The present invention relates to a method for fermenta¬ tion of especially highly viscous substrate in a fermen¬ tation tank with a controlled mixing of air and an arrangement for mixing air into the substrate.

It is necessary to supply air to the fermentation tank in order to make the microorganisms grow in aerobic fermentations. Such arrangements for supply of air may be designed in many ways. One known arrangement for supply of air consists of a tube coil provided with holes which is arranged close to the bottom of the fermentation tank. The curtain of air bubbles which is formed is then distributed in the tank by means of a mixer of turbine type.

Air addition must take place also when waste liquids are purified biologically. In this connection surface aerators as well as different kinds of submerged aeration arrangements are used. In SE 351 198 there is described an arrangement comprising an impeller and an unperforated tube for addition of gas which tube leads to the vicinity of the impeller boss.

When this kind of aerator is used the liquid flows through a distinct aeration zone, where the air is powerfully mixed with the liquid. The mixture of air and liquid then flows towards the surface. The liquid which passes the aeration zone is saturated with oxygen, but it is only a smaller part of the liquid in the fermenta¬ tion tank which passes the aeration arrangement. In the tank there are consequently partial volumes which are

saturated with oxygen and partial volumes which miss oxygen. It is very energy demanding to dimension the capacity for mixing in such way that the whole volume in the tank is brought to flow past the aeration arrange- ment.

In industrial fermentations with uniform cell cultures there are many possibilities to control the fermenta¬ tion. In spontaneous fermentations or wild fermentations as biological purification of waste water or liquid com¬ posting control of the addition of oxygen is one of few possible control methods.

According to the invention there is proposed a method where the addition of air is controlled, which method may be used in industrial fermentations where one desires to control the addition of oxygen to the liquid as well as different kinds of wild fermentations, where one needs to control the fermentation course and/or the formation of by-products.

The proposed method is mainly characterized in that the addition of air to the substrate takes places in two steps. In the first step a small part of the substrate is mixed with a larger part of air in such a way that a foam comprising air bubbles is formed. In the second step the formed foam is distributed and mixed into the substrate in a mixing zone close to the bottom in the fermentation tank, at which very small air bubbles are obtained, the rising speed of which is so low that the degree of utilization of oxygen in percent of the added amount of oxygen is above 20 %.

In a fermentation tank, where the method of the inven- tion is used, air bubbles are distributed evenly in the

tank at a certain moment. They all move upwards towards the surface with a varying speed. New air bubbles are supplied at the bottom of the tank all the time. Not only the air bubbles but also the surrounding liquid move upwards towards the surface. Liquid which has reached the surface and has been impoverished of air moves downwards towards the bottom. In this way a self- controlling stable system is obtained. In the zone where the foam formation takes place the conditions are turbu- lent, but for the rest only a limited mixing effect is needed. Foam consists of a network of thin liquid lamellas which surround small limited volumes of gas. When the foam is formed energy is added to the liquid which corresponds to the surface tension in the newly formed surfaces of liquid on both sides of the foam lamella. According to the invention the foam is formed in one moment and in the second the gas volumes which are present in the foam are distributed in the sur¬ rounding substrate. The addition of air may therefore take place with a low energy consumption.

The mixing of foam may take place by forcing formed foam into the substrate by way of tube coils close to the bottom of the tank.

The air bubbles in the formed foam may also be distri¬ buted in the substrate by hydromechanical working. Such a hydromechanical working may take place with means, which by way of a shearing action divide the foam in connection with the mixing.

The air bubbles in the foam may alternatively be divided further prior to the mixing in the fermentation tank. A smaller diameter of the foam bubbles demands an increase in the amount of liquid present in the surrounding

liquid lamella. A mechanical reduction of the size of the foam bubbles must therefore be combined with an addition of liquid. This takes place most simply by a repeated remixing of already formed foam and liquid.

With such a two-step mixing of air very small air bubbles may be obtained. The diameter of the air bubbles is in the interval between some tenth part of a milli¬ meter up to some millimeters. The diameter of the air bubbles is suitably below one millimeter. Such small air bubbles have a low rising speed which means that they are available for a longer time for the microorganisms in the fermentation tank. The more viscous the substrate is the slower is the rising speed. Many small air bubbles also give smaller average distance between the air bubbles. The contact area between gas-liquid is large, which also results in a high and constant degree of utilization of oxygen.

Foam forming means may if so is desired also be added to the substrate in the fermentation tank.

A device for mixing of air into highly viscous substrate according to the invention comprises a tube for addition of air to a rotating, foam forming means which is arranged submerged in the substrate in the fermentation tank. The means comprises two elements where at least one of the elements has a side with a surface structure with lowered and/or raised parts. The elements are arranged to be movable in a transversal way in relation to each other with a mutual distance corresponding to the thickness of the desired liquid film. If the parts are considered to be lowered or raised depends on the plane defined as the main plane. The form should anyhow be such that the surface of the raised parts is situated

on a small distance = the desired thickness of the liquid film from the most adjacent surface of the other element. The spaces which are formed in the lowered parts in the structure are connected to the air pipe and surrounding substrate such that a foam is formed between the elements. Means to distribute the formed foam in the substrate are arranged close to the bottom of the tank.

When the foam is formed a liquid is stretched to thin lamellas. If a larger amount of foam of a highly viscous liquid should be produced, an arrangement is needed which continuously creates thin liquid lamellas by mechanically shearing the liquid to a thin film which immediately is exposed to the air or the gas which one desires to surround. According to the invention a suit¬ able design of the surface structure of at least one of the elements may be achieved in many different ways. The structure may be formed with regular or irregular bars, grooves, rods or tube pieces. The vital thing is that the element has an upper surface which in corporation with the corresponding surface of the other element may form a thin liquid film when the elements move in relation to each other. The surface structure of the element shall also contain lowered parts intended to contain air in order to facilitate foam forming.

The elements are movable in a transversal way in rela¬ tion to each other, which means that one of the elements may be movable and the other stationary or both may be movable. The simpliest arrangement from a constructive point is that one of the elements is stationary. In the slit between the stationary and the movable element, the liquid is sheared into a thin film. Since the structure has a form with lowered or raised parts, the liquid layer is exposed to a lowered part, that is a space

which is mainly filled with air and earlier formed foam. For each passage a number of foam bubbles consequently is formed.

The simpliest way to create the necessary movement to form foam is based on rotation.

The foam forming means is suitably designed such that it also distributes the formed foam in the substrate. If it is desirable the foam formation may take place on some other place in the fermentation tank.

The formation of foam advantageously takes place between a rotating part and a stationary part. If the rotating part is shaped as a cylindrical part within a stationary house, the movable or the stationary part or both are provided with grooves or rods with a pitch having a screw form.

The foam forming means is with advantage shaped such that the two elements consist of one stationary and one rotating disc, which both have a structure with lowered and/or raised parts in the shape of radial rods, at which the space between the discs is connected to the tube for air addition and holes for addition of substrate are arranged in the stationary and/or in the rotating disc. The formed foam is pumped into the surrounding liquid by influence of the rotation.

The foam forming means may have a little diameter, which at a certain speed of the motor gives a high shearing speed. At the same time there is obtained an efficient distribution of foam in the liquid.

The distance between lowered and/or raised parts in the surface structure may be 1-100 mm, preferably 10-30 mm.

When carrying through the method according to the invention the degree of utilization of oxygen in the added air is above 20 %. How well the oxygen may be utilized depends on many factors, for example the shape of the tank, the viscosity of the substrate, the amount of microorganisms and their demands for oxygen. In many processes it is desirable to be able to use the oxygen in an optimal way. When fermentating liquid manure one wants to obtain such a high degree of oxygen utilization as 90-95 % in order to avoid emission of ammonia. When fermentating for production of energy (heat energy) one wants to avoid unnecessarily large amounts of air since the heating and moisturizing of the air which passes the substrate constitute a loss of energy.

The method according to the invention is suitable for example for a fermentation where the substrate consists of liquid manure with a high content of dispersed material. By adding the air in the form of finely divided air bubbles the fermentation may be carried through with controlled aerobic conditions, which results in an retardation of the formation of ammonia such that the emission of ammonia to the surrounding atmosphere may be kept low.

The manure which is obtained when holding animals must be treated in order not to be considered as a waste problem. Manure in solid form has since long been composted. Due to the rise of temperature to 40-60° C which takes place during the composting the amount of pathogenic bacterias, parasites and weeds is diminished. The smell is also made more agreable. Liquid manure may

also be composted, but the composting demands addition of air to the liquid.

Both when composting solid and liquid manure there is problem with losses of ammonia to the departing air. Earlier the ammonia emission has been considered as mainly a loss of nitrogen. With the increased knowledge of the dangerous effect on the environment of the ammonia it is necessary to lower the emission of ammonia. This has taken place by filtering the leaving air which has been brought to pass peat beds or acid water solutions. One has also tried to add clay or to increase the amount of straw in order to increase the amount of degradable carbohydrates, which should result in an increased concentration of carbonate and hydrogen carbonate ions. One has also tried to switch between aerobic and anaerobic fermentation by varying the amount of added air. During the aerobic fermentation an increase in pH and in formation of ammonia are obtained, while the anaerobic fermentation results in a lowering of the pH and binding of the ammonia.

None of the methods described above has worked well in practice.

By using the method according to the invention the emission of ammonia to the environment may be reduced considerably. One possible explanation of this effect is that a large part of the nitrogen in the fresh manure is bound as urea. Urea is decomposed by the enzyme ureas to ammonia and carbonate ions. At rised pH values the de¬ composing takes place by strictly aerobic microorganisms which demand a good access of oxygen. If the oxygen concentration is limited the decomposing is hindered. According to the present invention there are air bubbles

distributed in the whole volume of liquid. The oxygen from the air bubbles is dissolved into the liquid to a constant but low level.

An arrangement for mixing air according to the invention is described in the attached drawing, fig 1 of which shows the foam forming means seen partly in section, partly from the side and fig 2 of which shows a section between the both discs, as well as the disc 2 seen from above.

The arrangement comprises a tube 1 for addition of air to a foam forming means 2 which is submerged in the sub¬ strate in a fermentation tank (not shown). An upper stationary disc 3 is connected to the tube 2 for addition of air. Under the stationary disc 3 there is a rotating disc 4. Both the stationary and the rotating disc have radial grooves or rods 5 (fig 2) . In the form which is shown in figure 1 and 2 both the stationary and the movable disc have holes for addition of substrate. If the substrate contains large amounts of suspended material for example plant fibres, the means is designed with holes only in the rotating disc. The holes are suitably formed such that they are self cleaning.

The rotating disc may be provided with a sieve arrange¬ ment 8 which also rotates. The proportions between air and liquid in the foam (foam density) is controlled by the air flow through the pipe and the liquid flow through the holes together with the characteristica of the pump wheel of the discs 2 and 3 (peripheral speed and the design of the flow channels).

The pore size of the foam and consequently the size of the air bubbles in the fermentation tank is controlled

by the slit height between the stationary and the rota¬ ting disc and of the shearing speed of the liquid between the discs.

The example given below describes how the method accor¬ ding to the invention is carried through when composting liquid manure. The fermentation tank has a volume of 10 m ³ with a cylindrical form and with a bottom in the form of a frustum of a cone. The diameter was 2,2 m. From the start the tank was unisolated but during the series of trials it was shown that an isolation was demanded. The first trials were carried through with air mixers of the marks Aldo 1004kW and Biojet 2,2 kW. The latter gives a better result than the first but the result was still unsatisfactory.

Data for two fermentations of liquid manure carried through according to the method of the invention are given below. Trial 1 Trial 2

Time 5 days Nov -90 9 days Feb -91

Reaction temperature 45-52° C 43-50° C Temp. distribution in the tank +1-0° C +1-0° C Added amount of air 3,1-2,4 kg/H 4,7-3,2 kg/h

Added amount of oxygen 0,6-0,5 kg 0 ₂ /h

Utilization of oxygen 90-100 % 80-90 % Addition of energy

W/m ³ volume of the reactor 140 140 Loss of ammonia 0,3 g/h 0,5 g/h pH-value in the liquid 7,9-8,1 8,1-8,3 Dissolved oxygen in the liquid 0,5-0,8 ppm 0,2-0,6 ppm Energy consumption kWh/m ³ for treated liquid 16 16

By letting the leaving air containing ammonia pass a heat exchanger where it is cooled by air which is sucked into the tank the emission of ammonia may be lowered further. Condensed water comprising ammonia ions is then returned to the tank.

In liquid manure from cattle the viscosity may be as high as 2000 cP. As a mean value during a composting process a value of the viscosity <~^ 300 cP may be used. An air bubble of a diameter of 1 mm then has a rising speed of 1,82 mm/sek (Navier-Stok). If the composting tank contains an amount of liquid with a height of 2 m the mean holding time is around 18 min, if the movement of the bubble only depends on differences in density. Such a long contact time makes the utilization degree of oxygen high. The volume air which is mixed into the substrate may therefore be relatively small. Only 15-20 % of the air flow which earlier used air mixers added, is sufficient to give a satisfactory content of air in the liquid. Even if the power requirement per m ³ mixed air increases when the size of the bubbles is smaller, 'the net power may be low due to the reduction of the air flow.

A detailed description has been made above of a specific application of the method according to the invention. Another application is fermentations where one desires to control or influence the fermentation as for example when utilizing reaction heat from aerobic degradation.

The necessary amounts of nitrogen, potassium and phosphor are then added in the form of liquid manure, latrine or industrial waste. The amount of carbohydrate which is needed for energy production is added in the form of fibrous waste or plant material. A substrate

with a high content of energy per volume unit has inferior liquid properties. It is difficult to carry through an addition of oxygen such that the degree of utilization of oxygen is high and the losses of heat to air flowing through the liquid are kept low.

The method according to the invention may with advantage also be used when the fermentation results in products with a high viscosity.