Procedia Computer Science 52 ( 2015 ) 730 – 737

1877-0509 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Conference Program Chairsdoi: 10.1016/j.procs.2015.05.086

ScienceDirectAvailable online at www.sciencedirect.com

The 5th International Conference on Sustainable Energy Information Technology (SEIT 2015)

Modeling of the anaerobic digestion of organic waste for biogas production

Fedailaine M a,b*, Moussi Ka, Khitous Ma, Abada Sa, Saber Ma, Tirichine Na

aCentre de Développement des Energies Renouvelables CDER, BP. 62 Route de l’observatoire, Bouzaréah 16340 Algiers, Algeria b Laboratoire de stockage et valorisation des énergies renouvelables, LSVER. Faculté de chimie, USTHB, BP 32 El Alia, Bab Ezzouar,

Algiers, Algeria

Abstract

Anaerobic digestion is a biological process in which the organic material is converted by microorganisms to methane and carbon dioxide (biogas) in the absence of oxygen. This process is interesting but the control on industrial scale spontaneous biological reactions requires good knowledge of the phenomena involved. The search for appropriate models to be use in control theory is now a high priority to optimize fermentation processes and solve important problems, such as the development of renewable energy from biodegradable organic waste. The aim of this study is modeling of biokinetics of anaerobic digestion on several aspects such as microbial activity, substrate degradation and methane production. For this, we developed a mathematical model based on mass balances on biomass, the organic substrate and biogas. This model is then simulate on Matlab using the experimental data from the literature and comparison between other models and our experimental results. The sensitivity of the model to the process parameters was study by varying the initial concentration of the biomass and the dose of the organic substrate. © 2015 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Conference Program Chairs.

Keywords:Anaerobic Digestion, Modelling, biogas

1. Introduction Anaerobic Digestion is a natural process where biodegradable material (biomass) is transformed by micro organisms to biogas in the absence of oxygen. 1The search for appropriate models to be use in control theory is now a major priority for optimizing the fermentation process and solve important problems, such as renewable energy development from biodegradable organic waste. 2Microbiological, biochemical and technological point of view, anaerobic digestion is generally composed of four major stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. During this stage, microorganisms convert the hydrogen and acetic acid to methane gas and carbon dioxide. 3 The bacteria responsible for this conversion are call methanogens and are strict anaerobes.Several mathematical models associated with these phenomena are present in the literature but are often very complex and not suitable for control. 4

* Corresponding author. Tel.: +213-219-01503; fax+213-219-01560. E-mail address: m.fedailaine@cder.dz

© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Conference Program Chairs

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The modeling of a bioprocess is a virtual representation of biological, physical and chemical processes taking place in the digester. From this and a numerical calculation software, it is possible to simulate, quickly and cheaply, different treatments scenarios taking into account and evaluating the impact of input variations (quantities and qualities) and operation. Models can be classify according to their mode of conception: (i) the "black-box" models if the process is too complex. 5 However, the modeling of biochemical processes remains difficult because there is no biological laws or universal models; unlike physics, where known and validated models for centuries (Principles of thermodynamics, ideal gas laws etc.) can be the basis for the construction of mechanistic models. Bacterial growth models are essentially empirical. In this context, we have established a biokinetic model based on mass balances on the substrate, biomass and methane production in order to predict the observed behavior of anaerobic digestion and better understand the internal phenomena that occur within digester. The model is simulate on Matlab software introducing experimental parameters of the literature. 2. Development Model

Modeling the anaerobic digestion can follow the time evolution of the composition of the biogas produced during the transformation of the organic matter. The hydrolysis step is to solubilize the substrate; It is done through extracellular enzymes excreted by certain bacteria. This is not a biological process, because there is no metabolism. 6 The acidogenic, acetogenesis and methanogenesis are metabolic steps where the organic substrate is consume and converted by bacteria. Biokinetics is describe by three phenomena, substrate consumption, growth and bacterial decay, methane production and inhibition of bacterial activity.7

3. Mathematical Model

Making material balances on the substrate, biomass and biogas, we have established a biokinetic model to describe the work of the digester. The model was simulated by the Matlab software. The simplifying assumptions to consider are: (1) The digester is a sealed reactor; (2) A perfect agitation within the reactor; (3) A biochemical reaction in the reactor; (4) A uniform in the reactor; (5) An established transitional arrangements; (6) The growth kinetics obeys the substrate inhibition model (Model Haldane); (7) The factor limiting bacterial growth is the organic substrate; (8) The suspended biomass contributes to the biodegradation of the substrate.The diagram below shows the principle of operation of a biogas digester:

.

Qni, Si, Xi

Fig. 1. Diagram of operation of an anaerobic digester

Qi, Qf : are the input and output flows of the liquid and the flow of the biogas produced, and Qg respectively(L/ d). Si , Sf : concentrations of the substrate at the inlet and the outlet (g/L). Xi ,Xf : biomass concentration at the inlet and the outlet (g/L). Z : concentration of methane in the biogas (g/L). V : volume of the digester (L). The balance sheets of material on biomass, the decomposition of the substrate and methane production accompanied by other gases (CO2, H2, NH3) are based on the mass conservation law. Taking into account the simplifying assumptions and neglecting the endogenous decay of microorganisms, mass balance equations are describe as:

3.1. Mass balance on biomass

The mass balance of biomass is write as follows:

(1)

V

Qi, Si, Xi Qf, Sf ,Xf

Qg ,Z CH4

initial final

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Dividing by the Volume of the substrate and assuming a constant rate (Qi= Qf= Q) and (Xf=X), we obtain the following equation:

(2)

We note : The Haldane relationship: (3)

where: D: is the dilution rate (day-1); Kd: is the rate of detachment of microorganisms (day-1); μ : is the rate of growth of anaerobic microorganisms (day-1). μmax: rate of growth of anaerobic microorganisms (day-1); Ks : half saturation constant (g/L); KI : coefficient of inhibition (g/L).

3.2. Mass balance on the substrate

During the conversion of the organic substrate in methane, a part of the substrate is use for the formation of new cells and provide the energy needed for growth and maintenance of microorganisms. The substrate is write as

(4)

where: Sf=S : (Substrat final is substrate instantaneous S) Yx: coefficient of production of new cells (g/g); Ksx : substrate degradation rate required for the growth of microorganisms (g/g); Kmx : substrate degradation rate required to maintain microorganisms (g/g); Ys : biogas coefficient (g/g). Dividing by the volume Vt , we obtain the following equation:

(5)

3.3. Mass balance on biogas

(6)

Methane production is negligible at the beginning and the end of the process. So we can write:

and (7) Where K is the organic substrate conversion of methane. It is given by the following relationship:

(8)

Yp : methane production ratio (g/g). For the other constituent biogas (CO2, H2, NH3), we can write:

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(9)

(10)

(11)

The following lists all constants and parameters of the model. Table 1. Theoretical values of the different parameters of the model. Parametre Symbole Value and unit Reference Initial concentration of the organic substrate S0 4 g/L 8 Initial biomass concentration X0 2 g/L 9 Maximum specific growth rate μm 0.35 jour-1 10 Dilution rate D 0.029 jour-1 11 Detachment rate of microorganisms Kd 0.02 jour-1 10 Mass transfer coefficient at the substrate-liquid interface Ks 150 g/L 12

Factor inhibition Ki 0.5 g/L 13 New cell production ratio Yx 0.82 g/g 13 Substrate degradation rate for the growth of microorganisms Ksx 0.983 g/g 11

Substrate degradation rate for the maintenance of microorganisms Kmx 0.4 g/g 11

Coefficient production YS 4.35 g/g 11 CH4 production coefficient YCH4 0.27 g/g 11 CO2 production coefficient YCO2 0.4 g/g - H2 production coefficient YH2 0.03 g/g - NH3 production coefficient YNH3 0.01 g/g - 4. Results and discussion

4.1. Biokineticmodeling anaerobic digestion

Figures 2 (a) and 3 (b) show the evolution of the concentration of the organic substrate (S) and microorganisms (X) versus time. The first step corresponds to the consumption of the organic substrate by microorganisms whose concentration decreased slightly. The results also show a production of methane as the major component at a concentration of about 1.82 g / L with a low concentration of other gases, such as: [CO2] = 0.16 g/L [H2] = 0.012 g/L and [NH3] = 0.004 g/L. The biological activity is important; it corresponds to the period of the decay of microorganisms. It therefore leads to an increased production of methane. Whereas when the organic substrate is completely consume, biodegradation is complete. Microorganisms need for maintenance phenomena, production of new cells and elimination of bacteria as a function of time. The comparison with model of Nakhla G above concentration and Chui S-F with our experimental high substrate concentration in figure 2 (c) we obtain the concordance between them. 13 14.In the figure 3 (d), we have the volume accumulation biogas experimental (48.05 L) in the same order with the theoretical volume (49.56 L). The obtained results are show in the following table 2 and fig. 2.and 3. :

Table 2.Comparison with our and other model and experimental measurements. Element Model

simulation Nakhla G 200613 Chiu S-F

201314 Experimental work 2014

S0 (g/L) 4 7.85 15.5 25 X0 (g/L) 2 2.833 4.6 12 Biogas Experimental (L) - - 48.05 Biogas theoretical CH4: 1.83g/L

CO2: 0.16 H2: 0.012 NH3:0.0042

CH4:3.95 g/L CO2:0.36 H2: 0.027 NH3:0.0091

CH4:8.87 CO2:0.81 H2: 0.06 NH3:0.02

CH4:15.5 g/L CO2:10.4 g/L 49.65 L H2: 0.1 g/L NH3:0.03 g/L

Volume of reactor (L) - 4 - 10

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Fig. 2. Evolution of the concentrations of Fig. 3. Evolution in the concentration of the biogaswith biomass and substrate with time (a), (c). time and volume accumulation (b),(d), (e). 4.2. Sensitivity analysis of the model

4.2.1. Variation of the initial concentration of the biomass

The results obtained show that the initial biomass concentration has a significant influence on the degradation of the substrate, production of methane and bacterial population. We also note a good simulation of the work of the digester over time. The initial biomass concentration decreases slightly during the first thirty days. This decrease is more significant at relatively high initial concentrations of 3.75 to 5 g/L. Then, it keeps almost constant after one month of operation. The degradation of the substrate increases rapidly during the first ten days of anaerobic digestion and then gradually decreases until equilibrium is reached after one month. This is due to a complete

(a) (b)

(c) (d)

(e)

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degradation of the organic matter that increases significantly with the initial concentration of microorganisms. Against by, the production of methane increases linearly with the initial concentration of the biomass.

When the initial concentration of the biomass increased from 1 to 5 g/L, the production of methane increases from 1.31 to 2.35 respectively. This is due to a strong or high deterioration of the organic substrate methane in the presence of a large number of microorganisms

Fig. 4. Effect of initial biomass concentration on: (a) Bacterial population,(b) Degradation of the substrate and

(c) Production of methane.

4.2.2. Variation of the initial dose of the substrate

In this case, we varied the initial dose of the substrate from 2 to 10 g/L. The following table shows the

initial conditions and the concentration of methane produced after 80 days of running the digester: The followings Fig. 5.Show the effect of the initial dose of the substrate on the biomass concentration, degradation of the substrate and production of methane, respectively. Increasing the initial dose of the substrate causes a rapid decrease of the initial concentration of microorganisms and an increase in methane production during the forty days of operation. By against the degradation of the substrate increases during the first twenty days, and then gradually decreases until reaching a complete degradation of the substrate after 80 days of fermentation. This degradation is quite large with the initial doses of the highest substrate (7.5 and 10 g/L).

This behaviour can be explain by decay phenomena and detachment of microorganisms following

fermentation reactions of organic matter in the presence of a large amount of the organic substrate to be degraded, thereby producing a rapid degradation of the substrate and an increase in biogas as well.

(b)

(c)

(a)

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Fig. 5. Effect of dose on the substrate: (a) Concentration of biomass,(b) Degradation of the substrate and

(c) Production of methane.

5. Conclusion

Through this study, the following conclusions can be drawn: • The biokinetic model that has established allowed to monitor the operation of the digester over time,

including, the degradation of organic matter, evolution of biomass, biogas composition and methane production.

• The model of the sensitivity study of the process parameters, namely the initial biomass concentration and dose of the substrate showed that the increase is favourable on the anaerobic digestion.

• The results of the simulation helps to design a biogas unit and prevent a possible maintenance and digester feed.

• Finally, we have seen our model presents a concordance with other models.

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