WO2010071417A2

WO2010071417A2 - A method for biological anaerobic treatment of water, and a pond

Info

Publication number: WO2010071417A2
Application number: PCT/NL2009/000262
Authority: WO
Inventors: Alberto Cohen
Original assignee: Ecofys Investments B.V.
Priority date: 2008-12-19
Filing date: 2009-12-18
Publication date: 2010-06-24
Also published as: NL1036342C2; BRPI0923179B1; BRPI0923179A2; WO2010071417A3

Abstract

The invention relates to a method for biological anaerobic treatment of wastewater using an Up-flow Anaerobic Sludge Blanket (UASB) process. According to the present invention, the method involves the use of a pond comprising a flexible polymer-based gas-impermeable cover, and a baffle below the flexible polymer-based gas-impermeable cover suitable for deflecting methane-comprising gas-bubbles rising in the body of water, the baffle being inclined at an angle to the horizontal of 40°-70°, the baffle defining a settling zone above the baffle and an uplift-zone below the baffle, the baffle allowing for the passage of water above the baffle from the uplift-zone to the settling zone and the passage of settled material from the settling zone into the body of water below the baffle, wherein the wastewater is subjected to micro-organisms in the pond causing methane comprising gas bubbles to form and to rise to the surface of the pond below the cover; water entrained by rising methane-comprising gas bubbles and the gas bubbles are deflected in the uplift-zone by the baffle resulting in a stream of water passing from the deflecting zone to the settling zone; gas from gas-bubbles breaking through the surface of the body of water below the cover is collected and removed to yield methane- comprising gas; and micro-organisms comprising particles in the water in the settling zone above the baffle are allowed to settle and pass from the settling zone into the body of water below the baffle.

Description

A method for biological anaerobic treatment of water, and a pond

The present invention relates to a method for biological anaerobic treatment of wastewater using an Up-flow Anaerobic Sludge Blanket (UASB) process.

World-wide large volumes of wastewater containing organic material are produced, and this wastewater must be purified before being discharged. Anaerobic biological treatment is a powerful albeit slow process requiring long residence times of the wastewater inside the anaerobic reactor, usually in the order of 7-10 days. The process can be accelerated using an UASB, because this technique allows for the retention of active biomass that contains micro-organisms, which micro-organisms are responsible for the break down of the organic material in the wastewater. The retention means that a higher concentra- tion of micro-organisms is present because the rate at which active biomass is washed out of the reactor is reduced. The higher concentration of micro-organisms makes lower residence time of the waste water in the reactor possible. This reduces the cost of the treatment . The UASB technique is performed using reactors suffering from a number of drawbacks. Amongst others, the reactors suffer from corrosion due by acidic sulphur-containing compounds resulting from the oxidation of sulphur-containing compounds generated in the anaerobic digestion process. This may require costly repairs. The object of the present invention is to provide a method for the biological anaerobic treatment of wastewater using an Up-flow AΠT aerobic Sludge Blanket process that is cheaper in terms of capital outlay and/or operating costs.

To this end, the invention provides a method for biological an- aerobic treatment of wastewater using an Up-flow Anaerobic Sludge Blanket (UASB) process, the method involving the use of a pond, said pond comprising a bank, a pond bottom, and a body of water, wherein the pond comprises - a flexible polymer-based gas-impermeable cover, said cover covering at least a part of the water surface of the body of water, an area between the water surface and the cover defining a chamber for collecting methane-comprising gas resulting from the biological treatment; a conduit in connection with the chamber for removing methane- comprising gas from said chamber,

- a conduit for supplying wastewater containing organic material to be subjected to the biological treatment to the body of water, said conduit having an outlet opening below the covered part of the body of water;

- a discharge passage for removing water subjected to the biological treatment; and - a baffle below the flexible polymer-based gas-impermeable cover suitable for deflecting methane-comprising gas-bubbles rising in the body of water, the baffle being inclined at an angle to the horizontal of 40°-70°, the baffle defining a settling zone above the baffle and an uplift-zone below the baffle, the baffle allowing for the pas- sage of water above the baffle from the uplift-zone to the settling zone and the passage of settled material from the settling zone into the body of water below the baffle, wherein

- the wastewater to be treated is introduced into the body of water below the cover;

- the wastewater is subjected to micro-organisms in the pond causing methane-comprising gas bubbles to form and to rise to the surface of the pond below the cover;

- water entrained by rising methane-comprising gas bubbles and the gas-bubbles are deflected in the uplift-zone by the baffle resulting in a stream of water, the stream of water passing from the deflecting zone to the settling zone;

- gas from gas-bubbles breaking through the surface of the body of water below the cover is collected in the chamber and removed from the chamber via the conduit for removing methane-comprising gas using a pump ;

- micro-organisms-comprising particles in the water in the settling zone above the baffle are allowed to settle and pass from the settling zone into the body of water below the baffle; and - water subjected to biological treatment and subjected to settling is removed from the pond via the discharge passage for removing water subjected to the biological treatment. By forfeiting an actual reactor tank, and using a flexible polymer-based cover instead of a fixed, rigid chamber for the collection of gas below the surface of the water, both the cost of investment and the sensitivity to corrosion are reduced. The methane- comprising gas collected in the chamber between the lower side of the cover and the surface of the body of water is substantially at atmospheric pressure, eliminating the need for a costly sub-surface rigid gas collection chamber. The methane-comprising gas is removed using a pump, which will in general also be used to increase the pressure of the methane-comprising gas exhausted by the pump to the pressure necessary for storing or use of the methane-comprising gas. Wastewater will be introduced below the cover within an area that is a vertical projection of the cover. It is recommended that wastewater is not introduced at a horizontal distance less than 4 meters from the perime- ter of the cover. If the cover and/or the bottom of the pond is provided with a wall with a vertical height of at least 1 meter and near the circumference of the cover, it is recommended that wastewater is introduced at a horizontal distance not less than 2 meters from said wall. The ordinary person skilled in the art is quite capable of de- termining the loss of undigested organic material (for example by determining the Biological Oxygen Demand) and consider whether that is acceptable. If not, the wall could be higher and/or the distance to the wall or the perimeter could be increased and/or the rate at which waste water to be treated is supplied could be reduced. In the pre- sent application the relative term "below" not just refers to a relative height, but also requires the lower object to be within the area formed by the (downward) vertical projection of the higher object. Similarly, the term "above" means both higher than and over the lower object. The term "seals against" encompasses the cover dipping into the body of water at an angle other than 0°, in other words, the cover may penetrate the water surface. To prevent the formation of oxygen in the pond by algae, the cover preferably does not transmit light. The vertical component of the baffle is generally at least 30 cm, preferably at least 50 cm, more preferably at least 1 m, ensuring sufficient volume to allow settling to occur.

According to a preferred embodiment, the wastewater is introduced in the body of water below the baffle and at less than 1 m from the bottom of the pond. This helps to ensure that water is sufficiently treated before being discharged (from the pond) .

According to a preferred embodiment, the discharge passage for removing water subjected to the biological treatment is located near the surface of the body of water in a secondary settling zone.

This helps to ensure that water discharged from the pond is sufficiently treated and loss of biologically active particulate material is reduced. This secondary settling zone where water is extracted from the pond is preferably located near the periphery of the cover, and more preferably removed from the pond from a settling zone in a part of the pond not covered by the cover. If there are vertically extending walls as described above, the settling zone is preferably outside those walls. Settled biological material from the secondary settling zone may be re-introduced in the area below the cover or extracted using a vacuum sludge extraction pump and disposed of.

This reduces the investment and operational (maintenance) costs .

The present invention also relates to a pond suitable for the method according to the invention. More specifically, the pond is a pond for biological anaerobic treatment of wastewater, said pond comprising a bank, a pond bottom, and a body of water, wherein the pond comprises

- a flexible polymer-based gas-impermeable cover, said cover covering at least a part of the water surface of the body of water, an area between the water surface and the cover defining a chamber for collecting methane-comprising gas resulting from the biological treatment;

- a conduit in connection with the chamber for removing methane- comprising gas from said chamber, - a conduit for supplying water containing organic material to be subjected to the biological treatment to the body of water, said conduit having an outlet opening below the covered part of the body of water; and

- a discharge passage for removing water subjected to the biological treatment; wherein the pond further comprises

- a baffle suitable for deflecting methane-comprising gas-bubbles rising in the body of water, the baffle being inclined at an angle to the horizontal of 40° -70°, the baffle defining a settling zone above the baffle and an uplift-zone below the baffle, the baffle allowing for the passage of water above the baffle from the uplift-zone to the settling zone and the passage of settled material from the settling zone into the body of water below the baffle.

Such a pond is very favourable for use in the method according to the invention, and avoids to a large extent the use of materials sensitive to corrosion. The discharge passage may be of any type, such as a channel, a pipe, or a hose. According to a favourable embodiment, two adjacent baffles taper towards at a location near the surface of the body of water where water is allowed to pass from the uplift-zone to the settling zone.

This allows for shearing force to be generated, easing the separation of gas attached to particulate material for release to the chamber, and also facilitating the settling of the particles in the settling zone above at least one of the baffles. The passage opening may be vertical, horizontal or at an angle with the horizontal.

According to a preferred embodiment, the discharge passage for removing water subjected to the biological treatment is located in a secondary settling zone.

This helps to ensure that particulate matter and more specifically active biomass is retained by sedimentation and hence elimination from the water that is to be discharged from the pond.

More preferably, the secondary settling zone for removal of wa- ter subject to the biological treatment is in a part of the pond not covered by the cover.

Thus the cover needs no special design to accommodate for the removal of treated wastewater, and any maintenance thereof is eliminated. This secondary settling zone where water is extracted from the pond is preferably located near the periphery of the cover, and more preferably removed from the pond from a settling zone in a part of the pond not covered by the cover. If there are vertically extending walls as described above, the settling zone is preferably outside those walls, thus helping to keep biological material under the cover.

According to a favourable embodiment, a floating element is provided below the cover. Such a floating element will perform at least one of the following functions: a) help the cover keep afloat if the cover has a density higher than that of water; b) help define the location where the gas is collected (i.e. where the chambers for collecting methane- comprising gas are present) .

A convenient and low-cost example of the floating element is a polystyrene beam.

According to a favourable embodiment, the baffle is suspended from at least one of the cover and the floating element. This allows avoidance of any rigid under-water structure to keep the gas-deflecting baffles in place, as required by state of the art UASB reactors.

According to a preferred embodiment, the baffles are suspended from the cover by providing a polymer-comprising membrane with a si- nusoidal shape, with passages for water at a top part of the sinusoi- dally shaped membrane and passages for settled material at the bottom part of the sinusoidally shaped membrane.

This is a simple design that can be manufactured cheaply.

According to a favourable embodiment, the baffles are made of flexible polymer-based gas-impermeable material.

This material is advantageously the same as that of the cover.

For the best purification of the wastewater, it is preferred that the outlet opening of the conduit for supplying water containing organic material to be subjected to the biological treatment is below the baffle.

Finally, the present invention relates to a method of providing a pond for biological anaerobic treatment of wastewater according to the invention, said pond comprising a bank, a pond bottom, a body of water and a flexible, polymer-based, gas-impermeable cover for cover- ing at least part of the body of water, wherein the method comprises the steps of

- providing a zigzag folded, flexible, polymer-based sheet above the pond bottom, said zigzag folded, flexible, polymer-based sheet having lower folds and higher folds and openings at both the lower and higher folds, and

- extending the zigzag folded, flexible, polymer-based sheet so as to increase the distance between adjacent lower folds and between adjacent higher folds. The shape and position of the zigzag folded sheet may be retained in various ways, for example using weights (in particular at the lower folds), floating elements such as polystyrene beams (in particular at the higher folds) , ropes extending towards poles in the pond bottom and/or to the bank of the pond, rigid ribs connected to the zigzag folded sheet etc.

According to a preferred embodiment, at least some of the lower openings are openings created by cutting through the zigzag folded, flexible, polymer-based sheet leaving flaps that extend in a direc- tion from the higher fold towards the lower fold, at least partially preventing gas bubbles from rising through the openings at the lower fold.

The present invention will now be elucidated with reference to the accompanying drawings, wherein fig. 1 represents a cross-sectional view of a pond provided with a cover for performing an Up-flow Anaerobic Sludge Blanket (UASB) process; fig. 2 a detailed cross-sectional view of a part of a pond provided with a cover for performing an Up-flow Anaerobic Sludge Blanket (UASB) process; fig. 3 represents a top view of the pond of fig. 1, with a cut out; fig. 4 displays the layout of supply conduits at the bottom of the pond; fig. 5 is a perspective view on a pond comprising baffles made using a long sheet with cut-outs; fig 6 is a plan view of a flexible, polymer-based sheet as can be used in a method for preparing a pond according to the invention; and fig. 7a-e show how a pond according to the invention may be constructed using the sheet of fig. 6.

Fig. 1 shows an artificial pond 1 comprising a body of water 2 surrounded by dikes 3. Part of the body of water 2 is covered with a flexible polymer-based gas-impermeable cover 4 made of plastic or synthetic rubber as known in the art. A suitable material is black geomembrane liner (1 mm polypropylene) obtainable from Inter-Mexico Representaciones, S. A. de CV, Tijuana, Mexico. A liquid waste stream to be treated is introduced via a supply line 5 by means of a pump 6 to nozzles 7 (fig. 2) below the cover 4. The wastewater is for instance wastewater from agriculture, food and agricultural processing industry or liquid manure from farming opera- tions. At the bottom of the pond there is a sludge blanket comprising anaerobic bacteria capable of converting organic substances, such as carbohydrates, fat and proteins into methane-comprising gas. As a result of the biochemical action of the anaerobic bacteria organic components contained in the wastewater are converted into methane- comprising gas. The methane-comprising gas will form gas bubbles G (see Fig. 2) which have a tendency to rise towards the surface of the body of water 2 in the pond 1 and collect in a space 10 underneath the cover 4 above the surface of the body of water 2.

The methane-comprising gas is extracted from the space 10 through a conduit 11 by the use of a gas pump 78, which creates a reduced pressure in the conduit 11 so that the gas will have the tendency to flow through the conduit 11 in the direction of the gas pump 78. To prevent water from entering the conduit 11, a ball float may be provided. If it is desired to avoid the connection of a conduit 11 to the cover 4, the conduit 11 may be placed underneath the cover 4 with the inlet of the conduit 11 located in the space 10.

The space 10 is not in open contact with the atmosphere and is in the embodiment shown defined by the lower side of the cover 4 and the surface of the body of water 2, in an area within the perimeter of where the cover 4 seals against the body of water 2.

The cover 4 may be provided with weights 13, for example in the shape of beams, to ensure that the cover 4 remains sealed against the body of water 2 at pre-defined locations.

In accordance with a preferred embodiment and as shown in fig. 2, the cover 4 is provided at or near its circumference with a downwardly extending gas barrier wall 13, said barrier preventing the escape of gas below the cover 4. The wall 13 is forced down under the water surface by weights 14 attached to the lower edge of the wall 13 so that the wall 13 protrudes into the body of water 2. The wall 13 acts like a skirt or curtain. To make sure the cover 4 doesn't sink because of the weight, flotation elements 15, such as polystyrene beams, may be provided. Alternatively or in addition, there may also be a wall 16 extending from the bottom of the pond 1 (or from near the bottom of the pond 1) upward to impede the escape of bacterial sludge-particles. To achieve the desired orientation, the wall 16 may be provided with a floatation element 17, for example a beam of polystyrene, or a hollow PVC pipe sealed at both ends.

The walls 13 and or 16 do not prevent passage of biologically treated wastewater from below the cover 4 to the body of water 2 outside the area covered by cover 4. Because the walls 13 and 16 merely have to impede the escape of methane-comprising gas, they do not need to be made of gas-impermeable material, they may comprise holes and do not need to stretch over the full circumference of the cover 4.

In accordance with the present invention, baffles 20 are provided below the flexible polymer-based gas-impermeable cover 4, which baffles 20 are suitable for deflecting methane-comprising gas-bubbles rising in the body of water 2. The baffles 20 will be discussed in detail with reference to fig. 3. A typical pond will have a depth of at least 5 m in which case the sludge bed may take up 2 m, the settling zone 2 m, and an intermediate zone for separating the sludge bed from the settling zone so as to allow proper circulation of water of 1 m.

Fig. 3 shows a top view of the pond 1 of fig. 2. In the embodiment shown, the cover 4 is held in place by cables anchored to upright hardwood posts 49 in the bottom of the pond. There is a rectan- gular cut-out in the drawing showing the baffles 20 and openings 21 and 22.

Fig. 3 also indicates schematically the position of upright hardwood posts 50, used to attach the inclined baffles 20 as explained below. To explain the invention in more detail, again reference is made to fig. 2, which is very schematic and not to scale. It displays part of a pond with a smaller-scale cover 4. Wastewater is introduced via nozzles 7 into a sludge blanket S. Gas bubbles G evolve and rise towards the surface of the body of water 2. Generally, the gas ad- heres to particulate material, which comprises bacteria for which it is desired to keep them available at a high concentration so as to treat the wastewater quickly. In other words, it is desired to keep the particulate material below the cover and not flush it away with treated wastewater.

The baffles 20 are inclined at an angle α to the horizontal of 40°-70°, the baffle defining a settling zone SZ above the baffle and an uplift-zone US below the baffle, the baffles 20 allowing for the passage of water present below the baffles 20 from the uplift-zone UZ to the settling zone SZ above the baffles 20 and the passage of settled material from the settling zone SZ back into the body of water 2 below the baffles 20 where it may settle to the sludge blanket at the bottom of the pond 1. The vertical component of the baffle 20 is 2 m (from lower edge of the baffle to the edge of the baffle where water can enter the settling zone) .

To separate the gas and the particulate material, water of the body of water below the cover 4 rises together with the gas bubbles and is deflected by the inclined baffles 20. The water passes through a narrow opening 21, subjecting the particulate material carrying gas-bubbles to shear forces. The gas separated is collected in space 10 and removed via conduit 11. The particulate material settling in the settling zone SZ returns via an opening 22 to the body of water below the cover and to the sludge blanket S. Arrows in the body of water 2 schematically depict the flow of the water under the cover 4.

The baffles 20 are suspended in the body of water using posts

50 provided in the pond bottom. Plastic ropes 51 are provided between the posts 50, and pass through holes in the baffles 20. Shifting of the baffles 20 from their desired position may be prevented using knots that are bigger than the holes, for example. Any movement of the baffles 20 may help to pass any sludge on top of the baffles 20 to be transported through openings 22.

Treated water can escape from below the cover 4. In general, the same amount of treated wastewater will be removed from the pond 1 as wastewater is introduced under the cover 4. In general, wastewater will be introduced below the cover 4 at a couple of meters from the perimeter or walls 13 and 16. Biologically active sludge escaping from below the cover 4 settles in a secondary settling zone (at the location of reference line 2 in fig. 1) . Excess sludge will be ex^¬ tracted from the pond 1.

Figure 4 displays the layout of supply conduits 150 at the bottom of the pond 1 below the cover 4 (fig. 1) . The liquid waste stream to be treated is introduced in the pond via the nozzles 7. Generally, there will be a manifold (not shown) after pump 6, the manifold supplying the supply lines 150.

A typical pond will have a volume of >100 m³, and a depth of 3- 10 m. The surface area of the cover will generally be >30 m², but larger is better.

Fig. 5 is a perspective view on a pond 501 comprising a body of water 502 surrounded by banks 503. A few details are left out and are discussed with reference to later figures. Part of the body of water 502 is covered by a gas-impermeable cover 504 of material described for the cover 4 of fig. 1. There are supply lines 505 for supplying wastewater to be treated in the pond 501 below the cover 504. Below the cover 504 above the water surface there is a conduit 511 for collecting methane-containing gas generated in the pond by degradation of waste in the wastewater. Below the cover 504 there are baffles 520 made using a long sheet of zig-zag folded flexible polymer. There are lower folds 531 and higher folds 532. The lower folds are provided with cut-out holes 533 to allow sludge settling in a settling zone SZ to pass towards the bottom of the pond 501. The cutouts 533 are U- shaped with little or no loss of material of the polymer, so as to result in a protruding part 534 that helps to prevent that gas bubbles rise through the cut-out holes 533 (which are the result of the cutting out process) into the settling zone SZ. The higher folds 532 are provided wit cutouts 535 for the passage of gas bubbles and wastewater that contains sludge that has to settle in the settling zone. In the embodiment shown, there are polystyrene beams 517 to ensure that the higher folds 532 of the baffles 520 are at the surface of the body of water 502. The baffles 520 are held at the desired position using rope 781 (see fig. 7) . Fig 6 is a plan view of a flexible, polymer-based sheet 660 as can be used in a method for preparing a pond according to the invention. The sheet 660 will be folded along fold lines 661 for forming a lower fold 631 and folded in the opposite direction along fold lines 662 to form a higher fold 632. U-shaped slits 663 are cut to provide for protruding parts 634 that serve the purpose as explained for protruding parts 534 as explained in reference to fig. 5. There are cut^¬ out holes 635 that serve as the cutout holes 535 that will allow for the passage of gas and liquid to be subjected to settling. Fig. 7a-e show a how a pond according to the invention may be constructed using the flexible, polymer-based sheet of fig. 6. More specifically fig. 7a shows a dry pond 701 in which is provided with conduits 705 for introducing wastewater to be processed into the pond 701. Walls 716 are erected to keep sludge introduced or developed below a cover 704 (to be discussed later) . Water is introduced into the dry pond 701. A zig-zag folded polymer sheet 760 is introduced into the partially filled pond, and a first end of a rope 781 is attached to bank 703, and a second end of said rope 781 is attached to bank 703' . Thus the zig-zag folded polymer sheet 760 is extended to provide properly angled baffles 720 (fig. 7c) and to maintain the baffles 720 at a desired location in the pond 701. Knots may be used in rope 781 to assure proper distance between adjacent baffles. On shore, polystyrene beams 717 are introduced from the side of the sheet 760. They provide enough buoyancy to keep a conduit 711 for collecting gas substantially above the water surface of the body of water 702 (fig. 7d) . Although the conduit 711 is drawn higher than the polystyrene beams 717. These conduits 711 are placed in the same longitudinal direction as the sheet 760. They may be kept in place using rope (not shown) passed through holes (not shown) near the higher folds. Subsequently, the conduits 711 and sheets 760 are covered with cover 704 (fig. 7e), which may have a skirt 713.

Claims

C L A I M S

1. A method for biological anaerobic treatment of wastewater using an Up-flow Anaerobic Sludge Blanket (UASB) process, the method involving the use of a pond, said pond comprising a bank, a pond bottom, and a body of water, wherein the pond comprises

- a conduit in connection with the chamber for removing methane- comprising gas from said chamber, - a conduit for supplying wastewater containing organic material to be subjected to the biological treatment to the body of water, said conduit having an outlet opening below the covered part of the body of water;

- a discharge passage for removing water subjected to the biological treatment; and

- a baffle below the flexible polymer-based gas-impermeable cover suitable for deflecting methane-comprising gas-bubbles rising in the body of water, the baffle being inclined at an angle to the horizon- tal of 40°-70°, the baffle defining a settling zone above the baffle and an uplift-zone below the baffle, the baffle allowing for the passage of water above the baffle from the uplift-zone to the settling zone and the passage of settled material from the settling zone into the body of water below the baffle, wherein

- gas from gas-bubbles breaking through the surface of the body of water below the cover is collected in the chamber and removed from the chamber via the conduit for removing methane-comprising gas using a pump;

- micro-organisms-comprising particles in the water in the settling zone above the baffle are allowed to settle and pass from the settling zone into the body of water below the baffle; and - water subjected to biological treatment and subjected to settling is removed from the pond via the discharge passage for removing water subjected to the biological treatment.

2. The method according to claim 1, wherein the wastewater is intro- duced in the body of water below the baffle and at less than 1 m from the bottom of the pond.

3. The method according to claim 1 or 2, wherein the discharge passage for removing water subjected to the biological treatment is lo- cated near the surface of the body of water in a settling zone.

4. The method according to claim 3, wherein water subjected to the biological treatment is removed from the pond from a settling zone in a part of the pond not covered by the cover.

5. A pond for biological anaerobic treatment of wastewater, said pond comprising a bank, a pond bottom, and a body of water, wherein the pond comprises

- a conduit in connection with the chamber for removing methane- comprising gas from said chamber,

- a conduit for supplying water containing organic material to be subjected to the biological treatment to the body of water, said con- duit having an outlet opening below the covered part of the body of water; and

- a baffle suitable for deflecting methane-comprising gas-bubbles rising in the body of water, the baffle being inclined at an angle to the horizontal of 40°-70°, the baffle defining a settling zone above the baffle and an uplift-zone below the baffle, the baffle allowing for the passage of water above the baffle from the uplift-zone to the settling zone and the passage of settled material from the settling zone into the body of water below the baffle.

6. The pond according to claim 5, wherein two adjacent baffles taper towards at a location near the surface of the body of water where water is allowed to pass from the uplift-zone to the settling zone.

7. The pond according to claim 5 of 6, wherein the discharge passage for removing water subjected to the biological treatment is located in a secondary settling zone for removal of water subject to the biological treatment.

8. The pond according to claim 7, wherein the secondary settling zone for removal of water subject to the biological treatment is in a part of the pond not covered by the cover.

9. The pond according to any of the claims 5 to 8, wherein a floating element is provided below the cover.

10. The pond according to claim 9, wherein the floating element is a polystyrene beam.

11. The pond according to any of the claims 5 to 10, wherein the baf^¬ fle is suspended from at least one of the cover and the floating ele- ment .

12. The pond according to claim 11, wherein baffles are suspended from the cover by providing a polymer-comprising membrane with a si- nusoidal shape, with passages for water at a top part of the sinusoi- dally shaped membrane and passages for settled material at the bottom part of the sinusoidally shaped membrane.

13. The pond according to any of the claims 5 to 12, wherein the baffles are made of flexible polymer-based gas-impermeable material.

14. The pond according to any of the claims 5 to 13, wherein the outlet opening of the conduit for supplying water containing organic ma- terial to be subjected to the biological treatment is below the baffle.

15. A method of providing a pond for biological anaerobic treatment of wastewater according to the invention, said pond comprising a bank, a pond bottom, a body of water and a flexible, polymer-based, gas-impermeable cover for covering at least part of the body of water, wherein the method comprises the steps of

- extending the zigzag folded, flexible, polymer-based sheet so as to increase the distance between adjacent lower folds and between adjacent higher folds.