CA1085520A

CA1085520A - Handling of waste material

Info

Publication number: CA1085520A
Application number: CA270,602A
Authority: CA
Inventors: Victor Lawson
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-01-28
Filing date: 1977-01-27
Publication date: 1980-09-09
Also published as: AU516110B2; DE2703571A1; GB1571886A; DE2703571C2; FR2339434A1; BR7700496A; FR2339434B1; JPS52107175A; BE850811A; AU2141177A; JPS60100099U; IT1075062B

Abstract

HANDLING OF WASTE MATERIAL

ABSTRACT OF THE DISCLOSURE

Refuse, such as domestic refuse, containing inorganic and organic material is treated to separate off the inorganic material, after which the organic material is assimilated in a digestor by a microorganism to produce a combustible gas, and a dilute slurry emerging from the digestor is concentrated to recover a concentrated slurry or cake of mainly organic content, which can be utilised.

Description

~085520 BACKGROUND OF THE INVENTION
This invention relates to a process for handling waste materials.
At present much domestic refuse is collected and taken a considerable distance from the point of collection to an area where it is used as a landfilling material.
These filled areas, after allowing for settlement and after providing a layer of topsoil, can be used as playing fields or parkland.
Currently landfill sites in many areas are bocoming filled, and new sites difficult to obtain. The generally used alternative to landfilling is incineration which is becoming expensive owing to high capital cost, involves compliance with many stringent environmental requirements and lacks the opportunity for recycling of useful materials. With increasing transportation costs, it is desirable to minimize the quantity of material which needs to be transported. Also it is desirable to remove certain constituent~ of the waste material before disposing of the waste material.
SUMMARY OF T~E INVENTION
According to the present invention there is provided a process for the production of a concentrated slurry suitable for use in the production of a self-binding product in the form of paper, board or fuel, withthe contemporaneous consumption of domestic refuse con-taining both organic and inorganic materials and of a sewage sludge, which process comprises:-diluting the domestic refuse with water and

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. ~ . , -breaking up the refuse;
separating off from the diluted, broken-up refuse, the larger pieces or refuse containing the majority by weight of the inorganic material originally present in the refuse;
passing the diluted, broken-up refuse from which the larger pieces have been separated and which contains the majority by weight of the organic material originally present in the refuse, to a digestor containing micro-organisms capable of assimilating the organic materialpresent, and maintaining the digestor at a temperature effective for producing a combustible hydrocarbon gas;
passing to the digestor a sewage sludge;
allowing the microorganisms to feed on the sewage sludge and on said diluted, broken-up refuse from which the larger pieces have been separated and to produce a combustible hydrocarbon gas, the contents of the digestor being agitated to produce a uniform dilute slurry;
burning part of the resulting combustible hydro-carbon gas to provide heat required to maintain thecontents of the digestor at the required operating temperature;
withdrawing a dilute slurry from the digestor;
dewatering the dilute slurry withdrawn from the digestor to produce both a concentrated slurry and a more dilute slurry;
withdrawing said concentrated slurry from the dewatering stage; and recycling at least part of said more dilute slurry ~ _3_ '; ,~.
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to the digestor.
The term "inorganic material" appearing in this specification is intended to cover all inorganic material present unless otherwise stated, but is not intended to include any water present.
Depending on the organic materials present in the digestor and on the choice of microorganisms, different combustible gases can be produced by the microorganisms.
Whilst propane and ~utane could be produced, it is particularly convenient to produce methane.
Frequently the inorganic material in the refuse will contain magnetisable metals, hereinafter referred to as "ferrous metals", as well as non-magnetisable metals -3a-., . `,, ~ : , ~
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hereinafter referred to as "non-ferrous metals". It is to be understood that the "ferrous metals" can include magnetisable metals other than iron.
The majority by weight of the inorganic material separated off from the diluted, broken-up refuse can be subjected to magnetic separation, to separate off ferrous metals present, which can be baled or shredded as desired.
If there is any plastic material originally present in the refuse, the majority may be separated off from the diluted, broken-up refuse with the majority of the inorganic material. Much of this plastics material can be separated from the inorganic material in an air classifier : which uses a blower. The recovered plastics material can then be baled or granulated, as desired.
The separated inorganic material, from which the ferrous metals and plastics material have been separated, can '; be collected and used as a landfill material. This landfill material can weigh approximately only one quarter of the original refuse, which can ensure a considerable saving in the transportation costs.
As will be explained hereinbelow in much greater detail, it may be preferred for economic reasons to have , part of the plant for carrying out the process operating continuously and to have another part of the plant operating for only a portion of each day. In this case it might be necessary to have a buffer tank disposed upstream of the digestor, so that the portion of the refuse being passed to ,`";'~ ., ' ' ' '' ' . ~ ' :., the digestor can collect in the tank during the afore-mentioned portion of the day and can be fed continuously to the digestor on a 24-hour per day basis.
Preferably the diluted, broken-up refuse passed to the digestor has a solids content in the range from 8 to 15 percent by weight.
If desired the raw sewage sludge can be fed to the buffer tank, which is kept stirred continuously, and the sludge in the resulting aqueous suspension forwarded to the digestor. None or some of the organic solids originally present in the sewage sludge may be assimilated by the microorganisms in the digestor. The sewage sludge can serve as a source of fresh microorganisms. The dilute slurry from the digestor is centrifuged to produce a con-centrated slurry which might have a solids content of,for example, 40% by weight. This concentrated slurry can be used as a raw material in the production of paper or board, as a fuel, as a basis for animal feedstuff sub~ect to the removal of any dangerous constituents such as glass, metal or plastics material in a sorting step prior to digestion, or as a soil conditioner or peat substitute.
The sorting step referred to above can be effected in a flotation apparatus, for example, a froth flotation tank, to remove glass and metals. The separated materials can then be passed to the stream of mainly inorganic material destined for landfilling purposes.
If desired, the centrate (i.e. the supernatant liquid remaining after the centrifuging) can be sent to a tank provided - - .
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': :, 1~85520 with a stirrer, and part of the centrate recycled to the aforementioned buffer tank. The remainder of the centrate can be passed to a treatment plant and part of the resulting treated water can be recycled to be used as the water required to dilute the original refuse. Other parts of the resulting treated water can be discharged to the sewer.
The gaseous product of the digestor can include carbon dioxide in addition to methane or other combustible gas and this product can be treated to absorb and hence remove the carbon dioxide and to dry the remaining combustible gas which can then be pumped to a storage vessel and discharged from there for industrial use and for providing heat to maintain the content of the digestor at the desired operating temperature .
DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention and to show how the same may be carried into effect, reference will no-Y be made, by way of example, to the accompanying drawings in which:
Figure 1 shows in outline form a plant suitable for carrying out the process of the present invention;
and F1gure 2 shows in outline form a different plant for carrying out the process of the present invention.
In Figure 1 of the drawing there is shown a conveyor ; belt 1 for conveying refuse to a separating device 2 such as a . .
wet pulping device for reducing the size of the solid materials ' :~

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108~520 present and for separating much of the solid material present from other material in the refuse. A pipe 3 is provided for conveying water to the device. The solid materials separated in the device 2 are conveyed by conveyor ~ to a magnetic separator 5 and any magnetisable material separated by the sepærator 5 is sent to a metal baler or shredder 6.
The solid material from which the magnetisable material has been separated is transferred to an air classifier in the form of a blower 7 for separating off the plastics materials which are transferred to a baler or granulator 8. The remaining material is collected in a skip 9 and can be used as land fill.
The other material in the device 2, i.e. that not separated off towards the conveyor 4, is fed in a pipe 10 to a larger buffer tank 11 provided with a stirrer 12.
Leading to the tank 11 are five pipes, namely a pipe 13 for feeding a raw sewage sludge to the tank 11, a pipe 14 for feeding water to the tank 11 during start-up, a pipe 15 used sometimes for feeding organic waste stream and liquid in proportion to the feed in pipe 10 to the tank 11, a pipe 16 for feeding pH control agents to the tank 11 and a pipe 17 for re-cycling centrate (described in detail below) to the tank 11.
A pipe 18 conveys slurry from tank 11 to a digestor 19 which is provided with a stirrer 20. In the digestor 19, microorganisms feed on the organic materials in the slurry and .. . . . .
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- :~ ,, . . ',, , 1~85520 produce carbon dioxide and methane. These two gases are conveyed in a pipe 21 to a gas cleaning and drying station 22 where the carbon dioxide is removed by absorption and the remaining methane is purified and dried, any solid material being collected and discharged through a pipe 22A as a sludge which can be used as landfill.
The purified methane is pumped by a pump 23 to a storage vessel 24 from which part can be withdrawn through a pipe 25 for external use and from which part can be conveyed through a pipe 26 to a burner 27 used to generate heat required to maintain the slurry in the digestor 19 at the optimum temperature.
Slurry in the digestor 19 is withdrawn and transferred by a pipe 28 to centrifuge 29 for concentrating the slurry.
~5 The concentrated slurry is discharged through a pipe 30 and can, possibly after drying, be used as a source of organic material.
The centrate from the centrifuge29 is conveyed in a pipe 31 to a centrate tank 32 provided with a stirrer 33.
Part of the centrate is recycled via the aforementioned pipe 17 to the buffer tank 11 whereas the remainder is transferred via a pipe 34 to a treatment station 35. In the treatment station 35, the water is treated appropriately so that some may be recycled via a pipe 36 to a tank 37 and to device 2, some discharg~ to the public sewer through a pipe 38 and some discharged as slurry through pipe 39 for landfill purposes.
If desired, the material being sent in pipe 10 from separator 2 to tank 11, can be sent via a pipe 41 to a froth ~, I

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flotation tank 42 to separate off glass and metals which can be transferred by conveyor 43 to blower 7, whilst the rest of the material is returned to pipe 10 via a pipe 44.
Part of the plant described above requires a work force and supervision and it could be arranged for this part to be operated eight hours per day, seven days a week. The other part of the plant could be arranged to operate autom-atically 24 hours per day, seven days a week, as it requires only minimal supervision and no labour force. In fact, those components numbered 1 to 10 and 13 to 17 could be operated only 8 hours per day, whereas the remaining components could be operated 24 hours per day. Thus, apart from the tank 11 and stirrer 12, all components to the left of the broken line 40 in the drawing operate only 8 hours per day, and, apart from fluid flow through pipe 17, all components to the right of the broken line 40 in the drawing operate 24 hours per day.
Here, it will be appreciated that the microorganisms in the digestor 19 cannot satisfactorily be regulated to operate only an 8-hour day; in any case, it is more efficient to have this part of the plant operating all the time. Similarly, from the point of view of payment of the labour force, it is most efficient to have the part of the plant requiring a labour force operating for only 8 hours per day.
Referring now to Figure 2 of the accompanying drawings, the components numbered 1 to 10 and 19 to 44 therein are identical to the corresponding numbered components shown in Figure 1. In Figure 2, however, the pipe 10 (instead of leading , _9_ .. , ,: : , . .- - , , , . , , . - , - : . ~
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1~855ZO

to a buffer tank, as i.n Figure 1) leads to a hopper 51 from the bottom of which a pipe 58 conveys waste to the digestor 19. Also leading to the digestor 19 are five other pipes, namely a pipe 53 for feeding raw sewage sludge, a pipe 54 for feeding raw sewage sludge during start-up, a pipe 55 used sometir.~es for feeding organic sludge and other liquid waste . or sludges as required, a pipe 56 for feeding pH control. agents, and a pipe 57 for recycling centrate. It can be seen that the pipes 53 to 57 correspond in function approximately 10 to pipes 13 to 17 of Figure 1. In the plant of Figure 2 the hopper 51 replaces the buffer tank 11 of Figure 1, and the pipes 53, 54, 55, 56 and 57 are now to the right of the broken line 40.
. The present invention will now be illustrated by the following Example, in which references to tons are to Imperial tons, and references to gallons are to Imperial gallons.
EXAMPLE
The process described in this Example was carried out in a plant as illustrated in Figure 1 of the drawing, with components 1 to 10 and 13 to 17 being in use 8 hours per day and the remaining components being in use 24 hours per : day. The plant was operated 7 days a week for 50 weeks per year, and the annual tonnages given below are calculated on 2S this basis.
Domestic refuse was fed to the conveyor 1 at a rate of 8 tons per hour, equivalent to 64 tons per day and to 22~00 :,.
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~0855Z0 tons per annum (with 7 day week and a 50-week year).
The composition of this domestic refuse fed to conveyor 1 was, in tons per hour, as follows:-ferrous metals 0.256 non-ferrous metals 0.256 glass 0.640 other inorganic materials 1.344 plastics materials 0.192 other organic materials 3.712 water (inherent) 1.600 total 8.000 Water was fed at a rate of 3.6 tons per hour to the device 2 via pipe 3, this being equivalent to a rate of 10080 tons per annum. Thus the total quantity of . water fed to the device 2 via conveyor 1 and pipe 3 was 5.200 tons per hour.
Most of the inorganic and plastics materials fed to the device 2 were separated off and fed to the :. 20 conveyor 4; the remainder, which was fed through the pipe 10 : to tank 11, had the following composition, expressed in tons per hour:-, ferrous metal 0.016 ; non-ferrous metal 0.016 glass 0.040 '- ~ other inorganic ~ materials 0.640 .

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plastics materials 0.032 other organic materials 3.600 water 4.800 total 9.144 The materials fed to the conveyor 4 passed the magnetic separator 5 which separated off the majority of the ferrous material and forwarded it to a metal baler or : shredder 6 which operated at the rate of 0.224 ton per hour, equivalent to 627.2 tons per annum.
The remainder of the material fed to the conveyor 4 was transferred to the air classifier which included the blower 7, which was responsible for separating off the ; majority of the plastics materials present. The latter were conveyed to a baler or granulator 8 which operated at the rate of 0.12 ton per hour, equivalent to 336 tons per annum.
The residue of the material, i.e. that not separated off by blower 7, was transferred to a skip 9 at the rate of 2.112 tons per hour, equivalent to 5913 tons per annum and corresponding to 26.39% by weight of the refuse starting material. The composition of this residue in skip 9 was, on a ton per hour basis, as follows:-ferrous metals 0.016 non-ferrous metals 0.240 l glass 0.600 "~ 25 other inorganic materials 0.704 plastics materials 0.040 -~ . . . ..
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other organic materials 0.112 water 0.400 total 2.112 This residue was suitable for direct use as landfill material.
The tank 11, in which the stirrer 12 operates 24 hours per day, has a capacity of 200 000 gallons and was capable of supplying the digestor 19 for 20 hours without addition of material through pipes 10, 13, 14, 15, 16 and 17, it being appreciated that these six pipes are out of operation 16 hours per day.
During the more active 8-hours part of the day, - the tank 11 received, in addition to the material in pipe 10 (specified above), 7.3922 tons per hour of a 3% solids raw sewage sludge via pipe 13, water via pipe 14 (but only during start-up), p~ control agents via pipe 16 and centrate , recycled at the rate of 28.27 tons per hour for 8 hours per day via pipe 17. It was also possible to add via pipe 15 an organic sludge and liquid comparable in those in the ~ 20 refuse, but none was added in this Example.
r From tank 11 to digestor 19 was fed via pipe 18 for 24 hours per day a mixture which had the following composition, ; expressed as tons per hour:-' ferrous metal 0.0053 non-ferrous metal 0.0053 glass 0.0133 other inorganic ` materials 0.2133 ~' ~ ,. ..
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plastics materials 0.0107 sewage solids0.0739 other organic materials 1.2000 water 13.3853 total 14.9071 The digestor 19 was in the form of two chambers each having a capacity of 65000cubic feet. The stirrer 20 operated 24 hours per day. For efficient operation the digestor 19 required a heat input of 1498670 British thermal units (BTU) per hour, which heat was supplied by the burner 27.
From the digestor 19 were led off in pipe 21 carbon dioxide at the rate of 8895 cubic feet per hour and methane at the rate of 8824 cubic feet per hour. The carbon dioxide was separated off in the station 22 and other impurities were separated off and discharged through a pipe 22A as sludge for use as a landfilling material. The purifled methane was pumped by pump 23 at the rate of 8824 cubic feet ... ..
per hour to the storage vessel 24 which thus received 211776 cubic feet of methane per 24 hour day. This fuel intake is equivalent to 8.82 x 10 BTU per hour, or 88.24 therms per hour, corresponding to 741216 therms per annum. The majority (approximately seven-ninths) of the methane was discharged through pipe 25 for external use (equivalent to 68.24 therms per hour, or 573216 therms per annum), and 2000 cubic feet per hour of methane was fed via pipe 26 to the burner 27, which was capable of providing 2 x 106 BTU per hour, where the methane ; -14-, - ' ., . ,, : ' , -, :.

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10855~0 was burned to provide approximately 1.5 x 106 BTU per hour used to heat the contents of the digestor 19.
From a bottom region of the digestor 19 were led off a dilute slurry which had a composition (expressed as tons per hour) identical to the mixture in pipe 18 except that the content of "other organic materials" had fallen from 1.2000 to 0.6047. This slurry was transferred via pipe 28 to a centrifuge 29 which operated at approximately 54 gallons per minute. The centrifuge 29 produced a 40% slurry which had the following composition expressed as tons per hour:-ferrous metals 0.0053 non-ferrous metals 0.0053 glass 0.0133 other inorganic materials 0.1600 plastics materials 0.0107 sewage solids 0.0739 other organic materials 0.5879 water 1.2846 total 2.1410 This total 40% slurry in pipe 30 was produced in an amount of 51 3840 tons per 24-hour day (calculated in the wet state), the latter being equivalent to 17984 tons per annum. This slurry could be used in the production of paper or board, or as a fuel, or as a basis for an animal feedstuff after removal of the metals, glass and plastics materials.

':; ' ' ' ' ' 1~85520 The centrate from the centrifuge 29 was conveyed in a pipe 31 at a rate of 12.1708 tons per hour, over a 24-hour ~ay, to the tank 32. This centrate had the following composition, expressed as tons per hour:-inorganic material 0.0533 organic material 0.0168 water 12.1007 total 12.1708 The stirrer 33 in the tank 32 was operated continuously and the tank 32 had a capacity of 55000 gallons and was thus able to receive centrate from pipe - 31 for up to 20 hours. ~1uch of the centrate in tank 32 -; was recycled via pipe 17 to buffer tank 11 at-the rate ~' of 28.1848 tons per hour for 8 hours per day; the remainder was sent via pipe 34 to the treatment station 35 at the rate of 2.7758 tons per hour over a 24-hour day. From the station 35 water was recycled at a rate of 1.2 tons per hour over a 24-hour day to tank 37 via pipe 36 and from there to pipe 3 at a rate of 3.6 tons per hour over an 8-hour day. Also from the station 35, 1.5578 tons per hour of waste material was discharged via pipe 38 to the sewer.
In summary, on an annual basis over a 50-week year, there were fed to the plant 22400 tons of refuse, 10080 tons of water and 20700 tons of 3% sewage sludge, and there were obtained from the plant 627 tons of ferrous ~ materials, 336 tons of plastics materials, 5913 tons of ,, ! -- I 6-l.
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landfill material, 17984 tons of slurry, 23316 tons of waste water (as measured in pipe 34) and 57321600 cubic feet of methane (equivalent to 573216 therms per annum), as well as some carbon dioxide and small amounts of additional waste material.
Of the 23316 tons of waste water, 10080 tons were recycled to the device 2 and the remainder drained off.
When the slurry in pipe 30 is intended to be used as a basis for an animal feedstuff, it is most practicable prior to the digestion to remove the metals, glass and plastics materials from the material being trans-ferred from the device 2 to the tank 11, for example by use - of the froth flotation tank 42. It will be appreciated that in the foregoing example, the process was carried out without employing such a tank 42.
The plant can be operated efficiently with the contents of the digestor 19 at a temperature in the range from 35C to 65C, and in the case of the foregoing Example the temperature of the contents of the digestor was 60C.
In the foregoing Example one of the microorganisms present in the digestor 19 was Methanobacillus sp Omelianski.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the production of a concentrated slurry suitable for use in the production of a self-binding product in the form of paper, board or fuel, with the contemporaneous consumption of domestic refuse con-taining both organic and inorganic materials and of a sewage sludge, which process comprises:-diluting the domestic refuse with water and breaking up the refuse;
separating off from the diluted, broken-up refuse, the larger pieces of refuse containing the majority by weight of the inorganic material originally present in the refuse;
passing the diluted, broken-up refuse from which the larger pieces have been separated and which contains the majority by weight of the organic material originally present in the refuse, to a digestor containing micro-organisms capable of assimilating the organic material present, and maintaining the digestor at a temperature effective for producing a combustible hydrocarbon gas;
passing to the digestor a sewage sludge;
allowing the microorganisms to feed on the sewage sludge and on said diluted, broken-up refuse from which the larger pieces have been separated and to produce a combustible hydrocarbon gas, the contents of the digestor being agitated to produce a uniform dilute slurry;
burning part of the resulting combustible hydro-carbon gas to provide heat required to maintain the contents of the digestor at the required operating temperature;
withdrawing a dilute slurry from the digestor;
dewatering the dilute slurry withdrawn from the digestor to produce both a concentrated slurry and a more dilute slurry;
withdrawing said concentrated slurry from the dewatering stage; and recycling at least part of said more dilute slurry to the digestor.

2. A process according to claim 1, wherein the dewatering of the dilute slurry is effected by centrifuging.

3. A process according to claim 1, wherein the inorganic material contains a magnetisable metal separated by magnetic separation.

4. A process according to claim 3, wherein the separated magnetisable material is baled or shredded.

5. A process according to claim 1, wherein the refuse contains a plastics material which is separated off by air classification.

6. A process according to claim 1, wherein the process is effected in a plant of which part generally operates for only a portion of each day, and another part generally operates continuously.

7. A process according to claim 1, wherein the com-bustible gas produced by the microorganisms is selected from the group consisting of methane, propane and butane.

8. A process according to claim 1, wherein the diluted, broken-up refuse passed to the digestor has a solids content in the range from 8 to 15 percent by weight.