Influence of Environmental Parameters on the Composting Kinetic of Lignocellulosic Residues

By Bueno, P Yanez, R; Ariza, J; Diaz, M J

The kinetic of the thermophilic composting process of trimming residues was studied under several parameters [moisture (40-70%), aeration (0.2-0.61 air/kg min) and particle size (1-5 cm)] in order to determine the best composting conditions to ensure an optimum composting design. Under those conditions, suitable temperature evolutions have been found for all the reactors. The kinetic model proposed by Whang and Meenaghan was used and two first-order kinetic equations were used for describing the composting process. A central composite experimental design was used and a second-order polynomial model consisting of the three selected independent process variables was found to accurately describe (the differences between the experimental values and those estimated by using the equations never exceeded 10% of the former) the kinetic process. The magnitude of the constants among the studied conditions varies among 5 and 13.9 for 1/K^sub 1^ and 0.0015 and 0.0035 for K^sub 2^. Both, moisture and particle size affects positively and negatively the composting process and low effect of aeration (among studied values) was found. The values of 1/K^sub 1^ and K^sub 2^ obtained showed higher values for both parameters under 55% moisture content, 5cm particle size and 0.21^sub air^/min kg. Introduction

Lignocellulosic materials are promising renewable resources of the terrestrial ecosystem and have been used for many biological and industrial purposes. Moreover, the necessary harvesting for suitable regrowth of plants provides trimming residues. Trimming residues are leaves and branches of 0.5-5 cm thickness. The main environmental problem for trimming residues present is the high risk of wildfires. A major initiative is underway to remove excess fuel material in high-risk areas, but the lack of markets for these wastes is a problem.

The levels of nutrients of this waste can be beneficial factors in recycling this waste for agricultural purposes using composting processes.

Composting is a low-cost way of recycling organic matter. The most active phase of composting can be described through a first order reaction (Jhorar et al. 1991; Keener et al. 1996) catalysed by enzymes, and the enzyme kinetic concepts may be applicable in the modelling of this phase (Haug 1993, Hamelers 2004). In the case of the trimming residue compost, the information obtained could be used for the optimization of the process.

The main factors in optimization and control of composting include: environmental parameters (temperature, moisture content, pH, aeration) and nature of substrate (C/N ratio, particle size and nutrient content).

Among environmental parameters, temperature is a key factor in composting, which has frequently been used to judge the efficiency and degree of stabilisation of the treatment process, (Jeris and Regan 1973a, Diaz et al. 2003) and pH control is not usually applied in practical composting operations (Gray et al. 2971; Nakasakieffl/ . 2993).

A fairly small initial particle size aids rapid decomposition by providing greater surface area for microbial attack (Gray et al. 2971). However, if the particle size is too small, air circulation through the pile is inhibited, free air space in the system decreased and this reduced oxygen diffusion (Jesis and Regan 1973b, Haug 1993). To control the compost feed material, a balance between diffusion transport and oxygen supply is necessary.

References about factors such as C/N ratio, moisture content, temperature, particle size, pH, mixing and turning, and aeration rate that can influence the kinetic of composting have been reported by several authors in inductive and deductive models (Jhorar et al. 2991, Hamoda, et al. 2998; Bari et al. 2000; Agamuthu, et al. 2000; Hamelers 2004). Nevertheless inductive models, in which it observes the influence of more than two variables simultaneously, have not been found.

Moreover, knowledge of composting kinetics and its parameters are necessary to obtain the optimum design and operate composting facilities.

The main objective of this work is to determine the relative influence of moisture, aeration, particle size and time on the kinetic rate constants to obtain the most favourable conditions for optimum composting of trimming residues.

Materials and Methods

Composting Process

Lignocellulosic waste (legume trimming residues, LTR), collected from mixed legume used in soil restoration, was mixed to achieve uniform feed material and chipped at three different particle size levels. About 20 kg mixture was placed in each reactor. The most relevant chemical characteristics of the raw materials and final composts are reported in Table 1.

Cylindrical composting reactors were formed from an acrylic column (0.5 m in diameter, Im in depth). To minimize the conductive heat loss along the reactor wall, it was insulated with polyurethane foam. Two sensors for temperature check (K thermocouples, TMC6-HA) were placed at the center and on top of each reactor. An additional temperature sensor was placed outside the reactor to obtain environmental temperature (Protimeter-MMS-Plus). Temperatures were recorded every 0.5 days in each reactor by two data-loggers (HOBO U12-006). Compressed air was introduced to the bottom of each reactor and evenly distributed to the waste mixture through a perforated plate.

TABLE 1.

Chemical characterisationa of the raw materials (trimming residues, TR) and the final composts (R1-R15, t=78 days).

Independent variables were established following a three levels central composite experimental design. Moisture content, aeration and particle size was established following a central composite experimental design. The moisture (M), aeration (A), particle size (PZ) and time (t) used in the different experiments of the factorial design were 40, 55 and 70% (Madejon et al. 2002, Zavala and Funamizu 2005), 0.2, 0.4 and 0.6 1 air/(kg min) (Ekinci et al. 2004, Kulcu and Yaldiz 2004, Yamada and Kawase 2006), 1,3 and 5 cm (Haug 1993, Gray et al. 2971) and from 1 to 90 days respectively. To ensure and sustain initial conditions, water losses were compensated by the addition of water during active composting.

Experimental Design for the Incubation Process, Statistical Analysis and Kinetic Model

In order to relate the dependent (temperature and organic matter) and independent (aeration (A), moisture (M), particle size (PZ) and time (t)) variables with the minimum possible number of experiments, a 2^sup n^ central composite factor design was used (Montgomery 1991, Aknhazarova and Kafarov 1982). The central combination for the experimental design was as follows: t = 20 days, A=0.4 mg/(l kg), M=55% and PZ=3 cm. The results were subjected to multiple linear regressions as implemented in SPSS system package.

In that form, time (t): -1 (0 days), 0 (20 days), 1 (40 days); aeration (A): -1 (0.2 1 air/min kg ), 0 (0.4 1 air/min kg), 1 (0.6 1 air/min kg), moisture (M), -1 (40%), 0 (55%), 1 (70%) and particle size (P), -1 (1 cm ), 0 (3 cm), 1 (5 cm) were used.

The independent variables used in the equations relating to both types of variable were those having a statistically significant coefficient (viz. those not exceeding a significance level of 0.05 in Student’s t-test and having a 95% confidence interval excluding zero).

The plot 1/R vs. 1/C permits the graphical estimation of the constants, K^sub 1^ and K^sub 2^.

Analytical Methods

Samples were collected every day in the early composting and every week in the mesophilic and maturation stages, dried (60[degrees]C) and ground (0-0.5 mm). Moisture was determined by drying at 105[degrees]C to constant weight. The mixtures were analyzed for the following parameters: pH (1:5 w/v) using a pH electrode, total organic matter (OM) by loss on ignition (550[degrees]C for 5h) (Allison 1965) and carbon was estimated as OM/ 1.8 (Haug 1993). Carbon was also expressed on ash basis: C(g)/ Ash(g), total P (acid digest) using the ascorbic acid method (APHA 1995), total K, Ca and Mg (acid digest) by atomic absorption spectrophotometry (APHA 1995), Kjeldahl-N (Bremen 1996) and NH^sub 4^^sup +^-N, NO^sub 3^-N using the KCl extraction method (Mulvaney 1996).

The chemical characteristics of the trimming residues and the final characterisation of the compost are shown in Table 1.

Results and Discussion

Temperature Profiles

The temperature profiles of the reactors with similar conditions except for particle size are presented in Figures 1a (1 cm), 1b (3 cm) and 1c (5 cm). As expected, the temperature of the composting reactors began to rise soon after the establishment of composting conditions. The variation observed show a natural tendency of elevation during the first 15 days in all the reactors, and decline as the transformation process takes place. The low temperature peak is believed to be partially the result of relatively low ambient temperatures during the experiment period. For all experiments, thermophilic temperatures (>45[degrees]C) were observed after the second day of experiments. The temperature profiles of reactors in Figure 1a are, in general, higher than those found for Figures 1b and 1c during a period of 5 to 30 days of composting. This fact could be due to the significant effect of the particle size on the development of the temperature (Haug 1993). Probably in the low particle size values, the compactation of the substrates in the pile made difficult the correct distribution of the air inside the reactor. On the other hand, the high particle size one could give rise to a smaller surface available for the microbial growth. FIGURE 1. Temperature evolution in reactors as a function of particle size (1cm Figure a), 3cm Figure b) and 5 cm Figure c.

FIGURE 2.1/K1 evolution as a function of particle size and aeration at three levels of moisture.

FIGURE 3. K2 evolution as a function of particle size and aeration at three levels of moisture.

After this period temperatures within the reactors began to drop to levels below 45[degrees]C for all the reactors. The temperatures observed for the reactors with 1 cm particle size shows higher values than those found for the reactors with 5 cm of particle size.

For all the reactors, a drop at the nineteenth day of composting is showed with the temperature remaining low during two days when it began to rise again. This was due to adverse weather conditions and strong rains during those days.

The higher temperatures were obtained for RlO (Figure 1), R9 (Figure 2) and R6 (Figure 3) and it was found at 59 and 57 and 53[degrees]C, respectively.

Changes of Organic Matter

During the incubation, organic matter content decreased during composting for all reactors (Table 2). As usual, the decrease is more pronounced during the initial stages of the process. The composting of most substrates is characterised by an initial period of rapid degradation followed by a longer period of low degradation (Robinzon et al. 2999). Increments on organic matter of 8.33%, 4.94% 4.73% and 4.46% for R7, R4, R2 and R14 respectively were recorded after 40 days of composting. And slight differences were found in the organic matter evolution for R9, R11, R6 and R1 for the different reactors. In a similar way to that found in temperature evolution, the reactors with low particle size (R1, R2, R9, R11, R12) are those that lower organic matter evolution have been found.

TABLE 2.

Organic matter evolution of the reactors following the selected experimental design.

TABLE 3.

Values of a=1/K^sub 2^, b= K^sub 1^/K^sub 2^, regression coefficients, K^sub 1^, K^sub 2^ and 1/K^sub 1^.

TABLE 4.

Equations yielded for each dependent variable.

Kinetic Constants Modelization

For the estimation of the kinetic constants, only the first 40 days data (thermophilic temperatures) were used, when the maximum degradation rates was observed.

From organic matter data versus time for each reactor, the Lineweaver-Burke plots (1/R and 1/C) (figure not shown) were linearly correlated with high regression coefficients (r) (Table 2). From these plots, the values of the kinetic constants were obtained (Table 2).

From the regression coefficients could be deduced that the kinetic model proposed by Whang and Meenaghan (1980) seems to be adequate to describe the thermophilic phase of composting process of trimming residues.

Substituting the values of the independent variables for each dependent variable in Table 3 and applying a multiple linear regression analysis for each one of the dependent variables of that table as a function of the independent variables, the polynomials mathematical models were obtained (Table 4). These equations can be used to estimate the variation of dependent variables (1/K1 and K2) with changes in the independent variables (moisture, aeration or particle size) over the ranges considered. Only the terms with statistically significant coefficients are shown according to the proposed methodology.

On the other hand, identifying the independent variables with the strongest and weakest influence on the dependent variables in equations is not so easy since the former contain quadratic terms and the others involve interactions between two independent variables. Then, to determine the values of the independent variables giving the 1/K^sub 1^ and K^sub 2^ evolution, the response surfaces for each variable were plotted three levels of the most strongly independent variable (Figures 2 and 3).

As can be seen in Figure 2, the moisture content is the variable most strongly influencing the 1/K^sub 1^ evolution, whereas aeration has the lowest effect on that evolution. Higher values of 1/K when the moisture composting is 55% are found. Lower values of 1/K^sub 1^ for 70% and for 40% were observed. Minimum values for 3 cm particle size with respect to 1 and 5 cm are found for all experimented moistures. Furthermore, an optimum moisture and particle size (1 or 5 cm) increased the stability of the substrate-microorganism complex, probably due to optimal balance among these parameter and free air space helping the proliferation of the microorganism population.

55% of moisture and 5cm particle size (among the selected aeration rates) have presented the higher values of this parameter, suggesting their composting is more difficult under the rest of composting conditions.

According to that found for 1/K^sub 1^ and as can be seen from Figure 3, the K^sub 2^ values were less influenced by aeration than by the moisture and particle size. Although, in K^sub 2^, a low positive effect of aeration is found. This value depends on the operational parameters of the process such as temperature, moisture, particle size, aeration and chemical conditions. That is to say, low increments on K^sub 2^ values were found among high and low aeration levels. Similar evolution for 55%, 70% and 40% moisture content has been found for K^sub 2^ with respect to 1/K^sub 1^. Particle size has both positive and negative effects in the studied range. In that form, the lower values for K^sub 2^ (minimum degradation have been found for medium particle size (3 cm).

The optimal balance among the parameters (among the studied range) show that a optimal K^sub 2^ is found at 55% of moisture content, 1cm of particle size and 0.61^sub air^/min kg of aeration. It probably due to the optimal free air space-moisture-exposed surface for microorganisms balance found under those conditions. However similar values on K2 is found under 55% moisture content, 5cm of particle size and 0.2 1^sub air^./min kg (optimal values for maximum 1/K^sub 1^). The values found under the selected conditions were similar to those found for similar residues (Bari et al. 2000, Agamuthu et al. 2000).

Conclusions

The results of experiments indicated that the composting process of trimming residues is technically feasible and could be considered as an ecological way to recycle these wastes.

Among the studied composting conditions, [time (0-78 days), aeration (0.2-0.6 1 air/min), moisture (40-70%) and particle size (1- 5 cm)] suitable temperatures evolution have been found for all the reactors.

The kinetic model proposed is useful to describe the most active phase (0-40 days) of the co-composting of legume trimming residues.

Both, moisture and particle size affects positively and negatively the composting process and a low effect of aeration (among the studied range) is found.

The values of 1/K^sub 1^ and K^sub 2^ obtained using this model, showed higher values for both parameters under 55% of moisture content, 5cm of particle size and 0.21^sub air^/min kg of aeration.

Acknowledgements

The authors acknowledge financial support from the CICYT (Science and Technology Inter Ministerial Commission, Spanish Government)- FEDER, project number CTQ2004-06564-C0404/PPQ. R. Yanez wants also to acknowledge “Ministerio de Educacion y Ciencia” for financial support under contract program “Juan de la Cierva”. Special thanks to the IRNAS (CSIC) for his collaboration.

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P. Bueno, R. Yanez, J. Ariza and M.J. Diaz

Departamento de Ingenieria Quimica, Facultad de Ciencias Experimentales, Campus El Carmen,

Universidad de Huelva, Huelva, Spain

Copyright J.G. Press Inc. Spring 2008

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