Natural floatation during anaerobic digestion of high strength wastes

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<ul><li><p>Biomass 6 (1985) 223-234 </p><p>Natural Floatation during Anaerobic Digestion of High Strength Wastes </p><p>J. D. Finck* and G. Goma </p><p>Institut National des Sciences Appliqu6es, D6partement de G6nie Biochimique et Alimentaire, ERA-CNRS 879, Avenue de Rangueil, </p><p>31077 Toulouse Cedex, France. </p><p>(Received: 17 February, 1984) </p><p>ABSTRACT </p><p>The tendency of vegetable fibres to float during the anaerobic digestion process has been studied for two types of substrate, cow dung and domestic refuse. A mathematical model has been developed which, on the basis of a knowledge of the initial dry matter content and the degree of progress of the reaction, enables the reaction volume occupied by the floating phase to be determined accurately. It is thus possible to determine the critical dry matter content beyond which fermentation can no longer take place due to limitations on diffusion, lack of water and other technical reasons. </p><p>Key words: Anaerobic digestion, biogas, domestic refuse, floatation, mathematical model, straw. </p><p>INTRODUCTION </p><p>One of the basic problems of most digesters used in the treatment of very concentrated matter 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 </p><p>*Present address: Elf Bio Recherches, La Grande Borde, BP 62, Lab6ge, 31320 Castanet Tolosan, France. </p><p>223 Biornass 0144-4565/85/$03.30 - Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain </p></li><li><p>224 J. D. Finck, G. Goma </p><p>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 the almost exclusive use of batch digesters based on the Ducellier Isman aerobic pre-fermentation pro- cess. 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 </p><p>Even in batch processes the floatation of fibres raises other technical problems. The large increase in volume as the result of 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. </p><p>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 35C 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. </p><p>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. </p><p>MATERIALS AND METHODS </p><p>Substrates </p><p>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. </p></li><li><p>Floatation during anaerobic digestion of wastes </p><p>TABLE 1 Principal Characteristics of Two Solid Substrates (Cow Dung and Pre-sifted </p><p>Domestic Refuse) Used in the Experiments </p><p>225 </p><p>Substrate Cow dung Pre-sifted domestic refuse </p><p>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 </p><p>Reactors </p><p>The study of the changes in the volume of phases during digestion, related to the production of biogas, was carried out in a fermenter made of 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 production of biogas was measured by means of a Schlumberger water displacement meter and reduced to standard pressure and temperature conditions. </p><p>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 </p><p>Analytical procedures </p><p>The dry matter 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 105C for 12 h. The volatile solids were determined by calcination of the dry matter at 550C for 6 h. Cellulose, hemicellulose and lignin were determined according to the procedure described by Van Soest. 9 </p></li><li><p>J. D. Finck, G. Goma 226 </p><p>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 </p><p>tank; 7 - discharge; 8 - continuous feed inlet; 9 - gas meter. </p><p>RESULTS </p><p>Phase separation during the anaerobic digestion process of two sofid type substrates </p><p>The batch anaerobic digestion was carried out at a temperature of 35C in a transparent reactor shown in Fig. 1. The manure was not seeded at all beforehand, but the domestic refuse was seeded to 1% with liquors derived from a cont inuous digester treating cow manure containing 8.7% dry matter. 7 </p><p>At the beginning o f fermentat ion, a large increase in the volume of liquid mass was observed due to biogas bubbles being trapped in the straw network which very rapidly appeared on the surface. This increase </p></li><li><p>Floatation during anaerobic digestion of wastes 227 </p><p>100 _ </p><p>50_ </p><p>Fig. 2. </p><p>% Vol. </p><p>~ A </p><p>B </p><p>- - -4C t(~) </p><p>J I I I I I I I </p><p>~0 20 3o 40 so 60 70 e0 90 </p><p>Batch digestion of cow dung: phase changes against time. A - Floating solids; B - liquid phase; C - sludge. </p><p>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 of these curves was substantially the same in the case of domestic refuse. </p><p>The surface scum consisted largely of straw, cellulose waste and pos- sibly other residues whose density is close to that of water. The produc- tion of biogas is probably responsible for this natural floatation, for it was not observed when the same experiment was performed at 4C when fermentation was prevented. </p><p>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. </p></li><li><p>228 J. D. Finck, G. Goma </p><p>OjS- </p><p>0,25- </p><p>Fig. 3. </p><p>VF VT </p><p>C D </p><p>~_ DR </p><p>A </p><p>I 1 I 0, 25 0, 5 0,75 </p><p>VF/V T against degree of progress A of reaction at 35C and 10% initial dry matter content: C.D. - cow dung; D.R. - domestic refuse. </p><p>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 quantity of biogas formed. The latter is represented by the degree of progress (A) of the reaction, which is defined as the quantity of biogas produced related to the pro- duction capacity at infinite retention time (AT). This production capacity is approached by a double reciprocal of the cumulated biogas production versus digestion time, or: </p><p>1 _ f (1 ) P (t) </p><p>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 </p><p>accumulation of the floating solids in the upper phase than in the case </p></li><li><p>Floatation during anaerobic digestion of wastes 229 </p><p>of domestic refuse. With an identical initial dry matter content, domestic refuse occupies 3 1% less reaction volume than cow dung at the begin- ning 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. </p><p>MODELLING </p><p>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. </p><p>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). </p><p>This basic parameter can be expressed in an equation: </p><p>Volume of floating solids (VF /VT) t = </p><p>Total apparent volume </p><p>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): </p><p>(VF/VT)lO = In(0.0836DMi + 1) (1) </p><p>If, secondly, the changes in the VF/VT ratio are represented in accord- ance with the degree of progress A of the reaction, it is found that, when DMi = 10, the following equation is obtained: </p><p>(VF/VT)t =- 0.269A + 0.667 (2) </p><p>The values used for the development of this equation were calculated from the data in Fig. 2. </p><p>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 lignocellu- loses, of the hydrolytic bacterial populations is in the floating solids and not in the liquid phase, which represents a dead volume. </p></li><li><p>230 J. D. Finck, G. Goma </p><p>Fig. 4. </p><p>VF x p '~;"T </p><p>DM, I 1 I </p><p>5 10 15 </p><p>exp(Vr/VT) against initial dry matter content DMi for cow dung. </p><p>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. </p><p>We confirmed that (Vv/V~)e/(Vv/VT)IO is independent of the initial dry matter content DMi. By combining eqns (1) and (2), one obtains: </p><p>(Vv/VT)t (VV/VT)10 </p><p>=- 0.444A + 1.1 (3) </p><p>By introducing eqn (1) into the denominator, it is possible to define a general model of the form: </p><p>(Vv/Vx)t = ln(0-083DMi + 1) (1-1 -- 0.444A) (4) </p></li><li><p>Floatation during anaerobic digestion of wastes 231 </p><p>This final representation is valid under the following conditions: (1) the substrate is cow manure; (2) most of 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 continuous 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 of progress of the reaction. </p><p>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: </p><p>(Vv/VT)t = [ ln (aDMi - 1)] (b - 0.444A) (5) </p><p>where a and b are constants dependent on the substrate. </p><p>0.I </p><p>0.3 </p><p>0-5 </p><p>0-7 </p><p>0,5_ </p><p>I l I </p><p>5 10 15 </p><p>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. </p><p>DH i </p></li><li><p>232 J. D. Finck, G. Goma </p><p>TECHNOLOGICAL IMPLICATIONS </p><p>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 there- fore 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. </p><p>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. </p><p>In this case, eqn (3) gives a maximum initial dry matter 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 matter 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 treatment where the breakdown of organic matter is more rapid. 6 </p><p>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 </p></li><li><p>Floatation durin...</p></li></ul>

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