3. Results and discussion
3.3. Degradation of particulates and corresponding monomers
For more complex substrates, including hydrolytic and acidogenic substrates, parameter estimation only using MPR curves was difficult due to the lack of information on intermediate dynamics (H2, VFAs, etc.). Consequently, in this section, only the ability of default ADM1 acidogenic parameter set was evaluated.
3.3.1. Degradation of glucose and cellulose
MPR curves obtained after addition of glucose in WAS and PS acclimated sludge are shown in Figure 42. This figure compares these results with ADM1 simulation results using the default parameter set (km_su =30 kgCOD.kgCOD-1.d-1 and Ks_su = 0.5 kgCOD.m-3). For both anaerobic inoculums, the first MPR peak was due to a very rapid acidogenesis leading to production of H2, which is rapidly converted into methane. The rest of the MPR curve corresponds to the conversion of VFAs produced into methane. First, the total volume of
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methane produced by glucose degradation in the batch experiments was 38 and 28% lower than the volume of methane produced in the simulation of PS and WAS acclimated inoculums respectively. These differences may be due to stoichiometric issues related to the production of alternative products (ethanol for example) or long-term storage phenomena (at the end of the batch experiment, HPLC analysis detected no residual glucose in the soluble phase of the media). Consequently, in the simulation, the initial concentration of Ssu in batch experiments was modified to induce total methane production consistent with that in the experiments. The glucose acidogenesis rate was significantly overestimated by default ADM1 parameters for PS acclimated sludge and underestimated for WAS acclimated sludge. Nevertheless, in Figure 42, the comparison of the MPR curves due to cellulose, glucose, propionate and acetate addition (main intermediate compounds for carbohydrate degradation) shows that, for two inoculums, the rate-limiting stages in saccharide degradation are cellulose hydrolysis, acetotrophic methanogenesis and, to a lesser extent, propionate acetogenesis for PS acclimated inoculum. Problems in simulating glucose degradation were already highlighted by Batstone et al. (2006). There are many possible reasons for these difficulties including variable stoichiometry (Rodriguez et al., 2006), and pH and H2 regulation. As glucose acidogenesis is not rate limiting, these simulation problems do not arise during modelling of a continuous reactor. However, they can strongly impact the simulation of batch systems.
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Figure 42 : MPR curves obtained for batch experiments related to carbohydrate degradation in PS acclimated inoculum (A) and WAS acclimated inoculum (B).
Simulation of MPR curve due to glucose addition with ADM1 default parameters for acidogenesis.
A B
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3.3.2. Degradation of amino acids (AA) and casein
The MPR curves obtained after addition of an amino acid in WAS and PS acclimated sludge are shown on Figure 43 with the main intermediate compounds for protein degradation. This figure compares these results with simulation results with ADM1 using the default parameter set (km_aa =50 kgCOD.kgCOD-1.d-1 and Ks_aa = 0.3 kgCOD.m-3). For the two inoculums, the first MPR peak was again due to the production of hydrogen and its immediate conversion into methane. This MPR peak was not as clear in the experimental MPR curves.
For this reason, the AA degradation kinetics of both inoculums was overestimated by the model. In contrast to glucose fermentation, the volume of methane produced by AA degradation in batch experiments matches that produced in the simulation of both inoculums.
For the WAS acclimated inoculum, the MPR curve obtained after amino acid addition showed several production peaks. In particular, an additional MPR peak occurred at day 3. This phenomenon can be explained by different amino acid degradation rates. Indeed, Ganesh Kumar et al. (2008) identified lower acidogenesis rates for aromatic AA, which represent about 10% of the amino acids from casein, than for non-aromatic AA. In addition, casein hydrolysis and amino-acid acidogenesis are not rate-limiting stages in the anaerobic degradation of proteins. Acetotrophic methanogenesis was strongly rate limiting for both inoculums and propionate degradation was slightly rate limiting for PS acclimated sludge.
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Figure 43 : MPR curves obtained for batch experiments related to protein degradation in PS acclimated inoculum (A) and WAS acclimated inoculum (B).
Simulation of MPR curve due to amino acid addition with ADM1 default parameters for acidogenesis.
A B
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3.3.3. Degradation of oleate and triolein
MPR curves obtained after addition of oleate to WAS and PS acclimated inoculums are shown in Figure 44. Results showed that with PS acclimated inoculum, total methane production due to oleate degradation was 17% lower in the experiment than in the simulation.
Incomplete COD degradation may be due to the adsorption of this substrate onto biomass aggregates and other components. For PS acclimated inoculum, the MPR dynamics during oleate degradation were quite satisfactorily simulated. But some phenomena were not represented: the first slow increase in MPR and the slow decrease in MPR at the end of the curve which was probably due to low substrate accessibility (hydrophobic material) and the fact that the substrate attached to the glass surface of the reactor. Concerning WAS acclimated inoculum, the degradation kinetics was significantly underestimated. The delay in uptake at the start of the pulse, which could be due to adaptation by the biomass, was again related to issues not included in the model. Figure 44 compares MPR curves due to triolein, oleate and acetate. With PS acclimated sludge, oleate acidogenesis was the only rate-limiting stage in the degradation of triolein. With WAS acclimated sludge, inhibition of triolein degradation occurred and acetotrophic methanogenesis appears to be the rate-limiting stage for oleate degradation.
Figure 44 : MPR curves obtained for batch experiments related to lipid degradation in PS acclimated inoculum (A) and WAS acclimated inoculum (B).
Simulation of MPR curve due to oleate addition with ADM1 default parameters for acidogenesis.
A B
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