Results and discussion

2.1.2.4.1.Analytical results Experiment reproducibility

The curves of CO₂ production rates were similar among duplicates for each waste (data not shown). The absolute mean difference between duplicates was of 14 %, 12 %, 6.8 % and 12

% respectively for MAN, ADMAN, BIO and ADBIO. Measured peaks in CO₂ production rate were slightly different between duplicates, especially for BIO (6.8 and 5.7 gC-CO₂.100g^-1 initial TC.day^-1), but the cumulated CO2 productions were very similar (differences between replicates smaller than 3% of initial TC for all four wastes). Furthermore, similar times to reach a peak in CO2 production rate (difference between duplicates smaller than 1.2 day for all four wastes) indicate similar microbial kinetics between duplicates. Thus, the cells monitored in parallel for each waste were considered as having the same biodegradation kinetics.

Respiration kinetics

For the four organic wastes, respiration kinetics curves (Figure 14) were rather different. The non-digested wastes (MAN, BIO) demonstrated the highest respiration peaks. Cumulated CO2

productions were computed to compare the total amounts of C degraded during the biodegradation. At the end of the biodegradation process, 60% of the initial TC for MAN and 36% for BIO were emitted as CO₂. The digested organic wastes (ADMAN, ADBIO) displayed lower respiration rates corresponding to smaller cumulated CO2 productions: 28 % of the initial TC for ADMAN and 23% for ADBIO. These values are in the typical range for substrate degradation in composting. Indeed, Eghball et al. (1997) measured C loss ranging from 46 to 62 % during the composting of cattle manures. Depending on their composition, Francou et al. (2008) found that between 37.1 % and 61.8 % of organic matter was mineralized during composting of reconstructed biowaste mixtures. Our results thus fell in the lower range found by these authors. Few data are available about degradation of OM during the composting of the solid fraction of anaerobic digestates. Teglia (2011) measured carbon degradation ranged between 13 and 49 % during composting of solid digestate from agricultural wastes and urban biowastes.

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Figure 14 : The rate of CO2 productions and OM fractions evolutions during aerobic biodegradation of the four organic wastes. (a): fattening steers solid manure (MAN). (b): solid phase of the fattening steers solid manure anaerobic digestate (ADMAN); (c): biowaste and green waste mixture (BIO); (d): solid phase of the digestate from the biowaste and green waste mixture (ADBIO). LIC: Lignin and cutin-like fraction – CEL: Cellulose-like fraction – HEM: Hemicellulose-like fraction – SOLNDF: Soluble in neutral detergent fraction – SOLH2O: Soluble in hot water fraction - dCO2: CO2 production rate – TC: Total Carbon.

The shape of the respiration curves also showed important differences between the four organic wastes. The MAN showed a high initial CO2 production rate and fast peak shortly after being placed in the respiration cell presumably linked to the growth of a microbial community because of directly available substrate. Then, a rapid decrease in microbial activity was observed, which presumably reflected the exhaustion of the highly available compounds. A second high CO2 peak occurred after 2 days of biodegradation leading to the hypothesis of a second fraction of OM being available for microbial growth after hydrolysis.

The BIO showed one high peak in CO₂ production rate, appearing after a slowly increasing

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Fractions (gC.100g-1initial TC )

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Fractions (gC.100g-1initial TC )

85 hours of biodegradation, probably due to the limited amount of substrate available for microbial growth and/or to a small initial microbial biomass in this waste.

The ADMAN showed a rapid peak in CO2 production rate but about twice as small as that of the non-digested MAN. After six days, a second peak appeared, resulting from the growth of a different microbial community and/or to the slow hydrolysis of an OM fraction becoming available for biodegradation.

The ADBIO showed a rapid peak in CO2 production rate, roughly equivalent to that observed for ADMAN. A 5 day plateau was followed by a slow drop in CO2 production rate.

Evolution of the OM fractions

Figure 14 presents the evolution of the various OM fractions during biodegradation. First of all, it must be noted that at some experimental points the sum of the OM fractions were higher than at the previous experimental point or even higher than the initial total carbon content of the waste. Such inconsistencies were probably due to cumulated sampling and analytical uncertainties influencing the global mass balances.

For all organic wastes, the SOLH2O fraction showed an important evolution. Based on the initial TC content, it varied from 26% to 6.3% for MAN, from 22% to 13% for ADMAN, and from 14% to 4.1% for BIO. Accordingly, HPLC results (data not shown; Tremier et al., 2014) demonstrated that two compartments can be analyzed in SOLH2O: low molecules (<1.5 kDa) and heavy molecules (>1.5 kDa) which were simultaneously or successively fed from the hydrolysis of the solid portion, and consumed for microbial growth during biodegradation, as also observed by Said-Pullicino et al. (2007). The SOLH2O fraction was well degraded at the end of the process for the MAN and BIO, leading to its entire consumption in the latter.

During the entire process, the SOLnd fraction showed moderate variations from 12 to 18%

and from 17 to 23% of the initial TC for MAN and BIO, respectively, but showed more heterogeneity for both digestates. Finally, the quantity of SOLnd fraction in the final products varied by only 5.0% at the most for MAN and BIO, as compared to that of the raw waste, on the basis of initial TC. The ADMAN was different, demonstrating a drop from 20 to 4.6% of initial TC in the SOLnd fraction, probably responsible for the first respiration peak. This later observation confirms the hypothesis that the SOLnd fraction has both a biodegradable and a recalcitrant fraction (Peltre et al., 2010).

For the four experimental wastes, the HEM fraction was continuously degraded supporting the hypothesis of a simple first order degradation rate. The degradation was slowest for ADBIO with the smallest initial HEM fraction. The amplitude of the biodegradation was very

86 similar for the three other experimental wastes starting at 18%, 17% and 17% of initial TC, respectively, and finishing at 5.3%, 3.8% and 4.5% for MAN, ADMAN and BIO. Thus, between 71 and 78% of initial HEM was degraded for those three experimental wastes.

The CEL fraction of the four organic wastes was also largely degraded, but with a delay till the second respiration peak for MAN and ADMAN, attesting again that the first peak was linked to the soluble OM fractions and not to the solid OM fractions. Lopez-Gonzalez et al.

(2013) also reported a delay in cellulose degradation as other readily biodegradable compounds were preferentially degraded during the first stage. The total amplitude of CEL biodegradation was larger for MAN and ADMAN, at 79% and 86% of initial CEL, compared to that for BIO and ADBIO at 44 and 48%. Francou et al. (2008) and Doublet et al. (2011) also found variable intensities for CEL biodegradation during composting, depending on initial amount of cellulose and on the substrate origin. As between 50 % (BIO) and 73 % (ADBIO) of degraded C originated from HEM and CEL, these two fractions were responsible for the major part of total CO2 production for all four experimental wastes.

The LIC was slowly degraded, mainly during the last days of biodegradation, as also reported during a composting experiment by Paradelo et al. (2013). The LIC fraction was degraded to a smaller extent (27, 57 and 33 % of initial amount respectively for MAN, ADMAN and BIO) than the other OM fractions which explains its increase in concentration during biodegradation for all experimental wastes, except for ADBIO for which the LIC proportion remained approximately constant.

2.1.2.4.2.Modeling results

These experimental data, detailed above, were used to calibrate the biodegradation model.

Simulation of the biodegradation with optimized parameters

• Simulation of the respiration kinetics

Optimized parameters values are presented in Tableau 7, based on values of the initial OM fractions obtained from laboratory analyses. In Figure 15, CO2 production rates are simulated for the four experimental wastes using the model calibrated with these optimized values.

87

Figure 15 : Measured and simulated CO2 production during aerobic biodegradation of the four organic wastes: (a) fattening steer solid manure (MAN); (b) solid phase of the fattening steer solid manure anaerobic digestate (ADMAN); (c) biowaste and green waste mixture (BIO); (d) solid phase of the digestate from the biowaste and green waste mixture (ADBIO). dCO2: CO2 production rate – TC: Total Carbon.

The general shape of the curves were reasonably well predicted by the model for all four experimental wastes, leading to an error under 5% of the initial TC, in predicting the total amount of CO₂ produced (except for BIO, 7% of initial TC). The RMSE calculated for the CO2 kinetics were 0.36, 0.41, 0.37 and 0.25 gC.100g^-1 initial TC.day^-1 respectively for MAN, ADMAN, BIO, ADBIO, which corresponds to MAPE of 15%, 61%, 30% and 27%, respectively.

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88

• Simulation of the OM fractions during biodegradation

Depending on the type of waste and the OM fraction concerned, the predicted dynamics and final OM fraction values were more or less in agreement with the experimental data. Figure 16 shows, as an example, the simulated and measured dynamics of the OM fractions for MAN.

Figure 16 : Measured and simulated evolutions of the OM fractions during biodegradation of the fattening steer solid manure (MAN). (a) SOLH2O: Soluble in hot water fraction; (b) SOLnd: Soluble in neutral detergent fraction; (c) HEM:

Hemicellulose-like fraction; (d) CEL: Cellulose-like fraction; (e) LIC: Lignin and cutin-like fraction. TC: Total Carbon

0 5 10 15 20 25 30 35

051015202530 _Measured

Simulated

Time (Day)

SOLH2O (% of initial TC)

(a)

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SOLnd (% of initial TC)

(b)

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HEM (% of initial TC)

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89 Tableau 10 shows the RMSE and MAPE calculated for each fraction of the four experimental wastes.

Tableau 10 : Statistics demonstrating the accuracy of the calibrated model. MAN: finishing steer solid manure. ADMAN:

solid phase of the finishing steer solid manure anaerobic digestate. BIO: biowaste and green waste mixture. ADBIO: solid phase of the digestate from the biowaste and green waste mixture. SOLH2O: Soluble in hot water fraction; SOLnd:

Soluble in neutral detergent fraction; HEM: Hemicellulose-like fraction; CEL: Cellulose-like fraction; LIC: Lignin and cutin-like fraction. RMSE: Root mean square error; MAPE: Mean absolute prediction error. TC: Total Carbon.

The most difficult fraction to predict was SOLH2O, as attested by the high RMSE and MAPE (Tableau 10). The prediction errors for SOLnd, HEM, CEL and LIC were generally smaller, with MAPE ranging from 5.8 to 39 % and were considered acceptable, considering the heterogeneity and complexity of the biodegradation processes. The highest relative errors (Tableau 10) on these fractions were obtained for the CEL fraction of ADMAN and for the LIC fraction of BIO and were respectively due to the delay observed for the degradation of CEL in ADMAN that the first order equations could not simulate and to experimental uncertainties on LIC for BIO.

When calculated based on the final TC for all four experimental wastes, the predicted OM fractions corresponded to that measured (Figure 17) with a mean of the absolute relative errors standing at 66, 28, 17, 22 and 18 %, respectively for SOLH2O, SOLnd, HEM, CEL and LIC. As expected, the SOLH2O fraction demonstrated the highest error as it is computed from the measurement of the others OM fractions, probably leading to an error accumulation.

Furthermore, no curve fitting was performed for this fraction.

Best final composition prediction was obtained with BIO and ADBIO, with a mean of the absolute relative errors on the five OM fractions of 10 % and 15 %, respectively. For MAN and ADMAN, this error was of the order of 32% and 43%, respectively, because of important errors in predicting the SOLH2O fraction. The MAPE were thus quite high for SOLH2O, however when expressed on the basis of TC, errors are acceptable, as attested by the RMSE ranging from 4.2 to 20 gC.100g^-1 initial TC (Tableau 10).

RMSE (gC.100g^-1 initia l TC) MAPE (%) RMSE (gC.100g^-1 ini tia l TC) MAPE (%) RMSE (gC.100g^-1 i ni ti a l TC) MAPE (%) RMSE (gC.100g^-1 i niti a l TC) MAPE (%)

SOLH2O 10 84 20 92 4.3 NA 4.2 NA

SOLnd 3.5 20 2.6 30 3.1 14 14 38

HEM 1.9 12 1.1 12 2.8 23 0.92 6.9

CEL 2.8 12 11 31 6.3 21 4.0 12

LIC 1.1 5.8 2.9 11 9.1 39 5.0 9.8

MAN ADMAN BIO ADBIO

90

Figure 17 : Measured and simulated final quality of the four organic wastes. MAN: finishing steer solid manure. ADMAN:

solid phase of the finishing steer solid manure anaerobic digestate. BIO: biowaste and green waste mixture. ADBIO: solid phase of the digestate from the biowaste and green waste mixture. LIC: Lignin and cutin-like fraction – CEL: Cellulose-like fraction – HEM: Hemicellulose-like fraction – SOLnd: Soluble in neutral detergent fraction – SOLH2O: Soluble in hot water fraction – TC: Total Carbon.

Discussion on the initial OM fractions and the model parameter values

• Initial OM fractions

The main characterization differences distinguishing each four experimental wastes were: 1) the SOLH2O fraction, and; 2) the LIC fraction. The total SOLH2O experimentally determined was highest for MAN at 26.0 % TC and for BIO at 7.30 % TC (Tableau 8).

Anaerobic digestion lead to a decrease in the value of this fraction. However, model calibration produced equivalent initial fractions of directly available substrate for microbial growth (SOLH2O1) in MAN and in BIO (Tableau 7), and slightly lower values for the digested wastes. The SOLH2O fraction could not be experimentally quantified for ADBIO, but calibration produced a value of 3% TC. Initial biomasses X₁ and X₂ were higher for MAN and ADMAN as compared to BIO and ADBIO. Their concentration respects the value reported by Gattinger et al. (2004) for fresh cattle-manure compost of 1.2 gC.100g^-1 dry matter and for fresh biowaste compost of 0.3 gC.100g^-1 dry matter. The X₁/X₂ ratio roughly equaled 1, for all four experimental wastes.

• Rate coefficients for solid OM hydrolysis

The KSOLnd-B coefficient was quite similar among the experimental wastes (coefficient of variation of 15%) with a mean value of 0.083 day^-1 for all four experimental wastes, slightly

0

Measured Predicted Measured Predicted Measured Predicted Measured Predicted

MAN ADMAN BIO ADBIO

91 higher than that obtained by Zhang et al. (2012) for the fast degrading SOL fraction of composts of different origins. The value obtained from the experimental data represents likely the maximum rate for SOLnd-B degradation, considering the optimum experimental temperature, humidity and aeration environment for biodegradation. The KHEM value was very similar for MAN and ADMAN, slightly lower for BIO and much lower for ADBIO, with a mean value of 0.034 day^-1 for all four wastes (Tableau 7). Zhang et al. (2012) obtained a lower value of 0.019 day^-1. The KCEL value ranged from 0.021 day^-1 for ADMAN to 0.046 day^-1 for MAN, that demonstrated to be all close to the mean value of 0.040 day^-1. The KLIC

value was quite similar for MAN, BIO and ADBIO, namely between 0.0093 day^-1 and 0.012 day^-1, but slightly higher for ADMAN (0.029 day^-1), with a mean value of 0.015 day^-1, compared to the much lower value obtained by Zhang et al. (2012) of 0.0001 day^-1.

• Kinetics of the hydrolysis of SOLH2O2

The rate coefficient for SOLH2O₂ hydrolysis (K_h) was similar for MAN, ADMAN and ADBIO and about 2.5 times higher for BIO. The mean value for all four wastes of 6.7 day^-1_, was slightly higher than the range obtained by Tremier et al. (2005), of 2.4 day^-1 to 4.8 day^-1 for the hydrolysis of the slowly-biodegradable fraction into the easily-biodegradable fraction, and higher than the typical Activated Sludge Models (ASM) with a cited value of 3 day^-1 at 20°C. The limitation coefficient for the hydrolysis of SOLH2O₂ (K_MH) was rather variable, ranging from 1.8 to 18, compared to 6.5 as obtained by Tremier et al. (2005) for the hydrolysis limitation coefficient of the slowly-biodegradable fraction into the easily-biodegradable fraction.

• Microbial-related parameters

The growth rate value for µ1 was comparable for MAN and ADMAN on one hand (4.5 day^-1), and ADBIO (5.3 day^-1) on the other hand. As BIO demonstrated a lag-phase, its µ1 was lower and equaled to2.8 day^-1. The growth rate value for µ2 was comparable for MAN and ADMAN, ranging from 6.5 day^-1 to 8.1 day^-1. µ₂ was also comparable for BIO and ADBIO, ranging from 2.0 day^-1 to 3.0 day^-1_. For BIO, only one marked respiration peak was measured, which is explained by its similar µ₁ and µ₂ values (2.8 day^-1 and 2.0 day^-1).

The ranges for µ1 and µ2 respectivelycorresponded to that found in the literature. Indeed, Zhang et al. (2012) measured a value for a one-biomass-model ranging from 4 day^-1 to 10 day

-1. Sole-Mauri et al. (2007) used a value of 4.8 day^-1 for mesophilic bacteria and 2.4 day^-1 for fungi. Tremier et al. (2005) obtained a value ranging from 3.1 day^-1 to 8.2 day⁻¹, depending

92 on temperature. Stombaugh and Nokes (1996) used a value of 4.8 day^-1 while Hamelers (2004) used value of 6 day^-1.

The coefficient b1 was quite heterogeneous between wastes, ranging from 0.20 day^-1 for ADMAN to 1.2 day^-1 for ADBIO. The mean value for all four wastes was 0.68 day^-1. Sole-Mauri et al. (2007) proposed values of 0.24 day^-1 to 0.72 day^-1, depending on the microbial communities. Zhang et al. (2012) calculated values of 0.1 day^-1 and 0.63 day^-1. Tremier et al.

(2005) calculated values ranging from 1.2 to 3.12 day^-1.

The coefficient b2 was of the same range as b1 with a mean for all four wastes of 0.81 day^-1, but with more heterogeneity, ranging from 0.60 day^-1 for ADBIO to 1.3 day^-1 for ADMAN. In each digested wastes, the two microbial biomasses have quite different growth and death parameters, explaining the two observed respiration peaks for these wastes.

The coefficients K_B1 and K_B2 ranged respectively from 0.15 gC.100 g^-1 initial TC to 0.49 gC.100 g^-1 initial TC and from 0.14 gC.100 g^-1 initial TC to 0.54 gC.100 g^-1 initial TC. As described in Tableau 7, KB1 and KB2 are Monod saturation coefficients limiting microbial growth when substrate becomes scarce. If the substrate used for microbial growth (SOLH2O₁) equals the KBi values, then effective microbial growth rates would equal half of the specific maximum growth rates (Equation 3). Therefore, KB1 and KB2 might not be interpreted independently from substrate concentration. In the present work, KB1 and KB2 represent from 3 % to 40 % of the initial SOLH2O1for all four wastes. Based on the Activated Sludge Models (ASM) developed by Gujer et al. (1999), Tremier et al. (2005) used slightly lower values, equivalent to 0.5 to 1.5 % of initial directly available substrates. Kaiser (1996) used a value of 2 % of the total mass of waste for the microbial populations of the different wastes modelled. However, Zhang et al. (2012) calculated a mean value of 101 gC.100 g^-1 of initial TC, meaning that the maximum growth rate was never achieved in the composting environment.

• The coefficients for biomass growth yield

For each experimental waste, calculated values for Y₁ and Y₂ were very close, namely 0.60 to 0.70, with mean values of 0.66 and 0.65, respectively. These values are higher than that of Zhang et al. (2012), namely 0.5, but agree with the mean value of 0.68 used by Tremier et al.

(2005) and proposed for the ASM (Gujer et al., 1999).

• The coefficient for recycled dead biomass in SOLnd-R

93 The coefficient f₁ was similar for all four experimental wastes, ranging from 0.38 to 0.45.

Tremier et al. (2005) used a smaller value of 0.2 for the recycling of dead biomass into inert OM.

Model sensitivity

To evaluate the model sensitivity, the parameters values obtained by calibration for MAN (Tableau 7) were used as starting values around which each parameter was successively modified to + 20%, thus allowing the calculation of the sensitivity coefficients (Equation 11;

Figure 18).

Figure 18 : Sensitivity of the simulated cumulated CO2 to variation in parameters and initial fractions values. The sensitivity coefficient is defined in equation 11 and parameters and initial fractions in Tableau 7.

The predicted cumulated CO₂ emissions were particularly sensitive to the parameters linked to microbial activity (Figure 18). Initial fractions, and particularly CEL, LIC and HEM, also highly influenced the CO2 emissions. Indeed, a 20% variation of any of this fraction lead to a 5-to-8% of the simulated CO2 production. As they defined the part of C mineralized versus assimilated, the assimilation yields (Y1 and Y2) also had important effects on the simulated CO2 emissions (sensitivity coefficients of 5% and 6% respectively). The initial water soluble fractions had moderate influence on the emitted CO2 whereas the rate coefficients for OM hydrolysis and the initial microbial biomasses were found to have the smallest influence.

These results are in agreement with those of Zhang et al. (2012) who also found that microbial kinetics and initial fractions highly determine the cumulated amounts of CO2. However, when considering the CO2 production rates (i.e. considering the timing and the amplitude of the

94 influent initial fraction (sensitivity coefficient of 7%) whereas the initial solid OM fractions were poorly related to the production rates (sensitivity coefficients lower than 5%). To

94 influent initial fraction (sensitivity coefficient of 7%) whereas the initial solid OM fractions were poorly related to the production rates (sensitivity coefficients lower than 5%). To

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