Download - Natural floatation during anaerobic digestion of high strength wastes

Biomass 6 (1985) 223-234

Natural Floatation during Anaerobic Digestion of High Strength Wastes

J. D. Finck* and G. Goma

Institut National des Sciences Appliqu6es, D6partement de G6nie Biochimique et Alimentaire, ERA-CNRS 879, Avenue de Rangueil,

31077 Toulouse Cedex, France.

(Received: 17 February, 1984)

ABSTRACT

The tendency o f vegetable fibres to float during the anaerobic digestion process has been studied for two types o f substrate, cow dung and domestic refuse. A mathematical model has been developed which, on the basis o f a knowledge o f the initial dry matter content and the degree o f progress o f the reaction, enables the reaction volume occupied by the floating phase to be determined accurately. I t is thus possible to determine the critical dry matter content beyond which fermentation can no longer take place due to limitations on diffusion, lack o f water and other technical reasons.

Key words: Anaerobic digestion, biogas, domestic refuse, floatation, mathematical model, straw.

INTRODUCTION

One of the basic problems of most digesters used in the treatment of very concentrated mat ter that has not been resolved satisfactorily so far is the formation of a surface crust which can become very thick and impervious to gases. 1' 2 This crust not only reduces the reaction volume, which causes significant problems in sewage treatment plants, but also

*Present address: Elf Bio Recherches, La Grande Borde, BP 62, Lab6ge, 31320 Castanet Tolosan, France.

223 Biornass 0144-4565/85/$03.30 - © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

224 J. D. Finck, G. Goma

reduces the release of biogas formed, which in extreme cases gives rise to explosion risks, a In France (where 80% of the biogas potential comes from cow manure) the difficulty of treating solid residues due to such crust formation has resulted in t h e almost exclusive use of batch digesters based on the Ducellier Isman aerobic pre-fermentation process. 4 This uses three tanks connected in parallel, designed to regulate the gas production output . The inherent productivity level of this process is low, being 0-5-1 m 3 biogas m -3 day-1. 5

Even in batch processes the floatation of fibres raises other technical problems. The large increase in volume as the result o f trapped gases has necessitated the installation of metal anti-floating bars in many cases. 2 The floatation of fibres is not the only problem in the treatment of solid wastes. According to Jewell 6 fermentation is inhibited as soon as the dry matter content in the reactor has reached about 30% in weight.

The first continuous reactor based on the floatation properties of straw was developed by Elf Bio Recherches and the Institut National des Sciences Appliqu6es, Toulouse. 7 The productivity at 35°C is 1-76 m 3 m -3 day -1 for a loading rate of 6.7 kg of volatile solids m -3 day -1 without any agitation at that temperature.

This paper describes the changes in the phases of a digester during fermentation with no agitation and derives a mathematical model applicable to the digestion of solid wastes capable of floating. This model makes it possible, by theoretical extrapolation, to determine a critical dry matter content above which digestion slows down.

MATERIALS AND METHODS

Substrates

The cow manure was supplied by the Ecole Nationale Sup6rieure Agro- nomique, Toulouse. It came from an open shed breeding establishment, where 4 kg of straw are provided per animal per day. The domestic refuse, which came from Avignon, was supplied by the Soci6t6 Carene Valorga of Montpellier. It had previously been sifted for the removal of metals and large pieces of plastic. The basic characteristics of these two substrates are given in Table 1.

Floatation during anaerobic digestion of wastes

TABLE 1 Principal Characteristics of Two Solid Substrates (Cow Dung and Pre-sifted

Domestic Refuse) Used in the Experiments

225

Substrate Cow dung Pre-sifted domestic refuse

Initial dry matter (% w/w) 10 10 Volatile solids (% dry matter) 80.7 - Cellulose 28.3 11-41 Hemicelluloses 25.1 1.65 Lignin (% dry matter) 6.4 4.23 Initial pH 7.1 7.6

Reactors

The study of the changes in the volume o f phases during digestion, related to the production o f biogas, was carried out in a fermenter made o f transparent PVC with an effective capacity of 60 litres (Fig. 1). The height/diameter ratio was 4, the diameter being 300 mm. An Eheim centrifugal pump (output : 40 litres rain -1) circulated the liquid phase sequentially, after it had passed through a gridded strainer. Heating was by circulating hot water inside a pipe of semi-circular cross-section, fitted around the reactor. The product ion of biogas was measured by means of a Schlumberger water displacement meter and reduced to standard pressure and temperature conditions.

Additional batch process studies were carried out in 2-1itre glass bottles connected to an electric pulse gas meter, fitted in accordance with the principle described by Moletta. 8

Analytical procedures

The dry mat ter content was determined by measuring the loss of weight after heating a 50 g sample placed in a silica evaporating dish in an oven at 105°C for 12 h. The volatile solids were determined by calcination of the dry matter at 550°C for 6 h. Cellulose, hemicellulose and lignin were determined according to the procedure described by Van Soest. 9

J. D. Finck, G. Goma 226

Fig. 1. Transparent 100-1itre PVC reactor for determining the floatability of solids in cow dung and domestic refuse. 1 - PVC tank; 2 - heating pipe; 3 - liquid phase recycling circuit; 4- temperature sensor; 5 -temperature regulator; 6 - pre-heating

tank; 7 - discharge; 8 - continuous feed inlet; 9 - gas meter.

R E S U L T S

Phase separa t ion dur ing the anaerobic digestion process of two sofid type substrates

The ba tch anaerobic digestion was carried out at a t em p e ra tu r e o f 35°C in a t ransparen t r eac to r shown in Fig. 1. The manure was no t seeded at all be fo rehand , bu t the domest ic refuse was seeded to 1% with l iquors derived f rom a con t inuous digester t reat ing cow manure conta in ing 8.7% dry mat te r . 7

At the beginning o f f e rmen ta t ion , a large increase in the vo lume o f liquid mass was observed due to biogas bubbles being t r apped in the straw ne twork which very rapidly appeared on the surface. This increase

Floatation during anaerobic digestion of wastes 227

100 _

5 0 _

Fig. 2.

% Vol .

~ A

B

- - - 4 C t(~)

J I I I I I I I

~0 20 3o 40 so 60 70 e0 90

Batch digestion of cow dung: phase changes against time. A - Floating solids; B - liquid phase; C - sludge.

in the apparent volume was 26% for cow dung and 24% for domestic refuse. At the same time, three distinct phases were observed (Fig. 2): the sludge, an intermediate aqueous phase free from solid particles and a scum consisting of floating solids which can form a crust in the long term. The form o f these curves was substantially the same in the case of domestic refuse.

The surface scum consisted largely of straw, cellulose waste and pos- sibly other residues whose density is close to that of water. The production of biogas is probably responsible for this natural floatation, for it was not observed when the same experiment was performed at 4°C when fermentation was prevented.

In the text that follows, we shall call the surface phase 'floating solids', in contrast to the liquid phase as such, it being understood that this particle scum is not free of water. The sludge, which consisted mainly of already-digested small size straws, was easily pumpable.

2 2 8 J. D. Finck, G. Goma

OjS-

0,25-

Fig. 3.

VF VT

C D

~ _ D R

A

I 1 I • 0, 25 0, 5 0,75

VF/V T against degree of progress A of reaction at 35°C and 10% initial dry matter content: C.D. - cow dung; D.R. - domestic refuse.

Once the surface phase was well formed after 4 to 5 days, it was found (Fig. 3) that a gradual reduction of its apparent volume occurred which was directly proportional to the quanti ty of biogas formed. The latter is represented by the degree of progress (A) of the reaction, which is defined as the quanti ty of biogas produced related to the production capacity at infinite retention time (AT). This production capacity is approached by a double reciprocal of the cumulated biogas production versus digestion time, or:

1 _ f ( 1 ) P (t)

For cow dung, AT was found to be 513 litres kg -1 dry matter introduced. It is also seen (Fig. 3) that with cow dung there is a more marked

accumulation of the floating solids in the upper phase than in the case


of domestic refuse. With an identical initial dry matter content, domestic refuse occupies 3 1% less reaction volume than cow dung at the beginning of the digestion. This property can be related to the lower viscosity of mixtures containing domestic refuse, and therefore pumpability and hence a simplified anaerobic treatment from the technical point of view.

MODELLING

A second series of experiments was performed in order to obtain the ratio of the floating solids to the total apparent volume for various dry matter contents. These determinations were made in a battery of 2-1itre bottles (c.f. Materials and Methods) after 10 days digestion, the substrate being cow dung.

We confirmed that the ratio of floating solids to the total apparent volume, between two bottles containing different concentrations is independent of the digestion duration (not shown).

This basic parameter can be expressed in an equation:

Volume of floating solids ( V F / V T ) t =

Total apparent volume

It can be seen that exp(VF/VT) is directly proportional to the initial dry matter content DM i (% in weight) (Fig. 4), represented by the following equation (t = 10 days):

(VF/VT)lO = In(0.0836DMi + 1) (1)

If, secondly, the changes in the VF/VT ratio are represented in accordance with the degree of progress A of the reaction, it is found that, when DMi = 10, the following equation is obtained:

(VF/VT)t = - 0.269A + 0.667 (2)

The values used for the development of this equation were calculated from the data in Fig. 2.

It follows from eqn (2) that the reduction in the volume of floating solids is directly proportional to the degree of progress of the reaction and therefore to the production of biogas at time t. This clearly indi- cates that practically all the 'primary' substrate, in this case lignocelluloses, of the hydrolytic bacterial populations is in the floating solids and not in the liquid phase, which represents a dead volume.

2 3 0 J. D. Finck, G. Goma

Fig. 4.

VF • x p '~;"T

DM, I 1 I

5 10 15

exp(Vr/VT) against initial dry matter content DMi for cow dung.

The intermediate metabolites produced (organic acids, glucose) are transferred into the liquid of the actual methanization process.l° The transfer of the soluble intermediate metabolites, ' removed' from the lignocelluloses, obviously does not cause any increase in the volume of the liquid phase.

We confirmed that (Vv/V~)e/(Vv/VT)IO is independent of the initial dry mat ter content DMi. By combining eqns (1) and (2), one obtains:

(Vv/VT)t (VV/VT)10

= - 0.444A + 1.1 (3)

By introducing eqn (1) into the denominator, it is possible to define a general model of the form:

(Vv/Vx)t = ln(0-083DMi + 1) (1-1 -- 0.444A) (4)


This final representation is valid under the following conditions: (1) the substrate is cow manure; (2) most o f the biogas produced is derived from the breakdown of floating solids; and (3) the degree of progress A of the reaction is between 0-095 and 0.7, which agrees well with the yields obtained in the cont inuous digestion of lignocellulose products. Figure 5 gives a theoretical representation of eqn (4), in which the evolution of (VF/VT) t is shown against the initial dry matter content for various degrees o f progress o f the reaction.

It will be noticed (Fig. 3) that the slope of the-solids breakdown curve against the degree of progress of the reaction is the same for domestic refuse as for cow dung. Equation (4) would therefore appear to be extendable to all floating solids, in the form:

(Vv / VT) t = [ l n ( a D M i - 1)] ( b - 0.444A) (5)

where a and b are constants dependent on the substrate.

0.I

0.3

0-5

0-7

0 , 5 _

I l I

5 10 15

Fig. 5. Theoretical representation (eqn (4)) of the evolution of (VF/VT) t against degree of progress A of reaction and dry matter content for cow dung.

DH i


TECHNOLOGICAL IMPLICATIONS

It follows from Fig. 3 that the limiting step from a kinetics point of view, that is cellulolysis, develops in the floating solids phase since the reduction of the apparent volume of the latter is directly proportional to the cumulated production of biogas. It can be deduced from this observation that the actual liquid phase, even if the methanogenic reactions occur within it, is not of basic importance in the overall process. The intermediate metabolites produced, in particular organic acids, do not accumulate. Their conversion to methane and CO2 therefore occurs without any kinetic limitation, which implies that it is possible to reduce the volume of the liquid phase substantially to a minimum in order to limit the reaction volume, and therefore also appreciably the cost of the anaerobic digesters treating products with high solids contents.

Provision should therefore be made, for example, for 10% of the total apparent volume to be reserved for the aqueous phase and the sludge to provide a sufficient support for the floatation of floating solids. Thus, for a continuous reactor treating cow manure, it is possible to calculate the maximum dry matter content in the feed for a degree of progress A of the reaction by making Vv/V T = 0.9. For this type of substrate, the value of A is taken to be 0.62, that is a typical yield of 400 litres biogas kg -1 volatile solids input.

In this case, eqn (3) gives a maximum initial dry mat ter content in the feed or a DM i value of 23-7%. Due to technical limitations and pumping in particular, it will not be possible to introduce concentrated dung with a dry mat ter content above about 23.7%. Hence there are only two possibilities of working such substrates continuously at greater concentrations: (1) increasing VF/VT, or working an entirely solid product, which is not possible with the continuous process at present, for technological reasons, and (2) increasing A, for example by thermophile t reatment where the breakdown of organic mat ter is more rapid. 6

We confirmed that the ratio VF/VT = f ( A ) , is independent of temperature. Equation (3) makes it possible to determine the moment from which the content of a reactor consists entirely of floating solids, without any liquid support phase (VF/VT = 1). For the preceding example, the DMi value was found to be 28-25%. It is interesting to note that this value is very close to the concentration threshold from


which Jewell 6 observed fermentation inhibition for a similar cellulose- rich substrate. It is suggested that this inhibition was due to lack of water and therefore to a poor accessibility of the soluble enzymes and free bacteria, limited in the aqueous carrier phase, to the vegetable polymers. It would be interesting to determine whether the absolute value of the inhibition threshold is the same for all substrates. If the (Vv /VT)a value is not the same for all substrates, this threshold should be different. It should in particular be greater for domestic refuse, which subsides much more than cow manure (Fig. 3).

CONCLUSIONS

The tendency of straw and other solid particles to float during digestion is generally considered as a source of technical problems. We have shown 7, n that it is possible to make use of this tendency in an appro- priate reactor.

Expressing this phenomenon in an equation enables the changes in the hydrodynamic behaviour of a complex mixture during anaerobic digestion to be described by determining three basic parameters: (1) the initial dry matter content DMi, (2) the degree of progress of the reaction A, and (3) the ratio of the volume of floating solids to the rest of the reaction medium.

Apart from the purely descriptive aspects of this phenomenon, this model enables digesters to be designed precisely by stating the fractions to be occupied by the floating solids, the liquid phase and the sludge.

Determining the physical behaviour of the reaction media more closely provides new possibilities at the engineering design level and is therefore a contribution towards the reduction of anaerobic digestion investment costs.

ACKNOWLEDGEMENTS

The work in this paper was financed jointly by the AFME (Agence Franqaise pour la Maitrise de l'Energie) and Elf Aquitaine (Elf Bio Recherches). The authors thank Didier Payen and Michel Bruche for their technical assistance.


R E F E R E N C E S

1. Barlett, H. D. (1979). Agricultural anaerobic digesters; design and operation, Penn State University, Bulletin 827.

2. Lagrange, B. (1979). Biomethane. 2: Principes, techniques, utilisations, EDI SUD, Aix en Provence.

3. Dudley, T. C. (1980). Sludge digestion and gas utilisation in the metropolitan public health division of the Thames Water Authority. First International Course on Anaerobic Digestion, Europe Council, Dijon, December.

4. Ducellier, G. & Isman, M. (1941). Premier aperqu sur le gaz de fumier. L'Agria, 8 March.

5. Nyns, E. J., Naveau, H. P., Chome, R. & Bertrand, Y. (1979). Digesters. A worldwide review. The First International Symposium on Anaerobic Digestion, Cardiff, 17-21 September.

6. Jewell, W. (1981). Agricultural and high strength wastes. Second International Symposium on Anaerobic Digestion, Travemi~nde, 6-11 September.

7. Fink, J. D. & Goma, G. (1982). Anaerobic degradation of lignocellulosic materials- description of a new continuous process. Energy from biomass. 2nd E.C conference, eds A. Strub, P. Chattier and G. Schleser, Applied Science Publishers Ltd, London, p. 1033.

8. Moletta, R. & Albagnac, G. (1982). A gas meter for low rates of gas flow. Application to the methane fermentation. Biotechnol. Letters, 4 (5), 319-22.

9. Van Soest, P. J. (1963). Use of detergents in the analysis in fibrous feeds, lI. A rapid method for the determination of fiber and lignin. J. AOAC, 46 (5), 829-35.

10. Varel, V. H., Isaacson, H. R. & Bryant, M. P. (1976). Thermophilic methane production from cattle waste. Appl. Environm. Microbiol., 33,298-307.

11. Fink, J. D. & Goma, G. (1983). Les principes fondamentaux des m~canismes. Biomasse Actualit~s. Biogaz, num6ro special, 5-11.

Download - Natural floatation during anaerobic digestion of high strength wastes

Top Related