GB2190370A - Process for purification of effluents and treatment of sludge - Google Patents

Abstract

A process for the purification of effluents, more particularly communal sewage, and the treatment of the sludge, in which the raw effluent is fed into a high-load adsorption stage A followed by a low-load culturing stage B, and the surplus sludge from the adsorption stage and the culturing stage is pre-thickened and fed into a sludge treatment stage having at least one acidification reactor 6 having a solid-bed 7 and at least one a methane reactor 9, having a solid-bed 10 the merely thickened but not dewatered surplus sludge from the adsorption stage A and the undewatered surplus sludge from the low-load culturing stage B being subjected to a mechanical fines removal with the aid of a rake and/or screen 2b, the fines-free sludge mixture being made more fluid by pasteurisation heating 3 and acidification 6, and the fluidised sludge mixture being further treated in the solid-bed methane reactor 9 at a dry substance content exceeding 100 kg/m<3>. Cell digesting substances may be added to the fines free sludge (eg lysin). <IMAGE>

Description

SPECIFICATION Process for purification of effluents and treatment of sludge This invention relates to a process for the purification of effluents, more particularly corn- munal sewage, and the treatment of the sludge, in which raw effluent is fed into a high-load adsorption stage followed by a low load culturing stage, and the surplus sludges from the adsorption and culturing stages are blended and pre-thickened before feeding as a surplus sludge mixture into a sludge treatment train having at least one acidification reactor and at least one methane reactor. The surplus sludges from the adsorption and culturing stages respctively can obviously be thickened separately.In the context of the process of the invention, the effluents are purified by the so-called adsorption technology (cf. "Korres pondenz Abwasser", vol. 30, no. 7, p 452).

The basic concepts of adsorption technology and the present technology involve no me chanical pre-cleaning. However, its provision is possible within the context of the invention.

In a known process of this type, the surplus sludges from the high-load adsorption stage and the low-load culturing stage are first prethickened and then dewatered, for example with the aid of a decanter or a dewatering screen unit, before feeding into the acidification reactor and the methane reactor. The sludge mixture is also frequently blended with raw sludge from mechanical pre-cleaning applied to the incoming effluent. The solids content (dry substance) of the sludge mixture when treated in the methane reactor averages 45kg/m3 and can reach a maximum of 70 to 90kg/m3. It is impossible to treat the sludge from conventional clarification systems in methane and/or acidification reactors of the solid-bed type, because of the high risk of blockage.On the other hand, it is common knowledge that one cannot operate at a dry substance content exceeding 50 to 60kg/m3 without adopting the known method of adding a granular material as a bacterial substrate.

The object of the invention is to adapt the known process so that the methane reactor can be operated consistently at a much increased dry substance context, even without adding any granular material as a substrate for the bacteria.

According to the present invention, the surplus sludge mixture, before dewatering is subjected to mechanical fines removal with the aid of a rake and/or a screen, the fines-free sludge mixture is made more fluid by pasteurisation heating at 70 to 750C and acidification at 50 to 55 C, and the fluidised sludge mixture is treated further in at least one methane reactor that has a solid-bed, at a dry substance content exceeding 100kg/m3. The preferred temperatures are about 700C for the pasteurisation heating and about 500C for the acidification. Preferably the acidification is car ried out in at least one acidification reactor that has a solid-bed.It is obvious that the fines-free sludge mixture, having been pasteu rised by heating in the first stage, can then be further fluidised in a second stage in at least one acidification reactor. However, fluidisation by pasteurisation and acidification can be car ried out within the acidification reactor. In the process of the invention, the effluent circulat ing in the acidification and/or methane reactors is highly mobile. A high proportion (around 80 to 85%) of the bacterial and/or anaaerobic culture sludge is firmly attached to the solid-bed substrate, though it is included in the dry substance content as specified above. The highly mobile mixture of effluent and sludge at a suspended solids content of 30 to 50kg/m3 (=3 to 5% dry substance) is further treated in the at least one solid-bed methane reactor.The total culture mass available is increased by providing additional colonisation surfaces for sessile bacteria in the solid beds, in the methane reactor at least and preferably in the acidification reactor as well. The sludge mixture can be treated in the solid-bed methane reactor at dry substance contents of 150 to 300kg/m3.

The invention utilises the fact that after purification in a system having a high-load adsorption stage and a low-load culturing stage it is possible by mechanical fines removal with the aid of a rake and/or drum screen to produce a sludge mixture which can then be treated as a highly concentrated effluent in solid-bed reactors without risk of blockage, making use of the further fluidisation brought about by the pasteurisation and acidification treatments. If raw sludge is formed in a coarse pre-cleaning unit, it can similarly be subjected to fines removal and then added to the sludge mixture.

Alternatively, one could include a screening rake at the start of the effluent route, beyond and/or ahead of the usual rake or sand trap, to provide mechanical fines removal at this stage. Fines removal is preferably carried out at a "grain size limit" not exceeding 3mm.

The term "grain size limit" signifies that the maximum apertures between the rake or screen bars or the meshes are 3mm wide or less. One could alternatively operate at a coarser size limit, provided a fine screen is incorporated in the sludge return cycle. It is obvious that cell-digesting substances such as lysin can be added to the fines-free slduge mixture. In order to regulate the normal operating parameters, the sludge mixture can be subjected to internal and/or external circulation in the acidification reactor and/or the solid-bed methane reactor. Internal circulation signifies as it were agitation, irrespective of the means whereby agitation is brought about. One can for example inject the evolved methane gas.

External circulation signifies a closed-circuit system of suitable pipelines.

The accuring advantages are to be seen briefly in that the sludge mixture can be treated consistently in the methane reactor at a substantially increased dry substance content, using the solid-bed type of methane reactor. The addition of granular substrate material is not necessary. In particular, a notably higher degree of toxic matter degradation (=COD) can be attained by adding coal. As a result, the area and bulk of the sludge treatment system is significantly reduced. Further advantages accrue in detail. They consist in that the dewatering stage essential according to prior art, involving decanters, dewatering screens and/or scren-belt presses, can be dispensed with. The pasteurisation stage hydrolyses the sludge mixture and partly destroys the cells, producing higher mobility.Post-thickening beyond the acidification reactor can be dispensed with, though some dewatering can still be advantageous at this stage, and can easily be provided. It is obvious that activated coal can be added ahead of the solid-bed methane reactor, so as to retain and/or biologically destroy toxic substances. Moreover, lime can be added ahead of the methane reactor, if the acidity of the effluent/sludge mixture is too high. Because of the reduced bulk, it is possible to use and standardise steel vessels to construct the apparatus. Furthermore, the yield of methane gas is increased.

A typical embodiment of the invention will now be described with reference to the accompanying drawing which is a flowsheet thereof.

The process flowsheet shows, at the top, an effluent purification plant having an adsorption stage A and a culturing stage B. The raw effluent advances from left to right, and the purified effluent is discharged to the right. The pipes SR are provided for sludge recirculation.

The sludge discharge pipes SA terminate in a supply line 1 for the surplus sludge treatment system of the invention.

The lower part of the flowsheet shows the supply line 1 for surplus sludge from the highload adsorption stage and the low-load culturing stage, a pre-thickener 2a followed by a mechanical fines removal unit 2b, having a fine rake and/or a fine-mesh screen, an immersionburner pasteurising unit 3 and a pump 5 in the discharge pipe 4. One could operate with two separate pre-thickeners, one for the surplus sludge from the adsorption stage and the other for that from the culturing stage. The acidification reactor 6 comes next and has a solid bed 7. Unless the acidification reactor is mounted high up, the discharge pipe 8 will have an additional pump 5, so that the effluent/sludge mixture is either pumped or drained into the methane reactor 9, which is enclosed and has a solid bed 10. There follow a postthickener 11 with sludge recycling means and a dewatering unit 12.The liquid discharge is sent along pipes 13 for clarification after adding the usual chemicals for P-reduction. The post-thickener carries another return line 14, leading back to the pump 5 already referred to, ahead of the solid-bed methane reactor.

Evolved gas from the methane reactor 9 is extracted through a pipe 15 and taken to a gas-holder 16. The gas-holder 16 can also receive gas extractable from the acidification reactor 6, by way of a pipe 19. A pipe 17 is provided for the return of the gas from the gas-holder 16 to the methane reactor 10 for the purposes of internal circulation. A gas escape shaft 18 is provided. Lime or coal can be added at a point 20. The pipe 1 receives undewatered surplus sludge from the adsorption stage and undewatered surplus sludge from the low-load culturing stage in the preceding purification plant, as a sludge mixture after mechanical fines removal. The fines-free sludge mixture is pasteurised and thus initially made more fluid with the aid of the immersion burner in item 3.The pasteurised sludge mixture is then made even more mobile by partial cell destruction in the acidification reactor 6, so that it can be further treated in the solidbed methane reactor 9 at an average solids content comprising sessile and suspended culture masses in excess of 100kg/m3 dry substance).

For such a typical embodiment, one can assume that an effluent purification plant serving 100,000 inhabitants (or their equivalent) and having an adsorption stage and culture basins has a daily output of 200m3 surplus sludge.

The sludge from the effluent stream, already pre-cleaned by a rake (not shown), is fed through the supply line 1 into a pre-thickener 2a of capacity about 200m3. After thickening to a solids content of about 4-5%, this fully uniformly constituted and homogeneous sludge passes through the fine rake and/or fine-mesh screen unit 2b, to obtain a sludge-water mixture free from fibres. At relatively low flow rates around 30cm/s, the sludge/water mixture behaves like a highly concentrated organic suspension comparable with sewage. The treatment in the effluent stream has already lowered the BOD5 concentration of the effluent from 300mg/l to about 5mg/l on the one hand, whereas the fluid sludge/water phase on the other hand has been raised to about 15,000 to 20,000mg/l, i.e., concentrated by a factor of 50 to 70.

This sludge/water mixture (200m3) is heated to 700C in the immersion burner unit 3, which holds about 10m3 and gives a dwell period of about half an hour. At a steady rate over the 24-hour period the 200m3 of sludge/water mixture is pumped at a temperature of 50-55 C (mesophil) into the acidification stage 6/7, which holds about 300m3 and gives a dwell period of 1 to 1.5 days. Here the organic compounds are degraded and the sludge undergoes further fluidisation accompanied by pasteurisation. The sludge/water mixture is internally circulated in stage 6/7 by injecting methane gas at 17, and the solid-bed reactor is thus kept functioning.

At a temperature of about 330C (mesophil), the sludge/water mixture then passes through the solid-bed methane reactor 9/10, the capcity of which is about 2,400m3, giving a dwell period of about 10 to 12 days. The line 17 recirculates gas from the gas-holder 16 for internal circulation of the sludge through the solid beds 7 and 10. After degassing the digested sludge in the gas escape shaft 18, it is fed into a post-thickener 11, where it remains for about 1 to 1.5 days.

In order to sustain the external culture mass cycle and activate the biological processes, a proportion of the thickened sludge is fed back through the pipe 14 to the methane reactor, while the residue is fed to the sludge dewatering unit 12. The liquid discharge is taken along a pipe 13 to the clarifier inlet.

In the hydrolysis phase, which in the embodiment shown takes place in the acidification reactor 6, polymers are degraded by exoenzymes into fragments which are then broken down in the acidification phase, to alcohols, CO2,H2 and organic acids for example. The hydrolysing and acidifying bacteria have a brief life cycle. An acid medium is more favourable for the acidifying bacteria.

In the methane-producing phase, which in the embodiment shown takes place in the methane reactor 9, the methanogenic bacteria convert only acetates, H2 and CO2 directly to CO4. Other organic acids, alcohols and the like must first be converted to acetates by acetogenic bacteria, and for reasons of energy requirement acetogenic and methoanogenic bacteria can only exist in close physical contact.

Both types of organism have a long life cylce.

A neutral to slightly alkaline medium is favourable for methanogenic bacteria.

Claims (6)

1. A process for the purification of effluents, more particularly communal sewage, and the treatment of the sludge, in which raw effluent is fed into a high-load adsorption stage followed by a low-load culturing stage, and the surplus sludges from the adsorption and culturing stage are blended and pre-thickened before feeding as a surplus sludge mixture into a sludge treatment train having at least one acidification reactor and at least one methane reactor, and wherein surplus mixture before dewatering is subjected to mechanical fines removal with the aid pf a rake and/or a screen, the fines-free sludge mixture is made more fluid by pasteurisation heating at 70 to 750C and acidification at 50 to 55 C, and the fluidised sludge mixture is treated further in at least one methane reactor having a solid-bed, at a dry substance content exceeding 100kg/m3.

2. A process as in Claim 1, wherein the acidification is carried out in at least one acidification reactor having a solid-bed.

3. A process as in either of Claims 1 to 2, wherein the effluent-sludge mixture is treated in the solid-bed methane reactor at dry substance contents of 150 to 300kg/m3.

4. A process as in any one of Claims 1 to 3, wherein the fines removal is carried out at a "grain size limit" not exceeding 3mm.

5. A process as in any one of Claims 1 to 4, wherein the sludge mixture is subjected to internal and/or external circulation in the acidification reactor and/or the solid-bed methane reactor.

6. A process for the purification of effluents, and the treatment of the sludge substantially as hereinbefore described with reference to the accompanying drawing.