NL1036342C2

NL1036342C2 - A method for biological anaerobic treatment of water, and a pond.

Info

Publication number: NL1036342C2
Application number: NL1036342A
Authority: NL
Inventors: Alberto Cohen
Original assignee: Econcern N V
Priority date: 2008-12-19
Filing date: 2008-12-19
Publication date: 2010-06-22
Also published as: WO2010071417A2; BRPI0923179B1; BRPI0923179A2; WO2010071417A3

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 5 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 10 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- 15 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 .

20 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, his may require costly repairs.

25 The object of the present invention is to provide a method for the biological anaerobic treatment of wastewater using an Up-flow Anaerobic 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-30 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 35 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 ; 1036342 2 - a conduit in connection with the chamber for removing methanecomprising 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 5 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 10 - 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-15 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 20 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 25 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 30 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 35 - 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.

3

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-5 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 10 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-15 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-20 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-25 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 30 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.

According to a preferred embodiment, the wastewater is intro-35 duced 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).

4

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 5 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 verti-10 cally 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) 15 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, 20 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 treat- 25 ment; - a conduit in connection with the chamber for removing methanecomprising 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- 30 duit 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 35 - 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 5 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 5 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 10 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 15 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 specifi-20 cally 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 water subject to the biological treatment is in a part of the pond not covered by the cover.

2 5 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 30 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 35 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 6 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.

5 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.

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

15 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 20 that the outlet opening of the conduit for supplying water containing organic material to be subjected to the biological treatment is below the baffle.

The present invention will now be elucidated with reference to the accompanying drawings, wherein 25 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 30 (UASB) process; fig. 3 represents a top view of the pond of fig. 1, with a cut out; and fig. 4 displays the layout of supply conduits at the bottom of the pond.

35 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 7 geomembrane liner (1 mm polypropylene) obtainable from Inter-Mexico Representaciones, S.A, de C.V, 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.

5 The wastewater is for instance wastewater from agriculture, food and agricultural processing industry or liquid manure from farming operations. 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 re-10 suit of the biochemical action of the anaerobic bacteria organic components contained in the wastewater are converted into methanecomprising 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 15 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 20 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 25 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 30 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 35 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 8 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 5 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 10 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.

15 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.

20 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.

25 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 30 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 adheres to particulate material, which comprises bacteria for which it is desired to keep them available at a high concentration so as to 35 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.

9

The baffles 20 are inclined at an angle a 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 5 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.

To separate the gas and the particulate material, water of the 10 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 15 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 20 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.

25 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 30 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 extracted 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 35 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.

5 10 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.

1036342

Claims

A method for biological anaerobic treatment of waste water using an Anaerobic sludge blanket with upward flow (UASB) process, the method comprising the use of a pond, said pond having a bank, a pond bottom, and a water bed body, wherein the pond comprises - a flexible polymer-based gas impermeable cover, said cover covering at least a portion of the water surface of the water body, and wherein an area between the water surface and the cover is a chamber for defines the collection of methane-containing gas resulting from the biological treatment; 15. a conduit communicating with the methane-containing gas removal chamber from said chamber; - a conduit for feeding waste water to be subjected to biological treatment containing organic material to the body of water, said water conduit has an outlet opening below the covered part of the body of water; - a drainage passage for removing water subjected to biological treatment; and - a baffle under the flexible polymer-based gas-insensitive cover that is suitable for guiding methane-containing gas bubbles that rise in the water body in another direction, the baffle being at an angle to the horizontal from 40 ° -70 ”, the baffle defining a settling zone above the baffle and a rising zone below the baffle, the baffle defining the passage of water over the baffle from the rising zone to the settling zone and the passage of settled material from the settling zone allows in the body of water below the partition, wherein - the waste water to be treated is introduced into the body of water below the cover 35; - the waste water is subjected to microorganisms in the pond, whereby methane-containing gas bubbles are formed and rise to the surface of the pond under the cover; 1036342 - water is entrained by rising methane-containing gas bubbles and the gas bubbles in the rise zone are led through the bulkhead in one direction, resulting in a stream of water, the stream of water going from the zone for leading in another direction 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 a pump for removing methane-containing gas using a pump; 10. particles comprising microorganisms in the water in the sedimentation zone above the bulkhead may sink and move from the sedimentation zone in the body of water below the bulkhead; and - biological treatment and settling water is removed from the pond via the drainage passage for removing biological treatment water.

Method according to claim 1, wherein the waste water is introduced into the body of water below the partition and at less than 1 m from the bottom of the pond. 20

Method according to claim 1 or 2, wherein the drainage passage for removing water subjected to the biological treatment is located near the surface of the body of water in a settling zone. 25

The method of claim 3, wherein water subjected to biological treatment is removed from the pond from a settling zone in a portion of the pond that is not covered by the cover. 30

5. Pond for biological anaerobic treatment of waste water, said pond comprising a bank, a pond bottom, and a body of water, the pond comprising - a flexible polymer-based gas impermeable cover, said cover at least one covers part of the water surface of the body of water, and wherein an area between the water surface and the cover defines a chamber for collecting methane-containing gas that is produced during the biological treatment; - a conduit communicating with the methane-containing gas removal chamber from said chamber, 5. a conduit for supplying waste water to be subjected to biological treatment containing organic material to the body of water, wherein the said conduit has an outlet opening below the covered part of the body of water; - a drainage passage for removing water subjected to the biological treatment; the pond further comprising - a baffle suitable for guiding methane-containing gas bubbles that rise into the body of water in another direction, the baffle being at an angle to the horizontal of 40 ° -70 °, wherein the baffle defines a settling zone above the baffle and a rise zone below the baffle, the baffle allowing the passage of water above the baffle from the rising zone to the settling zone and the passage of settled material from the settling zone into the body of water below the baffle. 20

A pond according to claim 5, wherein two adjacent bulkheads tapering towards each other at a location near the surface of the body of water where water may pass from the rising zone to the settling zone. 25

A pond according to claim 5 or 6, wherein the drainage passage for removing water subjected to biological treatment is in a secondary settling zone for removal of water subjected to biological treatment. 30

The pond of claim 7, wherein the secondary settling zone for removal of water subjected to biological treatment is in a portion of the pond that is not covered by the cover. 35

The pond of any one of claims 5 to 8, wherein a floating element is provided under the cover.

The pond of claim 9, wherein the floating element is a polystyrene beam.

A pond according to any of claims 5 to 10, wherein the baffle 5 is suspended from at least one of the cover or the floating element.

12. Pond as claimed in claim 11, wherein partitions are suspended from the cover by providing a polymer-containing membrane with a sinusoidal shape, with passages for water at an upper part of the sinusoidal shaped membrane and passages for settled material at the lower part of the sinusoidally formed membrane.

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

14. Pond as claimed in any of the claims 5 to 13, wherein the outlet opening of the conduit for supplying water containing organic material to be subjected to the biological treatment is under the partition. 1036342