The present invention relates to a method for reducing sludge, a sedimentation tank suitable for

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- 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,

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 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 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 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.

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 .

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 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 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.

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 holding anoxic sludge in a container, in particular a sedimentation tank, wherein the anoxic sludge comprises oligochaete-treated sludge. Additionally or alternatively, a method is provided for reducing sludge comprising 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 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, particularly oligochaete-material which comprises

oligochaete-treated sludge, a reduction of more than 50% in the amount of sludge could be achieved. Furthermore, it was found that when sludge was fed to oligochaeta and the oligochaete-treated sludge was held as anoxic sludge in a container, an overall reduction of almost 70% in the amount of sludge could be 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 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 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 include primary sludge. In this embodiment, wherein

combinations of sludge are held in the container, it is particular preferred that the contents of the container comprise at least oligochaete-treated sludge, preferably in combination with secondary sludge but also any of the other types of sludge mentioned herein.

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

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 obtained from oxygen-treated sewage or wastewater. Active sludge particles 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 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 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,

(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 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

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 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 releases an increased amount of previously inaccessible sludge, or material or compounds comprised by the 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, partial digestion of sludge streams and/or reduction of particle size of sludge due to the passing of sludge particles through the gastrointestinal tract of these worms likely adds to the improved sludge reduction in the container.

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

containment thereof in the sedimentation tank. It is also conceived possible that the oligochaetes that grow on the digesting of the sludge, or parts of these oligochates, and also the contents of their gastrointestinal track, decompose in the sedimentation tank thereby further contributing, e.g. in a 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 amounts to between 150-450 m ³ . Fermentation takes place naturally but is facilitated by the increased availability of the organic fraction sludge facilitated by the

oligochaetes .

d) Anoxic oxidation of solids by denitrification, i.e. the mineralisation of solids by using nitrite or nitrate as electron acceptor. Consequently, CO ₂ , 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 reactions and/or other such biochemical reactions. Such reactions lead to reduction of sludge as the reaction products comprise soluble minerals or gaseous (N ₂ ) products or H ₂ O.

g) Presence of bacteria which are able to use 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 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 to improved or prolonged sludge reduction. For example, release of partially digested sludge would lead to increased accessibility of dry matter for subsequent conversion processes which in turn would allow more efficient

denitrification, fermentation, leaching and the like, ultimately leading to increased sludge reduction.

Results presented herein show that holding of oligochaete-treated sludge under anoxic conditions allows anaerobic digestion to efficiently proceed under unique conditions comprising, high enzyme levels, including high lipase and protease levels in combination with small floe sizes, which so far have not been reported nor investigated. It is likely that this unique combination caused by

predation by aquatic oligochaetes followed by anaerobic digestion under anoxic conditions allows a biodegradation of sludge to an extent which so far had not been reported under the prevailing conditions of ambient temperatures and duration .

Irrespective of the precise underlying mechanisms, this is the first disclosure that holding sludge in a container at ambient temperature and contacting the sludge, in particular secondary sludge, with oligochaete material, in particular aquatic oligochaetes and more particular with oligochate-treated sludge, provides a reduction of about 50- 70% dry matter, whereas in conventional systems secondary sludge digested under mesophilic conditions degrades for about 30%. It is currently believed that 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 allow digestion to proceed in a psychrophilic and/or mesophylic manner as the sludge is held at ambient temperature. Alternatively, the temperature of the sludge may be controlled such that it is kept at a temperature which lies between 4 to 45 ⁰ C, preferably between 4 to 36 ⁰ C, more preferably 10 to 28 ⁰ C, most preferably 10 to 2O ⁰ C. The container comprises between 5 to 120 g/1 dry matter,

preferably between 15 to 50, or 15 to 30 g/1 dry matter. The retention time of the sludge contained in the container is between 20 to 150 days, preferably 40 to 150 days, more preferably between 60 to 90 days. Obviously, longer

retention times, such as up to 200 days or even longer, would be possible but the efficiency of the present

invention does not require longer retention times.

As is understood by the skilled person, shorter retention times suffice at higher temperatures for the same level of degradation or reduction of dry matter as obtained at lower temperatures.

Furthermore, the content, i.e. sludge, of the container is thus allowed to settle and not aerated, meaning the sludge is in essence anoxic. Hence, it is understood by the skilled person that the sludge comprised by the

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 oxygen is however present in the sludge, for example from nitrate, which is a byproducte from the predation by the ologochaetes, and 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 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 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 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 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.

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

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 for such material or for the predation of sludge preferably comprise aquatic oligochaetes, in particular nestling aquatic oligochates, such as species from the Naididae family, such as Aulophorus furcatus, but also other species from other families such as the Aeolosomatidae,

Lumbriculidae, Tubificidae, free swimming or sessile aquatic oligochaetes, but also non-aquatic oligochaetes from the Lumbricidea 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 used, this is meant in the context of the oligochaete material and oligochaete species as mentioned according to the invention. In particular, aquatic sessile or nestling oligochaetes are preferred according to the present invention for the provision of oligochaete-treated sludge.

Additionally or alternatively according to yet another preferred embodiment of the invention, sludge is 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 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 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 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 .

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 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

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 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 calculated by dividing the volume of aeration tank by the influent flow rate, wherein volume is in m and influent flow rate is in m ³ /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 invention wherein an aqueous medium is used comprise water or preferably sludge, or a combination thereof.

In a preferred embodiment of the invention, the sludge retention time divided by the hydraulic retention time is between 2 to 70 preferably between 4 to 50, most preferably between 15 to 35. In more conventional anaerobic digestion systems, this ratio is close to or equals 1, meaning such systems are not flushed as is presently done.

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 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 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.

Another advantage of adding an aqueous medium to the container is that it flushes out any inhibiting

material, compounds or chemicals resulting from the

digestion process thereby sustaining optimal conditions for further digestion and degradation

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 invention is primary sludge. The sludge according to this embodiment comprises between 0.1 to 8.0, preferably 0.1 to 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 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 water to be discharged is clear and merely needs removal of ammonium, phosphate and/or nitrogen, before disposal to the environment.

In another preferred embodiment, the oligochaete- treated sludge is first gravitationally thickened before being added to the container. Dewatering using mechanical means such as a membrane, centrifuge, drum or screw drum or thickening table is also possible. An advantage of first dewatering is that the container, or sedimentation tank, can be smaller and would still provide enhanced sludge reduction as found.

An additional advantage of adding oligochaete- treated sludge is that oligochaete-material as herein 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 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 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 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 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 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 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 mechanical stirring means, such as a mixer, agitator, shaker, stirrer, such as a magnetic or mechanical stirrer, or stirring using gasses which allows gentle mixing or blending of the content of the container thereby increasing contact between active components and convertible matter, increasing 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, 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 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.

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.

According to another embodiment of the invention, the added sludge comprises between 0.1 to 8, preferably 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 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 to 50, preferably 20 to 80 g/1.

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 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 as described which is suitable for holding sludge, in particular anoxic oligochaete-treated sludge.

In a preferred embodiment, the container further comprises sludge selected from the group consisting of primary sludge, secondary sludge, digested sludge, sludge derived from wastewater treatment or a combination thereof.

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 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 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 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 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 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 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 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 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 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 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. 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 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 m /h to a sedimentation tank with a capacity of 900 m ³ . The sludge is gravitationally dewatered in the sedimentation tank wherein the sludge is allowed to settle 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 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 m /h and the tank is 900 m . The

oligochaete-treated sludge is added to the sedimentation 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 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 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 ⁰ C. The sludge comprised by the sedimentation tank is 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 tonnes of sludge (900 m ³ 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 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.

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 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 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) .

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 60-70%. Sludge reduction herein is measured as the amount of recovered sludge (in kg dry matter) as a function of the total amount of added sludge.

Table 1: Sludge input and output of sedimentation tank comprising anoxic sludge. The first column indicates the number of days the sludge was contained in the

sedimentation 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 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 from 7 non-overlapping periods of time.

Example 3

Sludge Volume Index (SVI) of oligochaete-treated and untreated sludge was measured by allowing sludge to settle in a 1 litre graduated cylinder for 30 minutes and dividing the volume of settled sludge by the concentration of total solids of the sludge.

Results show that the average SVI of sludge predated by at least Aulophorus furcatus is significantly lower with 118.5 ± 15.9 ml/g total solids compared to 194.6 ± 73.9 ml/g for untreated sludge. Besides that these results show that the structure of the oligochaete-treated sludge is significantly altered, oligochaete-treated sludge is stabilized compared to untreated sludge as can be derived from the lower standard deviation of the treated sample. Example 4

Subsequently, sludge floc-size of about a 1000 floes of oligochaete-treated and untreated sludge was determined using microscopic analyses.

Whereas average floe area and average floe radius of untreated sludge amount to 16.500 ± 3300 μm ² and 72 ± 7.2 μm, respectively, average floe area and average floe radius of oligochaete-treated sludge are 4200 ± 350 μm ² and 37 ± 1.5, respectively.

Example 5

Chemical Oxygen Demand (COD) was measured using

Dr. Lange test kit LCK614, in accordance to the provided instructions. Untreated and oligochaete-treated sludge were allowed to settle for at least 30 minutes. A 2 ml sample was taken from the water phase of the settled sludge.

COD from treated sludge was measured at 145.7 ±

16.7 mg/L which is significantly increased compared to untreated sludge which was measured at 66.5 ± 10.8 mg/L.

These results indicate that oligochaetes cause a structural modification of the sludge by their feeding behaviour which can be further exploited by the present invention .