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Study of performance criteria of serial configuration of two chemostats

  • This paper deals with thorough analysis of serial configurations of two chemostats. We establish an in-depth mathematical study of all possible steady states, and we compare the performances of the two serial interconnected chemostats with the performances of a single one. The comparison is evaluated under three different criteria. We analyze, at steady state, the minimization of the output substrate concentration, the productivity of the biomass and the biogas flow rate. We determine specific conditions, which depend on the biological parameters, the operating parameters of the model and the distribution of the total volume. These necessary and sufficient conditions provide the most efficient serial configuration of two chemostats versus one. Complementarily, this mainly helps to discern when it is not advisable to use the serial configuration instead of a simple chemostat, depending on: the considered criterion, the operating parameters fixed by the operator and the distribution of the volumes into the two tanks.

    Citation: Manel Dali Youcef, Alain Rapaport, Tewfik Sari. Study of performance criteria of serial configuration of two chemostats[J]. Mathematical Biosciences and Engineering, 2020, 17(6): 6278-6309. doi: 10.3934/mbe.2020332

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  • This paper deals with thorough analysis of serial configurations of two chemostats. We establish an in-depth mathematical study of all possible steady states, and we compare the performances of the two serial interconnected chemostats with the performances of a single one. The comparison is evaluated under three different criteria. We analyze, at steady state, the minimization of the output substrate concentration, the productivity of the biomass and the biogas flow rate. We determine specific conditions, which depend on the biological parameters, the operating parameters of the model and the distribution of the total volume. These necessary and sufficient conditions provide the most efficient serial configuration of two chemostats versus one. Complementarily, this mainly helps to discern when it is not advisable to use the serial configuration instead of a simple chemostat, depending on: the considered criterion, the operating parameters fixed by the operator and the distribution of the volumes into the two tanks.


    The chemostat device was invented concomitantly by Monod [1] and Novick & Szilard [2] in 1950. Widely used as a biochemical laboratory-pilot, it consists essentially in a continuously-fed bioreactor characterized by the equality of the input and the output flow rates. It is designed as a vessel in which different microorganisms grow, also called continuous culture of microorganisms. Its importance for the continuous culture of microorganisms has been reported in several books and publications, among them [3,4,5,6,7]. In other words, the classical model of the chemostat consists of a perfect mixed media at a constant temperature, a constant pH, a filtered feed and a unique flow rate. Although this model is used for industrial applications with continuously-fed bioreactors such as waste-water treatment, see for instance [8], in physical reality, industrial applications which use large bioreactors hardly satisfy the assumption of the perfect mixed media. Several mathematical representations of the spatial heterogeneity have been studied in the literature with partial differential equations, see for instance [9,10]. However, discrete spatial representations, such as the gradostat model [6,11], are also a way to represent spatial heterogeneity [12,13,14]. Serial configurations, as a simple gradostat, have received a great interest in the literature in view of optimizing bioprocesses. Indeed, it has been shown that having two tanks (or more) in series (each of them being assumed to be perfectly mixed) can produce the same substrate conversion than a single vessel, but with a significant lower total volume, and thus a lower residence time. Serial configurations have been also studied in view of ecological insight, see for instance [15,16]. In this paper, we propose to revisit the serial configuration of two chemostats in series with a constant total volume V, as shown in Figure 1. We focus on the analysis of the performance at steady-state for different criteria with the aim of drawing comparisons with the single chemostat. Notice that these different criteria of comparison are known in the literature, see for instance [3]. However, to our knowledge, a complete and deep analysis of all possible configurations for a general class of growth functions and the various criteria is missing in the literature, which is the aim of the present work.

    Figure 1.  The serial configuration of two chemostats. The output substrate concentration at steady state Soutr measures the performance of the system to convert the substrate Sin.

    It is well known [4,6] that, for the simple chemostat, the output concentration at steady steady Sout is independent of the input concentration Sin, provided that there is no washout, see also (2.5). This property is no longer satisfied when there is a spatial structure, see for instance [15] and the references therein. Since Sout measures the performances of the chemostat to convert the substrate S, our purpose is to distinguish which configuration guarantees the minimal output substrate concentration at steady state. Actually, reducing the output substrate concentration is one of the biological objectives in waste-water treatments and this minimizing problem is well known in the literature. The novelty of our work is that Sout is considered as a function which depends on the three operating parameters: the input substrate concentration, the dilution rate and the volume of each chemostat. In fact, what has already been treated, see for instance [17,18,19], corresponds to the case where the input substrate concentration Sin is fixed and the total volume V can be chosen. Thus, we give conditions which involve the input substrate concentration Sin and ensure the optimal way to slice the two serial reactors volume. These conditions can ensure a lower output substrate concentration.

    Our study is somehow a generalization of the main results presented in [16]. The conditions that we found are necessary and sufficient to reduce the output substrate concentration in contrast of the result in [16] where the given conditions are only sufficient. In addition, the originality of this article consists in comparing both configurations according to two other performance indexes which are the productivity of the biomass and the biogas flow rate. The biogas flow rate represents the quantity of natural gas per unit of time produced by the decomposition of organic matter in absence of oxygen and the productivity of the biomass represents the amount of biomass per unit of time produced by the decomposition of organic matter. The productivity of the biomass of several configurations including the serial device of two interconnected chemostats has been graphically and numerically analyzed in [12,20]. However, these two criteria have not yet been deeply mathematically analyzed. The global analysis shows that the different performance criteria involve the same performance threshold. This threshold is explicitly defined by a function which depends on the dilution rate D. It defines the set of the values of Sin and D that allow or not a better performance of the serial configuration with two chemostats. Several numerical applications are given to illustrate all the results of the study.

    This paper is organized as follows. Section 2 presents the model. Subsequently, the main part of the paper constituting section 3 is dedicated to the study of the equilibria and the performance analysis of the configuration. Indeed, the output substrate concentration, the productivity of the biomass and the biogas flow rate are respectively treated in sections 3.1, 3.2 and 3.3. Next, the operating diagram of the model is depicted in section 4. Afterwards, several numerical simulations illustrating the results of our analysis and using some specific growth functions are represented in section 5. Finally, section 6 contains a global conclusion. Most of the proofs corresponding of the theorems and propositions stated along the paper are proved in Appendixes A, B, C and D. Firstly, Appendix A contains the proof related to the existence and the stability of steady states. Secondly, Appendixes B and C contain respectively the proofs related to the output substrate concentration, the productivity of the biomass and the biogas flow rate. Finally, Appendix D contains proofs related to some of technical results of the paper.

    If S and X denote respectively the substrate and the biomass concentration in a single chemostat of volume V, the input flow rate Q and the input concentration of substrate Sin, their time evolution are modeled by the following system of ordinary differential equations:

    ˙S=D(SinS)f(S)X/Y˙X=DX+f(S)X (2.1)

    where Y is the yield conversion of substrate into biomass, f() the specific growth rate of the microorganisms that is assumed null at S=0 and to be increasing for S>0, and D=Q/V is the dilution rate. Without loss of generality, one can assume Y=1 in Eq (2.1) by using the change of variable x=X/Y. System (2.1) becomes

    ˙S=D(SinS)f(S)x˙x=Dx+f(S)x. (2.2)

    The detailed mathematical analysis of the model (2.2) may be found in [4,6]. Let us recall classical results about the asymptotic behavior of (2.2). We define

    m:=supS>0f(S), (m may be +). (2.3)

    As f is increasing then the break-even concentration is defined by

    λ(D):=f1(D) when 0D<m. (2.4)

    When Sin>λ(D) (or, equivalently, f(Sin)>D), any solution of (2.2) with S(0)0 and x(0)>0 converges toward the positive steady state E1=(λ(D),Sinλ(D)). On the contrary, when Dm or Sinλ(D) (or, equivalently, f(Sin)D), any solution of (2.2) with S(0)0 and x(0)0 converges toward the wash-out steady state E0=(Sin,0). Thus, the output concentration at steady state Sout(Sin,D) is given by

    Sout(Sin,D)={SinifDf(Sin)λ(D)ifD<f(Sin). (2.5)

    We consider now the serial interconnected chemostats, where the volume V is divided into two volumes, rV and (1r)V with r(0,1), as shown in Figure 1, with Q the flow rate and Sin the input substrate concentration in the first chemostat. The mathematical model is given by the following equations:

    ˙S1=Dr(SinS1)f(S1)x1˙x1=Drx1+f(S1)x1˙S2=D1r(S1S2)f(S2)x2˙x2=D1r(x1x2)+f(S2)x2. (2.6)

    The dilution rates of the first and second chemostats are D/r and D/(1r), respectively, where D is the dilution rate, defined by D:=Q/V, of the single chemostat of volume V and flow rate Q. For the limiting cases r=0 and r=1, these equations are not valid. Indeed, the limiting cases correspond to the single chemostat model defined by (2.2).

    In [16], the mathematical analysis of (2.6) was performed for a linear growth function f(S)=aS (a>0) and numerical simulations were given for a Monod growth function f(S)=mS/(K+S). The results of [16] were extended to Monod growth function and for increasing and concave growth function in [21].

    Remark 1. The main result in [16,21], see also [15], predicts that there exists a threshold Sin1 such that for SinSin1, the output Soutr(Sin,D), which is the output density of the substrate at steady state, satisfies Soutr(Sin,D)>λ(D), for all r(0,1) and, if Sin>Sin1, there exists a threshold r1(0,1), such that Soutr(Sin,D)<λ(D) if and only if r1<r<1.

    As it was noticed in (2.5), for a single chemostat, one has Sout(Sin,D)=λ(D). Therefore, if SinSin1, the serial configuration is always less efficient than the single chemostat of the same total volume V. In contrast, for Sin>Sin1 and r large enough (i.e. r>r1), the serial configuration is more efficient than the single chemostat.

    In this paper, we extend this result to general increasing growth functions where the concavity of f is not required and we provide explicit formulas for the thresholds Sin1(D) and r1(Sin,D). Hence, we consider a growth function satisfying only the following qualitative property:

    Assumption 1. The function f is C1, with f(0)=0 and f(S)>0 for all S>0.

    The following result is classical in the mathematical theory of the chemostat and is left to the reader.

    Lemma 1. The solutions (S1(t),x1(t),S2(t),x2(t)) of (2.6) with nonnegative initial conditions, exist for all t0, are nonnegative, bounded and limt+(Si(t)+xi(t))=Sin for i=1,2.

    The existence and stability of steady states of (2.6) are given by the following result. We use the abbreviation LES for locally exponentially stable and GAS for globally asymptotically stable in the positive orthant.

    Theorem 1. Assume that Assumption 1 is satisfied. The steady states of (2.6) are:

    The washout steady state E0=(Sin,0,Sin,0) which always exists. It is GAS if and only if

    Dmax{r,1r}f(Sin). (2.7)

    It is LES if and only if: D>max{r,1r}f(Sin).

    The steady state E1=(Sin,0,¯S2,Sin¯S2) of washout in the first chemostat but not in the second one, where ¯S2 is given by ¯S2=λ(D/(1r)). This steady state exists if and only if D<(1r)f(Sin). It is GAS if and only if

    rf(Sin)D<(1r)f(Sin). (2.8)

    It is LES if and only if: rf(Sin)<D<(1r)f(Sin).

    The steady state E2=(S1,SinS1,S2,SinS2) of persistence of the species in both chemostats, where S1 is given by S1=λ(D/r) and S2=S2(Sin,D,r) is the unique solution of the equation

    h(S2)=f(S2)withh(S2):=D(S1S2)(1r)(SinS2). (2.9)

    This steady state exists if and only if D<rf(Sin). It is GAS and LES whenever it exists.

    Proof. The proof is given in Appendix A.

    Remark 2. Transcritical bifurcations occur in the limit cases D=rf(Sin) and D=(1r)f(Sin).

    1. For 0<r<1/2, we have a transcritical bifurcation of E0 and E1 when D=(1r)f(Sin) and a transcritical bifurcation of E1 and E2 when D=rf(Sin).

    2. For 1/2<r<1, we have a transcritical bifurcation of E0 and E2 when D=rf(Sin) and a transcritical bifurcation of E0 and E1 when D=(1r)f(Sin).

    3. For r=1/2 and D=f(Sin)/2, we have transcritical bifurcations of E0 and E1, and E0 and E2, simultaneously.

    Figure 2(a) shows the functions f and h, and the solution S2=S2(Sin,D,r) of the Eq (2.9), which is unique since f is strictly increasing and the graph of h is a hyperbola. If Sin,1>Sin,2, then hi(S2)=D(S1S2)(1r)(Sin,iS2), i=1,2, satisfies h2(S2)>h1(S2), for all S2(0,S1), as shown in Figure 2(b). Therefore, we have the following result:

    Figure 2.  (a): Graphical illustration of Eq (2.9). (b): The result of Proposition 1 with Si2=S2(Sin,i,D,r), i=1,2.

    Proposition 1. Let Sin1 and Sin2 be two different input substrate concentrations. If Sin,1>Sin,2>0 then for all r(D/f(Sin,2),1) and D>0, one has S2(Sin,1,D,r)<S2(Sin,2,D,r).

    Proof. The proof is given in Appendix B.1.

    In this section we give the expressions of the output substrate concentration at steady state, the productivity of the biomass and the biogas flow rate, for the serial configuration of two chemostats.

    Let us consider the dependency of the output substrate concentration with respect to the dilution rate D and the input concentration Sin. As stated in Theorem 2.1, for 0<r<1, the output substrate concentration at steady state is given by the formulas:

    Soutr(Sin,D)={Sinifmax{r,1r}f(Sin)Dλ(D/(1r))ifrf(Sin)D(1r)f(Sin)S2(Sin,D,r)ifD<rf(Sin). (3.1)

    Although Soutr(Sin,D) is defined by (3.1) only for 0<r<1, we extend it, by continuity, for r=0 and r=1 by

    Sout0(Sin,D)=Sout1(Sin,D)=Sout(Sin,D). (3.2)

    The continuity follows from the facts that limr1S2(Sin,D,r)=λ(D) and the second case, where Soutr(Sin,D)=λ(D/(1r)), is possible only if 0r1/2.

    We have to compare Soutr(Sin,D), given by (3.1) and (3.2), with Sout(Sin,D), given by (2.5). Let r(0,1) be fixed. Let gr:[0,rm)R, where m is given by (2.3), be defined by

    gr(D):=λ(D)+λ(D/r)λ(D)1r. (3.3)

    The following result asserts that the serial configuration of two chemostats of volumes rV and (1r)V respectively, shown in Figure 1, is more efficient than the simple chemostat of volume V, if and only if Sin>gr(D).

    Theorem 2. For any r(0,1), one has Soutr(Sin,D)<Sout(Sin,D) if and only if Sin>gr(D).

    Proof. The proof is given in Appendix B.2

    We need the following assumption, which is satisfied by any concave growth function, but also by Hill function, which is not concave, as it is shown in section 5.

    Assumption 2. For every D[0,m), the function r(D/m,1)gr(D)R is strictly decreasing.

    Let g:[0,m)R be defined by

    g(D):=λ(D)+Df(λ(D)). (3.4)

    We have the following result:

    Lemma 2. Assume that Assumptions 1 and 2 are satisfied. For all (Sin,D) verifying the condition Sin>g(D), there exists a unique r1=r1(Sin,D) (0,1) such that Sin=gr1(D). One has r>r1(Sin,D) if and only if Sin>gr(D).

    Proof. The proof is given in Appendix B.3.

    We can state now our main result which compares Soutr(Sin,D) and Sout(Sin,D).

    Theorem 3. Assume that Assumptions 1 and 2 are satisfied.

    If Sing(D) then for any r(0,1), Soutr(Sin,D)>Sout(Sin,D).

    If Sin>g(D) then Soutr(Sin,D)<Sout(Sin,D) if and only if r1(Sin,D)<r<1 with r1(Sin,D) defined in Lemma 2.

    The equality is fulfilled for r=0, r=r1 and r=1.

    Proof. The proof is given in Appendix B.4.

    Lemma 2 and Theorem 3 give analytical expression for the thresholds Sin1 and r1 mentioned in Remark 1. Indeed, we have Sin1=g(D) and r1 depends on D and Sin, and is given implicitly by equation Sin=gr1(D). In section 5, we give explicit formulas for r1(Sin,D) in the cases of linear growth functions, see (5.1), or Monod growth functions, see (5.2). To have a better understanding of the role of the parameter r, we also analyze the function rSoutr(Sin,D) when Sin and D are fixed. According to the conditions on Sin and D, related to the global stability of the equilibria, several cases must be distinguished. The following result encompasses the whole possible cases.

    Proposition 2. Let D>0 and Sin>0. We denote by r0 the ratio r0=D/f(Sin).

    1) If Sinλ(D) then for any r[0,1], one has Soutr(Sin,D)=Sout(Sin,D)=Sin.

    2) If λ(D)<Sin<λ(2D) then one has 12<r0<1 and

    Soutr(Sin,D)={λ(D/(1r))if0r1r0Sinif1r0rr0S2(Sin,D,r)ifr0r1. (3.5)

    3) If λ(2D)Sin then one has 0<r012 and

    Soutr(Sin,D)={λ(D/(1r))if0rr0S2(Sin,D,r)ifr0r1. (3.6)

    Proof. The proof is given in Appendix B.5.

    For a deeper analysis, we consider the functions DSoutr(Sin,D) and DSout(Sin,D) where we fix the input substrate density Sin and the parameter r. We add the following assumption, which is satisfied by concave growth functions and also by Hill functions as it is shown in section 5.

    Assumption 3. For every r(0,1), the function D(0,rm)gr(D)R is strictly increasing.

    We have the following result:

    Proposition 3. Assume that Assumptions 1 and 3 are satisfied. For any r(0,1) and Sin>0, there exists a critical value Dr=Dr(Sin), which is the unique solution of the implicit equation Sin=gr(D), such that the serial configuration of two interconnected chemostats is more efficient than a simple chemostat if and only if 0<D<Dr(Sin). That is to say, for any 0<D<Dr(Sin), one has Soutr(Sin,D)<Sout(Sin,D).

    Proof. The proof is given in Appendix B.6.

    The result of Proposition 3 is illustrated by Figure 3. In this Figure, the critical value Dr=Dr(Sin) is depicted for various values of r and Sin, illustrating then Proposition 1 which asserts that, for a fixed dilution rate D, the output substrate concentration decreases when Sin increases.

    Figure 3.  The output substrate concentration of the serial device and the simple chemostat are respectively represented by the red and the blue curves. Dir is the implicit solution of Sin,i=gr(D), i=1,2. The output substrate concentration of the serial device (in red) decreases as Sin increases.

    The following Lemmas 3 and 4 provide sufficient conditions for Assumption 2 and 3 to be satisfied. These conditions are useful for the applications given in section 5. For this purpose, we consider the function γ defined by

    γ(r,D):=gr(D) where dom(γ)={(r,D):0<r<1,0<D<rm}, (3.7)

    which consists simply in considering gr(D), given by (3.3), as a function of both variables r and D. If γr(r,D)<0 for all (r,D)dom(γ), then Assumption 2 is satisfied. Similarly, if γD(r,D)>0 for all (r,D)dom(γ), then Assumption 3 is satisfied. The following lemmas give equivalent conditions, and also sufficient conditions, for partial derivatives of γ to have their signs as indicated above.

    Lemma 3. For D(0,m), let lD be defined on (D/m,1] by lD(r)=λ(D/r). The following conditions are equivalent

    1. For all (r,D)dom(γ), γr(r,D)<0.

    2. For all D(0,m) and r(D/m,1), lD(1)>lD(r)+(1r)lD(r).

    If lD is strictly convex on (D/m,1], then the condition 2 is satisfied. If, in addition, f is twice derivable, then lD is twice derivable and the following conditions are equivalent

    3. For all D(0,m) and r(D/m,1], lD(r)>0.

    4. For all S>0, f(S)f(S)<2(f(S))2.

    Proof. The proof is given in Appendix D.1.

    Lemma 4. The following conditions are equivalent

    1. For all (r,D)dom(γ), γD(r,D)>0.

    2. For all (r,D)dom(γ), f(λ(D/r))<f(λ(D))/r2.

    If f is decreasing, then the condition 2 is satisfied.

    Proof. The proof is given in Appendix D.2.

    Remark 3. If the growth function is twice derivable and satisfies f(S)0 for all S>0, then the condition 4 in Lemma 3 and the condition 2 in Lemma 4 are satisfied. Thus, Assumptions 2 and 3 are satisfied. Therefore, our results apply for concave growth functions. The previous Lemmas allow to consider a non-concave growth function such as the Hill function as shown in section 5.3.

    Let us consider the dependency of the productivity of the biomass with respect to the dilution rate D and the input concentration Sin. Recall that for a simple chemostat the output biomass at steady state is given by xout=SinSout. Thus, the productivity of a single chemostat is defined by

    P(Sin,D):=Qxout(Sin,D)={0 if Df(Sin)VD(Sinλ(D)) if D<f(Sin). (3.8)

    Let Dopt(Sin) be the dilution rate which maximizes P(Sin,D) i.e.

    Dopt(Sin):=argmax0Df(Sin)P(Sin,D). (3.9)

    Assumption 4. The dilution rate Dopt(Sin) defined by (3.9) is unique.

    Proposition 4. The dilution rate Dopt(Sin) defined by (3.9) is the solution of equation Sin=g(D) where g is defined by (3.4).

    Proof. The proof is given in Appendix C.1.

    The productivity of the two serial interconnected chemostats at steady-state is defined as

    Pr(Sin,D):=Qxoutr(Sin,D). (3.10)

    with xoutr=SinSoutr. Using the definitions (3.1) and (3.2) of Soutr(Sin,D), for r(0,1) we have

    Pr(Sin,D)={0 if max{r,1r}f(Sin)DVD(Sinλ(D/(1r))) if rf(Sin)D(1r)f(Sin)VD(SinS2(Sin,D,r)) if D<rf(Sin) (3.11)

    and Pr(Sin,D)=P(Sin,D), when r=0 and r=1. As a consequence of Theorem 3 we obtain the following result.

    Corollary 1. Assume that Assumptions 1 and 2 are satisfied.

    If Sing(D) then for any r(0,1), Pr(Sin,D)<P(Sin,D).

    If Sin>g(D) then Pr(Sin,D)>P(Sin,D) if and only if r(r1,1), where r1=r1(Sin,D) is the unique solution of Sin=gr(D)

    and Pr(Sin,D)=P(Sin,D) for r=0, r=r1 and r=1.

    Proof. The proof is given in Appendix C.2.

    This Corollary ensures that if Sin>g(D) and for any r1<r<1, the productivity of the biomass of the serial configuration is larger than the one of the simple chemostat. These conditions, related to the productivity of the biomass, are the same conditions that arose in the case of the minimization of the output substrate concentration, see section 3.1. We illustrate this Corollary in section 4 in Figure 8.

    Figure 4.  The biogas flow rate of the serial configuration of two chemostats (in light blue) and the one of the single chemostat (in black).
    Figure 5.  The operating diagram of two interconnected chemostats in serial depending on the parameter r.
    Figure 6.  Regions in the operating plan with different behaviors of the mapping rSoutr(Sin,D) where (Sin,D) is fixed.
    Figure 7.  The map rSoutr(Sin,D) (in red) in the regions J1, J2, J3 and J4 compared to rSout(Sin,D) (in blue). The value r1 is the unique solution of Sin=gr(D) and r0=D/f(Sin).
    Figure 8.  The map rPr(Sin,D) (in light blue) in the regions J1, J2, J3 and J4 compared to rP(Sin,D) (in black). The value r1 is the unique solution of Sin=gr(D) and r0=D/f(Sin).

    Let us consider the dependency of the biogas flow rate with respect to the dilution rate D and the input concentration Sin. Recall that for a simple chemostat the output biomass at steady state is given by xout=SinSout. Classically, the biogas flow rate at steady-state of the simple chemostat model is defined as

    G(Sin,D):=Vxoutf(Sout)={0 if Df(Sin)VD(Sinλ(D)) if D<f(Sin). (3.12)

    The biogas flow rate of the serial configuration of two chemostats is the sum with the same propositional coefficient kept equal to one, which is thus defined as

    Gr(Sin,D):=2i=1Vixout,if(Sout,i). (3.13)

    with Vi the volume, xout,i the output steady-state biomass and Sout,i the output steady-state substrate concentration, all corresponding to the tank i=1,2. In this respect, for r=0 and r=1 we have Gr(Sin,D)=G(Sin,D) and when r(0,1) it is formulated by

    Gr(Sin,D)={0 if max{r,1r}f(Sin)DVD(Sinλ(D/(1r))) if rf(Sin)D(1r)f(Sin)VD(Sinλ(D/r))+V(1r)f(S2)(SinS2) if D<rf(Sin). (3.14)

    Proposition 5. For any D[0,m), Sin>0 and r(0,1), one has Gr(Sin,D)=Pr(Sin,D).

    Proof. The proof is given in Appendix C.3.

    We know that for a single chemostat, the biogas flow rate and the productivity of the biomass at steady state are identical. Proposition 5 asserts this same conclusion in the case of two serial interconnected chemostats. Thereby, we deduce that analyzing the productivity of the biomass or the biogas flow rate at the steady state of two interconnected chemostats are equivalent. In this respect, Corollary 1 and the following result are verified for both performance criteria.

    Proposition 6. Let Sin>0. Let Gmax(Sin):=maxD(0,f(Sin))G(Sin,D). For any D>0 and r(0,1), one has Gr(Sin,D)<Gmax(Sin).

    Proof. The proof is given in Appendix C.4.

    The two functions DGr(Sin,D) and DG(Sin,D) are depicted in Figure 4. It shows that, for fixed values Sin and r, the biogas production of the serial configuration of two chemostats is more efficient than the one of the single chemostat if and only if 0<D<Dr with Dr solution of Sin=gr(D), as it was proved in Proposition 3. In addition, Proposition 6 guarantees that the biogas flow rate of the serial device will never exceed the maximal biogas flow rate of the single chemostat. In other words, the extrema of the blue curve of the serial configuration will never exceed the extremum of the black curve of the simple chemostat.

    This result has been graphically shown in [12] and [20] for the productivity of the biomass in the particular case of the Monod growth function. The simulations depicted in these references predicted that spatialization as we proposed it, does not give a better productivity of the biomass than a simple chemostat. According to Proposition 5, we know that at steady-state, the biogas flow rate and the productivity of the biomass are the same, which explains why predictions of the authors of [12] and [20] correspond to Proposition 6.

    The operating diagram is the bifurcation diagram for which the values of the biological parameters are fixed. The various regions of the operating diagram reflect qualitatively different dynamics. The operating parameters which are the input concentration Sin and the dilution rate D of the chemostat can be chosen by the practitioners and the behavior of the model is discussed with respect to them. In contrast, the biological parameters are the ones of the growth function since they depend on the organisms, the substrates and the conversion rate Y, and are usually estimated in the laboratory.

    Let the curves Φr and Φ1r in the (Sin,D) positive plane be defined by

    Φr:={(Sin,D):D=rf(Sin)}andΦ1r:={(Sin,D):D=(1r)f(Sin)}. (4.1)

    The curves Φr and Φ1r split the positive plane (Sin,D) in several regions denoted I0(r), I1(r), I2(r) and I3(r) defined by:

    I0(r):={(Sin,D):max{r,1r}f(Sin)D},I1(r):={(Sin,D):rf(Sin)D<(1r)f(Sin)},0r<12,I2(r):={(Sin,D):0<D<min{r,1r}f(Sin)},I3(r):={(Sin,D):(1r)f(Sin)D<rf(Sin)},12<r1.

    We fix r in (0,1) and we depict in the plane (Sin,D) the regions in which the solution of system (2.6), with positive initial condition, globally converges towards one of the steady states E0, E1 or E2. In the case 0r<12 [res. 12<r1], the regions I0(r), I1(r) and I2(r) [res. I0(r), I2(r) and I3(r)] form a partition of the positive plane. The region I1(r) for 12r1 [res. I3(r) for 0r12] is empty. The behavior of the system in each region is given in Table 1. Let the curve Γr in the positive plane (Sin,D) be defined by

    Γr:={(Sin,D):Sin=gr(D)} (4.2)
    Table 1.  Stability of the steady states in the various regions of the operating diagram. The letter U means that the steady state is unstable. The letters GAS mean that the steady state is globally asymptotically stable in the positive orthant. No letter means that the steady state does not exist.
    I0(r) I1(r) I2(r) I3(r)
    E0 GAS U U U
    E1 GAS U
    E2 GAS GAS

     | Show Table
    DownLoad: CSV

    with gr defined by (3.3).

    Lemma 5. For all r(0,1) the curve Φr defined by (4.1) is always above the curve Γr defined by (4.2) in the plane (Sin,D).

    Proof. The proof is given in Appendix D.3.

    In this respect, for any growth function f verifying Assumption 1, the operating diagram of system (2.6) looks like Figure 5. Thus, according to Theorem 2, for the minimization of the output substrate concentration criterion, the serial configuration is more efficient than the simple chemostat if and only if Sin>gr(D) i.e. if and only if (Sin,D) is strictly below the curve Γr, see Figure 5. Let use the operating plane to give a better understanding of the results of Proposition 2 on the behavior of the function rSoutr(Sin,D), according to (Sin,D). To this end, we consider the curves Φ1, Γ and Φ1/2 in the operating plane defined by:

    Φ1/2:={(Sin,D):Sin=λ(2D)},Φ1:={(Sin,D):Sin=λ(D)} (4.3)
     and Γ={(Sin,D):Sin=g(D)} (4.4)

    with g defined by (3.4).

    Curves Γ and Φ1/2 lie below curve Φ1. Therefore, curves Γ, Φ1/2 and Φ1 split the operating plane into at most five regions defined by:

    J0:={(Sin,D):Sinλ(D)},J1:={(Sin,D):λ(D)<Sinmin(g(D),λ(2D))},J2:={(Sin,D):g(D)<Sin<λ(2D)},J3:={(Sin,D):max(g(D),λ(2D))Sin},J4:={(Sin,D):λ(2D)<Sin<g(D)}. (4.5)

    These regions are shown in Figure 6 which is given for an illustrative example but does not correspond to any particular growth function.

    Regions J0, J1 and J3 always exist and are connected. However, regions J2 and J4 do not necessarily exist and if they exist, in general, they are not necessarily connected, depending on the relative positions of curves Γ and Φ1/2. For instance, for linear growth rates, Γ=Φ1/2 and regions J2 and J4 do not exist (see section 5.1); for Monod growth function, curve Γ is above curve Φ1/2 and region J4 does not exist (see section 5.2); for Hill growth function, regions J2 and J4 both exist (see section 5.3) and are connected. Notice that for plotting operating diagrams we must choose the growth function f and the values of the biological parameters, see Figures 9 and 14. We can state now the main result on function rSoutr(Sin,D), for (Sin,D)Ji, i=0,...,4.

    Figure 9.  Regions in the operating plane with f defined by f(S)=S in (a) and f(S)=6S/(5+S) in (b). The dashed blue line D=1 indicates the respective critical values Sin0, Sin1 and Sin2 of Figures 10 and 11.
    Figure 10.  (a): The function rSoutr(Sin,D) with f(S)=S, D=1, r1(4,1)=0.333, r1(3,1)=0.5 and r1(2.5,1)=0.666. (b): For D=1, the critical values corresponding to the passageways between the regions Ji, i=0,1,3 are Sin0=1 and Sin1=2.
    Figure 11.  (a): The function rSoutr(Sin,D) with f(S)=6S/(5+S), D=1, r1(4,1)=0.5, r1(3,1)=0.666 and r1(2.5,1)=0.833. (b): The critical values corresponding to the passageways between the regions Ji, i=0,1,2,3 are Sin0=1, Sin1=2.2 and Sin2=2.5.
    Figure 12.  The curves Φr and Φ1r are defined by (4.1). The curves Γr and Γ are respectively defined by (4.2) and (3.7). The curve Δr of maximal productivity, defined by (5.3), is obtained numerically with f(S)=6S/(5+S), V=1, and (a): r=0.295, (b): r=0.75.
    Figure 13.  The productivity of the biomass, of the serial configuration with Doptr=Doptr(Sin) defined in (5.3) and Pmaxr=Pr(Sin,Doptr), corresponding to the Figure 12(a).
    Figure 14.  The five regions in the operating plane where f(S)=8S2/(5+S2). The blue dashed lines D=3 and D=1 indicate respectively the critical values Sin0, Sin1 and Sin2, of schemes (a) and (b) of Figure 15.
    Figure 15.  The function rSoutr(Sin,D) with f(S)=8S2/(5+S2) and D1=1.5279. (a): D=3, r1(9,3)=0.43, r1(5,3)=0.56, r1(3.87,3)=0.72, Sin0=1.73, Sin1=3.11 and Sin2=3.87. (b): D=1, r1(2.5,1)=0.28, r1(2,1)=0.38, r1(1.5,1)=0.70, Sin0=0.85, Sin1=1.33 and Sin2=1.29.

    Proposition 7. Let Ji, i=0,1,...4 be defined by (4.5). The behavior of function rSoutr(Sin,D), according to (Sin,D) is as follows:

    If (Sin,D)J0 then, for all r[0,1], Soutr(Sin,D)=Sout(Sin,D)=Sin.

    If (Sin,D)J1 then when λ(D)<Sin<λ(2D), Soutr(Sin,D) is given by (3.5) and when Sin=λ(2D), Soutr(Sin,D) is given by (3.6). In addition, for all r(0,1), Soutr(Sin,D)>Sout(Sin,D). The equality is fulfilled for r=0 and r=1, see Figure 7(a).

    If (Sin,D)J2 then Soutr(Sin,D) is given by (3.5) and Soutr(Sin,D)<Sout(Sin,D) if and only if r(r1,1) where r1=r1(Sin,D) is the unique solution of Sin=gr(D). The equality is fulfilled for r=0, r=r1 and r=1, see Figure 7(b).

    If (Sin,D)J3 then Soutr(Sin,D) is given by (3.6) and Soutr(Sin,D)<Sout(Sin,D) if and only if Sin>g(D) and r(r1,1) where r1=r1(Sin,D) is the unique solution of Sin=gr(D). The equality is fulfilled for r=0, r=r1 and r=1, see Figure 7(c).

    If (Sin,D)J4 then Soutr(Sin,D) is given by (3.6) and for all r(0,1), Soutr(Sin,D)>Sout(Sin,D). The equality is fulfilled for r=0 and r=1, see Figure 7 (d).

    Proof. The proof is given in Appendix B.7

    According to the regions depicted in Figure 6, we obtain Figure 7 which covers the whole possible cases of the behavior of the function rSoutr(Sin,D). Thusly, we can minimize the output substrate concentration at the steady state by using a serial configuration of two interconnected chemostats instead of one chemostat if (Sin,D) is fixed in the regions J2 or J3 (i.e. Sin>g(D)) and for r1<r<1.

    We have previously shown that Corollary 1 is a consequence of Theorem 3 and one can see in Proof C.2 of Corollary 1 that comparing the two quantities Pr(Sin,D) and P(Sin,D) involves the comparison of the two quantities Soutr(Sin,D) and Sout(Sin,D). That is why, the curves representing the productivity of the biomass depicted in Figure 8 are analogous to the curves of Figure 7. In Figure 8, we fix r(0,1) and we plot the functions rPr(Sin,D) and rP(Sin,D) for (Sin,D) fixed in the regions J1, J2, J3 and J4. As in the case of the output substrate concentration, it is shown that the productivity of the biomass or the biogas flow rate of the serial configuration is larger than the one of the simple chemostat if and only if r(r1,1) and (Sin,D) is fixed in one of the regions J2 or J3.

    In this section, we consider three different kinetics: the linear function, the Monod function and the Hill function. Table 2 gives the analytical expressions of most of the results previously presented. These expressions show that an analytical study of the different performance criteria is possible.

    Table 2.  Analytical expressions obtained for a linear, Monod and Hill (with p=2) growth functions.
    Functions gr(D) g(D) λ(2D)
    f(S)=aS, a>0 D(1+r)ar 2Da 2Da
    f(S)=mSK+S, DK(m(1+r)D)(mD)(mrD) KD(2mD)(mD)2 2KDm2D
    f(S)=mS2K2+S2 KD1r(1rmDrmD) K2D(mD)3(3m2D) K2Dm2D

     | Show Table
    DownLoad: CSV

    We consider f as a linear function defined by f(S)=aS. According to Table 2, remark that λ(2D)=g(D) then, the curves Φ1/2 and Γ defined respectively by (4.3) and (3.7) merge and constitute only one curve. The behavior of the maps rSoutr(Sin,D) and rSout(Sin,D) or rPr(Sin,D) and rP(Sin,D) depends on the position of (Sin,D) in the three regions Ji, i=0,1,3 represented in Figure 9(a). These regions are defined by

    J0={(Sin,D):Sinλ(D)},J1={(Sin,D):λ(D)<Sinλ(2D)},J3={(Sin,D):λ(2D)Sin}.

    For a fixed value of D, the passageway form the region J0 to J1 is defined by the critical value Sin0=λ(D) and the passageway form the region J1 to J3 is defined by the critical value Sin1=g(D)=λ(2D) as shown in Figures 9(a) and 10(b). As stated in Lemma 2, for any Sin>Sin1 there exists a threshold r1=r1(Sin,D) solution of Sin=gr(D) which is explicitly given by

    r1(Sin,D)=DaSinD. (5.1)

    Then, according to the three performance criteria which are the minimization of the output substrate concentration, the maximization of the productivity of the biomass and the maximization of the biogas flow rate, the serial configuration is more efficient than the simple chemostat if and only if Sin>Sin1 and r(r1,1). This result is illustrated in Figure 10 for minimization of the output substrate concentration criterion. Figure 10(a) should be compared with Figure 6 of [16], where the part of the curves represented in Figure 10(a) corresponding to r>r0, for which Soutr(Sin,D)=S2(Sin,D,r), are depicted. Indeed, in [16], the authors were only interested in the case where the positive equilibrium E2 is GAS. The threshold Sin1=2 shown in Figure 6 of [16] is given by Sin1=g(1) and for any Sin>2 the threshold r1(Sin,D) is explicitly given by (5.1).

    The Monod function is given by f(S)=mS/(K+S), see the second line of Table 2.

    Lemma 6. The curve Γ is located strictly above the curve Φ1/2 in the (Sin,D) plane.

    Proof. The proof in given in the Appendix D.4

    Thus, considering a Monod function induces four regions Ji, i=0,1,2,3 in the operating plane, that describe the behaviors of the maps rSoutr(Sin,D) and rPr(Sin,D), which depend on the position of (Sin,D) in these regions, as depicted in Figure 9(b). The behavior of the map rSoutr(Sin,D) through these regions is depicted in Figure 11(a). For a fixed dilution rate D, the limit curves Φ1, Γ and Φ1/2 define critical values denoted Sin0=λ(D), Sin1=g(D) and Sin2=λ(2D), that respectively characterize the passageways between the regions Jii=0,1,2,3, see Figures 9(b) and 11(b). As stated in Lemma 2, for any Sin>Sin1 there exists a threshold r1=r1(Sin,D) solution of Sin=gr(D) which is explicitly given by

    r1(Sin,D)=D(K+Sin)(mD)m(SinmD(K+Sin)). (5.2)

    Then, according to the three studied performance criteria, the serial configuration is more efficient than the simple chemostat if and only if Sin>Sin1 and r1<r<1. Figure 11(a) should be compared with Figure 9 of [16], where the part of the curves represented in Figure 11(a) corresponding to r>r0, for which Soutr(Sin,D)=S2(Sin,D,r), are depicted. Indeed, in [16], the authors were only interested to the case where the positive equilibrium E2 is GAS. If D=1 as shown in Figure 9(b), the threshold Sin1 is given by Sin1=g(1)=2.2 and for any Sin>2.2 the threshold r1 is explicitly given by (5.2). Notice that Figures 10(a) and Figure 11(a) illustrate Proposition 1. As stated in this Proposition, when D is fixed, one can remark that when increasing Sin, the output substrate concentration at the steady state decreases. Thus, the minimum of the curve rSoutr(Sin,D), representing the optimal point that gives the best possible serial configuration, decreases as Sin>Sin1=g(D) and Sin increases.

    For the purpose of comparing the productivity of the biomass of both configurations, for a fixed r(0,1), we characterize the operating parameters (Sin,D) that allow the optimal biomass productivity of the serial configuration. Let Δr be the curve defined by

    Δr:={(Sin,Doptr(Sin)):Doptr(Sin)=argmax0Df(Sin)Pr(Sin,D)}. (5.3)

    where Pr is defined by (3.11). This curve is obtained numerically and depicted in the operating plane (Sin,D), see Figure 12(a), (b). For the values of the parameters used in Figure 12(a), corresponding to the case 0<r<1/2, there exits a threshold Sin0.84 such that for 0<Sin<0.84, the maximum of Pr(Sin,D) is reached when Pr(Sin,D)=VD(SinS2(Sin,D,r)), and for Sin>0.84, it is reached when Pr(Sin,D)=VD(Sinλ(D/(1r))), as shown in Figure 13. Therefore, for 0<Sin<0.84, the maximum of Pr(Sin,D) is reached when E2 is stable, i.e. when D<rf(Sin), as illustrated for Sin=0.4 in Figure 13(a). That is why, for 0<Sin<0.84, the curve Δr is strictly below the curve Φr. In contrast, for Sin>0.84 the maximum of Pr(Sin,D) is reached when E1 is stable, i.e., when Drf(Sin), as illustrated for Sin=1.2 in Figure 13(c). That is why, for Sin>0.84, the curve Δr is strictly above the curve Φr. In the limit case Sin=0.84, both maxima of Pr(Sin,D) are equal, as shown in Figure 13(b). This corresponds to the leap of the curve Δr, shown in Figure 12(a). On the other hand, for 1/2<r<1, the equilibrium E1 cannot be stable and Pr(Sin,D)=VD(SinS2(Sin,D,r)), whenever it is positive. Therefore, its maximum is reached when the positive equilibrium E2 is stable, that is why, the curve Δr is strictly below the curve Φr, see Figure 12(b).

    According to Proposition 4, Γ is the curve of equation D=Dopt(Sin), where Dopt(Sin) is defined in (3.9). In other words, Dopt(Sin) is the optimal dilution rate corresponding to the maximal productivity of the biomass, of the simple chemostat. We observe on Figure 12 that Δr is strictly below the curve Γ. Hence Dopt(Sin)>Doptr(Sin), as it was also depicted in Figure 4. We conjecture that this property is always verified.

    For all p>1, the non-concave Hill function is given by f(S)=mSp/(Kp+Sp).

    Proposition 8. The Hill function verifies Assumptions 2 and 3.

    Proof. The proof is given in Appendix D.5

    Proposition 8 shows that we can use effectively a non-concave growth function in our analysis. In the following, we consider the case where p=2, see third line of Table 2.

    Lemma 7. Let us denote D1:=m(35)/4.

    If 0<D<D1 then the curve Φ1/2 defined by (4.3) is strictly above the curve Γ defined in (4.4). In contrast, if D1<D<m2 then the curve Φ1/2 is strictly below the curve Γ.

    Proof. The proof is given in Appendix D.6

    According to Lemma 7, considering an Hill function with p=2 induces five regions Ji, i=0,1,2,3,4 in the operating plane, defined in (4.5), that describe the behavior of the maps rSoutr(Sin,D) and rSout(Sin,D) or rPr(Sin,D) and rP(Sin,D), which depends on the position of (Sin,D) in these regions (see Figure 14). For a fixed dilution rate D, the limit curves Φ1, Γ and Φ1/2 define critical values denoted Sin0=λ(D), Sin1=g(D) and Sin2=λ(2D) that characterize the passageways between the different regions Ji, i=0,1,2,3,4. Notice that, if D<D1, as shown in Figure 14(b), where D1 is defined in Lemma 7 then, we have Sin1>Sin2 and the behavior of the maps rSoutr(Sin,D) is as depicted in Figure 15(b). Remark that, in this case, the region where Soutr(Sin,D)=Sin disappears before the emergence of the threshold r1 solution of Sin=gr(D), that is, before the emergence of the region where the serial configuration is more efficient than the simple chemostat i.e. Soutr(Sin,D)<Sout(Sin,D). On the other hand, if D>D1, as shown in Figure 14(a) then, we have Sin1<Sin2 and the behavior of the maps rSoutr(Sin,D) is as depicted in Figure 15(a).

    As stated in Lemma 2, for any Sin>Sin1, there exists a threshold r1=r1(Sin,D) solution of Sin=gr(D) such that, for r1<r<1, the performance of the serial configuration is more efficient than the one of the simple chemostat. In other words, the output substrate concentration at steady-state of the serial configuration is lesser than the one of the simple chemostat if and only if (Sin,D)J2J3 and r(r1,1).

    This work presents an in-depth mathematical study of a model of two serial interconnected chemostats with one species and a monotonic growth function. We analyze, at steady-state, three different performance criteria: the minimization of the output substrate concentration, the maximization of the productivity of the biomass and the maximization of the biogas flow rate. The aim is to compare with the performance of the single chemostat. A part of this paper extends some of the results published in [16] and presented in the thesis [21]. In these both references, the concavity of the function f is a required assumption but this assumption is not necessary in our analysis. The thorough study of our model reveals three main results. First, we provide an explicit expression depending on the dilution rate D, that represents the threshold Sin1=g(D) on the input concentration for the performance. We deduce that there exists a configuration of two tanks that is better than a single tank. Actually, through the optimization of the distribution of the volume V and the threshold Sin1, we distinguish which configuration is the best. Secondly, we infer that maximizing the production of the biomass is equivalent to maximize the biogas flow rate at steady-state even in the case of a serial device of two interconnected chemostats. At the end, we obtain the same conditions for the three performance criteria. Thus, reducing the output substrate concentration, maximizing the production of the biomass or maximizing the biogas flow rate at steady state involve the same conditions and the same threshold Sin1. These conditions are necessary and sufficient to allow the best performance, and they are characterized by the input concentration Sin, the dilution rate D and the parameter r. Finally, for deeper understanding, we depict the corresponding operating diagram of the model which describes the behavior of the steady states. This diagram presents the conditions which induce an optimal configuration with regions characterized by the parameter r and the operating parameters Sin and D.

    To broaden and deepen the present work, a forthcoming paper will present the analysis of performance, of an extension, of the model of two serial interconnected chemostats, with death rates. This future work will also include a comparison with the simple chemostat with death rate.

    The first author thanks the Algerian Government for her PhD grant. The authors thank the Euro-Mediterranean research network TREASURE (http://www.inra.fr/treasure) for support. The authors thank Jérôme Harmand for valuable and fruitful exchanges, and the anonymous reviewers for their constructive comments which have greatly improved this work.

    System (2.6) has a cascade structure. Let us consider zi(t)=Si(t)+xi(t) (i=1,2) then, we have the following system:

    ˙z1=Dr(Sinz1)˙x1=Drx1+f(z1x1)x1˙z2=D1r(z1z2)˙x2=D1r(x1x2)+f(z2x2)x2. (A.1)

    One can easily show that limt+zi(t)=Sin (i=1,2). Therefore (x1(t),x2(t)) satisfies an asymptotically autonomous dynamics, whose limiting system

    ˙x1=Drx1+f(Sinx1)x1˙x2=D1r(x1x2)+f(Sinx2)x2 (A.2)

    is defined in the square Σ:=[0,Sin]×[0,Sin]. System (A.2) has a cascade structure. It admits at most three equilibria:

    e0:=(0,0),e1:=(0,Sinλ(D/(1r))) and e2:=(Sinλ(D/r),x2)

    with x2(0,Sin) a solution, if it exists, of equation

    φ(x2)=Sinλ(D/r)withφ(x2):=x2(1r)D1f(Sinx2)x2. (A.3)

    The equilibria E0, E1 and E2 of (2.6) corresponding to e0, e1 and e2, respectively, have the same values xi, i=1,2, and their corresponding Si are given by Si=Sinxi, i=1,2. Note that e0, e1 and e2 give

    (S1,S2)=(Sin,Sin),(S1,S2)=(Sin,λ(D/(1r)))and(S1,S2)=(λ(D/r),S2),

    where S2=Sinx2. This proves that one has ¯S2=λ(D/(1r)) and S1=λ(D/r) as stated in the Theorem. The equilibrium e0, and hence the corresponding equilibrium E0, always exists. The equilibrium e1, exists if and only if Sinλ(D/(1r))>0, that is D<(1r)f(Sin), which is the condition of existence of E1 in the Theorem. For the existence and uniqueness of e2, note that x2 is a solution of (1.3), if and only if S2=Sinx2 satisfies f(S2)=h(S2), which proves (2.9). The function h is positive, strictly decreasing and h(S1)=0, where S1=λ(D/r), if and only if Sin>λ(D/r), see Figure 2. Thus, as f is strictly increasing (see Assumption 1), there exists a unique solution of h(S2)=f(S2) denoted S2 in [0,S1). Therefore, the equilibrium e2 exists if and only if Sin>λ(D/r), that is D<rf(Sin), which is the condition of existence of E2 in the statement of the Theorem.

    For the local stability, the Jacobian matrix associated to system (A.2) is defined by

    J:=(D/r+f(Sinx1)f(Sinx1)x10D/(1r)D/(1r)+f(Sinx2)f(Sinx2)x2)

    The eigenvalues of this triangular matrix are its diagonal elements. For e0, the eigenvalues are D/r+f(Sin) and D/(1r)+f(Sin). Therefore e0, and hence E0, is LES if and only if D>max{r,1r}f(Sin). For e1, the eigenvalues are D/r+f(Sin) and f(λ(D/(1r)))(Sinλ(D/(1r))). The second eigenvalue is negative if and only if D<(1r)f(Sin), that is, e1 exists. Therefore e1, and hence E1, is LES if and only if rf(Sin)<D<(1r)f(Sin). For e2, the eigenvalues are f(λ(D/r))(Sinλ(D/r)) and D/(1r)+f(S2)f(S2)x2, where x2=SinS2, and S2 is the solution of Eq (2.9). As it is seen in Figure 2 (a), 0<S2<S1>Sin. Hence (S1S2)/(SinS2)<1. Thus, we have h(S2)<D/(1r) and, according to (2.9), this is equivalent to f(S2)<D/(1r). Consequently, the second eigenvalue is negative, which proves that e2, and hence E2, is LES, if and only if the first eigenvalue is negative. This condition is equivalent to Sin>λ(D/r), i.e D<rf(Sin), which is the condition of existence of E2.

    For the global asymptotic stability we use phase plane arguments, as in the proof of Proposition 7 in [22], or in section 2.1.2.3 of [4]. We give the details of the proof when e2 exists. The case where e2 does not exist but e1 exists and the case where neither e2 nor e1 exist are similar. The isoclines x1=Sinλ(D/r) and x1=φ(x2), where φ is defined by (A.3), separate the interior of Σ into four regions defined by

    I:˙x1<0,˙x2<0,II:˙x1>0,˙x2<0,III:˙x1>0,˙x2>0,IV:˙x1<0,˙x2>0.

    Two cases must be distinguished, according to the existence, or not of e_1 , see Figure 1. We consider the case where e_1 exists. The case where it does not exist is similar. The isocline x_1 = \varphi(x_2) is as shown in Figure 16 (b) , that is, it is the graph of a strictly increasing function. Indeed, using the definition (A.3) of \varphi , we have

    \varphi'(x_2) = 1-\frac{1-r}{D}f(S^{in}-x_2)+\frac{1-r}{D}f'(S^{in}-x_2)x_2.
    Figure 16.  Global stability of the equilibrium e_2 . (a) : e_1 does not exist. (b) e_1 exists.

    Note that \varphi'(0) = 1-\frac{1-r}{D}f(S^{in}) . Therefore, e_1 exists if and only if \varphi'(0) < 0 as shown in the Figure. For x_2\in(\overline{x}_2, S^{in}) , where \overline{x}_2 = S^{in}-\lambda(D/(1-r)) is the x_2 component of e_1 , we have

    \varphi'(x_2) \gt 1-\frac{1-r}{D}f(S^{in}-x_2) \gt 1-\frac{1-r}{D}f(S^{in}-\overline{x}_2) = 0,

    which proves that \varphi is strictly increasing. The vector field associated to (A.2) is horizontal if x_1 = \varphi(x_2) and vertical if x_1 = 0 or x_1 = S^{in}-\lambda(D/r) . It is directed as shown in the Figure. Assume first that \left(x_1(0), x_2(0)\right)\in I\cup III . These regions are positively invariant. Since in I [resp. III ], x_1(t) and x_2(t) are strictly decreasing [resp. increasing], the following limits exist:

    \begin{equation} \lim\limits_{t\to+\infty}x_1(t) = x_{1\infty}, \quad \lim\limits_{t\to+\infty}x_2(t) = x_{2\infty}. \end{equation} (A.4)

    Therefore, \left(x_{1\infty}, x_{2\infty}\right) is an equilibrium of (A.2), which belongs to the closure \overline{I} or the closure \overline{III} . Since e_0 , e_1 and e_2 (resp. e_2 ) are the only steady states in \overline{III} (resp. \overline{I} ) and, since e_1 attracts only solution with x_1(0) = 0 and e_0 attracts no solutions with positive initial conditions, it follows that

    \begin{equation} e_2 = \left(x_{1\infty}, x_{2\infty}\right). \end{equation} (A.5)

    Assume now that (x_1(0), x_2(0))\in IV . If (x_1(t), x_2(t)) remains in IV for all t > 0 then x_1(\cdot) is strictly decreasing and x_2(\cdot) is strictly increasing. Thus, the limits (1.4) exist. Hence, \left(x_{1\infty}, x_{2\infty}\right) is an equilibrium of (A.2), which belongs to the closure \overline{IV} . Since e_2 is the only equilibrium in \overline{IV} , we conclude that (A.5) holds. If (x_1(t), x_2(t)) leaves the region IV , then it can only enter in the region I . Hence, as shown previously, it necessarily tends to e_2 and hence, (A.5) holds. The same argument shows that any solution starting with initial condition in II always remains in II and then converges to e_2 or leaves the region II , then enters necessarily in region III , and then, as shown previously, it tends to e_2 . Therefore, e_2 is GAS in the interior of \Sigma . Using the theory of asymptotically autonomous systems (see Appendix F in [6]), we deduce that E_2 is GAS if and only if it exists.

    Let S_2^{*i} = S_2^{*}(S^{in, i}, D, r) , i = 1, 2 . Suppose that S_2^{*1} \geq S_2^{*2} . Since f is increasing then, we have f(S_2^{*1}) \geq f(S_2^{*2}) . Since f(S_2^{*1}) = h_1(S_2^{*1}) and f(S_2^{*2}) = h_2(S_2^{*2}) then, we have h_1(S_2^{*1}) \geq h_2(S_2^{*2}) . Since h_2 > h_1 then, we have h_2(S_2^{*2}) > h_1(S_2^{*2}) . Since h_1 is decreasing then, we have h_1(S_2^{*2})\geq h_1(S_2^{*1}) . Therefore, we have h_1(S_2^{*1}) > h_1(S_2^{*1}) which is a contradiction. Hence S_2^{*1} < S_2^{*2} .

    Recall that S_2^*(S^{in}, D, r) is the unique solution of Eq (2.9). Let us first prove that

    \begin{equation} S_2^*(S^{in}, D, r) \lt \lambda(D) \quad\mbox{ if and only if }\quad S^{in} \gt g_r(D). \end{equation} (B.1)

    Since f is strictly increasing and h is strictly decreasing then, S_2^*(S^{in}, D, r) < \lambda(D) is equivalent to h(\lambda(D)) < f(\lambda(D)) = D . Thus, using the definition of h , the condition h(\lambda(D)) < D is written as

    \frac{D\left(\lambda(D/r)-\lambda(D)\right)}{(1-r)\left(S^{in}-\lambda(D)\right)} \lt D,

    which is equivalent to S^{in} > \lambda(D)+\left(\lambda(D/r)-\lambda(D)\right)/(1-r) . Hence, according to the definition (3.3) of g_r , this is equivalent to S^{in} > g_r(D) . Notice also that the function g_r , defined by (3.3), satisfies

    \begin{equation} g_r(D) = \lambda\left(D/r\right)+\dfrac{r\left(\lambda(D/r)-\lambda\left(D\right)\right)}{1-r}. \end{equation} (B.2)

    Therefore, one has g_r(D) > \lambda(D/r) .

    Let us go now to the proof of the Theorem. Assume that S^{in} > g_r(D) . Then, S^{in} > \lambda(D/r) > \lambda(D) , so that, as shown by (2.5) and (3.1), we have

    \begin{equation} S^{out}_r(S^{in}, D) = S_2^*(S^{in}, D, r) \quad\mbox{ and }\quad S^{out}(S^{in}, D) = \lambda(D). \end{equation} (B.3)

    Therefore, using (B.1), we have S^{out}_r(S^{in}, D) < S^{out}(S^{in}, D) . Assume now that S^{in}\leq g_r(D) . When r < 1/2 , three cases must be distinguished. First, if \lambda(D) < \lambda(D/r) < S^{in}\leq g_r(D) , then, by (2.5) and (3.1), we obtain (B.3). Hence, using (B.1), we have S^{out}_r(S^{in}, D)\geq S^{out}(S^{in}, D) . Secondly, if \lambda(D) < \lambda(D/(1-r)) < S^{in}\leq \lambda(D/r) then, by (2.5) and (3.1), S^{out}_r(S^{in}, D) = \lambda(D/(1-r)) and S^{out}(S^{in}, D) = \lambda(D) . Therefore, we have S^{out}_r(S^{in}, D) > S^{out}(S^{in}, D) . Finally, if S^{in}\leq\lambda(D) , then S^{out}_r(S^{in}, D) = S^{out}(S^{in}, D) = S^{in} . When r\geq 1/2 , the proof is similar, excepted that we must distinguish only two cases, \lambda(D) < S^{in}\leq \lambda(D/r) and S^{in}\leq\lambda(D) .

    In conclusion, for any r\in(0, 1) , S^{out}_r(S^{in}, D) < S^{out}(S^{in}, D) if and only if S^{in} > g_r(D) .

    Let D < m . From Assumption 2, the function r\in(D/m, 1)\mapsto g_r(D) is strictly decreasing. From Assumption 1, we have \lim_{r\to D/m}\lambda(D/r) = \lambda(m) = +\infty . Thus, \lim_{r\to D/m}g_r(D) = +\infty . Using L'Hôspital's rule, one has \lim_{r\to 1}g_r(D) = g(D) . Then, using Intermediate Value Theorem, we deduce that for S^{in} > g(D) there exists a unique r_1 = r_1(S^{in}, D) in (0, 1) such that S^{in} = g_{r_1}(D) . Since the function r\mapsto g_r(D) is strictly decreasing then, r > r_1(S^{in}, D) if and only if S^{in} = g_{r_1}(D) > g_r(D) which ends the proof of the Lemma.

    ● From Assumptions 1 and 2, the function r\in(D/m, 1)\mapsto g_r(D) is strictly decreasing. Thus, for any r \in (0, 1) , g(D) < g_r(D) . If S^{in}\leq g(D) then S^{in} < g_r(D) and according to Theorem 2 we deduce that S^{out}_r(S^{in}, D) > S^{out}(S^{in}, D) .

    ● If S^{in} > g(D) then according to Lemma 2, there exists a unique r_1 = r_1(S^{in}, D) in (0, 1) such that S^{in} = g_{r_1}(D) , where for all r > r_1 , we have S^{in} > g_r(D) . Thus, according to Theorem 2 we deduce that S^{out}_r(S^{in}, D) < S^{out}(S^{in}, D) .

    The equality in the limiting cases r = 0 and r = 1 is already verified, see (3.2).

    If r = r_1 then S^{in} = g_{r_1}(D) . According to (B.2), one has \lambda(D/r_1) < g_{r_1}(D) , that is, \lambda(D/r_1) < S^{in} . Thus, one has S^{out}_{r_1}(S^{in}, D) = S_2^*(S^{in}, D, r_1) where S_2^*(S^{in}, D, r_1) is the unique solution of h(S_2)|_{r = r_1} = f(S_2) . Consequently, one has S_2^*(S^{in}, D, r_1) = \lambda(D) if and only if h(\lambda(D))|_{r = r_1} = f(\lambda(D)) , which is equivalent to D\left(\lambda(D/r_1)-\lambda(D)\right)/\left((S^{in}-\lambda(D))(1-r_1)\right) = D , that is, \lambda(D/r_1)-\lambda(D) = (1-r_1)(S^{in}-\lambda(D)) . Consequently, one obtains that \lambda(D)+\left(\lambda(D/r_1)-\lambda(D)\right)/(1-r_1) = S^{in} , which is equivalent to g_{r_1}(D) = S^{in} . This ends the proof of the Theorem.

    Let us consider r_0 = D/f(S^{in}) i.e. S^{in} = \lambda(D/r_0) .

    1) When S^{in}\leq\lambda(D) one has, for all r\in(0, 1) , \lambda(D)\leq\min\{\lambda(D/(1-r)), \lambda(D/r)\} i.e. S^{in}\leq\min\{\lambda(D/(1-r)), \lambda(D/r)\} . Then, according to (3.1) one has S^{out}_r(S^{in}, D) = S^{in} .

    2) When \lambda(D) < S^{in} < \lambda(2D) , one has r_0\in (1/2, 1) . Firstly, if 0\leq r\leq 1-r_0 then, one has \lambda(D/(1-r))\leq\lambda(D/r_0)\leq\lambda(D/r) i.e. \lambda(D/(1-r))\leq S^{in}\leq\lambda(D/r) . This is equivalent to rf(S^{in})\leq D\leq (1-r)f(S^{in}) . According to (3.1), one has S^{out}_r(S^{in}, D) = \lambda(D/(1-r)) . Secondly, if 1-r_0 \leq r \leq r_0 , one has \lambda(D/r_0)\leq \min\{\lambda(D/(1-r)), \lambda(D/r)\} i.e. S^{in}\leq \min\{\lambda(D/(1-r)), \lambda(D/r)\} . According to (3.1), one has S^{out}_r(S^{in}, D) = S^{in} . Finally, if r_0 < r\leq1 , one has \lambda(D/r)\leq \lambda(D/r_0) i.e. \lambda(D/r)\leq S^{in} then, according to (3.1), one has S^{out}_r(S^{in}, D) = S_2^*(S^{in}, D, r) . These all prove (3.5).

    3) When \lambda(2D)\leq S^{in} one has r_0\in (0, 1/2] . If 0\leq r\leq r_0 then \lambda(D/(1-r))\leq\lambda(D/r_0)\leq\lambda(D/r) i.e. \lambda(D/(1-r))\leq S^{in}\leq\lambda(D/r) . According to (3.1), one has S^{out}_r(S^{in}, D) = \lambda(D/(1-r)) . If r_0\leq r\leq1 then \lambda(D/r)\leq\lambda(D/r_0) i.e. \lambda(D/r)\leq S^{in} . According to (3.1), one has S^{out}_r(S^{in}, D) = S_2^*(S^{in}, D, r) . These all prove (3.6).

    Let r\in(0, 1) . Form Assumption 3, the function D\in[0, rm)\mapsto g_r(D) is strictly increasing. From Assumption 1, we have \lim_{D\rightarrow rm}\lambda(D/r) = \lambda(m) = +\infty . Thus, \lim_{D\rightarrow rm}g_r(D) = +\infty and g_r(0) = 0 . Then, using Intermediate Value Theorem, we deduce that for S^{in} > 0 there exists a unique D_r = D_r(S^{in}) in [0, rm) such that S^{in} = g_r(D_r) . Since the function D\mapsto g_r(D) is strictly increasing then, 0 < D < D_r(S^{in}) if and only if 0 < g_r(D) < g_r(D_r) = S^{in} . Consequently, according to Theorem 2, g_r(D) < S^{in} if and only if S^{out}_r(S^{in}, D) < S^{out}(S^{in}, D) , which ends the proof of the Proposition.

    The result is a direct consequence of Proposition 2 and Theorem 3. We give the details for regions J_1 and J_2 . The proof for other regions is similar.

    If (S^{in}, D)\in J_1 then, according to (4.5), \lambda(D) < S^{in}\leq\min\left(g(D), \lambda(2D)\right) . Therefore, \lambda(D) < S^{in}\leq\lambda(2D) . When \lambda(D) < S^{in} < \lambda(2D) , from Proposition 2, S^{out}_r(S^{in}, D) is given by (3.5) and if S^{in} = \lambda(2D) then S^{out}_r(S^{in}, D) is given by (3.6). Now, using S^{in}\leq g(D) , from Theorem 3, we have for all r\in(0, 1) , S^{out}_r(S^{in}, D) > S^{out}(S^{in}, D) .

    If (S^{in}, D)\in J_2 then, according to (4.5), g(D) < S^{in} < \lambda(2D) . Therefore, \lambda(D) < S^{in} < \lambda(2D) and from Proposition 2, S^{out}_r(S^{in}, D) is given by (3.5). Now, using g(D) < S^{in} , from Lemma 2, there exists a threshold r_1 , defined as the unique solution of S^{in} = g_r(D) . Therefore, from Theorem 3, S^{out}_r(S^{in}, D) < S^{out}(S^{in}, D) if and only if r\in(r_1, 1) and equality holds if and only if r = 0 , r = r_1 or r = 1 .

    The equation P(S^{in}, D) = 0 admits the two roots D = 0 and D = f(S^{in}) . For all D > 0 , P(S^{in}, D) is positive if and only if S^{in} > \lambda(D) . In addition, for all D < f(S^{in}) we have

    \frac{\partial P}{\partial D}(S^{in}, D) = V\left(S^{in}-\lambda(D)-\frac{D}{f'(\lambda(D))}\right) = V(S^{in}-g(D))

    with g defined by (3.4). Thus, \frac{\partial P}{\partial D}(S^{in}, D) = 0 is verified if and only if S^{in} = g(D) . Consequently, using Assumption 4, D^{opt} defined in (3.9) is the unique solution of S^{in} = g(D) .

    One knows that x^{out}_r(S^{in}, D) = S^{in}-S^{out}_r(S^{in}, D) and x^{out}(S^{in}, D) = S^{in}-S^{out}(S^{in}, D) . Firstly, if S^{in}\leq g(D) then according to Theorem 3, for any r\in(0, 1) , one has S^{out}_r(S^{in}, D) > S^{out}(S^{in}, D) . Thus, for any r\in(0, 1) , one has x^{out}_r(S^{in}, D) < x^{out}(S^{in}, D) . Consequently, for any r\in(0, 1) , one has P_r(S^{in}, D) < P(S^{in}, D) . Secondly, if S^{in} > g(D) then according to Theorem 3, one has S^{out}_r(S^{in}, D) < S^{out}(S^{in}, D) if and only if r_1 < r < 1 with r_1 defined in Lemma 2. Then, one has x^{out}_r(S^{in}, D) > x^{out}(S^{in}, D) if and only if r_1 < r < 1 . Consequently, one has P_r(S^{in}, D) > P(S^{in}, D) if and only if r_1 < r < 1 . Finally, if r = 0 , r = r_1 or r = 1 , then one has S^{out}_r(S^{in}, D) = S^{out}(S^{in}, D) . Thus, for r = 0, r_1, 1 one has x^{out}_r(S^{in}, D) = x^{out}(S^{in}, D) . Consequently, if r = 0 , r = r_1 or r = 1 then, P_r(S^{in}, D) = P(S^{in}, D) , which ends the proof of the Corollary.

    Let V be a fixed volume. In the following, we use the respective definitions (3.11) and (3.14) of P_r and G_r . In both cases: \max\{r, 1-r\}f(S^{in})\leq D and rf(S^{in})\leq D \leq(1-r)f(S^{in}) , it is clear that G_r(S^{in}, D) = P_r(S^{in}, D) . In addition, if D < rf(S^{in}) then

    G_r(S^{in}, D) = VD(S^{in}-\lambda(D/r))+V(1-r)f(S^{*}_2)(S^{in}-S^{*}_2)

    with S_2^* the unique solution of (2.9). According to this equation, G_r can be written as

    G_r(S^{in}, D) = VD(S^{in}-\lambda(D/r))+VD(\lambda(D/r)-S_2^*).

    Thus, we deduce that G_r(S^{in}, D) = P_r(S^{in}, D) = VD(S^{in}-S_2^*) and consequently, for any r\in(0, 1) , we have G_r(S^{in}, D) = P_r(S^{in}, D) .

    Let V be a fixed volume and S^{in} > 0 . Let us consider the function \varphi(S) = f(S)(S^{in}-S) . Considering the change of variable S = \lambda(D) , one can easily verify that \varphi'(S) = 0 is equivalent to S^{in}-g(D) = 0 . According to Assumption 4, \varphi admits a unique maximum. We maximize the biogas flow rate at steady-state with respect to D . On the one hand, the biogas flow rate of the simple chemostat is defined by G(S^{in}, D) = V\varphi(S^{out}(D)) with S^{out} defined by (2.5). Then, the maximal biogas flow rate of the simple chemostat is G^{max}(S^{in}) = V\max_{D\in(0, f(S^{in}))} \varphi(S^{out}(D)) . Since the map \lambda defines a homeomorphism from [0, f(S^{in})] to [0, S^{in}] then \max_{D\in(0, f(S^{in}))}\varphi(S^{out}(D)) = \max_{S\in(0, S^{in})}\varphi(S) . On the other hand, as S^*_1 > S^*_2 and using the definition (3.13) of G_r , the biogas flow rate of the two serial interconnected chemostats at steady-state is defined by G_r(S^{in}, D) = rV\varphi(S_1^*)+(1-r)V\varphi(S_2^*) with S^*_1 = \lambda\left({D}/{r}\right) and S_2^* the unique solution of (2.9). In addition, as for all D < f(S^{in}) we have \varphi(S^*_i(D)) < \max_{S\in(0, S^{in})}\varphi(S) , i = 1, 2 then, we have

    G_r(S^{in}, D) \lt rV\max\limits_{S\in(0, S^{in})}\varphi(S)+(1-r)V\max\limits_{S\in(0, S^{in})}\varphi(S).

    Hence, we deduce that G_r(S^{in}, D) < V\max_{S\in(0, S^{in})}\varphi(S) which is equivalent to G_r(S^{in}, D) < G^{max}(S^{in}) . This completes the proof of the Proposition.

    Using l_D(r) = \lambda(D/r) , \gamma(r, D) = g_r(D) , defined by (3.3), is given by

    \gamma(r, D) = l_D(1)+\frac{l_D(r)-l_D(1)}{1-r}

    The partial derivative, with respect to r of \gamma is given then by

    \frac{\partial\gamma}{\partial r} (r, D) = \frac{l_D'(r)(1-r)+ l_D(r)-l_D(1)}{(1-r)^2}.

    Therefore, \frac{\partial\gamma}{\partial r} (r, D) < 0 if and only if l_D(1) > l_D(r)+(1-r)l_D'(r) , which proves the equivalence of conditions 1 and 2 of the Lemma.

    Moreover, if l_D is strictly convex on (D/m, 1] then for all s and r in (D/m, 1] , if s\neq r , then

    l_D(s) \gt l_D(r)+(s-r)l_D'(r).

    Taking s = 1 and r\in(D/m, 1) one obtains the condition 2.

    Assume now that f , and hence l_D , are twice derivable. Using \lambda'(D) = 1/f'(\lambda(D)) and \lambda''(D) = -f''(\lambda(D))/\left(f'(\lambda(D))\right)^3 , we can write

    l_D''(r) = \frac{2D}{r^3}\lambda'\left({D}/{r}\right)+ \frac{D^2}{r^4}\lambda''\left({D}/{r}\right) = \frac{D}{r^3\left(f'(\lambda(D/r))\right)^3} \left(2\left(f'\left(\lambda\left({D}/{r}\right)\right)\right)^2-({D}/{r})f''\left(\lambda\left({D}/{r}\right)\right)\right)

    Therefore, the condition 3 is equivalent to the following condition:

    \begin{equation} \mbox{For all } D\in(0, m) \mbox{ and } r\in(D/m, 1], Df''\left(\lambda\left(D/r\right)\right)/r \lt 2(f'\left(\lambda\left(D/r\right)\right))^2 \end{equation} (D.1)

    Using the notation S = \lambda(D/r) , which is the same as f(S) = D/r , the condition () is equivalent to : for all S > 0 , f(S)f''(S) < 2\left(f'(S)\right)^2 , which is the condition 4 in the Lemma.

    Using \lambda'(D) = 1/f'(\lambda(D)) , the partial derivative, with respect to D of \gamma(r, D) = g_r(D) , defined by (3.3), is given by

    \frac{\partial\gamma}{\partial D} (r, D) = \lambda'(D)+\dfrac{1}{1-r}\left(\dfrac{1}{r}\lambda'(D/r)-\lambda'(D)\right) = \dfrac{f'\left(\lambda(D)\right)-r^2f'\left(\lambda(D/r)\right)}{r(1-r)f'\left(\lambda(D)\right)f'\left(\lambda(D/r)\right)}.

    Therefore, \frac{\partial\gamma}{\partial D} (r, D) > 0 if and only if f'\left(\lambda(D/r)\right) < f'\left(\lambda(D)\right)/r^2 , which proves the equivalence of conditions 1 and 2 of the Lemma.

    Moreover, since 1/r > 1 and \lambda is strictly increasing, then \lambda(D/r) > \lambda(D) . Thus, if f' is decreasing, we have f'\left(\lambda(D/r)\right)\leq f'\left(\lambda(D)\right) < f'\left(\lambda(D)\right)/r^2 , which proves condition 2 of the Lemma.

    As 0 < r < 1 and \lambda is a strictly increasing function then, we have D/r > D and \lambda(D/r) > \lambda(D) . Consequently, using the definition (B.2) of g_r , we have g_r(D) > \lambda(D/r) . According to the respective definitions (4.1) and (4.2) of the curves \Phi_r and \Gamma_r , we deduce that the curve \Phi_r is always above the curve \Gamma_r .

    The curves \Phi_{1/2} and \Gamma are respectively defined by (4.3) and (4.4). Let us define the function H:\left[0, \frac{m}{2}\right)\mapsto \mathbb{R} such that H(D) = \lambda(2D)-g(D) . According to Table \ref{TableApplicationsLinearAndMonod}, for a Monod function, the function H is defined explicitly by H(D) = \frac{KmD^2}{(m-D)^2(m-2D)} . Then, for any D\in \left[0, \frac{m}{2}\right) one has H(D) > 0 . Thus, for any D\in \left[0, \frac{m}{2}\right) , one has \lambda(2D) > g(D) which means that \Gamma is always located strictly above \Phi_{1/2} .

    Let us prove that the Hill function satisfies Assumption 2. Straightforward computations show that

    F(S): = \frac{f(S)f''(S)}{\left(f'(S)\right)^2} = \frac{p-1-(p+1)(S/K)^p}{p}

    Hence, for every p\geq 1 , F'(S) = -\frac{p+1}{K^p}S^{p-1} < 0 and F(0) = \frac{p-1}{p} < 1 , which proves that F(S) < 1 for all S > 0 . Therefore, condition 4 of Lemma 3 is satisfied, which is a sufficient condition for Assumption 2 to hold.

    Let us prove now that the Hill function satisfies Assumption 3. It is equivalent to prove that it satisfies the condition 2 of Lemma 4. Straightforward computations show that

    \lambda(D) = \left(\frac{K^pD}{m-D}\right)^\frac{1}{p} \quad\mbox{ and }\quad f'(\lambda(D)) = \frac{p}{m}\left(\frac{D^{p-1}}{K^p}\right)^{\frac{1}{p}}(m-D)^{\frac{p+1}{p}}

    We have 0 < r < 1 and D < rm then, obviously, we have 0 < m-D/r < m-D and 0 < rm-D < m-D . Thus, we obtain the following inequality

    \left(m-D/r\right)(rm-D)^{\frac{1}{p}} \lt (m-D)(m-D)^\frac{1}{p}.

    Straightforward calculations give

    \frac{1}{r^2m}(rm-D)^{\frac{P+1}{p}} \lt \frac{1}{r m}(m-D)^{\frac{p+1}{p}}.

    Consequently, we have

    \frac{p}{r^2m}\left(\frac{D^{p-1}}{K^p}\right)^{\frac{1}{p}}(rm-D)^{\frac{p+1}{p}} \lt \frac{p}{rm}\left(\frac{D^{p-1}}{K^p}\right)^{\frac{1}{p}}(m-D)^{\frac{p+1}{p}}

    which is equivalent to f'(\lambda(D/r)) < f'(\lambda(D))/r and induces f'(\lambda(D/r)) < f'(\lambda(D))/r^2 . This completes the proof of the Proposition.

    Let the function H: \left[0, \frac{m}{2}\right) \mapsto \mathbb{R} be defined by H(D): = \lambda(2D)-g(D) . According to the analytical expressions of Table 2, we have

    H(D) = K\sqrt{D}\left(\sqrt{\frac{2}{m-2D}}-\frac{3m-2D}{2(m-D)^{\frac{3}{2}}}\right).

    Thus, H(D) > 0 gives 4mD^2-6m^2D+m^3 < 0 . The equation 4mD^2-6m^2D+m^3 = 0 admits the two roots D_1 = \frac{3-\sqrt{5}}{4}m and D_2 = \frac{3+\sqrt{5}}{4}m such that 0 < D_1 < \frac{m}{2} and \frac{m}{2} < D_2 . Therefore, for any D\in \left(D_1, \frac{m}{2}\right) we have, H(D) > 0 which means that for any D\in \left(D_1, \frac{m}{2}\right) , the curve \Phi_{1/2} is strictly below the curve \Gamma .



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