charge; likewise, many small bubbles are 
more advantageous, as compared to a few large bubbles, because of the 
greater total contact surface of gas/water. In addition, the contact time 
plays a role, and specifically the gas bubbles having been introduced into 
a greater depth have available--as a result of the longer ascending path 
to the water surface--a correspondingly longer contact time. On the other 
hand, it must be taken into account, of course, that the size of the gas 
bubbles, and therewith the contact time, is dependent upon the respective 
water depth and the pressure prevailing there. 
The energy consumption needed for the gas charge in the known waste water 
ventilating basins is on the order of 0.7-1.5 kWh per kilogram of oxygen 
being introduced and constitutes a significant portion of the operational 
costs of a sewage treatment plant. 
In conceptualizing a sewage ventilating basin, the following physical 
conditions must be taken into consideration for an optimal oxygen 
introduction or charge: 
--the presence in minute bubbles of the gas to be added 
--a high pressure during the contact time 
--a long contact time 
--the uniform reach or inclusion of the entire quantity of sewage. 
A good, minute-bubble gas introduction method consists in pressing the gas 
being supplied by a blower or compressor through a finely porous body, 
which may have any desired shape, under water into the basin. The porous 
body may consist, for example, of a ceramic, or a corrosion-resistant 
material. The uplift or upward pressure velocity of the gas bubbles 
issuing from the porous surface into the water depends upon the size of 
the bubbles and the viscosity of the sewage. Thus, for example, smaller 
gas bubbles are frequently passed by larger ones so that, during their 
rising in stagnant water, larger and fewer bubbles appear at the surface 
than are produced below, which is disadvantageous because of the upwardly 
decreasing total contact surface of gas/water. 
This natural phenomenon may be countered, for example, in that the large 
bubbles are dispersed over and over again by means of producing a high 
turbulence in the water. 
A turbulence may be brought about by deflecting the path of the bubbles 
with static means, for example by installing tilted baffle plates at whose 
edges turbulent zones are generated. More effective, however, would be 
mechanical devices, such as, for example, oppositely-directed paddles or 
propellers which, in addition to generating turbulence, will also directly 
break up large bubbles. 
The pressure at which the oxygen introduction or charge is to take place is 
determined, in an open basin, by the water depth wherein, as is well 
known, the pressure increases linearly with the depth. The pressure in the 
gas bubbles corresponds precisely to the pressure of the ambient water in 
the respective depth. In an ascending gas bubble, therefore, the pressure 
decreases and the volume increases at the same time; in other words, 
rising bubbles expand and the contact surface of the individual bubble 
also increases. 
Basically, two different possibilities exist for obtaining a high pressure 
in sewage, namely on the one hand the generation of high pressure within 
closed containers by means of either hydraulic or pneumatic systems, and 
on the other hand the utilization of great water depths in ventilating 
towers or ventilating shafts. 
While in sewage towers the raw sewage as well as the residual sludge must 
be pumped up to the pressure head of the tower, this expenditure may be 
obviated in cases where a subterranean shaft can be sunk. Here the sewage 
inflow, the residual sludge feed, and the water drainage will hardly 
occasion any energy costs other than those which arise in the customary 
flat basin plants. Yet the energy expenditure involved in the introduction 
of oxygen into the water is relatively great. 
In a sewage tower and in a deep sewage shaft, the water circulation plays 
an important part. The cross-sectional dimensions are relatively small as 
compared to the water depth, and the water does not circulate 
independently since no high temperatures or differences in density occur. 
In actual operation it is therefore not sufficient simply to replace any 
oxygen-enriched water discharging from the top with inflowing water being 
introduced from above because such inflowing water could practically not 
flow into the lower zones where the rational oxygen charge or introduction 
can take place. 
For this reason the water circulation must be artificially produced and/or 
enhanced. When the oxygen is introduced at the tower or shaft bottom, the 
untreated sewage must be brought to this bottom area, and precisely 
together with the residual sludge from the subsequent purification. The 
shaft content must be adjusted or coordinated to the quantity of sewage to 
provide a residence time of the sewage within the container which is 
appropriate with respect to the container, corresponding to the 
decomposition output. 
In sewage shafts, the sewage feed line and the air or oxygen line are 
expediently installed in the interior of the shaft. Instead of causing the 
water to flow just once upwardly from below and then have it flow into the 
subsequent purification basin, it may also be advisable to circulate the 
water being present in the shaft repeatedly in the vertical direction. 
This may be accomplished, for example, in that the tower or shaft is 
subdivided into two or more like or unlike shafts by means of vertical 
walls or pipes, and the circulation is effected with feed pumps. For the 
purpose of a good intermixture, the raw sewage is advantageously added to 
the revolution in a downwardly-directed stream. In order to achieve as 
long as possible a contact time of the gas bubbles in the water, the air 
is brought to a high pressure by means of a compressor and thereafter 
introduced into the sewage as much as possible in proximity to the bottom 
of the shaft and as much as possible in minute bubbles. 
This requires a high energy expenditure for the compressor so that the 
operational costs of known sewage treatment plants with sewage ventilating 
basins are relatively high. 
SUMMARY OF THE INVENTION 
It is now the object of the present invention to eliminate this 
disadvantage. It is intended to create a sewage ventilating basin of the 
type mentioned hereinbefore wherein the energy expenditure for the 
introduction of oxygen is reduced while the effectiveness of this 
introduction is simultaneously improved. 
In accordance with the present invention, this object is obtained by virtue 
of the fact that devices for the fine-bubble introduction of a gas are 
disposed in an upper part of the pipe, and that the flow velocity of the 
sewage in the pipe directly below this upper part is greater than the 
uplift velocity of the gas bubbles being suspended in the flow at the same 
place. 
Achieved with this provision of the sewage ventilating basin is a very 
fine-bubble oxygen introduction into the sewage, i.e. the total gas bubble 
surface is large, which results in a correspondingly high diffusion 
velocity of the oxygen in the water. The uplift velocity of the small gas 
bubbles is very low so that a long contact time is obtained. Due to the 
high turbulence and microturbulence of the water, the oxygen-saturated 
boundary layers of the small gas bubbles are rapidly renewed, and the rate 
of solution and diffusion velocity is influenced advantageously. The good 
intermixture and revolution of the sewage leads to a homogeneous 
distribution of the pollutants, of the oxygen, and of the sludge, and 
contributes to the realization of a balanced decomposition of the organic 
pollutants. Since the air and/or the oxygen is not introduced in the 
bottom zone but in an upper zone into the downwardly-directed pipe, the 
gas pressure is lower than in the case of the direct introduction of the 
gas into the bottom zone, as in the prior art sewage treatment plants, so 
that the energy expenditure is considerably smaller. 
Lastly, it should be pointed out that the afore-mentioned measures not only 
will not impair the microorganisms which decompose the pollutants, but the 
so-called substrate breathing as well as the endogenous breathing will be 
accelerated and intensified. As a result thereof, shorter residence times 
of the sewage within the activator container and/or smaller containers are 
required. 
Experiments have shown that with the inventive sewage ventilating basin it 
is possible to introduce into the waste water per kWh a multiple of oxygen 
as compared to known sewage treatment plants.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The sewage ventilating shaft illustrated in FIG. 1 belongs to the category 
of the open types of construction in which the water surface is subjected 
to atmospheric pressure. Also possible are types of construction in the 
form of towers above the ground, as well as shafts disposed 
subterraneously or in intermediate layers, which are suitable for 
relatively large quantities of water with varying pollutant 
concentrations. The water depths are between 10 and 50 meters, but deeper 
shafts are equally possible. The diameters amount to approximately 1.5 to 
3 meters, with greater diameters also being possible, however. 
The main constituent of the plant according to FIG. 1 is the sewage 
container 1 which is sunk into the ground 2 and containing sewage 21 being 
treated. The inflow of the raw sewage 3 takes place by way of the channel 
4 which terminates into the container 1 above the water surface 5. 
Disposed next to the container 1 and equally sunk into the ground 2 is a 
subsequent purification basin 6 whose overflow is in operative connection 
with the discharge channel 7. Three vertical tubes 8 projecting into the 
container 1 and in which a downwardly-directed waste water or sewage flow 
9 is produced by means of water feed pumps 10, connect the upper part of 
the container 1 with its bottom zone 11 and carry surface waste water to 
the bottom. 
The water feed pumps 10 may be electrically-driven pre-rotation pumps known 
per se. Disposed in the upper region 12 of the tube 8 is a plurality of 
successively-arranged narrowing or constricting tube lengths 13a-13d in 
the form of Venturi tubes. Within these lengths 13, the flow velocity V is 
accelerated due to the injector effect, which results in a lower static 
pressure. An air supply line 14 terminates in the tube 8 within the area 
of the first tube-narrowing length 13a so that the air 16 is suctioned in 
and intensively mixed with the waste water jet directly below the first 
tube 13a. In one of the subsequent tube-narrowing or constricting lengths 
or paths 13c, the inlet connecting piece 15 terminates in the tube 8 which 
supplies the residual sludge 18 by way of the line 19. The residual sludge 
18 is equally suctioned in due to the flow 9 and intensively mixed with 
the sewage flowing downwardly. Instead of air 16, pure oxygen 17, or air 
enriched with oxygen could also be introduced into the flow in the same 
manner. 
Through the tube 8 being thereafter widened again flows therewith a mixture 
of raw sewage 3, circulating sewage water 21 recycled sludge 18, and air 
16 and/or oxygen 17. The stationary uplift velocity V.sub.A of the gas 
bubbles is highest directly below the narrowing lengths or paths 13 
because at this point 20 a lower pressure still prevails than at a 
lower-positioned point near the container bottom, and the volume of the 
gas bubbles is still greatest here. In order that the gas bubbles 16, 17 
being suspended in the flow 9 be concomitantly transported downwardly, the 
flow velocity V of the water at the point 20 must be slightly higher than 
the stationary uplift velocity V.sub.A of the gas bubbles. During the 
transportation downwardly, the gas bubbles enter zones with a linearly 
increasing pressure and therewith become smaller so that their uplift 
velocity decreases. It is true, however, that due to the lower gas volume 
portion of the mixture, the density thereof will slightly rise, 
additionally supported by the fact that because of the diffusion of oxygen 
in the water an additional gas volume shrinkage will occur. With a 
constant cross section of the tube 8, there arises therefore a slight 
velocity decrease which, in turn, will lead again to a pressure increase 
and density increase of the mixture. In the concrete case, however, these 
changes are relatively small. 
Still, with long tubes and a sufficiently long contact time, an almost 
complete oxygen passage into the water may be achieved already within the 
tube 8. Essential for the contact time is the velocity difference 
.DELTA.V=V-V.sub.A of the downwardly-directed flow and/or of the 
upwardly-directed uplift, whereby of course this difference must be 
positive so that all of the suctioned-in gas bubbles are conveyed 
downwardly. When this difference is great, the gas bubbles reach the lower 
tube end quickly, but if it is small, the long contact time being sought 
is achieved. 
After reaching the tube mouth 22 within the container bottom zone 11, the 
mixture in the sewage container 1 flows upwardly. The gas bubbles will 
then flow toward the water surface 5 and will thereby enter zones having a 
decreasing pressure; i.e. the reverse procedures with respect to bubble 
size and mixture density will take place as compared to those in the 
downwardly-directed flow within the tube 8. Since the size of the gas 
bubbles is variable, the larger will move upwardly faster than the smaller 
so that they will be combined with each other and so that individual 
larger bubbles are formed. 
For the purpose of maintaining the fine-bubble condition of the gas on its 
relatively long path within the container, it may be advantageous to 
provide intensive turbulence zones 26 at either one or several points. 
Used for producing the turbulence are mechanical means which cover the 
entire shaft cross section but generate a very small intensity of 
turbulence density and thus require little energy. Achieved by virtue of 
these turbulence zones 26 is the fact that larger bubble formations at 
various points are broken up in order to maintain the fine-bubble 
condition of the gas over the entire water depth. 
As turbulence generators serve, for example, the propeller devices 23 shown 
in FIGS. 1 and 2. Each device 23 is provided with a packet or set of 
twelve propeller blades 24 which are secured to the drive shaft 25. The 
set of blades has a small thickness in order that the layer thickness of 
the horizontal turbulence zone 26 remain as small as possible. The thin 
drive shaft 25 is positioned within three mounting supports 27 and is 
driven by means of the geared engine 29 mounted on the container cover 28. 
Because of the higher velocity of the blades toward their outer ends, they 
may become thinner outwardly from the shaft 25 in order that the layer 
thickness of the turbulence zone 26 remain approximately the same over the 
entire diameter. The actually undesirable rotation of the entire container 
or vessel content is countered by the three tubes 8 acting as baffle 
plates. In order to avoid sludge deposits, cover plates 30 are disposed 
between the tubes and the inner container wall so as to cover the lost 
corner spaces. 
In order to render it possible to remove from the container the propeller 
devices 23 together with the drive shaft 25 for purposes of inspection, 
three guide rods 31 are fastened to the inner container wall. The outer 
ends of the mounting supports 27 are slidingly positioned on these guide 
rods so that the drive shaft 25 together with the propeller devices 23 may 
be removed from the container 1 in a simple manner. 
Water nozzles 32 may be fastened to the container cover 28 in a manner 
known per se for possibly combating foam above the water level 5 of the 
container. The spray water is removed from the drainage channel 7 by way 
of the line 33. Further disposed at the top of the container cover 28 is a 
waste gas connecting piece 34 designed for removing the used air. 
A part of the ventilated sewage 21 circulating downwardly through the tubes 
8 and upwardly within the container 1 is continuously removed from the 
container 1 and flows into the subsequent purification basin 6 by way of 
the pipe lines 35. A part of the sludge 18 being deposited in the 
subsequent purification basin 6 is supplied again to the sewage 
circulation or cycle via the siphon 36 and the residual sludge line 19. 
The oxygen tank 37 serves for the supply of the plant with pure oxygen 17 
which may be employed either alternately or together with air 16. 
The dimensioning or design of the entire plant should be made individually 
for each respective case because, in addition to the quantity and 
composition of the waste water to be treated other conditions, for example 
the building site may play an important role for the container. The 
container size, the residence time of the water within the container, the 
pump output, the circulation velocity, the tube diameter, etc. must be 
coordinated with respect to each other in the dimensioning or design so as 
to provide an optimal ventilation process. The influences of the various 
physical and biological parameters are determined on the basis of 
experimental results. The intensive ventilation carried out in the 
described plant leads to a significant heat production within the 
container which is relatively well insulated in the ground. It would 
therefore be possible to utilize the increased water temperature, for 
example by means of a heat pump.