NL2003191C2

NL2003191C2 - Method for reducing sludge.

Info

Publication number: NL2003191C2
Application number: NL2003191A
Authority: NL
Inventors: Gerrit Antoni Schouwenburg
Original assignee: Elsdon B V
Priority date: 2009-07-13
Filing date: 2009-07-13
Publication date: 2011-01-17
Also published as: EP2453737A1; WO2011008084A1

Description

Method for reducing sludge

The present invention relates to a method for reducing sludge, a sedimentation tank suitable for 5 performing the method and the use thereof.

Sludge is a solid or semi-solid residue left after processing of domestic and/or industrial waste comprised by wastewater, in particular sewage. Such processing can for example be performed in a sewage plant or wastewater-10 purification plant. Contemporary processing of wastewater, such as sewage, can comprise several treatments of biological, mechanical and/or chemical nature. Such biological treatment can include aerobic and/or anaerobic conversion processes, digestion, decomposition, 15 fermentation, or the like, whereas mechanical or chemical treatments include purification steps, extraction of substances, filtering, thickening, dewatering, drying, or the like.

One of the main purposes of the biological 20 treatment steps of wastewater or sewage is to reduce the amount of residue for disposal by progressively converting organic matter comprised by the wastewater or sewage into a solid or semi-solid mass (i.e. sludge). Different compounds are obtained or produced during the conversion processes 25 such as water, minerals, nutrients and/or biogas. The sludge as finally obtained from the subjected wastewater or sewage comprises of in essence decomposed or digested organic matter, inorganic matter and water.

This sludge as finally obtained needs to be 30 disposed of in a safe and effective manner for which some possibilities are available. Depending on the quality of the sludge and prevailing legislation, disposal of sludge can be by incineration, land filling, or use as soil conditioner.

2

Besides that sludge is obtained from wastewater treatment, an effluent stream of water is produced which, when sufficiently clean, can be discharged to the environment.

5 Biological treatment of sludge using oligochaete worms has been described previously. However, such methods are expensive when considering the costs per kilogram sludge treated. Such high expenses are caused by, for example, the requirement to use expensive reactors, the need for input of 10 significant amounts of heat to provide suitable conditions to sustain populations of oligochaete worms, the need for aeration during sludge treatment, or combinations thereof.

Besides the economic requirement to reduce costs for purifying wastewater or sewage, the need for more 15 efficient and effective methods of reducing sludge is becoming an important issue as well. As prices for disposing sludge are high and may be expected to increase, the need to develop more effective methods by which it is possible to increase reduction of sludge more and more present.

20 The present invention has for its object the provision of a solution to the above-mentioned problems by a method and device for reducing sludge.

This goal is achieved according to the invention by the provision of a method for reducing sludge, comprising 25 holding anoxic sludge in a container, in particular a sedimentation tank, and contacting the anoxic sludge with oligochaete material.

An advantage of the present invention is that a low-cost, labour-extensive, effective method of reducing 30 sludge is provided. During the research that led to the invention it was found that when anoxic sludge was held in a container and was contacted with oligochaete material, a reduction of up to 70% in the amount of sludge could be 3 achieved. This is the first time such high reduction of dry matter content of sludge is disclosed in this field of research. The fact that such low quantities of sludge remain after contacting sludge with oligochaete material according 5 to the invention, makes the present invention particularly effective and cost-efficient. A direct consequence of the effective reduction of sludge is that the amount of sludge which needs to be disposed of is reduced which is an important factor which contributes to the economical benefit 10 of the present invention.

The sludge which is held in the container can comprise of sludge derived from wastewater treatment, secondary sludge, digested sludge, oligochaete-treated sludge, or a combination thereof. It is also intended to 15 include primary sludge.

The terms primary sludge, digested sludge and secondary sludge herein are all used in its art-recognized meaning. Primary sludge herein comprises the settled fraction obtained after primary sedimentation of raw 20 domestic sewage or industrial wastewater before being subjected to further processing steps. The term secondary sludge, also known as activated sludge or surplus sludge, herein comprises sludge which is obtained after treatment of primary sludge with biological floe. Active sludge particles 25 are largely composed of saprotrophic bacteria, and may include protozoan flora, such as amoebae, Spirotrichs, Peritrichs including Vorticellids, and a range of other filter feeding species and/or motile and sedentary Rotifers. Digested sludge is sludge of which the for fermentation 30 accessible content has in essence been completely fermented. Such sludge is for example derived from primary or secondary sludge which is subsequently digested.

It is currently hypothesized that the surprising 4 high efficiency of sludge reduction as obtained by the present invention is based on improved release of compounds comprised by sludge particles, subsequent increased exposure or accessibility of such compounds to biological, 5 (bio) chemical and/or physical degradation processes which allows significant improvement and prolonged sludge reduction in a container. In this model, the reduction of sludge in the container results from a combination of effective micro-organism activities, prolonged effects 10 induced by oligochaete activities, increased enzymatic activities, release of bacterial content resulting from oligochaete feeding on those bacteria, increased physical decomposition of sludge particles which allows further degradation thereof and/or occurrence of particular 15 biochemical reactions in the sedimentation tank or from compounds formed as a result of digestion or partial digestion of sludge by oligochaetes.

The demonstrated effect of sludge reduction by holding sludge in a container and contacting sludge with 20 aquatic oligochaete material can be a result of the following underlying combination of processes which are hypothesized to allow improved sludge reduction in the container.

a) Pre-treatment of sludge by oligochaete worms 25 releases an increased amount of previously inaccessible sludge. Subsequent containing in and/or adding to a container of this oligochaete-treated sludge allows prolonged oligochaete-induced sludge treatment in the tank and subsequent sludge reduction. For example, dry matter 30 content of sludge streams or size of sludge particle may be reduced due to the passing of sludge particles through the gastrointestinal tract of these worms adding to the improved sludge reduction in the container.

5 b) During sludge pre-treatment, bacteria or enzymes may be excreted by the oligochaetes which subsequently end up in the sedimentation tank. Such bacteria or enzymes may further reduce the sludge during the 5 containment thereof in the sedimentation tank. It is also conceived possible that the from oligochaete-treated sludge originating oligochaetes or parts thereof, but also the contents of their gastrointestinal track, decompose in the sedimentation tank thereby further contributing, e.g. in a 10 synergistic manner, to the reduction process which takes place in the container.

c) Increased fermentation of organic matter leading to methane and carbon dioxide production. The amount of methane as formed per tonne sludge organic dry matter 15 amounts to between 150-450 m3. Fermentation takes place naturally but is facilitated by the increased release of sludge .

d) Anoxic oxidation of solids by denitrification, i.e. the mineralisation of solids by using nitrite or 20 nitrate as electron acceptor or DNRA. Consequently, CO2, ammonium and nitrogen gas are produced.

e) Leaching, i.e. dissolving of inorganic salts leading to reduction of dry matter.

f) Other biochemical reactions, e.g. Sharon 25 reactions and/or other such biochemical reactions. Such reactions lead to reduction of sludge as the reaction products comprise soluble minerals or gaseous (N2) products or H2O.

g) Presence of bacteria which are able to use 30 nitrite and/or nitrate as a source of energy which further aids in the reduction of sludge.

By analysing and reconstructing the presently disclosed invention, it becomes clear that pre-treatment of 6 sludge with oligochaete material, in combination with microbial activity originating from the sludge contained in the container, and the (bio)chemical and physical processes that occur herein, a synergistic effect is obtained leading 5 to improved or prolonged sludge reduction. For example, release of sludge would lead to increased accessibility of dry matter for subseguent conversion processes which in turn would allow more efficient denitrification, fermentation, leaching and the like, ultimately leading to increased 10 sludge reduction.

Irrespective of the precise underlying mechanisms, this is the first disclosure that holding of sludge in a container and contacting the sludge with oligochaete material, in particular aquatic oligochaetes, provides a 15 reduction of about 70% dry matter or more. Such an effect has not been disclosed previously, nor these measures to achieve this effect.

The conditions under which the sludge is held in the container comprise temperatures, such as between 4 to 20 45°C, more preferably between 4 to 36°C, most preferably between 20 to 28°C. The container comprises between 5 to 120 g/1 dry matter, preferably between 15 to 50 g/1 dry matter. The retention time of the sludge contained in the container is between 40 to 150 days, preferably between 60 to 90 days. 25 Obviously, longer retention times would be possible but the efficiency of the present invention does not require longer retention times. Furthermore, the content, i.e. sludge, of the container is in essence anoxic. Hence, it is understood by the skilled person that the sludge comprised by the 30 container is "anoxic sludge". A consequence of the contents of the container being anoxic is that the conditions for oligochaete worms, aquatic oligochaetes in particular, are such that life of these worms cannot be sustained. Bound 7 oxygen is however present in the sludge, for example from nitrate, which microbes can use as electron acceptor for mineralization of solids. From the above it follows that the conditions in the container are adverse for oligochaete 5 worms in such a manner that growth and development of such worms, or development of a population of such worms, is not a likely possibility.

A container according to the invention can be any sedimentation tank, storage device, pond or lagoon that be 10 used to hold, keep or contain and/or collect sludge to allow the sludge to settle resulting in the formation of a sediment. Such devices are for example used in a sewage plant or wastewater-purification plant. Consequently, the term "container" herein is meant to include any suitable 15 device for holding sludge such as a sedimentation tank, settling tank, sludge tank, digester, storage tank or other holding tank but also any suitable lagoon or pond.

According to a preferred embodiment of the invention, oligochaete material is added to the sludge 20 holding container. An advantage of addition of oligochaete material to the container is that the enhanced reduction of sludge as obtained can be further supported, prolonged or improved to allow more effective sludge reduction according to the invention to take place.

25 The oligochaete material according to another preferred embodiment of the invention can comprise intact oligochaetes, severed oligochaetes, debris from oligochaetes, oligochaete extract, excrements of oligochaetes, parts of oligochaetes or a combination 30 thereof. An advantage of using such oligochaete material is that enzymes and/or bacteria capable of converting or reducing sludge that are comprised by the worms are also added to the container. Species that can be used as a source 8 for such material preferably comprise aquatic oligochaetes, such as species from the Naididae family, such as Aulophorus furcatus, but also other species from other families such as the Aeolosomatidae, Tubificidae, free swimming or sessile 5 aquatic oligochaetes, but also non-aquatic oligochaetes from the Lumbriculidae and Lubricidea family such as earthworms, terrestrial worms or any other suitable oligochaete worm that can be used to reduce sludge is intended useful. Wherever in the text the term "oligochaete material" is 10 used, this is meant in the context of the oligochaete material and oligochaete species as mentioned according to the invention.

Additionally or alternatively according to yet another preferred embodiment of the invention, sludge is 15 added to the sludge holding container. Such sludge added to the container comprises of primary sludge, secondary sludge, digested sludge, oligochaete-treated sludge, sludge derived from wastewater treatment or a combination thereof.

Oligochaete-treated sludge as used herein is 20 intended to comprise sludge derived from wastewater treatment, secondary sludge, digested sludge or a combination thereof, which has been treated in a conditioned environment with oligochaetes as meant herein in such a manner that these worms were allowed to reduce the sludge by 25 feeding thereon. One such treatment of sludge with oligochaetes, in this case aquatic oligochaetes, which can be used to obtain oligochaete-treated sludge is comprised by the invention as disclosed in PCT/NL2006/000076. However, besides the particular aquatic oligochaetes used according 30 to the invention disclosed therein, any oligochaete worm that can be used to reduce sludge is intended useful for reducing sludge in a container according to the present invention.

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An advantage of adding, or containing, oligochaete-treated sludge is that oligochaetes are first allowed to treat the sludge by passage of sludge through the digestive tract of these worms. Such treatment likely 5 improves accessibility for further biological degradation of organic matter or release of inorganic matter contained in sludge particles which contributes to the reduction of sludge. Also any combination of oligochaete-treated sludge with primary or secondary sludge is preferred as 10 oligochaete-treated sludge contains oligochaete material which can contribute to subsequent reduction of sludge held in the container.

In another embodiment of the invention, an aqueous medium is added to the sludge containing container in an 15 amount such that water comprised by the aqueous medium has a hydraulic retention time of between 5 to 500 hours, preferably 10 to 96 hours. The hydraulic retention time herein is a measure of the average length of time that water remains in the container. Hydraulic retention time is 20 calculated by dividing the volume of aeration tank by the influent flow rate, wherein volume is in m3 and influent flow rate is in m3/h. The aqueous medium could be added in a batch-wise manner or preferably continuously such that a continuous flow of water is generated. Embodiments of the 25 invention wherein an aqueous medium is used comprise water or preferably sludge, or a combination thereof.

An advantage of adding an aqueous medium to the container is that a flow of water is created through the container as a result of which slow release of inorganic 30 material such as salts, i.e. leaching, is promoted thereby reducing the amount of dry matter.

Another advantage of adding an aqueous medium to the container is that gentle movement of the content of the 10 container is caused which likely contributes to a better distribution of the content of the container and/or better distribution of added oligochaete material through the sludge in the container.

5 In a preferred embodiment of the invention, the sludge which is added as the aqueous medium comprises secondary sludge, digested sludge, oligochaete-treated sludge, sludge derived from wastewater treatment or a combination thereof. Also intended useful for practicing the 10 invention is primary sludge. The sludge according to this embodiment comprises between 0.1 and 5.0 g/1 dry matter.

An advantage of adding sludge as an aqueous medium is that particles contained by the sludge are retained in the container by filtering. Such filtering is caused by 15 large aggregates of dry matter, such as a blanket of dry matter which is floating in the container or submerged aggregated dry matter in the container, through which the sludge flows. Particles comprised by the sludge are consequently retained in the container by filtering. The 20 water to be discharged is clear and merely needs removal of ammonium, phosphate and/or nitrogen, before disposal to the environment.

An additional advantage of adding oligochaete-treated sludge is that oligochaete-material as herein 25 described is added to the container allowing suitable sludge reduction.

In yet another preferred embodiment of the invention, the aqueous medium is added to the container at any depth lower than 70%, preferably lower than 80%, more 30 preferably lower than 90% of the average depth of the container. The addition of aqueous medium to the container preferably occurs at such a depth that the flow of water through the container extends over a distance stretching 11 across the container, preferably over the diagonal distance of the container. Also, filtering through aggregates of dry matter, such as a blanket of dry matter which is floating in the container or submerged aggregated dry matter in the 5 container, is enhanced by discharging the sludge at a sufficient depth as disclosed.

It will be clear to the skilled person that in order to allow such preferred flow of water through the container, an inlet for allowing addition of the aqueous 10 medium to the tank and an outlet for allowing discharge of the effluent water stream should be positioned such that the flow of water occurs through in essence the entire or most of the container.

It will also be clear to the skilled person that 15 discharging of water from the container occurs preferably by overflowing of the tank. Consequently, the inlet for filling of the tank is preferably positioned in close proximity of the bottom and/or a side of the container according to the invention. Other suitable embodiments are also possible 20 wherein the water flows through the container over a shorter distance .

In another embodiment of the present invention, the content of the container is gently stirred using a stirring means. Such a stirring means can comprise a 25 mechanical stirring means, such as a mixer, agitator, shaker, stirrer, such as a magnetic or mechanical stirrer, airlift which allows gentle mixing or blending of the content of the container thereby increasing contact between active components and convertible matter, increasing 30 diffusion and/or reducing aggregate formation such that sludge reduction is increased, for example by more efficient conversion of organic matter and/or leaching of salts. The sludge in the container is also brought into motion, 12 although the effect is subtle, as a result of escape of gas bubbles formed by fermentation, occurrence of exothermic reactions, active transport and the like. Additional stirring of the container can lead to a more even 5 distribution of the content of the container, better distribution of added oligochaete material through the sludge in the container and/or improved settling of sludge. This prevents or diminishes formation of channels through which aqueous media comprising sludge particles flow.

10 Reduction of such channels forces newly added aqueous media comprising sludge particles through sludge thereby acting as filter. It will be clear to the skilled person that stirring of the content of the container should be such that dewatering of the sludge is not compromised.

15 According to another embodiment of the invention, the added sludge comprises between 0.1 to 5.0 g/1 dry matter. Such sludge comprises sludge derived from wastewater treatment, secondary sludge, digested sludge, oligochaete-treated sludge, or a combination thereof. Also intended 20 useful for practicing the invention is primary sludge.

In yet another embodiment of the invention, sludge contained in the container is mechanically dewatered. Mechanical dewatering has the advantage that sludge can be obtained with higher dry matter content than 20-50 g/1.

25 It is further contemplated that combining measures and/or more efficiently applying particular measures such as increasing the stirring of the content of the container, or adding more bacteria, enzymes and/or oligochaete material, or more efficiently releasing the content of bacteria 30 through the action of oligochaete worms, a further increase in the reduction of sludge to 85 or 90% is possible.

Another aspect of the present invention relates to a sedimentation tank or container for performing the method 13 as described suitable for holding sludge. In a preferred embodiment, the sedimentation tank comprises an inlet for filling of the sedimentation tank with sludge. Also preferred is that the sedimentation tank comprises an outlet 5 or discharging means for discharging an effluent water stream out of the sedimentation tank.

The inlet is positioned at a depth lower than 70%, preferably lower than 80%, more preferably lower than 90% of the average depth of the sedimentation tank. The outlet 10 preferably comprises a vertically positioned outlet in close proximity to a side of the tank, along in essence the entire side or along most of the depth of the sedimentation tank. The vertical outlet is preferably provided over in essence its entirety with a plurality of independently operable 15 outlets. This configuration allows discharge of the effluent water stream along in essence the entire depth of the sedimentation tank. Preferably, the effluent water is discharged by overflow of the tank or through any of the plurality of independently operable outlets positioned along 20 the depth of the sedimentation tank. When the inlet is positioned in close proximity to the bottom of the tank, an in essence diagonal flow of water is created in the sedimentation tank.

The content of the sedimentation tank can be 25 further agitated, blended, mixed or stirred when the tank is provided with a stirring means.

In a more preferred embodiment, the inlet is combined with a stirring means such that the sludge and/or aqueous medium entering the sedimentation tank causes a slow 30 distribution of newly added material and the content of the tank. For example, a stirring means which scrapes the bottom of the tank with vertical bars, such as in a thickening tank, wherein the scraper comprises one or more outlets for 14 allowing material to enter the tank. While the scraper slowly moves in a circular motion in close proximity to the bottom of the tank, sludge is then allowed to enter the tank. The vertical bars gently disturb the sludge while the 5 scraper moves around, cause of gentle stirring of the content of the tank and simultaneously allow better dewatering of the sludge.

Finally, the invention relates to the use of a sedimentation tank for reduction of sludge comprising 10 containing sludge in a sedimentation tank and contacting the sludge with oligochaete material.

In a preferred embodiment, the sludge is primary sludge, secondary sludge, digested sludge, oligochaete-treated sludge, sludge derived from wastewater treatment or 15 a combination thereof.

The invention is further illustrated by the following examples which are in no way meant to restrict the invention in any manner.

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Example 1 A continuous effluent stream of oligochaete-treated sludge, derived from a reactor to which secondary sludge was added and which comprised a population of nestling aquatic 25 oligochaetes (Aulophorus furcatus) , with a dry matter content of between 0.1 and 3.0 g/1 was added with a rate of between 10-30 m3/h to a sedimentation tank with a capacity of 900 m . The sludge is gravitationally dewatered in the sedimentation tank wherein the sludge is allowed to settle 30 by way of which the dry matter content is raised to about 30g/l. The effluent stream of water is clear and of sufficient quality for further treatment, i.e. removal of 15 dissolved nitrogen and phosphate in an aeration tank or the like. The hydraulic retention time is about 3.5 days as the average flow is 11 m3/h and the tank is 900 m3) . The oligochaete-treated sludge is added to the sedimentation 5 tank in close proximity to the bottom thereof, at a depth of 2.8 meter of the total depth of 3 meter.

In the sedimentation tank a cover of sludge is formed which acts as filter to filter out sediment from newly added sludge. This cover, or blanket, is continuously 10 flushed with water flowing from the inlet of the tank to the surface after which the water subsequently overflows out of the tank creating an effluent stream. The sedimentation tank also forms submerged larger aggregates of sludge particles and there among flowing water. Consequently, the process of 15 filtering and flushing is expected to be amenable for further optimization. The sludge retention time is up to 135 days for the sludge which was first added. The sedimentation tank was placed under ambient temperatures of between 4 to 25°C. The sludge comprised by the sedimentation tank is 20 essentially anoxic.

It was found that approximately 20-25 tonnes dry matter of sludge was recovered from the sedimentation tank whereas a total amount of 50 tonnes dry matter was added.

The theoretical capacity of the sedimentation tank is 27 25 tonnes of sludge (900 m3 and 30g/l). Apparently, very effective and extended decomposition of sludge takes place in the tank. Table 1 provides a more detailed overview of the results.

The ash-content of the recovered sludge was 30 slightly higher than the ash-content of the sludge which was added to the sedimentation tank but less than expected. The amount of ashes was around 40% in the sedimentation tank compared to around 30% at the start of the process.

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Surprisingly, the reduction of sludge as demonstrated herein is very effective with reduction of sludge of 40 and up to about 70%. Sludge reduction herein is measured as the amount of recovered sludge (in kg dry 5 matter) as a function of the total amount of added sludge.

Example 2.

The experimental set-up and conditions for reducing sludge were similar to the conditions as mentioned in example 1 with the exception that the sludge added to the 10 sedimentation tank was a mixture of oligochaete-treated sludge and wastewater-derived or secondary sludge. The percentage wastewater-derived or secondary sludge was between about 13 and about 50 percent by weight of the total amount of sludge (cf table 1).

15 Also the reduction of sludge by adding oligochaete-treated sludge and wastewater-derived or secondary sludge is very effective with sludge reduction percentages of up to about 70%. Sludge reduction herein is measured as the amount of recovered sludge (in kg dry 20 matter) as a function of the total amount of added sludge.

___Sedimentation tank __

Period IN from IN Sludge (days) worm reactor direct IN total OUT decrease__%_ _84__44946___44946 14742 30204 67% _65__23590___23590 16356 7234 31% _70__29845___29845 20832__9013 30% 137__48135 7775 55910 23256 32654 58% _75__18750 14776 33526 16207 17319 52% _90__31933 13455 45388 22680 22708 50% 122 27254 21299 48552 18202 30350 63%

Table 1: Sludge input and output of sedimentation tank comprising anoxic sludge. The first column indicates the 25 number of days the sludge was contained in the sedimentation 17 tank. The second column indicates the amount of oligochaete-treated sludge in kg which is added from the oligochaete-comprising reactor. The third column indicates the amount of sludge derived from wastewater treatment in kg without prior 5 oligochaete-treatment. The fourth and fifth columns indicate the total amount of sludge in kg added and retrieve from the sedimentation tank respectively. The sixth and seventh columns indicate the decrease of the amount sludge in kg and as percentage by weight respectively. Results were obtained 10 from 7 non-overlapping periods of time.

Claims

A method for reducing sludge, comprising keeping anoxic sludge in a container, in particular a sedimentation tank, and bringing the anoxic sludge into contact with material from oligochaetes.

The method of claim 1, wherein oligochaete material is added to the container.

The method according to claim 1 or 2, wherein the oligochaetes material comprises intact oligochaetes, damaged oligochaetes, oligochaetes remains, oligochaetes extract, oligochaetes secretions, parts of oligochaetes or a combination thereof.

The method of any one of claims 1-3, wherein sludge is added to the container.

5. Method as claimed in any of the claims 1-4, wherein an aqueous medium is added to the container in such an amount that the water comprising the aqueous medium has a hydraulic retention time of between 5-500 hours, preferably 10-96 hours .

The method of claim 5, wherein the aqueous medium comprises sludge or water or a combination thereof.

A method according to claim 5 or 6, wherein the sludge is primary sludge, secondary sludge, digested sludge, oligochaeten-treated sludge, sludge from waste water treatment or a combination thereof.

A method according to any of claims 5-7, wherein the aqueous medium is delivered to the sedimentation tank at a depth lower than 70%, preferably lower than 80%, more preferably lower than 90% of the average depth of the sedimentation tank .

A method according to any one of claims 1-8, wherein the contents of the sedimentation tank are gently stirred by means of a stirrer.

The method according to any of claims 1-9, 5, wherein the added sludge comprises between 0.1 - 5.0 g / l dry matter.

11. Method according to any of claims 1-10, wherein the added sludge and / or sludge in the sedimentation tank is primary sludge, secondary sludge, digested sludge, oligochaeten-treated sludge, sludge from waste water treatment or a combination thereof .

12. A method according to any one of claims 1-11, wherein the oligochaetes are aquatic oligochaetes, in particular aquatic nesting oligochaetes, such as species selected from the group consisting of species from the Naididae family, such as Aulophorus furcatus and / or species from the Aeolosomatidae family; and / or ground oligochaetes, in particular Lumbriculidae and / or Lubricidea.

A method according to any one of claims 1-12, wherein the container comprises between 5 - 120 g / l dry matter, preferably between 15 - 50 g / l dry matter.

14. Method as claimed in any of the claims 1-13, wherein the sludge in the container has a temperature of between 4 - 45 ° C, preferably between 4 - 36 ° C, most preferably between 20 - 28 ° C.

A method according to any one of claims 1-14, wherein the retention time of the sludge in the container is 40 - 150 days, preferably 60 - 90 days.

A sedimentation tank for carrying out the method according to any of claims 1-15, suitable for holding anoxic sludge, wherein the sludge is primary sludge, secondary sludge, digested sludge, oligochaeten-treated sludge, sludge from wastewater treatment or combination thereof.

17. Sedimentation tank according to claim 16, comprising a feed for filling the sedimentation tank with sludge.

A sedimentation tank according to claim 16 or 17, comprising a drain for draining water.

A sedimentation tank according to any one of claims 16 to 18, comprising a stirring device.

A sedimentation tank according to any one of claims 16-19, wherein the feed is placed at a depth lower than 70%, preferably lower than 80%, more preferably lower than 90% of the average depth of the sedimentation tank.

Use of a sedimentation tank for reducing sludge comprising maintaining anoxic sludge in a sedimentation tank and bringing the sludge into contact with material from oligochaetes.

22. Use according to claim 21, wherein the sludge is primary sludge, secondary sludge, digested sludge, oligochaeten-treated sludge, sludge from waste water treatment or a combination thereof.