EP1855998A1

EP1855998A1 - Method for reducing sludge using a population of oligochaeta held in a reactor, a device therefor, and the use of nestling oligochaeta for reducing sludge in a reactor

Info

Publication number: EP1855998A1
Application number: EP20060716619
Authority: EP
Inventors: Gerrit Antoni Van Schouwenburg; Evert Jan Leeuw
Original assignee: SR Technologie BV
Current assignee: ELSDON B.V.
Priority date: 2005-02-16
Filing date: 2006-02-14
Publication date: 2007-11-21
Also published as: WO2006088356A1; NL1028290C2

Abstract

The invention relates to a method for reducing sludge, comprising steps for: i) holding a population of oligochaeta in a reactor, in a reactor medium flowing through the reactor, under conditions which are optimal for oligochaeta in respect of metabolism and reproduction; and ii) adding sludge to the reactor medium, to a device for performing the method, comprising a reactor, means for supplying and discharging reactor medium and sludge, and a carrier suitable for carrying nestling oligochaeta, and to the use of nestling oligochaeta, preferably Aulophorus furcatus, in the reduction of sludge in a reactor.

Description

METHOD FOR REDUCING SLUDGE USING A POPULATION OF OLIGOCHAETA HELD IN A REACTOR, A DEVICE THEREFOR, AND THE USE OF NESTLING OLIGOCHAETA FOR REDUCING SLUDGE IN A REACTOR

The present invention relates to a method and to a device for reducing sludge, and to the use of nestling oligochaeta (in particular aquatic oligochaeta) for use in such a method and device according to the invention. In the biological treatment of domestic and industrial waste there is created sludge, in particular sewage sludge. The excess of sewage sludge, the so-called surplus sludge, must be discharged regularly. This discharge of surplus sludge is accompanied by increasing costs for landfill, wet oxidation or incineration.

In a treatment plant the domestic and industrial waste water is subjected to an aerobic reduction using bacteria. The surplus sludge therefore consists largely of dead and living bacteria cells, collected organic and inorganic particles, as well as protozoa and also higher organisms .

In Environmental Science Technology 2003, 37, 3171_3180 Wei et al describe experiments for reducing sludge using oligochaeta. Use is made of a membrane bioreactor or a conventional active sludge reactor. In the membrane bioreactor the sludge is aerated and reduced in aerobic bacterial manner. Water passes through the membrane and is discharged. It is stated that oligochaeta do not contribute toward a sludge reduction or an improvement in the settling characteristics of the sludge. In the conventional active sludge reactor worm bloom occurs from time to time, whereby the sludge yield in the aerobic bacterial reduction is greatly reduced.

Wei et al, Water Res. 2003 Nov; 37 (18) :4453_67 states that oligochaeta can be used in water treatment plants if the problem of the unstable and non-controllable worm growth is solved.

In Stromen, no. 10, 6 June 2003, Voorter describes that sewage sludge can be reduced by making use of free- swimming worms, reportedly free swimming worms turned out difficult to cultivate. On laboratory scale worms (tubifex) are cultivated in containers which are 4 metres long, 30 cm wide and 30 cm high. Sludge settles and the worms must eventually be removed from the separated sludge by sieving. Further research is deemed necessary to arrive at a pilot plant which can process sufficient sludge rapidly enough.

The present invention has for its object to provide a method and device for reducing sludge in which a population of oligochaeta can be held in controlled and managed manner, whereby for the first time a method is provided with which sludge can be reduced on industrial scale.

This is achieved according to the invention with a method for reducing sludge, comprising steps for: i) holding a population of oligochaeta in a reactor, in a reactor medium flowing through the reactor, under conditions which are optimal for oligochaeta in respect of metabolism and reproduction; and ii) adding sludge to the reactor medium. The sludge which is used in the method according to the invention can comprise all manner of sludge, such as sea sludge, river sludge and sludge from water treatment plants. In this latter case this sludge can be surplus sludge, so- called end of pipe sludge, or sludge which is extracted from the sludge from the settling tank returned to the aeration tank.

Oligochaeta which can be used according to the invention include, among others, Naididae, Aeolosomatidae, Tubificidae (tubifex) . These are worms. A drawback of free- swimming worms is that they are carried along by flowing reactor medium and flushed from the reactor. A population of oligochaeta is a very large number of worms which can live in a reactor, metabolize sludge and reproduce therein. In the method and device according to the invention there is a particular application of a nestling worm, the Aulophorus furcatus which belongs to the family of the Naididae. This is a freshwater-nestling, free-swimming worm which prefers warm places and is tolerant to low oxygen levels and organic contaminants. A particular characteristic of Aulophorus furcatus is that it can nestle in the reactor and under normal conditions is not flushed out with the flowing reactor medium.

In the method and device according to the invention the population of oligochaeta is held under optimal conditions for their metabolism and reproduction. These optimal conditions comprise a relatively high temperature of generally 20 to 30 "C, such as 25°C. The oxygen content amounts generally to between 1 and 6 mg O₂/litre. The sludge concentration must be relatively low and is generally from 0.5 to 4 grams of dry substance per litre, preferably around 1 gram per litre. Furthermore, the population of oligochaeta must not be exposed to high shear forces . In the case of high shear forces the worms display flight behaviour and a risk of undesired exit from the reactor.

According to a preferred embodiment of the method according to the invention, nestling oligochaeta are used for the reduction of sludge, in particular Aulophorus furcatus. In order to provide these nestling oligochaeta with optimum conditions the reactor is provided with a carrier on which the oligochaeta can nestle. Such a carrier preferably has a suspended net structure with for instance a mesh width of 0.2 to 10 cm, in general 0.5 to 2 cm mesh width. Nestling oligochaeta attach to the carrier. As the population grows there forms a tangle of attached oligochaeta and congeners incorporated in the tangle. A tangle contains many thousands of millions of worms per cm², depending upon the species which is washed with flowing reactor medium in which the sludge is situated, and this sludge is reduced. Optimal conditions may require aeration of the reactor medium. According to another embodiment of the method according to the invention, the preferably nestling oligochaeta are held in a reactor containing a thin layer of flowing reactor medium such that air is supplied by means of diffusion. The advantage is achieved here of forgoing means for supplying air, and moreover means for separating worms out of reactor medium leaving the reactor. According to a preferred embodiment, the reactor takes the form of a gutter with a reactor medium layer of 0.1 to 3 cm, preferably 0.2 to 2 cm, more preferably 0.3 to 1 cm. The thickness of the layer of reactor medium is chosen such that by means of diffusion sufficient air enters the reactor medium for the prevailing optimal conditions for the oligochaeta. These dimensions also depend on prevailing temperature and on the speed of the flowing reactor medium. As stated above, it is further recommended that the oligochaeta are nestling oligochaeta such as Aulophorus furcatus . According to another embodiment of the method according to the invention, air is supplied to the reactor medium. In this case use can be made of a higher reactor medium layer. In order to avoid an undesirably great deposition of sludge in the reactor, sludge must in this method be swirled up so that it can be carried along by the reactor medium flowing through the reactor. According to a preferred embodiment, the reactor takes the form of a gutter with a reactor medium layer of 5 to 50 cm, preferably 5 to 30 cm, more preferably 10 to 20 cm. In this case non-nestling oligochaeta can leave the reactor as a result of the prevailing flow, and it is therefore recommended that the oligochaeta are separated out of the reactor medium leaving the reactor and fed back to the reactor. A readily applicable type of oligochaeta is tubifex. Exit from the reactor is minimal in the case of nestling oligochaeta.

It is noted that in order to create a flowing reactor medium it is possible to place the reactor at an incline (for instance 2%) , thereby realizing a sufficiently flowing medium. It is otherwise possible to make use of pumps or air to generate the flow.

Another aspect of the present invention relates to a device for performing the above described method according to the invention. The device comprises a reactor, means for supplying and discharging reactor medium and sludge. In the case that it contains nestling oligochaeta, the reactor is also provided with a carrier suitable for these nestling oligochaeta. The carrier preferably has a net structure which is placed suspended in the reactor. A particular embodiment of this reactor is a gutter in which the reactor medium has a thickness such that sufficient absorption of oxygen is possible by means of diffusion. In another embodiment extra oxygen has to be added by means with which air can be introduced into the reactor medium. Means are then also required in the reactor for swirling up sludge that is settling . The reactor may be a packed biological reactor, in which the colonies of aquatic oligochaeta form the biological active particles which may be adhered to the carrier means, such as nettings. For aeration purposes and in order to maintain flowing reactor medium in the reactor, use can also be made of different aerating means. Use is preferably made of a so- called air-lift or of disc aerators. The air-lift aerator has the advantage of an optimal aeration with minimal shear forces.

Another aspect of the invention is the use of a vortex in separating means for separating worms out of reactor medium. This vortex sets reactor medium into a swirling movement. The worms present display flight behaviour, form into tangles which settle easily under the force of gravity.

Finally, the present invention relates to the use of nestling oligochaeta in the reduction of sludge. This relates particularly to the naturally occurring Aulophorus furcatus . Mentioned and other features of the method and device according to the invention will be further elucidated hereinbelow on the basis of a number of exemplary embodiments, which are only given by way of example without the invention being deemed limited thereto. Reference is herein made to the accompanying drawings in which: figure 1 shows in schematic cross-section a first exemplary embodiment of the device according to the invention; figure 2 shows a perspective view of a carrier for use in the device according to figure 1; figure 3 shows in cut-away view and on a different scale another device for performing the method according to the invention; figure 4 shows a view according to arrow IV in figure 3 of the device of figure 3; and figure 5 shows a flow diagram of yet another device for performing the method according to the invention.

Figure 1 shows a device 1 for reducing sludge according to the invention. Device 1 comprises a reactor 2 having the form of a gutter. The gutter has for instance a length of 10 metres, a width of 60 cm and an incline of 2%. Water 3 and sludge 4 (1 gram dry substance per litre) are added to reactor 2. Reactor medium 5 has a layer thickness of 10 mm and, due to the incline, flows at a speed of about 500 litres/hour over the bottom of reactor 2 in which a carrier material 6 is disposed. This carrier material creates differences in flow (cascade/pool) in the gutter.

As shown in figure 2, carrier material 6 can consist of a concrete mesh 7 arranged on a bottom 8. The mesh width is 5 cm.

The conditions in reactor 1 of figure 1 are such that Aulophorus furcatus adheres well to carrier material 6, is either not flushed out or only to a degree such that through reproduction the population does not decrease. Due to the adhering characteristics it is not necessary to recover worms from the reactor medium leaving reactor 1. If desired, sludge from reactor medium 9 leaving reactor 1 can be fed back to sludge feed 4.

The gutters can be arranged adjacently of each other but also one above another. This results in a space-saving. Stacked gutters can also be accommodated in a cassette profile and enclosed by a container, whereby optimum conditions, and in particular conditions for temperature, can be readily adjusted and maintained.

Figures 3 and 4 show a packed bed reactor 10 according to the invention. Reactor 10 comprises a silo-like tank 11 with a support frame 12 on which are mounted suspended carriers 13 with a net structure. Reactor 10 is intended for reducing sludge using nestling oligochaeta, in particular Aulophorus furcatus . The nestling oligochaeta nestle on carriers 13, particularly in and round the net structure. Flowing reactor medium containing the sludge washes around carriers 13. The flow in the reactor is enhanced by aerators 15 arranged on bottom 14. Heating elements 16 are further suspended in reactor 10 for adjusting the temperature of the reactor medium present in the reactor. The reactor medium and the sludge can be added to reactor 10 in a manner not shown. When bottom aerators 15 are used, the reactor medium provided with sludge flows upward from below along carriers 13. If air-lift aerators are used, the reactor medium provided with sludge is transferred to and flows downward along carriers 13 via a distribution system.

Instead of the use of bottom aerators 15, it is possible to make use of air-lift aeration. During this experiment Aulophorus furcatus nestled in and in between net structures suspended close to one another. In this nestling mass sludge is captured and eventually tubifex nestled in this nestling mass, at places forming large tangled colonies.

Finally, figure 5 shows a device 17 for performing the method according to the invention. In this case reactor 18 takes the form of a gutter with a reactor medium layer therein of 5 to 50 cm, depending on the environmental conditions. The incline is 2%. Gutter 18 has a length of 10 metres, a width of 60 cm. Sludge is supplied from a supply container 19 via conduit 20 in a concentration of about 1 gram dry substance per litre. Aerated and cleaned reactor medium is further supplied via conduit 21 and untreated aerated reactor medium via conduit 22.

The reactor medium leaving reactor 18 via conduit 23 is supplied to a vortex unit 24 in which the reactor medium is subjected to swirling, whereby aeration occurs. The aerated reactor water is fed back via conduit 22. The remaining part of the reactor medium is discharged to a settling tank or separating apparatus 26 via conduit 25. Cleaned water 27 discharged via conduit 27 is partially fed back to reactor 18 via conduit 21, and the remaining part is discharged via conduit 28. In vortex unit 25 oligochaeta can be separated via conduit 29. All or a part of the separated oligochaeta can be fed back to the reactor using reactor medium. Sludge can optionally be discharged via conduit 30. In a manner which is not shown, reactor 18 is also provided with a swirl mechanism for periodically swirling up and transporting the sludge through reactor 18.

Claims

1. Method for reducing sludge, comprising steps for: i) holding a population of in particular aquatic oligochaeta in a reactor, in a reactor medium flowing through the reactor, under conditions which are optimal for oligochaeta in respect of metabolism and reproduction; and ii) adding sludge to the reactor medium.

2. Method as claimed in claim 1, wherein the oligochaeta are nestling oligochaeta, such as Aulophorus furcatus, and are held adhered to a carrier in the reactor.

3. Method as claimed in claim 1, wherein the carrier has a suspended net structure .

4. Method as claimed in claims 1-3, wherein the reactor comprises a thin layer of flowing reactor medium such that air is added by means of diffusion.

5. Method as claimed in claim 4, wherein the reactor takes the form of a gutter with a reactor medium layer of 0.1 to 3 cm, preferably 0.2 to 2 cm, more preferably 0.3 to 1 cm.

6. Method as claimed in claim 4 or 5, wherein the oligochaeta are nestling oligochaeta, such as Aulophorus furcatus.

7. Method as claimed in claim 1, wherein air is added to the reactor medium and sludge settling in the reactor is swirled up into the flowing reactor medium.

8. Method as claimed in claim 7, wherein the reactor takes the form of a gutter with a reactor medium layer of 5 to 50 cm, preferably 5 to 30 cm, more preferably 10 to 20 cm.

9. Method as claimed in claim 7 or 8 , wherein oligochaeta are separated out of the reactor medium leaving the reactor and fed back to the reactor.

10. Method as claimed in claims 7-9, wherein the oligochaeta are not free-swimming, such as tubifex.

11. Device for performing the method as claimed in claims 1-10, comprising a reactor, means for supplying and discharging reactor medium and sludge, and a carrier suitable for carrying nestling oligochaeta.

12. Device as claimed in claim 11, wherein the carrier has a net structure.

13. Device as claimed in claim 11 or 12, wherein the reactor takes the form of a gutter with a reactor medium layer of 0.1 to 3 cm, preferably 0.2 to 2 cm, more preferably 0.3 to 1 cm.

14. Device as claimed in claim 11 or 12, wherein the reactor is a packed bed reactor of carriers.

15. Device for performing the method as claimed in claims 7-10, comprising a reactor, means for supplying and discharging reactor medium and sludge, means for adding oxygen to the reactor medium, means for swirling up sludge that is settling, wherein the reactor takes the form of a gutter with a reactor medium layer of 5 to 50 cm, preferably 5 to 30 cm, more preferably 10 to 20 cm.

16. Device as claimed in claims 11-15, which is provided with separating means for separating oligochaeta from reactor medium, which separating means take the form of a vortex separator.

17. Use of nestling oligochaeta, preferably Aulophorus furcatus, in the reduction of sludge in a reactor.