Acta Biotechnol. 10 (1990) 5, 473-475 Akademie-Verlag Berlin

Concentration of Microorganisms in Microbial Suspensions

Akademie der Wissenschaften der DDR Institut fur Biotechnologie, Leipzig PermoserstraDe 15, Leipzig 7050, G.D.R.

Summary

For technical research and development with high cell densities the volume concentration plays a n important role. On the basis of simple determinations of a density concentration relationship a method is described which allows a better calculation of the real density of biomass and of dry matter as used till now. As results of simple relations for spherical particles an average water con- tent and the volume fraction may be derived.

Introduction

The commonly used measures of the concentration of microorganisms in microbial SUS- pensions are the content of dry matter of biomass per volume unit and the optical den- sity within the range of LAMBERT-BEERS law. In concentrations of technical interest the volume fraction occupied by biomass plays an important role in the case of fermenta- tion, for the primary recovery stage or in recycling procedures [l]. The models used up to now are too simple to describe the real situation satisfactory [2-51. Otherwise the specific weight of the cell itself and the specific volume of cell are also of interest in tech- nical scale fields. In this paper a method will be described to determine the real concen- trations of biomass in the area of high cell density. By use of a simple model of spherical cells a formula may be used to calculate the volume fraction of the cells. In processes like aggregation, flocculation or agglutination and in fermentation steps with immobilized whole cells or with a flocculent growth of the microorganisms (e.g. in waste-water treatment) the described relationship may be applied.

Theory

The volume fraction of a solid s in a suspension is calculated by

es. - e l

es -e l 9% =

after [6] . In microbial suspensions the cell as the solid is composed by biomass dry matter in a swollen state and the water phase. With the assumption of a constant density of

7 Acta Biotechnol. 10 (1890) 5

474 Acta Biotechnol. 10 (1990) 5

the whole cell (which may be reached under steady-state conditions) the distribution of dry matter and water phase is determined by eq. (2)

@S = TB* @B* + ql* . @f*. (2) Substitution of eq. (2) in eq. (1) yields the density of the whole suspension

whereas el* = el. Thus the density of the dry matter biomass can be calculated from the plot of esU against X B . Using the real density of the biomass itself the maximum concentration reached by cen- trifugation (according to [2]) or any other concentration can be corrected by eq. (4)

Assuming the maximum hexagonal package of spherical particles the volume fraction Q ) ~ ( ~ ~ ~ ) was calculated to 0.742 [7] an average water content of the cell (including the water around the cell in the GUY-CHAPMAN layer) is calculable by

or

wit.h

The real density of the cell under these conditions is a measure of the relation of dry matter of biomass and of water content. On the other hand the reciprocal value of the density (as the specific weight per volume unit) is a measureof the necessary volume per weight unit, that means the average specific volume of the cell. On this way the volume fract.ion as a concentration of cells in a microbial suspension can be calculated by eq. (8)

Results and Conclusions

The results of the determination and calculation of the above described values of .diffe- rent strains of the Institute of Biotechnology are summarized in Tab. 1.

Tab. 1. Biomass density and water content of different microbial populations

Xo. Culture

1 Methan-utilizing bacteria 1.07 1.53 85 0.932 2 Methan-utilizing bacteria 1.10 1.40 73 0.867 3 Methanol-utilizing bacteria 1.14 1.57 74 0.936 1 Methanol-utilizing bacteria 1.14 1.54 70 0.948 5 Methanol-utilizing yeast 1.06 1.45 77 0.924

PATz, R., Concentration of Microorganisms 47 5

It can be shown that i) the real density of the cell itself and of the dry biomass is different to 1.0 g/ccm,

ii) in the case of Calculation with higher biomass concentrations the density relationship should be known and used,

iii) under the abough-mentioned conditions an average water content of different strains can be calculated which is in a good agreement with theoretical assumptions,

iv) a simple relation exists for the calculation of the volume fraction of the cells deter- mined with the dry matter grarimetrical method.

Nomenclature

qs - volume fraction of solid S esc - density of the whole suspension 0, - density of dispersing agent es - density of solid S ql* - volume fraction of the water phase in S gl* - density of water phase in S eB - density of dry matter biomass (identical to es*) XB - dry matter biomass per volume unit mg - dry matter biomass of the sample mSa - mass of whole suspension of the sample V s - specific volume of S

Received June 6, 1989

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References

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[7] SONNTAG, H. : Lehrbuch der Kolloidwissenschaften. Berlin: VEB Deutscher Verlag der Wissen-

[8] PATz, R., ROTHER, R., PICKERT, H.: Acta Biotechnol., in press.

Verlag fur Grundstoffindustrie, 1982.

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