GB2220411A

GB2220411A - Anaerobic digestion of sewage sludge

Info

Publication number: GB2220411A
Application number: GB8906893A
Authority: GB
Inventors: Yoshio Kobayashi
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-03-30
Filing date: 1989-03-28
Publication date: 1990-01-10
Anticipated expiration: 2009-03-28
Also published as: GB8906893D0; GB2220411B

Abstract

An anerobic digestion process for sewage sludge in which a part of the digested sludge is recirculated and kneaded with the incoming raw sludge. The raw sludge is dewatered to a solid concentration of at least 10 %, and may be heat treated before being mixed with the recirculated sludge. The digestion process produces methane gas and may take place in one or two stages. In the latter it is the digested sludge from the first stage that is recirculated. <IMAGE>

Description

TITLE OF THE INVENTION Anaerobic Digestion Process for Sewage Sludhe BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an anaerobic digestion process for sewage sludge.

Description of the Background Art In general, sludge is discharged from a sewage-treatment plant in a state ha concentration of about 2 . In a conventional countermeasure fo reducing the amount of such discharge: the sludge is guicec to a digestor chamber for methane fermentation.

In such a conventional method, however, the Igestor hamber having large capacity is required of fermentation is slow and the sludge concentration is very dilute. Further, the conventional method is Inferior in economic efficiency since sewage gas yield thereof is 40 to 50 % at the most, with the result that considerable part of sewage gas generated through digestion is spent for heating the sludge.

In sludge digestion technique, the first important point to improve further is miniaturization of a digestor chamber, and the second point is betterment of heat balance in the process as well as improvement in sewage gas generation efficiency. An attempt has recently been made to enrich the sludge for increasing its solid concentration of about 2 % to 4 to 6 %, in order to reduce the capacity of the digestor chamber and to improve heat balance so as to be able to recover energy through a sewage gas power generation system. However, sufficiently economic efficiency cannot yet be obtained through such enricment, and awaited is further improved performance.

Generally known methods of improving efficie@ methane fermentation are that of making fermentation at a high temperature of 50 to 5500 and that of heating sludgn in advance before methane fermentation. In the former method, the speed of fermentation is increased to 2.0 to 2.5 times that in ordinary methane fermentation caused at 35 C.In the latter case, it is said that digestib can be improved with a raise of 50 to 60 % as compared with that in the ordinary case by previously heat-treating raw sludge at a temperature of 120 to 18000. These methods are considerably effective in order to improve efficiency of sludge digestion. if the concer.l-ati of the sludge is about 4 to 6 %, however, the heating temperature is limited to about 350C, since large quantities of heat is required for such heating.Thus, methane fermentation cannot be caused at a temperature of 50 to 55 C and raw sludge cannot be heated in advance to 120 to 1800C in an ordinary sewage-treatment plant, since large quantities of heat are required for such treatment, SUMMARY OF THE INVENTION An object of the present invention is to provide an anaerobic digestion process for sewage sludge, which can improve the generation efficiency of sewage gas, while reducing the capacity of a digestor chamber.

The present invention provides a sewage sludge anaerobic digestion process for circulating part digested sludge after methane fermentation and adding raw sludge to the part of the digested sludge to au;- methane fermentation. The inventive process comprises steps of dewatering the raw sludge into solid concentration of at least 10 %, adding the dewatered raw sludge to the part or the divested sludge for homogeneously kneading the with each other, and causing methane fermentation of the kneaded sludge at a predetermined temperature. The present inventIon is characterized in that the dewatered raw sludge is employed instead of conventional dilute sewage sludge.

According to the present invention, the raw sludge is preferably dewatered into solid concentration of at least 15 %, more preferably at least 20 %. Furthermore, the dewatered raw sludge is preferably heated at a temperature of 120 to 180 C before it is added to the digested sludge. In this case, methane fermentation is preferably performed at a high temperature fermentation region of 50 to 550C at which a higher fermentation rate is obtained.

The present invention is also applicable to a so-called two-phase anaerobic digestion process, in which a methane fermentation step is separated into an acid formation step and a methanation step. In this case, raw sludge is dewatered into solid concentration of at least 10 %, and the dewatered sludge is added to the digested sludge after acid formation fermentation to be homogeneously kneaded with the same. Than the kneaded sludge is subjected to acid formation fermentation, an products of acid formation fermentation are guides to a methanatlon chamber to generate methane. The products of acid formation fermentation are separated in the fr s ; filtrate, by filtering the digested sludge after acid formation fermentation.Such products of acid formation fermentation contained in the obtained filtrate are subjected to methanation fermentation by contact with a fie Diocatalyst in methanation chamber. The term "acid formation fermentation" indicates fermentation of generating lower fatty acid, alcohol and carbon dioxide gas etc. from organic substances contained in the sludge.

In the two-phase anaerobic digestion process, improved efficiency can be obtained by previously heating the raw sludge and fermenting at a high temperature range.

According to such a two-phase anaerobic digestion process, the kneading condition of high-viscosity raw sludge and seed sludge becomes easier and higher efficiency can be obtained.

If solid concentration of sludge exceeds 4 to 6 %, viscosity thereof is abruptly increased, while consumption of stirring power of conventional methane fermentation vessel equipped with a general stirrer becomes too large to be economical if solid concentration exceeds 10 -O.

Furthermore, stirring cannot be performed by the nral stirrer if the solid concentration exceeds 15 % Therefore, it has generally been consIdered that p@actical concentration is not more than 4 to 6 %. Thus, no methane fermentation has generally been made through use or sludge having solid concentration of at least 10 %, since such methane fermentation has been considered impossible.

The inventor has found that, even if solid concentration of raw sludge exceeds 20 %, methane fermentation can be efficiently made by sufficienly increasing concentration of a ferment bacteria and uniformly and homogeneously mixing the ferment bacteria into the raw sludge. According to the present invention, a small amount of highly concentrated raw sludge is added to and homogeneously mixed with highly concentrated seed sludge, i.e., digested sludge, and the mixture is maintained at a constant temperature in a digestor chamber, thereby to cause methane fermentation.

According to the present invention, a digestor chamber can be reduced in capacity as compared with the conventional art, since raw sludge is dewatered into solid concentration of at least 10 %. If the capacity of the digestor chamber is similar to that in the conventional art, a larger amount of raw sludge can be digested.

According to the present invention, the amount f water to be heated is reduced as compared with the conventional art since the sludge has high solid cocencrato. Therefore, according to the present invention, It is possible to perform the heat treatment or raw sludge and the high-temperature fermentation which have not been adopted practically so far.

A sewage-treatment plant may be provided with equipment of a sewage gas power generation. According ço the present invention, sludge can be heated through exhaust heat generated in the power generation system...

Thus, the present invention is particularly availal for such a treatment plant.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the amounts of gas generated in a Fill and Draw method experiment; Fig. 2 illustrates methane contents in the gas generated in the Fill and Draw method experiment; Fig. 3 is a process drawing showing a first embodiment of the present invention; Fig. 4 is a process drawing showing a secona embodiment of the present invention; Fig. 5 is a perspeelve view showing a digestor chamber employed In a bench scale exper lmen t along the process shown in Fig. 4; Fig. Q Is a partially fragmented perspectIve view showing a digestor chamber in the case of applying the present invention to a large scale plant; and Fig. 7 is a process drawing showing a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fill and Draw Method Experiment The present invention has been studied through a Fill and Draw method experiment.

Seed sludge was prepared by dewatering the sludge digested at 50 to 55 0C into solid concentration of 15 E. Raw sludge was dewatered into solid concentration of 21 %.

Heat-treated sludge was prepared by heating raw sludge at about 1700C for 30 minutes.

Example 1 Seed sludge of 200 g and raw sludge of 2.6 g were introduced into a hermetically sealed laboratory kneader of about 1000 ml in content volume which has an exhaust port, and substitution was performed in a pot of the adr by nitrogen gas. Then, kneading was performed for five minutes and thereafter the kneader was dipped and stood in a constant-temperature water bath of 53 C.

Digested gas thus generated was collected in a scavenging bottle, and the amount of generated gas and the tn content were measured on the next day. Then, 2.6 g or the sludge was extracted from the pot of the kneader, ard sludge was newly added by 2.6 g and kneaded for five minutes. Such operation was performed once a day, and this experiment was continued for 16 days. Fig. 1 shows the amount of generated gas and Fig. 2 shows the methane content in the generated gas.

Reference Example 1 Seed sludge of 200 g and raw sludge of 2.6 g were introduced into a conical flask of 300 ml, and stirred and -mixed by a glass rod with substitution in the flask by nitrogen gas. This conical flask was covered wit a rubber stopper provided with a digested gas extraction port, and dipped and stood in a constant temperature bath of 53 0C. Digested gas thus generated was collected in a scavenging bottle, and the methane content was measured on the next day. Then, 2.6 g of the sludge was extracted from the flask, and raw sludge was newly added by 2.6 g and stirred and mixed by the glass rod. Such operation was performed once a day, and this experiment was continued for 16 days. Figs. 1 and 2 shows the resits thus obtained.

Example 2 The raw sludge employed in Example 1 was replaced by heat-treated slr ge t to measure the mount of gc..e~- gas and the methane content in the generated gas similarly to example 1. Figs. 1 and 2 show the results.

Reference Example 2 The raw sludge employed in reference example 1 was replaced by heat-treated sludge, which was mixed Into seed sludge and digested similarly to reference example 1, to measure the amount of generated gas and the methane content. Figs. 1 and 2 show the results thus obtained.

As shown in Fig. 1, the amount of gas generated in Example 1, in which the sludge was sufficiently mixed in the laboratory kneader, was extremely higher than that in reference example 1, in which stirring was insufficient. This amount of generated gas was an amount per organic substances of 1 g contained in the sludge. The amount of gas generated in reference example 1 was at a level substantially identical to that in general digestion performed in low concentration.

In Example 2, the raw sludge was heat-treated in advance. As shown in Fig. 1, the amount of gas generated in Example 2 was higher than that in Example 1. Thus, it is understood that sewage gas yield was improved b, neat treatment, to generate a large amount of gas.

Further, it is understood that the sludge must be sufficiently mixed also, from comparison example 2 reference example 2 employing the hea--tre=ed sludge.

Bench Scale Experiment Fig. 3 is a process drawing showing a first embodiment of the present invention. Referring to Fig 3, raw sludge dewatered into solid concentration of at least 15 % is introduced into a kneader 1, to which part of digested sludge is fed back. The raw sludge is mixed into tne digested sludge, serving as seed sludge, kneader 1. The kneader 1 is formed by that capable of kneading highly viscous substances, such as a co-kneader, a ribbon mixer or guillotine mixer, for example. Sledge obtained by kneading the raw sludge with the digested sludge in the kneader 1 is supplied to a digestor chamber 2. In the digestor chamber 2, digestion is performed and sewage gas thus generated is extracted.A great part of the slflge digested for a predetermined period is fed back to the kneader 1, while the remaining part is disposed.

The large mixing ratio of the digested sludge/raw sludge in the kneader 1 provides the stable operation. The preferable ratio thereof, which depends on the efficiency of kneader, is selected from the range of 1 to 20.

It IS said that methane fermentation is adapted to form methane through two processes of forming organic acid, alcohol etc. from organic substances of substrates contained in sludge and forming methane from the organic acid and alcohol. If ferment bacteria concentration iii raw sludge is low or raw sludge is heterogeneously mixes with seed sludge and ferment bacteria concentratlo locally reduced, formation of organic acid and alcohol becomes dominant to inhibit activity of methanation bacteria.The rate of methane fermentation depends Dn the methanation process, and it is said that methanation bacteria are extremely sensitive to a pH value and 'g-anc acid and alcohol concentration. Therefore, it is necessary to regularly maintain high concentration of the methanation bacteria in the raw sludge and to mix the raw sludge with the seed sludge so that concentration of the organic acid and alcohol around the methanation bacteria is not in excess of a certain limit. In such mixing of the raw sludge and the seed sludge, it is preferable to increase initial concentration of the ferment bacteria by increasing the amount of the seed sludge to be circulated, in order to prevent excessive increase in concentration of organic acid and alcohol.Thus, equilibrium in symbiotic relation between acid forming bacteria and the methanation bacteria is maintained.

Fig. 4 Is a process drawing showing a second embodiment of the present invention, in which raw sludge is hear-treated before the same is kneaded with sludge. Referring to Fig. 4, dewatered raw sludge supplied to a sludge preheater 3, to be preheated tneiein.

Then the raw sludge is supplied to a heater 4, and heated to a tcmerature of 120 to l80C. This heater 4 can perform heating with steam, for example. Such steam heating can be made by steam recovered from waste heat of a sewage gas power generation system. The heated sludge is then introduced into a first flash tank 5, and cooled to 100 C. Saturated steam in the first flash tank fed back to the sludge preheater 1, to heat the raw sludge. Then the raw sludge is introduced into a second flash tank 6, and cooled to 550C, for example.

The cooled raw sludge is supplied into a kneader 1, and kneaded with part of circulated digested sludge, which serves as seed sludge. The kneaded sludge is fed to a digestor chamber 2, and methane fermentation is made at a temperature of 50 to 550C, for example. sewage gas thus generated is extracted from an extraction port, and a large part of the sludge digested for a predetermined period is fed back to the kneader 1, while the remaining part is disposed.

The apparatus shown in Fig. 4 was employed to make A wench scale experIment. Fig. 5 shows a tube type digestor chamber employed in this experiment. Referring to Fig. 5, a tube 12 Is provided in a jacket 11. A slate s:pp'Sv nozzle 13 an a sewage gas exhaust port 15 are provided in an upper end of the tube 12, arid a sludge exhaust port i4 is provided in the bottom portion of the jacket 11.

Sludge kneadec In the kneader 1 equipped with a heating jacket is supplied to the tube 12 through the sludge supply nozzle 13. The interior of the tube 12 is maintained at a predetermined temperature by the jacket 11. The sludge is digested and downwardly moved in the tje 12. The sludge thus moved in the tube 12 within a constant period is exhausted from the sludge exhaust port 14. Gas generated by such digestion is exhausted from the sewage gas exhaust port 15 provided in the upper portion of the tube 12.

A tube 12 of 160 mm in diameter and 3,500mm in length was employed and heated by warm water flowing in the jacket 11 so that its interior was at a temperature of 50 to 550C. The rate for supplying sludge from the sludge supply nozzle 13 was adjusted so that the sludge passed through the tube 12 in one day.

Within the kneader 1 heated to 50 to 55 0C, raw sludge of 0,84 kg, which was prepared by in advance heat-treating raw sludge of solid concentration 21 % at 160 C for 30 minutes, was kneaded with seed sludge, i.e., digested sludge of 12.0 kg. The sludge mixture thus obtained çer into the tube i2 once every 6 hours intermittently the sludge supply nozzole 13 This bench scale test was operated for 15 days, to measure the amount of gas generated by digestion t methane content in the generated gas. As the result, gas was generated at the rate of about 750 l per organic substances of 1 kg contained in the raw sludge. The amount of the gas thus generated in this system was considerably large than that in an ordinary anaer digestion process, which is about 500 per organic substances of 1 kg. The methane content was about 60 %, which was substantially similar to that in the conventional anaerobic digestion process.It is obvious that the digestion process according to the present invention is excellent also in bench scale test under conditions further approximate to those for actual operation, since the amount of the generated gas per unit organic substances is extremely improved as compared with that in the conventional digestion process.

Fig. 6 is a partially fragmented perspective view showing an exemplary digestor chamber employed in the case of applying the present invention to a large scale plant.

According to the present invention, sludge cannot te digested in a conventional stirring vessel since the same has high viscosity. In one of effective methods.

therefore, the sludge which is kneaded in advance well in kneaded is digested during movement in a tube, as shown in Fig. 5. Thus, an apparatus having a plurality of as sown 1n Fig. 6 can be employed in a large scale plant.

Referring to Fig. 6, a plurality of tubes 22 are contained in a jacket 21. The tubes 22 are 0.2 to 1.0 m in diameter and 10 to 30 m in length, for example. The jacket 21 is provided in its bottom portion with a sludge supply port 2 9 t which has sludge distribution nozzles 24 in correspondence to the respective tubes 22. A warm water inlet port 27 and a sludge exhaust port 25 are further provided in the bottom portion of the jacket 21. A sewage gas outlet port 26 for extracting generated gas and a wanm water outlet port 28 are provided in an upper portion of the jacket 21.

Sludge supplied from the sludge supply port 23 is pushed into the respective tubes 22 through the sludge distribution nozzles 24. The sludge is gradually upwardly moved within the tubes 22 to overflow the upper ends of the tubes 22, and then downwardly moved along the outer walls of the tubes 22. The sludge thus downwardly moved along the outer walls of the tubes 22 is exhausted from the sludge exhaust port 25, so that a great part thereo is fed back to the kneadpr 1 and the remaining part disposed.

The sludge IS digested du@ing the upward movement within the tubes 22 and the eewnward movement along the outer walls of the tubes 22. Due to such a system, a long residence time can be ensured even if the tubes 22 are short. warm aater supplied from the warm water Inln Bt 27 passes through the outer wall of the jacket 21, to be discharged from the warm water outlet port 28. The interior of the jacket 21 is maintained at a predetermines temperature by this warm water.

Through employment of the digestor chamber having a plurality of tubes as shown in Fig. 6, the speed ot movement of the sludge within the tubes can be uniformalized, thereby to prevent ununiform movement of the sludge such as short-circuit flow.

It is to be noted that the digestor chamber shown in Fig. 6 is a mere example of a vessel employable in the present invention, and the present invention is not restricted to the digestor chamber shown in Fig. 6.

As hereinabove described, the present invention is also applicable to a two-phase anaerobic digestion process, In which methane fermentation is separate into an acid formation step and a methanation step. Fig. 7 is a process drawing showing an embodiment in which the present invention is applied to such a two-phase anaerozic digestion process. Referring to Fig. 7, dewatered raw sludge is supplied to a kneader 31, to be kneaded with a larse part of digested sludge which is effluent from n acid digester vessel 32.The kneaded sludge is supplied to an acic digestor vessel 32, to be subjected to manly an acid formation step in a methane fermentation. A great part of the digested sludge after fermentation is fed back to the kneader 31, and the remaining part is filtered by a filter 33. Organic substances contained in the raw sludge are converted into lower fatty acid, alcohol, carbon dioxide etc. and solubilized through acid formation fermentation. Thus, such lower fatty acid etc. are dissolved in the filtrate. Solids generated in the llter 33 are disposed. Then the filtrate is supplied to a methanation fermentation vessel 34, in which a biocatalyst for causing methanation fermentation is fixed, to be subjected to methanation fermentation.Methane gas thus generated is extracted and the filtrate passing through the methanation fermentation vessel 34 is disposed. In this embodiment, tube type digestor chambers shown in Figs. 5 and 6 can be employed as the acid digestor 32.

urthermore, in this embodiment, raw sludge may ee heat-treated in advance before the same is kneaded with sPed sludge in a similar manner to the process shown Fig. 4.

As hereinabove described, the digestion process according to the present invention is also applicable two-phase aerobic digestion process.

Although the present invention has been described and illustrated in detail, it is clearly understood that fle same is by way of illustration and example only and 1 not to be taken by way of limitation, the spirit and scope or the present invention being limited only by the terms ot the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An anaerobic digestion process for sewage sludge for circulating part of digested sludge after methane fermentation and adding raw sludge to said part of said digested sludge for causing methane fermentation, said process comprising the steps of: dewatering said raw sludge into solid concentration of at least 1 ; adding said dewatered raw sludge to said part of said digested sludge and homogeneously kneading the saone ' & tn each other; and ausing methane fermentation of said kneaded sludge at a prescribed temperature.

2. A process in accordance with claim 1, wherein said dewatering step comprises a step of dewatering said raw sludge into solid concentration of at least 15 %.

3. m Process in accordance with claim 1, wherein said dewatering step comprises a step of dewatering said raw sludge into solid concentration of at least 20 %.

4. A process in accordance with claim 1, wherein said fermentation step comprises a step of causing methane fermentation at a temperature of 50 to 550C.

5. An anaerobic digestion process for sewage sludge for circulating part of digested sludge after methane fermentation and adding raw sludge to said part of said digested sludge for causing methane fermentation, sai process comprising the steps of: dewatering said raw sludge into solid concentration of at least Q 0 %; heat-treating said dewatered raw sludge at a temperature of 120 to 180 C; adding said heat-treated raw sludge to said digested sludge and homogeneously kneading the same with each other; and causing methane fermentation of said kneaded sludge at a prescribed temperature.

6. A process in accordance wit claim 5, wherein said dewatering step comprises a step of dewatering said raw sludge into solid concentration of at least 15 g.

7. A process in accordance with claim 5, wherein said dewatering step comprises a step of dewatering said raw sludge into solid concentration of at least 20 %.

8. A process in accordance with claim 5, wherein said fermentation step comprises a step of causing methane fermentation at a temperature of 50 to 550C.

9. A two-phase anaerobic digestion process for sing methane fermentation in separation into an acid formation process and a methanation process, said acid formation process comprising the steps of: dewatering raw sludge into solid concettiuii of d least 10 t.

adding said dewatered raw sludge to part of digested sludge after acid formation fermentation and homogeneous lv kneading the same with each other; and subjecting said kneaded sludge to acid format Ion fermentation, methanation process comprising a step of fermenting products resulting from said acid formation fermentation to generate methane.

10. A process in accordance with claim 9, wherein said methanation process comprises the steps of: separating said products resulting from said acid formation fermentation in the form of a filtrate by filtering said sludge after said acid formation fermentation, and generating methane from said products resulting from said acid formation fermentation through a fixed biocatalyst.

11. A process in accordance with claim 9, wherein said acid formation process further comprises a step of heat-treating said dewatered raw sludge at a temperature of 120 to 180 0C.

12. A process in accordance with claim 9, wherein.

said aid for.,.ation process comprises a step of causing acid formation fermentation at a temperature of 50 to 55 C.

13. A process in accordance with claim 9, wherein said dewatering step comprises a step of dewatering said raw sludge into solid concentration of at least 15 %.

14. A process in accordance with claim 9, wherein said dewatering step comprises a step of dewatering said raw sludge into solid concentration of at least 20 %.