BIOFILM CARRIER FOR WATER AND WASTE WATER PURIFICATION

Abstract: The invention refers to a carrier element to be used as a carrier for a microbial film in a biological water or waste water purification process in which the carrier elements are kept moving in the water. The surface of the carrier element is partly protected against collision with the surfaces of other carrier elements. The carrier element has flow passages therethrough and is moreover designed to allow good flow of water therethrough also after a microbial film having been established on the surfaces. The carrier element has a length, width and/or height which exceeds 1.5 cm, and the protected area is larger than 100 m<2>/m<3> of the volume of the carrier element. The cross-sectional area of the passage openings in the boundary surface towards the surroudings of the element comprises at least 35 % of the boundary surface towards the surroundings of a homogenous body having the same dimensions. The invention also relates to the use of such carrier elements for the treatment of waste water from the wood-processing industry, the waste water containing at least 100 mg/l suspended matter as measured on a GF/A-filter, as well as a reactor for biological water or waste water purification, comprising a reactor tank (20) which contains carrier elements (25) according to the invention and wherein a grid (26) covering the bottom of the tank is provided over aerators (21) at said bottom.

Description: BIOFILM CARRIER FOR WATER AND WASTE WATER PURIFICATION The invention relates to a biofilm carrier for water or waste water purification. In biological purification of water or waste water the water is passed through some type of reactor wherein micro-organisms are utilized to convert impurities existing in the water to harmless end products such as carbon dioxide and water. The purification i.a. can be performed under supply of air (aerobically) or without supply of air (anaerobically) . In order to increase the efficiency of the purification a high content of active micro-organisms in the process commonly is aimed at so that said organisms are not allowed to escape with the purified water, either by the micro-organisms being allowed to grow in suspension in the reactor and being separated from the water in a separation step after the reactor and being returned to the reactor (e.g. the activated sludge process) , or by some type of carrier material being introduced into the process, on the surfaces of which the micro-organisms can grow as a biofilm and thus can be maintained in the process (the biofilm process) . The biofilm process has a number of advantages as compared with the activated sludge process and other processes with suspended biomass. Among other things higher loads can be used and the processes are substantially less sensitive to variations and disturbances. Most conventional biofilm processes are based on the purification reactor being packed with carrier material in the shape of filler bodies or blocks which are fixed to be stationary in the process. These process embodiments have the drawback that there is a risk of the carrier material being stopped by biomass or other particulate material and that dead zones may be established in the process where the contact between the water and the active micro-organisms is very bad. In another type of biofilm process a carrier material is utilized which is held in suspension and movement in the process. The carrier material with micro-organisms growing thereon is maintained in the process by outgoing water being allowed to pass through a strainer or grid having an opening diameter or slot width which is so small that the carrier material cannot pass therethrough. The advantage of this type of process is i.a. that the risk for stopping of the carrier material and the establishment of dead zones is substantially reduced. Different types of carrier material are available for this type of process : foamed rubber pieces (EP-A-0 142 123) , rings of non-woven plastic material (Haikangijutu Vol.29 (4) : pp 64-69, 1987), and pieces of extruded plastic hoses the surface of which is enlarged by mechanical processing (Haikangijutu vol. 29(4) : pp 64-69, 1987) , intermixing of material which provides a rough surface (Water Environment Research vol. 64(7) : pp 884-889) , or introduction of inside partitions and outside fins (WO-A-91/11396) . These materials have in common that they have a density which is close to the density of water so as to be easily held in suspension. Although processes with carrier elements of these types often can be operated at high loads all prior art embodiments of carrier elements have, however, drawbacks which substantially limit the capacity of the process, particularly in aerobic processes wherein the supply of oxygen to the working micro-organisms is of decisive importance for the purification efficiency. In foamed rubber pieces and rings of non-woven material the pores often are blocked by growing biomass so that the active surface in contact with waste water and air will be reduced. The carrier elements which have been produced by extruding a plastic hose are small, < 1 cm, in order that o <s o o m w XΛ _ υ5 m o ε. o. o i For said purpose the carrier element has obtained the characterizing features of claim 1 so as to combine a large size with a large surface protected against wear and with a very good flow of both water and air therethrough. The carrier element according to the invention therefore provides great advantages as compared with prior art carrier materials i.a. by a considerably greater supply of oxygen and by largely reduced risks for stopping of strainers or grids for retaining the carrier material. The invention also relates to the use of such carrier elements for the treatment of waste water from the wood- processing industry, the waste water containing at least 100 mg/1 suspended matter as measured on a GF/A-filter. Furthermore, the invention relates to a reactor for biological water and waste water purification using the carrier elements according to the invention. In order to explain the invention in more detail illustrative embodiments thereof will be described below reference being made to the accompanying drawings in which FIGS. 1 and 2 are perspective views of a first embodiment of the carrier element according to the invention as seen from one and the other end, respectively, FIG. 3 is an end view of the carrier element in FIGS. 1 and 2 as seen from the same end as in FIG. 1, FIG. 4 is an end view of the carrier element in FIGS. 1 and 2 as seen from the same end as in FIG. 2, FIGS. 5 and 6 are corresponding perspective views as FIGS. 1 and 2 of a second embodiment, FIGS. 7 and 8 are corresponding perspective views as FIGS. 1 and 2 of a third embodiment, and FIG. 9 is a diagrammatic perspective view of a reactor wherein the carrier elements can be used. The carrier element in all embodiments shown can be described generally as a number of walls arranged and elements, and on these surfaces a microbial film can grow in the biological water purification process. The totally protected surface should be larger than 100 m /m volume of the carrier element, preferably larger than 200 m /m 3 , particulary larger than 275 m /m 3 . If the carrier elements are designed so as to have a very large surface, >500 m /m , it can, however, be difficult to avoid that the passages through the carrier element will be so narrow that they are stopped by the growth. In aerobic processes it is customery to use carrier elements having a density close to the density of water, but according to the invention it has been found that it is advantageous to use carrier elements having a higher density due to the fact that carrier elements having a density close to the density of water without resistance will be carried along by the flowing water and rapidly will escape from raising air bubbles; it follows that water easily will be stationary inside the elements and that air seldom will pass through the inner passages of the carrier element. Carrier elements having a higher density on the contrary tend to move down into the water and resist water and air flow upwards in the process so that water and air will be forced through the inner passages of the carrier element the biofilm being supplied with oxygen. At the same time the air bubbles will be broken down into smaller bubbles, which also increases the transfer of oxygen from air to water. The flow through the carrier elements will be increased at increased density of the carrier element but at densities which are too high, >1.40 kg/dm , the agitation of the material in the process will be made difficult so that the total efficiency of the process will be reduced. Very good efficiency has been obtained at a density of the carrier elements ranging from 1.20 to 1.30 kg/dm 3 where the flow through the carrier elements is high and the mixing is still good. The high density combined o o © in β\ W υ © v. O 5. w υ co t-~ © in «N 1Λ O © © i?) β\ W XΛ u σ» o

Claims: CLAIMS 1. Carrier element for use as a carrier for microbial film in a biological water or waste water purification process wherein the carrier elements are kept moving in the water, the surface of the carrier element being partly protected against collision with the surfaces of other carrier elements and the carrier element having flow passages and moreover being designed to allow good flow of water therethrough also after a microbial film having been established on the surfaces, c h a r a c t e r i z e d in that the carrier element has a length, width and/or height exceeding 1.5 cm, preferably in the interval ranging from 2.5 to 10 cm, particularly in the interval ranging from 3 to 6 cm, that the protected surface is larger than 100 m /m carrier element volume, preferably larger than 200 m /m carrier element volume, particularly larger than 275 m /m carrier element volume, and that the cross- sectional area of the passage openings in the boundary surface towards the surroundings of the element is at least 35 %, preferably at least 40 %, particularly at least 50 % of the boundary surface towards the surroundings of a homogenous body having the same dimensions. 2. Carrier element as in claim c h a r a c t e r¬ i z e d in that the passage openings in the boundary surface of the element are arranged such that water and air can flow into the element from all directions. 3. Carrier element as in claim 1 or 2, c h a r a c t e r i z e d in that the carrier element is criss-crossed by passages (15, 16, 17, 18) in such a manner that water can flow through the element in all three dimensions. 4. Carrier element as in any of claims 1 to 3, c h a r a c t e r i z e d in that it is built up of radially directed walls (11, 12, 11', 12') which are interconnected to a structure similar to a turbine wheel and open centrally. 5. Carrier element as in claim 4, c h a r a c t e r¬ i z e d in that the walls (11, 12, 11', 12') are interconnected by means of an annular portion (13, 14) at each end of the carrier element. 6. Carrier element as in claim 4 or 5, c h a r a c t e r i z e d in that the walls (11, 12) are interconnected centrally by means of a tube (18) centrally of the carrier element. 7. Carrier element as in any of claims 4 to 6, c h a r a c t e r i z e d in that the passages between the walls (11, 12) at one end of the carrier element are alternatingly open and closed by means of walls (19) . 8. Carrier element as in any of claims 4 to 7, c h a r a c t e r i z e d in that the walls (11, 12, 11' , 12') have alternatingly greater and smaller radial length. 9. Carrier element as in any of claims 1 to 8, c h a r a c t e r i z e d in that the walls (11', 12') are helical. 10. Carrier element according to any of claims 1 to 9, c h a r a c t e r i z e d in that the density of the carrier element is in the interval ranging from 0.92 to 1.40 kg/dm 3 , preferably in the interval ranging from 0.94 to 1.35 kg/dm 3 , particularly in the interval ranging from 1.10 to 1.30 kg/dm 3 . 11. Carrier element as in any of claims 1 o 10 c h a r a c t e r i z e d in that the carrier element is made by injection moulding of plastics or composite. 12. Use of the carrier element as in any of claims 1 to 11 for treatment of waste water from the wood-processing industry containing at least 100 mg/1 suspended matter as measured on a GF/A filter. 13. Reactor for biological water or waste water purification comprising a tank (20) containing suspended carrier elements (25) c h a r a c t e r i z e d in that a grid (26) covering the bottom and having a slot width which is smaller than the smallest outside dimension of the carrier element is provided under the portion of the tank receiving the carrier elements, spaced from the bottom of the tank a distance which allows access to the space under the grid when the water has been drained from the tank. 14. Reactor as in claim 13 c h a r a c t e r i z e d in that the grid (26) is provided as an outlet grid for separating the carrier elements (25) from water which is drained from the tank via a bottom outlet (24) .

Inventors: GUNNARSSON LARS AAKE HANS, LOEFQUIST ANDERS ERIK, MALMQVIST AASA ELSA, WELANDER THOMAS GUNNAR, GUNNARSSON, LARS, AAKE, HANS, LOEFQUIST, ANDERS, ERIK, MALMQVIST, AASA ELSA, WELANDER, THOMAS, GUNNAR
Applicants: ANOX HOLDING AB, GUNNARSSON LARS AAKE HANS, LOEFQUIST ANDERS ERIK, MALMQVIST AASA ELSA, WELANDER THOMAS GUNNAR, ANOX AB, GUNNARSSON, LARS, AAKE, HANS, LOEFQUIST, ANDERS, ERIK, MALMQVIST, AASA ELSA, WELANDER, THOMAS, GUNNAR
Application year: 1995
Main classification: C02F3
IPC codes: B01J19, C02F3