This article proposes a methane production approach through sequenced anaerobic digestion of kitchen waste, determines the hydrolysis constants and reaction orders at both low total solid (TS) concentrations and high TS concentrations using the initial rate method, and examines the population growth model and first-order hydrolysis model. The findings indicate that the first-order hydrolysis model better reflects the kinetic process of gas production. During the experiment, all the influential factors of anaerobic fermentation retained their optimal values. The hydrolysis constants and reaction orders at low TS concentrations are then employed to demonstrate that the first-order gas production model can describe the kinetics of the gas production process. At low TS concentrations, the hydrolysis constants and reaction orders demonstrated opposite trends, with both stabilizing after 24 days at 0.99 and 1.1252, respectively. At high TS concentrations, the hydrolysis constants and the reaction orders stabilized at 0.98 (after 18 days) and 0.3507 (after 14 days), respectively. Given sufficient reaction time, the hydrolysis involved in anaerobic fermentation of kitchen waste can be regarded as a first-order reaction in terms of reaction kinetics. This study serves as a good reference for future studies regarding the kinetics of anaerobic digestion of kitchen waste.
Kitchen waste constitutes a key part of municipal waste, making up as much as 30% to 50% of municipal solid waste according to the National Environment Bulletin [
The first step in investigating reaction kinetics is to determine the order of the reaction, which is an indicator of the effect of reactant concentrations on reaction rates, as well as a key parameter for studying the reaction mechanism. Four approaches have so far been proposed for determining reaction order: the integration method, the differential method, the half-life method, and the initial rate method [
Kitchen waste was obtained from the canteen of a local university. Nondegradable substances such as fishbone and disposable chopsticks were removed, and the waste was then cut into 1 cm × 1 cm × 0.5 cm cubes and stored at 4°C. The total solid (TS) concentration and volatile solid (VS) concentration were 23.31% and 92.84%, respectively. Sewage sludge used as inoculum was obtained from a local sewage plant and treated at mild temperatures. The TS concentration, VS concentration, and carbon-to-nitrogen (C/N) ratio of the sewage sludge were 11.26%, 77.79%, and 7.41, respectively.
The customized reactor consisted of three 1 L wide mouth bottles used as a reaction bottle, gas collection bottle, and water collection bottle. For the three low TS concentration tests, 17.8 g, 60.7 g, and 103.6 g raw materials were mixed with 300 mL sludge in the reaction bottle. Water was added as needed so that the solutions in all reaction bottles reached 1 L. In these cases, the initial TS concentrations were 4%, 5%, and 6%, respectively. For the three high TS concentration tests, 330.7 g, 352.1 g, and 373.6 g raw materials were mixed with 150 mL sludge in the reaction bottle. Water was added as needed so that solutions in all reaction bottles reached 500 mL. In these cases, the initial TS concentrations were 19%, 20%, and 21%, respectively. High purity N2 was then injected into each reactor for 5 min to eject air. The reaction bottles and gas collection bottles were connected by glass tubes and pretreated latex tubes, followed by sealing using rubber stoppers and sealant. Thermostatic water baths were used to maintain the designated temperature. Each experiment was designed to group 3 parallel samples. After adding water to the fermentation reactor to the level (1L), all reaction bottles were incubated at 37°C in the water bath for 30 d, during which period stirring was conducted twice a day. The pH values of the solutions and gas produced were measured daily to avoid issues such as the inhibition phenomenon.
During the anaerobic fermentation process, all of the influential factors retained their optimal values. Specifically, the fermentation tank was heated in water to maintain an internal temperature of 37°C, which is ideal for anaerobic fermentation. The pH values of the solutions were adjusted to fall within a range of 6.8 to 7.2. In addition, the fermentation tank was shaken twice a day for purposes of stirring, and it was sealed at all times.
The products in the TS concentration group and the VS concentration group were heated to 103–105°C and 600°C, respectively. The pH values of the solutions were determined using a digital pH meter. The volume of the produced gas was measured using the saturated salt water replacement method.
The logistic equation is written as follows:
The modified Gompertz equation is copied as follows:
The
A first-order gas production model [
In this way,
The procedures of the initial rate method are as follows: Assuming the reaction follows
The initial rate method is based on different concentrations of reactants. In this study, the concentration of one reactant was assigned three different values for each group, while the concentrations of the other reactants remained constant. As the experiments proceeded, the concentrations of reactants and products were measured regularly.
Anaerobic fermentation refers to a process in which methane is produced from organics; therefore, the amount of methane produced can be recorded and used to investigate the hydrolysis constant and reaction order through the initial rate method. If
Entropy is a state function used to describe and characterize the degree of chaos in a system. The entropy change of a process is only related to the system’s initial state and final state, regardless of the approach or method.
The complicated composition of kitchen waste makes the complete
Standard Gibbs free energy change when using glucose as fermentation substrate and bacteria for hydrolysis, acid production, and fermentation.
Reaction equation (pH = 7, |
|
|
---|---|---|
C6H12O6 + 4H2O + 2NAD+ |
|
>0 |
C6H12O6 + 2NADH |
|
>0 |
C6H12O6 + 4H2O |
|
>0 |
C6H12O6 + 2H2O |
|
>0 |
C6H12O6 + 2H2O + 2NADH |
|
>0 |
C6H12O6 |
|
>0 |
The data in Table
Therefore, the entropy values of these reactions are all greater than zero, and the processes increase entropy.
The standard Gibbs free energy change when using hydrogen-producing acetogens for the metabolism of organic acids and alcohols is shown in Table
Standard Gibbs free energy change when using hydrogen-producing acetogens for metabolism of organic acids and alcohols.
Reaction equation (pH = 7, |
|
|
---|---|---|
CH3CH2OH + H2O |
+9.6 > 0 | <0 |
CH3CH2COO− + 3H2O |
+76.1 > 0 | <0 |
CH3CH2COO− + 2 |
+72.4 > 0 | <0 |
CH3CH2CH2COO− + 2H2O |
+48.1 > 0 | <0 |
CH3CH2CH2COO− + |
+45.5 > 0 | <0 |
CH3CH2CH2CH2COO− + 2H2O |
+25.1 > 0 | <0 |
CH3CHOHCOO− + 2H2O |
−4.2 < 0 | >0 |
According to Table
Furthermore, the numerical values of
The standard Gibbs free energy change when using methanogens for the metabolism of intermediates is explained in Table
Standard Gibbs free energy change when using methanogens for metabolism of intermediates.
Reaction equation (pH = 7, |
|
|
---|---|---|
4CH3CH2COO− + 3H2O |
−102.0 < 0 | >0 |
2CH3CH2CH2COO− + |
−39.4 < 0 | >0 |
CH3COOH |
−31.0 < 0 | >0 |
4HCOOH |
−130.1 < 0 | >0 |
4H2 + |
−135.6 < 0 | >0 |
2CH3CH2OH + CO2 |
−116.3 < 0 | >0 |
CH3OH + H2 |
−112.5 < 0 | >0 |
4CH3OH |
−104.9 < 0 | >0 |
4CH3NH2 + 2H2O |
−75.0 < 0 | >0 |
2(CH3)2NH + 2H2O |
−73.2 < 0 | >0 |
4(CH3)3N + 6H2O |
−74.3 < 0 | >0 |
2(CH3)2S + 2H2O |
−73.8 < 0 | >0 |
The data in Table
The anaerobic fermentation process of kitchen waste with initial TS concentrations of 4%, 5%, and 6% was analyzed using a population growth model. Nonlinear fitting with the software Origin established the fitting parameters for the logistic equation and modified Gompertz equation describing the anaerobic fermentation of kitchen waste at different initial TS concentrations (see Tables
Fitting parameters for logistic equation.
TS/% |
|
|
|
|
---|---|---|---|---|
(mL/gVS) | (mL/gVS/d) | |||
4 | 480.60 | 21.91 | −7.42 | 0.95125 |
5 | 534.81 | 42.48 | −1.18 | 0.97202 |
6 | 503.78 | 32.24 | −1.17 | 0.99414 |
Fitting parameters for modified Gompertz equation.
TS (%) |
|
|
|
|
---|---|---|---|---|
(mL/gVS) | (mL/gVS/d) | |||
4 | 485.10 | 26.52 | −5.53 | 0.95981 |
5 | 540.94 | 32.18 | −4.95 | 0.98597 |
6 | 513.09 | 23.67 | −6.34 | 0.99705 |
Tables
Because kitchen waste contains a great deal of readily decomposable organic starches like rice and steamed buns, as well as a moderate amount of organic protein like lean meat and eggs, the ratio between carbon and nitrogen during the anaerobic fermentation process is always appropriate. This not only accelerates the hydrolysis reaction but also benefits the growth and reproduction of microbes, thereby ensuring that the reaction proceeds smoothly. In this way, the experiment can generate biogas from the beginning, without any time delay.
Table
Parameters of anaerobic fermentation of kitchen waste at different TS concentrations predicted by the proposed first-order gas production model.
Initial TS concentration | Parameter | ||
---|---|---|---|
|
|
| |
4% | 4.8109 | 0.2179 | 0.9930 |
5% | 4.1292 | 0.1170 | 0.9938 |
6% | 4.2131 | 0.1430 | 0.9965 |
The results revealed that the hydrolysis constants corresponding to TS concentrations of 4%, 5%, and 6% were 0.2179, 0.1170, and 0.1430, respectively. Meanwhile, the values of
According to the formulas and experimental data from the population growth model and first-order gas production model, both models achieve a satisfying fitting effect for the anaerobic fermentation and biogas production process of kitchen waste with low TS concentrations. In this study, the population growth model always yielded correlation coefficient
Let the groups whose initial TS concentrations are 4%, 5%, and 6% be defined as Groups
Hydrolysis constant and reaction order of anaerobic fermentation of kitchen waste at low TS concentrations.
The data indicates that the hydrolysis constant and reaction order exhibit opposite trends, although both stabilize eventually. The reaction order decreased during the first three days to a minimum at 0.6822, increased from Day 4 to Day 17, and finally decreased gradually until stabilizing at 0.99 from Day 24. The hydrolysis constant increased during the first six days, decreased from Day 7 to Day 17, and then increased steadily until stabilizing at 1.1252 from Day 24. Therefore, the hydrolysis of kitchen waste with initial TS concentrations of 4%, 5%, and 6% can be described by the first-order hydrolysis dynamic equations proposed.
In most studies concerning kitchen waste digestion, the first-order hydrolysis constant is obtained based on continuous dry fermentation. For instance, Wu et al. investigated anaerobic digestion of kitchen waste mixed with pig manure at mild temperatures [
The average hydrolysis constants and reaction orders for anaerobic digestion of kitchen waste with initial TS concentrations of 19%, 20%, and 21%, from the initial moment to a specific point, were obtained and shown in Figure
Hydrolysis constant and reaction order of anaerobic fermentation of kitchen waste at high TS concentrations.
As the data shows, the reaction order dropped from 0.8027 to 0.4552 during the first three days and then increased to 1.2511 on Day 5. Afterwards, the hydrolysis constant fluctuated and stabilized at 0.98 after Day 18. In contrast, the hydrolysis constant increased during the first four days until it reached 1.1479; then, it decreased from Day 5 to Day 8, increased again, and stabilized at 0.3507 after Day 14. These results suggest that hydrolysis of kitchen waste with initial TS concentrations of 19%, 20%, and 21% can be described by the first-order hydrolysis dynamic equations proposed.
(
(
(
(
(
The authors declare that they have no conflicts of interest.
This work was funded by the National Science and Technology support (no. 2014BAC24B01) and the Cultivation Plan for Youth Agricultural Science and Technology Innovative Talents of Liaoning Province support (no. 2014016).