NO772611L - PROCEDURES FOR THE TREATMENT OF BIODEGRADABLE ORGANIC MATERIAL - Google Patents

Abstract

Treatment of biodegradable material.

Description

The invention relates to the treatment of biodegradable organic material such as carbohydrates and protein-containing substances.

Solutions and suspensions of carbohydrates often occur as effluents from food factories and paper mills. Carbohydrates in avlbp from food factories also contain a significant amount of sugars which are usually found in solution although insoluble components such as starches and cellulosic substances which are in suspension may also occur. Proteinaceous material can also be found. Carbohydrates in waste water from paper mills, however, usually only contain insoluble components in suspension. Solutions and suspensions and other biodegradable organic substances can be formed as by-products or effluents from many types of chemical plants.

Such sewage liquids are difficult to get rid of. Sometimes they are temporarily stored in tanks where solids will partially settle out and are then fed to sewers for further treatment in normal sewage treatment plants or fed to rivers or other waterways. The authorities usually require payment for the treatment of these unwanted sewage liquids or for the right to discharge the sewage into a river. When solids are separated in settling tanks, these substances are buried in holes in the ground specially excavated for these purposes. This has also required significant costs and is a poor solution from an environmental protection point of view.

Later, certain factory plants of the type in question have been equipped with their own sewage treatment plants, which include stirred sludge tanks, biological filters and sedimentation tanks, but these are expensive to build and operate and occupy relatively large areas. ,

Various methods have been proposed for treating such sewage biologically in digesters, but usually these are only economical when the sewage is relatively concentrated and do not work satisfactorily or at all when the sewage liquid is diluted, which is often the case. Usually, the drain must also be sterile.

The aim of the present invention is to provide a method for treating sewage of the type mentioned and which is particularly effective for treating diluted sewage.

The present method is a biological method and can result in the formation of biological products of the type usually called a biomass. The biomass can be used as a basis for useful and marketable products such as animal feed.

It is therefore conceivable that the procedure can be operated on

such a way that it provides economic benefits instead of the losses usually associated with methods of disposal of such sewage liquids.

Although the invention has mainly been developed with regard to the treatment of sewage, it is not limited to the treatment of sewage liquids: as such, i.e. to waste products from other manufacturing processes, but can be used for the treatment of other solutions or suspensions of biodegradable organic matter . According to the invention, one can also use solutions or suspensions that have been specially prepared for such treatment, in order to thereby "produce the biomass in an efficient manner."

Production of biomass by tower fermentation has been discussed. Fermentation towers or suspension towers are fermentation tanks that have an upright column-like working chamber containing a microorganism that digests the biodegradable material fed up through the chamber, and this construction has been used for the technical production of liquids such as alcohol from sugar (e.g. in brewing) , acetic acid from alcohol (for example in vinegar production) and citric acid from molasses and other carbohydrates. Various continuous, semi-continuous and batch fermentation processes, both aerobic and anaerobes, which have used different yeasts, fungi and bacteria in fermentation towers have been proposed. In such continuous and semi-continuous processes, the biodegradable solution or suspension and usually a gas containing oxygen has been quickly up through the tower and the formed liquid and gas is quickly out of the working chamber or fermentation chamber through separate outlets. Problems usually arise with the formation of foam on the surface of the liquid in the chamber and an expansion chamber is usually arranged above the liquid outlet from the working chamber where the foam will settle and the microorganisms that would otherwise be washed out with the foam fall back into the working chamber when the foam breaks down and where the gas from the process is fed out of the expansion chamber through a separate outlet located at or near the top of the top of the expansion chamber.

In these fermentation processes for the production of liquids, the fermentation conditions are adjusted so that the growth of the microorganisms is minimized and the microorganisms are kept in the working chamber. It has been proposed to change the conditions so that the growth of certain microorganisms is favored for the production of a biomass that can be fed out together with the liquid from the working chamber and separated from the liquid and form a usable product. Although this can be achieved, it has been found that the kinetic conditions of continuous and semi-continuous processes are so complicated and the knowledge of these so incomplete that such methods have not been used so far.

For the technical production of biomass, processes (hereafter often called "older methods") have been used until now, which have been based on "air-loft" fermentation, "pressure-cycle" fermentation or a reactor with a "stirred tank". Most often, a method with a stirred tank reactor is preferred since this method relatively easily reaches equilibrium, a steady state. In the Rbre tank process, a continuous stream of a solution or suspension of biodegradable organic material containing all the necessary salts and nitrogenous substances required for microorganism growth is introduced into a tank containing a microorganism that can live satisfactorily on the biodegradable material , air is introduced into the solution or suspension and the tank contents are thoroughly mixed using an electric stirrer. During the treatment, the micro-organisms remove at least most of the biodegradable organic material and as the micro-organisms grow or rather multiply, the surplus of micro-organisms is drained out together with the liquid that leaves the tank. The bottled microorganism, or biomass, can be separated and after further treatment used as animal feed.

However, the present invention offers advantages

in relation to this older method, as will be seen in the following.

According to the present invention, a method for treating biodegradable organic matter is provided where a liquid medium containing the degradable material is caused to flow up through an upright working chamber having an aspect ratio of not less than 3:1, where a flocculating microorganism is cultivated as is capable of digesting at least part of the biodegradable organic material and oxygen-containing gas is introduced into the chamber to enable the microorganism to develop, and is characterized by the fact that the microorganism is essentially flocculating throughout the chamber and that it the resulting mixture of treated medium, gas and excess microorganism is drained off through a common outlet at the top of the chamber.

An important difference between the method according to the invention and the previously known tower fermentation processes is that the treated medium, excess microorganism (biomass) that is formed by the growth of organisms in the chamber and any other liquid or solid substances that are formed in the process or that form residues from the starting materials are drained off through the same outlet as the gas. The mixture of gas/liquid/solids, hereinafter called the "reaction product", is preferably drained off near or at the top of the chamber. The upper part of the chamber preferably has a tapered shape, e.g. like a truncated cone or dome shape, where the reaction product outlet is at the top. It has been found that such a shape prevents or reduces blocking or clogging of the outlet with solids in the reaction product and contributes to advantageous kinetic conditions. Another advantageous embodiment is that the reaction product is driven out through an opening which is essentially shaped like an inverted U.

With the method of the invention, foaming is not a problem, but can, on the other hand, be beneficial and desirable. It has been found that the gas acts as a ceiling pump for the solids and liquids that are part of the reaction product and lifts them up through the outlet. At low dilution (defined later), relatively large amounts of foamed liquid will be forced out through the opening at intervals so that the liquid level in the chamber may drop below the top of the chamber at alternating intervals, leaving a gas-filled space. Even if a related process takes place with great dilution, the emptying will happen more often and the volume of the gas-filled space will be smaller. The liquid level will then most often extend into the lower part of the outlet rib and not leave a gas-filled space in the chamber itself. By degree of dilution is meant here the ratio between (A) the volume of liquid medium containing biodegradable material and any auxiliary substances that favor the growth of the microorganisms, which are introduced into the working chamber per hour in relation to (B) the liquid volume in the working chamber.

The gas that is introduced into the working chamber can be oxygen alone or a mixture of oxygen and other gases that normally do not participate in the chemical reactions that take place in the working chamber. In particular, air will be able to form this oxygen source. In practice, it is found that the use of oxygen alone is less effective and more expensive than the use of air, as experiments have shown that when oxygen is used instead of air, the necessary amount of oxygen will be about half of the amount of air, even if only approx. one fifth of the air consists of oxygen.

It is assumed that the reason for the improvement achieved when a mixture of oxygen and another gas is used is as follows: For a given volume of oxygen introduced into the working chamber per time unit, the total gas volume introduced per time unit bkes. The gas forms bubbles and blisters in the liquid and thus reduces the volume weight of the liquid content in the working chamber. This in turn leads to an increased difference between the density of the flocculated micro-organisms and the effective density of the mixture around the micro-organism flocculates so that the gravitational effect on the flocculates is reduced and thereby makes them better retained in the chamber. Another advantage of using a mixture of oxygen and other gases is that the increase in the volume of gas introduced into the chamber per unit of time tends to improve the circulation of the chamber contents and consequently the degree of reaction.

Because the gas will rise up through the liquid in the chamber and because it is beneficial that the entire contents of the chamber are sufficiently oxygenated, it is an advantage that the gas is introduced near the chamber bottom. The gas should be distributed through the liquid in the form of small bubbles both for the above reason and to provide rapid dissolution of the oxygen. It is therefore preferred that the gas is not introduced through a single nozzle, but through a distributor which forms small bubbles. With an appropriate construction, the gas is brought up through a perforated plate at the bottom of the working chamber, as the liquid is above the plate and is not allowed to fall below it. When the chamber is not too large, the plate can advantageously be a sintered glass disc. However, a sinter glass disc may be too weak to support the weight of the liquid in a stiffer chamber and in this case a metal plate or plastic plate with separate holes can be used.

If the gas is introduced at too low a rate, microorganism growth will be inhibited due to a lack of oxygen and the process will not proceed satisfactorily. If more oxygen is introduced than is necessary, with respect to the microorganisms,

the amount of oxygen dissolved in the liquid will rise considerably.

It is therefore relatively easy to introduce too much oxygen and then to reduce the amount of strain until the amount of dissolved oxygen suddenly drops to close to zero (but not zero). This is the preferred input quantity.

A suitable measure of the input quantity (input quantity '

per unit of time) of gas in the working chamber will in the following be called "surface velocity of the gas". This is the volume of gas introduced per time unit divided by the cross-sectional area of the working chamber. It has been found experimentally that for any given system type the most favorable surface velocity will be approximately constant and independent of the chamber volume. This is for air

numbers preferably around 1-10 cmsec.<->"'", a typical value for

small working chambers are 2 cmsec. In general, it is found that higher numbers can be used in working chambers with larger volumes. When the surface velocity of the gas rises to a certain maximum value, the system becomes unstable and will not be able to be operated satisfactorily, and this maximum value increases with increasing working chamber volume.

The microorganisms used in accordance with the present invention must be flocculating, i.e. with respect to fungi approximately spherical colonies of hyphae and with respect to yeast approximately spherical aggregates of cells. It has been observed that most often the flocculants of microorganisms will develop from single cells or small cell clusters and form flocculants in the form of beads or granules whose surface may appear smooth or may have protruding threads or hyphae. All these types of flocculants will eventually tend to break up into individual cells or clusters, each of which in turn forms the basis for new flocculants. It has been found that the morphology of the microorganisms can be regulated in a simple way in connection with the present invention and that special conditions are not usually necessary. If desired, the flocculation can be favored or accelerated in known ways by means of flocculation agents, which for some yeasts are aluminum chloride or calcium chloride. Due to the simple morphological regulation, the method has a high degree of effectiveness within a wide range of biodegradable solutions and suspensions, concentrations and production quantities.

The microorganisms are predominantly flocculating throughout the working chamber. The majority of the microorganisms must thus be present in flocculent form and preferably at least 75% and preferably as much as possible of the microorganisms in flocculent form. This is in contrast to the conditions in the rebar tank process where the agitator will break up the flocs at least in the vicinity of the agitator blades.

A typical flocculant size can be 0.5 - 20 mm, especially 2 - 10 mm. Large flocculants are preferred as these can be more easily prevented from being washed out too early from the chamber.

The microorganism can be of a single type or a mixture of two or more types. Preferably there is only one type of micro-organism in the working chamber. The microorganism feeds on the biodegradable nutrients and metabolizes these into a proteinaceous biomass. Suitable microorganisms for adaptation to particular biodegradable substances to be treated can be found by experimentation. Flocculating yeast can be used, but the method of the invention is particularly advantageous when filamentous .fungi are used. A typical example of filamentous fungus is Aspergillus niger. However, although A, niger easily digests the sugar, it will not easily digest long-chain carbohydrates such as starches and cellulosic substances or proteinaceous compounds. Other microorganisms will, however, be able to digest at least some of the long-chain carbohydrates and a typical microorganism of this type is Trichoderma viride. Strains of microorganisms particularly suitable for particular biodegradable materials can be selected on the basis of experiments" Other filamentous fungi that can be used are Sporotrichum thermopile, Penicillium roquefortii, Geotrichum candidum, Thizopus spp., Mucor spp. and Fusarium spp. Among flocculating yeasts that have been used are Saccharomyces cerevisiae NCYC 1026 and Saccharomyces carlsbergensis (uvarum).

It is assumed that when carrying out the method of the invention, the relative quantity of flocculates in relation to individual cells and cell clusters will be greater in the working chamber than in the biomass that is drained from the working chamber. The microorganism flocculates will therefore tend to be held back in the working chamber probably due to gravity,

even if the concentration of micro-organisms from one part of the working chamber to another does not vary significantly due to turbulence in the chamber caused by the gas stirring the chamber contents sufficiently so that the contents are distributed evenly throughout the chamber. It has been found that in certain cases, e.g. in connection with relatively large working chambers, the concentration of microorganisms may vary from place to place in the chamber, but when equilibrium is reached, the concentration at one place will tend to remain substantially constant even if the strength of dissolution or suspension of biodegradable material varies.

Another phenomenon that has been observed is that the microorganisms tend, especially filamentous microorganisms, to capture insoluble particles that cannot be digested and carry these with them when they leave the working chamber. This will help to prevent the build-up of such particles in the working chamber.

The biodegradable material that can be treated according to the method of the invention can be or be based on e.g. effluent from the following types of food factories: dairies and cheese factories, potato processing factories such as factories for the production of potatoes and other products based on potatoes, factories for the processing of other starchy vegetables such as in the production of confectionery and confectionery, facilities for the treatment of beans or peas, e.g. e.g. for the canning of these vegetables, factories for the production of palm oil and sugar factories and sugar processing facilities that produce e.g. candies, mineral water and caramel or sugar charcoal. The invention can also be applied to the treatment of waste from fermentation plants such as Breeding fluids containing organic acids such as citric acid and acetic acid. Optionally, biodegradable solutions and suspensions can be prepared especially for treatment by the present method.

The liquid medium is usually water.

In order for the microorganism to thrive, it must also be supplied with relatively small amounts of nitrogen-containing substances and even smaller amounts of certain salts. These compounds and salts are known in the art. Suitable substances may be found originally in the solution or suspension to be treated, especially if it consists of effluents from certain types of food factories, but if some or all of these are absent, they must be introduced into the mixture. Preferably, they are added to the solution or suspension before the treatment, although they could at least theoretically be added to the material during the treatment. For the sake of simplicity, the solution or suspension together with the necessary nitrogen-containing substances and salts of the type described will be called "starting material". In order for the operation to take place satisfactorily, the starting material must be clean, at least in the sense that it is not significantly contaminated with toxic substances and does not contain large amounts of polluting microorganisms. However, it is not usually necessary to sterilize the starting material as it has been discovered that foreign microorganisms that are introduced into the working chamber together with the starting material are not able to compete with the selected microorganisms and are washed out of the chamber because they have been given the opportunity to establish themselves . This is particularly evident at high levels of dilution. When filamentous fungi are used, the pH will further drop considerably due to acid formation during the growth of the fungus and this increased acidity will inhibit the growth of competing microorganisms such as yeasts and bacteria. When using the known older methods, it is usually necessary to sterilize the starting material to prevent unwanted microorganisms from entering the working chamber and growing there in competition with the selected microorganism.

It has also been found that if the present process reaches equilibrium and the supply of starting material stops for a certain period, the microorganisms continue to live in the working chamber and the process can be restarted satisfactorily without special precautions. If it e.g. starting material is not introduced for 48 or 60 hours, which may be the case if the process is used to treat sewage from a factory that is closed over Thursday and Sunday, the process can normally be restarted without difficulty. During this time, no flow of liquid takes place through the working chamber and foreign microorganisms will be able to grow and establish themselves in much higher concentrations than when the process is in normal operation. Nevertheless, it is usually found that when the process is restarted, the foreign microorganisms are washed away rather quickly and the process returns to approximately the original state of equilibrium.

With the help of the method of the invention, and particularly when filamentous fungus is used as a microorganism, a state can be reached which is such that if the degree of dilution alone were increased gradually, the concentration of microorganisms in the working chamber would drop steadily. The method is very effective when used under these conditions. In order to determine whether a special method is operated under this condition, the concentration of microorganisms in the chamber is measured for varying degrees of dilution when equilibrium is reached in each case.

The mentioned condition does not occur when using

ter processes similar to the stirrup tank process. If the concentration of biodegradable material in the starting material that is fed into the tank, (i.e. the working chamber), at a constant degree of dilution in this known stirred tank process is increased until the concentration of microorganisms in the tank cannot be increased further, by increasing the concentration of biodegradable material, and the degree of dilution then increase gradually,

it is found that the concentration of micro-organisms in the tank initially remains essentially unchanged and that when a certain critical degree of dilution is reached, the concentration of micro-organisms drops sharply due to the fact that the micro-organisms are suddenly no longer able to resist the straining force so that the majority are suddenly washed away out of the tank and process

is therefore no longer usable.

This critical dilution rate depends on a number of factors, an important factor of which is the type of microorganism used, but typically the critical dilution rate for the stirred tank process will be in the range of 0.1 - 0.5 h""^. Although the method according to the present invention also has a critical degree of dilution, this will usually occur after a uniform reduction of the microorganism concentration mentioned above

for and is much higher than for the stirrup tank process. E.g. can the present method be operated at dilution rates of

up to approx. 7 h<_1>.

In contrast to the stirred tank process and similar methods, it has also been found by carrying out the present method that, at least when the concentrations of biological

degradable material in the starting material lies within the normal range, the concentration of microorganisms in the working chamber does not vary significantly when concentra-

The concentration of biodegradable material in the starting material varies, even if the microorganism concentration varies with the degree of dilution and possibly the composition of the starting material. If the concentration or strength of the starting material is reduced to a very low level, proper-

another time when there is an insufficient amount of biological

degradable material in the starting material to enable

create a steady and continuous growth of microorganisms in the

the pickling chamber, and the method will no longer operate efficiently. At low concentrations of biodegradable material in the starting material and at a given degree of dilution, the microorganisms can develop in the working chamber, but the growth can be so slow that very little surplus of microorganisms is formed. As the concentration of the starting material increases further and at the same degree of dilution, the growth of the microorganisms increases, but as the concentration of microorganisms in the working chamber remains constant, the process produces biomass in increasing quantities. When the concentration of the starting material reaches a certain value, for the same degree of dilution, the production of biomass reaches a maximum value and the surplus of biodegradable material is emptied with the product.

When using the present method, it is generally found that this maximum productivity increases steadily with increasing degree of dilution until the micro-organism starts to be washed out of the chamber at a high degree of dilution (e.g. at least 3.h"^). This is in contrast to the stirred tank process where, although the productivity increases much faster with increasing degree of dilution until the maximum value is reached, the productivity then falls rapidly to a very low value which occurs at the critical degree of dilution which is generally much lower than the critical degree of dilution for the present method Fig. 1 and 2 in the attached drawings shows typical curves and is intended to illustrate some of the differences between the present method and the stirred tank process. On the curves Fig. 1 and 2 the scale is linear and starts from zero where the axes intersect. In each diagram the abscissa is a measure of the degree of dilution and the scale is the same in both cases In Fig. 1, the ordinate is a must for the concentration of micro-organisms for wheat r given degree of dilution, while in fig. 2, the ordinate is a measure of maximum productivity at a given degree of dilution. In both figures, the solid curve denotes a typical method according to the present invention, while the dashed line denotes a typical retank process. Fig. 1 shows how the concentration of micro-organisms in the working chamber (under equilibrium conditions) decreases steadily with increasing degree of dilution and illustrates the fact that the maximum concentration of micro-organisms in the reaction tank may well be significantly higher than according to the present method over a certain range of degrees of dilution. However, it can be seen that the maximum microorganism concentration in the stirred tank process varies with the strength (concentration) of the starting material, so that the dashed line only reproduces figures for a given starting concentration. In a similar manner, fig. 2 that the productivity for the stirred tank process is greater than for the present method within the mentioned area. It can therefore sometimes be beneficial to use the stirred tank process when the degree of dilution falls within this range.

Due to the fact that maximum productivity increases with increasing degree of dilution, and there are other advantages associated with high degrees of dilution, it is normally advantageous to operate the process at a relatively high degree of dilution. This means that the microorganism concentration in the working chamber is relatively small and that the concentration of starting material must be correspondingly low. If the method is used for the treatment of relatively concentrated starting material, it may therefore be beneficial to dilute it for treatment.

Thus, the proposed method is particularly suitable for treating diluted starting material, e.g. solutions or suspensions containing from 0.1 - 20 g.l<->^ of biodegradable material. Preferably, the starting material contains at least 0.5 and especially 1-10 g.l<->"^ biodegradable material. However, higher concentrations such as, for example, 100 g.l""'" can be used if desired in some cases.

When carrying out this method, the real speed of the starting material flowing through the working chamber is relatively small. For this reason, it is not decisive that the starting material is supplied to the working chamber continuously and at a constant speed. The material can be fed into the chamber at intervals, e.g. semi-continuously or at irregular intervals or a mixture of these systems, provided that the way in which the system operates does not change significantly from the operating mode when the material is fed continuously and at a constant rate per unit of time.

Although the starting material will usually be introduced into the working chamber near its lower end, such a system is not the only one. E.g. the starting material can be introduced halfway up the working chamber. The most important factor in determining where the starting material is to be introduced is the need to ensure sufficient circulation of the material in the chamber and to avoid areas where the material will stay for much longer than the average residence time.

The present method is advantageously operated in such a way that, other factors being constant, if the concentration of biodegradable organic material in the starting material were to increase, an excess of biodegradable material would be formed which would be emptied with the final product#Although it is usually advantageous to operate the method in such a way that little or no biodegradable material is emptied with the product, this is not decisive and it may sometimes be desirable that such emptying occurs. In that case, the product that leaves the working chamber can be used as starting material or as a basis for starting material in a subsequent process which is either of the same type or of a different type, e.g. of an older known type. In a similar way, the starting material for the present method can consist of material that comes from other treatments.

As previously mentioned, the working chamber must have an aspect ratio of at least 3:1. The "aspect ratio" here denotes the ratio between the height of the chamber and its diameter when the chamber is in the form of a straight cylinder. When the working chamber has a different shape, the aspect ratio for such a chamber will be that of a chamber with a right circular cylinder and which is operated in an equivalent manner. This makes it possible to determine the aspect ratio of a non-cylindrical working chamber experimentally.

The format ratio for the working chamber should not be below 3:1, as the process cannot normally be operated efficiently at a lower ratio and will be close to the stirred tank process, which is characterized by the fact that the maximum concentration of microorganisms is essentially independent of the degree of dilution of

the critical degree of dilution is reached. When implementing the present invention, the format ratio is preferably at least 5:1 and preferably 7:1 to 15:1. The best range is 10:1 to 12:1. When the aspect ratio is higher than approx. 15:1, the working chamber begins to resemble a rudder and there is a risk that the microorganisms can be washed out of the chamber at low dilution

degrees. However, that situation is different from the condition that occurs in the stirred tank process in that the concentration of microorganisms, until the leaching takes place, decreases steadily with increasing degree of dilution.

The size of the working chamber depends on the amount of starting material to be processed per unit of time and of the concentration (strength) of the starting material. As explained earlier, the concentration of the starting material determines the maximum degree of dilution. which can be used if you want minimal biodegradable material emptied with the product, and the maximum degree of dilution together with the volume of the starting material per unit of time again determines the volume of the working chamber. Since it is often advantageous to work at relatively high dilution rates, the working chamber need not be large.

In contrast to the stirred tank process, mechanical devices will not be beneficial in the present method, and such expenses are therefore saved and reproducibility is increased.

Furthermore, a certain amount of back-mixing can be beneficial and, therefore, paddles and perforated plates are not required in the working chamber, in contrast to other tower fermentation processes.

Typical apparatus for carrying out the invention is schematically shown in fig.'3. The apparatus consists of a container 10 where the majority of the container internally forms the working chamber 11 in the form of a straight circular cylinder with a vertical axis. The lower boundary of the working chamber is indicated by a perforated plate 12 near the lower end of the container, as the plate distributes the air from an air intake 13 in the lower end of the container. The starting material is introduced through an inlet 14 just above the plate. The top of the chamber 15 is dome-shaped and is connected at the top with an outlet pipe 16 with an ombby U shape,

for previously described reasons. A water jacket 17 guides heated or cooled water around the container to keep the contents at the desired temperature. Various aids are illustrated: a lower sampling opening 18, thermometer 19, thermistor 20, a probe 21 for measuring the oxygen concentration dissolved in the liquid in the chamber, a pH meter 22, upper sampling opening 23, opening 24 for introducing material into the working chamber

moret and a pH reference electrode 25.

The container can be made of suitable material such as e.g. plastic. It has been found that the contents of the working chamber usually become acidic and that it can reach a pH as low as 1.5. The container lining must therefore be chosen so that it does not decompose under these acidic conditions. If desired, the product from the container can be treated with lime or treated in another way so that it is neutralized or made less acidic, however, such treatment is usually unnecessary.

Depending on the type of starting material and factors such as probable variations of the concentration with time, the present method can be used either alone or in combination with other processes. E.g. a relatively concentrated effluent or similar can be treated quickly in the stirred tank process, but in such a way that certain sugars or sugar-containing compounds are emptied out with the product, and after separation of the biomass the product can be treated according to the present method. Alternatively, the first of two treatment steps can also take place according to the present method. Here, too, it may be beneficial to use the present method as a prior step in an existing treatment system. It can e.g. It may happen that a food factory has its own system for treating sewage liquids. If the facility is then expanded, the sewage may overload the existing treatment system. In order to avoid building a new system in parallel with an existing system, it may be economically advantageous to treat all sewage according to the present method and to direct the discharge from the present method to the already built system.

The proposed method can be used to reduce the BOD (biological oxygen demand) and the solids content in industrial sewage so that it becomes more suitable for ordinary sewerage.

The biomass in the product can be separated from the rest of the product in a suitable way. For example the separation can take place by gravity in a sedimentation tank, although one difficulty is that part of the biomass tends to float and must be skimmed off from the tank. Alternatively or additionally, the biomass or the remaining biomass can be separated by centrifugation or filtration, e.g. in a rotating vacuum filter. Fixed pairs of ticks embedded in the biomass may remain there or be separated from it. After separation, the biomass can be used and pulverized or pressed into pellets for storage or processing.

The relatively large microorganisms that can be used in the present method make it possible to separate the biomass easily by filtration. Furthermore, the morphology of the microorganisms is such that colloidal and suspended solids can be carried along in such a way that these can also be recovered by normal filtration techniques so that complicated separation processes are avoided.

The biomass's protein content will depend on the starting material and the microorganism used. The method of the invention is particularly suitable for the production of protein-rich biomass (e.g. at least 30 g.lOO g"<1>). The proteins exhibit a broad amino acid spectrum and generally have a higher nutritional value than protein derived from most vegetables and grain types. The method of the invention can used for the production of biomass suitable for food for humans or animals, eg fish, livestock or livestock, or as a fertilizer or soil conditioner.

The invention shall be illustrated by the following examples.

Example 1

Effluent from a dairy was treated according to the present invention in apparatus of a similar type as shown in fig. 3. The effluent contained 2.5 g.l<-1> of solids on a weight basis (the solids are the components that remain when the liquid phase is evaporated), of which 65% by weight was sucrose and the rest, 35% by weight, milk solids such as lactose, protein are like casein, salts and vitamins. The sewage was treated in a container with a working chamber with a volume of approx. 1000 liters and a format ratio of 10:1, as the working chamber was 5 m high and 50 cm in diameter. The working chamber contained a strain of Aspergillus niger which digested sucrose and part of the lactose. To ensure that the microorganism had sufficient nitrogen, ammonium nitrate was added to the effluent in a concentration of .0.2 g.l<->^ and in addition disodium hydrogen phosphate in a concentration of 0.05 g.l<->^. The effluent was quickly passed through the working chamber at a dilution rate of 0.17 h""<*>"and the chamber contents were kept

at 30°C. The microorganism's activity emits heat and it was not usually necessary to introduce a lot of extra heat to

keep the temperature at this level. Air was rushed through the chamber' with a surface velocity of 2 cm.sec."''". After reaching a state of equilibrium, it was found that the concentration of microorganisms in the chamber was 2.0 g.l"<1> (measured as dry weight), while the concentration of microorganisms in the effluent was slightly below 1.0 g.l"<1>. At least 90% of the casein was captured by the microorganisms and removed with them from the chamber. pH dropped to 2.9. No effort was made to sterilize the effluent, but few unwanted microorganisms were found in the chamber. The microorganisms in the emptied product were separated using a vibrating sieve. Alternatively, a centrifuge or a vacuum sieve could have been used. The concentration of solids in the final product was 0.2 g.l"<1> on a weight basis.

Example 2

A series of experiments of a similar nature to example 1 was carried out, but with varying concentrations of solids in the starting material and varying degrees of dilution. In each case, the starting material was a solution of sucrose in water, together with the usual smaller amounts of nitrogenous substances and other salts. The starting material was autoclaved to sterilize it. The experiments were carried out in a chamber with a volume of 10.5 1. The results

goes off fig.' 4 and 5.

In fig. 4 is the abscissa, a measure of the degree of dilution (h-1) and the ordinate a measure of the concentration of microorganisms in the working chamber (g.l<->^) after reaching a state of equilibrium (steady state).

The different symbols denote the following concentrations of sucrose in the starting material:

O 55 g.l<-1>

© 27.5 g.l"<1>

O 10.0 g.l"<1>

5.0 g.l"<1>

2.5 g.l"<1>

At 2.0 g.l"1, equilibrium had not been reached.

It will be seen that within the experimental margin of error the concentration of microorganisms in the working chamber was independent of the sucrose concentration in the starting material. It can also be seen that the concentration of microorganisms in the working chamber fell steadily with increasing degree of dilution,

In fig. 5, the abscissa is again a measure of the degree of dilution (h~^), but the ordinate is a measure of the productivity, i.e. the weight of microorganisms (measured as dry weight) in the product that is drained from the working chamber per volume unit of the chamber per time unit (g.l<-1>.h-<1>).

It will be seen that at a given degree of dilution the productivity will increase with increasing concentration of sucrose until a maximum value is reached, which maximum value is independent of the sucrose concentration. You will also see that maximum productivity increases with increasing degree of dilution.

In addition, one can easily calculate that when treating a given amount of sucrose per unit of time, greater productivity can often be achieved by increasing the dilution of the starting material and increasing the degree of dilution for the tower accordingly so that the same amount of sucrose enters the container per unit of time.

Example 3

A series of experiments of a similar type to example 2 were carried out, but with the difference that the sucrose solution was not sterilized and that only a solution with a single concentration was used, namely 2.5 g.l<-1>. The experiments were carried out at similar dilution rates as in example 2 and further at much higher dilution rates. The results are shown graphically in fig. 6 and 7. In fig. 6 is the abscissa. a measure of the degree of dilution (h<->^) and the ordinate a measure of the concentration of microorganisms (g.l"''") in the working chamber. In fig. 7, the abscissa is a measure of the degree of dilution (h""<*>") and the ordinate is a measure of productivity (g.l "'".h "*").

The first part of the curve in fig. 6 as far as point A generally corresponds to the curve in fig. 4, although the real concentrations of microorganisms at different degrees of dilution differ from these concentrations in example 2 on

because the solution was not sterilized.

After the curve has flattened out, the micro-organism concentration begins to fall gradually with increasing degree of dilution. Above a degree of dilution of approx.-1.5 h"<1>, the curve falls downward again and higher than a degree of dilution of approx. 3.0 h"*1, the concentration of microorganisms in the working chamber falls more rapidly, but still fairly evenly.

The corresponding productivity figures appear in fig. 7. You will see here that the increase in productivity with the degree of dilution continues until the degree of dilution reaches a value of approx. 3.0 h"<1>after which productivity again falls. Higher than a dilution rate of about 3.0 h"<1>leaching begins to occur, but this is much less marked than when using the stirred tank process.

It must be emphasized that these dilution rates are

much higher than the dilution rates used in the stirred tank process.

Example 4

The following experiment illustrates the variation in the microorganism's concentration in the working chamber with the variation of the surface velocity of the gas (gfh). The starting material was as in example 2, the sucrose concentration being 27.5 g.l""''. The starting material was introduced into a work chamber with format ratio 12:1 and volume 10.5 1 in apparatus as shown in fig. 3, at a degree of dilution of 0.088 h""<1>. The temperature was 30°C. After establishment of equilibrium, the microorganism concentration (Aspergillus niger) in the working chamber was measured as tbrr weight.

Example 5

Effluent from a palm oil mill was treated according to the invention. The sewage comes from a factory that processes coconuts for the production of coconut oil. In this treatment, coconut butter is ground up in the presence of water and the resulting mixture is steam distilled. The distillate consists of a mixture of coconut oil and a water fraction that differs from the first. The coconut oil is drained off and the water fraction is mixed with the distillate residue or sludge. It is this mixture of sludge and water fraction that makes up the sewage. Until now, the sewage, which mainly consists of cellulose, fibers and sugar, has quickly been discharged into rivers.

The effluent used for the experiments contained 8.38 g.l"<1> of total solids, of which the carbohydrate content was 4.48 g.l""<*>". Ammonium sulfate and sodium dihydrogen orthophosphate were added to this waste in equal amounts of 1/10 of the carbohydrate weight. The sewage was treated in apparatus as in fig. 3 with a working chamber with a volume of 10.5 1 and an aspect ratio of 12:1. Air was supplied with a surface velocity of 2 cm.sec."<1> and the temperature in the working chamber was 30°C.

At a dilution rate of 0.10 h after reaching steady state, the total amount of filterable solids in the working chamber (measured as dry weight) was 6.7 g.l"<1> and the microorganism content (A. niger) (measured as dry weight) was 5 .79 g.l"<1>.

At a dilution rate of 0.20 h~\ after reaching steady state, the concentration of total filterable solids in the working chamber (dry weight) was 3.5 g.l"<1> and the micro-organism content (dry weight) 2.93 g.l"<1> .

Claims (47)

Process for treating biodegradable organic material where a liquid mixture containing said material is caused to flow upwards through an upright working chamber with a format ratio of at least 3:1 where flocculating microorganisms capable of digesting at least part of the biodegradable material are grown in the working chamber and oxygen-containing gas is introduced into the chamber so that the microorganism can develop, characterized in that the microorganism is predominantly flocculating throughout the chamber and the resulting mixture of treated liquid medium, gas and excess microorganisms is emptied out through a common outlet at the top of the chamber. 2. Method as stated in claim 1, characterized in that the formed mixture is emptied near or at the very top of the chamber. 3. Method as stated in claim 1 or 2, characterized in that the upper part of the chamber is conically tapered. 4. Method as set forth in claim 1, 2 or 3, characterized in that the upper part of the chamber has the shape of a straight-cut cone or dome. 5. Method as stated in claim 3 or 4, k a r a!Jk-terized. in that the common outlet is arranged at the top point of the upper part of the chamber. 6. Method as stated in one or more of the above claims, characterized in that the joint outlet has the shape of an inverted U-shaped rudder. Method as specified in one or more of the above requirements, characterized in that the process is controlled so that if the degree of dilution (as defined) was increased. gradually, the concentration of microorganisms in the chamber would decrease steadily. 8. Method as stated in one or more of the above claims, characterized in that the process is controlled so that, when other factors are constant and if the concentration of biodegradable material in the liquid medium was increased, an excess of biodegradable material would be formed which. was exhausted with the resulting mixture. 9. Method as stated in one or more of the above claims, characterized in that only one type of flocculating microorganism is cultivated in the working chamber. 10. Method as stated in one or more of the above claims, characterized in that the flocculating microorganisms are a filamentous fungus. 11. Method as stated in claim 10, characterized in that said filamentous fungus is Aspergillus niger. 12. Method as stated in claim 10, characterized in that said filamentous fungus is Trichoderma viride, Sporotrichum thermopile, Penicillium roquefortii, Geotrichum candidum, Rhizopus spp., Mucor spp. or Fusarium spp. 13. Method as stated in claims 1 - 9, characterized in that the microorganism is a flocculating yeast. 14. Method as stated in one or more of the above claims, characterized in that the biodegradable material in the liquid medium is or is based on effluent from a food factory. 15. Method as specified in one or more of the above claims, characterized in that the biodegradable material in the liquid medium is or is based on effluent from a coconut oil factory. 16. Method as specified in one or more of the above claims, characterized in that the liquid medium is water. 17. Method as stated in one or more of the above claims, characterized in that the biodegradable material is in solution or suspension in the liquid medium. 18. Method as stated in one or more of the above claims, characterized in that the liquid medium also contains nitrogen-containing compounds and salts which enable the microorganism to thrive in the chamber. 19. Method as stated in one or more of the above claims, characterized in that the chamber has an aspect ratio of at least 5:1. 20. Method as stated in claim 19, characterized in that the format ratio is between 7:1 and 15:1. 21. Method as stated in claim 20, characterized in that the format ratio is in the range 10:1 to 12:1. 22. Method as stated in one or more of the above claims, characterized in that the gas is a mixture of oxygen and another gas. 23. Method as stated in claim 22, characterized in that the gas is 24. Method as specified in one or more of the above claims, characterized in that the gas is introduced at or near the bottom of the chamber. 25. Method as specified in one or more of the preceding claims, characterized in that the gas is distributed in the liquid in the form of small bubbles or blisters. 26. Method as specified in one or more of the above claims, characterized in that the gas has such a flow rate that the amount of oxygen dissolved in the liquid medium is close to zero. 27. Method as stated in one or more of the above claims, characterized in that the gas is air and that the surface velocity, i.e. the volume of gas introduced per unit of time divided by the working chamber's cross-sectional area is between 1 and 10 cm.sec." <1> . 28. Method as specified in one or more of the above claims, characterized in that the biodegradable material in the liquid medium is introduced into the chamber at the lower end or close to it. 29. Method as stated in one or more of the above claims, characterized in that the amount of biodegradable material in the liquid medium is from 0.1 - 20 g.l"1. 30. Method as stated in claim 29, characterized in that the amount of biodegradable material in the liquid medium is from 0.5 - 20 g.l"1. 31: Method as stated in claim 30, characterized in that the amount of biodegradable material in the liquid medium is from 1-10 g.l" <1> . 32. Method as stated in claims 1 - 28, characterized in that the amount of biodegradable material in the liquid medium is up to 100 g.l" <1> . 33. Method as specified in one or more of the above claims, characterized in that the majority of the microorganisms are in flocculating form. 34. Method as stated in claim 33, characterized in that 75% of the microorganisms are in flocculent form. 35. Method as stated in one or more of the above claims, characterized in that as much as possible of the microorganism is in flocculent form. 36. Method as stated in one or more of the above claims, characterized in that the floc size for the microorganisms is mainly in the range of 0.5 to 20 mm. 37. Method as stated in claim 36, characterized in that the floccule size for the microorganism is mainly in the range 2 - 10 mm. 38. Method as stated in one or more of the above claims, characterized in that the chamber contents are not removed with mechanical devices. 39. Method as specified in one or more of the above claims, characterized in that backmixing of the chamber contents is not prevented by guide plates or perforated plates. 40. Method as stated in one or more of the above claims, characterized in that the degree of dilution (as defined) is over 0.5 h" <1> . 41. Method as stated in one or more of the above claims, characterized in that the apparatus used is essentially as described in fig. 3. 42. Method as stated in claim 1 and substantially as described in examples 1-5. 43. Method according to claim 1 and essentially as described. 44. Biomass produced, according to one or more of the above requirements. 45. Biomass according to claim 44, used as animal feed. 46. Biomass as specified in claim 44, used as fertiliser. 47. Biomass according to claim 44, used as a soil conditioner.