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Anaerobic digestion of corn silage was studied in a semi-continuous anaerobic plug flow reactor (PFR) at mesophilic condition. This work study the influence of increase in organic loading rate (OLR) on biogas production and other important parameters during anaerobic digestion. Varied OLR in the range of 3.5–8.5 kg m−3 d−1 (volatile solids (VS) fed basis)-volatile fatty acids (VFA) and FOS/TAC value increased with OLR, was stable up to 6.5 kg m−3 d−1, and became critically unstable when OLR was 8.5 kg m−3 d−1. An increase in biogas production was perceived with each upsurge in OLR, although CH4 concentration decreased and H2S content increased under high loading. The PFR lost its stability at 8.5 kg m−3 d−1 OLR that was apparently observed by decline in biogas yield and CH4 content.

Different heating processes were applied to pretreat the pulp and paper mill sludge to enhance the hydrolysis stage. Hot air oven, autoclave, microwave and water bath were used for comparison. Chemical analyses exposed that all the pretreatment methods have shown improvement in solubilisation of organic matter. Among the pretreatments studied, heat transfer through hot air (80 °C for 90 min) mode had the highest impact on sludge solubilisation. There was an increase in cellulose (P = 0.0096), acid soluble (P = 0.0053) and insoluble lignin (P = 0.0012), but decrease in hemicellulose (P = 0.0009) content after pretreatment. The XRD and FT-IR spectroscopic characterisation shows the development of aliphatic, unsaturated and carbonyl carbon functionalities in the pretreated samples at higher severities. FESEM picture also confirms the change in structure after pretreatment. Thus, pretreatments contribute to disruption of the lignocellulosic structure making the cellulose easily accessible to acidogenic microorganisms.

The methane production rate increases with a decrease in moisture content up to the optimum threshold level and diminish in a batch reactor fed with the lignocellulosic material in solid state anaerobic digestion. The interpretation on two-faceted effect of moisture content could not be elucidated by the conventional anaerobic digestion model no 1. This study proposed that the decreased moisture content causes augmented mass diffusion resistance by the accumulation of hydrolytic product. This could lead to the reduced production of methane gas. Based on the hypothesis, a new solid state anaerobic digestion model was developed based on mass diffusion limitation and hydrolysis inhibition. The better degree of agreement between model simulation and the literature as well as experimental data verified that the deterioration in the volumetric rate of methane production. This could be attributed to hydrolysis inhibition as a result of limited mass diffusion in solid state anaerobic digestion.

This novel work is focused on evaluating the electrohydrolysis pretreatment conditions (applied voltage and time) and anaerobic digestion process for the biological bioconversion of pulp and paper mill sludge into biogas in batch assay. The pretreatment at 15 V for 45 min shows highest impact on sludge solubilization. The XRD and FT-IR spectroscopic characterization shows the development of aliphatic, unsaturated and carbonyl carbon functionalities in the pretreated samples. FESEM picture also qualities the change in alteration of structure after pretreatment. Batch anaerobic bioreactor was carried out to determine the efficacy of electrohydrolysis pretreated and untreated pulp and paper mill sludge. The methane production potential was increased from 274 ± 5 to 301 ± 4 mL CH4/g VS after electrohydrolysis pretreatment.

The application of anaerobic digestion is mounting worldwide because of its economic and environmental benefits. As a concern, the rate and extent of the anaerobic digestion of pulp and paper mill sludge was tested using the biochemical methane potential (BMP) with different food/microorganisms (F/M) ratio. The study was evaluated at five different F/M ratios (i.e. 0.5, 1.0, 1.5, 2.0, 2.5). In addition to that reaction kinetics and the influence of biodegradability were also evaluated for the biomethane potential of substrates. The highest cumulative methane yield was obtained in F/M ratio 2.0 with 3.4 L followed by F/M ratio 1.5 and 2.5 with 3.3 L and 2.9 L, respectively. The maximum methane production rate 3.66 L CH4 was perceived as per kinetics. The biomass activity may be affected due to the high substrate concentration during BMP test of lower F/M ratios. On the other hand, F/M ratio greater than 2.0, methane yield was decreased due to increased volatile fatty acid production reduces the reactor pH and inhibits methanogens activity.

Swine manure and corn stover are abundant agricultural wastes which contribute to greenhouse gas (GHG) emissions, nutrient runoff leading to eutrophication, and a biosafety risk with respect to improper swine manure handling. Anaerobic co-digestion (AcoD) of swine manure and corn stover can mitigate these negative impacts while producing biogas as a renewable energy source. Semi-continuous mesophilic plug flow reactor (PFR operation) was studied during a step-wise increase in organic loading rate (OLR) over the range of 0.25–4.7 kg volatile solids added (VS) m−3 d−1, which corresponded to total solids content (TS) of 1.5–9.0%. Process stability was observed at all OLR, with the highest total biogas yield and methane content of 0.674 ± 0.06 m−3 kg−1 and 62%, respectively, at 0.25 kg m−3 d−1. As OLR and TS increased, VS reduction decreased and volatile fatty acids (VFA) increased due to shorter hydraulic retention times (HRT). Hygienization potential was assayed using fecal indicator bacteria (FIB), with some groups being reduced (E. coli, fecal coliforms) and others not (Clostridia spp., fecal enterococci). Lignocellulolytic enzyme activity trended upward as OLR was increased, highlighting changes in microbial activity in response to feeding rate.

Kinetic study of plug flow reactor digesting corn silage was studied under the mesophilic condition (37 ± 2 °C). The effect of organic loading rate (OLR) on biogas yield and its production was tested in a long-term experimental study. Both the biogas yield (0.68–0.39 m3/kg) and methane content in the biogas (66–58%) were reduced with the increase in OLR (3.5–8.5 kg VS /m3٠d). In this study, the maximum biogas yield (ym) as 0.716 m3/kg and first-order reaction rate constant (k) values as 0.0689 per day was obtained. The developed kinetic model could be used to design the dimension for a plug flow reactor digesting different organic wastes such as agricultural residue, animal manures, and biogas crops.