WO2010137971A1

WO2010137971A1 - System and method for treating an aqueous waste stream

Info

Publication number: WO2010137971A1
Application number: PCT/NL2010/050292
Authority: WO
Inventors: Tim Lucas George Hendrickx; Hellen Johannes Hubertina Elissen; Bernard Gerhard Temmink; Cees Jan Nico Buisman
Original assignee: STICHTING WETSUS CENTRE OF EXCELLENCE FOR SUSTAINABLE WATER TECHNOLOGY
Current assignee: STICHTING WETSUS CENTRE OF EXCELLENCE FOR SUSTAINABLE WATER TECHNOLOGY
Priority date: 2009-05-27
Filing date: 2010-05-18
Publication date: 2010-12-02
Anticipated expiration: 2011-11-27
Also published as: NL2002938C2

Abstract

The invention relates to a system and method for treating an aqueous waste stream. The system comprises a predation reactor, comprising a support for supporting aquatic worms, wherein the support is configured such that the worms can be positioned substantially in a horizontal position, and supply means for providing sludge from the waste stream to the predation reactor.

Description

System and method for treating an aqueous waste stream

The present invention relates to a system for treating an aqueous waste stream. More specifically, the system treats waste streams, such as sludges from industrial and domestic waste water.

Existing systems for treating industrial and domestic waste water use micro-organisms, wherein waste matter is converted into harmless products and at the same time consumed as a substrate. The floes from the micro-organisms can be separated using a post-settling device, or a membrane separating device.

In another existing system, an aqueous waste stream is separated into an effluent and waste sludge. The waste sludge is converted into predated waste sludge in a predation reactor that comprises a support provided with worms. The waste sludge acts as feed stream for the worms. The object of the present invention is to improve the efficiency of the system for treating an aqueous waste stream using a predation reactor provided with worms.

This object is achieved with the system according to the invention, the system comprising:

- a predation reactor, comprising a support for supporting aquatic worms, wherein the support is configured such that the worms can be positioned substantially in a horizontal position; and supply means for providing sludge from the waste stream to the predation reactor.

By configuring the predation reactor such that the worms can be positioned substantially in horizontal position, instead of vertically, the position of the worms is closer to the natural position, which is head down. This requires providing the worms on a support or carrier, with the support to be placed vertically instead of horizontally. Experiments have shown an increased growth rate for the net worm biomass by using the horizontal positioned worms in relation to the vertically positioned worms. For example, the growth yield was significantly higher.

For a specific configuration wherein only the position of the worms was changed experiments showed an increased growth yield from 0,20 g dwi/g TSS digested for the horizontally positioned worms versus 0,13 g dwi/g TSS digested for the substantially vertically positioned worms. This significantly improves the overall efficiency of the system.

A further advantage of the system according to the invention is that in the substantially horizontal position of the worms less worms fall out of the carrier material.

Experiments have shown a 87% reduction. This results in more worms being available to treat the waste stream and therefore this will contribute to improving the overall efficiency of the system. The system according to the invention will improve the overall efficiency of the reduction of an aqueous waste stream, such as sludge from an industrial or domestic waste stream. Furthermore, as an additional advantage, as the system contributes to a significant increase in growth yield, the system enhances possibilities for using the worms for other applications. For example, it is possible to produce worms that are fed on a waste stream or sludge, to be used as fish feed. This provides an alternative to the use of fish oil and fish meal thereby contributing to the prevention of the extinction of some fish species. Another possibility would be to grow a specific compound by the worms that can be used for several purposes, including use as fertilizer, coatings and adhesives, taste enhancer. This further improves the overall efficiency of the system according to the invention.

In a preferred embodiment according to the present invention, the support comprises cylinders of a mesh material.

By providing cylinders with a circular or other shape, in a substantially vertical direction in the system when in use, the worms can be positioned in a substantially horizontal direction. Preferably, the cylinder is made of a mesh material with as one of the preferred embodiments a mesh size of 350 μm with as a preferred diameter of the cylinder 4 cm. Suitable carrier material includes carriers provided with openings including pores or prefaces, growing whereon the worms may establish, flexible sponge like materials, such as RECTICEL^® or other mesh like materials.

The worms are positioned in the mesh material. For treating an aqueous waste stream the worms provided in the predation reactor are aquatic worms. With the sludge preferably provided on one side of the worms, the worms may grow and replicate. Separation of the sludge and the worms provides added value to such purification systems over systems wherein the worm biomass is recirculated.

The worms nest in the mesh material feeding on constituents of the sludge, which flows along and/or through the pores of the mesh material, thereby increasing the worm biomass in the porous carrier.

In a preferred embodiment according to the present invention, the reactor comprises a number of supports.

By increasing the number of supports resulting in more than one support in every predation reactor, the reactor volume can be more efficiently used. Preferably, the supports are placed in a parallel configuration. In a presently preferred embodiment, the preferably mesh cylinders are provided at a distance of 4 cm. For a given configuration of the system, this increases the amount of carrier material for a given volume of the predation reactor. For example, in the present and preferred embodiment 22.5 m² of carrier material can be provided efficiently in 1 m³ of reactor volume. In contrast, to realize a similar efficiency in a similar prior art predation reactor, which is provided with a horizontal support or carrier, the sections of a water and sludge compartment have to be stacked requiring each section to be only 4 cm high, 2 cm for each of the two compartments. This limited height would make sludge distribution and faeces collection extremely complicated. Therefore, the substantially vertical positioned parallel cylinders further improve the overall efficiency of the system according to the invention.

A further advantage achieved with the system according to the invention is the ability to combine the control of the non-consumed sludge and the effluent with worm metabolise, thereby excluding the need to control two levels in the reactor, one for the water compartment and one for a sludge compartment. Also, a separate control is prevented for the return of both the effluent and non-consumed sludge to the WWTP. This significantly simplifies the system according to the invention thereby contributing to the overall efficiency thereof.

Preferably, the ratio between the support surface for carrying the aquatic worms and reactor volume is above 10m^"1 (m²/m³) , and preferably above 20m^"1 (m²/m³) . Experiments have shown that by using a substantially vertical configuration of the supports of carrier material, preferably using mesh cylinders placed in parallel, the reactor volume is used more efficiently. A ratio above 10m^"1 already achieves a significant improvement in the overall efficiency of the system according to the invention. Experiments have shown that a ratio above 20m^"1 even further increases the overall efficiency of the system according to the invention. In a preferred embodiment according to the present invention the system further comprises an aeration device to supply the worms with oxygen.

The system according to the invention provides a relatively easy implementation of an aeration device to supply oxygen to the water compartment and thereby supply the worms with oxygen. By providing the support or carrier material substantially vertical, air bubbles cannot get trapped under this carrier material. Oxygen uptake by the worms is thereby improved. In addition, the surface area of the support or carrier material is used more efficiently. This improves the overall efficiency of the system of the invention .

In a further preferred embodiment according to the present invention, the system further comprises faeces removal means.

By providing the support or carrier material in a substantially vertical configuration, the collection of worm faeces is made more efficient. One collection system for a large number of mesh cylinders would suffice, instead of separate collection system for each horizontal layers in the stack in prior art systems. This advantage is specifically interesting for a parallel configuration of mesh cylinders.

In a further preferred embodiment according to the present invention, the aquatic worms are of the class of Oligochaeta.

Worms of the class of Oligochaeta have proven to be extremely efficient for application in reducing and compacting an aqueous waste stream or sludge. Furthermore, as an additional beneficial effect, this class of worms is capable of motion by swimming that makes the separation of the worms from the carrier material or support more easing by using a so called escape reflex. Such an escape reflex is a neurophysiologic reaction which occurs in certain worms in response to exposure to sub lethal concentrations of toxins or touching. This reflex enables a swimming motion and enables removal of the worms from the carrier material.

Preferably, the worms are selected from the family of Lumbriculidae or Naididae, such as Nais variabilis or Nais simplex. More preferably, the worms are selected from the genus Lumbriculus, such as the species Lumbriculus variegatus or from the genus Dero, such as the species Dero digitata. Experiments have shown that especially the Lumbriculus variegatus effectively reduces waste sludge.

Furthermore, they have proven to exhibit a relatively stable growth on sludge as compared to other worms, and replicate asexually, making the processing of a predation reaction more easy. The present invention also relates to a method for the treatment of an aqueous waste stream, comprising the steps of:

- providing a predation reactor comprising a support for aquatic worms, wherein the support is configured such that the worms can be positioned substantially in a horizontal position; and

- supplying a sludge from the waste stream to the predation reactor.

Such method provides the same effects and advantages as those stated with reference to the system.

Further advantages, features and details of the invention are elucidated on basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings wherein:

Figure 1 shows a prior art system;

Figure 2 shows a vertical configuration for the worms according to the prior art;

Figure 3A and B shows a horizontal configuration for the worms according to the invention; Figure 4 shows experimental results for Cumulative amounts of added waste sludge and collected worm faeces from a continuous worm reactor, which received all the waste sludge from a lab-scale activated sludge system; and

Figure 5 shows experimental results for Ammonia and phosphate load removed with the outflow from the worm reactor and with the worm faeces.

In a system (figure 1) an aqueous waste water stream 4 is fed to a bioreactor 6 provided with a post-settling device, flotation device or membrane separation device 8. The water that is separated off by the separation devices leaves bioreactor 6 as an aqueous effluent stream 10. The excess sludge that is formed during the biological treatment is fed to a predation reactor 12 as waste sludge 14 and then predated in a predation reactor. In addition to or instead of the waste sludge from the bioreactor, sludge produced during pre-settling of an aqueous waste stream or sludge originating from a fermenter can also be fed to predation reactor 12. An oxygen-comprising water stream 16 is also fed to predation reactor 12, for which purpose aqueous effluent 10 from the bioreactor can optionally be used. The stream leaves the reactor via outlet 18 and is removed or is recirculated, after it has passed through an aerator, as input 16 to predation reactor 12. The predated waste sludge 20 is removed or recirculated to the bioreactor 6. The effluent 22 from predation reactor 12 is therefore removed or recirculated to the bioreactor 6 or to the predation reactor 12. During the predation of waste sludge, the biomass of the aquatic worms increases. The increase is about 5% -20%, calculated on the basis of weight of the original amount of waste sludge and expressed in dry matter. The excess mass of worms is harvested and can, for example, appropriate be used in fish food, and as a raw material for agricultural chemicals in adhesives, as a toxicity organism, in compositions comprising surface-active matter, in coatings, in biodegradable plastics, as a source of enzymes, as detergents, as a high-protein additive, or as a fertiliser, or is recirculated tot the bioreactor 6.

The waste sludge 14 is fed, along with the sessile worms, to the predation reactor 12, which is provided with a support 24 that preferably comprises a fine-mesh separation device, above the support 26. The waste sludge is predated by the worms in the support. The predated waste sludge 20 leaves the predation reactor 12 at the bottom of the support 24 and the effluent comprising non-predated sludge leaves the reactor at the top of the support 24. The support 24 therefore also has a separation function. The oxygen- comprising water 15 is fed to the bottom of the support 24 and also leaves the reactor via outlet 22 at the bottom thereof.

The reactor system 26 (Figure 2) comprises a beaker or sludge compartment 28 containing both waste sludge 30 and worms 32. The open side of the beaker is covered with a carrier material 34, through which the worms can protrude their tails. The beaker 28 is placed in the water compartment 36, at least partially submerged therein, with carrier material 34 facing downwards. By aerating water compartment 36 with aerator 38, the worms position themselves in carrier material 34. For example, the worm species L. variegatus feeds with its head, but respires and defecates via its tail. As a result, the worms keep their heads in the sludge compartment 28 and protrude their tails into the water compartment 36. The carrier material 34 therefore acts as both a support material for the worms 32 and a separation layer between the waste sludge 30 and the worm faeces 40. Optionally, an oxygen measurement (not shown) is provided in the water compartment 36 to monitor and/or control the reactor system 26.

The worms feed from the waste sludge on one side of the carrier material, whilst their tails protrude through the carrier so that they defecate on the other side. The worms position themselves this way as they use their tails to take up oxygen, which is provided on that side of the carrier. In this configuration, oxygen can efficiently be supplied to the worms, without the waste sludge consuming a considerable part of the supplied oxygen. Additionally, the intention is to harvest the worms from the system, thus obtaining a valuable protein-rich product from the waste sludge.

An alternative system 42 (figure 3) according to the invention is configured according to the characteristics given in Table 1.

Table 1: Dimensions of the worm reactor

mesh size carrier material μm 350 mesh cylinders # 3 surface area cm² 1257 (x 3) height mesh cylinder cm 100 diameter mesh cylinder cm 4 volume sludge compartment L 1.3 volume water compartment L 31 Worms (in the illustrated embodiment 29.8 g wet weight (ww) ) can be introduced in the worm reactor 44 via the open top 46 of the mesh cylinders 48. Waste sludge 50 from the activated sludge system 52, comprising a settler 54 and an aeration tank 56, is directly pumped to the inlet 58 of the sludge compartment, i.e. the bottom of the mesh cylinders 48. Effluent 60 from the activated sludge system 62 is collected in an overflowing bucket 64, from where it is pumped to the inlet of the water compartment. Optionally, the water compartment was aerated using a diffuser 70, with in the illustrated embodiment an air flow rate of about 690 mL/min inside a pipe. This visibly creates some mixing of the effluent 62 in the water compartment, which could distribute dissolved oxygen throughout worm reactor 44, but at the same time allowed worm faeces 66 to settle .

Experiments

Experiments were performed using the reactor described above and shown in figure 3 with the aquatic worm L. variegatus . In a first experiment, the feasibility of worm growth in the new reactor configuration was tested in a small reactor. In a second experiment a larger worm reactor was operated, which received all the waste sludge produced by a lab-scale activated sludge system. The larger reactor was operated for a period of 8 weeks, in which faeces production and nutrients release were measured. Worm growth was determined at the end of the experimental run.

Total, Volatile and Fixed Suspended Solids (TSS, VSS and FSS) were determined according to Standard Methods known to the skilled person using black ribbon filters (12-25 μm, Schleicher and Schuell) . Chemical Oxygen Demand (COD) , total nitrogen (total N) , total phosphorus (total P) and total ammonia (NH₄ + NH₃) were determined according to Standard Methods kown to the skilled person using Dr Lange^® test kits. Nitrate (NO3) and phosphate (PO₄) were determined according to Standard Methods known to the skilled person using ion chromatography (Metrohm 761 Compact IC).

Sludge and effluent from a lab-scale activated sludge system were used in the experiments, in a setup shown in figure 3. This system treated pre-settled domestic sewage in a completely mixed aeration tank (50 L) followed by a settler. Waste water was frequently analysed for total COD, total N and total P. Every other day, sludge was analysed for TSS and effluent was analysed for total COD, soluble COD, ammonia, nitrate and phosphate. The system was operated at a sludge retention time (SRT) of 18 days by wasting a fixed volume of sludge directly from the aeration tank. The waste water flow rate was 78 L/d with an average total COD of 408 mg/L, which resulted in an average organic loading rate of 0.16 g COD/ (g TSS-d).

Experiment 1

In a first experiment, the feasibility of worm growth in the new reactor configuration was tested in a small reactor 44 with Leeuwarden WWTP sludge. In the first experiment with the new reactor configuration sludge and effluent from the Leeuwarden WWTP was used. Sludge was first sieved (1 mm mesh) and effluent was filtered over black ribbon filters (12-25 μm. Schleicher and Schuell) before being used in the experiments. A small version of the reactor shown in Figure 3 was used, with only one mesh cylinder with a diameter of 4 cm and a height of 30 cm. The mesh size of the carrier material was 350 μm. Fresh sludge was added every 1-3 days and was recirculated over the mesh cylinder. The effluent in the water compartment was replaced once a week. At the same time, worms were removed from the mesh to weigh them. Their wet weight (ww) was determined by placing the worms on a polyamide mesh material (150 μm) . By pushing paper towelling against the back of the mesh, adhering water was removed from the worms. Worms fallen from the carrier into the water compartment were not accounted for .

In a period of 40 days, the worm biomass in the reactor increased from 9.8 to 18 g ww. This showed that net worm growth rate (0.015 d^'1) was possible also in this configuration, even higher than in a horizontal carrier

(0.009-0.013 d^"1) , but stil 11l below rates found for non- immobilised worms (0.026 d^'1;

Experiment 2

For comparative analysis results of a sequencing batch experiment with a horizontal carrier material were measured. Effluent was first filtered over black ribbon filters (12-25 μm, Schleicher and Schuell) . Worms were counted and their wet weight (ww) was determined. The carrier material used in the experiments had a mesh size of 350 μm. With sludge (and effluent) from the lab-scale activated sludge system, these experiments showed a sludge consumption rate of 138 mg TSS/ (g ww-d) when a worm density of 1.2 kg ww/m² was used. TSS reduction in the batch experiments was 11% (16% based on VSS) . Based on these results, a worm reactor to treat the waste sludge from the lab-scale activated sludge system would require 58 g ww of worm biomass and a carrier surface area of 485 cm². The large continuous worm reactor according to the present invention that was used offered ample surface area

(3770 cm²), thereby avoiding worm density to become a limiting factor. The larger reactor was started with a lower amount of worm biomass (29.8 g ww) to demonstrate that worm growth would be possible.

In the experiment 29.8 g ww of worms were introduced in the reactor44. The effluent flow rate through the water compartment of the worm reactor 44 was decreased stepwise from 43 L/d to 2.8 L/d. Worm faeces 50 were pumped from the bottom of the water compartment at a rate of 1.2 L/d.

In the experiments with the system shown in figure 3, the outflow 68 from the worm reactor 44 was collected and analyzed for total COD, soluble COD, ammonia, nitrate and phosphate. Sludge that was not consumed by the worms was not found in the worm outlet, but formed a sludge bed inside the mesh cylinders. Collected worm faeces were analyzed for TSS, total COD and its supernatant for total COD, ammonia, nitrate and phosphate. Waste sludge and worm faeces were occasionally analyzed for total N and total P. In case experiments have to be performed in system 44, at the end of the experimental run, all the worms in the mesh cylinders were collected and their wet weight was determined. Temperature and dissolved oxygen (DO) concentration in the water compartment of the worm reactor were measured using an optical dissolved oxygen measurement probe (Oxymax W COS61, Endress and Hauser) (not shown) .

The larger continuous worm reactor was operated without any problems during the entire experimental period of nearly

8 weeks. The cumulative amounts of waste sludge fed to the worm reactor and collected worm faeces are shown in Figure 4

(cumulative TSS in gram, time in weeks, and waste sludge in indicated with filled diamonds ♦, and worm faeces out indicated with open diamonds 0) . In total 431 g TSS of waste sludge was fed to the worm reactor and 167 g TSS was collected as worm faeces. However, sludge accumulation was observed as a sludge bed in the mesh cylinders, which was expected since the reactor was started with an insufficient amount of worms. The amount of sludge consumed by the worms was therefore estimated from the amount of collected worm faeces and the TSS reduction (11 %) found in the batch experiments. This resulted in an estimated total sludge consumption of 187 g TSS and a total sludge digestion by the worms of 20 g TSS. The sludge consumption rate of 110 mg TSS/ (g ww-d) during the last days of operation, was lower than the 138 mg TSS/(g ww-d) in the sequencing batch experiment. This could be explained by the DO concentration of 6.7 mg/L in the water compartment, which was below an optimum concentration of 8.1 mg/L for the worms.

Worm biomass The worm reactor started with 29.8 g ww of worms divided over the three mesh cylinders. At the end of the 8 weeks of operation, 49.5 g ww of worms was found in the mesh cylinders. During operation of the worm reactor a total of 6.7 g ww of worms was collected with the worm faeces (worms that had fallen from the mesh) . Thus, a total worm growth of 26.8 g ww was observed, which corresponded with a yield of 0.20 g dw/g TSS digested by the worms. This is higher than the yield of 0.13 g dw/g TSS digested found in the continuous worm reactor with a horizontal carrier material. The average worm net biomass growth rate was 0.014 d^"1, which is only slightly lower than the growth rate found in the feasibility experiment.

Visual inspection of the mesh cylinders showed that worms were situated along the entire sludge bed inside each mesh cylinder. By the end of the experiment the total sludge bed height in each cylinder had increased to 20-45 cm.

However, most of the worms (~ 80 %) were situated in the top ~ 10 cm of the sludge bed. This corresponded to a worm density of 1.1 kg ww/m² carrier material, which matched the stable worm density found in sequencing batch experiments with the same carrier material.

Nutrients

On average the sludge from the activated sludge system contained 48.6 mg total N/g TSS and 14.9 mg total P/g TSS. The collected worm faeces contained 40.4 mg total N/g TSS and 15.7 mg total P/g TSS. Similar to experiments with sludge from a municipal WWTP, the phosphorus content of the faeces was higher than in the sludge, whereas the nitrogen content was lower. Based on a mass balance over the worm reactor, an ammonia release of 7.8 mg NH₄-N/g TSS digested and a phosphate release of 20 mg PO₄-P/g TSS digested were calculated. For phosphate this was similar to what was found for sludge from a municipal WWTP, but for ammonia this was much lower than the 58 mg NH₄-N/g TSS digested found in those experiments. However, at the same time nitrate was produced in the worm reactor and the combined release of ammonia and nitrate amounted to 64 mg N/g TSS digested by the worms. This showed that nitrification of the released ammonia had taken place in the worm reactor, which was observed throughout the entire experimental run. Whether this occurred in or near the sludge bed in the mesh cylinders or by nitrifiers in the worm faeces, was not further investigated. Nitrification can also be expected to take place in a full scale worm reactor. Not only will this result in an increase of the oxygen demand of the worm reactor, but also in a decrease of the internal ammonia load on the WWTP. For the worm reactor, an advantage of nitrification in the worm reactor is that less effluent is required to keep the ammonia concentration low enough (< 0.1 mg N/L of unionised ammonia) to prevent inhibition of the sludge consumption rate by the worms.

Analysis of the supernatant of the worm faeces showed that the total ammonia concentration was high, up to 9.2 mg N/L. At the same time, the total ammonia concentration in the outlet of the worm reactor was very low (< 0.5 mg N/L) and appeared to be independent of the effluent replacement flow rate, which was decreased stepwise from 42 L/d to 2.8 L/d. This showed that the ammonia load released by the worms (and that was not nitrified), was removed from the reactor mainly with the supernatant of the worm faeces (Figure 5A in mg NH₄-N/d in time in weeks, with out with worm faeces indicated with open diamonds 0, and worm reactor outflow indicated with filled diamonds ♦) , thus keeping the ammonia concentration in the water compartment low. Possibly, ammonia adsorbed to worm faeces (2.9 mg NH₄-N/g TSS of faeces) . Another reason for the ammonia release could be some mineralization of the worm faeces after collection from the worm reactor. For phosphate the average concentration in the supernatant of the worm faeces was 7.2 mg PO4-P/L, only somewhat higher than in the outlet of the worm reactor (6.0 mg PO4-P/L) . The phosphate load removed with the faeces remained constant during the entire experimental period

(Figure 5B in mg PO₄-P/d in time in weeks, with out with worm faeces indicated with open diamonds 0, and worm reactor outflow indicated with filled diamonds ♦) ) . The stepwise decrease in effluent replacement rate from 14 l/d in the first time period, via 7.2 l/d in the second time period, to 2.8 l/d in the third time period (with a constant phosphate concentration) caused the decrease in the phosphate load in the outflow from the worm reactor (Figure 5) . Air inpu t

Inside the worm reactor the effluent in the water compartment was aerated with a single bubble diffuser. Oxygen was also introduced to the reactor with the effluent, which had an average dissolved oxygen (DO) concentration of 5.8 mg/L. In the first weeks, the average DO concentration in the reactor was 7.7 mg/L, whilst during the last couple of weeks, this had dropped to 6.7 mg/L. This may have been caused by the increased oxygen consumption by the worms (as the amount of worm biomass increased over time), by the lower input of DO with the effluent that was pumped into the worm reactor (this flow rate was decreased stepwise over time) , by oxygen consumption due to nitrification of the ammonia that was released by the worms and by the respiration of the sludge bed in the mesh cylinders. The temperature was constant at 20.6 ± 0.9⁰C and could therefore not have caused the decrease in DO concentration.

Results The new reactor configuration with a vertically placed carrier material demonstrated a stable operation over a period of 8 weeks. Although only a low TSS reduction of 11% was found, worm faeces with a high settleability were continuously produced. The latter was shown to be the main benefit of a worm reactor.

The new configuration has several advantages over the initial configuration with a horizontal carrier material. Most importantly, a higher net growth rate of 0.014 d^'1 over 8 weeks was achieved, compared to 0.013 d^'1 over 3 weeks for the horizontal carrier reactor. Despite their horizontal orientation, still some worms fell from the carrier material. But this was less than 0.5% per day of the amount of worms in the mesh cylinders, which was much lower than the 4% found in sequencing batch experiments with the same, but horizontally orientated, mesh material.

A further advantage is the efficient use of reactor volume. With the new configuration 22.5 m² of carrier material per m³ of reactor volume can be achieved by spacing the mesh cylinders at a distance of 4 cm. In contrast, to achieve 22.5 m²/m³ in a reactor with a flat horizontal carrier, sections consisting of a water- and sludge compartment would have to be stacked. Each section could then only be 4 cm high; 2 cm for the sludge compartment and

2 cm for the water compartment. Such a small height would make sludge distribution and faeces collection very difficult .

Furthermore, the new configuration has practical advantages with respect to faeces collection (one collection system for a large number of mesh cylinders) and aeration

(air bubbles cannot get trapped under the carrier material) .

Additionally, the combined collection of non-consumed sludge and effluent with worm metabolites excludes the need to control two levels in the reactor (one in the water compartment and one in the sludge compartment) . A separate collection is not required as both effluent and non-consumed sludge need to be returned to the WWTP.

A worm reactor such as the one described appears to be most interesting especially for smaller WWTPs with relatively high sludge handling costs. As an example, the results from the current experiments (110 mg TSS/ (g ww-d) and 1.1 kg ww/m²) were used to estimate the size of a worm reactor for a 35,000 p.e. WWTP. For a waste sludge production of 1600 kg TSS/d, the footprint of the worm reactor would be 195 m² (assuming 22.5 m²/m³ and a height of

3 m) . This is roughly one tenth of the surface area of a settler of such a WWTP. This additional surface space is expected to be available at a small WWTP, which is generally not located in densely populated areas.

The present invention is by no means limited to the above described embodiments thereof. The rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.

Claims

1. System for treating an aqueous waste stream, comprising : - a predation reactor, comprising a support for supporting aquatic worms, wherein the support is configured such that the worms can be positioned substantially in a horizontal position; and supply means for providing sludge from the waste stream to the predation reactor.

2. System according to claim 1, wherein the supports comprise cylinders of a mesh material.

3. System according to claim 1 or 2, wherein the reactor comprises a number of supports.

4. System according to claim 2, wherein the supports are provided in a parallel configuration.

5. System according to any of claims 1-4, wherein the ratio between effective support surface and reactor volume is above 10 m^"1, and preferably above 20 m^"1.

6. System according to any of the claims 1-5, further comprising an aeration device to supply the worms with oxygen .

7. System according to any of the claims 1-6, further comprising faeces removal means.

8. System according to any of the claims 1-7, wherein the aquatic worms are of the class of Oligochaeta.

9. System according to claim 8, wherein the worms are of the species Lumbriculus Variegatus .

10. Method for the treatment of an aqueous waste stream, comprising the steps of: providing a predation reactor comprising a support for aquatic worms, wherein the support is configured such that the worms can be positioned subtantially in a horizontal position; and - supplying a sludge from the waste stream to the predation reactor.