# Investigating the Effects of Aerobic Hydrolysis on Scum Layer Formation during the Anaerobic Digestion of Corn Stalk Particles

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Experimental Materials

#### 2.2. Experimental Design

#### 2.2.1. Water Absorption Test of Corn Stalks

_{2}were measured using a low-field nuclear magnetic resonance (NMR) analyser with a magnetic field strength and proton resonance frequency of 0.47 T and 20 MHz, respectively, and the application programme Carr–Purcell–Meiboom–Gill (CPMG). The T

_{2}distribution spectrum was obtained by inverting the T

_{2}data using the Contin algorithm until the weight change remained constant for two consecutive times. NMR analysis was used to determine the water existing state and the migration process in the process of water absorption of the corn straw before and after treatment.

#### 2.2.2. Aerobic Hydrolysis and Anaerobic Digestion Experiments

_{2}) was then purged for 5 min to ensure an anaerobic environment. After purging, the stalks were sealed with a rubber stopper and placed in a constant temperature (37 °C ± 1 °C) water bath for anaerobic digestion experiments. The reactor was shaken manually twice every morning and evening for 15 s each time during the experiment. The gas volume and composition were measured once a day, and the thickness of the scum layer was measured every day until the end of the experiment.

#### 2.3. Measurement Method

#### 2.4. Mathematical Model and Data Analysis

#### 2.4.1. Peleg Mathematical Model

_{0}(%) is the initial moisture content of the sample; M

_{t}(%) is the moisture content of the sample at time t; K

_{1}is the rate constant (h•%

^{−1}); and K

_{2}is the capacity constant (%

^{−1}). In the formula, ± is taken as + to indicate water absorption, and − is taken as water loss. Equation (2) is deformed to obtain Equation (3). To obtain the linear equation, t is taken as the independent variable and t/(M

_{t}− M

_{0}) as the dependent variable. The intercept and slope of the equation are the rate constant K

_{1}and capacity constant K

_{2}, respectively.

#### 2.4.2. Arrhenius Equation

_{1}) as the dependent variable, the linear equation, activation energy E, and frequency factor K

_{0}of the stalks’ water absorption process can be obtained.

_{0}is the frequency factor (h

^{−1}), e is the activation energy (kJ/mol), R is the gas constant (8.3145 kJ/(mol·K)), and t is the absolute temperature (k).

#### 2.4.3. Methane Production Model

^{−1}VS); P∞ is the maximum cumulative gas yield (mL g

^{−1}VS); Rm is the maximum gas production rate (mL g

^{−1}VS day); λ is the lag period (d); t is the digestion time; and e is the Euler constant (2.718282).

## 3. Results

#### 3.1. Effect of Corn Stalks Conditioning

#### 3.2. Water Absorption Characteristics of Corn Stalks

#### 3.2.1. Changes of the Water Absorption Rate of Corn Stalks

#### 3.2.2. Establishment and Analysis of Water Absorption Kinetic Parameters of Corn Stalks

^{2}of the untreated stalks was greater than 0.98 and the relative error was 0.332–3.069%, while the R

^{2}of the conditioned stalks was greater than 0.99 and the relative error was 0.013–0.089%, indicating that the fitting effect of the dynamic equation of the water absorption process of the conditioned stalks was better than that of the untreated stalks. In addition, K

_{1}and K

_{2}showed a downward trend with the increase in temperature. Taking the temperature as the independent variable and the two water absorption parameters of the quenched and tempered stalks as the dependent variable, the linear curve (Figure 2) and linear equation (Equations (7) and (8)) of K

_{1}and K

_{2}with respect to temperature can be obtained.

^{2}= 0.8091)

^{2}= 0.9019)

_{1}and K

_{2}have a negative correlation with temperature, indicating that under the condition of 25–50 °C, with the increase of temperature, the higher the water absorption rate of the stalks, the greater the moisture content of the stalks when reaching equilibrium. Bringing Formulas (7) and (8) into Formula (2), the water absorption kinetic equation of the conditioned stalks at 25–50 °C can be obtained: Mt = 6.94 + t/[0.0123 − 0.001T + (0.0125 − 0.0009t) t], where 6.94 is the initial water content of the conditioned stalks M

_{0}(%), T is the temperature (°C), and t is the time (H). This equation reflects the relationship between the water content of the conditioned stalks and the temperature and time in the process of water absorption. Additionally, it can predict the water content at any time under the water absorption temperature of 25–50 °C.

#### 3.2.3. Fitting of the Arrhenius Equation and Activation Energy

_{1}and temperature. The equation expressed 1/T × 10

^{3}as the independent variable and ln(1/K

_{1}) as the dependent variable. Figure 3 shows the fitting curve. The linear equation with a good fitting degree was obtained, and the correlation coefficient was 0.8218. Therefore, the activation energy E of stalks during soaking was 0.9345 kJ/mol, and the frequency factor K

_{0}was 77.3 h

^{−}

^{1}. Thus, the Arrhenius equation of modified corn stalks was obtained: 1/K

_{1}= 77.3 × exp

^{(}

^{−}

^{0.9345/T)}, from which the relationship between the rate constant K

_{1}and temperature can be further obtained.

#### 3.2.4. Water Existing State and the Migration Process of Corn Stalks in the Process of Water Absorption

#### 3.3. Aerobic Hydrolysis Stage

#### 3.3.1. pH and VFAs

#### 3.3.2. Lignocellulose

#### 3.3.3. Fourier Transform Infrared Analysis

^{−1}, showing strong absorption bands at 3404 and 1054 cm

^{−1}. In contrast, the relative intensity of the absorption peaks of the quenched and tempered stalks at these two wavenumbers was significantly lower than that of the untreated stalks, indicating that the destruction degree of the hydrogen bond between the cellulose molecules of the quenched and tempered stalks was higher than that of the untreated stalks, and the water molecules were more easily absorbed, causing the cellulose to swell. At the same time, the long-chain fatty acid structure on the surface of the stalks was damaged by aerobic hydrolysis. The lattice structure of lignocellulose also changed, promoting microorganisms to degrade the corn stalks more quickly. At 1732 and 1426 cm

^{−1}, the characteristic peaks gradually weakened with the increase in hydrolysis time. The peak value of milled stalks at 1732 cm

^{−1}disappeared. With the increase in hydrolysis time, the absorption peak disappeared more clearly, indicating that a large amount of hemicellulose in the stalks was de-acetylated, and external factors easily destroyed hemicellulose due to its unstable structure. Then, degradation occurs. The intensity of the 1245 cm

^{−1}absorption peak decreased gradually during the hydrolysis process, indicating that the C–O bond of lignin also breaks during the hydrolysis process.

#### 3.3.4. Microstructure

#### 3.4. Anaerobic Digestion Stage

#### 3.4.1. Change of Stalks’ Scum Layer

#### 3.4.2. Cumulative Methane Production and Volumetric Methane Production Rate

^{2}of the Gompertz model was higher than 0.986. The maximum cumulative methane yield of the untreated and conditioned stalks occurred under the condition of hydrolysis for 12 h. The fitting results are consistent with reality and can accurately reflect the change of cumulative methane yield in the digestion process.

## 4. Discussion

^{1}H nucleus in the magnetic field. Hydrogen protons mainly come from water molecules. Therefore, the characteristics of material water distribution and water migration can be analysed by measuring the change in the transverse relaxation time of hydrogen protons in the process of changing from a high-energy to a low-energy state [26]. It can be seen from the analysis of the above results that the conditioning treatment of corn stalks can destroy its internal structure to a certain extent, resulting in less bound water in the stalks and higher water freedom. At the same time, after the structure is damaged, it also speeds up the process of water evaporation and volatilises the free water in the free state. This is consistent with the conclusion that the conditioning treatment of corn stalks accelerates water evaporation, as studied by Li Wenzhe and others [20]

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Appendix A

**Figure A1.**Structure of adjusting material device [11].

**Figure A2.**The variation of water absorption of various corn stovers. (

**a**) represents untreated stalks and (

**b**) represents milled treated stalks.

**Figure A3.**SEM imaging of corn stalks. (

**a**,

**b**) are SEM images of untreated stalks and adjusted stalks hydrolysed for 12 h at 35 ± 1 °C.

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**Figure 1.**SEM imaging of corn stalks. (

**a**) represents untreated stalks and (

**b**) represents milled treated stalks.

**Figure 2.**Variation relationship between parameters K

_{1}(

**a**) and K

_{2}(

**b**) of Peleg equation of quenched and tempered stalks and temperature.

**Figure 4.**Water distribution of corn stalks during water absorption. The relaxation time (T

_{2}) of 1–10 ms refers to bound water, 10–100 ms represents non-flowing water, and 100–1000 ms represents free water.

**Figure 5.**Variation of pH and VFAs with hydrolysis time. (

**a**) represents untreated stalks and (

**b**) represents milled treated stalks.

**Figure 6.**Variation of lignocellulose degradation rate with hydrolysis time. (

**a**) represents untreated stalks and (

**b**) represents milled treated stalks.

**Figure 7.**FTIR diagram of corn stalks at different hydrolysis times. (

**a**) represents untreated stalks and (

**b**) represents milled treated stalks.

**Figure 8.**Volume ratio of scum layer during anaerobic digestion of corn stalks. (

**a**) represents untreated stalks and (

**b**) represents milled treated stalks.

**Figure 9.**Changes of cumulative methane production, volumetric methane production rate, and volumetric methane production rate during anaerobic digestion of corn stalks. (

**a**,

**c**,

**e**) represents untreated stalks and (

**b**,

**d**,

**f**) represents milled treated stalks.

Test Index | Unit | Untreated Stalks | Milled Stalks Particles | Inoculum |
---|---|---|---|---|

TS | % | 90.96 ± 0.52 | 93.06 ± 0.33 | 4.60 ± 0.02 |

VS | % | 88.16 ± 0.98 | 89.37 ± 0.65 | 2.81 ± 0.01 |

pH | - | - | 7.23 ± 0.02 | |

Cellulose | %TS | 36.96 ± 0.11 | 32.09 ± 0.09 | - |

Hemicellulose | %TS | 26.62 ± 0.05 | 23.25 ± 0.06 | - |

Lignin | %TS | 4.26 ± 0.03 | 3.54 ± 0.02 | - |

Time (h) | Pm (mL g^{−1} VS) | Rm (mL g^{−1} VS d) | R^{2} | |
---|---|---|---|---|

Untreated | 4 | 168.41 ± 1.61 | 13.38 ± 0.49 | 0.997 |

8 | 241.75 ± 1.09 | 19.12 ± 0.32 | 0.999 | |

12 | 252.43 ± 1.42 | 19.55 ± 0.41 | 0.999 | |

16 | 212.33 ± 1.62 | 15.30 ± 0.40 | 0.998 | |

Conditioning treatment of stalks | 4 | 231.72 ± 3.20 | 24.67 ± 1.77 | 0.986 |

8 | 292.65 ± 3.77 | 26.23 ± 1.53 | 0.991 | |

12 | 320.55 ± 4.09 | 25.80 ± 1.36 | 0.992 | |

16 | 253.85 ± 3.56 | 24.15 ± 1.64 | 0.988 |

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**MDPI and ACS Style**

Jiao, H.; Li, W.; Jing, H.; Wang, M.; Li, P.; Sun, Y.
Investigating the Effects of Aerobic Hydrolysis on Scum Layer Formation during the Anaerobic Digestion of Corn Stalk Particles. *Sustainability* **2022**, *14*, 6497.
https://doi.org/10.3390/su14116497

**AMA Style**

Jiao H, Li W, Jing H, Wang M, Li P, Sun Y.
Investigating the Effects of Aerobic Hydrolysis on Scum Layer Formation during the Anaerobic Digestion of Corn Stalk Particles. *Sustainability*. 2022; 14(11):6497.
https://doi.org/10.3390/su14116497

**Chicago/Turabian Style**

Jiao, Hao, Wenzhe Li, Hongjing Jing, Ming Wang, Pengfei Li, and Yong Sun.
2022. "Investigating the Effects of Aerobic Hydrolysis on Scum Layer Formation during the Anaerobic Digestion of Corn Stalk Particles" *Sustainability* 14, no. 11: 6497.
https://doi.org/10.3390/su14116497