GB2053882A - Separation of biomass from particulate media - Google Patents

Abstract

Biomass growing on the surface of particulate material in an aqueous medium is detached and separated from the particulate material, and obtained as a relatively concentrated sludge by first removing some of the aqueous medium before the biomass detachment step. The thickened mixture of particulate material with attached biomass and aqueous medium is then subjected to shear forces to detach the biomass, and the particulate material is then separated from the mixture. The method is particularly useful where expanded or fluidised beds of the particulate material are used, such as in the water and sewage treatment industries. Apparatus is described for carrying out the method.

Description

SPECIFICATION Separation of biomass from particulate media This invention relates to the separation of biomass from particulate media to the surfaces of which the biomass is attached.

It is known to use micro-organisms attached to the surfaces of a particulate material to effect bio chemical reactions on fluids which are brought into contact with the particulate material. Usually, but not in every case, the particulate material forms a fluidised or expanded bed in a reactor and the fluid is passed therethrough. In the water industry, this general technique is used to remove undesirable substances from the water (or other liquid such as sewage effluent), the microorganism being naturally selected in accordance with the function desired.

In the use of such particulate materials over a period of time, the micro-organisms grow as films of biomass on the surface of the particles and the bed expands in volume as the film thickness increases. It is necessary to periodically remove the excess biomass from a fraction of the bed in order to prevent carryover of the coated particles in the effluent. This is most conveniently done by removing the biomass from the most thickly coated particles which rise to the top of the bed.

The cleaned media is returned to the bed. This separation step has been effected in a variety of ways. A bed of particulate material may be backwashed or otherwise flushed with fluid to detach and wash out the excess biomass. Air scouring has been used. More recently, with the advent of expanded or fluidised beds of sand or like material, the same has been subjected to shear forces to strip off biomass attached thereto.

Whilst these various techniques have in general provided satisfactory separation of the biomass, they have all resulted in a relatively weak sludge by-product. That is to say, the separated biomass is obtained as a suspension in an aqueous liquid, the suspension having a dry solids content of only about 1 to 2% by weight. Such suspensions or sludges need treatment to concentrate them before disposal in order to reduce transport and disposal costs.

We have now devised a method of separating excess biomass from a particulate medium to which it is attached, by which the separated biomass can be obtained as a much more concentrated sludge.

In one aspect, the invention provides a method of separating biomass from a finely divided particulate material to which it is attached, the particulate material being in suspension in an aqueous liquid, which comprises the steps in sequence of: a) reducing the liquid content of the suspension; b) subjecting the suspension from step (a) to shear to detach biomass from the particulate material; and c) separating the particulate material from the mixture of detached biomass and liquid.

The method of the invention is particularly useful in water and sewage treatment and, accordingly, in one preferred aspect of the invention there is provided a method of treating an aqueous effluent (as herein defined) in which the effluent is passed through a bed of a finely divided particulate material attached to which are microorganisms, the micro-organisms effecting the treatment of the effluent, characterised in that at least part of the particulate material is periodically removed from the bed in suspension in said effluent, the liquid content of the suspension is reduced, the suspension is then subjected to shear to detach biomass from the particulate material, and the particulate material is then separated from the detached biomass and effluent for return to said bed.

The invention also provides apparatus useful for this method, which comprises a vessel for containing a bed of particulate material having micro-organisms attached thereto, means for passing aqueous effluent through a said bed in the vessel, means for periodically removing at least part of a said bed, suspended in effluent, from the vessel to a separator to reduce the liquid content thereof, means for subjecting the suspension to shear to detach biomass from the particulate matter, means for separating the particulate matter from the detached biomass and means for returning the separated particulate matter to the vessel.

By "aqueous effluent" we mean any aqueous liquid containing biodegradable material whereby such material can be degraded or utilised for growth by micro-organisms attached to a particulate material. In water and sewage treatments, these materials will principally be oxidisable carbonaceous compounds, ammonia and inorganic nitrates.

Hereinafter, the invention will be more particularly described with reference to the preferred aspect but it is to be understood that the invention is not limited to use in water and sewage treatment processes.

In recent years, attention has been focussed on the possibility of using expanded or fluidised beds of finely divided material in the treatment of water and sewage. U.K. patent specifications nos.

1430410 and 1433582 refer to the use of fluidised beds for the removal of nitrate and the removal of organic carbon and oxidation of ammonia, respectively. Reference should be made to these specifications for further details. The method of the present invention is particularly useful where fluidised (or expanded) beds of finely divided particulate material are used. The nature of the particulate material is not critical but it should of course be inert and finely divided so that shear can be applied. Typically, the particulate material will be of a size 0.2 to 2mm. Suitable material include sand and anthracite.

In utilising the invention in conjuction with a fluidised bed of particulate matter, the method of the invention may advantageously be automated.

One such arrangement is as follows.

The required amount of particulate solid (e.g.

biomass coated sand) is pumped or drained by gravity flow from the fluidised bed reactor into a tank fitted with a high speed mixer. The mixture is allowed to settle for a short period of time (1-2 minutes). The biomass-coated sand rapidly settles to the bottom of the tank and then occupies 20~40% of the total volume. Most of the supernatant water is then decanted - an adjustable siphon arrangement being a preferred method of doing this. Sufficient supernatant is left for the uptake of the biomass. The ratio of settled coated sand to supernatant water is preferably in the range 5:1 to 10:1. This mixture is then stirred for the necessary period of time for biomass detachment. A stirring period of 21 5 minutes has been used with coated sand having varying degrees of biomass attachment.After this has been done, the slurry of media and sludge is passed to a media/sludge separation unit. A vibrating sieve has been used and is preferred, but separation could also be effected in, for example, (i) a centrifuge, (ij) a hydrocyclone, (iii) an elutrication column or (iv) on a modified belt press.

The under-flow from the sieve is a media-free waste sludge which may be disposed of by conventional sludge treatment methods. It may be necessary for the process to be repeated once or twice more in order to produce a clean medium which is suitable for return to the fluidised bed without incrementing the effluent suspended solids. If it is necessary to repeat this stage, a small quantity of water or effluent must be added to the media.

In order that the invention may be more fully understood, reference is made to the accompanying drawing which shows diagrammatically one arrangement for carrying out the method of the invention.

Referring to the drawing, there is shown a fluidised bed 1 of particulate material (sand) having micro-organisms attached thereto. An optical level probe 2 is provided in the top of the fluidised bed 1. A conduit 20, with a valve 3, connects the top of bed 1 to a shear cell 4 fitted with a stirrer 8, and a level probe 7. A siphon pipe 6 dips into cell 4 and communicates, externally of cell 4, with primary sedimentation tanks (not shown). A conduit 21 is also provided from above the top of the particulate matter in bed 1 to the cell 4 via a valve 15. A valved outlet 9 from cell 4 opens into a feed hopper 10 from which a screw pump 11 communicates with a single deck vibrating sieve 12. From sieve 12 there is provided a valved waste sludge conduit 13 and, as an extension thereof, a valved liquid conduit 16, communicating with the siphon pipe 6 externally of cell 4.Dashed lines 17 show a second position into which sand exit pipe 14 from sieve 12 can be placed, so as to discharge treated sand into fluidised bed 1. Optical level probes 2 and 7 are connected to control valve 3 and stirrer 8, respectively.

In use, when the sand in the bed 1 has expanded to level A, the optical level probe 2 senses this and opens valve 3. This allows coated sand and effluent to flow slowly from the reactor into the shear cell 4 until the level of the liquid in the cell has risen to B when the probe 5 senses this and shuts valve 3. The automatic siphon 6 starts to drain supernatant liquid from the shear cell when the liquid level reaches B. The siphon is adjustable in length to accommodate different ratios of supernatant liquid and coated media. The siphoned supernatant liquid is recycled to the primary settling tank from which the feed to the fluidised bed is taken. When the liquid reaches level C, the level probe 7 switches on the stirrer 8 which runs for a preset time (in the range 0.5-5.0 min).When the stirrer is switched off, automatic valve 9 opens allowing the sheared media/biomass slurry to fall into the feed hopper 10 of the screw pump 1 1 which conveys the slurry up to the single deck vibrating sieve 12. The vibrating sieve is started by a signal from the stirrer 8 as it switches off.

The biomass sludge passes through the sieve deck and is collected as a waste sludge via 13.

The sand may need further cleaning, and if so it is returned to the shear cell via pipe 14. When all the sand has been returned valve 15 opens allowing a small quantity of effluent (about 10~20% of sand volume) to be passed to the shear cell. The quantity is determined by using a preset timer to control valve 15. The cycle is then repeated as before. The media entering the shear cell is now much cleaner and just requires a final rinse. This is done by allowing the liquid level to rise to C. This time when the media/biomass mixture is passed through the vibrating sieve the underflow is very dilute and is recycled to the primary settling tank via valve 16 taking with it only a small quantity of biomass. The sand is now clean and is returned to the fluidised bed reactor via 14 which has been swung round to position 17 over the fluidised bed.

In the method of the invention, the suspension of particulate solids in liquid is thickened before shearing. This not only provides the substantial advantage of more efficient shear, and hence more efficient detachment of biomass, but also results eventually in a more concentrated biomass sludge by-product for disposal. For example, in the embodiment described, a biomass sludge of 5% to 7% dry solids w/w can be obtained. The production of such sludges (instead of the much weaker sludges of the prior art) enables a large cost saving to be made in the dewatering and disposal of the sludge.

The method of the invention can be used to advantage in the fluidised and expanded bed processes described and claimed in our copending application no. 21190/77 to which reference should be made for further details.

Claims (17)

1. A method of separating biomass from a finely divided particulate material to which it is attached, the particulate material being in suspension in an aqueous liquid, which comprises the steps in sequence of: a) reducing the liquid content of the suspension; b) subjecting the suspension from step (a) to shear to detach biomass from the particulate material; and c) separating the particulate material from the mixture of detached biomass and liquid.

2. A method of treating an aqueous effluent (as herein defined) in which the effluent is passed through a bed of finely divided particulate material attached to which are micro-organisms, the micro-organisms effecting the treatment of the effluent, characterised in that at least part of the particulate material is periodically removed from the bed in suspension in said effluent, the liquid content of the suspension is reduced, the suspension is then subjected to shear to detach biomass from the particulate material, and the particulate material is then separated from the detached biomass and effluent for return to said bed.

3. A method according to claim 2 wherein said bed is an expanded or fluidised bed.

4. A method according to claim 2 or 3 wherein the treatment of the aqueous effluent comprises removal of organic carbon therefrom by the microorganisms attached to the particulate material.

5. A method according to claim 2 or 3 wherein the treatment of the aqueous effluent comprises oxidation of nitrogen compounds thereon by the micro-organisms attached to the particulate material.

6. A method according to claim 2 or 3 wherein the treatment of the aqueous effluent comprises denitrification, the effluent containing a sufficient amount of a carbon source to allow oxidised nitrogen therein to be converted to nitrogen by the micro-organisms.

7. A method according to any of claims 2 to 6, wherein the particulate material is separated from the detached biomass by use of a vibrating sieve.

8. A method according to any of claims 2 to 7 wherein the suspension removed from the bed is allowed to settle and supernatant effluent liquid is removed therefrom to reduce the liquid content thereof.

9. A method according to any of claims 2 to 8 wherein aqueous effluent is treated on a continuous basis, and wherein periodically a portion of the particulate material is removed from, and subsequently after separation from detached biomass is returned to, the bed.

10. A method of separating biomass from a finely divided particulate material to which it is attached substantially as herein described with reference to the accompanying drawing.

11. A method of treating an aqueous effluent (as herein defined) substantially as herein decribed with reference to the accompanying drawing.

12. Apparatus for carrying out the method of claim 2, which comprises a vessel for containing a bed of particulate material, having microorganisms attached thereto, means for passing aqueous effluent through a said bed in the vessel, means for periodically removing at least part of a said bed suspended in effluent, from the vessel to a separator to reduce the liquid content thereof, means for subjecting the suspension to shear to detach biomass from the particulate matter, means for separating the particulate matter from the detached biomass and means for returning the separated particulate matter to the vessel.

13. Apparatus according to claim 12 wherein the said separator comprises a settling tank and means for removing supernatant liquid therefrom to reduce the liquid content of the suspension therein.

14. Apparatus according to claim 13 wherein said means for removing supernatant liquid comprises a siphon.

15. Apparatus according to claim 12, 13 or 14 wherein said separating means comprises a vibrating sieve.

16. Apparatus according to any of claims 12 to 1 5 which include a screw pump or conveyor for transferring the suspension after shear to said separating means.

17. Apparatus according to claim 12 substantially as herein described with reference to the accompanying drawing.