# Kinetic Study of the Anaerobic Digestion of Recycled Paper Mill Effluent (RPME) by Using a Novel Modified Anaerobic Hybrid Baffled (MAHB) Reactor

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## Abstract

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^{−1}, respectively.

## 1. Introduction

^{−1}and ${K}_{s}$ value of 0.4028 g COD L

^{−1}. Lokshina et al. [8] used the Monod model to evaluate the kinetic coefficients of low-temperature acetoclastic methanogenesis for lake sediments. The result indicated that the inhibition constant ${K}_{1}$ and half-saturation constant ${K}_{s}$ values are 110 and 103 mM, respectively. Chen and Hashimoto [9] suggested that the Contois kinetic model was more suitable than the Monod model for describing the performance of the anaerobic digestion process in treating dairy wastewater. This suggestion was based on the assumption that in the Contois kinetic model, a direct relationship exists between influent and effluent substrate concentrations. Abu-Reesh [10] fitted the experimental data of AD of Labaneh whey to the Contois kinetic model and obtained a kinetic constant value of 0.065 day

^{−1}and K_s of 1.27 g L

^{−1}with 1.29 value of error obtained from nonlinear curve fitting of the model. Vavilin et al. [11] stated that in treating complex solid waste, the Contois kinetic model is preferable when considering the optimal design of a two-phase anaerobic digestion system. Veeken and Hamelers [12] used Contois kinetics with inhibition of 30 g of volatile fatty acid (VFA) per liter, which yielded an adequate result in treating biowaste. Meanwhile, Veeken et al. [13] elucidated the VFA inhibition mechanism by designing a set of experiments for treating organic solid waste. The result showed that no inhibition by non-ionized VFA or VFA can be measured at pH between 5 and 7 and that acidic pH was the inhibitor factor. Other researchers have also viewed a specific growth rate assuming Monod kinetics with substrate inhibition. The demanding task of determining kinetic data to describe the anaerobic acetate-to-methane conversion has restricted the implementation of this model. Variability in obtaining maximum growth rates still occurred in experiments involving identical cultures of Methanosarcina barkeri, strain 227, and the substrate acetate [14]. This variability might be the reason why few studies were performed by implementing Monod kinetics with substrate inhibition.

## 2. Materials and Methods

#### 2.1. Equipment

#### 2.2. Inoculum and Wastewater Preparation

#### 2.3. Kinetic Study

^{−1}until reaching steady state at each condition.

^{−1}. For the methanogenesis kinetics, four different feeding COD concentrations of 1000, 2000, 3000, and 4000 mg COD L

^{−1}at the hydraulic retention time (HRT) of 1–7 days were considered. The samples from each operating condition were collected at constant time interval (every 2 weeks of operation times). The effluent from the MAHB reactor was collected and analyzed for COD, solid concentration (i.e., TS, total volatile solid (VS), TSS, and VSS), VFA, methane production rate, and pH for every two subsequent days. To estimate the reaction kinetics, the experimental data were plotted as a relationship between substrate concentration and specific growth rate.

^{−1}), and S is the vs. concentration. Hence, the conversion coefficient of vs. into product was denoted as ∝ and applied to the kinetic equation to obtain the following equation:

_{0}and S

_{o}are the initial product and substrate concentrations, respectively.

^{−1}. The biomass concentration inside the reactor was assumed constant during the steady-state conditions. Hence, the mass balances of the reactor were given as

_{4}day

^{−1}), μ is the specific microbial growth rate (per day), and X is the biomass concentration. Substituting the Monod model (Equation (4)) into Equation (15) yielded

^{−1}), and S is the effluent substrate concentration (g COD L

^{−1}). Then, assuming that the substrate is almost depleted (${K}_{s}\gg S$) and $X$ is constant throughout the system, Equation (16) became

_{4}day

^{−1}) could be expressed as

^{−1}), $S$ is the effluent COD concentration, and Q is the volumetric feed flow rate (L day

^{−1}).

#### 2.4. Analytical Method

^{−1}, a column temperature of 28 °C, a detector temperature of 38 °C, and an injector temperature of 128 °C. VFAs were measured using esterification methods. Triplicate samples were collected for each parameter reading to increase the precision of the results, and only the average value was reported throughout this study. VSS was measured in accordance with the Standard Methods [18], while COD was measured using Spectrophotometer DR-2800 in accordance with the reactor digestion method [19]. The MAHB reactor was monitored every 2 days for COD and the biogas produced and weekly for VFA. Samples were collected for analysis from each of the five compartments of the MAHB reactor at HRT of 1, 3, 5, and 7 days as the system achieved its steady state.

## 3. Results

#### 3.1. Kinetic Study of Anaerobic Digestion by Using an MAHB Reactor

#### 3.1.1. Hydrolysis Kinetics

^{2}values of 0.9617, 0.9008, 0.9485, and 0.9839 for the initial feeding concentrations of 1000, 2000, 3000, and 4000 mg COD L

^{−1}, respectively. The values of kinetic coefficient obtained and summarized in Table 2 clearly indicated that first-order rate coefficients k were highest at low feeding concentrations and that the substrate conversion coefficient (α S

_{o}) increased as the feeding concentration increased. This phenomenon might be due to a high feeding concentration providing substantial substrate particles that collide with the microorganism per unit time, which leads to frequent reactions between them. As a result, the substrate conversion coefficient increased as the feeding concentration increased. The decrease in the first-order rate coefficient might be due to the excessive available amount of adsorption sites of particulate substrate because the hydrolysis rate is controlled by enzyme kinetics [21].

^{−1}. Previous study reported that the conversion coefficient for a control reactor was 0.13 g COD g VSS

^{−1}. For enzymatic treatment of solid waste, the conversion coefficient determined ranged from 0.23 g COD g VSS

^{−1}to 0.27 g COD g VSS

^{−1}. From the result, hydrolysis process was the rate-limiting step, which made the assumption possible. The reason was that the rate coefficient values were less than 0.5, which implied that hydrolysis/acidogenesis was the rate-limiting step, as previously suggested by Momoh et al. [22].

_{o}= 7315 mL and k = 0.0117 day

^{−1}. From the result obtained, the experimental data exhibited a good agreement and fitted reasonably well with the first-order kinetics, which yielded the following substance conversion equation:

#### 3.1.2. Kinetics of Acetogenesis

^{−1}. Figure 3a,b were plotted using Equations (7) and (8), respectively. From the results shown in Figure 3, VFA degradation began when the system started, and higher sequences of degradation were obtained at higher feed flow rates, followed by other feed flow rates in descending patterns. This finding indicated that once the acetogenic bacteria underwent the acclimatization process, VFA degradation started. The kinetics was determined using the Monod kinetic model. ${k}_{max.VFA}$ and ${k}_{s.VFA}$ values (from the slope) for VFA were determined using the Monod model, as depicted in Figure 3a,b, respectively.

^{−1}VSS day

^{−1}reported by Skiadas et al. [23]. In addition, ${K}_{S,VFA}$ values are close to the value of 0.28 g VFA L

^{−3}recorded by Romli et al. [24] and the value of 0.15 g VFA L

^{−3}determined by Vavilin and Lockshina [25]. Comparison of the results obtained from both methods indicated that the differences between the methods were less than 10% (5.2%, 3.3%, 3.4%, and 0.7% for the feed flow rates of 58.0, 19.3, 11.6, and 8.29 L day

^{−1}, respectively) for constant ${k}_{max.VFA}$. values. This finding confirmed that the proposed method was suitable in determining the specific maximum degradation rate. For saturation constant ${K}_{s,VFA,}$. the difference between the methods was not more than 37.9% at the highest feed flow rate of 58 L day

^{−1}.

#### 3.1.3. Kinetics of Methanogenesis

^{−1}at HRT of 1–7 days. Figure 6 presents the plot of effluent COD concentration versus the inverse HRT at each set of data. All data showed an R

^{2}value higher than 0.80 for different feeding concentrations. The results implied that an increase in feeding COD concentration yielded an increase in effluent COD concentration. Given that Figure 6 shows high R

^{2}values, the variation in methane production rate $({r}_{M})$. as the function of effluent COD was then plotted (Figure 7).

_{4}g

^{−1}COD day

^{−1}at the influent COD concentration of 4000 mg L

^{−1}.

_{4}g

^{−1}COD day

^{−1}at a high feeding COD concentration. This phenomenon might be due to the high specific activity of microorganisms inside the reactor. The hybrid system (biofilm or attached microorganism) applied inside the MAHB reactor allowed to metabolize the intermediate products, such as VFA. The apparent rate constant (K) was further interrelated with the concentration of microorganisms (X) (in terms of VSS), as shown in Figure 9. The experimental data fitted well with an R

^{2}value of 0.89. However, in contrast with $K=\left[\frac{{Y}_{M}{\mu}_{m}}{{Y}_{s}{K}_{s}}\right]X$, a nonzero intercept was observed. This condition might be due to the inability to distinguish other suspended organic matter, as well as true microorganisms.

^{2}value of 0.93. The theoretical data were calculated by multiplying the effluent substrate concentration (S) in terms of COD with the apparent rate constant (K). High correlation values indicated insignificant difference between the theoretical and experimental values.

_{M}was calculated using Equation (19) to yield a value of 0.0645 L CH

_{4}g COD

^{−1}. This value was lower than the previous results obtained by Belhadj et al. [28] (0.245 L CH

_{4}g COD

^{−1}) and Zinatizadeh et al. [27] (0.3251 L CH

_{4}g COD

^{−1}). The lower value of methane yield coefficient might be attributed to the differences in the types of wastewater (RPME) and the lower substrate concentration (below 4000 mg L

^{−1}COD concentration) compared with those in previous studies, which presented a high sewage sludge concentration of 50,000 mg L

^{−1}[28] and a palm oil mill effluent concentration of 34,000 mg L

^{−1}[27]. However, the present study clearly indicated that anaerobic digestion could be a good option for degrading the available feedstock (RPME). The kinetic parameters in each step are summarized in Table 5.

## 4. Conclusions

^{−1}. For acetogenesis kinetics, Monod and integral show similar ${K}_{s,VFA}$ and ${K}_{max,VFA}$ values with TIC values equal and lower than 0.203 for all cases studied. For methanogenesis kinetics, ${Y}_{M}$ obtained is 0.0645 L CH

_{4}g COD

^{−1}.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Time profiles of the VFA concentration during RPME mesophilic anaerobic degradation at different initial wastewater concentrations (in terms of mg VFA L

^{−1}). Symbols refer to the experimental data, and dash lines refer to the predictions by using Equation (2) with k = 0.0356 ± 0.004 day

^{−1}and α = 0.206 ± 0.0084 g VFA g VSS

^{−1}.

**Figure 2.**Time profile of the methane volume released during anaerobic sludge and RPME effluent degradation under mesophilic conditions (35 ± 2 °C). Symbols refer to the experimental data and dash lines to the model predictions with k = 0.0356 ± 0.004 day

^{−1}and α S

_{o}= 327.9 ± 21 mL.

**Figure 5.**Comparison of the results of the calculation method with the experimental data for VFA degradation at different feed flow rates. Symbols refer to the Monod kinetic model, and the dash line refers to the integral method.

**Figure 7.**Variation in methane production rate as a function of effluent biodegradable substrate concentration.

Anaerobic Phase | Parameters | Model Used |
---|---|---|

Hydrolysis | Data Feeding Concentration: 1000, 2000, 3000 and 4000 mg COD L^{−1} at HRT of 7 days | First order kinetics model |

Acetogenesis | Feed flow rates: 58, 19.3,11.6 and 8.29 L day^{−1} | Monod kinetic and integral method |

Methanogenesis | Feeding Concentration: 1000, 2000, 3000 and 4000 mg COD L^{−1} at HRT in a range of 1–7 days | Monod kinetic model |

Feeding Concentration, (mg COD L^{−1}) | Substrate Conversions Coefficient, α S_{o} (mL) | First Order Rate Coefficient, k (Day^{−1}) |
---|---|---|

1000 | 8.682 | 0.1040 |

2000 | 51.564 | 0.0440 |

3000 | 29.974 | 0.0578 |

4000 | 24.210 | 0.0643 |

Feed Flow Rate (L Day^{−1}) | Monod Model | Integral Method | ||
---|---|---|---|---|

${\mathit{K}}_{\mathit{s}\mathit{,}\mathit{V}\mathit{F}\mathit{A}}$ (g VFA L−1) | ${\mathit{K}}_{\mathit{m}\mathit{a}\mathit{x}\mathit{,}\mathit{V}\mathit{F}\mathit{A}}$ (mg VFA mg−1 VSS Day−1) | ${\mathit{K}}_{\mathit{s}\mathit{,}\mathit{V}\mathit{F}\mathit{A}}$ (g VFA L−1) | ${\mathit{K}}_{\mathit{m}\mathit{a}\mathit{x}\mathit{,}\mathit{V}\mathit{F}\mathit{A}}$ (mg VFA mg−1 VSS Day−1) | |

58.0 | 0.29 | 13.35 | 0.18 | 12.66 |

19.3 | 0.15 | 19.83 | 0.10 | 19.17 |

11.6 | 0.15 | 16.75 | 0.12 | 16.18 |

8.29 | 0.10 | 10.36 | 0.090 | 10.43 |

Feed Flow Rate (L Day^{−1}) | TIC | |
---|---|---|

Monod | Integral | |

58.0 | 0.203 | 0.201 |

19.3 | 0.036 | 0.035 |

11.6 | 0.059 | 0.057 |

8.29 | 0.025 | 0.026 |

AD Phase | Kinetic Parameters | |
---|---|---|

Hydrolysis | α S_{o} | 7315 mL |

k | 0.0117 day^{−1} | |

Acetogenesis | Monod Model ${K}_{s,VFA}$ ${K}_{max,VFA}$ Integral Method ${K}_{s,VFA}$ ${K}_{max,VFA}$ | 0.10–0.29 g VFA L ^{−1}10.36–19.83 mg VFA mg-1 VSS day ^{−1}0.090–0.18 g VFA L ^{−1}10.43–19.17 mg VFA mg ^{−1} VSS day^{−1} |

Methanogenesis | Y_{M} | 0.0645 L CH_{4} g COD^{−1} |

K | 4.03 L CH_{4} g^{−1} COD day^{−1} |

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**MDPI and ACS Style**

Hassan, S.R.; Hung, Y.-T.; Dahlan, I.; Abdul Aziz, H.
Kinetic Study of the Anaerobic Digestion of Recycled Paper Mill Effluent (RPME) by Using a Novel Modified Anaerobic Hybrid Baffled (MAHB) Reactor. *Water* **2022**, *14*, 390.
https://doi.org/10.3390/w14030390

**AMA Style**

Hassan SR, Hung Y-T, Dahlan I, Abdul Aziz H.
Kinetic Study of the Anaerobic Digestion of Recycled Paper Mill Effluent (RPME) by Using a Novel Modified Anaerobic Hybrid Baffled (MAHB) Reactor. *Water*. 2022; 14(3):390.
https://doi.org/10.3390/w14030390

**Chicago/Turabian Style**

Hassan, Siti Roshayu, Yung-Tse Hung, Irvan Dahlan, and Hamidi Abdul Aziz.
2022. "Kinetic Study of the Anaerobic Digestion of Recycled Paper Mill Effluent (RPME) by Using a Novel Modified Anaerobic Hybrid Baffled (MAHB) Reactor" *Water* 14, no. 3: 390.
https://doi.org/10.3390/w14030390