DEVICE AND METHOD FOR PURIFICATION OF WASTE WATER

Abstract: The sewage from single homes or small agglomerations can be treated in a miniaturised water purification plant (1), consisting of an aerobic, biological purification step (7), surrounded by at least one sedimentation step (4 and 5). The aerobic biological purification step preferably takes place in an exchangeable bio-reactor cassette (14) having at least two filter beds (18 and 23) with an intermediate discontinuity or gap (22) for aeration. The bio-reactor can also be assembled from standardised segments of filter material, enclosed in a permeable, synthetic or organic mesh/cloth and optionally supported on a support structure, fitted into the space (7).

Description: Device and method for purification of waste water The present invention concerns compact devices for the purification of wastewater from single homes and small agglomerations, often referred to as miniature water purification plants. Background of the invention Today it is required that all wastewater are subjected to a purification step before they are led into the recipient. In densely populated areas, this is taken care of by the municipal sewage system, leading the wastewater from homes, industries and other sources to large wastewater treatment plants. These wastewater treatment plants comprise mechanical, chemical and biological treatment steps, ensuring a high degree of purification. A large wastewater treatment plant is often stable and reliable by virtue of its size and offers possibilities for continuos supervision and control of the processes. On the other hand, a large wastewater treatment plant requires a considerable investment and can involve high operating costs and maintenance costs. In smaller communities and in sparsely built-up areas, a network of sewers is not always possible to implement due to economic and practical reasons. The water purification needs of isolated houses, agricultural estates, recreational homes etc must be taken care of by other means. A frequently applied solution is the installation of a three-compartment septic tank, where a mechanical purification is achieved. A very large number of septic tanks are currently in use world-wide. Traditional septic tanks are however plagued by many problems, such as bad smell, the need of regular emptying etc. Most importantly, the degree of purification is insufficient. Further, a conventional three-compartment septic tank lacks provisions for reliable sampling and for the implementation of a chemical purification step. Lately, the environmental regulations have raised the requirement on purification. It is now desirable - and in some regions even required - that the traditional septic tanks are supplemented with an arrangement for infiltration, a so called sub-surface filter or sand filter trench. Requirements for regular sampling and validation of the purification have been discussed. Different alternative solutions have been presented, such as large sub-surface filters, using filter media such as sand, gravel, peat and expanded clay. These are however less suitable due to their space requirement and the risk of contamination of the ground water. WO 93/02015 discloses a compact device for the treatment of sewage-effluent, wherein the effluent within one joint, closed circuit is exposed to a filtering process, together with both an aerobic and an anaerobic treatment. In the device according to WO 93/02015, a first sludge filter and an aerobic zone, consisting of filter material contained in sacks, are situated above the anaerobic zone forming an extra sludge filtering zone. Obviously, the sequence of purification steps is different from that of the present invention. The construction is not optimised for simplifying maintenance and the change of filter material remains labour consuming. Further, the construction of the device according to WO 93/02015 does not allow easy and unhindered sludge removal from the anaerobic zone. The present invention therefore sets out to solve the problems indicated above and to make available a compact, simple and reliable plant for water purification and in particular a plant comprising means for mechanical, biological and, as a preferred embodiment, also chemical purification. The plant according to the present invention should be possible to use in combination with a septic tank and similar existing installations or, according to a preferred embodiment of the invention, completely replace the traditional septic tanks. Summary of the invention The above stated problems and difficulties will be solved by the present invention as defined in the attached claims, i.e. an aerobic bio-reactor for water purification. According to one embodiment of the invention, a complete miniaturised water purification plant is provided, comprising means for mechanical purification or sludge separation, biological purification and, in preferred embodiments, also chemical purification. By arranging a biological purification step surrounded by the means for mechanical treatment and, optionally also means for chemical treatment, a compact and reliable solution is achieved. It is thereby guaranteed that the microorganisms participating in the biological purification step are kept at an even temperature suitable for their function and survival. The plant according to the present invention also allows the regular taking of samples and control of the quality of the purified water. Further, the filter material is easily changed, adapted for different purification needs, supplemented etc. Additionally, part of the used filter material can be composted or used in land fill applications. Further advantages of the present invention will become evident from a closer study of the description and drawings. Short summary of the drawings The invention will be described in closer detail in the following description, examples and attached drawings, in which Fig. 1 shows a schematic view from above, showing an embodiment having a central biological step, surrounded by means for mechanical treatment; Fig. 2 shows schematically an embodiment having a central biological step, surrounded by means for mechanical and chemical treatment; Fig. 3 shows a schematic cross section of a miniaturised water purification plant according to the invention; Fig. 4 shows schematically the construction of an exchangeable cassette for use in a plant according to the invention. Description of the invention The following terms will be used throughout the description and are hereby given the following definitions: The term "aerobic" is used to define conditions which support the proliferation of organisms requiring molecular oxygen as well as organisms able to grow either in the presence or in the absence of molecular oxygen, including organisms that require a low partial pressure of oxygen. The term "fibrous" is used to denote different materials that substantially consist of elongate components, either entangled or separate, such as fibres, threads and the like. Examples of fibrous materials include cellulose fibres, for example in the form of wood chips, different fractions of peat, mineral wool and the like. The term "particulate" is used to denote materials that substantially consist of separate particles, such as different fractions of sand and gravel, ceramic fillers, crushed concrete or crushed bricks, expanded clay products etc. The term "mechanical purification" is meant to include means and process steps for the removal of components in the wastewater based on their size, specific weight, sedimentation properties etc. Sedimentation is one example of mechanical purification. The term "chemical purification" is meant to include means and process steps for the removal of components in the wastewater based on their chemical behaviour, for example by the addition of coagulants, flocculation agents etc. A chemical purification step is preferably followed by a sedimentation step. The term "biological purification" is meant to include means and process steps for the microbial digestion of nutrients contained in the wastewater. The miniaturised water purification plant according to the invention is characterised in that it comprises at least one sedimentation step and one aerobic, biological purification step within one container, through which the water to be purified flows by gravity. Preferably, as shown in Fig. 1 , the sewage water enters the purification plant 1 through an inlet 2 and enters a first sedimentation step 4, from which the overflow is led to a second sedimentation step 5 and further to a third sedimentation step 6. The overflow from step 6 is then led into the biological reactor 7. Purified water is released into the environment through outlet 8. As can be seen from Fig. 1, the sedimentation steps surround the aerobic, biological purification step. This arrangement helps to keep the central biological step at temperatures suitable for the proliferation of the microbes providing for the aerobic digestion of the organic components in the wastewater. According to a second embodiment, shown in Fig. 2, a chemical purification step is added by including an elongate vessel around the periphery of the container (not shown) or as another concentric compartment 3 of the purification plant 1. Suitable chemicals are then added to the in-flow of sewage, mixed with and reacting with the components of the sewage during a duration determined by the length of the flow path in said vessel. By arranging the vessel in the shape of a spiral around or inside the container, a long flow path is achieved. In the embodiment, illustrated in Fig. 2, the wastewater enters the purification plant through an inlet 2 and passes via a first sedimentation step 4 before being led over to the chemical step 3. The wastewater is the led through the chemical step along a flow path, which gives a retention time in the chemical step of desired duration, and is then passed over into the second sedimentation step 5 (passage not shown). Fig. 3 shows a schematic cross section of a purification plant 1 having an inlet 2. and sedimentation steps 4 and 5, the third step not shown. The central, biological purification step 7 has an outlet 8 and is closed with a lid or cover 9. A conduit 12 leads air to a discontinuity in the biological reactor. Each sedimentation step 4 and 5 has a manhole 10 and 1 1 for removing sludge. The corresponding manhole for the third sedimentation step is not shown. The plant has means for facilitating lifting, for example lifting ears 13 (only one is shown) for lifting the plant, securing it during transport and finally, for anchoring it upon installation. It is evident from Fig. 3 that the construction of the miniaturised water purification plant, according to the present invention, provides direct access to all the compartments of the plant. Through the manholes 10 and 1 1 , sludge can be removed from the sedimentation steps. The filter material in the centrally located space 7 for the biological step can easily be inspected and replaced. When the biological step is constructed as a filter cassette, as described further in the description, the used cassette is easily removed vertically and a new inserted quickly and with minimal disturbance of the functioning of the plant. In Fig. 3, the space 7 reserved for the biological purification step, can be filled with filter material to obtain a filter which functions according to the following principles: the sewage is introduced near the top of space 7, through means for spreading the liquid evenly over the surface of a underlying - first filter, which can consist of a homogenous filter bed or two or more layers of different filter materials, under which - a first discontinuity or gap provides aeration of the liquid, precipitating through the first filter, before reaching an underlying - second filter which can consist of a homogenous filter bed or two or more layers of different filter materials, under which a second discontinuity or gap provides aeration and prevents over-saturation of the second filter, whereupon - the purified liquid exits the biological purification step through an outlet. The biological purification step can be constructed by assembling filter elements, supporting structures and the like, directly into the space 7, provided that the above principles are followed. The two discontinuities or gaps have been found to be of considerable importance not only for aeration purposes but also for preventing the filters from becoming over-saturated with liquid. The above principles can be followed even if the loose filter material is loaded into the space 7, provided that suitable spacers and/or supporting structures are used. It is also possible to enclose the different components in building blocks of a water permeable material, having a cylindrical or other shape, fitting into the space 7. Preferably, the biological purification step is arranged in an exchangeable filter cassette, such as the cassette 14 shown in Fig. 4. This filter cassette comprises at least one filter bed, into which the clarified sewage is infiltrated. When infiltrated with sewage, this filter bed will quickly become colonised by a flora of microbes decomposing the nutrients in the sewage. According to a preferred embodiment of the present invention, the biological step consists of at least one filter cassette 14 having an upper and a lower filter bed, separated by a discontinuity or gap 22 for allowing air to enter and a similar discontinuity or gap 25 below said lower filter bed, in order to keep the lowest part of the filter constantly above the highest water level at the bottom of the biological step. Fig. 4 shows a cross section of a filter cassette 14 according to a preferred embodiment of the invention. The wastewater having passed through the sedimentation steps and - optionally - a chemical treatment step, enters the biological purification step via an inlet 15, connected to a tube or grid of tubes 16 having holes for evenly distributing the clarified wastewater over the filter beds. The wastewater first enters an upper filter bed, which here consists of a first layer 17 of a fibrous filter material, for example mineral wool. Mineral wool has shown to be very suitable, as it offers a large area to be colonised by bacteria digesting the nutrients in the wastewater. Below this layer, a second layer of a particulate filter material, for example expanded clay granules, such as suitable products from the Leca® product range, Filtralite® granules, sand, gravel or the like, is situated. This layer is preferably supported on a perforated support 19, for example a sieve. A third layer 20, rests on a second perforated support 21. This third layer is preferably a fibrous filter, for example mineral wool. Between the supporting structure or perforated support 21, and the second filter bed 23, a discontinuity or gap 22 is arranged. Air is led into this gap via the tube 12, shown in Fig. 3. The second or lower filter bed 23 consists of a particulate filter material, for example sand, gravel or expanded clay granules, such as suitable products from the Leca® product range, Filtralite® etc. The composition of the particulate material is chosen depending on the quality of the wastewater to be purified and the required degree of purification. By selection of particular minerals, this second filter bed can be used to remove specific components in the wastewater. for example to lower the phosphorus content of the wastewater. The second or lower filter bed 23 rests on a perforated support structure 25, a certain distance above the bottom of the water purification plant. This distance is 3 to 5 cm or more, but at least enough to prevent the filter bed 23 from being in contact with water on the bottom of the purification plant. Finally, the purified water exits the plant via outlet 8. In outlet 8, arrangements for the collection of samples can be made. Samples taken at this point give a very accurate picture of the degree of purification achieved, as no further purification, such as infiltration in underground sand filters etc. is required. The cassette 14 thus consists of at least one upper filter bed 18, and one lower filter bed 23, suspended above the bottom of the plant, thus forming a second discontinuity or gap 25. The second gap 25 helps to lead air to the lower filter bed, but more importantly, helps to prevent over-saturation or "ponding" in the filters. Further, according to preferred embodiments of the invention, the biological purification step comprises a smell absorbing filter 26, resting on a support structure 27 above the inlet 15. The smell absorbing filter 26 preferably consists of peat enclosed in flat, cylinder shaped pillows of perforated material, a mesh of synthetic or organic fibres, a wire cloth or the like. In a cassette structure according to the one shown in Fig. 4, the smell absorbing filter 26 may rest on a perforated structure 27 simplifying the removal of this filter alone, to allow inspection of the upper filter bed. The upper filter bed is preferably covered by a pre-filter (not shown), for example a mesh or filter cloth, protecting the underlying filter bed from particulate matter which can pass through the sedimentation steps at high flow. This pre- filter can then be changed at intervals shorter that the intervals for changing the entire filter cassette 14. In colder climates, the smell absorbing filter has a dual function. On the one hand, it absorbs and neutralises smells emanating from the wastewater and the microbiological digestion of the nutrients. On the other hand, it provides insulation of the bioreactor and helps to keep it a temperature, suitable for the functioning and proliferation of the microbes. The fibrous filter material in the filter bed or beds 17 an 20 is chosen among the following: mineral wool, cellulose, wood chips, bark, peat etc. It is preferred that the filter material is enclosed in compartments with perforated or otherwise permeable delimiting structures. The fibrous filter material is preferably chosen among organic filter materials, suitable for degradation through composting. However, mineral wool has proven to be a very suitable filter material and in particulate blocks of mineral wool manufactured with a press force of 80 to 100 kg. It is part of the present invention, that the entire biologic purification step is delivered as an exchangeable component or cassette 14, in other words a bioreactor. which can be easily inserted in and removed from the space 7 in the miniaturised water purification plant 1. This is the preferred embodiment, comprising the possibility of making available different bioreactors having differing filter design and filter material, depending on the intended use and purification requirements. This also gives the possibility of "upgrading" the water purification plant when the quality of the incoming wastewater or the purification requirements change. This upgrading can be performed without any changes to the structure of the plant, by simply removing the cassette and replacing it with another. Such cassettes can be manufactured from a series of basket-like supporting structures, enclosed in a cylindrical structure. Preferably, said cassette has loops or other means, facilitating its lifting when transporting, removing or installing a cassette. Further, the cassette has means for guiding the cassette into right position in the space 7. These means are preferably constituted by a longitudinal groove, corresponding to a vertical ridge along the inner surface of the space 7 of the miniaturised plant 1, guiding the cassette into right position with respect to the inlet 15 and the air-duct 12. Naturally, the cassette can have a longitudinal ridge or ridges, fitting into one or several vertical grooves in the walls of the space 7. According to another embodiment, the bioreactor consists of interchangeable elements, which can be combined to a bioreactor having the required characteristics. It is also conceived, that the bioreactor in itself can function as a purification plant for less contaminated waste water or as a final purification step for water leaving a traditional septic tank. In that case, a suitable outer container having an inlet, corresponding to the inlet 15, and an outlet, corresponding to the outlet 8, both of Fig. 4, is connected to the existing septic tank. A bioreactor according to the invention is then assembled in this container from elements or installed as a cassette, such as the cassette 14 in Fig. 4. The aerobic bioreactor according to the invention is characterised in that it comprises at least one filter bed with an underlying discontinuity or gap for allowing the inflow of air. Preferably the bioreactor consists of an upper and a lower filtration bed. separated by a discontinuity between the two filter beds and having an air-inlet, connected to the outside air. Although the invention has been described with regard to its preferred embodiments, which constitute the best mode presently known to the inventors, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention as set forth in the claims appended hereto.

Claims: Claims 1. Water purification plant (1 ) comprising means for mechanical (4, 5, 6) and biological purification (7) of wastewater within one container, having an inlet (2) and an outlet (8), characterized in that the means for biological purification is surrounded by the means for mechanical purification, and that the means for biological purification includes an upper filter bed (18) and a lower filter bed (23) with an intermediate air gap (22) and an air gap (25) below the lower filter bed. 2. Water purification plant according to claim 1, characterized in that it further comprises means for chemical purification (3) within or around the periphery of the container. 3. Waste water purification plant according to claim 1, characterized in that the means for biological purification comprises - means (16) for spreading the wastewater evenly over the filters; - at least one upper filter bed (18); - an intermediate air gap (22); - a lower filter bed (23), and - a lower air gap (25), separating the lower level of the filter (23) from the water level at the bottom of the means biological purification (7). 4. Waste water purification plant according to claim 3, characterized in that it has an air-intake (12) connected to the intermediate air gap (22). 5. Waste water purification plant according to claim 3, characterized in that the means for biological purification further comprises a fibrous filter layer (17) above the upper filter bed (18). 6. Waste water purification plant according to claim 3, characterized in that the means for biological purification further comprises a fibrous filter layer (20) below the upper filter bed (18). 7. Waste water purification plant according to claim 3, characterized in that the means for biological purification further comprises a smell absorbing filter (26) above the means (16) for spreading the wastewater evenly over the filters. 8. Waste water purification plant according to claim 3, characterized in that the upper filter bed (18) consists of a particulate filter material, chosen from sand, gravel, and expanded clay granules. 9. Waste water purification plant according to claim 3, characterized in that the lower filter bed (23) consists of a particulate filter material, chosen from sand, gravel, and expanded clay granules. 10. Water purification plant according to any one of claims 1 - 9, characterized in that the means for biological purification are enclosed in a cassette (14) comprising - at least one upper filter bed (18); - an intermediate air gap (22); - a lower filter bed (23), and - distance means for creating a lower air gap (25), separating the lower level of the filter (23) from the water level at the bottom of the means biological purification (7). 1 1. Cassette for use in a water purification plant, characterized in that said cassette comprises - at least one upper filter bed (18); - an intermediate air gap (22); - a lower filter bed (23), and - distance means for creating a lower air gap (25), separating the lower level of the filter (23) from the water level at the bottom of the means biological purification (7). 12. Cassette according to claim 11, characterized in that the means for biological purification further comprises a fibrous filter layer (17) above the upper filter bed (18). 13. Cassette according to claim 11, characterized in that the means for biological purification further comprises a fibrous filter layer (20) below the upper filter bed (18). 14. Cassette according to claim 11, characterized in that the means for biological purification further comprises a smell absorbing filter (26) above the means (16) for spreading the wastewater evenly over the filters. 15. Cassette according to claim 1 1, characterized in that the upper filter bed (18) consists of a particulate filter material, chosen from sand, gravel, and expanded clay granules. 16. Cassette according to claim 1 1, characterized in that the lower filter bed (23) consists of a particulate filter material, chosen from sand, gravel, and expanded clay granules. 17. Cassette according to claim 1 1, characterized in that the upper filter bed (18) is further protected by a pre-filter.

Inventors: ESTOLA KARL-ERIK, ESTOLA, KARL-ERIK
Applicants: ESTOLA KARL ERIK, SKANDINAVISK ECOTECH AB, ESTOLA, KARL-ERIK
Application year: 2000
Main classification: C02F3
IPC codes: C02F3, C02F9