CA1128462A - Process for the concentration of biomasses - Google Patents

Abstract

PROCESS FOR THE CONCENTRATION OF BIOMASSES
Abstract of the disclosure:

Biomass is concentrated by coagulation of the solids of a fermentation suspension. The fermentation suspension is exposed to the action of an electric current at 20 to 100°C. The current density amounts up to 10mA/cm2, and the frequency of the current is in the range from 0 to 2000 Hz.
Direct current is used preferably. The process may be per-formed discontinuously in a trough cell or continuously in a flow cell.

Description

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- 2 - HOE 78/F 072 The present invenSion relates to a process for the concentration of bio~ass.
Aerobic fermentation processes are being used to an increasing extent in the industry for the purpose of converting organic crude substances such as sugar- anc' starch-containing media, n-paraffins, methanoi and ethanol into biological products of great importance such as enzymes, metabolites or single cell proteins - / German Offenlegungsschriften 1,442,051; 2,423,766;
10 2,407,740; Soviet Patent 498,940 ard Japanese Patent 50-677~ _7.
A common feature of microbiological processes of this type is that they provide as a fer~entation product an aqueous suspension having a biomass content in the form of soiids of up to 5 %. The biomass is produced in gassed fermenting vessels, in which the optimal growth conditions for the microorganisms such as temperature, pH value, ionic concentration, substrate concentration, oxygen partial pressure and turbulence are adjusted in batchwise or continuous operation.
Prior to working up the fermentation mass and to obtaining the desired products, the solids must fre-quently be separated from the aqueous fermentation medium, for example hy sedimentation processes or by using separators such as sewage sludge centrifuges etc.
However, these apparatuses are efficient only, when the solid to be separated has a suf`ficiently high difference in density to the solution.
: 29 In the case of biological cell suspensions, however, '' ' ~ ,.

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- 3 -this difference in density is often very low. ~n thatevent the sedin~entation behavior is determined in parti-cular by the size and the form of the susp-nded partic~
les. For example, when separating bacteria having a particle size of 1 /u or less, the expenditure on me-chanical energy and costs for the separat ng apparatuses may be reduced, when bringing the individual particles to aggregate and, consequently, increasing considerahlv their si~e.
Various microorganisms have a .~pontaneous tendency to flocculation, this phenomenon being, consequently, a racial characteristic thereof. In other cases floccula-tion may be induced-upon completion of the fermenta-tion and prior to the ~echanical concentration-by addi-tional mea-~ures, for example by changing the pH, by add-- ing electrolytes / cf. J. Weigl, Elektrokinetische Grenzflachenvorgange, Verlag Chemie, Weinheim 1977, pa-ges 88 et seq; F. Reiff et al, Editors, Die Hefen, Vo-; lume I, Verlag Hans Carl, ~urnberg 1960, page 238 7, by adding polymers / cf. B. Alkinson and J. S. Daoud in Adv. Biolog. Eng. Vol. II, Springer Verlag, 1974, pa-ge 48 _7 or water-soluble solvents or finally by in-creasing the temperature to 50 to 200C / German Of-fenlegungsschrift 2,310,041_7.
h prerequisite for a sedimentation or separation in any known flocculation processes, except the tempe-rature process, which is useful to a limited degree on-ly, is the addition of` flocculation agent. The latter 29 is in most cases undesired in the further working up ~ .. .. , .. .. ~ . .. . .. ....... ... . .. .. .
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and must hence be separated with great expenditure.
Moreover processes are known involving a removal of pollutants from aqueous solutions by the action of an electric current. For example, organic pollutants-containing waste waters may be purified by oxi-dation with anodically formed oxygen on large area elec~rodes~ In this process the pollutants undergo a chemical alteration ~French Patent 2,316,196).
Furthermore variants are known, wherein fine gas bubbles that have been formed by electrolysis, combine with the suspended particles optionally preflocculated by a pH changement or by the addition of polyvalent cations or mono- or polymer flocculation agents, and float these particles. The formulation of gas bubbles by electrolysis, however, requires a correspondingly high energy expenditure [J. 5J111~528-0; German Offenlegungsschriften 2,5QO,455; 2,545,875 and 2,545,9Q9; Food Technology, Febr. 197~, page 18].
However, as a consequence of 10tation, voluminous eoams are ob~ained that are difficult to convey under pressure.
Furthermore highly diluted particle suspensions may be clarified b~ pressing them through an electric field of a direct current of 100 to 500 volts and by stimulating the individual particles to migrate in this field.
This process is efficient only or aqueous solutions with very low solids contents (50mg/1l ~cf. Soviet Patent 523-713~.
For the puri$ication of food wastes the so-called "electrocoagula-tion" process ~as been proposed ~cf. Food Technology, Febr. 1974, page 18) which involves a floccu-~Z~34~Z
- 5 - HOE 7~/F 072 lation and flotation caused by gas formation and cation formation by the action of electric current on the waste water. However, this method requires the same expenditure on time as a process using chemical flocculation agents and represents hence no particular technical progress.
It was, consequently, an object of the present in-vention to provide a process that permits the concentra-tion of biomass in suspensions, the density of the latter . , being only insignificantly lower than that of the biomass, without the addition of flocculation agents or other foreign matter, without the influence of gas bubbles and while avoiding thermal damage.
This object is achieved by a process for the concen-tration of biomass by coagulation of the solids of a fer-mentation suspension by the actior. of electric current, which comprises exposing the suspension to the action of electric current having a frequency from 0 to 2000 Hz, preferably from O to 50 Hz up to a current density of 10 mAtcm2, at a temperature from 20 to 100C.
By the indication G to 2000 Hz it is intended to ex- ~ ;
. press that both direct current and alternating current can be applied for performing the process of the inven-tion.
- In contradistinction to the hitherto ~nown processes the individual particles to be coaeulated in the process of the invention react with one anothe~ when applying an adequately adjusted current, independe~r of an electroly-tic gas formation on the electrodes and in particular in a 29 current/voltage range, in which this gas formation does .

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not yet occur.
The invention will now be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a view in cross-section of a trough cell used for a discontinuous operation;
~ igure 2 is a vi0w in cross-section of a slow cell used for Q
continuous operation;
Figure 3 is a view in cToss-section of a specific orm of flow cell, namely a tubular cell.
The process of the invention is generally performed in the following manner: The suspension i5 introduced into a suitable reaction vessel, whereupon the desired temperature is adjusted and the desired current is passed through. The current quantity required for the coagulation depends on the nature and on the concentration of the fermentation suspension used in each case. The concentration according to the invention may be carried out both in continuous and in discontinuous manner. ~or a discontinuous operation a trough cell Ccf. figure 1I may be used, by way of example. The trough cell consists of the actual cell container 1, which is surrounded by a jacket and which is covered by a tightly closing cover 2. The cover is provided with current supplies for electrodes C3~ and C4~, with inlet C5) for the suspension, outlet C6~ for gases and thermomoter C9~. The gas outlet may be connected with a reflux condenser Cnot shown), where evaporating portions of the suspension may be condensed. By inlet ~7~ at the bottom of the jacket and outlet C8~ at the top of the jacket a cyclic process with heating or cooling liquid may be set up. The temperature of the suspension is moni-tored by thermometer C9~ or a thermosensor. The two electrodes 3 (anode~
and 4 (cathode) are arranged at a distance from 0.5 to 50 mm, preferably from 1 to 15 mm, from each other.

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-l~Z~3~62 The electrodes may be, by way of example, nets or plates of palladium or platinum or noble metal coated metals, preferably titanium, mixed oxide coated metals Cas anodes~, preferably titanium anodes, or slotted or unslot-- 6a -B
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~Z8~i2 ted plates of graphite. The electrodes may be arranged vertically as well as horizontally. Furthermpre it has proved advantageous to arrange several electrode pairs, for example in the form of a block-type combination of angled or not-angled capillary split-electrodes with or wiihout vibration of the electrodes. While passing the current through, the suspension may be mixed, preferably by a stirrer, for example a magnetic-type stirrer (10) or by recyc]ing the suspension, in particular in the case of block-type combinations. In continuous operation cover (2) of the electrolysis cell (1) is provided with a further aperture for the continuous circulation of the suspension.
A part of the circulated suspension is separated in each case for the purpose of working it up while a corres-ponding portion of fresh suspension is added.
A further appropriate reaction apparacus, in parti-cular for continuous operation, is a flow cell. Plate cells (cf. figure 2) are especially useful. They consist of a cell element made of plastic or steel and having a rectangular cross-section (1). In the simplest embodiment : :
of such a cell two plate-shaped electrodes, namely anode (3) and cathode (4) are arranged at a distance from 1 to 60 mm inside the cell. The suspension heated to the de-sired temperature is fed to the cell by inlet t5) and is withdrawn through outlet (12). The suspension may be passed through the cell once or repeatedly.
A further ~uitable flow cell is a ~ubular cell (cf.
figure 3). This cell consists of a cell element of plastic 29 or steel (1) wherein two electrodes, anode (3) and ca-~ " . ~ .. . ~ . . . .. ..
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thode (4) are arranged concentrically at a dis/:ance frorn 1 to 60 mm fron. one another. The suspension having the desired temperatur,e is fed to the cell throllgh inlet (5) and is withdrawn through outlet (12). The suspension may be passed through the cell once or repeatedly.
Generally the process is carried out under normal pressure. Ho~ever, elevated pressure ~nay be used alterna-tively.
By the term "biomass" there is to be understood the mass of microorganisms that is present in a fermentation solution during fermentation processes in the form of a solid consisting of` individual particles.
As microorganisms there are generally used bacteria, yeasts and fungi. Examples o~ micr~ rganisrns of this type are for example methanol-utilizing bacteria of the genus Methylomonas, for example the strain Methylomonas clara ATCC 31,226, yeasts such as the strain Candida lipo~ytica ATCC 20,383 that may be obtained by cultivation on n-pa-raffins in the presence of an aqueous nutrient medium, or fungi that are generally used for the known production of antibiotics, for example of the strain Penicillium chryso-genum.
- The invention will be illustrated in the accompanying drawings representing sectional views of electrolysis cells and in the following examples E X A M P L ~ 1:
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The straitl ~let'nylormonas clara ATCC 31,226 was culti-vated under aerobic conditions in a nutrient medium con-29 taining methanol as the carbon source, ammonia as the ni-, .
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trogen source, phosphate, iron and magnesium sa:ts and other usual tr~ce elemeots. A 500 ml portion of` the bacte-ria suspension formed having a solids content of 1.1 weight ~ was introduced into a trough cell (cf. fi~:ure 1).
Two concentrically arranged platinum wire gauze cylinder 3 of 24 and 36 mm diameter respectively, of 95 mm heighc and having 225 meshes/cm2 plunged into thi~s suspension. The outer electrode functioned as an anode. The temperatur-~
was maintained at 35C during electrolysis. Upon switch- , ing on the direct current, the curre;t intensity in test 1 was 0.1 A, which signifies an average current density, calculated on the anode area, of 0.93 mA/cm2.
After 15 minutes the current was switched off. The calculated average cell voltage was 1.8 volts. The con-tents of the cell were then transferred to a sedimentationvessel.
The clear supernatant was decanted and the sedimcnt ~as subjected to the further processing. Tests 2 to 5 were carried out in analogous manner as test 1. In tests 3 to 5, however, alternating current having a frequency of 20 Hz (test 3), 200 Hz (test 4) and 2000 Hz (test 5) was app-lied instead of direct current. In that case, the corres-ponding current and voltage values can he considered as effective values. In Table 1 the test results are compared with the results of a blank test. Test 3 and 4 show the effective current and voltage values.

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Test Current Current density Voltage Sedirrlent vo]ume after No. ~A 7 rmA/cm 7 ~V_7 ~ml 7 30' 60' 90' 1 0.1 0.93 1.8 not measurable 50 ml 50r;l 2 0.01 0.0g3 1.6 " ab~e 45 ml 3a) 0.1& ) 1.~7 ) 0 5b) " 45 ml 4-:ml 4C) o.3b) 2.79b~ 0.5b) " na~Sur- 40 .rn 5d) O. 18b~ 1.67b~ 0.5~)) " a~~Sur- 1~0 ~
cont- 0 0 0 " ~sur a~eur rol a) frequency 20 Hz, b) effective value, c) frequency 200 Hz, d) frequency 2000 H3 It cai: be clearly seen from the table that the sedi mer,ted volume of the electrically treated samples is by far higher than that of untreated samples.
E X A M P L E 2:
A hydrocarbon utilizing strain of Candida lipolytlca 5 ATCC ~0,3&3 was cultivated on n-paraffins in the presence of an aqueous nutrient medium and an oxygen-containing gas. A 500 ml portion of the resulting suspension having a solids content of 2 ~leight % was introduced into a trough cell (cf. figure 1) and treated as described in Example 1.
Successively the supernatant was decanted, the se-dlrmern was drie~ and weighed. The dry weight of the biomass obtaine~, calcll1ated on the cell mass contained in the suspension, is a measule for a g-ood flocculation.
Table 2 shows the poltions of biomass obtained for 5 .. . :

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- ll - HOE 7~/ ~ 072 tests, in dependence on the current intensity, the volta--ge, the current density and the time.

Test Current Current den- Voltage Time Biomass sity used obtained No. ~A-J rmA/cm2_7 ~V_7 rmin_/ ~g_7 / g l 0.01 0.093 1.5 lO lo.~ g.o 2 0.01 0.093 1.6 15 lO.o 9.4 3 0.05 0.465 1.7 15 10.0 9.6

4- 0.1 0.93 1.8 15 lO.0 9.8

5. 0.1 0.93 1.8 30 10.0 9.8 con- O - - - 10.0 5.0 trol The distinctly higher portion of biomass obtained of the electrically treated samples in comparison to that of the untreated sample can be clearly seen.
E X A ~l P L E 3:
The strain Penicillium chrysogenurrl ATCC 10,238 was cultivated in usual manner under aerobic conditions in a - nutrient medium containing lactose, liquid cornsteep, phosphate, carbonate and magnesium sulfate. Using the electrodes described in Example 1, test 1 direct current was applied for a predetermined period of time and there-after the filterability of the suspension was determined by measuring the filtrate volume/time.
Test 1:
A 500 ml portiorl of a myce~ suspension having a solids ,.. ~.............................................. ~ ~

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' 284~i2 - 12 - HOE 7~/F 072 content of 10 weight ~ was electrolyzed for 15 minutes at 20C with 0.005 A. Thereafter the suspension was filtered (diameter of the filter 11 cm, paper, a vacuum of 15 torrs) and the volume of the filtrate was measured after 1 m~nute.
The results of tests 1 to 5 of Example 3 are sum-marized in Table 1 and compared with that of a blank test.
TABLE 3:
Test Current Current den- Voltage Time Filtrate volume No. sity -~-A~ ~mA/cm2/ ~V7 /min7 /ml/min7 1 0.005 0.0465 1.4 15 5 2 0.01 0.093 1.5 15 30 3 0.05 0.465 1.6 15 85 4 0.1 0.93 1.7 15 90 0.1 0.93 1.7 10 90 con- O 0 0 3 trol The by far greater filtrate volume of the electri-cally treated samples as compared to the untreated sample~
is obvious. :

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the concentration of biomass by coagulation of the solids of a fermentation suspension by the action of electric current, which comprises exposing the suspension bo the action of electric current having a frequency from 0 to 2000 Hz, up to a current density of 10 mA/cm2, at a temperature from 20 to 100°C.

2. The process as claimed in claim 1, which comprises operating discontinuously in a trough cell.

3. The process as claimed in claim 1, which comprises operating continuously in a flow cell.

4. The process as claimed in claims 1, 2 or 3 wherein said electric current has a frequency of from 0 to 50 Hz.

5. The process as claimed in claims 1, 2 or 3 wherein said electric current has a frequency of a Hz, direct current.

6. The process of claim 2 wherein said discontinuous operation is carried out with stirring or circulating of the suspension.