Method and reactor for biological purification of waste water

Abstract

Present invention relates to a method and apparatus for purification of water, said method comprises the steps of feeding the water into a reactor (4) through one or more inlet tubes (1) or inlet zones and feed water an substrate through carrier elements for bio film (5) which have a large protected surface (>200 m2/m3 carrier elements) and large pore volume (>60%), and that the carrier elements are fluidized for the removal of waste sludge, wherein the ratio of charge of the elements (5) by normal duty corresponds to an amount corresponding to 90%-100%, more preferred 92%-100%, and most preferred 92%-99% of the vet volume of the reactor (4), said carrier elements (5) is kept substantially at rest or hindered movement between the times surplus sludge is removed, and that the carrier elements being fluidised for removal of surplus sludge, said carrier elements (5) having a specific gravity in the area of 0.8-1.4, more preferred 0.90-1.1 and most preferred 0.93-0.97, and feed the treated water to one or more outlet zones (7) and one or more outlet tubes (2). The invention also comprises a reactor for performing the method.

Claims

1. A method for biological water treatment, the method comprising: a contaminant-removal process using a reactor, the reactor comprising: an inlet; a treated-water outlet at a first height, the first height corresponding to a first reactor volume; a plurality of biofilm carrier elements having a protected surface area of >200 m.sup.2/m.sup.3, a pore volume of >60%, and a specific weight of 0.8-1.4 kg/dm.sup.3, and that fill at least 90% of the first reactor volume; a sludge outlet at a second height of the reactor, wherein the second height is greater than the first height and corresponds to a second reactor volume that is greater than the first reactor volume; wherein the contaminant-removal process comprises: leading water to be treated into the reactor via the inlet; outputting treated water from the reactor via the treated-water outlet; and wherein a height of the plurality of biofilm carrier elements does not exceed the first height during the contaminant-removal process; and a sludge-removal process comprising: while continuing the leading of the water to be treated into the reactor, closing the treated-water outlet, the closing causing the biofilm carrier elements to fill the reactor to less than 85% of the second reactor volume; opening the sludge outlet; dislodging sludge from the biofilm carrier elements via turbulence; and removing dislodged sludge from the reactor via the sludge outlet.

2. The method of claim 1, the method comprising: after closing the treated-water outlet and opening the sludge outlet and after a level of the water has reached the sludge outlet, closing the inlet; and after the turbulence has ended, opening the inlet.

3. The method of claim 1, wherein the water is continuously supplied into the reactor through the inlet during the contaminant-removal process and the sludge-removal process.

4. The method of claim 1, the method comprising leading the water to be treated via the inlet into the reactor during the turbulence.

5. The method of claim 1, wherein a degree of filling for the plurality of biofilm carrier elements corresponds to 92%-100% of the first reactor volume during at least part of the contaminant-removal process.

6. The method of claim 1, wherein a degree of filling for the plurality of biofilm carrier elements corresponds to 92%-99% of the first reactor volume during at least part of the contaminant-removal process.

7. The method of claim 1, wherein the plurality of biofilm carrier elements have a specific weight of 0.9-1.1 kg/dm.sup.3.

8. The method of claim 1, wherein the plurality of biofilm carrier elements have a specific weight of 0.93-0.97 kg/dm.sup.3.

9. A reactor for biological water treatment, the reactor comprising: an inlet that carries a continuous stream of water to be treated; an outlet for treated water positioned at a first height of the reactor; wherein a contaminant-removal process comprises the outlet for treated water being open; wherein a sludge-removal process comprises the outlet for treated water being closed; an outlet for sludge positioned at a second height of the reactor; wherein the contaminant-removal process comprises the outlet for sludge being closed; wherein the sludge-removal process comprises the outlet for sludge being open; wherein the second height is greater than the first height; wherein the first height corresponds to a first reactor volume and the second height corresponds to a second reactor volume; and wherein the first reactor volume is less than the second reactor volume; a plurality of biofilm carrier elements disposed within the reactor; wherein the plurality of biofilm carrier elements fill at least 90% of the first reactor volume during at least part of the contaminant-removal process and less than 85% during at least part of the sludge-removal process; wherein the biofilm carrier elements have a protected surface area of >200 m.sup.2/m.sup.3, a pore volume of >60%, and a specific weight of 0.8-1.4 kg/dm.sup.3; and a mechanism that creates turbulence in the reactor.

10. The reactor according to claim 9, comprising an appliance adjacent each of the outlet for treated water and the outlet for sludge to retain the plurality of biofilm carrier elements within the reactor.

11. The reactor according to claim 9, the reactor comprising a mechanism for transport of the water to be treated and oxygen in an aerobic purification process.

12. The reactor according to claim 9, comprising a mechanism for transport of the water to be treated in an anaerobic and anoxic purification process.

13. The reactor according to claim 9, wherein the mechanism that creates turbulence comprises at least one of an air blower, a circular pump, and a mechanical stirrer.

14. The reactor according to claim 9, wherein at least some of the plurality of biofilm carrier elements exceed the first height during the sludge-removal process.

Description

(1) The invention will be explained in the following in more detail with the help of an embodiment example with reference to the enclosed figures, where:

(2) FIG. 1A shows schematically normal operation of the biofilm reactor according to the present invention;

(3) FIG. 1B shows schematically sludge coming loose and being washed out at continuous supply of water to the biofilm reactor;

(4) FIG. 2 A shows a figure corresponding to FIG. 1A and shows the biofilm reactor during normal operation;

(5) FIG. 2B shows surplus sludge coming loose on stopping the supply of water;

(6) FIG. 2C shows the washing out of surplus sludge;

(7) FIGS. 3A, 3B show schematically a section of a biofilm reactor according to the present invention.

(8) Standard operating procedure for the new biofilm process with continuous supply of water and intermittent removal of sludge is outlined in FIGS. 1 A-B. The biofilm reactor has an inlet pipe (1), an outlet pipe with a valve (2) for biologically purified water, and an outlet pipe with a valve (3) for removal of sludge. During normal operation (A) one can have from 90 to 100% filling of the biofilm medium and restricted or nearly no movement of the medium. Biofilm erosion due to collision between biofilm elements will be very small and the concentration of suspended material out of the reactor will be very low.

(9) When one wishes to remove sludge, one first closes the valve for outlet of biologically purified water (2) and opens the valve for removal of sludge (3). When the water level rises up to the level of the pipe (3), one ensures very turbulent conditions in the reactor (FIG. 1B) so that loose biomass, sedimented particles (particles can sediment inside the biofilm elements) and the outer layer of biofilm is torn off and is suspended in the liquid. This assumes that the water level in the reactor increases so much that the degree of filling falls below about 85% and that the biofilm elements are moving rapidly. The necessary turbulence can be set up by blowing in air, with the use of mechanical stirrers or by circular pumping. The required time for the loosening of particulate material can be from 1 minute to about ½ hour, dependent on the shape of the reactor and the strength of the turbulence. Thereafter, sufficient incoming water must pass through the reactor to get the sludge transported out of the reactor through pipe (3). The necessary amount of water to transport the sludge out of the reactor, and thus the volume of sludge water, will normally be from 1 to 3 times the reactor volume, dependent on how low the content of suspended material must be as one again returns to normal operation by opening the valve on pipe (2) (FIG. 1A).

(10) Standard operating procedure for the new biofilm process with intermittent supply of water and intermittent removal of sludge is outlined in FIG. 2. The biofilm reactor has an inlet pipe with a valve (1), an outlet pipe with a valve (2) for biologically purified water and an outlet pipe with a valve (3) for removal of sludge. During normal operation (A) one can have from 90 to 100% filling of the biofilm medium and restricted or nearly no movement of the medium. Biofilm erosion due to collision between the biofilm elements will be very low and the concentration of suspended solids out of the reactor will be very low.

(11) When one wishes to remove sludge, one first closes the valve for outlet of biologically purified water (2) and opens the valve for removal of sludge (3). When the water level has risen to the level of the pipe (3), one shuts the valve on the inlet line (1). One ensures very turbulent conditions in the reactor (FIG. 2 B) so that loose biomass, sedimented particles (particles can sediment inside the biofilm elements) and the outer layer of biofilm is torn off and is suspended in the liquid. This assumes that the water level in the reactor increases so much that the degree of filling falls below about 85% and that the biofilm elements are moving rapidly. The necessary turbulence can be set up by blowing in air, with the use of mechanical stirrers or by circular pumping. The required time for the loosening of particulate material can be from 1 minute to about ½ hour, dependent on the shape of the reactor and the strength of the turbulence in the reactor.

(12) When sufficient amount of suspended material is in suspension one opens the valve on the inlet line (1) at the same time as one continues with the turbulent conditions in the reactor. Surplus sludge will then be transported out of pipe (3) as shown in FIG. 2 C. The necessary amount of water to transport the sludge out of the reactor, and thus the volume of sludge water, will normally be from 1 to 3 times the reactor volume, dependent on how low the content of suspended material must be as one again returns to normal operation by opening the valve on pipe (2) and shutting the valve in pipe (3) (FIG. 2 A).

(13) The reactors must have an outlet arrangement that prevents that the biofilm elements can leave the reactor, at the same time as purified water and sludge can be led out through pipe (2) and pipe (3), respectively.

(14) In one embodiment the reactor comprises a mixing mechanism for transport of the water and substrate and which supplies oxygen to an aerobic process at the same time. Examples of mixing mechanisms will be diffuser aerators and ejector aerators.

(15) In another embodiment the reactor comprises a mixing mechanism for transport of the water and the substrate in an anaerobic and in an anoxic process. Examples of mixing mechanisms will be mechanical stirrers, circular pumping and anaerobic gas agitation.

(16) In relation to active sludge processes, the present invention has many advantages. There is no need for pumping of recycled sludge. There is no risk of discharge of sludge. The concentration of suspended material out of the bioreactor is low. Thus, the particle load on the sludge separation step will be low and one can use many alternative sludge separation processes, such as, for example, sedimentation, flotation, fine sieving or filtration. The bioreactor can handle considerably higher loads than an activated sludge process, so that the necessary bioreactor volume is considerably smaller and one gets a compact purification plant. In an aerobic process the biofilm elements in the present invention will break up large gas bubbles, reduce the velocity of all the gas bubbles and increase the distance the gas bubbles must travel to get to the surface of the liquid in the reactor. Thereby, one achieves a considerably better oxygen transfer and a lower energy consumption than in an activated sludge process.

(17) The present invention also has many advantages with regard to other biofilm processes. Submerged biological filters with a stationary biofilm medium and without back-flushing have problems with blocking and channel formation, in addition to that there is no access to the diffusion aerators at the bottom of the reactors. When there is a need to have access to the diffusion aerators at the bottom of the reactors in the present invention, the biofilm elements can simply be shovelled, sucked or pumped out of the reactors. Furthermore, the present invention has a higher specific biofilm surface area and a considerably higher capacity than the submerged biological filters mentioned above, so that the bioreactor becomes more compact.

(18) Compared to BAF processes, the present invention has the advantage that one does not have to have basins to store water that shall be used for the back-flushing. One can also have a continuous supply of water to the present invention. Furthermore, the present invention tolerates waste waters with a higher concentration of suspended material than what the BAF processes tolerate. With the present invention one has more freedom in the choice of bioreactor shapes and forms. BAF processes have a high pressure drop, while the present invention has a negligible pressure drop across the bioreactor.

(19) In relation to “moving bed” processes, the present invention has a greater extent of filling of biofilm elements. This results in an increased biofilm surface area. In “moving bed” processes, the biofilm elements move around freely and follow the flow pattern of the water in the reactor. This means that the velocity gradient between the biofilm elements and the water is relatively small. In the present invention, the biofilm elements have hindered or no movement and the velocity gradient between the biofilm elements and the water becomes greater. This results in a better transfer of substrate and oxygen to the biofilm so that the rates of reaction increase. Together with an increased biofilm surface area, this means that the present invention leads to a very compact process. The oxygen transfer is also better than in a “moving bed” process. In a “moving bed” process the gas bubbles are, to some extent, slowed down by the biofilm elements, but because the biofilm elements are largely following the water stream that is created by the air bubbles, the effect is considerably smaller than in the present invention where the biofilm elements have a limited or no movement. The present invention will thereby have up to 50% higher specific oxygen transfer than a “moving bed” process.

(20) With the present invention one can achieve, with a powerful turbulence for the washing out of the excess sludge, a somewhat shorter sludge age and somewhat more sludge than in a conventional “moving bed” process. A high sludge production was previously regarded as a disadvantage, now it is viewed as an advantage. A higher biological sludge production means a lower energy consumption, in that the oxygen requirement and thus the need for air is lower. At the washout of sludge as described in the present invention the need for oxygen will be typically reduced by 10 to 20%. If one has degradation tanks on the purification plant, more biological sludge will mean more energy recovery in the form of biogas.

(21) Compared with fluidized bed processes, the present invention is considerably simpler to construct and operate. The energy costs are considerably lower than for a fluidized bed process, because of the high pump costs to keep the biofilm medium (normally sand) fluidized.

(22) The present invention and associated method for removal of excess sludge will have many advantages compared with other biofilm processes: Removal of excess sludge is brought about by the incoming waste water. Other processes with back-flushing use costly, already purified, waste water. In addition, they need a storage basin for the purified water that shall be used for the back-flushing. The technique for back-flushing is very simple. The pressure drop is minimal. Depending on the chosen operating method and the frequency of washout of sludge, one can get a low concentration of suspended solids (SS) out of the reactor (pipe 2 in FIG. 1 and FIG. 2). A thinner biofilm, which one gets from regular washing, is normally more efficient than a thick biofilm. Particles that are in the incoming waste water will, to a large extent, be absorbed in the biofilm between each washing so that one will have a low SS in the outflow. A lower SS in the outflow than one will achieve in trickling filters, submerged biofilters, bio-rotors or moving bed reactors opens for many possibilities: If one has not very stringent requirements (for example, secondary cleaning requirements for BOF and KOF) the outlet (pipe 2) can go directly to the recipient. The outlet can go to a process for separation of particles. This can be sedimentation or flotation as for other biofilm processes. However, the low SS concentration from the present invention opens for the use of micro-sieves or sand filters for the final separation. With the other biofilm processes mentioned above the particle load will be too great for a sand filter. Excess sludge (pipe 3 in FIG. 1 and FIG. 2) can go; back to the pre-sedimentation for separation together with mechanical sludge; to a thickener (conventional or mechanical); to a fine sieve; or to a small flotation installation. In large purification plants with many parallel lines, a small separation step (for example, a fine sieve or flotation installation) can serve the whole plant in that one washes out the excess sludge from one reactor at a time and distributes the load of excess sludge between the subsequent separation steps over the whole of the 24 hour period. If required, the supply of water and discharge of biological purified water can be continuous, in that one lets the wash water (pipe 3) pass a separation step (for example, a fine sieve) in connection with the washing, where the sludge particles go further to sludge treatment and the water phase goes to the recipient or to further purification.

(23) The design of the reactors (4) (see FIGS. 3 A and 3B) represents no limitation for the invention, but it will typically have a flat bottom and vertical walls. The effective depth of the reactor (4) will typically be in the area 1.5 to 12 meters, normally 3.0 to 8.0 meters. The choice of material for the manufacture of the reactor (4) is of no importance for the process and can be chosen freely.

(24) The inflow of water to the reactor (4) can comprise one or more inlet zones, typically arranged with pipes (1) or channel constructions. In aerobic reactors, the water can either enter at the top of the reactor so that one has a water level gap (see FIG. 3 A) or one can have a submerged inlet (see FIG. 3B). For reactors with anoxic or anaerobic processes it is important to avoid entry of oxygen into the water that will occur with an open gap in the water level, and the inlet must therefore be submersed or at the same level as the surface of the water in the reactor during normal operation. Even with a submerged inlet pipe one can lead the water into the reactor by gravitation, also in connection with the removal of sludge, in that the water level in a previous process step or tank lies higher than the highest water level in the reactor. In such case, one will have a filled inlet pipe under pressure. This is illustrated in that in the FIGS. 1, 2 and 3B a curved inlet pipe is shown that extends above the maximum water level in the reactor. Water can also be pumped into the reactor through a submerged inlet pipe with a non-return valve.

(25) The direction of flow of water through the reactor (4) can be both horizontal and vertical.

(26) The outlet of water from the reactor can comprise one or more outlet zones (7), typically with an arrangement to keep the biofilm elements (5) in place in the reactor. The outlet arrangement will typically be characterised in that a construction with openings is used that are smaller than the linear dimensions of the biofilm elements (5).

(27) The aeration system in an aerobic reactor shall ensure that oxygen is supplied to the bioprocess and sufficient energy is provided to tear off loose excess sludge and keep the sludge in suspension in connection with the washing process. The aeration system will typically be placed at the bottom of the reactor (4) and be arranged so that the air is distributed in the largest part of the horizontal extent of the reactor (4).