GB2054549A

GB2054549A - Process and apparatus for the anaerobic treatment of sewage

Info

Publication number: GB2054549A
Application number: GB8019829A
Authority: GB
Original assignee: Grosse W D
Current assignee: Grosse W D
Priority date: 1979-06-18
Filing date: 1980-06-17
Publication date: 1981-02-18
Also published as: BE883862A; ATA319080A; DE2924465C2; FR2459206A1; CA1137658A; CH647482A5; AT379570B; DE2924465B1; GB2054549B; JPS565193A

Abstract

Anaerobic treatment of sewage and any other solids optionally added includes comminuting and homogenizing the sewage and any other solid matter and then passing it upwardly through a biological sand contact bed 14 in a container 4 at a temperature above 40 DEG C to purify the sewage and solids and form sewage gas, after which at least a portion of the purified sewage is recirculated through indirect heat exchanger 2 to preheat untreated sewage passing through the heat exchanger 2 prior to treatment thereof in the container 4. <IMAGE>

Description

SPECIFICATION Process and apparatus for the anaerobic treatment of sewage A process for the anaerobic treatment of sewage is disclosed by the German Offenlegungsschrift 2531 598. This anaerobic treatment is carried out in a contact bed reactor in which coal, coke, slag, porous stone, e.g. pumice, or ceramic is used as a bed for the anaerobic bacteria. The treatment is carried out at temperatures between 100 and 40 C, preferably between 200 and 30 C, and the duration of treatment is specified as being 10 minutes to 10 hours, preferably 0.5 to 3 hours. This process is primarily only directed to the putrefaction of sewage without any great importance being given to the sewage gas inherently produced as a by-product.

Anyone unfamiliar with sewage treatment in general, may refer to the text SEWAGE TREATMENT, 2nd Edition, published by John Wiley & Sons, Inc., New York (1956), this text by reference being hereby made a part of the present application.

In contrast, the object of the present invention is to further develop a process of the aforementioned type and to provide an apparatus for practicing this process so that the amount of. sewage gas obtained is substantially increased while saving as much energy as possible.

The essential features of the process of the invention are: the thermophilic treatment at a temperature of above 40 C, preferably 500 to 555C, which in contrast to the mesophilic process which is carried out at temperature from 280 to 350C, produces about 80% more sewage gas; and the execution of the anaerobic treatment in a sand contact bed where due to the high heat transfer coefficients of sand, the sand layer permits an even temperature which protects the bacteria against fluctuations in temperature of more than + 0.5 C. An essential factor for a short duration of the treatment is the comminution and homogenization of the solid matter contained in the sewage as well as the preheating thereof which is carried out according to the invention in a manner to save energy by transferring the heat from the putrefied sewage and solid matter mixture. The large amount of sewage gas produced hereby can be used for heating purposes or as a source of energy for power engines. The apparatus provides an effective means for effecting these and other steps.

According to a preferred embodiment of the process according to this invention, organic waste can be added to the sewage prior to the comminution and homogenization of the solid matter contained therein, in order to increase the yield of sewage gas further. Materials particularly suited as organic waste to be added to the sewage, are grass, leaves, straw, agricultural waste and slaughterhouse waste products. These organic waste materials are comminuted together with the solid matter already contained in the sewage and homogeneously distributed in the sewage, whereby a particle size of less than 1 mm is aimed at. Should the sewage thicken excessively due to the organic waste added, fresh or industrial water can be added in order to reestablish the degree of fluidity sufficient to the anaerobic treatment in the sand contact bed.

The organic waste materials added to the sewage provide an additional source of food for the anaerobic bacteria. The sewage gas produced during the decomposition of the organic waste due to the anaerobic bacteria adds to the amount of sewage gas produced in the usual putrefaction of sewage sludge.

The process according to the invention can be carried out either in cycles or continuously. In the case of fixed cycle operation, new sewage~~possibly with added organic waste~~runs into a heat exchanger, in which it is preheated by the transfer of heat from the mixture of sewage and solid matter already putrefied. The sewage is circulated through the heat exchanger by means of a gear pump, for example, which comminutes and homogeneously distributes the solid matter and organic waste contained in the sewage, until a sufficient degree of preheating, comminution and homogenization has been reached. The preheated sewage is then passed into the sand contact bed, the volume of which increases by about 30%, whereby a corresponding column of putrefied sludge, sewage water and surface scum situated in the upper portion of the treatment container is pressed into a centrifuge.The centrifuge separates the sewage sludge and the sewage water from the sewage gas which is discharged separately.

The new sewage conducted into the treatment container is gradually putrefied by bacteria adhering to the grains of sand in the sand contact bed.

When the putrefaction has ended, the swirling of the sand in the bioreactor is interrupted and in the state of rest an arrangement of layers is automatically brought about due to the different specific gravities. The upper layer is composed of surface scum (foam from gas and light materials with adherent gas bubbles), the sewage water, in which the biological process no longer progresses, lies under this. Underneath the sewage water is the so-called thin sludge and directly on the contact bed lies a layer of thickened older sewage sludge which, because of its bacteria content, is needed for inoculating into the mixing with the newly added sewage.

When the new sewage is added, the upper scum layer is removed from the reactor holding the sand bed, after being separated into sewage gas and a putrefied mixture of sewage and solid matter, and the newly added sewage is mixed with the thickened older sewage sludge lying on the contact bed by way of a swirling action of the sand contact bed, caused by upward flow through the sand bed.

A fluid bed composed of sand has the advantage over coal, slag, porous stone or ceramic, as known from the German Offenlegungsschrift 2531 598 that the swirling is substantially more intensive and the friction within the fluid bed encourages gas bubbles to rise. This particularly applies for the gas formed in the sand bed during a pause for continuing putrefication. The production of gas is at its greatest shortly after the process has begun, but does not cease during the standstill.

The comminution and homogenization of the solid matter and possibly organic waste in the sewage and its preheating in conjunction with the elevated processing temperature produces a considerably higher yield of sewage gas than was previously possible with the known process. The activity of the bacteria is at its highest at temperature of between 500 and 550C since then optimum living conditions are provided for the bacteria. The gas production is almost exhausted at a process temperature of over 55 C.

According to an advantageous embodiment of the process of the invention, the addition of new sludge, which has not yet putrefied, into the sand contact bed and the separate removal of the putrefied sludge, sewage water and sewage gas ail take place continuously. This requires circulation of the contents of the treatment container by way of a bypass pipe from its upper portion to a pump where newly added sewage is continuously admixed and is pumped back into the lower portion of the treatment container. Due to new sewage being constantly added, the floating putrefied material is pressed upwardly into the centrifuge, to be separated into putrefied sewage and solid matter mixture and sewage gas.

In order to conduct the sewage several times through the system for the comminution and homogenization of the solid matter contained in it and also for sufficient preheating, a bypass pipe can run from the comminuting and homogenizing means to the upper part of the heat exchanger according to an advantageous development of the apparatus of the invention. A flow heater can be connected into the bypass pipe to ensure additional preheating of the sewage.

Further, a bypass pipe can run from the upper part of the treatment container into its lower part, this permitting circulation of the contents of the treatment container.

In an advantageous embodiment a disk centrifuge is arranged in the upper part of the treatment container for separating the putrefied sewage and solid matter mixture and the sewage gas.

A preferred form of the apparatus is schematically illustrated in side elevation by the accompanying drawing and which is described hereinbelow as follows: New sewage, which has possibly organic waste mixed with it, flows into the heat exchanger 2 through the inlet 1. In the heat exchanger 2 the sewage absorbs the heat from the putrefied sewage sludge which flows into the heat exchanger 2 through a pipe 3 out of the upper portion of the treatment container 4 and passes through the heat exchanger 2 through a separate chamber or spiral tube 5, whereby it does not mix with the newly supplied sewage but is discharged from the heat exchanger 2 through a pipe 6 after transferring its heat.

Atthe lower part of the heat exchanger 2 a pipe 7 leads to a gear pump 8, in which the solid matter, and possibly organic waste, in the sewage is comminuted and homogeneously distributed therein. The sewage can be circulated several times through the heat exchanger 2 by the pump 8 via a bypass pipe 9 in which a flow heater 10 can be connected to provide additional heat, until a sufficient degree of comminution, homogenization and preheating has been reached. The homogenized and preheated sewage then passes through a pipe 11 into the lower part of the treatment container 4 which is provided with a jacket heating means 12.

The sewage then flows through a porous or perforated floor 13 into a sand layer 14 positioned on this floor and which has been provided with anaerobic bacteria. The height of the sand bed is dimensioned according to the contents of the treatment container 4 or to the flow velocity of the pump 8, so that the critical flow velocity is exceeded, so that the sand bed is swirled and forms a sand contact bed 14, whereby the swirling motion for each passage is maintained for about 15 or 20 minutes. During this time the biological reaction of the anaerobic bacteria which have settled on the surface of the individual grains of sand is achieved with the pumped in sewage. The depth of the sandbed and the size of the sand grains determine the available area of the reactive surfaces. The coarser solid components in the water are held back in the lower portion of the sand contact bed 14.The gas bubbles can escape freely to the surface due to the swirling. Due to the gas bubbles joining with the sewage sludge, emulsion colloids are formed during and afterthe putrefaction process which are deposited on the surface of the water above the sand contact bed 14 in the form of scum 15.

In the upper part of the treatment container 4 a disk centrifuge 16 is arranged into which the surface scum 15, sewage water and thin sludge is forced before swirling of the sand contact bed 14 is started.

The sewage gas is separated off by way of the disk centrifuge 16. The sewage sludge is passed through the pipe 3 into the heat exchanger 2 and the sewage gas leaves the container 4 via pipe 17 and passes into a container 18, from which it can be transferred into a pressure vessel 20 by way of a compressor 19 and stored there.

Thirty-five thousand liters of sewage gas can be produced in this way from approximately 50 kg dry organic waste material. As the average CH4 content of the sewage gas amounts to 60%, 1 Nm3 of gas has a heat content of approximately 6,000 kcal. Thus, from 50 kg of organic waste material approximately 210,000 kcal can be reached per day, only about 8,500 kcal of which is necessary for the heating or for compensating the heat loss of the process.

A treatment container with a volume of 1 to 2 Nm3 is sufficient in the case of 50 kg of organic waste to cover the heat requirements of a household. After the sewage gas has been desulphurized, it is equally suitable for use in operating a gas engine for powering a heat pump.

Claims

1. A process for the anaerobic treatment of sewage which includes comminuting and homogenizing the sewage and any other solid matter optionally added thereto, subjecting the sewage and any other solid material to the anaerobic treatment in a container con taining a bacteria sand contact bed at a temperature above 400C to produce, in the container, a putrefied sewage and a sewage gas, allowing the sewage gas to discharge from the container, and recirculating at least a fraction of the hot putrefied sewage from the treatment container into an indirect heat exchanger to transfer heat to untreated sewage prior to treatment thereof.

2. Process for the anaerobic treatment of sewage contailling solid matter in which for anaerobic treatment the sewage is conducted together wjith the solid matter contained therein upwards through a bacteria contact bed at a temperature above room temperature, the sewage, after remaining in the contact bed for a sufficient time to putrefy, is removed together with the putrefied solid matter -from the upper part of the contact bed by conducting new sewage into the lower part of said bed, and the putrefied sewage and solid matter and the sewage gas produced therefrom are continually separately discharged from above the contact bed; wherein the improvement comprises the sewage being prehe ated and the solid matter contained therein commi nuted and homogenizing before being conducted through said bed, the sewage being subject to the anaerobic treatment in a sand contact bed at a temperature of over 40 C, and the new sewage added to said putrefied sewage and solid matter being preheated by means of the mixture of sewage and solid matter removed from the sand contact bed.

3. Process according to claim 1 or claim 2 in which organic waste is added to the sewage prior to the communition and homogenization of the solid matter contained therein.

4. Process according to any one of the preceding claims in which the anaerobic treatment is carried out at a temperature in the range of 50#to 55 C.

5. Process according to any one of the preceding claims in which the adding of the new sewage, which has not yet putrefied, into the sand contact bed and the separate discharge of the putrefied sludge sewage water and sewage gas take place continuously.

6. Apparatus for carrying out an anaerobic pro cess and comprising a heated treatment container in which a bacteria bed is positioned and which has an outlet in its upper portion for sewage gas produced during treatment and a separate outlet for the putrefied sewage and solid matter mixture, wherein the improvement comprises a heat exchanger with connecting points for the supply and discharge of the sewage which has not yet putrefied and with a separate conduit for a putrefied sewage and solid matter mixture, which runs to a comminution and homogenizing means which is connected to the lower portion of said treatment container, the latter being provided with a sand contact bed.

7. Apparatus according to claim 6 in which a perforated or porous floor is arranged in the lower portion of said treatment container, above which floor the bacteria sand bed is disposed and below which an inletforthe sewage from said heat exchanger is positioned.

8. Apparatus according to claim 6 or claim 7 in which the comminuting and homogenizing means is a gear pump.

9. Apparatus according to claim 6, claim 7 or claim 8 in which a bypass pipe runs from the comminuting and homogenizing means to the upper part of the heat exchanger.

10. Apparatus according to claim 7 in which a flow heater is connected into the bypass pipe.

11. Apparatus according to any one of claims 6 to 10 in which a bypass pipe runs from the upper part of the treatment container into its lower part, said bypass pipe enabling the contents of said treatment container to be circulated.

12. Apparatus according to any one of claims 6 to 11 in which a disk centrifuge is arranged in the upper part of the treatment container to separate the putrefied sewage and solid matter from the sewage gas.

13. Process according to any one of claims 1 to 5 substantially herein described and as illustrated with reference to the accompanying drawing.

14. Apparatus according to any one of claim 6 to 12 substantially as herein described and as illustrated with reference to the accompanying drawing.