GB1589190A

GB1589190A - Single stage process for continuously introducing oxygen containing gases into sewage containing activated sludge or fermentation broths

Info

Publication number: GB1589190A
Application number: GB5234/78A
Authority: GB
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1977-02-09
Filing date: 1978-02-09
Publication date: 1981-05-07
Also published as: NL7801455A; FR2380226B1; DE2705243A1; ATA84778A; BE863791A; AU517718B2; AT356025B; IT7847946A0; CA1106301A; CH630046A5; AU3311978A; IN148310B; JPS5399659A; FR2380226A1; JPS5726840B2

Abstract

In this method, the oxygen-containing gas (5) is introduced into the wastewater or the broth (6) by injectors having diameters of the propellant jet nozzle (1) </= 20 mm. In the mixing space (2), the gas (5) is mixed with the propellant (6). The injectors are impinged by a gas flow rate of 5 to 100 eff.m<3>/h. The propellant jet flow rate is 15 to 60% by volume of the gas flow rate, measured in eff.m<3>. <IMAGE>

Description

(54) SINGLE STAGE PROCESS FOR CONTINUOUSLY INTRODUCING OXYGEN CONTAINING GASES INTO SEWAGE CONTAINING ACTIVATED SLUDGE OR FERMENTATION BROTHS (71) We, BAYER AKTIENGESELLSCHAFT a body corporate organised under the laws of Germany of 509 Leverkusen, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement.

In order to operate a plant for purifying sewage biologically, the quantity of oxygen required for the metabolism of the micro-organisms must be introduced into the activated sludge tank, and the concentration of the oxygen must be maintained at a predetermined value so that the bacterial culture can work aerobically. The oxygen required for this purpose is supplied to the liquid from a gaseous phase. The rate at which the oxygen from the gas enters the liquid depends both on the size of the gas/liquid interphase and upon the turbulence prevailing therein, and in a directly proportional manner on the difference in concentration A c between the maximum saturation concentration of 2 obtainable and the 02 concentration prevailing in the liquid.

In order to increase the interfacial area various devices may be used, for example surface aerators, or the gas, generally air, is fed into the liquid and optionally dispersed in it by means of stirrers or jets of liquid (for example from an injector or two component nozzle): In the first case, normal pressure prevails in the gas-liquid contacting area while in the second case tanks or towers are usually used so that a hydrostatic pressure is maintained at the point where the gas is dispersed.

The rate at which the oxygen enters the liquid may be accelerated considerably by increasing the difference in concentrationAc. In practice, Ac can only be increased by raising the partial pressure of the Q in the gas. This may be effected either by raising the pressure of the system or by using oxygen or oxygen-enriched air instead of air. The use of pure oxygen or oxygen-enriched air requires, for economic reasons, that the outlet gas stream is substantially impoverished in oxygen. When the plant is operated under normal pressure this oxygen starvation is obtained by recycling the gas stream through the liquid several times and/or by feeding the gas through successive absorption stages (so-called multi-stage activated sludge tanks). To obtain a narrow residence time distribution for the gas throughput by the use of a multi-stage system is the subject of the process described in German Auslegeschrift No. 2,032,535.

The pressure of the system may also be raised in order to increase the concentration difference by constructing the active sludge tank in the form of a tower. However. if a high hydrostatic pressure is built-up by a high liquid column, this results in the disadvantages: the sewage has to be pumped to the corresponding liquid head and the gas has to be compressed to the same system pressure. If the gas-liquid contacting is carried out in high towers using nozzles which are only fed by gas and positioned close to the floor, a high energy consumption is necessary for the required oxygen upstake, owing to the poorly suppressed bubble coalescence, which causes a low O utilisation of the air.

One object of the invention is to provide an improved single-step process for treating sewage in high activated sludge tanks, which overcomes the above mentioned disadvantages in the known processes and thus ensures that the oxygen uptake efficiency.

[kgO2/kWh] is as high as possible. Under some circumstances it may be desirable that the process should also be capable of lowering the oxygen content of the gas in a single absorption stage sufficiently to provide a residual gas containing from 6 to 9% oxygen by volume which may be thermically deodorized at about 1,00() C after adding fuel (for example fuel oil or natural gas).

According to the present invention there is provided a process for continuously introducing a gas containing more than 20% by volume of oxygen into sewage containing activated sludge, the gas being substantially consumed by the sewage in a single absorption stage in an activated sludge tank, wherein the gas is introduced into the sewage, which is under its own hydrostatic pressure, at locations which are at hydrostatic pressures of between 0.9 and 3 bar wherein the pressure of the gas introduced is from 0.01 to 0.5 m water column above the hydrostatic pressure of the gas inlet, wherein the gas is introduced by means of injectors having propulsion jet nozzles whose diameters are less than or equal to 20 mm, wherein each of the injectors are subjected to a gas throughput of from 5 to 100 m3/h, wherein the propulsion jet throughput is from 15 to 60% by volume of the gas throughput m3, and the injectors are arranged equidistantly or in equidistant clusters and wherein one injector is provided per 1 to 25 m2 of the floor area of the activated sludge tank, from 0 to 1 m above the floor, the gas throughput being measured at the pressure and temperature prevailing in the tank. For convenience the gas throughput as measured aforesaid is referred to below as being in "effective" m3/h. The propulsion jet nozzles preferably have diameters which are 8 to 16 mm. Preferably one injector per 1 to 5 m2 of the floor area of the tank is provided, preferably at locations of identical hydrostatic pressure.

The process according to the invention is preferably carried out in towers which are approximately 10 to 30 m, particularly preferably 15 to 20 m, high and of any suitable cross-section. The height to diameter ratio is preferably from 5 to 0.5. Air is preferably used as the oxygen-containing gas in the process but oxygen-enriched air or industrial oxygen may also be used. The process, according to the invention may be carried out particularly conveniently in an apparatus having settling chambers arranged concentrically around the activated sludge tank, the settling chambers being connected communicating with degassing and flocculation cyclones via inlet pipes. An apparatus of this type forms the subject of a prior U.K. Patent Application No. 50071/76. (Serial No. 1568516) The injectors which are preferably circular or slit-shaped have individual outlet cross-sectional areas of, from about 2 to 15 cm2. When a slit-shaped injector outlet is used its length to width ratio is from about 5:1 to about 10:1. The gas containing oxygen is introduced into the activated sludge tank at a throughput of from 5 to 100 effective m /h by the injectors. The cross-sectional throughput of the injector outlet is from 2 to 8, preferably from 2 to 4 effective m3 gas/cm2h. The liquid throughput through the propulsion jet nozzle of the injector is 15 to 60 % by volume of the gas throughput this being expressed in effective m3. The velocities of the propulsion jets are between about 10 to 20 m/sec., preferably between 10 to 15 m/sec.

Injectors having propulsion jet nozzles whose diameters are S 20 mm, preferably from 8 to 16 mm and whose individual outlet cross-sections are from 2 to 15 cm2 are suitable for carrying out the process according to the invention. In a particularly preferred embodiment, injectors are used having nozzles whose outlet cross-sections are slit-shaped with a length to width ratio of between 5:1 to 10:1, and the diameter of the propulsion jet nozzle is between tO and 20 mm. In these injectors, the mixing chamber is such that it changes from a preferably circular or oval inlet cross-section into a slit-shaped outlet cross-section. This produces converging surface boundaries in the mixing chamber which result in high shear rates at the boundary layer and promote the formation of very fine primary gas bubbles. With injectors of this type, the gas/liquid dispersion leaves the injector as a flat belt and mixes more readily into the surrounding liquid.

An injector of this type is illustrated in the accompanying drawing in such a way that the interior of the apparatus can be seen through the cut-away parts of the wall. The reference numerals used therein have the following meanings: 1 propulsion jet nozzle 2 mixing chamber 3 mixing chamber inlet of circular cross-section 4 mixing chamber outlet of slit-shaped cross-section 5 gas inlet 6 liquid inlet All injectors are preferably installed at points of equal hydrostatic pressure, when they are arranged individually and are positioned at intervals I of from 1 to 6 m, preferably 1 to 2 m. A greater interval is used if clusters of injectors are used. If a cluster contains z injectors, then the interval from cluster to cluster is increased by a factor I z, given the same closeness of distribution (floor area per injector). If the injectors are arranged in clusters, then the injector outlets should be positioned in such a way that the outflowing free jets of gas/liquid disturb each other as little as possible since this could lead to an undesirable bubble coalescence. The hydrostatic pressure where the gas is introduced is between 0.9 and 3 bar, and the gas is introduced at a pressure of from 0.01 to ().5 m water column above the hydrostatic pressure of the gas inlet.

It is particularly advantageous to use sewage containing activated sludge as propulsion liquid for the injectors.

It has surprisingly been found that the task posed according to the invention may be solved excellently by using special injectors with small diameter propulsion jet nozzles (t1 620 mm). With these injectors, it is only necessary to have small propulsion jet throughputs which flow through the injector at a relatively low velocity and which cause small pressure losses on the liquid side. This is particularly important from the energy point of view. It is simple to install a filtration unit upstream of these low throughput propulsion jets to remove coarse particles from the sewage. Thus there is no danger of the injector becoming blocked in carrying out the process according to the invention.

The invention is illustrated below by way of example of an apparatus for biologically purifying sewage, using air.

An activated sludge tank which has a liquid volume of 10,000 m3 is charged with 1,000 m3 of sewage per hour. This plant requires 20 tons of oxygen daily or 833 kg O,/h. The O, concentration required in the liquid is 1 mg per litre and the water temperature is 25"C. The liquid height is 10 m and the cross-sectional area of the floor of the activated sludge tank is 1,000 m while in case B the liquid height is 20 m and the cross section of the floor 500 m2.

In both cases, injectors whose propulsion jet nozzles have a diameter of 8 mm (cross-section of the mixing chamber outlet: 2.3 cm-) are arranged equidistantly from one another at intervals of 2 m. In the container which is 20 m high therefore only half as many injectors are present as in the container with a floor of half the cross section.

The following parameters apply in case A: Liquid height: 10 m Number of injectors: 318 Gas throughput through all injectors in effective m3/h: 6200 Liquid throughput through all injectors in m3/h: 1330 Proportion of O2 in the waste gas: 12 % by volume The over-all efficiency of the oxygen uptake is 2.5 kg O2/kWh, when the efficiency of the compressor used is 0.6 and that of the liquid pumps is 0.75.

The following operating parameters apply in case B: Liquid height: 20 m Number of injectors: 15') Gas throughput through all injectors in effective m3t: 3975 Liquid throughput through all injectors in m'/h: 690 Proportion of 02 in the waste gas: 6.3 % by volume The over-all efficiency of the oxygen uptake is 2.9 kg O2/kWh, the efficiencies of the compressor and pump being the same as in case A.

WHAT WE CLAIM IS: 1. A process for continuously introducing a gas containing more than 20% by volume of oxygen into sewage containing activated sludge, the gas being substantially consumed by the sewage in a single absorption stage in an activated sludge tank. wherein the gas is introduced into the sewage, which is under its own hydrostatic pressure, at locations which are at hydrostatic pressures of between 0.9 and 3 bar wherein the pressure of the gas introduced is from 0.01 to 0.5 m water column above the hydrostatic pressure of the gas inlet, wherein the gas is introduced by means of injectors having propulsion jet nozzles whose diameters are less than or equal to 20 mm, wherein each of the injectors are subjected to a gas throughput of from 5 to 100 rn/h, wherein the propulsion jet throughput is from 15 to 60% by volume of the gas throughput m3, and the injectors are arranged equidistantly or in equidistant clusters and wherein one injector is provided per 1 to 25 m2 of the floor area of the activated sludge tank, from 0 to 1 m above the floor, the gas throughput being measured at the pressure and temperature prevailing in the tank.

2. A process according to claim 1, wherein the diameter of the propulsion jet nozzles are from 8 to 16 mm.

3. A process according to claim 1 or 2. wherein one injector is provided per 1 to 5 m2 of floor area.

4. A process according to any preceding claim, wherein the injectors are arranged at positions of equal hydrostatic pressure.

5. A process according to any preceding claim wherein injectors are used which have individual outlets whose cross-sections are from 2 to 15 cm2.

6. A process according to any preceding claim. wherein the cross-sectional load of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the hydrostatic pressure of the gas inlet. It is particularly advantageous to use sewage containing activated sludge as propulsion liquid for the injectors. It has surprisingly been found that the task posed according to the invention may be solved excellently by using special injectors with small diameter propulsion jet nozzles (t1 620 mm). With these injectors, it is only necessary to have small propulsion jet throughputs which flow through the injector at a relatively low velocity and which cause small pressure losses on the liquid side. This is particularly important from the energy point of view. It is simple to install a filtration unit upstream of these low throughput propulsion jets to remove coarse particles from the sewage. Thus there is no danger of the injector becoming blocked in carrying out the process according to the invention. The invention is illustrated below by way of example of an apparatus for biologically purifying sewage, using air. An activated sludge tank which has a liquid volume of 10,000 m3 is charged with 1,000 m3 of sewage per hour. This plant requires 20 tons of oxygen daily or 833 kg O,/h. The O, concentration required in the liquid is 1 mg per litre and the water temperature is 25"C. The liquid height is 10 m and the cross-sectional area of the floor of the activated sludge tank is 1,000 m while in case B the liquid height is 20 m and the cross section of the floor 500 m2. In both cases, injectors whose propulsion jet nozzles have a diameter of 8 mm (cross-section of the mixing chamber outlet: 2.3 cm-) are arranged equidistantly from one another at intervals of 2 m. In the container which is 20 m high therefore only half as many injectors are present as in the container with a floor of half the cross section. The following parameters apply in case A: Liquid height: 10 m Number of injectors: 318 Gas throughput through all injectors in effective m3/h: 6200 Liquid throughput through all injectors in m3/h: 1330 Proportion of O2 in the waste gas: 12 % by volume The over-all efficiency of the oxygen uptake is 2.5 kg O2/kWh, when the efficiency of the compressor used is 0.6 and that of the liquid pumps is 0.75. The following operating parameters apply in case B: Liquid height: 20 m Number of injectors: 15') Gas throughput through all injectors in effective m3t: 3975 Liquid throughput through all injectors in m'/h: 690 Proportion of 02 in the waste gas: 6.3 % by volume The over-all efficiency of the oxygen uptake is 2.9 kg O2/kWh, the efficiencies of the compressor and pump being the same as in case A. WHAT WE CLAIM IS:

1. A process for continuously introducing a gas containing more than 20% by volume of oxygen into sewage containing activated sludge, the gas being substantially consumed by the sewage in a single absorption stage in an activated sludge tank. wherein the gas is introduced into the sewage, which is under its own hydrostatic pressure, at locations which are at hydrostatic pressures of between 0.9 and 3 bar wherein the pressure of the gas introduced is from 0.01 to 0.5 m water column above the hydrostatic pressure of the gas inlet, wherein the gas is introduced by means of injectors having propulsion jet nozzles whose diameters are less than or equal to 20 mm, wherein each of the injectors are subjected to a gas throughput of from 5 to 100 rn/h, wherein the propulsion jet throughput is from 15 to 60% by volume of the gas throughput m3, and the injectors are arranged equidistantly or in equidistant clusters and wherein one injector is provided per 1 to 25 m2 of the floor area of the activated sludge tank, from 0 to 1 m above the floor, the gas throughput being measured at the pressure and temperature prevailing in the tank.

injector outlet is between 2 and 8 migas/cm'h, the gas throughput being measured as aforesaid.

7. A process according to claim 6, wherein the said load is between 2 and 4 m3 gas/cm2h.

8. A process according to any preceding claim. wherein the liquid throughput through the propulsion jet nozzle of each injector is from 15 to 60So by volume of the gas throughput, the gas throughput being measured as aforesaid.

9. A process according to any preceding claim. wherein the velocities of the propulsion jet are between 10 and 20 m/sec.

10. A process according to claim 9. wherein the said velocities are between 10 and 15m/sec.

11. A process according to any preceding claim wherein the injectors have outlets which are slit-shaped and whose length to width ratio is from 5:1 to 10:1.

12. A process for continuously introducing a gas into sewage. substantially as herein described with reference to Example A or Example B.

13. A process for continuously introducing a gas into sewage, substantially as herein described with reference to the accompanyings drawing.