GB2204056A

GB2204056A - Digestion chamber

Info

Publication number: GB2204056A
Application number: GB8808938A
Authority: GB
Inventors: Stig Arvid Henrikson
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-05-01
Filing date: 1988-04-15
Publication date: 1988-11-02
Also published as: GB8808938D0; ES2006924A6; DE3814442A1; GB8710438D0

Abstract

A digestion chamber for use in the anaerobic degradation of a liquid mixture of finely divided refuse and a liquid to form digestion gas. The chamber 9 is closed and generally rectangular and has a feed screw 10 extending longitudinally along its bottom from an inlet and to an outlet end. The chamber has a gas dome 14 in its roof in which digestion gas formed in the chamber can collect. This gas can be pumped back into the chamber to agitate the mixture in the chamber. <IMAGE>

Description

DIGESTION CHAMBER.

The present invention relates to the digestion of refuse. In the digestion of refuse, refuse is ground to small particles in a hammer mill or the like and mixed with a liquid to form a liquid mixture which in a heated closed chamber is subjected to treatment such that anaerobic degradation or digestion takes place, with the formation of methane gas which is recovered for producing energy.

Refuse and sludge management in large urban areas is today one of the major environmental problems with which the present invention is concerned.

According to the present invention there is provided a digestion chamber for the anaerobic degradation of a liquid mixture of finely divided refuse and a liquid, for the formation of digestion gas, comprising a generally rectangular closed chamber having at or toward its bottom a feed screw extending generally longitudinally of the chamber and arranged to transport the mixture lying at the chamber bottom from an inlet end of the chamber to its outlet end, said chamber having at its upper side at least one gas dome for collecting the digestion gas formed in the chamber, which gas is at least partially fed back to the chamber by pumping means for agitation of the mixture therein.

The digestion chamber can be used in plant in which liquid with which the refuse particles are mixed is water. The liquid can be sludge from a sewage treatment plant. The sludge may suitably consist of excess sludge from a sewage treatment plant serving a community or the like, the sludge being mixed with the municipal waste from the same community.

In using the present apparatus the initial amount of refuse and sludge can be reduced to about 1/3 by weight while a still larger reduction by volume is obtained. The residue can easily be removed and deposited on a dump.

The invention will now be described by way of example only, with particular reference to the accompanying drawings. In the drawings: Figure 1 shows a refuse digestion plant from one side, partly in cross-section, Figure 2 shows in section a digestion chamber and mixing trough used in the plant, Figure 3 is a plan view of the same parts partly in section, Figure 4 is a cross-section of the digestion chamber taken along the line IV-IV in Figure 2, and Figure 5 shows on a larger scale and in cross-section parts of the digestion chamber as seen from one side.

As appears from Figure 1, refuse is tipped from a vehicle into a hammer mill 1 where the refuse -is ground into small particles whose size is of the order of about 1 cm. From the mill 1, the refuse is moved by means of a conveyor device to a basin 2 in which the refuse is deposited. A conduit 3 for sewage sludge from a sewage treatment plant 4 discharges into the basin 2. If the plant is not associated with a sewage treatment plant, the conduit 3 may be used for conducting water.

The admixture of the small refuse particles with the sludge is conducted with maximum efficiency.

Thus, mixing is carried out in a horizontal mixing trough 5 by means of a mixing device 6 which is mounted at the bottom of the trough 5 in the longitudinal direction thereof. The mixing device mixes refuse particles and sludge in such proportions as to produce a mixture of viscous consistency. The mixing device 6 is for instance a twin-shaft mixer extending in the longitudinal direction of the trough 5.

The ratio of the amount of refuse to the amount of sludge should be such that the dry solids content of the mixture is in the order of 5-10%, giving a mixture of the desired viscous consistency. The sludge surface in the trough 5 is maintained a certain distance above the upper parts of the twin-shaft mixer so as to minimise the amount of air introduced in the mixture when the refuse and sludge are slowly mixed by means of the twin-shaft mixer 6. The length of the mixing trough 5 may vary within wide limits, but typically is 5-10 m.

The mixing trough 5 opens at its outlet end into a pumping station 7 provided with an agitator. The mixture of refuse and sludge is withdrawn from the pumping station 7 downwards from a central part of the bottom thereof, whereupon a pump 8 supplies the mixture to a closed digestion chamber 9 in which it is introduced at the bottom of the inlet end. More exactly, the chamber 9 is horizontal with a rectangular plane shape.

The closed rectangular chambers 9 (Figure 4) in which the mixture of refuse and sludge is treated under anaerobic conditions, i.e. without the supply of oxygen and under heating, are provided at their bottoms with a longitudinal feed screw 10 conveying the mixture, having a thicker consistency towards the bottom, in the direction of the outlet end of the chamber. The feed screws 10 are, thus, arranged to slowly convey the contents of the digestion chamber from the inlet to the outlet side, where another feed screw hauls the digested material out of the digestion chamber 9. This arrangement makes it possible to put out smaller particles of metal, glass and the like from the digestion chamber 9 which in conventional constructions are difficult to put out. The provision of storing central feed screws 10 makes the plant robust so that it can handle substantially any kind of refuse.

As appears best from Figure 4 the horizontal digestion chamber 9 with three vertical screening walls is divided into four equally large chambers.

The screening walls being designed as baffles, the four chambers are in total hydraulic connection with each other. Thus, the plan is not to be understood as four separate chambers, but as one single chamber.

The bottom and top portions of the rectangular chambers 9 are designed, as seen in cross-section, with upper sloping walls forming arches 11. Provided along one long side of the chambers 9 are pipes 12 which are designed at their lower ends as air-lift pumps the injection nozzles 13 of which are connected to low-pressure compressors or the like on the delivery side thereof. The suction side of the compressors is connected to a gas dome 14 mounted in the roof of the chamber 9. Thus, the compressors draw digestion gas from the gas dome 14 and pump the gas down to the nozzles of the above-mentioned air-lift pumps in the liquid mass, thus causing mixing of the liquid mass and activation of the formation of digestion gas. Thus no movable parts in the chamber 9 are required therefor.

The gas dome 14 is located on the roof of the chamber 9 nearer the inlet end thereof than the outlet end. The opening of the chamber 9 is round, having a diameter of the order of 2 m. Optionally, the chamber can have two or more gas domes 14.

Due to the action of an overflow 16, the sludge surface 15 will be located in the circular opening just below the gas dome 14, where liquid sludge will accumulate. In order to prevent an embarrassing layer of liquid sludge from building up, a funnel-shaped device 17 is provided just below the sludge surface.

the device 17 is connected at its bottom to a vertical pipe 18 descending to the lower zone of the chamber 9.

The lower end of the pipe is connected to a U-shaped pipe 19 the free end of which is directed upwardly.

On the upwardly directed end there is mounted a nozzle 20 for injecting digestion gas. The upwardly directed end then services as an air-lift pump, sucking off the liquid sludge through the funnel-shaped device 17.

The digestion gas to the air-lift pump is drawn from the same compressor that cooperates with the agitator described above.

In order to obtain a maximum amount of methane gas, the liquid mixture in the chamber 9 should be alkaline, i.e. have a pH above 7. This is achieved by mixing suitably finely divided lime, CaO or Ca(OH)2, or soda lye, Na(OH)2, into the refuse and sludge liquid. This is carried out in the elongate trough 5 by means of the twin-shaft mixer 6. pH sensors are mounted in the chamber 9 so as to allow automatic adjustment of the pH value in the chamber 9.

Iron chloride can be added to the liquid in the mixing trough 5 in order to ensure more extensive dewatering of the sludge mixture in the chamber 9. In fact, the iron chloride brings about flocculation of sludge particles.

At the outlet end of the mixing trough 5, liquid sludge is scraped off from the mixture which, as described above, is thereafter pumped to the closed chamber 9. The liquid sludge is scraped off by means of a scraper device 21.

In the process of degradation in the chambers 9, water is formed which is withdrawn from the chambers 9 via the overflow which, as illustrated, comprises three pipes 23 descending vertically into the chambers 9 and having open lower ends which are located on different levels. Alternatively, the three pipes may be replaced by a telescopic tube the lower end of which can be adjusted to the water contents of the chambers 9. The water discharged through the overflow is then recycled through conduits 24 to the sewage treatment plant for purification and treatment together with waste water therein. All the water or part of it may instead be recycled through the conduit 25 to the mixing trough 5. If the plant is not used in combination with a sewage treatment plant, the water passing the overflow should normally be recycled to the mixing trough 5.

It is important that the pipes 12 with their nozzles 13 for injecting digestion gas for mixing the refuse and sludge mass are so positioned that there is obtained an agitation-free zone where the overflow 16 with its descending pipes 23 is located. In this manner, it is ensured that sludge liquor can be discharged through the overflow.

In the treatment of sludge and refuse in the digestion or degradation chambers 9, anaerobic degradation of the organic material takes place, the digestion gas being emitted and the refuse and sludge mass losing the major part of its organic contents.

The degraded sludge is discharged at the outlet end 26 and conveyed up to a level 27 outside the chambers 9 that is located above the sludge surface 15 in the gas dome 14. The upward conveyance of the sludge is carried out by means of a feed screw 28 and a certain dewatering of the degraded sludge product is here performed. The residue, the degraded refuse and sludge mass, is collected on a water-draining conveyor belt 29.

In a correctly operating plant, the residual sludge is odourless and can be removed, e.g. in containers and used as filling material or be deposited on a dump.

The digestion gas is purified and the thus purified methane gas is supplied to a gasometer. The methane gas suitably powers gas producers, thus generating electric energy. The cooling water from the gas motor is preferably used, via a heat exchanger, to heat the contents of the digestion chamber to a suitable temperature.

Since the production of methane gas from the joint digestion of refuse and sludge is very large as compared with the production of gas when digesting sludge alone, being in the order of 5-10 times as large, the energy produced in the joint digestion process will entail annual proceeds considerably exceeding the annual costs in respect of installation, depreciation, interest, maintenance and operation.

EXAMPLE OF CALCULATION It is assumed that the refuse and sludge degradation plant is served by a sewage treatment plant, with complete biological purification, for 100,000 people. According to the publication "Kommunf6rbundet, Slambehandling" ("Association of Local Authorities, Sludge Treatement"), p. 26, the gas production is 30 l/p.eq. ~ d, i.e.

a daily amount of digestion gas of 100,000 ~ 0.03 3,000 m3/d Assuming that the refuse contains 0.8 kg organic matter per person and day, then there will be produced, according to the publication "Vattenreningsteknik" ("Water Treatment Technology") p. 72, an amount of 0.6 m3 of digestion gas per kg organic matter, i.e.

a daily amount of digestion gas of 100,000 ~ 0.8 ~ 0.6 48,000 m3/d 3 Total amount of digestion gas 51,000 m According to this publication "Vattenreningsteknik), it is possible to recover per m3 of digestion gas 1.8 kWh as electric energy and 9 MJ as heat. (For heating purposes, it is stated that if the heat is used for heating the reactor contents and it is assumed that the minimum temperature is sufficiently high (300C), it is possible to heat 109.63.106 = #228 m3/h water with a temperature 20-249103 increase of 20 C).

Possible electric energy 1.8 ~ 51,000 = 91,800 kWh/d i.e. a constant power output of 3,825 kW (-5,200 h.p.) Possible heat energy 9 ~ 51,000 459,000 MJ (109. 63 106kcal)

Claims

CLAIMS: 1. A digestion chamber for the anaerobic degradation of a liquid mixture of finely divided refuse and a liquid, for the formation of digestion gas, comprising a generally rectangular closed chamber having at or toward its bottom a feed screw extending generally longitudinally of the chamber and arranged to transport the mixture lying at the chamber bottom from an inlet end of the chamber to its outlet end, said chamber having at its upper side at least one gas dome for collecting the digestion gas formed in the chamber, which gas is at least partially fed back to the chamber by pumping means for agitation of the mixture therein.
2. A digestion chamber as claimed in claim 1, wherein the liquid mixture is sludge.
3. A digestion chamber as claimed in claim 1 or claim 2, wherein said pumping means comprise air-lift pumps.
4. A digestion chamber as claimed in claim 3, wherein the air-lift pumps comprises substantially vertical pipes provided along one side of the chamber, said pipes extending from the lower part of the chamber to the upper part thereof.
5. A digestion chamber substantially as hereinbefore described with reference to the accompanying drawings.