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Anaerobic digestion of pulp and paper mill sludge


Academic year: 2023

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This is to prove that the thesis entitled "Anaerobic digestion of pulp and paper mill sludge". Unlike anaerobic digestion of wastewater, anaerobic digestion of PPMS (both primary and biosludge) is still in its infancy (Meyer and Edwards, 2014).


One of the emerging pretreatment technologies prior to anaerobic digestion to improve the digestibility of sludge material with the increased production rate of biogas/methane (Elliott and Mahmood, 2007; Meyer and Edwards, 2014). Therefore, the aim of the present study was to treat the PPMS using anaerobic digestion.


Anaerobic digestion of PPMS has largely been performed by BMP assays, very few of which have been performed at bench scale. In contrast to the anaerobic digestion of wastewater, the anaerobic digestion of PPMS (both primary and biosludge) is still in its infancy (Meyer and Edward, 2014).


Anaerobic digestion could reduce the production of greenhouse gases and leachate in landfills and open dumps. This is mainly due to the inherent recalcitrant characteristics of the lignocellulosic content that exists in PPMS, making the rate of hydrolysis the rate-limiting step in anaerobic digestion.


Anaerobic digestion is a biological method for the treatment of organic matter, which involves the transformation and decomposition of organic content, leading to the production of renewable energy biogas, rich in methane, and stabilized digestate for agronomic purposes. Unlike wastewater treatment, anaerobic digestion of mill biosludge (waste activated sludge) and primary sludge is still in its infancy."


Primary sludge

Depending on the efficiency of the fiber recovery system, the primary sludge production rate varies. However, the anaerobic degradation of primary sludge is practically unexplored to date (Meyer and Edwards, 2014).

Secondary sludge or “Biosludge ”

In general, fiber boards with medium density are produced by mixing sludge from the de-inking mill and new mill sludge, which contains both the primary sludge and the secondary sludge at approx. 20. When comparing the ash content, primary sludge produced from the virgin mills has a lower ash content than deinking mill sludge.


Pretreatment of PPMS

  • Physical pretreatment
  • Chemical pretreatment
  • Biological pretreatment

The mechanical pretreatment is less effective than chemical pretreatment because it does not remove the lignin content (Zheng et al., 2014). Alkali addition has been shown to increase solubility of PPMS and improve biogas yield (Park et al., 2012).


On the antagonistic, Elliott and Mahmood (2007) recommend that PPMS could be more amenable to pretreatment due to the higher volatile fraction and reported that the residence time of sludge in a biogas reactor has been significantly reduced from 15-25 days to 7 days. The studies show that the specific methane yield from PPMS varies greatly between 50 and 429 mL/g VS fed/degraded.


Removal efficiency can be increased by co-digestion methods of different inoculums/substrates (Dhamodharan et al., 2015). In both studies (Puhakka et al., 1992; Karlsson et al., 2011), the loading rate of VS could be increased steadily over several months with relatively little effect on the specific methane yield.


  • Temperature
  • Volatile fatty acids (VFAs) and oxidation reduction potential (ORP)
  • Carbon/Nitrogen ratio
  • Toxic materials
  • Nutrients

Various nutrients including Na, K, Ca, Mg and others are found in the solvent influent and can be released due to the degradation of organic material or with added pH adjusting compounds. Sodium is an important methanogenic bacterium because it functions for the formation of ATP or the oxidation of NADH.


The moderate and strong inhibitory concentration for Na, K, Ca and Mg are included in Table 2.5. There is currently no literature available on biosolids generated from the anaerobic digestion that can also be used as soil conditioner (fertilizer).


A serious problem related to the fibrous biomass causing floating and stratification in LS-AD could be easily solved in SS-AD (Xu et al., 2013). The inhibition of hydrolysis by the accumulated sugar under low MC was a plausible elucidation for the SS-AD system ( Ge et al., 2016 ).



Therefore, large-scale and long-term experiments using the PPMS as a substrate can be conducted to exploit the microbial resource and its adaptability through detailed analyzes of the microbial community and physiology. In phase I, biochemical methane potential (BMP) test was conducted on the effects of different food/microorganism (F/M) ratios of the PPMS using cow dung as inoculum.


In phase II, the effects of different pretreatment techniques (thermal (autoclave, hot air oven, hot water bath, microwave), electrohydrolysis (application of current) and biological (isolated bacterial strain)) on the hydrolysis step were studied. In Phase IV, a laboratory-scale continuous reactor study was conducted at different organic loading rates (OLR) and its kinetics were studied.


In phase III, BMP test with different F/M ratio was investigated for the screened pretreatment followed by batch test with optimal F/M was performed for both control and the best pretreatment. Thymol blue indicator was added to the aspirator bottle to adjust the displacement was only due to NaOH solution.


Thermal pretreatment

  • Hot air oven
  • Autoclave
  • Hot water bath
  • Microwave oven

After reaching the appropriate temperature, another approach based on a time study, the prepared samples were kept for different times with the selected best temperature inside the pretreatment vessel (heating source). Once the best temperature was selected, the experiments were repeated at different exposure times and 120 min with the selected best temperature.

Electrohydrolysis pretreatment

By means of an isolated flash mixture rotated at 300 rpm, the sample stored in a plastic feed tank was brought into suspension. The speed of the flash mixture was maintained by the speed controller and the tachometer.

Biological pretreatment




Operation of a lab scale anaerobic auger plug flow reactor (AAPFR)

The auger fitted inside the reactor served as the transport device with the plug flow operation. The auger installed inside the reactor was equipped with a motor (with chain and sprocket) for rotation (5 rotations per day).


A 65-L Anaerobic Auger Flow Reactor (AAPFR) with a 50-L working volume as shown in the figure was used for the continuous study. Samples were taken once in a three-day period at different sampling openings (inlet, opening I, opening II and outlet) (Fig. 3.8 and 3.9).


  • Physico-Chemical analysis
  • Heavy metal analysis
  • Biochemical analysis
  • Instrumental characterization

Lignin was analyzed using the National Renewable Energy Laboratory (NREL) procedure (Templeton and Ehrman, 1995; Ehrman, 1996). The crystallinity index was measured using an empirical method (Gabhane et al., 2011) using the following equation 3.6.


  • Characterization of the PPMS
  • Anaerobic BMP assay results and discussions
  • Influence of F/M ratio on Specific Methanogenic Activity
  • Kinetic study
  • Conclusion

The rate of methane production values ​​can provide important information about the inoculum for the digestive system. Smaller changes for maximum methane production rate (Rm) with respect to F/M ratio other than 2.0.


  • Thermal pretreatment
    • The effect of sCOD and VFA in different heating processes
  • Electrohydrolysis pretreatment
    • Effect of sCOD and VFA with applied voltage
    • Effect of sCOD and VFA with time
    • Effect of current and time for applied voltage and time
  • Biological pretreatment
    • The effect of sCOD and VFA in different isolated bacterial strain . 65
  • Instrumental characterization
    • FESEM
    • FT-IR
    • XRD
  • Conclusions

The effect of applied voltage and time on sCOD and VFA in electrohydrolysis pretreatment. a) The effect of applied voltage on the sCOD and VFA for 20 min.) The effect of time on the sCOD and VFA at 15 V at different time intervals. The variation of sCOD and VFA in relation to isolated bacterial stain. a) Paenibacillus sp. the soluble compounds found in PPMS gained increased solubility, thus sCOD increased with rise in VFA in all four microbial culture pretreatments.


Thermal pretreatment (Hot air oven)

  • Effect of methane production after pretreatment
  • Theoretical methane yield and biodegradability
  • Influence of F/M ratio on specific methanogenic activity and
  • Energy assessment
  • Application of kinetic model

Based on the following Eq. 4.2), the theoretical methane yield and biodegradability of the PPMS were calculated. The energy determination of thermally pretreated PPMS was performed from the experimental results in BMP test.

Electrohydrolysis pretreatment (15 V for 45 min)

  • Effect of methane production after pretreatment
  • Influence of F/M ratio on specific methane yield (SMY) and
  • Energy assessment
  • Kinetic study

The energy output of the electrohydrolysis-pretreated PPMS was calculated from the highest methane yield based on Eq. Based on experimental results of the BMP test, the energy balance was estimated for the electrohydrolysis pretreated PPMS and subsequent anaerobic digestion.

Biological Pretreatment (Bacillus mojavensis (CDb1))

  • Effect of methane production after microbial pretreatment

4.15 (c)) concentrations of microbially pretreated PPMS were initially increased and then began to decrease as experienced in the previous thermal and electrohydrolysis BMP test. It was perceived in the microbial pretreated BMP study that the pH scale varied very short (6.0-7.5) compared to previously thermally pretreated (5.1-7.5) PPMS.

Batch studies for best pretreatment method with best F/M ratio

  • Effect of methane production
  • Effect of lignocellulose content in AD of PPMS

This is due to the hydrolysis (more or less effect by pretreatment) and the degree of acidogenesis by the fermentative bacteria present in the anaerobic reactor. During anaerobic metabolism, the cellulose content of PPMS gradually decreased due to the activity of acetogenic and methanogenic microorganisms.


Predicted methane production rate (Rm) from pretreated PPMS was higher from untreated PPMS for modified Gompertz compared to other two models. Based on three models, the highest methane production can be for pretreated electrohydrolysis followed by thermal pretreatment.

Acclimatization of AAPFR

The study mainly emphasized the effect of increasing the organic loading rate (OLR) on methane production in long-term experiments. Based on the long-term experimental data, further research was conducted to develop a kinetic model for AAPFR.

Operation of AAPFR at IITG, India

  • Methane production profile from AAPFR
  • Biogas content (%) (CH 4 and CO 2 ), pH, sCOD and VFA

Changes in biogas content (%) (CH4 and CO2), pH, sCOD and VFA concentrations in semi-continuous AAPFR. Thermal pretreatment allowed an improvement in methane production from 264 ± 4 mL CH4/g VS for the control to 310 ± 5 CH4/g VS for pretreated PPMS.

Operation of AAPFR at UoG, Canada

  • Methane production profile from AAPFR with varied OLR
  • pH profile from AAPFR
  • Volatile fatty acids (VFAs as FOS) profile from AAPFR
  • Buffering capacity (TAC) and FOS/TAC ratio profile from AAPFR 99
  • The composition of biogas at varied OLR in AAPFR operation

Variation of biogas production rate (a), pH rate (b), VFA concentrations (as FOS) (c), buffer capacity (as TAC) (d), FOS/TAC ratio (e) and VS reduction ( f) with OLR increased in the semi-continuous AAPFR experiment. However, at OLR 8.8 kg/m3/d, AAPFR experiences excessive high biomass content in the solvent.

Development of kinetic model for AAPFR

  • Application of kinetic model

The maximum biogas yield (ym) can be calculated from a single batch test, while the calculation of k can be obtained from long-term experiments in an AAPFR. The value for k can be obtained by dividing log[ym/(ym- y)] against HRTor 1/OLR. Therefore, the biogas yield y can be expressed as an absolute ratio p (p=y/ym), and the correlation between HRT and p comes from Eq.


However, a longer HRT will definitely reduce the utilization efficiency of the reactor and reduce the biogas production per day. The reactor lost stability at an OLR of 8.8 kg/m3/d, which was evident from the decrease in biogas yield and its methane content.

Overview of model development

Present study aimed to develop the mathematical model based on the influence of MC on limited mass diffusion rate leading to the hydrolysis inhibition caused by the deteriorated hydrolytic product during hydrolysis step of SS-AD system. The magnitude of hydrolysis inhibition caused by the internal mass diffusion resistance is one of the main differences in SS-AD and LS-AD.

Basic assumption and its physical process

The main difference between SS-AD and enzymatic hydrolysis is that SS-AD contains a large amount of sugar consumers than their production of hydrolytic product, when hydrolysis was a rate-limiting step. The mathematical model developed in this study for the SS-AD was able to interpret the influence of MC on the rate of CH4 production, which was based on limited mass diffusion causing hydrolysis inhibition by the collected hydrolytic product.

Development of mathematical model

  • Biological reaction model
  • Mass diffusion model
  • Mass balance model
  • Pseudo steady state model

Using the modified Gompertz model equation, the maximum CH4 production rate can be calculated from the equation. 6.13) Yt, Mmax,λ and t are the cumulative CH4 production during time t, maximum CH4 production. The maximum rate of CH4 production (Rmax) can be estimated by assuming that there was a negligible change in the initial concentration of substrate and inoculum as X0F and X0M by multiplying the CH4 yield coefficient (Y∆CH4/∆S) as indicated in Eq. 6.17), the known parameters such as XF0 and L can be calculated from the equation.

Verification of model

Developed model to estimate the maximum rate of CH4 production Rmax was also satisfactorily suitable data from the other literature data such as Le Hyaric et al. The parameter used for the simulation, listed in the table, is derived from a proposed theoretical hypothesis, used for the experimental data of present study and data by the Abbassi-Guendouz et al. 2014) for the statistical prediction which agreed well with each other.

Implication of model

This two-phase effect of MC can be better interpreted by the hydrolysis inhibition due to its mass diffusion. The hypothesis about inhibition mechanism in SS-AD due to mass diffusion resistances and its mathematical support is shown in Fig. 2008) reported that hydrolysis inhibition was caused by many hydrolytic products under a low MC.

Prediction of the model

By combining these three trends, it was concluded that at lower MC there was a severe inhibition of the hydrolytic product in SS-AD due to mass diffusion limitation. From this study, it was quantitatively detected that the existence of mass diffusion would lead to a region of inhibition of hydrolysis in SS-AD.


However, the thermal (hot air oven), electrohydrolysis and biological ((Bacillus mojavensis) pretreatment gave better results in solubilization rates measured in the form of sCOD and VFA. The better degree of agreement between model simulation and the literature as well as experimental data verified that the decline in the volumetric rate of methane production.


Screening of pretreatment methods to improve thermophilic anaerobic digestion of pulp and paper mill wastewater treatment secondary sludge. Aerobic composting and anaerobic digestion of pulp and paper mill sludge. Water science and technology.

Biochemistry and conversion steps in the anaerobic digestion process

Schematic representation of pulp and paper mill sludge production sites

Different pretreatment adopted in pulp and paper mill sludge

Experimental flow chart of the research work

The effect of temperature and time on COD and VFA different samples kept inside hot air oven and hot water bath. a) Effect of temperature on COD and VFA in hot air oven for 30 min.) Effect of time on COD and VFA in hot air oven at 80oCc) Effect of temperature on COD and VFA in hot water bath for 30 min) Effect of time on COD and VFA in hot water bath at 80oC. Thus, PPMS pretreated at 200oC for 2 minutes showed an increase of 39.4% sCOD (1.39 times) compared to untreated PPMS sample in this study. The effect of temperature and time on COD and VFA for different samples kept in an autoclave and microwave oven .a) The effect of temperature on COD and VFA in the autoclave for 20 min. b) The effect of time on COD and VFA in the autoclave for 110oCc) The effect of temperature on COD and VFA in the microwave for 2 min.

Source of substrate (PPMS) and filter press unit at the Nagaon paper mill (PPMS)

Schematic and experimental anaerobic setup used for the current study

Instrumentation used for the pretreatment study

A schematic diagram of the experimental setup of electrohydrolysis pretreatment

Batch reactor setup with water displacement system

Schematic view designed anaerobic auger plug flow reactor (AAPFR)

Pictorial view of continuous AAPFR with the gas measured through water

Pictorial view of continuous anaerobic auger plug flow reactor (AAPFR) at UoG,

Variation of VS, VFA, sCOD and pH in relation with methane production. a) Effect

The effect of temperature and time on COD and VFA different samples kept inside


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