WO2003072513A1

WO2003072513A1 - Membrane bioreactor

Info

Publication number: WO2003072513A1
Application number: PCT/NL2003/000146
Authority: WO
Inventors: Johannes Wouterus Van Groenestijn
Original assignee: Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek Tno
Priority date: 2002-02-28
Filing date: 2003-02-26
Publication date: 2003-09-04
Also published as: AU2003210073A1; NL1020091C2

Abstract

The invention relates to a membrane bioreactor for the purification of aqueous liquids, comprising a compartment (2) with membranes where purification takes place under aerobic conditions and a compartment (1) where anoxic conditions prevail in which a mixture of active sludge and said aqueous liquid circulates between the compartments, characterized in that the anoxic compartment (1) is placed in direct communication with and below the aerobic compartment (2). Such a reactor has the advantage that it is not necessary to use two separated reactors for the aerobic and the anoxic purification process, which enables a high purification efficiency and an effective use of space. The effective use of space results in a smaller 'footprint' (space occupation measured as bottom surface occupied by the reactor). Also provided is an optimum mixing of water and sludge throughout the content of the reactor, which enables a high sludge concentration. An additional advantage obtained by the vertical flow and aeration of the membranes is that the membranes soil or 'clog' less easily.

Description

MEMBRANE BIOREACTOR

The invention relates to a new membrane bioreactor and is in the field of the biological purification of aqueous liquids, preferably waste water, by means of membranes placed in reactors.

In biological waste purification, waste water is contacted in a so- called bioreactor with active sludge and aerated, which provides the removal of impurities. In traditional systems, after contact, water and sludge are separated again in large settling basins in which the sludge will slowly sink to the bottom. The drawback of this form of waste water purification is that only a sludge concentration of 3 grams of dry matter per liter can be maintained in the bioreactors.

In membrane bioreactors, water and sludge are separated by a membrane, thereby enabling the use of a much higher sludge concentration (and therefore a more efficient purification). These membranes can be placed outside the reactor (side stream) or within the bioreactors (submerged), in which connection the placement of vertical flat membrane plates (two membranes as sandwich with sucked-in clean water in the middle) has proved to be most effective. Clogging of the membranes is prevented by aeration with (air) bubbles. Mostly, this aerobic purification is preceded by an anoxic purification in which (biological) denitrification takes place.

To this end, the water/sludge mixture is located in an anoxic basin without membranes and is then introduced into the aerobic membrane bioreactor where a part of the water is sucked from the reactor via the membranes and another part of the water/sludge mixture is recirculated to the anoxic reactor for the removal of the nitrate formed. The invention relates to a membrane bioreactor for the purification of aqueous liquids, comprising a compartment with membranes where purification takes place under aerobic conditions and a compartment where anoxic conditions prevail in which a mixture of active sludge and said aqueous liquid circulates between the compartments.

A bioreactor in which both aerobic and anoxic conditions prevail is known from patent WO 00/37369. A drawback of this reactor, however, is that the aerobic and the anoxic conditions are not simultaneously present in the same reactor, but that these conditions alternate. This involves a loss of purification capacity.

Another reactor is known from patent US 5,702,604. It describes a reactor containing an aerobic and an anaerobic compartment in which the waste water is purified by methanogenic bacteria in an anaerobic phase and is further directed to the aerobic phase in which separation over membranes takes place. However, this is a separated sludge type system. It prevents aerobic sludge from entering the anaerobic compartment (through killing tanks), and by means of a packing or partitions, anaerobic sludge is prevented from entering the aeration zone. The drawback of this method is that extra technical measures are necessary for the sludge separation and that there is a risk of clogging of the filter beds.

The present invention removes the drawbacks of the known reactors. To this end, the membrane bioreactor for the purification of aqueous liquids, comprising a compartment with membranes where purification takes place under aerobic conditions and a compartment where anoxic conditions prevail in which a mixture of active sludge and said aqueous liquid circulates between the compartments is characterized in that the anoxic compartment is placed in direct communication with and below the aerobic compartment.

The membranes in the bioreactor are preferably flat plate membranes having a pore size of from 0.01 μm to 1 μm. The use of membranes having a small pore size (0.01 μm — 0.1 μm, so-called ultrafiltration membranes) less easily results in clogging of the membranes, and furthermore, the quality of the water filtered by the membranes is better because it contains fewer impurities in the form of floating dust. The presence of an aerobic and an anoxic phase in one and the same reactor is obtained by vertically placing the membranes in the upper part of the reactor and guiding along them air bubbles generated by an air source placed below the membranes. Positioned below this air bubble generator is the sludge in the anoxic part. The sludge consists of active sludge in which a biomass and other suspended and colloidal material are contained. The biomass present provides, while continuously feeding nutrients and oxygen, the conversion of biodegradable material. The final products, besides CO2 and water, are substantially biomass through sludge growth and nitrate. In the reactor according to the invention, this nitrate is broken down by denitrifying bacteria which in an anoxic medium proceed to nitrate breathing.

The influent and the returned sludge/water mixture from the aerobic zone can be distributed over the anoxic zone via a tube system. To remove coarse impurities, one or several large-meshed filters can be placed in the inflow tube of the waste water, or the influent can first be introduced into a presettling tank. If required, the inflowing liquid may be mixed in the anoxic part with underwater mixers. This mixing device is not necessary if the vertical flow velocity of the water is high enough. Subsequently, this water flows up to the aerobic zone. For a proper distribution, a horizontal plate with holes may be used to ensure that the water throughput is the same everywhere and is therefore evenly distributed. In this manner, the anoxic zone functions as pressure chamber. However, in case of a proper horizontal mixing of the anoxic zone through mixers, this horizontal plate may be superfluous. The mixture of water and sludge first passes a fine- meshed tube system from which air bubbles escape and then vertical membrane modules. Water, sludge and air bubbles pass the flat plates, and a part of the water is sucked through the membranes and discharged as clean water. Subsequently, the water/sludge mixture enters a zone above the membranes and is returned to the anoxic zone via a system of effluent gutters. These gutters are preferably half open and are flush with the water level. If the water level rises through feed from the bottom of the reactor, then the superfluous sludge/water mixture will flow over the edge of the gutter into the gutter. Another type of effluent discharge system, such as an overflow system with lateral discharge from the reactor, is, however, also conceivable without detracting from the invention itself or from the effectiveness of the reactor.

The top of the reactor may be open in the outside air, but may also be covered. In case of an airtight covering, however, it must be ensured that the air bubbling up from the reactor and any CO2 formed in the reactor are removed, so that no overpressure is created.

To regulate the influent flow of waste water and the effluent flow of the sludge/water mixture which is admixed, there are preferably arranged in the tube systems regulating mechanisms which can effect both a complete opening and a complete closure of said flows. By regulating the flow velocities of the two flows, an optimum mixing of new inflow of waste water can be obtained. The outflow of purified water from the membranes is preferably also regulable.

The purified water obtained from the outflow of the membranes may be used directly or may optionally be purified further, if this should be necessary.

Such a reactor has the advantage that it is not necessary to use two separated reactors for the aerobic and the anoxic purification process, which enables a high purification efficiency and an effective use of space. This effective use of space results in a smaller 'footprint' (space occupation measured as bottom surface occupied by the reactor). Also provided is an optimum mixing of water and sludge throughout the content of the reactor, which enables a high sludge concentration. An additional advantage obtained by the vertical flow and aeration of the membranes is that the membranes soil or 'clog' less easily. A requirement for the bacteria used in the sludge is that they are resistant to contact with air (oxygen). This makes methanobacteria and sulfate-reducing bacteria, which are obligately anaerobic, unsuitable for use in the bioreactor of the invention. However, denitrifying bacteria, such as those which normally occur in waste water purification and may be assumed to be known to those skilled in the art, are highly satisfactory in a reactor according to the invention.

It is also possible, however, to create a completely anoxic or anaerobic system using the reactor according to the invention. Anaerobic is, in this connection, the total absence of oxygen and nitrate in the medium, while under anoxic conditions oxygen is absent and nitrate is continuously fed. For such an anoxic or anaerobic system, no air or another oxygen- containing gas mixture should then be introduced into the reactor, but a gas mixture containing no oxygen, for instance nitrogen (N2) or biogas (substantially composed of methane). This enables the use of methanogenic bacteria or sulfate-reducing bacteria. In that case, however, the top of the reactor should be provided with a facility closing the reactor from the outside air, and in which, moreover, the gas bubbling up is recirculated to the gas inlet.

The present invention will hereinafter be explained in more detail with reference to an exemplary embodiment diagrammatically shown in

Fig. 1. The reactor is divided into an anoxic compartment (1) and an aerobic compartment (2) containing vertical flat plate membranes (3). The separation between the two compartments is effected by the presence of a fine tube system (4) generating air bubbles. If desired, for separation purposes, there may further be present a flat plate (21) provided with holes and placed below the air bubble generator (4). Feed of waste water to be purified occurs via an influent tube system (5) fed by a flow of waste water to be purified (11) and, via a tube (7) connected thereto, a return flow (14) of water/sludge mixture having traversed the reactor. Optionally, the anoxic zone (1) may further contain a mixing device (22) for continuously mixing water and sludge.

The air bubbles entering the reactor via tube system (4) are caused by feeding air or an oxygen-containing gas mixture (12). Separation of water and sludge takes place over the membranes (3), and a purified water flow (13) is removed from the reactor via tube system (8). The remaining water/sludge mixture is collected at the top of the reactor in effluent discharge gutters (6) and returned via line (7) to the anoxic zone.

For controlling the flow velocity, the inflow of waste water (11), the return flow of water/sludge mixture (14) and the outflow of purified water (13) are regulated by regulable pumps (33), (32) and (31), respectively.

Claims

1. A membrane bioreactor for the purification of aqueous liquids, comprising a compartment with membranes where purification takes place under aerobic conditions and a compartment where anoxic conditions prevail in which a mixture of active sludge and said aqueous liquid circulates between the compartments, characterized in that the anoxic compartment is placed in direct communication with and below the aerobic compartment.

2. A membrane bioreactor according to claim 1, characterized in that the aqueous liquid is mixed with active sludge.

3. a membrane bioreactor according to claim 2, characterized in that the sludge comprises denitrifying microorganisms.

4. A membrane bioreactor according to claim 2 or 3, characterized in that the aerobic zone comprises vertically placed flat plate membranes.

5. A membrane bioreactor according to claim 4, characterized in that air bubbles are guided along the membranes.

6. A membrane bioreactor according to any one of the preceding claims, characterized in that effluent from the aerobic compartment is returned to the anoxic compartment.

7. A membrane bioreactor according to any one of the preceding claims, characterized in that the water/sludge mixture in the anoxic compartment is mixed with underwater mixers.

8. A membrane bioreactor according to any one of the preceding claims, characterized in that between the anoxic and the aerobic compartment a horizontal plate provided with holes is arranged.

9. A membrane bioreactor according to any one of the preceding claims, characterized in that the sludge/water mixture leaves the reactor at the top by means of effluent gutters which are evenly distributed over the water surface.

10. The use of a membrane bioreactor according to any one of the preceding claims for the purification of waste water.