GB2105701A - Improvements in sewage aeration processes and apparatus for carrying out such processes - Google Patents

Abstract

Sewage aeration process and apparatus wherein substance to be treated is contained within a tank 1, preferably cylindrical in shape, the tank having means for entry of air to form bubbles within the substance in the tank 1 and means for producing recirculating flows through a coaxial open topped tube 2 leading to pumping mechanisms 4 which return the flows to the tank 1 at a lower level therein, through pipes 5. <IMAGE>

Description

SPECIFICATION Improvements in sewage aeration processes and apparatus for carrying out such processes This application relates to improvements in sewage aeration processes and apparatus for carrying out such processes.

The arrangement is such that when air is supplied to the substance under treatment in an aeration vessel absorption by the liquid of oxygen present in the air thus supplied is fascilitated for use in industry.

In sewage processes having an aerobic phase there is a step in which the liquid to be treated is given aeration treatment in order that, because of the solubility of the oxygenated gas in the water, the microorganisms having a vital need for oxygen can obtain it and thus digest the organic substance contaminating the water being treated.

Also, of course, in water aeration processes there are, in addition to specific parameters determining such procedure, such as the pressures of the liquid and solid phases, temperatures of the aqueous medium, salinity and so on, three phenomena which have considerable effect on the solubility of oxygen in the liquid.

These three phenomena are: Turbulence in the aqueous medium in the corresponding aeration tank or pond, as a means of increasing the solubility of oxygen in the liquid; The extent to which the incoming air is dispersed in the water under treatment in the corresponding aeration tank, since the greater the dispersion the greater the overall transfer, transfer then becoming possible in extensive zones of the total body of liquid being treated; The formation of surfactant foams on the exposed surface of the water in the aeration tank, which inhibits the transfer of atmospheric air to the liquids being treated.

It will be useful for the purposes of industrial sewage treatment with an aerobic phase, to have an aeration installation of a kind which can provide in its intended use, when air is supplied to the water under treatment near the bottom of the aeration tank in the form of a gas broken down into small bubbles, the following simultaneous phenomena which aid dissolution in the liquid being treated of the oxygen present in the incoming air and in the atmosphere and fascilitating industrial use, namely:: Local turbulence or eddies in the body of the aqueous medium being treated and contained in the corresponding aeration pond or tank; Wide dispersion in the body of the liquid being treated of the air which is supplied near the bottom of such pond or tank in the form of small bubbles; The dissemination, sucking-away and breakdown of any surfactant foams tending to form on the exposed surface of the water being treated in the aeration tank or pond.

It is the object of this invention to provide a sewage aeration process and apparatus for carrying out the process which meets these requirements.

According to the present invention there is provided a sewage aeration process comprising producing local turbulence or eddies in the substance to be treated (in aqueous medium), the substance being received in an appropriate vessel.

According to a further aspect, the invention resides in apparatus for sewage aeration comprising an aeration tank with a coaxial tube within it, the latter being connected by piping to mechanisms for producing recirculating flows which are returned to the tank through piping.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings wherein: Fig. 1 is a diagrammatic vertical section through apparatus constructed in accordance with the invention, and Fig. 2 is a diagrammatic plan view of the apparatus shown in Fig. 1.

As can be seen in the drawings, the apparatus includes a tank 1 which serves as an aeration pond; the tank is in the shape of a regular cylinder generated by a closed curve having a centre of symmetry or of revolution around a vertical axis; the tank may or may not have projections on its walls or base and the base may or may not be flat.

Disposed coaxially in the tank 1 is a duct 2 whose open top is disposed, in operation, below the exposed surface of the waters under treatment in the tank. Duct 2 extends through the base of the tank 1.

Below the tank 1 the duct 2 merges into a pipe 3 extending to mechanisms 4 adapted to produce sewage flows and so designed or chosen that their operation does not break up the flakes forming the cultures or colonies of microorganisms which digest organic material in the aerobic processes.

The mechanisms 4 may either be motor-driven axial flow or hydraulic friction pumps running at a speed that will not break up the microorganisms or any devices not causing such breakup.

The mechanisms 4 are connected to the inside of the tank 1 by way of a system of pipes 5 or closed ducts whose ends are disposed near the bottom of the tank and will be provided as required both in number and in direction, with components perpendicular to the tank axis of symmetry.

Together with the tank 1 and the elements, accessories and mechanisms referred to, all of which are characteristic, the system also comprises the conventional known facilities for performing the following functions: Supplying air in the form of small bubbles near the bottom of the tank to the water being treated; Means for recirculating activated sludge from secondary decanting to the tank; Providing a connection between the tank and the decanter and, where necessary, supplying the tank from previous treatment.

With the arrangement just described of an aeration tank 1 and of the elements, accessories and mechanisms mentioned, when the level of the water in the tank to be treated is above the open end of the coaxial duct 2 and the pumped recirculation facilities are started, recirculating flows are produced which go from the tank 1 by way of the duct 2, pipe 3, mechanisms 4, ducts 5 and from the terminations thereof back to the surface of the water being treated.

With the apparatus just described, taken together with the supply of air in the form of small bubbles near the bottom of the aeration tank (at a position and by means not shown) and with the recirculation of activated sludge from secondary decantation to the aeration tank, the complete operation of the system can be seen.

When the apparatus is started, production of the recirculating flows produces flows of water from the re-entry or return terminations of the ducts 5 towards the surface of the water in the tank 1. This rising flow will have a volume above the re-entry or return terminals or terminations, equal to the volume of the recirculating flows.

Also, that proportion of recirculated flow which has a component of its direction perpendicular to the tank axis of symmetry produces, in the waters undergoing treatment, because of inertia, hydraulic friction and the geometric shape of the tank produces a rotary movement around the tank axis of symmetry.

The hydraulic side of the circulation of the water is subject to the following dynamic actions: 1. The action of the flow of water rising towards the surface, resulting from the production, return and continuity of the recirculated flows; 2. Shape and reaction resistances opposed by the solid particles, which are always present in sewage, to entrainment by the rotary flow: 3. Shape and reaction resistances offered by the bubbled-in air to entrainment by the rotary flow; 4. Agitation produced by the bubbles of ail as they rise towards the water surface; 5. The actions of the returning recirculated flows, the components for which can be located anywhere that is required.

These dynamic actions, result from the operation of the apparatus and which therefore are operative continuously while the apparatus remains in operation, have persistent destabilizing effects on the rotary flow produce in the water in the aeration tank. Also, due to the continuous nature of the dynamic actions, the viscous forces produced cannot completely cancel out the destabilizing effects except by annulling them at their origin, this being impossible since if it was to occur, the viscosity of the sewage would be other than finite. Consequently, the reality of such actions and their continous nature are incompatible with a division of the rotary flow into streamlines or fluid layers differentiated from one another as in the case of theoretical laminar flow.

Since the rotary flow produced inside the aeration tank is neither laminar nor stable, the local velocity of the liquid increases when the destabilizing effects cause a convergence or approach of the surfaces of flows around a point, such velocity decreasing when the tendency is towards divergencies or separations. However, an increase in velocity must correspond to a decrease in pressure and vice versa; consequently, variations in pressure tend to increase the convergencies or divergencies produced in the flow surfaces by the destabilizing effects and, therefore, the increases and decreases of local velocity.This automatic process, which is a chain of cause and effect produces increase in velocity leading to decrease in pressure leading to further increase in velocity leading to further decrease in pressure etc. or decrease in velocity leading to increase in pressure leading to further decrease in velocity leading to further increase in pressure etc., these effects being present provided that recirculation is vigorous enough - i.e., that the means producing the recirculation are powerful enough. There are produced cutting forces (as a result of differences in local velocity between nearby points) which are so marked that they convert the disturbed zone into a plurality of vortices, dots of turbulence or eddies dissipating the power causing them and by their very action they cause an exchange of fluid between contiguous zones of the rotary flow inside the aeration tank.

The agitating effect of the spot turbulences or eddies, the diffusing effect of the exchange of fluid between contiguous zones of the rotary movement and the entrainment thereby of the incoming air bubbles clearly show effective dispersion of the air bubbles throughout the water in the aeration tank.

It is generally known, and it is adequately borne out by hydraulic theory, that a vortex phenomenon occurs above sinks close enough together on the exposed surfaces of liquids if the volume of water present in the sinks is great enough. If the recirculated volume is devised correspondingly for the operation of the installation described, it is obvious that such a phenomenon occurs above the open end of the coaxial duct 2.

In the operation of the apparatus, the vortex is produced on the surface of the water in the tank and the effects of the floating material being channelled towards the centre of the vortices and immersed thereby show that any foam appearing on the surface of the water in the tank is disseminated and sucked away.

The foam thus sucked away and mixed in the recirculating flow experiences therein, as they move, agitations which can be proved to inhibit the surfactant effect tending to agglomerate the foam.

As previously stated, the phenomena of eddies throughout the water contained in the tank, of dispersion of the incoming oxygenated gas and of the sucking-away and breakdown of foam are in the end dependent upon the strength of the recirculating flows produced. There is therefore no reasonable limit to these phenomena if, as occurs in practice, it is possible to produce strong enough recirculating flows to suit individual cases.

Consequently, it is proved that the sewage aeration plant of the kind to which this application relates, by virtue of design and operating features, provides in use the following simultaneous phenomena: a. Local turbulence or eddies in the body of the aqueous medium being treated in the aeration tank; b. Wide dispersion throughout the liquid under treatment of the air supplied in the form of small bubbles near the bottom of the aeration tank; c. The dissemination, sucking-away and breakdown of any surfactant foams tending to form on the exposed surface of the water under treatment in the aeration tank.

The apparatus may be modified in shape and as to materials in any way suitable for differing applications.

Claims (13)

1. Sewage aeration process comprising producing local turbulence or eddies in the substance to be treated (in aqueous medium), the substance being received in an appropriate vessel.

2. Sewage aeration process as claimed in claim 1 wherein air bubbles are introduced into the substance by appropriate means.

3. Sewage aeration process as claimed in claim 2 wherein movement of the body of substances produces exchanges of flow between adjacent zones of such movement and the entrainment thereby of the air bubbles supplied, with the result of wide dispersion of the air supplied to the substance being treated.

4. Sewage aeration process as claimed in any one of the preceding claims wherein any surfactantfoams tending to form on the exposed surface of the substance for treatment contained in the vessel are disseminated, sucked away and broken down.

5. Sewage aeration process as claimed in any one of the preceding claims wherein the recirculating and return flows are collected in an open end of a tube disposed coaxially within the vessel, which is cylindrical, the upper end of the tube being near the normal exposed surface of the substance being treated, vortex phenomenon being produced at the surface to disseminate and suck away any foam tending to form thereon.

6. Sewage aeration process as claimed in claim 5 in which such foam, when mixed by way of the tube with the recirculating flows in the tank, experiences actions tending to break it down and to inhibit its agglomeration.

7. Apparatus for sewage aeration comprising an aeration tank with a coaxial tube within it, the latter being connected by piping to mechanisms for producing recirculating flows which are returned to the tank through piping.

8. Apparatus as claimed in claim 7 wherein the coaxial tube has its upper end disposed at or below the normal substance level in the tank and it is connected from below the tank to said piping.

9. Apparatus as claimed in either of claims 7 and 8 wherein the mechanisms include pumps for delivering the liquid to the tank at positions near the bottom thereof.

10. Apparatus as claimed in any one of claims 7 to 9 in which the tank has the internal shape of a right cylinder generated by a closed curve having a centre of symmetry or revolution around a vertical axis, the mechanisms producing forced recirculating flows to return the substance to the aeration tank, being arranged to produce a rotary movement around the vertical axis of symmetry of the interior of the tank, the mechanisms producing such recirculating volumes as to induce local turbulence or eddies throughout the substance being treated.

11. Apparatus as claimed in any one of claims 7 to 10 wherein the tank has means for introduction of air to form bubbles in the substance, said air being introduced at or near the bottom of the tank.

12. Sewage aeration process substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

13. Apparatus for sewage aeration substantially as hereinbefore described with reference to and as shown in the accompanying drawings.