CA1103375A - Treatment of biodegradable material - Google Patents
Treatment of biodegradable materialInfo
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- CA1103375A CA1103375A CA282,414A CA282414A CA1103375A CA 1103375 A CA1103375 A CA 1103375A CA 282414 A CA282414 A CA 282414A CA 1103375 A CA1103375 A CA 1103375A
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Abstract
A B S T R A C T
A method of treating biodegradable organic material in which a liquid medium containing the material is caused to flow upwardly through an upright working chamber which has an aspect ratio of not less than 3:1, flocculent micro-organism capable of digesting at least a part of the biodegradable organic material being grown in the working chamber, gas comprising oxygen being introduced into the chamber to enable the micro-organism to grow, and the resultant mixture of treated medium, gas and biomass being discharged through a common outlet at the top of the chamber. The method is suitable for treating industrial effluent, especially of food industries, and the biomass produced may be used as a foodstuff.
A method of treating biodegradable organic material in which a liquid medium containing the material is caused to flow upwardly through an upright working chamber which has an aspect ratio of not less than 3:1, flocculent micro-organism capable of digesting at least a part of the biodegradable organic material being grown in the working chamber, gas comprising oxygen being introduced into the chamber to enable the micro-organism to grow, and the resultant mixture of treated medium, gas and biomass being discharged through a common outlet at the top of the chamber. The method is suitable for treating industrial effluent, especially of food industries, and the biomass produced may be used as a foodstuff.
Description
3~75 This invention is concerned with improvements in and relating to the treatment of biodegradable organic materials such as carbohydratecl and proteinaceous materials.
Solutions and suspensions of'carbohydrates fre~uently occur as effluents from food-procescZling plant and from paper-making plant. The carbohydrates in effluents from food-processing plant often include a significant proportion of sugars, which are usually in solution, though there may also be insoluble components such as starches and cellulosic materials which are in suspension. Proteinaceous materials may also be present. The carbohydrates in effluents from paper-maXing plant, however, usually con~ist almost entirely of insoluble components in suspension. Solutions and sus-pensions of other biodegradable organic materials may be produced as by-products or effluents from many different kinds of chemical pla~t.
Such effluents are difficult to dispose of. Sometimes they are stored temporarily in tanks, where solids may partially settle out, and they are then fed into the sewers for treatment in the normal s~wage-treatment plant, or fed into rivers or other water-ways. The authorities concerned usually demand payment for treating these unwanted effluents or for the right to dispose of the effluents in rivers. Where solids have been separated in settling tanks, these have been disposed of in waste-holes dug to receive them. That again has involved considerable co~t and is al80 poor practice from an en-vironmental point of view.
More recently some processing plants have been provided
Solutions and suspensions of'carbohydrates fre~uently occur as effluents from food-procescZling plant and from paper-making plant. The carbohydrates in effluents from food-processing plant often include a significant proportion of sugars, which are usually in solution, though there may also be insoluble components such as starches and cellulosic materials which are in suspension. Proteinaceous materials may also be present. The carbohydrates in effluents from paper-maXing plant, however, usually con~ist almost entirely of insoluble components in suspension. Solutions and sus-pensions of other biodegradable organic materials may be produced as by-products or effluents from many different kinds of chemical pla~t.
Such effluents are difficult to dispose of. Sometimes they are stored temporarily in tanks, where solids may partially settle out, and they are then fed into the sewers for treatment in the normal s~wage-treatment plant, or fed into rivers or other water-ways. The authorities concerned usually demand payment for treating these unwanted effluents or for the right to dispose of the effluents in rivers. Where solids have been separated in settling tanks, these have been disposed of in waste-holes dug to receive them. That again has involved considerable co~t and is al80 poor practice from an en-vironmental point of view.
More recently some processing plants have been provided
2. ~
3~S
with their own effluent treatment plant comprising activated sludge tanks, biological filters and lagoons, but these are expensive to install, run and maintain and they occupy relatively large areas of land.
Various methods have been considered for treating such effluents biologically in fermenters, but in general they are economic only when the effluent is relatively concentrated, and do not operate satisfactorily, or do not operate at all, when the effluents are dilute, as is often the case. Further, it is usually necessary for the effluents to be sterile.
An aim of the present invention is to provide a method of treating effluents of the kind referred to, and which i~ particularly efficient in treating dilute effluents.
The method that is the subject of the present invention i9 a biological method and may result in the production of biological products of the kind usually referred to as biomass.
The biomas~ may be u~ed as the ba~is for useful and saleable products such a~ animal food~tuff~. Thus it may well h~ppen that the method can be operated in ~uch a manner a~ to show a ~inancial profit in~tead of the 10~8 normally a~sociated with the method~ of effluent-dl~posal prevlou~ly employed.
Although the invention ha~ been developed prim~rily with a view to efflu0nt treatment it is not restricted to the treatment of effluents as ~uch, that is to the waste product~ of other manufacturing proce~ses,and it may be used in the treatment of other solutions or ~uspensions of biodegradable organic materials. The invention may in fact use solutions or suspensions specially prepared for treatment in order that the biomass may be readily obtained.
The production of biomass by tower fermentation has been considered. Tower fermenters, that is fermenters having an upright columnar working chamber containing a micro-organism which digests biodegradable material passing upwardly through the chamber, have been used in the commercial production of liquids such as alcohol from sugar (e.g. in brewing), acetic acid from alcohol (e.~. in vinegar production) and citric acid from molasses and other carbohydrates. Various continuous, semi-continuous and batch fermentation processes, both aerobic and anaerobic, employing various yeasts, fungi and bacteria in tower fer-menters, have been propo~ed. In these continuous and semi-continuou~ proce~se~, the biodegradable ~olution or suspen~ion and usually a ga~ compri~ing oxygen have been passed up through the tower, and the re~ultant liguid and gas have been di~charged from the working chamber through separate outlets. The tendoncy of the proces~s to produce a foam or froth on the surface of the liguid in tho chamber i~
u~ua~ly a problem and ~on~equently there i8 u~ually an 'expan~ion chamber' above the li~uid outlet from the working chamber, wherein the fr~th i~ allowed to ~ub~ide and the micro-organi~m, which would otherwi~e be dis-charged with the foam, i~ allowed to ~ettle back down into the working chamber a~ the foam collap~, the ga3 from the proce~ being di~charged from the expansion chamber through a separate outlet which i9 u3ually at or 3'75 near the top of the expansion chamber.
In these fermentation processes for producing liquids, the conditions are adjusted to minimise the growth of the micro-organism and to retain the micro-organism in the working chamber. It has been proposed to vary the conditions to encourage the growth of some micro-organisms to produce a biomass which can be discharged with the liquid from the working chamber and separated therefrom to result in a useful product. However, although this can be achieved, it is found that the kinetics of continuous and semi-continuous processes are so complex and knowledge of them is so incomplete that these processes are generally not used.
Hitherto, for the commercial production of biomass, processes (hereinafter referred to as 'the old processes') such as those involving the use of an 'air-lift' fermenter, a 'pressure cycle' fermenter or a 'stirred tank' reactor have been used. Generally a process involving a stirred tank reactor has been preferred since in that process a steady state i~ easily set up and maintained. In the stirred tank process a continuous flow of a solution or ~uspension of biodegradable organic material, containing any necessary salts and nitroganou~ ~ubstances required to encourage micro-organi~m growth, is admitted to a tank containing a micro-organism ~hich can live satisfactorily on the biode~radable material, air is introduced into the solution or suspension, and the contents of the tank are thoroughly mixed with the aid of a power-driven stirrer.
3'75 ~n the course of the treatment the micro-organism removes at least the major proportion of the biodegradable organic material and, as the micro-organism is continuously growing, an excess of micro-organism is continuously produced and the excess is discharged with the liquid leaving the tank.
This discharged micro-organism, or biomass, can be separated and after further treatment can be used as an animal foodstuff.
The present invention, however, offers advantages over the old process, as will be referred to hereinafter.
According to the present invention there is provided a method of treating biodegradable organic material in which a liquid medium containing the material is caused to flow upwardly through an upright working chamber which has an aspect ratio of not less than 3:1, flocculent micro-organism capable of digesting at least a part of the bio-degradable organic material being grown in the working chamber and gas compri~ing oxygen being introduced into the chamber to enable the micro-organism to grow, characteri~ed in that the micro-organi~m i8 predominantly floccu~ent throughout the chamber~ and the resultant mixture of treated modium, ga~ and ~urplu~ micro-organi~m i~ di~charged through a common outlet at the top of the chamber.
one important way in which the method of the invention is distinguished from the previously trie~
tower fermentation proce~e~ i~ that the treated medium, surplus micro-organism (biomass) produced by growth in the chamber and any other liquid or solid substance~
produced by the process or being residual from the starting materials, are discharged through the same outlet as the gas. This gas/liquid/solid mixture, which will hereinafter be referred to as 'the reaction product', is preferably discharged at or near the very top of the chamber. The upper part of the chamber is preferably of upwardly tapering shape, for example frusto-conical or dome-shaped, with the outlet for the reaction product at the apex.
I~ is found that such a shape prevents or reduces blocking of the outlet by the s~lids in the reac;tion product and contributes to the advantageous kinetics of the method.
Another advantageous embodiment is the discharge of the reaction product through an outlet which is shaped substantially in the form of an inverted-U.
In the method of the invention, frothing or foaming is not a problem; on the contrary it can be desirable.
It is found that the gas acts as a lift-pump for the solids and liquids of the reaction product, ~fting them up through the outlet. At low dilution rates (defined below) relatively large quantities of frothy liquid tend to be discharged intermittently so that the level of liquid in the chamber ma~ fall intermittently below the top of the chamber leaving a gas-filled space. Although a somewhat similar process occurs at high dilution rates, the discharge occurs more frequently and the volume of the gas-filled space is reduced. In fact the level of liquid may extend into the lower part of the outlet pipe leaving no gas-filled space in the chamber itself.
By dilution rate there is meant the ratio of (A) the volume of the liquid medium containing the biodegradable material and any auxiliary substances to enable the micro-organism to thrive, which is introduced into the working chamber per hour, to (B) the volume of liquid in the working chamber.
The gas introduced into the working chamber may consist of oxygen alone or a mixture of oxygen and some other gas, that other gas normally playing no part in the chemical reactions taking place in the working chamber.
In particular, air may constitute the source of oxygen.
It is found in practice that the use of oxygen alone i9 generally less efficient and less economic than the use of air, for it often happens that where oxygen is substituted for air the amount of oxygen required is about half the amount of air required, even though only about one fifth of the air is constituted by oxygen.
It is believed that the rea~on for the improvement that occurs when a mixture of oxygen and another gas i3 employed i5 as follows. For a given volume of oxygen introduced into the working chamber per unit time the total volume of gas introduced per unit time is increased.
The gas form~ bubbles in the liquid and thus reduces the bulk density of the fluid content~ of the working chamber.
This in turn leads to ~n increased difference between the density of the flocs o~ micro-organism and the effecti~e density of the medium surrounding the flocs, so that the gravitakional effect on the flocs is increased, thereby promoting their retention in the chamber. An additional advantage of using a mixture of oxygen and another gas is that the increase in the volume of gas introduced into the chamber per unit time tends to improve the circulation of the contents of the chamber and consequently the efficiency of the reaction.
Because the gas tends to rise through the liquid in the chamber, and because it is desirable for the whole contents of the chamber to be adequately oxygenated, it is preferred to introduce the gas at or near the bottom of the chamber. It is desirable that the gas should be distributed throughout the liquid in the form of small bubbles both for the reason indicated above and to promote the rapid solution of oxygen. It is therefore preferred not to introduce the gas through a single nozzle, but to introduce the gas through a distributor which causes it to form the desired small bubbles. In a convenient construction the gas is caused to pass through a perforated plate at the bottom of the working chamber, the li~uid remaining above the plate and not being permitted to pass below it. Where the chamber is not large the plate may conveniently comprise a sintered-glass disc. A disc of sintered glass however may be insufficiently strong to carry the weight of liquid in a large chamber, in which ca~e a metal or plastics plate with individually formed holes may be employed.
~f the gas is introduced at too low a rate, the growth of the micro-organism is inhibited through lack of oxygen and the process cannot be carried out satisfactorily.
If more oxygen is introduced than is required by the micro-organism the amount of oxygen dissolved in the liquid rises considerably. It is therefore a relatively simple matter to introduce oxygen at too high a rate and then to reduce that rate of flow until the amount of dissolved oxygen suddenly falls to near zero (but not zero). This is the preferred rate to be employed.
A convenient measure of the rate of introduction of gas into the working chamber is one that will be referred to herein as the superficial gas velocity. This is the volume of gas introduced per unit time divided by the cross-sectional area of the working chamber. It has been found experimentally that in any given type of system the preferred superficial gas velocity remains substantially constant and is independent of the volume of the chamber. For air the value i~ preferably between 1 and 10 cm.sec 1, a typical value for a small working chamber ~eing 2 cm.sec 1, In general it is found that higher values can be u~ed with working chambers of greater volumes. When the superficial gas velocity ri~es to a certain maximum value the system becomes unstable and ceases to operate satisfactorily, and this maximum value increases with an increasing volume of the working chamber.
The micro-organisms employed in the present invention must be flocculent, that is, with regard to fungi, approximately spherical colonies of the hyphae, 10 .
11~);~37~
and with regard to yeasts, approximately spherical aggregates of cells. It has been observed that in general the flocs of the micro-organism tend to develop from individual cells or small clusters of cells and lead to the formation of flocs in the form of pellets or granules the surface of which may appear smooth or may appear to have outwardly extending filaments or hyphae. Whatever their appearance the flocs tend eventually to break up to form individual cells or clusters each of which could form a basis for a new floc.
It is found that the morphology of the micro-organisms can be controlled easily in the method of the present invention and that usually no special conditions are necessary. If desired, flocculation may be enhanced by known means such as by the pre~ence of flocculating agents e.g.,for some yeasts, aluminium chloride or calcium chloride. Because of the ease of morphological control the method has a high efficiency over a wide range of biodegradable solution and suspen~ion concentrations and throughp~t r~tes.
The micro-organism is predominantly flocculent throughout the working chamber. The majority of the micro-organism 9hould be in flocculent form and ad-vantageou31y at least about 75 per cent, and preferably as much as posYible, of the mirro-organism i9 in flocculent form. This is in contrast with the stirred tank pxocess wherein the agitation of the stirrer tends to break up the flocs, at least in the vicinity of the stirrer blades.
A typical floc size might be 0.5 to ~0 ~m, especially 11 .
2 to 10 mm. Large flocs are preferred to prevent them being washed out prematurely from the chamber.
The micro-organism may be of a single kind or a mixture of two or more kinds. Preferably there is only one kind of micro-organism in the working chamber. The micro-organism feeds on the biodegradable nutrients and metabolizes them into proteinaceous biomass. Suitable micxo-organisms to suit the particular biodegradable materials to be treated can be found by experiment.
Flocculent yeasts may be employed but the method of the invention is particularly advantageous when employing filamentous fungi. One typical example of a filamentous fungus is Asperqillus niqer. However, although A. niqer readily digests sugar it does not readily digest long-chain carbohydrates 8uch as starches and cellulosic materials, or proteinaceous materials. Other micro-organisms, however, are capable of digesting at least some of the long-chain carbohydrates, a typical micro-organism of this kind being Trichoderma viride. Strains of micro-organisms particularly suited to partdcular materials may be selected as the result of experiment.
other filamentous fungi that may ~e used are SPorotrichum thermoPile, Penicillium roauefortii, Geotrichum candidum, Rhi~oPus spp~, Mucor spp. and Fusarium sppO Among flocculent yeasts that may be employed are SaccharomYces cerevisiae NCYC 1026 and SaccharomYces carlsber~ensis (uvarum).
It is believed that in carrying out the method 12.
in accordance with the present invention the proportion of flocs to individual cells and small clusters of cells is greater in the working chamber than in the biomass discharged from the working chamber. The flocs of micro-organism thus tend to be retained in the working chamber owing to, it is believed, the effect of gravity, even though the concentration of micro-organism between one part of the working chamber and another may not vary to any significant extent, and indeed the turbulence in the chamber resulting from the passage of the gas through it tends to stir the contents of the chamber sufficiently to cause the contents to be substantially uniform throughout the entire chamber.
It has been found that in certain circumstances, for example with relatively large working chambers, the con-centration of micro-organism may vary from place to place in the chamber, but once a steady state has been reached the concentration in any one place tends to remain sub-stantially con~tant, even if the strength of the solution or su~pension of biodegradable material varies.
Another phenomenon that has been observed is ~he tendency for the micro-organisms, particularly filamentous micro-organiCIms~ to trap any insoluble particles which cannot be digested and to carry these with them when they leave the working chamber. This tends to prevent any build-up of such particles in the working chamber.
The biodegradable material which may be treated by the method of the invention may be, or may be based on, 13.
'7S
for example, the effluents from the following types of food-processing plant: milk-processing plant; cheese-making plant; plant for processing potatoes, such as in the manufacture of potato-crisps and other potato-based products; plant for processing other starchy vegetables, such as in the manufacture of confectionery;
plant for processing beans or peas, such as in the canning of those vegetables; plant for producing palm oil;
and sugar-processing plant, such as plant for the production of sweetmeats, mineral waters and caramel. The invention m~y also be used in the treatment of effluents from fermentation plant, such as wastes containing organic acids such as citric acid and acetic acid. Alternatively, biodegradable solutions and suspensions may be specially prepared for treatment by the method. The liquid medium is usually water.
To enable the micro-organism to thrive it must also be supplied with relatively small quantities of nitrogen-containing substances, and still smaller quantities of certain salts. The nature of these substances and salts i9 well understood in the art. Suitable substances may be present initially in the solution or suspension to be treated, particularly if it comprises an ef1uent from certain typ~s of food-processing plant, but if some or all are absent they must be made available to the micro-organism. Preferably they are added to the solution or suspension before the treatment is undertaken, although at least in theory they could be added to the material 14.
'75 undergoing treatment. For convenience of description, the solution or suspension, together with the necessary nitrogen-containing substances and salts of the kind described above, will be referred to hereinafter as 'the starting material'.
For the satisfactory operation of the invention, the starting material should be pure, at least in the sense that it is not significantly contaminated with poisonous substances and does not contain a high proportion of contaminating micro-organisms. However,there is normally no need to sterilize the starting material for it is found that any foreign micro-organism introduced into the working chamber with the starting material is unable to compete with the selected micro-organism, and is washed from the chamber before it has had an opportunity to become established.
The effect is particularly noticeable at high dilution rates. Further, when filamentous fungi are employed the pH tends to fall considerably owing to the production of acids during growth of the fungi and thi~ increased acidity tends to inhibit the growth of competing micro-organisms such as yeasts and bacteria. When using the old processe~ on the other hand, it i~ normally nec~ssary to sterilize the starti~g material in order to prevent unwanted micro-organisms entering the working chamber and growing there- in competition with the selected micro-organism.
~t has also been found that if the present method has reached a steady state, and the suppl~ of the starting material is cut off for a period, the micro-organism 15.
3~75 continues to live in the working chamber and the method can be restarted satisfactorily without special steps being taken. For example if no starting material is introduced for 48 or 60 hours, as might be the case if it were used in treating effluent from a factory which was closed on Saturdays and Sundays, the process can normally be restarted without difficulty. During that period there is no flow of liquid through the working chamber, and any foreign micro-organisms present may be able to grow and establish them-selves in much ~arger concentrations than is the case when the method is in normal operation. ~evertheless it is generally found that when the method is restarted the foreign micro-organisms are washed away ~ite quickly, and the method returns to a steady state resembling the original steady state.
By means of the method of this invention, particularly when a filamentous fungus is employed as the micro-organism, it is pos~ible to obtain a state such that if the dilution rate alone were to be gradually increased the concentration of micro-organism in the working chamber would decrease steadily. The method i8 very efficient when opera~ed in this state. To determine whether a particular method is being oper~ted in this ~tate i8 a simple mat~er of measuring the concentration of micro-organism in the chamber for various dilution rates when a steady state has been reached in each instance.
This ~tate does not occur when using processes typified by the stirred tank process~ In that process 16.
if the concentration of biodegradable material in the starting material admitted to the tank (i.e. working chamber) at a constant dilution rate is increased until the con-centratiOn of micro-organism in the tank cannot be increased by any further increase in the concentration of the bio-degradable material, and then the dilution rate is increased gradually, it is found that the concentration of micro-organism in the tank initially remains substantially un-altered and then when a certain critical dilution rate is reached the concentration of micro-organism decreases sharply owing to the micro-organism suddenly being unable to resist the force of the flow so that the great majority of it is washed out suddenly from the tank, and the process thereby ceases to be useful.
This critical dilution rate depends on a number of factors, an important factor being the type of micro-organism employed, but typically the critical dilution rate for the stirred tank process would be in the range of from 0.1 to 0.5 h . Although the method of the present invention also has critical dilution rates these generally occur after the steady decrease in micro-organism concentration referred to above and are generally much higher than those of the stirred tank process. Fpr example, the present method can be operated at dilution rates of up to about Also in contrast to the stirred tank and similar processe~, in carrying out the present method it has been found that, at least when the concentration of biodegradable 3~5 material in the starting material is in the normal range~the concentration of micro-organism in the working chamber, while varying with the dilution rate and possibly with the composition of the starting material, does not vary to any significant extent with variation in the concentration of biodegradable material in the starting material, i.e. the strength of the starting material. Obviously, if the strength of the starting material is reduced to a very low level indeed there comes a time when there is insufficient bio-degradable material in the starting material to enable the steady and continuous growth of micro-organism to be sustained in the working chamber, and the method ceases to operate efficiently. At low strengths of starting material, and at a given dilution rate, the micro-organism may grow in the working chamber, but its growth may be so slow that very little excess micro-organism is produced. As the strength of the starting material is further increased, and at the same dilution rate, the growth of micro-organism increases, but as the concentration of micro-organism in the working chamber remains constant the method yields biomass at an increasing rate. When the strength of the starting material exceeds a certain value, for that same dilution rate, the production of biomass reaches a maximum value, and excess biodegradable material is discharged with the product.
When the present method is employed it is found in general that this maximum productivity increases steadily with increasing dilution rate until at high 3t75 dilution rates (e.g. at least 3 h 1) the micro-organism begins to be washed out from the chamber. This is in contrast with the stirred tank process, in which although the productivity rises much more rapidlv with increasing dilution rate until a maximum value is reached, therea~ter the productivity rapidly falls to a very low value indeed, this occurring at the critical dilution rate, a rate generally very much lower than the critical dilution rate of the method in accordance with the present invention.
The invention will be more clearly understood after reference tothe following detailed specification read in conjunction with the accompanying drawings wherein Fig. 1 is a diagrammatic graph illustrating the variation of micro-organism concentration with dilution rate;
Fig. 2 is a diagrammatic graph illustrating the variation of productivity with dilution rate;
Fig. 3 is a diagrammatic elevational view of apParatus suitable for use in the method of the present invention;
Fig. 4 is a graph showing the results of ex~eriments illustrating the variation of micro-organism concentration with dilution rate;
Fig. 5 is a graph showing the results of the exPerirlents illustrated by Fig. 4 but illustrating the variation of productivity with dilution rate;
Fig. 6 is a graph showing the results of further experi-ments illustrating the variation of micro-organism concentra-tion with dilution rate;
~ 19.
,1 3'75 Fiq. 7 is a qraph showing the results of the further experiments illustrated bv Fig. 6 but illustratinq the variation of productivitv with dilution rate.
Figures 1 and 2 of the accompanving drawings are typical curves and are intended to illustrate some of the distinctions between the present method and the stirred tank process. In each graph of Figures 1 and 2 the scales are linear and run from zero where the axes meet. In each graph the abscissa is a measure of the dilution rate, the scale being the same in both instances. In Figure 1 the ordinate is a measure of the concentration of micro-organism for each given dilution rate, while in ~iqure 2 the ordinate is a measure of the maximum Productivitv at each given dilution rate. In each Figure the con-tinuous curve represents a typical method of the present invention while the broken line represents a tvPical stirred tank process.
Figure 1 shows the way in which in the present method the micro-organism concentration (in a steadv state) in the working chamber decreases steadilv with increasing dilution rate and illustrates the ~act that the maximum concentration of micro-organism in the tank of the l9a.
3'75 stirred tank process may well be considerably higher than that of the present method over a range of dilution rates. It will be appreciated, however, that the maximum micro-organism concentration of the stirred tank process varies with the strength of the starting material, so the broken line merely represents the values for a given strength.~ Similarly, reference to Figure 2 shows that the productivity of the stirred tank process is greater than that of the present method in that range. Thus, it may sometimes be preferable to use the stirred tank process when the dilution rate is within the appropriate range.
In view of the fact that the maximum productivity increases with increasing dilution rate, and there are other advantages at high dilution rates, it is normally desirable to operate the new method at a relatively high dilution rate. This means that the micro-organism con-centration in the working chamber is relatively small, and that the strength of the starting material must be correspondingly low. If the method is used to treat a relatively strong starting material it may therefore be desirable to dilute it before treatment.
ThUs the method is particularly suitable for treating dilute starting material, for example ~olutions or suspension~ containing from 0.1 to 20 g.l 1 of bio-degradable materiaL Preferably the starting material contains at least 0.5, and especially from 1 to 10, gOl 1 of biodegradable materia~. However, higher concentrations, e.g. 100 g.l 1, may be employed if desired in some instances.
In carrying out the method the actual velocity of the starting material flowing through the working chamber is relatively low. For this reason it is not essential for the starting material ~ be supplied to the working chamber continuously and at a constant rate. The material may in fact be supplied to the chamber intermittently, i.e. semi-continuously, or at a non-constant rate, or both, provided that the manner in which the system operates does not differ significantly from the manner in which it operates when the material is supplied continuously and at a constant rate.
Although usually the start~ng material would be introduced into the working chamber at or near the lower end thereof such an arrangement is not essential. For example the starting material may be introduced half way up the working chamber. The most important factor in determining where the starting material is introduced is the need to ensure an adequate circulation of material in the chamber and the avoidance of places where the material would dwell for a period much longer than the average period.
rhe method is preferahly operated in such a manner that, other factors remaining constant, if the concen-tration of biodegradable organic material in the starting material were to be increased there would be an excess of biodegradable material which would be discharged with the product. Although it is usually desirable to arrange for the method to operate in such a manner that little or no 1 ~f~ 7 5 biodegradable material is discharged with the product this is not essential, and it may sometimes be desirable for this discharge to occur. In that case the product discharged from the working chamber may: be used as the starting - material, or as the basis for the starting material, of a subsequent process either of the same kind or of some other kind, such as one of the old processes. In a similar m~nner the starting material for the present method may comprise material discharged from another treatment process.
As stated hereinbefore, the working chamber should have an aspect ratio of not less than 3:1. The term 'aspect ratio' as used herein is the ratio of the height of the chamber to the diameter of the chamber, where the chamber is in the shape of a right circular cylinder.
Where the working chamber is of some other shape the aspect ratio of that chamber is the same as that of a chamber which is in the shape of a right circular cylinder and which operates in an equivalent manner.
This readily enables the aspect ratio of a non-cylindrical working cham~er to be determined by experiment.
The aspect ratio of the working chamber should not be less ~han 3:1, for below that value the method is normally inoperative and tends to resemble the stirrèd ~ànk process which is characterised in that the maximum concentration of micro-organism is substantially independent of the dilution rate until the critical dilution rate is reached.
In carrying out the present invention the aspect ratio 3'~S
is preferably not less than 5:1, and a preferred range is from 7:1 to 15:1. The range most preferred is from 10:1 to 12:1.
Where the aspect ratio is above about 15:1 the working chamber begins to resemble a pipe, and there is a danger that the micro-organism may be washed out of the chamber at low dilution rates. That situation, however, is different from that which occurs in the stirred tank process in that until the washout occurs the concentration of micro-organism in the working chamber decreases steadily with increasing dilution rate.
The size of the working chamber depends on the volume of starting material to be treated per unit time, and on the strength of the starting materia~. As explained above, the strength of the starting material determines the maximum dilution rate that can be used if there is to be a minimum of biodegradable organic material discharged with the produc~, and the maximum dilution rate, taken together with the volume of ~tarting material to be treated per unit time, in t~rn determines the volume of the working chamber. Since it is often useful to run the method at relatively high dilution rates the working chamber need not be ~ rge.
Moreover, unlike the stirred tank process, agitation by means of mechanical stirring means is not desired in the present method, and thus the cost of operation is reduced and the reliability is increased.
Furthermore, some back-mixing may be desirable and therefore the presence of baffles and perforated plates 23.
in the working chamber is not required, in contrast to other tower fermentation processes.
Typical apparatus for use in carrying out the invention is illustrated diagrammatically in Figure 3.
The apparatus comprises a vessel 10 of which most of the interior constitutes the working chamber 11, the working chamber being in the shape of a right circular cylinder with its axis vertical. The lower limit of the working chamber is defined by a perforated plate 12 near the lower end of the vessel, the plate serving to distribute air from an air inlet 13 at the lower end of the vessel.
The starting material is introduced through an inlet 14 a short way above the plate. The top of the chamber 15 is dome-shaped and is connected at its uppermost point to an outlet pipe 16 of inverted-U shape, for the reason described above. A water-jacket 17 enables heated or cooled water to flow around the vessel to assist in retaining its contents at the desired temperatures.
Various additional aids are illustrated: a lower sampling port 18, a thermometer 19, a thermistor 2~, a probe 21 for measuring the concentration of oxygen dissolved in the liquid in the chamber, a pH probe 22, an upper sampling port 23, a port 24 through which material can be int~duced into the working chamber, and a pH reference probe 25.
The ~essel may be made from any suitable material, and may for example be made of a plastics material. It i~ f~und that the content of the working chamber usually 24.
becomes acidic, and that it may reach a pH of as low as 1.5.
The lining of the vessel must therefore be chosen so that it will not be damaged by these acid conditions. If it is considered desirable the product issuing from the vessel may be treated with lime or in some other manner so as to neutralize it or make it less acid; however, such treatment is normally unnecessary.
Dependîng on the nature of the starting material, and such factors as the likely variations in its strength with time, the prese~nt method may be used either on its own or in combinatîon with other processes. For example a relatively strong effluent or the like may be treated first by the stirred tank process, but in such a manner that some sugars or sugar-based substances are discharged with the product, and after the separation of biomass, the product may then be treated by the present method.
Alternatively the first of the two treatment stages may be by the present method as well. Yet again it may be desirable to use the present method as a preliminary step in an existing treatment system~ It may happen, for example, that a food-manufacturing plant has its own effluent~treatment system. If the plant is then enlarged the ëffluent may overload the existing effluent-treatment system. To avoid the need for adding a further system in parallel with the existing one it may be commercially advantageous ~o treat all the effluent by the present method and to pass the outflow from the present method to the existing system~
3~5 The present method can be employed to reduce the BOD
(biological oxygen demand) and the solids content of an industrial effluent, thereby making it more suitable for conventional disposal.
The biomass in the product may be separated from the remainder of the product in any suitable manner. For example the separation may be effected gravitationally in a settling tank, though one difficulty is that some of the biomass tends to float and may have to be skimmed from the tank. Alternatively or in addition, the biomass or remaining biomass may be separated by a centrifuging process or by filtration such as by means of a rotary vacuum filter. Solid particles trapped with the biomass may remain with it, or separation may be effected.
After separation, the biomass may be dried and then powd~re~ or pelletised for storage or processing.
The relatively large micro-organisms which can be employed in the present invention enable the biomass to be separated easily by filtration. Moreover, the morphology of the micro-organism i8 such that colloidal and suspended solids may be entrained in such a way that they too can be recovered by normal filtration techniques, thereby ob~iating complicatad separation means.
The protein content of the biomass will depend on the starting materi~l and on the micro-organism employed.
The method of the invention is especially suitable for producing a protein-rich biomass (e.g. at least 300 g.lOOg ).
The protein exhibits a wide amino acid spectrum and ~6.
1~3~'75 generally has a higher nutritional value than protein derived from most vegetable and cereal sources. The method of the present invention may be employed to produce biomass which is useful as a foodstuff for humans or for non-human animals, for example fish, pets or farm animals, or as a fertilizer or soil improver.
The invention is illustrated in the following Examples.
EXAMPLE I
Effluent from a milk-processing plant was treated by a method embodying the present invention in an apparatus similar to that shown in Figure 3. The effluent contained 2.5 g.l 1 solids by weight (the solids being those constituents separable by evaporation of the liquid phase), of which 65% by weight wa8 ~ucrose and the remaining 35%
by weight were milk ~olids such as lactose, proteins including ca~ein, ealts and vitamins. The effluent was treated in a vesael with a working chamber of appr~ximately 1000 1 and having an aspect ratio of 10:1, the working cham~er being 5 m high and 50 cm in diameter. The working chamber contained a strain o~ AsPeraillu~ niqer which was readily able to dige3t sucrose and at least ~ome lactose. To ensure the micro-organism had sufficien~ nitrogen, ammonium nitrate was added to the e~fluent at a concentration of 0.2 g.l 1, and in addition disodium hydrogen phosphate was added at a concentration of 0.05 g.l . The effluent was passed through the working chamber at a dilution rate of 0.17 h 1, and the contents of the chamber were maintained at 30C.
The activity of the micro-organism generates heat, and it 3'75 was normally unnecessary to add much additional heat to maintain the temperature at the desired value. Air was passed through the chamber at a superficial gas veloc~ity of 2 cm.sec 1 When a ~eady state was reached it was found : that the concentration of micro-organism in the chamber was 2.0 g.l 1 (measured as dry weight), while the concentration of micro-organism in the effluent was slightly less than 1.0 g.l 1 At least 90/0 of the casein was trapped by the micro-organism and discharged with it from the chamber. The pH fell to 2.9. ~o steps were taken to sterilize the effluent, but few unwanted micro-organisms were observed in the chamber. The micro-organism in the discharged product was separated by means of a vibratory S~¢~
se~e. Alternatively a centrifuge could have been used, ~\~VQ
or a vacuum ~o~re could have been used. The concentration of solid~ in the di~charged product was 0.2 g.l 1 by weight.
~X~MPLE II
~ serie~ of experLment- was carried out, each being similar to the method de~cribad in Example I but with varying concentration~ of solids in the starting material and varying dilution rate~. In each instance the ~tarting material compri~ed a solution of sucrose in water, together with the u~ual ~mall ~uantities of nitrogen-containing materials and other salts. The ~tarting material wa~ treated in an autoclave to ~terilize it. The experiments were carried out in a cha~ber with a volume of 10. 5 1. The results are shown in Figure6 4 and 5.
In Figure ~ the ab~cissa is a measure of the dilution 28.
rate (h 1) and the ordinate a measure of the concentration d micro-organism in the working chamber (g.l 1) when a steady state was ac~ieved.
The various symbols repesent the following concen-trations of sucrose in the starting material:-o 55 g.l 27.5 g.l a lo.o g.l~l ~ 5.0 g.l-l ~ 2.5 g.l 1 At 2.0 g.l 1 a steady state was not achieved.
It will be observed that within the limits of experimental error the concentration of micro-organism in the working chamber was independent of the eucrose concentration in the starting material. It will also be ob~erved that the concentration of micro-organism in the working chamber decreased steadily with increasing values of dilution rate.
In Figure 5 the abscis~a is again a meaaure of the dilution rate (h 1), but t~e ordinate i9 a mea~ure of the productivity, that i~ the weight (mea~ured a~ dry weight) of micro-organi~m in the product dischar~ed from the working chamber por unit volume of the chamber per unit time (g.l lh 1)c Here it will be ob~erved that at ~ny given dilution rate the productivity increases with increasing con-centration o~ ~ucro~a until a maximum value i9 reach~d, that valuo bffing independent of the ~ucro~e concentration. It 29.
~;;L4'`~375 will also be observed that the maximum productivity increases with increasing values of dilution rate.
In addition it can readily be deduced that in treating a given quantity of sucrose per unit time a greater productivity can often be achieved by increasing the dilution of the starting material and increasing the dilution rate to the corresponding extent so that the same quantity of sucrose enters the vessel per unit time.
EXAMPLE III
A series of experiments was carried out generally similarly to those described in Example II, but differing from them only in that t.l~ su~rose solution was not s~erilized, and that a solution of only one strength was employed, that is 2.5 g.l . The experiments were carried out at dilution rates similar to those employed in the experiments of Example II and also at very much higher dilution rates. The ~esults are shown graphically in Figures 6 and 7. In Figure 6 the abscissa is a measure of the dilution rate (h 1) and the ordinate is a measure of the micro-organism concentrataon (g.1 ) in the working chamber. In Figure 7 the abscissa is a measure of the dilution rate (h 1) and the ordinate is a measure of the productivity (g.l l.h 1), The first part of the graph in Figure 6 a~ far as position 'A' is a representation generally corresponding to the graph of Fi~ure 4, though the actual concentrations of micro-organi~m at different dilution rates differ from tho~e in Example II due to the fact that the solution was 30.
3 ~337S
not sterilized.
After the curve has flattened out the micro-organism concentration continues to decrease gradually with increasing dilution rate. Above a dilution rate of about 1.5 h 1 the - curve turns downwards again and above a dilution rate of about 3.0 h 1 the concentration of micro-organism in the working chamber falls more rapidly, though still quite steadily.
The corresponding values of productivity are shown in Figure 7. Here it will be seen that the increase of productivity with dilution rate continues until the dilution rate reachesa value of about 3.0 h 1 after which the productivity falls again. Above a dilution rate of about 3.0 h 1 washout starts to occur, but the effect is ve~ much less sharply marked than it is when using the st;~red tank process.
It must be emphasised that these dilution rate~
are very much higher than the dilution rate~ u~ed in carrying out the stirred tank process.
EX~MPLE IV
The following experimental re~ults illustrate the variation in concentration of micro-organism in the working chamber with variation in the superficial ga~ velocity (sgv).
The starting material was similar to that described in Example II, the ~ucrose concentration being 27.5 g.l 1, The ~tarting material was introduced into the 10.5 1 12:1 aspect ratio working chamber of an apparatu~ similar to that of Figure 3 at a dilution rate of 0.088 h 1, The temperature was 30 C.
31.
1~''`;~375 When a steady state was achieved the concentration of micro-organism (A. niqer) in the working chamber was measured as dry weight.
sgv (cm.sec 1) concentration (g.l 1) .
1 2.39 2 3.60 3 5.27 EXAMPLE V
Palm mill effluent was treated by a method in accordance with the invention. The effluent arises from plant used in treating palm nuts to produce palm oil. In this treatment the palm nuts are ground in the presence of water, and the resultant mixture is steam-distilled.
The distillate comprises a mixture of palm oil and an aqueous fraction which separate from each other. The palm oil is removed and the aqueous fraction is mixed with the distillation residue or sludge. It is this mixture of sludge and aqueous fraction that constitutes the effluent. Hitherto, the effluent, which consists mainly of cellulose~ fibre and sugar, has been discharged into rivers.
The effluent used in the experiments contained 8.38 g.l 1 total solids of which the carbohydrate content was
with their own effluent treatment plant comprising activated sludge tanks, biological filters and lagoons, but these are expensive to install, run and maintain and they occupy relatively large areas of land.
Various methods have been considered for treating such effluents biologically in fermenters, but in general they are economic only when the effluent is relatively concentrated, and do not operate satisfactorily, or do not operate at all, when the effluents are dilute, as is often the case. Further, it is usually necessary for the effluents to be sterile.
An aim of the present invention is to provide a method of treating effluents of the kind referred to, and which i~ particularly efficient in treating dilute effluents.
The method that is the subject of the present invention i9 a biological method and may result in the production of biological products of the kind usually referred to as biomass.
The biomas~ may be u~ed as the ba~is for useful and saleable products such a~ animal food~tuff~. Thus it may well h~ppen that the method can be operated in ~uch a manner a~ to show a ~inancial profit in~tead of the 10~8 normally a~sociated with the method~ of effluent-dl~posal prevlou~ly employed.
Although the invention ha~ been developed prim~rily with a view to efflu0nt treatment it is not restricted to the treatment of effluents as ~uch, that is to the waste product~ of other manufacturing proce~ses,and it may be used in the treatment of other solutions or ~uspensions of biodegradable organic materials. The invention may in fact use solutions or suspensions specially prepared for treatment in order that the biomass may be readily obtained.
The production of biomass by tower fermentation has been considered. Tower fermenters, that is fermenters having an upright columnar working chamber containing a micro-organism which digests biodegradable material passing upwardly through the chamber, have been used in the commercial production of liquids such as alcohol from sugar (e.g. in brewing), acetic acid from alcohol (e.~. in vinegar production) and citric acid from molasses and other carbohydrates. Various continuous, semi-continuous and batch fermentation processes, both aerobic and anaerobic, employing various yeasts, fungi and bacteria in tower fer-menters, have been propo~ed. In these continuous and semi-continuou~ proce~se~, the biodegradable ~olution or suspen~ion and usually a ga~ compri~ing oxygen have been passed up through the tower, and the re~ultant liguid and gas have been di~charged from the working chamber through separate outlets. The tendoncy of the proces~s to produce a foam or froth on the surface of the liguid in tho chamber i~
u~ua~ly a problem and ~on~equently there i8 u~ually an 'expan~ion chamber' above the li~uid outlet from the working chamber, wherein the fr~th i~ allowed to ~ub~ide and the micro-organi~m, which would otherwi~e be dis-charged with the foam, i~ allowed to ~ettle back down into the working chamber a~ the foam collap~, the ga3 from the proce~ being di~charged from the expansion chamber through a separate outlet which i9 u3ually at or 3'75 near the top of the expansion chamber.
In these fermentation processes for producing liquids, the conditions are adjusted to minimise the growth of the micro-organism and to retain the micro-organism in the working chamber. It has been proposed to vary the conditions to encourage the growth of some micro-organisms to produce a biomass which can be discharged with the liquid from the working chamber and separated therefrom to result in a useful product. However, although this can be achieved, it is found that the kinetics of continuous and semi-continuous processes are so complex and knowledge of them is so incomplete that these processes are generally not used.
Hitherto, for the commercial production of biomass, processes (hereinafter referred to as 'the old processes') such as those involving the use of an 'air-lift' fermenter, a 'pressure cycle' fermenter or a 'stirred tank' reactor have been used. Generally a process involving a stirred tank reactor has been preferred since in that process a steady state i~ easily set up and maintained. In the stirred tank process a continuous flow of a solution or ~uspension of biodegradable organic material, containing any necessary salts and nitroganou~ ~ubstances required to encourage micro-organi~m growth, is admitted to a tank containing a micro-organism ~hich can live satisfactorily on the biode~radable material, air is introduced into the solution or suspension, and the contents of the tank are thoroughly mixed with the aid of a power-driven stirrer.
3'75 ~n the course of the treatment the micro-organism removes at least the major proportion of the biodegradable organic material and, as the micro-organism is continuously growing, an excess of micro-organism is continuously produced and the excess is discharged with the liquid leaving the tank.
This discharged micro-organism, or biomass, can be separated and after further treatment can be used as an animal foodstuff.
The present invention, however, offers advantages over the old process, as will be referred to hereinafter.
According to the present invention there is provided a method of treating biodegradable organic material in which a liquid medium containing the material is caused to flow upwardly through an upright working chamber which has an aspect ratio of not less than 3:1, flocculent micro-organism capable of digesting at least a part of the bio-degradable organic material being grown in the working chamber and gas compri~ing oxygen being introduced into the chamber to enable the micro-organism to grow, characteri~ed in that the micro-organi~m i8 predominantly floccu~ent throughout the chamber~ and the resultant mixture of treated modium, ga~ and ~urplu~ micro-organi~m i~ di~charged through a common outlet at the top of the chamber.
one important way in which the method of the invention is distinguished from the previously trie~
tower fermentation proce~e~ i~ that the treated medium, surplus micro-organism (biomass) produced by growth in the chamber and any other liquid or solid substance~
produced by the process or being residual from the starting materials, are discharged through the same outlet as the gas. This gas/liquid/solid mixture, which will hereinafter be referred to as 'the reaction product', is preferably discharged at or near the very top of the chamber. The upper part of the chamber is preferably of upwardly tapering shape, for example frusto-conical or dome-shaped, with the outlet for the reaction product at the apex.
I~ is found that such a shape prevents or reduces blocking of the outlet by the s~lids in the reac;tion product and contributes to the advantageous kinetics of the method.
Another advantageous embodiment is the discharge of the reaction product through an outlet which is shaped substantially in the form of an inverted-U.
In the method of the invention, frothing or foaming is not a problem; on the contrary it can be desirable.
It is found that the gas acts as a lift-pump for the solids and liquids of the reaction product, ~fting them up through the outlet. At low dilution rates (defined below) relatively large quantities of frothy liquid tend to be discharged intermittently so that the level of liquid in the chamber ma~ fall intermittently below the top of the chamber leaving a gas-filled space. Although a somewhat similar process occurs at high dilution rates, the discharge occurs more frequently and the volume of the gas-filled space is reduced. In fact the level of liquid may extend into the lower part of the outlet pipe leaving no gas-filled space in the chamber itself.
By dilution rate there is meant the ratio of (A) the volume of the liquid medium containing the biodegradable material and any auxiliary substances to enable the micro-organism to thrive, which is introduced into the working chamber per hour, to (B) the volume of liquid in the working chamber.
The gas introduced into the working chamber may consist of oxygen alone or a mixture of oxygen and some other gas, that other gas normally playing no part in the chemical reactions taking place in the working chamber.
In particular, air may constitute the source of oxygen.
It is found in practice that the use of oxygen alone i9 generally less efficient and less economic than the use of air, for it often happens that where oxygen is substituted for air the amount of oxygen required is about half the amount of air required, even though only about one fifth of the air is constituted by oxygen.
It is believed that the rea~on for the improvement that occurs when a mixture of oxygen and another gas i3 employed i5 as follows. For a given volume of oxygen introduced into the working chamber per unit time the total volume of gas introduced per unit time is increased.
The gas form~ bubbles in the liquid and thus reduces the bulk density of the fluid content~ of the working chamber.
This in turn leads to ~n increased difference between the density of the flocs o~ micro-organism and the effecti~e density of the medium surrounding the flocs, so that the gravitakional effect on the flocs is increased, thereby promoting their retention in the chamber. An additional advantage of using a mixture of oxygen and another gas is that the increase in the volume of gas introduced into the chamber per unit time tends to improve the circulation of the contents of the chamber and consequently the efficiency of the reaction.
Because the gas tends to rise through the liquid in the chamber, and because it is desirable for the whole contents of the chamber to be adequately oxygenated, it is preferred to introduce the gas at or near the bottom of the chamber. It is desirable that the gas should be distributed throughout the liquid in the form of small bubbles both for the reason indicated above and to promote the rapid solution of oxygen. It is therefore preferred not to introduce the gas through a single nozzle, but to introduce the gas through a distributor which causes it to form the desired small bubbles. In a convenient construction the gas is caused to pass through a perforated plate at the bottom of the working chamber, the li~uid remaining above the plate and not being permitted to pass below it. Where the chamber is not large the plate may conveniently comprise a sintered-glass disc. A disc of sintered glass however may be insufficiently strong to carry the weight of liquid in a large chamber, in which ca~e a metal or plastics plate with individually formed holes may be employed.
~f the gas is introduced at too low a rate, the growth of the micro-organism is inhibited through lack of oxygen and the process cannot be carried out satisfactorily.
If more oxygen is introduced than is required by the micro-organism the amount of oxygen dissolved in the liquid rises considerably. It is therefore a relatively simple matter to introduce oxygen at too high a rate and then to reduce that rate of flow until the amount of dissolved oxygen suddenly falls to near zero (but not zero). This is the preferred rate to be employed.
A convenient measure of the rate of introduction of gas into the working chamber is one that will be referred to herein as the superficial gas velocity. This is the volume of gas introduced per unit time divided by the cross-sectional area of the working chamber. It has been found experimentally that in any given type of system the preferred superficial gas velocity remains substantially constant and is independent of the volume of the chamber. For air the value i~ preferably between 1 and 10 cm.sec 1, a typical value for a small working chamber ~eing 2 cm.sec 1, In general it is found that higher values can be u~ed with working chambers of greater volumes. When the superficial gas velocity ri~es to a certain maximum value the system becomes unstable and ceases to operate satisfactorily, and this maximum value increases with an increasing volume of the working chamber.
The micro-organisms employed in the present invention must be flocculent, that is, with regard to fungi, approximately spherical colonies of the hyphae, 10 .
11~);~37~
and with regard to yeasts, approximately spherical aggregates of cells. It has been observed that in general the flocs of the micro-organism tend to develop from individual cells or small clusters of cells and lead to the formation of flocs in the form of pellets or granules the surface of which may appear smooth or may appear to have outwardly extending filaments or hyphae. Whatever their appearance the flocs tend eventually to break up to form individual cells or clusters each of which could form a basis for a new floc.
It is found that the morphology of the micro-organisms can be controlled easily in the method of the present invention and that usually no special conditions are necessary. If desired, flocculation may be enhanced by known means such as by the pre~ence of flocculating agents e.g.,for some yeasts, aluminium chloride or calcium chloride. Because of the ease of morphological control the method has a high efficiency over a wide range of biodegradable solution and suspen~ion concentrations and throughp~t r~tes.
The micro-organism is predominantly flocculent throughout the working chamber. The majority of the micro-organism 9hould be in flocculent form and ad-vantageou31y at least about 75 per cent, and preferably as much as posYible, of the mirro-organism i9 in flocculent form. This is in contrast with the stirred tank pxocess wherein the agitation of the stirrer tends to break up the flocs, at least in the vicinity of the stirrer blades.
A typical floc size might be 0.5 to ~0 ~m, especially 11 .
2 to 10 mm. Large flocs are preferred to prevent them being washed out prematurely from the chamber.
The micro-organism may be of a single kind or a mixture of two or more kinds. Preferably there is only one kind of micro-organism in the working chamber. The micro-organism feeds on the biodegradable nutrients and metabolizes them into proteinaceous biomass. Suitable micxo-organisms to suit the particular biodegradable materials to be treated can be found by experiment.
Flocculent yeasts may be employed but the method of the invention is particularly advantageous when employing filamentous fungi. One typical example of a filamentous fungus is Asperqillus niqer. However, although A. niqer readily digests sugar it does not readily digest long-chain carbohydrates 8uch as starches and cellulosic materials, or proteinaceous materials. Other micro-organisms, however, are capable of digesting at least some of the long-chain carbohydrates, a typical micro-organism of this kind being Trichoderma viride. Strains of micro-organisms particularly suited to partdcular materials may be selected as the result of experiment.
other filamentous fungi that may ~e used are SPorotrichum thermoPile, Penicillium roauefortii, Geotrichum candidum, Rhi~oPus spp~, Mucor spp. and Fusarium sppO Among flocculent yeasts that may be employed are SaccharomYces cerevisiae NCYC 1026 and SaccharomYces carlsber~ensis (uvarum).
It is believed that in carrying out the method 12.
in accordance with the present invention the proportion of flocs to individual cells and small clusters of cells is greater in the working chamber than in the biomass discharged from the working chamber. The flocs of micro-organism thus tend to be retained in the working chamber owing to, it is believed, the effect of gravity, even though the concentration of micro-organism between one part of the working chamber and another may not vary to any significant extent, and indeed the turbulence in the chamber resulting from the passage of the gas through it tends to stir the contents of the chamber sufficiently to cause the contents to be substantially uniform throughout the entire chamber.
It has been found that in certain circumstances, for example with relatively large working chambers, the con-centration of micro-organism may vary from place to place in the chamber, but once a steady state has been reached the concentration in any one place tends to remain sub-stantially con~tant, even if the strength of the solution or su~pension of biodegradable material varies.
Another phenomenon that has been observed is ~he tendency for the micro-organisms, particularly filamentous micro-organiCIms~ to trap any insoluble particles which cannot be digested and to carry these with them when they leave the working chamber. This tends to prevent any build-up of such particles in the working chamber.
The biodegradable material which may be treated by the method of the invention may be, or may be based on, 13.
'7S
for example, the effluents from the following types of food-processing plant: milk-processing plant; cheese-making plant; plant for processing potatoes, such as in the manufacture of potato-crisps and other potato-based products; plant for processing other starchy vegetables, such as in the manufacture of confectionery;
plant for processing beans or peas, such as in the canning of those vegetables; plant for producing palm oil;
and sugar-processing plant, such as plant for the production of sweetmeats, mineral waters and caramel. The invention m~y also be used in the treatment of effluents from fermentation plant, such as wastes containing organic acids such as citric acid and acetic acid. Alternatively, biodegradable solutions and suspensions may be specially prepared for treatment by the method. The liquid medium is usually water.
To enable the micro-organism to thrive it must also be supplied with relatively small quantities of nitrogen-containing substances, and still smaller quantities of certain salts. The nature of these substances and salts i9 well understood in the art. Suitable substances may be present initially in the solution or suspension to be treated, particularly if it comprises an ef1uent from certain typ~s of food-processing plant, but if some or all are absent they must be made available to the micro-organism. Preferably they are added to the solution or suspension before the treatment is undertaken, although at least in theory they could be added to the material 14.
'75 undergoing treatment. For convenience of description, the solution or suspension, together with the necessary nitrogen-containing substances and salts of the kind described above, will be referred to hereinafter as 'the starting material'.
For the satisfactory operation of the invention, the starting material should be pure, at least in the sense that it is not significantly contaminated with poisonous substances and does not contain a high proportion of contaminating micro-organisms. However,there is normally no need to sterilize the starting material for it is found that any foreign micro-organism introduced into the working chamber with the starting material is unable to compete with the selected micro-organism, and is washed from the chamber before it has had an opportunity to become established.
The effect is particularly noticeable at high dilution rates. Further, when filamentous fungi are employed the pH tends to fall considerably owing to the production of acids during growth of the fungi and thi~ increased acidity tends to inhibit the growth of competing micro-organisms such as yeasts and bacteria. When using the old processe~ on the other hand, it i~ normally nec~ssary to sterilize the starti~g material in order to prevent unwanted micro-organisms entering the working chamber and growing there- in competition with the selected micro-organism.
~t has also been found that if the present method has reached a steady state, and the suppl~ of the starting material is cut off for a period, the micro-organism 15.
3~75 continues to live in the working chamber and the method can be restarted satisfactorily without special steps being taken. For example if no starting material is introduced for 48 or 60 hours, as might be the case if it were used in treating effluent from a factory which was closed on Saturdays and Sundays, the process can normally be restarted without difficulty. During that period there is no flow of liquid through the working chamber, and any foreign micro-organisms present may be able to grow and establish them-selves in much ~arger concentrations than is the case when the method is in normal operation. ~evertheless it is generally found that when the method is restarted the foreign micro-organisms are washed away ~ite quickly, and the method returns to a steady state resembling the original steady state.
By means of the method of this invention, particularly when a filamentous fungus is employed as the micro-organism, it is pos~ible to obtain a state such that if the dilution rate alone were to be gradually increased the concentration of micro-organism in the working chamber would decrease steadily. The method i8 very efficient when opera~ed in this state. To determine whether a particular method is being oper~ted in this ~tate i8 a simple mat~er of measuring the concentration of micro-organism in the chamber for various dilution rates when a steady state has been reached in each instance.
This ~tate does not occur when using processes typified by the stirred tank process~ In that process 16.
if the concentration of biodegradable material in the starting material admitted to the tank (i.e. working chamber) at a constant dilution rate is increased until the con-centratiOn of micro-organism in the tank cannot be increased by any further increase in the concentration of the bio-degradable material, and then the dilution rate is increased gradually, it is found that the concentration of micro-organism in the tank initially remains substantially un-altered and then when a certain critical dilution rate is reached the concentration of micro-organism decreases sharply owing to the micro-organism suddenly being unable to resist the force of the flow so that the great majority of it is washed out suddenly from the tank, and the process thereby ceases to be useful.
This critical dilution rate depends on a number of factors, an important factor being the type of micro-organism employed, but typically the critical dilution rate for the stirred tank process would be in the range of from 0.1 to 0.5 h . Although the method of the present invention also has critical dilution rates these generally occur after the steady decrease in micro-organism concentration referred to above and are generally much higher than those of the stirred tank process. Fpr example, the present method can be operated at dilution rates of up to about Also in contrast to the stirred tank and similar processe~, in carrying out the present method it has been found that, at least when the concentration of biodegradable 3~5 material in the starting material is in the normal range~the concentration of micro-organism in the working chamber, while varying with the dilution rate and possibly with the composition of the starting material, does not vary to any significant extent with variation in the concentration of biodegradable material in the starting material, i.e. the strength of the starting material. Obviously, if the strength of the starting material is reduced to a very low level indeed there comes a time when there is insufficient bio-degradable material in the starting material to enable the steady and continuous growth of micro-organism to be sustained in the working chamber, and the method ceases to operate efficiently. At low strengths of starting material, and at a given dilution rate, the micro-organism may grow in the working chamber, but its growth may be so slow that very little excess micro-organism is produced. As the strength of the starting material is further increased, and at the same dilution rate, the growth of micro-organism increases, but as the concentration of micro-organism in the working chamber remains constant the method yields biomass at an increasing rate. When the strength of the starting material exceeds a certain value, for that same dilution rate, the production of biomass reaches a maximum value, and excess biodegradable material is discharged with the product.
When the present method is employed it is found in general that this maximum productivity increases steadily with increasing dilution rate until at high 3t75 dilution rates (e.g. at least 3 h 1) the micro-organism begins to be washed out from the chamber. This is in contrast with the stirred tank process, in which although the productivity rises much more rapidlv with increasing dilution rate until a maximum value is reached, therea~ter the productivity rapidly falls to a very low value indeed, this occurring at the critical dilution rate, a rate generally very much lower than the critical dilution rate of the method in accordance with the present invention.
The invention will be more clearly understood after reference tothe following detailed specification read in conjunction with the accompanying drawings wherein Fig. 1 is a diagrammatic graph illustrating the variation of micro-organism concentration with dilution rate;
Fig. 2 is a diagrammatic graph illustrating the variation of productivity with dilution rate;
Fig. 3 is a diagrammatic elevational view of apParatus suitable for use in the method of the present invention;
Fig. 4 is a graph showing the results of ex~eriments illustrating the variation of micro-organism concentration with dilution rate;
Fig. 5 is a graph showing the results of the exPerirlents illustrated by Fig. 4 but illustrating the variation of productivity with dilution rate;
Fig. 6 is a graph showing the results of further experi-ments illustrating the variation of micro-organism concentra-tion with dilution rate;
~ 19.
,1 3'75 Fiq. 7 is a qraph showing the results of the further experiments illustrated bv Fig. 6 but illustratinq the variation of productivitv with dilution rate.
Figures 1 and 2 of the accompanving drawings are typical curves and are intended to illustrate some of the distinctions between the present method and the stirred tank process. In each graph of Figures 1 and 2 the scales are linear and run from zero where the axes meet. In each graph the abscissa is a measure of the dilution rate, the scale being the same in both instances. In Figure 1 the ordinate is a measure of the concentration of micro-organism for each given dilution rate, while in ~iqure 2 the ordinate is a measure of the maximum Productivitv at each given dilution rate. In each Figure the con-tinuous curve represents a typical method of the present invention while the broken line represents a tvPical stirred tank process.
Figure 1 shows the way in which in the present method the micro-organism concentration (in a steadv state) in the working chamber decreases steadilv with increasing dilution rate and illustrates the ~act that the maximum concentration of micro-organism in the tank of the l9a.
3'75 stirred tank process may well be considerably higher than that of the present method over a range of dilution rates. It will be appreciated, however, that the maximum micro-organism concentration of the stirred tank process varies with the strength of the starting material, so the broken line merely represents the values for a given strength.~ Similarly, reference to Figure 2 shows that the productivity of the stirred tank process is greater than that of the present method in that range. Thus, it may sometimes be preferable to use the stirred tank process when the dilution rate is within the appropriate range.
In view of the fact that the maximum productivity increases with increasing dilution rate, and there are other advantages at high dilution rates, it is normally desirable to operate the new method at a relatively high dilution rate. This means that the micro-organism con-centration in the working chamber is relatively small, and that the strength of the starting material must be correspondingly low. If the method is used to treat a relatively strong starting material it may therefore be desirable to dilute it before treatment.
ThUs the method is particularly suitable for treating dilute starting material, for example ~olutions or suspension~ containing from 0.1 to 20 g.l 1 of bio-degradable materiaL Preferably the starting material contains at least 0.5, and especially from 1 to 10, gOl 1 of biodegradable materia~. However, higher concentrations, e.g. 100 g.l 1, may be employed if desired in some instances.
In carrying out the method the actual velocity of the starting material flowing through the working chamber is relatively low. For this reason it is not essential for the starting material ~ be supplied to the working chamber continuously and at a constant rate. The material may in fact be supplied to the chamber intermittently, i.e. semi-continuously, or at a non-constant rate, or both, provided that the manner in which the system operates does not differ significantly from the manner in which it operates when the material is supplied continuously and at a constant rate.
Although usually the start~ng material would be introduced into the working chamber at or near the lower end thereof such an arrangement is not essential. For example the starting material may be introduced half way up the working chamber. The most important factor in determining where the starting material is introduced is the need to ensure an adequate circulation of material in the chamber and the avoidance of places where the material would dwell for a period much longer than the average period.
rhe method is preferahly operated in such a manner that, other factors remaining constant, if the concen-tration of biodegradable organic material in the starting material were to be increased there would be an excess of biodegradable material which would be discharged with the product. Although it is usually desirable to arrange for the method to operate in such a manner that little or no 1 ~f~ 7 5 biodegradable material is discharged with the product this is not essential, and it may sometimes be desirable for this discharge to occur. In that case the product discharged from the working chamber may: be used as the starting - material, or as the basis for the starting material, of a subsequent process either of the same kind or of some other kind, such as one of the old processes. In a similar m~nner the starting material for the present method may comprise material discharged from another treatment process.
As stated hereinbefore, the working chamber should have an aspect ratio of not less than 3:1. The term 'aspect ratio' as used herein is the ratio of the height of the chamber to the diameter of the chamber, where the chamber is in the shape of a right circular cylinder.
Where the working chamber is of some other shape the aspect ratio of that chamber is the same as that of a chamber which is in the shape of a right circular cylinder and which operates in an equivalent manner.
This readily enables the aspect ratio of a non-cylindrical working cham~er to be determined by experiment.
The aspect ratio of the working chamber should not be less ~han 3:1, for below that value the method is normally inoperative and tends to resemble the stirrèd ~ànk process which is characterised in that the maximum concentration of micro-organism is substantially independent of the dilution rate until the critical dilution rate is reached.
In carrying out the present invention the aspect ratio 3'~S
is preferably not less than 5:1, and a preferred range is from 7:1 to 15:1. The range most preferred is from 10:1 to 12:1.
Where the aspect ratio is above about 15:1 the working chamber begins to resemble a pipe, and there is a danger that the micro-organism may be washed out of the chamber at low dilution rates. That situation, however, is different from that which occurs in the stirred tank process in that until the washout occurs the concentration of micro-organism in the working chamber decreases steadily with increasing dilution rate.
The size of the working chamber depends on the volume of starting material to be treated per unit time, and on the strength of the starting materia~. As explained above, the strength of the starting material determines the maximum dilution rate that can be used if there is to be a minimum of biodegradable organic material discharged with the produc~, and the maximum dilution rate, taken together with the volume of ~tarting material to be treated per unit time, in t~rn determines the volume of the working chamber. Since it is often useful to run the method at relatively high dilution rates the working chamber need not be ~ rge.
Moreover, unlike the stirred tank process, agitation by means of mechanical stirring means is not desired in the present method, and thus the cost of operation is reduced and the reliability is increased.
Furthermore, some back-mixing may be desirable and therefore the presence of baffles and perforated plates 23.
in the working chamber is not required, in contrast to other tower fermentation processes.
Typical apparatus for use in carrying out the invention is illustrated diagrammatically in Figure 3.
The apparatus comprises a vessel 10 of which most of the interior constitutes the working chamber 11, the working chamber being in the shape of a right circular cylinder with its axis vertical. The lower limit of the working chamber is defined by a perforated plate 12 near the lower end of the vessel, the plate serving to distribute air from an air inlet 13 at the lower end of the vessel.
The starting material is introduced through an inlet 14 a short way above the plate. The top of the chamber 15 is dome-shaped and is connected at its uppermost point to an outlet pipe 16 of inverted-U shape, for the reason described above. A water-jacket 17 enables heated or cooled water to flow around the vessel to assist in retaining its contents at the desired temperatures.
Various additional aids are illustrated: a lower sampling port 18, a thermometer 19, a thermistor 2~, a probe 21 for measuring the concentration of oxygen dissolved in the liquid in the chamber, a pH probe 22, an upper sampling port 23, a port 24 through which material can be int~duced into the working chamber, and a pH reference probe 25.
The ~essel may be made from any suitable material, and may for example be made of a plastics material. It i~ f~und that the content of the working chamber usually 24.
becomes acidic, and that it may reach a pH of as low as 1.5.
The lining of the vessel must therefore be chosen so that it will not be damaged by these acid conditions. If it is considered desirable the product issuing from the vessel may be treated with lime or in some other manner so as to neutralize it or make it less acid; however, such treatment is normally unnecessary.
Dependîng on the nature of the starting material, and such factors as the likely variations in its strength with time, the prese~nt method may be used either on its own or in combinatîon with other processes. For example a relatively strong effluent or the like may be treated first by the stirred tank process, but in such a manner that some sugars or sugar-based substances are discharged with the product, and after the separation of biomass, the product may then be treated by the present method.
Alternatively the first of the two treatment stages may be by the present method as well. Yet again it may be desirable to use the present method as a preliminary step in an existing treatment system~ It may happen, for example, that a food-manufacturing plant has its own effluent~treatment system. If the plant is then enlarged the ëffluent may overload the existing effluent-treatment system. To avoid the need for adding a further system in parallel with the existing one it may be commercially advantageous ~o treat all the effluent by the present method and to pass the outflow from the present method to the existing system~
3~5 The present method can be employed to reduce the BOD
(biological oxygen demand) and the solids content of an industrial effluent, thereby making it more suitable for conventional disposal.
The biomass in the product may be separated from the remainder of the product in any suitable manner. For example the separation may be effected gravitationally in a settling tank, though one difficulty is that some of the biomass tends to float and may have to be skimmed from the tank. Alternatively or in addition, the biomass or remaining biomass may be separated by a centrifuging process or by filtration such as by means of a rotary vacuum filter. Solid particles trapped with the biomass may remain with it, or separation may be effected.
After separation, the biomass may be dried and then powd~re~ or pelletised for storage or processing.
The relatively large micro-organisms which can be employed in the present invention enable the biomass to be separated easily by filtration. Moreover, the morphology of the micro-organism i8 such that colloidal and suspended solids may be entrained in such a way that they too can be recovered by normal filtration techniques, thereby ob~iating complicatad separation means.
The protein content of the biomass will depend on the starting materi~l and on the micro-organism employed.
The method of the invention is especially suitable for producing a protein-rich biomass (e.g. at least 300 g.lOOg ).
The protein exhibits a wide amino acid spectrum and ~6.
1~3~'75 generally has a higher nutritional value than protein derived from most vegetable and cereal sources. The method of the present invention may be employed to produce biomass which is useful as a foodstuff for humans or for non-human animals, for example fish, pets or farm animals, or as a fertilizer or soil improver.
The invention is illustrated in the following Examples.
EXAMPLE I
Effluent from a milk-processing plant was treated by a method embodying the present invention in an apparatus similar to that shown in Figure 3. The effluent contained 2.5 g.l 1 solids by weight (the solids being those constituents separable by evaporation of the liquid phase), of which 65% by weight wa8 ~ucrose and the remaining 35%
by weight were milk ~olids such as lactose, proteins including ca~ein, ealts and vitamins. The effluent was treated in a vesael with a working chamber of appr~ximately 1000 1 and having an aspect ratio of 10:1, the working cham~er being 5 m high and 50 cm in diameter. The working chamber contained a strain o~ AsPeraillu~ niqer which was readily able to dige3t sucrose and at least ~ome lactose. To ensure the micro-organism had sufficien~ nitrogen, ammonium nitrate was added to the e~fluent at a concentration of 0.2 g.l 1, and in addition disodium hydrogen phosphate was added at a concentration of 0.05 g.l . The effluent was passed through the working chamber at a dilution rate of 0.17 h 1, and the contents of the chamber were maintained at 30C.
The activity of the micro-organism generates heat, and it 3'75 was normally unnecessary to add much additional heat to maintain the temperature at the desired value. Air was passed through the chamber at a superficial gas veloc~ity of 2 cm.sec 1 When a ~eady state was reached it was found : that the concentration of micro-organism in the chamber was 2.0 g.l 1 (measured as dry weight), while the concentration of micro-organism in the effluent was slightly less than 1.0 g.l 1 At least 90/0 of the casein was trapped by the micro-organism and discharged with it from the chamber. The pH fell to 2.9. ~o steps were taken to sterilize the effluent, but few unwanted micro-organisms were observed in the chamber. The micro-organism in the discharged product was separated by means of a vibratory S~¢~
se~e. Alternatively a centrifuge could have been used, ~\~VQ
or a vacuum ~o~re could have been used. The concentration of solid~ in the di~charged product was 0.2 g.l 1 by weight.
~X~MPLE II
~ serie~ of experLment- was carried out, each being similar to the method de~cribad in Example I but with varying concentration~ of solids in the starting material and varying dilution rate~. In each instance the ~tarting material compri~ed a solution of sucrose in water, together with the u~ual ~mall ~uantities of nitrogen-containing materials and other salts. The ~tarting material wa~ treated in an autoclave to ~terilize it. The experiments were carried out in a cha~ber with a volume of 10. 5 1. The results are shown in Figure6 4 and 5.
In Figure ~ the ab~cissa is a measure of the dilution 28.
rate (h 1) and the ordinate a measure of the concentration d micro-organism in the working chamber (g.l 1) when a steady state was ac~ieved.
The various symbols repesent the following concen-trations of sucrose in the starting material:-o 55 g.l 27.5 g.l a lo.o g.l~l ~ 5.0 g.l-l ~ 2.5 g.l 1 At 2.0 g.l 1 a steady state was not achieved.
It will be observed that within the limits of experimental error the concentration of micro-organism in the working chamber was independent of the eucrose concentration in the starting material. It will also be ob~erved that the concentration of micro-organism in the working chamber decreased steadily with increasing values of dilution rate.
In Figure 5 the abscis~a is again a meaaure of the dilution rate (h 1), but t~e ordinate i9 a mea~ure of the productivity, that i~ the weight (mea~ured a~ dry weight) of micro-organi~m in the product dischar~ed from the working chamber por unit volume of the chamber per unit time (g.l lh 1)c Here it will be ob~erved that at ~ny given dilution rate the productivity increases with increasing con-centration o~ ~ucro~a until a maximum value i9 reach~d, that valuo bffing independent of the ~ucro~e concentration. It 29.
~;;L4'`~375 will also be observed that the maximum productivity increases with increasing values of dilution rate.
In addition it can readily be deduced that in treating a given quantity of sucrose per unit time a greater productivity can often be achieved by increasing the dilution of the starting material and increasing the dilution rate to the corresponding extent so that the same quantity of sucrose enters the vessel per unit time.
EXAMPLE III
A series of experiments was carried out generally similarly to those described in Example II, but differing from them only in that t.l~ su~rose solution was not s~erilized, and that a solution of only one strength was employed, that is 2.5 g.l . The experiments were carried out at dilution rates similar to those employed in the experiments of Example II and also at very much higher dilution rates. The ~esults are shown graphically in Figures 6 and 7. In Figure 6 the abscissa is a measure of the dilution rate (h 1) and the ordinate is a measure of the micro-organism concentrataon (g.1 ) in the working chamber. In Figure 7 the abscissa is a measure of the dilution rate (h 1) and the ordinate is a measure of the productivity (g.l l.h 1), The first part of the graph in Figure 6 a~ far as position 'A' is a representation generally corresponding to the graph of Fi~ure 4, though the actual concentrations of micro-organi~m at different dilution rates differ from tho~e in Example II due to the fact that the solution was 30.
3 ~337S
not sterilized.
After the curve has flattened out the micro-organism concentration continues to decrease gradually with increasing dilution rate. Above a dilution rate of about 1.5 h 1 the - curve turns downwards again and above a dilution rate of about 3.0 h 1 the concentration of micro-organism in the working chamber falls more rapidly, though still quite steadily.
The corresponding values of productivity are shown in Figure 7. Here it will be seen that the increase of productivity with dilution rate continues until the dilution rate reachesa value of about 3.0 h 1 after which the productivity falls again. Above a dilution rate of about 3.0 h 1 washout starts to occur, but the effect is ve~ much less sharply marked than it is when using the st;~red tank process.
It must be emphasised that these dilution rate~
are very much higher than the dilution rate~ u~ed in carrying out the stirred tank process.
EX~MPLE IV
The following experimental re~ults illustrate the variation in concentration of micro-organism in the working chamber with variation in the superficial ga~ velocity (sgv).
The starting material was similar to that described in Example II, the ~ucrose concentration being 27.5 g.l 1, The ~tarting material was introduced into the 10.5 1 12:1 aspect ratio working chamber of an apparatu~ similar to that of Figure 3 at a dilution rate of 0.088 h 1, The temperature was 30 C.
31.
1~''`;~375 When a steady state was achieved the concentration of micro-organism (A. niqer) in the working chamber was measured as dry weight.
sgv (cm.sec 1) concentration (g.l 1) .
1 2.39 2 3.60 3 5.27 EXAMPLE V
Palm mill effluent was treated by a method in accordance with the invention. The effluent arises from plant used in treating palm nuts to produce palm oil. In this treatment the palm nuts are ground in the presence of water, and the resultant mixture is steam-distilled.
The distillate comprises a mixture of palm oil and an aqueous fraction which separate from each other. The palm oil is removed and the aqueous fraction is mixed with the distillation residue or sludge. It is this mixture of sludge and aqueous fraction that constitutes the effluent. Hitherto, the effluent, which consists mainly of cellulose~ fibre and sugar, has been discharged into rivers.
The effluent used in the experiments contained 8.38 g.l 1 total solids of which the carbohydrate content was
4.48 g.l . Added to this effluent were ammonium sulphate and sodium dihydrogen orthophosphate in amounts each equal to one tenth the weight of the carbohydrate present. m e effluent was treated in an apparatus similar to that of Figure 3 having a working chamber of 10.5 ~1 capacity and an aspect ratio of 12:1. Air was supplied at a superficial ;~5 gas velocity of 2 cm.sec 1, and the temperature of the contents of the working chamber was maintained at 30C.
At a dilution rate of 0.10 h 1, when a steady state had been achieved, the total filterable solids in the working chamber (measured as dry weight) was 6.7 g.l and the micro-organism (A. ni~er) content (also measured as dry weight) was 5.79 g.l 1.
At a dilution rate of 0.20 h 1, when a steady state had been achieved, the total filterable solids in the working chamber (measured as dry weight) was 3.5 g.l and the micro-organism content (also measured as dry weight) was 2.93 g.l 1.
At a dilution rate of 0.10 h 1, when a steady state had been achieved, the total filterable solids in the working chamber (measured as dry weight) was 6.7 g.l and the micro-organism (A. ni~er) content (also measured as dry weight) was 5.79 g.l 1.
At a dilution rate of 0.20 h 1, when a steady state had been achieved, the total filterable solids in the working chamber (measured as dry weight) was 3.5 g.l and the micro-organism content (also measured as dry weight) was 2.93 g.l 1.
Claims (40)
1. A method of treating biodegradable organic material in which a liquid medium containing the material is caused to flow upwardly through an upright working chamber which has an aspect ratio of not less than 3:1, flocculent micro-organism capable of digesting at least a part of the biode-gradable organic material being grown in the working chamber and gas comprising oxygen being introduced into the chamber to enable the micro-organism to grow, characterised in that the micro-organism is predominantly flocculent throughout the chamber, and the resultant mixture of treated medium, gas and surplus micro-organism is discharged through a common outlet at the top of the chamber.
2. A method according to claim 1 in which the resultant mixture is discharged at or near the very top of the chamber.
3. A method according to claim 2 in which the flow of the resultant mixture is caused to converge at the upper part of the chamber by reason of the upwardly tapering shape of the upper part of the chamber through which the mixture flows.
4. A method according to claim 3 in which the upper part of the chamber has a frusto-conical or dome shape.
5. A method according to claim 3 or 4 in which the common outlet through which the liquid medium flows is at the apex of the upper part of the chamber.
6. A method according to claim 1, 2 or 3 in which the common outlet through which the liquid medium flows is in the form of an inverted U-shaped pipe.
7. A method according to claim 1, 2 or 3 which is oper-ated such that if the dilution rate were to be gradually increased, the concentration of the Micro-organism in the chamber would decrease steadily.
8. A method according to Claim 1, 2 or 3 which is operated such that, other factors being constant, if the concentration of the biodegradable material in the liquid medium were to be increased, there would be an excess of biodegradable material which would be discharged with the resultant mixture.
9. A method according to Claim 1, 2 or 3 in which only one kind of flocculent micro-organism is grown in the working chamber.
10. A method according to Claim 1, 2 or 3 in which the flocculent micro-organism is a filamentous fungus.
11. A method according to Claim 1, 2 or 3 in which the flocculent micro-organism is Asperqillus niger.
12. A method according to Claim 1, 2 or 3 in which the flocculent micro-organism is Trichoderma viride, Sporotrichum thermopile, Penicillium roquefortii, Geotrichum candidum, Rhizopus spp., Mucor spp. or Fusarium spp.
13. A method according to Claim 1, 2 or 3 in which the micro-organism is a flocculent yeast.
14. A method according to Claim 1, 2 or 3 in which the biodegradable material in the liquid medium is, or is based on, effluent from a food processing plant.
15. A method according to Claim 1, 2 or 3 in which the bio-degradable material in the liquid medium is, or is based on, effluent from a palm oil producing plant.
16. A method according to Claim 1, 2 or 3 in which the liquid medium is water.
17. A method according to Claim 1, 2 or 3 in which the biodegradable material is in solution or suspension in the liquid medium.
18. A method according to Claim 1, 2 or 3 in which the liquid medium also contains any nitrogen-containing substances and any salts which enable the micro-organism to thrive in the chamber.
19. A method according to Claim 1, 2 or 3 in which the chamber has an aspect ratio of not less than 5:1.
20. A method according to Claim 1, 2 or 3 in which the chamber has an aspect ratio in the range of from 7:1 to 15:1.
21. A method according to Claim 1, 2 or 3 in which the chamber has an aspect ratio in the range of from 10:1 to 12: 1.
22. A method according to Claim 1, 2 or 3 in which the gas is a mixture comprising oxygen and another gas.
23. A method according to Claim 1, 2 or 3 in which the gas is air.
24. A method according to Claim 1, 2 or 3 in which the gas is introduced at or near the bottom of the chamber.
25. A method according to Claim 1, 2 or 3 in which the gas is distributed throughout the liquid medium in the form of small bubbles.
26. A method according to Claim 1, 2 or 3 in which the gas is employed at a flow rate such that the amount of oxygen dissolved in the liquid medium is near zero.
27. A method according to Claim 1, 2 or 3 in which the gas is air and its superficial gas velocity, i.e., the volume of gas introduced per unit time divided by the cross-sectional area of the working chamber, is between 1 and 10 cm.sec 1.
28. A method according to Claim 1, 2 or 3 in which the biodegradable material in the liquid medium is introduced into the chamber at or near the lower end thereof.
29. A method according to Claim 1, 2 or 3 in which the amount of biodegradable material in the liquid medium is from 0.1 to 20 g.1-1.
30. A method according to Claim 1, 2 or 3 in which the amount of biodegradable material in the liquid medium is from 0.5 to 20 g.1-1.
31. A method according to Claim 1, 2 or 3 in which the amount of biodegradable material in the liquid medium is from 1 to 10 g.1-1.
32. A method according to Claim 1, 2 or 3 in which the amount of biodegradable material in the liquid medium is up to 100 g.1-1.
33. A method according to Claim 1, 2 or 3 in which the majority of the micro-organism is in flocculent form.
34. A method according to Claim 1, 2 or 3 in which at least 75% of the micro-organism is in flocculent form.
35. A method according to Claim 1, 2 or 3 in which as much as possible of the micro-organism is in flocculent form.
36. A method according to Claim 1, 2 or 3 in which the majority of the micro-organism flocs have floc sizes in the range 0.5 to 20 mm.
37. A method according to Claim 1, 2 or 3 in which the majority of the micro-organism flocs have floc sizes in the range 2 to 10 mm.
38. A method according to Claim 1, 2 or 3 in which the contents of the chamber are not agitated by mechanical stirring means.
39. A method according to Claim 1, 2 or 3 in which back-mixing of the contents of the chamber is not prevented by baffles or perforated plates.
40. A method according to Claim 1, 2 or 3 which is operated at a dilution rate of more than 0.5 h -1.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA282,414A CA1103375A (en) | 1977-07-11 | 1977-07-11 | Treatment of biodegradable material |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA282,414A CA1103375A (en) | 1977-07-11 | 1977-07-11 | Treatment of biodegradable material |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1103375A true CA1103375A (en) | 1981-06-16 |
Family
ID=4109106
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA282,414A Expired CA1103375A (en) | 1977-07-11 | 1977-07-11 | Treatment of biodegradable material |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1103375A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1991012208A1 (en) * | 1990-02-07 | 1991-08-22 | Sunds Defibrator Industries Aktiebolag | Treatment of wastewater |
-
1977
- 1977-07-11 CA CA282,414A patent/CA1103375A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1991012208A1 (en) * | 1990-02-07 | 1991-08-22 | Sunds Defibrator Industries Aktiebolag | Treatment of wastewater |
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