ANAEROBIC FERMENTATION

Abstract: In a method for the anaerobic fermentation of an organic waste liguid in which the liquid is fermented under anaerobic conditions in a packed fermentation vessel (1) containing an array of packing elements (6), the fermentation is carried out under conditions such that: (1) the flow, per unit area, of the waste liquid across a plane normal to the direction of flow is substantially uniform; and (2) the organic waste liquid flows smoothly over the surfaces of the array of packing elements with relatively few abrupt changes in the rate of flow. The liquid is preferably flowed upwardly through the array of packing elements which, in a preferred embodiment of the invention, comprises a dumped array of packing elements or rings having an aspect ratio of less than unity.

Description: ANAEROBIC FERMENTATION This invention is concerned with improvements in and relating to the anaerobic fermentation of organic liquids and to fermentation apparatus for use therein. It is well known to subject organic waste liquids to anaerobic fermentation (digestion) in order to purify the waste l quid (e.g. to reduce the biologic- oxygen demand (B.O. D. ) thereof), whereby gases, especially methane, are produced. In known digestion processes the the period of residence of the waste liquid is generally long, e.g. of the order of 10 - 20 days. Since it is often necessary Q to maintain the fermenting waste at a temperature above ambient, conventional anaerobic fermentation procedures may require the input of heat energy so that the overall energy yield from the process is low. It -is an object of the present invention to provide a process 5 for the anaerobic fermentation of an organic waste liquid in which ■ the fermentation may be carried out on a continuous basis with a relatively low average residence time, e.g. of the order of hours rather than days. In accordance with the present invention an organic waste ø liquid is fermented under anaerobic conditions in a packed fermentation vessel, that is a fermentation vessel containing an array of packing OMPI elements, whilst passing the organic waste liquid through the array of packing elements under conditions such that: (1) the flow rate, per unit area, of the organic waste liquid across a plane normal to the direction of flow of the organic liquid is.substantially uniform; and ( i) the organic waste l quid flows smoothly over the surfaces of the array of packing elements with relatively few abrupt changes in the rate of flow. It is currently preferred that the organic waste liquid be flowed upwardly through the array of packing elements and in the following description, reference wil be made only to th s mode of operation, although it is to be understood that, where appropriate, the description refers also to other modes of operation, for example where the organic waste liquid is flowed downwardly through the array of packing elements. In the following description reference will be made to the accompanying drawing which is a schematic cross-section through a fermentation apparatus for use in accordance with the invention. The fermentation apparatus shown in the drawing comprises a closed fermentation vessel 1 provided with, at the upper end thereof, a gas off-take conduit 2 provided with valve 3 and, in the lower end thereof, a gludge off- ake conduit 4 provided with valve 5. Mounted within vessel 1 is an array of packing elements - 3 - generally indicated at 6 and supported on perforated support 7. A waste liquid inlet pipe 8, provided with valve 9, is connected with liquid distributor means comprising a plurality of perforated pipes or troughs10 mounted in vessel below support 7. A diffuser or flow straightening device 11 is mounted intermediate pipes 8 and support 7. An inner circumferential wier 12 is mounted above the array of packing elements 6 and connects with treated liquid discharge pipe 14 which, in turn, connects with liquid seal 15, to retain gases with vessel 1. (Alternatively, for example, a lute system may be employed to retain gases within the vessel 1). In order to meet the first criterion, namely that the rate of flow of the organic waste liquid be substantially constant across a plane normal to the direction of flow of the waste liquid - i.e. approximate to plug flow through the fermenter, it is necessar that the organic waste liquid be introduced into the array of packing elements substantially uniformly across the lower surface thereof. In order to achieve this, the waste l quid is, in the embodimen shown in Figure 1, introduced into the fermentation vessel by means of a set of perforated pipes and, in order to achieve further uniformity, a flow straightener or diffuser 11 is mounted between the pipes 8 and the bottom of the array of packing elments 6. This diffuser may, for example, take the form of a perforated plate, a grid or a woven mesh such as of glass fibres. Thus, as will be appreciated, the rate of flow across the perforated - 4 - The array of packing elements 6 should be such that abrupt changes of the rate of flow of the organic waste liquid in contact with the surfaces of the array of packing elements are reduced This can, theoretically, best be achieved by forming the array of packing elements as an array of a plurality of vertical tubes extending from top to bottom of the array and such an array of tubes may be used in accordance with the invention. It should be noted however that such an array of tubes calls for very good initial distribution of the organic waste liquid since it does not in itself possess flow straightening properties, i.e. once a "plug" of liquid has entered a particular tube, it is contained therein until it emerges at the top of the tube. A similar effect may be achieved by means of an array of vertical sheets but this too will suffer from the same defect to some extent. Another way of providing an array of packing elements approxim to a set of tubes is to provide a stacked array of packing elements such as Raschig, Lessig or cross-partition rings. If such rings are packed in rows staggered relatively one to the othe, some degree of flow straightening may be achieved but it should be noted that it may prove very time-consuming to stack-pack such rings in a large fermentation vessel. . So-called "dumped" packings possess the advantage that they may simply be introduced into a reaction vessel by simply O PI being dumped therein and thus it is very simple to pack a vessel with dumped packing elements. However, by virtue of their mode of introduction, dumped packings tend to form a more or less random array. Generally speaking, such a random array will in many cases provide a packed array in which a liquid flowing through the array will be subjected to many abrupt changes of rate of flow when flowing over over the surfaces of the packings. This will particular be the case for dumped packings formed of conventional Raschig rings or solid cylinders and, although the position may be somewhat better, the same obtains for Pall-type rings and for saddle- type packings. Conventional Raschig and Pall rings have an aspect ratio (the ratio of their length to their diameter) of about unity and thus when dumped tend to form an array in which the packing elements are randomly or vertically oriented. On the other hand, packing elements having an aspect ratio of less than unity (e.g. less than 0.66, preferably from 0.5 to 0.25) tend to form a packed array in which the .elements tend to be horizontal, i.e. in the- case of simple rings, with the walls of the rings tending to the vertical rather than to the horizontal. It has been found that such packing elements are particularly suitable for use in accordance with the invention, possessing as they do the merits of being relatively easy to introduce into a vessel, being capable of flow straightening and, at the same time, meeting the criterion that the liquid flowing over the surfaces thereof is not subjected to too many abrupt changes of rate of flow. OMPI - - A useful, pragmatic, measure of the suitability of a packing element, for use- as a dumped packing for a fermentor in accordance with the invention, is the pressure drop across the packing when air is passed through it. As will be appreciated, the pressure drop of any form of packing is, inter alia, determined by its size, smaller packings having a greater pressure drop, or resistance to air flow, than larger rings and vice versa. Table 1 below sets out preferred maximum pressure drops for packings of various sizes for use in accordance with the inventio Table 1 O PI - 1 - Suitable packing elements meeting the above criteria include rings which may or may not be provided with apertures in their side walls and internal radial or diametric ribs, and Tellerette rings (packing.elements formed of a toroidal he!ex). tøi lst we do no wish to be limited by theoretical considerati we believe that the efficacy of the fermentation process in accordanc with the invention lies in the formation, on the surfaces of the array of packing elements, of a film of material comprising the anaerobic microorganisms which effect the anaerobic degradation of the waste l quid to be treated. It is further believed that by subject ng the surfaces of the packing elements to a relatively smooth flow of waste liquid it is possible not only to provide a more uniform and widespread covering of that film but to provide a more effective film. Thus, it is believed that-the thickness of the film is, inter alia, controlled by the rate of hydraulic shear of the liquid passing over the film and that consequently by achieving a rel tively uniform and smooth folow of liquid over the film it is possible to provide a substantially uniform film thickness, i.e. to provide a widespread covering of effective film over the surfaces of the packing elements, which is relatively free from "dead spots" so that the activity per unit surface is more uniform across a horizontal plane in the array of packed elements. _ OMPI Accordingly, another measure of the suitability of any particular form of packing for use in accordance with the invention may be the percentage of the free surface area of the packing which is covered with the film and thus, suitably the packing should be one capable, in use, of being covered with at least 40, preferably at least 75% of film. Again, whilst not wishing to be l mited by theoretical consideration, it is believed that anaerobic degradation, in many cases, takes place as step-wise operation; one microorganism degradi the waste to a certain extent and another microganism then taking the degradation a step further. . "" Since the flow of liquid through the fermentor will approxima to plug flow, this will allow .of horizontal stratification of the microorgan sms (or families of microorganisms) in the fermentor so that, for example, acidogenic microorganisms will be located in stratified ayers in the lower end of the fermentor and mεthanogen microorganisms will be similarly located in the upper parts of the fermentor. Further it is believed that optimum flow rates over such different films may differ and thus it is possible to use a packing formed of layers of packing elements of different surface area/unit volume or to employ a tapered fermentor so that the flow rate across a plane throught the packing, whilst being substantially uniform across that plane, differs from that across another plane. - 9 - Generally, the flow liquid rates through the fermentors n accordance with the invention, considered as the flow rate across a. plane normal to the direction of flow of the waste liquid, will be relatively low, e.g. of the order of 0.2 to 50 m/day, more especially 2 to 40 m/day. Even so such low flow rates can give rise to low (as compared with those the prior art) residence times; e.g. of the order of 24 hours or less, especially from 5 to 20 hours. Depending on the nature of the waste material to be treated, it may or may not be necessary to heat the liquid to effect fermentatio but in any event the fermentation is suitably carried out at a temperature of from 40 to 80 °C, preferably from 20 to 65 °C. The material flowing upwardly through the packing in the fermentor will, at least in the upper part of the packing also comprise gas, e.g. methane generated by the anaerobic fermentation. It is most. desirable that the packing allow for efficient release of gas from the surface thereof and a packing which does not lead to abrupt changes in flow rate over the surface thereof also allows for efficient release of the gas and for easy passage of the gas through the packing, without interfering with the film of anaerobic microorganisms thereof. In order to accommodate the increased overall flowrate of fluid (liquid and gas) in the upper part of the fermentor it may be desirable to form the upper part of the packing elements having a lower surface area per unit volume than that of the packing elements forming the lower part of the packing. The method and apparatus of the invention may be used to process a wide variety of organic waste liquids (by which term is meant aqueous solutions and/or suspensions or organic waste materials) such as domestic sewage liquors, wastes from food and beverage producing plants and from oil refineries. The fermentation serves two principal function namely (a) to reduce the B.O. D. of the waste liquid whilst (b) producing useful material, such as methane, from the waste material. If desired the treated waste liquid may be further purified by, for example, a conventional aerobic fermentation process. Since the anaerobic fermentation may, it is believed, take place as a stepwise operation as discussed above, the fermentation process in accordance with the invention may be carried out in a plurality of successive fer entors in each of which one step in the fermentation is carried out. The fermentor and fermentation-process of the invention may also be used for the anaerobic fermentation of l quid,organic material-containing, substrates other than waste liquids, for example waste liquids to which additional materials or nutrients have been added or other substrates. In other words, the process and apparatus may be used to carry out anaerobic fermentation of a wide variety of substrates by the action of microorganisms (such as bacteria or enzymes) which may be natural or genetically modified microorganisms.

Claims: CLAIMS:- 1. A method for the anaerobic fermentation of an organic waste liquid in which the waste liquid is fermented under anaerobic conditions in a packed fermentation vessel (1) containing an array of packing elements (6) characterized in that the fermentation is carried out by passing the waste liquid through the array of packi elements (6) under conditions such that: (1) the flow rate, per unit area, of the waste liquid across a plane normal to the direction of flow is substantially uniform; and (2) the organic waste liquid flows smoothly over the surfaces of the array of packing elements with relatively few abrupt changes in the rate of fl ow. 2. A method as claimed in claim 1 characterized in -that the was liquid is flowed upwardly through the array of packing elements. 3. A method as claimed in claim 1 or claim 2 characterized in that the waste liquid is introduced into the array of packing elements (6) via a diffuser or fl ow-strai tener (11). 4. A method as cl imed in any one of the preceding claims chara erized in that the array of packing elements (6) comprises an array of dumped packing elements having an aspect ratio of less than unit 5. A method as claimed in claim 4 characterized in that the packing elements have an aspect ratio of less than 0.66, preferably from 0.5 to 0.25.

Inventors: MCKEOWN KEVIN JOSEPH, MCKEOWN, KEVIN, JOSEPH
Applicants: MCKEOWN KEVIN JOSEPH, MCKEOWN, KEVIN, JOSEPH
Application year: 1982
Main classification: C02F3
IPC codes: C02F3, C02F11