GB2051769A

GB2051769A - Method for the aerobic digestion of sewage sludge

Info

Publication number: GB2051769A
Application number: GB8015396A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1979-05-11
Filing date: 1980-05-09
Publication date: 1981-01-21

Abstract

Sewage sludge is treated by continuously passing the sludge along an inlet into a sludge digestion zone; introducing oxygen or an oxygen-containing gas into the sludge in the digestion zone; aerobically digesting the sludge in the digestion zone at an elevated temperature which favours the growth of thermophilic bacteria; continuously passing at least partially digested sludge out of the digestion zone; maintaining sludge of different ages in the digestion zone, and heat-exchanging sludge passing along the inlet with at least partially digested, relatively warm sludge.

Description

SPECIFICATION Treatment of waste material This invention relates to treatment of waste material and in particular to treatment of sewage sludge or like material having a rancidly high biochemical oxygen demand.

It is known to circulate sludge from the primary or secondary sedimentation tanks of sewage plants through a further "digester" tank in which sludge is digested by bacterial action. What results is a liquid component which is recycled to the sewage plant, and a solids component of reduced bulk. It is preferred that the sediment should be in suitable condition and safe to discharge onto the land.

In conventional anaerobic digestion, the digested sludge will still contain pathogenic bacteria which can cause diseases in cattle and human beings (eg brucellosis and salmonella) even if treatment times as long as thirty days are employed. In order to reduce safely such treatment times, for example to about five days, it has been proposed that digestion should take place aerobically by dissolving air or oxygen in the sludge. The resultant biochemical reactions that take place are exothermic. As a result, the temperature of the sludge rises to about 50-60 degrees C. At such high operating temperatures the pathogenic bacteria may be reduced in number or totally eliminated. One disadvantage of this method is, however, that it is a batch method.There is thus an inherent risk that some of the sludge will pass out of the digestion tank after only having spent a relatively short time therein with the consequence that the effluent may well contain pathogenic bacteria.

In order to overcome this last mentioned disadvantage, it is proposed in our co-pending application 12946/76 Serial No. 1581432 that waste material such as sewage sludge should be digested by passing a stream of material along a path extending through a treatment zone, introducing an oxygen containing gas into the waste material and bringing a relatively hot, at least partially digested material in the stream into heat exchange relationship with relatively cold and relatively undigested material in the stream. An advantage of this method is that heat produced by the exothermic digestion process and retained in the outgoing digested material is used to preheat an incoming stream of relatively undigested waste material.Moreover, not only is the method capable of being operated continuously rather than as a batch process, but also the incoming material is raised more quickly to a temperature at which the process of digestion is promoted. Furthermore the continuous "plug" flow conditions that are made possible by the method will ensure equal treatment time to all the sludge, thereby avoiding the risk of discharging relatively untreated sludge.

The present invention relates to an improvement.

in the method described in our aforementioned co-pending application.

According to the present invention, there is provided a method of aerobically digesting sewage sludge (or like material) comprising the steps of: (a) continuously passing sludge along an inlet path into a main sludge digestion region; (b) introducing pure oxygen or an oxygencontaining gas mixture into the sludge in the main digestion region so as to maintain aerobic conditions within the sludge; (c) aerobically digesting sludge in the main digestion region at an elevated temperature which favours growth of thermophilic bacteria which take part in the digestion; (d) continuously passing at least partially digested sludge out of the sludge digestion region; (e) maintaining sludge of different ages (as hereinafter defined) in the main digestion region; (f) heat-exchanging sludge passing along the inlet path with at least partially-digested, relatively warm sludge.

The main sludge digestion region may be situated within a tank which communicates with the inlet path and with an outlet path, or may comprise a channel which is contiguous with the inlet path and an outlet path. Some digestion of sludge may take place along the inlet path andlor outlet path.

Preferably, the inlet path is defined by an extended inlet passage in which generally plug flow conditions are sustained. Preferably, the flow rate through the inlet path is such that it takes at least 24 hours for sludge entering the inlet path to reach the main digestion region. Typically, this period may be in the order of from 24 to 36 hours.

The sludge entering the inlet path may be taken from the primary or secondary settling tanks of a municipal sewage treatment plant. Typically, the incoming sludge will have a temperature a degree or two above ambient.

Preferably pure oxygen or oxygen-containing gas mixture is introduced intermittently or continuously into the sludge flowing along the inlet path in such a manner that substantially all the oxygen is taken up by the sludge. This is so as to maintain aerobic conditions in the sludge flowing along the inlet path.

Preferably, pure oxygen or a gas mixture rich in oxygen (eg. containing 80% or more by volume of oxygen) is used as the oxygenating gas. Typically, the apparatus or method according to UK patent specification 1455567 or UK patent specification 1 494731 is employed. If an oxygen-rich gas mixture is employed, its source may be a plant which produces such gas by preferentially absorbing either oxygen or nitrogen from air. If pure oxygen is used, its source may be a cryogenic air separation unit or a container of the gas in liquified state.

In the main sludge digestion region, the elevated temperature is typically sustained by the action of bacteria (in the sludge) respiring oxygen. Thus, it is possible to operate the process such that the chosen temperature in the main sludge digestion region is self-sustaining. No source of heat apart from this biochemical source is required, except, apart from, possibly, at the start of the process.

At ambient temperatures certain strains of aerobic bacteria tend to proliferate at the expense of others which remain passive. Thus, sewage sludge contains only a relatively small population of thermophilic bacteria at ambient temperature. However, at temperatures above 40 degrees C the thermophilic bacteria start to profilerate at the expense of others which remain passive or are killed.

Typically the main sludge digestion region is maintained at a temperature of at least 50 degrees C.

For example, the temperature could be in the range 60 to 80 degrees C.

By the age of any particular particle of sludge is meant the time it has been in the process. In contradistinction to the process described in our copending application 12946/76 Serial No. 1581432 strict plug flow conditions are not maintained throughout the method according to the invention.

Instead, some of the sludge is recycled to or in the sludge digestion region (which may comprise a channel contiguous with the inlet path and an outlet path) or the sludge digestion region may be provided by a tank or like vessel in which some mixing of sludge of different ages inevitably takes place, the tank having associated therewith an inlet conduit (or channel) and an outlet conduit (or channel) in which plug flow conditions are maintained. In the former embodiment, sludge entering the inlet path preferably takes at least five days before reaching the end of the outlet path.In the latter embodiment, the arrangement is preferably such that the average residence time of the sludge in the tank combined with the time ittakes the sludge to flowfrom the start of the inlet path to the tank and to flow along the outlet path from the tank is at least five days.

Typically, in both embodiments, extended outlet paths are provided. Typically, the time it takes the sludge to flow from one end to the other of the outlet path is equal to the time it takes the sludge to flow from one end to the other of the outlet path. Thus, the inlet and outlet paths may be of equal length. In examples of the process where the sludge is recycled, preferably about 60% by volume of the sludge is recycled. By this means, adequate populations of thermophilic bacteria are able to be maintained in the main sludge digestion region. Analogously, an example of the process where the main sludge digestion region is provided by a tank, the average time sludge is held in the tank is preferably greater than the time it takes the sludge to flow both from end to end of the inlet path and from end to end of the outlet path.This again makes it possible to achieve adequate populations of thermophilic bacteria in the main sludge digestion region.

Preferably, pure oxygen or an oxygen-rich gas mixture is introduced into the main sludge digestion region so as to maintain aerobic conditions. The requirements of the sludge for oxygen are considerably in excess of the requirements of, for example, primary and secondary sewage. Typically, the rate of introducing molcular oxygen into the process is from one to two percent by weight of the rate at which sludge passes through the process, depending on the biochemical oxygen demand of the sludge. The oxygen or oxygen-rich gas may be supplied to the sludge in the sludge digestion region either intermittently or continuously. The method of apparatus for introducing the oxygen or oxygen-righ gas to the sludge in the main digestion region may be of the kind used to introduce oxygen or oxygenrich gas into the sludge passing along the inlet path.

If desired, the same source of oxygen or oxygen-rich gas may be used to supply that gas both to the sludge flowing along the inlet path and the sludge in the main sludge digestion region.

If desired, sludge digestion may continue as the sludge flows along the outlet path which will typically be defined by a passage having a cross section narrower than that of the main sludge digestion region.

Preferably, at least part of the inlet path is such that incoming sludge is in heat exchange relation with at least part of the outlet path. Alternatively, or in addition, the sludge flowing along the inlet path may be heat exchanged with sludge in the sludge digestion region. The heat exchange helps to raise the temperature of the incoming sludge and thereby promote the increase in population of thermophilic bacteria in the sludge entering the main sludge digestion region.

If desired, surfaces across which the exchange of heat takes place may be formed of a heat conductive metal such as aluminium or stainless steel. It is an advantage of the method according to the invention that it is not necessary to use highly efficient heat exchangers. Typically, those surfaces across which it is not desired to exchange heat may be made of a heat-insulative material such as brick or breeze block.

The method accordinvg to the invention makes it possible, we believe, to perform a complete sludge digestion in a period which may be as short as five days.

The method according to the invention will now be described by way of example and with reference to the accompanying drawing in which: Figure 7 is a diagrammatic representation of the first apparatus for digesting sewage sludge, and Figure 2 is a diagrammatic representation of the second apparatus for digesting sewage sludge.

With reference to Figure 1 of the drawings, the illustrated apparatus comprises two generally spiral flow channels 2 and 4 for the passage of sludge.

Channels 2 and 4 share a common wall 6 such that sludge entering the channel 2 will flow spirally inwards until it reaches a central location 8 with which the channel 4 also communicates. This sludge will then flow spirally outwards until it reaches the outlet of the channel 4. The channel 2 has a relatively narrow inlet portion 14 in heat exchange relation through common wall 6 with a relatively narrow outlet portion 16 of the channel 4. Downstream of the inlet portion 14 the channel 2 has a portion 15 of wider cross-section which comes to an end at the central region 8. Upstream of the portion 1 6 the channel 4 has a portion 17 of wider cross-section which comes to an end at the central region 8.

Located in the common wall 6 is a passage 18 placing a downstream part of portion 17 in communication with an upstream part thereof. Just downstream of the passage 18 is a baffle 20 extending at least half the way across the channel 4.

The baffle 20 is as tall as the channel 4 and is positioned at an angle so as to deflect sludge from the portion 15 into the portion 17. In addition to, or instead of using the baffle 20, a pump (not shown) may deflect flow of sludge from the portion 16 into the portion 17.

In operation, the portion 14 defines the inlet path, the portions 15 and 17 main sludge digestion region, and the portion 16 the outlet path. The external walls defining the channels 2 and 4 may be of heatinsulated material such as brick or breeze blocks.

The channels have an open top but are provided with a cover so as to keep down evaporative losses of heat generated in the operation of the apparatus.

(The cover is not shown). The portions 14 and 16 are in heat exchange relationship with one another through the common wall 6.

In operation, incoming sludge to be digested enters the process at a temperature 1 or 2 degrees C above ambient temperature, is pumped by means of a suitable sludge pump (not shown) into the inlet portion 14. As it flows along this pipe so the sludge is raised in temperature by heat exchange with sludge leaving the process through the outlet portion 16. In order to maintain aerobic conditions in the sludge flowing along the portion 14, pure oxygen is dissolved therein at a downstream region indicated by the reference numeral 22. As the sludge gradually increases in temperature so conditions favouring an increase in the concentration of thermophilic bacteria begin to be created. It is these bacteria that are necessary for the effective digestion of the sludge.

When the sludge enters the portion 15 it may typically have a temperature close to the temperature that is maintained in the sludge digestion region. The sludge flows through the portion 15 toward the region 8. On its way its demands for oxygen are met by dissolving pure oxygen therein at two or more spaced apart regions indicated by the references 24. As the oxygen is dissolved so it is consumed by the thermophilic bacteria which multiply in number. These bacteria help to reduce the bulk of the sludge. This whole treatment is exothermic.

Thus, a temperature well above ambient is generated. Typically the temperature is in the range of 50 to 80 degrees C, the precise temperature depending on a number of factors including the rate of heat loss from the portions 15 and 17. The sludge reaches the region 8 and then flows back out through the portion 17. Further oxygen is dissolved in the sludge as it flows through the portion 17. This oxygen may be added at a location indicated by the arrows 26.

At least 50 per cent of the flow of sludge immediately upstream of the passage 18 is diverted or recycled to a more upstream part of the portion 17.

This helps to increase the concentration of thermophilic bacteria in the upstream part of the sludge digestion region and thereby helps the sludge digestion. When the sludge leaves the portion 17 it flows into the portion 16 and gives out heat to sludge passing through the portion 14. If necessary, the passage of the sludge through the outlet portion 16 may be assisted by a further sludge pump connected thereto (this pump is not shown). The resulting digested sludge may be separated into a liquid component which can be recycled to a sewage treatment plant and a solids component reduced in bulk. If desired, a solids component can be discharged onto the land.

Typically, the residence time of the sludge in the portion 14 may be arranged to be one and a half days. In addition, the residence time of the sludge in the outlet portion 16 may also be arranged to be one and a half days. The average time spent by sludge in the main sludge digestion region may be a further two days. This means, it is possible to ensure the sludge is kept at sufficiently high temperatures for sufficient time to enable adequate digestion to take place and to inhibit harmful bacteria which could cause diseases if present in significant quantity in the solids or sediment produced by the treatment process.

Typically, it may be arranged that the apparatus shown in Figure 1 of the drawings receives sludge for treatment at the rate of 50 litres per hour and discharges digested sludge at the same rate. It is intended to be operated continuously, ie over periods of weeks or months, the only break in operation being for routine cleaning and maintenance. If desired, there may be two plants running side by side so that one can continue to operate if the other is shut down temporarily.

In the apparatus shown in Figure 2, the sludge digestion region does not comprise spiral channels but instead a cylindrical tank. The tank has an inlet situated alongside an outlet so as to create generally circumferential or spiral flow of liquid therein.

However, because there are no partitions or walls in the tank to define discrete channels, ideal plug flow conditions cannot be created in the tank and there is inevitably some mixing of relatively newly received sludge with that which has been resident in the tank for some considerable period of time. This achieves the same effect as the recycling of the sludge through the passage 18 in operation of the apparatus shown in Figure 1.

Referring to Figure 2, a cylindrical tank 40 which may for example, be formed of stainless steel, has an inlet 42 alongside an outlet 44. An arcuate pipe 46 of extended length terminates in the inlet 42 and a similar acruate pipe 48 of extended length terminates in the outlet 44. The pipes 46 and 48 are of equal length and follow generally the outside wall of the tank 40. The pipes 46 and 48 may be made of heat conductive metal such as aluminium and are in exchange relationship with one another. If desired the non heat-exchanging surfaces of the pipes 44 be 6 and 46 may be lagged.

In operation, pipe 46 defines the inlet path along which sludge flows, the tank 40, the main sludge digestion region and the pipe 48 the outlet path along which the sludge flows.

Incoming sludge from, say, the secondary tanks of a municipal sewage treatment plant is pumped by means of a sludge pump (not shown) into the pipe 46. The sludge entering the pipe 46 typically has temperature of one or two degress C above ambient.

As the sludge flows along the pipe 46, so its temperature is raised by heat exchange with sludge flowing through the pipe 48 from the tank 40.

Typically, by the time the sludge flowing along the pipe 46 has reached the inlet 42 of the tank, its temperature may have been raised to within a degree or two C of the operating temperature of the tank. As the temperature of the sludge rises so does about 40 degrees C so conditions favouring the proliferation of thermophilic bacteria contained in the sludge begin to be created.

It is desirable to maintain the sludge in the pipe 46 in an aerobic condition. To this end, fine bubbles of pure oxygen are dissolved in the sludge in a downstream portion of the pipe 46 at a region indicated by the reference 50.

In the tank 40, generally circumferential flow conditions from the inlet 42 to the inlet 44 are set up.

Thus, as the sludge enters the tank 40, it will flow in a clockwise direction (as shown in Figure 2). The average residence time of the sludge in the tank 40 can be therefore arranged to be of relatively long duration.

As the sludge flows around the tank 40, in order to maintain aerobic conditions in the sludge, oxygen in the form of fine bubbles may be dissolved therein at several peripheral locations and toward the centre of the tank, the positions in which the oxygen is dissolved being indicated by references 52. As the thermophilic bacteria respire oxygen, so they multiply in number and take part in the biochemical digestion of the sludge. This process is exothermic and thus the necessary temperature favouring the proliferation of the thermophilic bacteria is created.

Typically, the tank 40 is open-topped but may have on it a cover so as to reduce evaporative losses.

Typically, the operating temperature in the tank may be in the range of 50 to 80 degrees C depending on a number of factors including the rate of heat loss from the tank. At such elevated temperatures, harmful bacteria tend to be rendered passive.

In the tank 40, there is an inevitable mixing of sludge of different ages. Thus some sludge that has been in the tank for a relatively long period would tend to diffuse into the sludge entering the tank. As the sludge which has been in the tank a relatively long period will have a greater population of thermophilic bacteria than the sludge that has just entered the tank. The migration of sludge will tend to assist in the digestion of the sludge entering the tank 40.

Migration of sludge will also tend to cause a small portion of the sludge which enters the tank 40 through the inlet 44 to have a relatively short residence time within the tank 40. Any such sludge included in the sludge leaving the tank 40 through the outlet 44 will tend to be digested as the sludge flows along the pipe 48. Thus, the sludge that leaves the pipe 48 can be in a condition such as its solids content may safely be discharged onto the land.

Liquid may be returned to the sewage treatment plant.

Typically it can be arranged that the residence time of the sludge in the pipe 46 is one and a half days and the residence time of the sludge in the pipe 48 is also one and a half days. Preferably, it is arranged that the average residence time of the sludge in the tank 40 is two days.

Typically, it can be arranged that the flow rate of sludge into the pipe 46 is ten litres per hour and the rate of flow of the sludge out of the pipe is also ten litres per hours.

The apparatus as shown in Figure 2 can be operated over continuous periods of days or months.

Claims

1. A method of aerobically digesting sewage sludge (or like material) comprising the steps of: (a) continuously passing sludge along an inlet path into a main sludge digestion region; (b) introducing pure oxygen or an oxygencontaining gas mixture into the sludge in the main digestion region so as to maintain aerobic conditions within the sludge; (c) aerobically digesting sludge in the main digestion region at an elevated temperature which favours growth of thermophilic bacteria which take part in the digestion; (d) continously passing at least partially digested sludge out of the main sludge digestion region; (e) maintaining sludge of different ages (as hereinbefore defined) in the main digestion region; (f) heat-exchanging sludge passing along the inlet path with at least partially-digested, relatively warm sludge.

2. A method as claimed in claim 1, in which the inlet path is defined by an extended passage in which generally plug flow conditions are sustained.

3. A method as claimed in claim 1 or 2, in which the flow rate through the inlet path is such that it takes at least 24 hours for sludge entering the inlet path to reach the main sludge digestion region.

4. A method as claimed in any of the preceding claims, in which the oxygen-containing gas mixture contains at least 80% by volume of oxygen.

5. A method as claimed in any of the preceding claims, in which the elevated temperature in the main sludge region is sustained by the action of bacteria (in the sludge) respiring oxygen.

6. A method as claimed in any preceding claims in which the main sludge digestion region is maintained at a temperature of at least 50"C.

7. A method as claimed in claim 6, in which the main sludge digestion region is maintained at a temperature in the range 60 to 80 C.

8. A method as claimed in any of the preceding claims, in which pure oxygen or oxygen-containing gas mixture is introduced intermittently or continuously into the sludge flowing along the inlet path.

9. A method as claimed in any of the preceding claims, in which pure oxygen or oxygen-containing gas mixture is introduced into the sludge in the main sludge digestion region so as to maintain aerobic condtions in the sludge.

10. A method as claimed in any preceding claim, in which sludge leaving the main sludge digestion region passes along an extended outlet path.

11. A method as claimed in any preceding claims, in which at least part of the inlet path is in heat exchange relationship with at least part of the outlet path such that incoming sludge is heated by the outgoing sludge.

12. A method as claimed in any preceding claim, in which sludge flowing along the inlet path is heat exchanged with sludge in the sludge digestion region.

13. A method as claimed in any of claims 10 to 12, in which the main sludge digestion region comprises a channel, and some of the sludge is recycled from the outlet path to the channel.

14. A method as claimed in claim 13, in which in upstream and downstream portions of the main sludge digestion region share a common wall in which there is a passage for recycling the sludge from the downstream to the upstream region.

15. A method as claimed in claim 13 or claim 14, in which sludge entering the inlet path takes five days before reaching the end of the outlet path.

16. A method as claimed in claim 14 or 15, in which about 60% by volume of the sludge is recycled.

17. A method as claimed in any of claims 10to 12, in which the main sludge digestion region is provided by a tank in which mixing of sludge of different ages takes place.

18. A method as claimed in claim 17, in which the average residence time of sludge in the tank combined with the time it takes sludge to flow from the start of the inlet path to the tank and from the tank to the end of the outlet path is at least 5 days.

19. A method as claimed in claim 17 or 18, in which the average time sludge is held in the tank is greater than the sum of the time it takes for sludge to flow down from the start of the inlet path to the tank and the time it takes for sludge to flow from the tank to the end of the outlet path.

20. A method of digesting sewage sludge (or like material) substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.