## 2 Materials and Methods
### Measurement of Volatile Fatty Acids and Methane Content  
The volatile fatty acids (VFAs) formed in the experiments were measured using an Agilent 7890 B, Santa Clara, CA, USA gas chromatograph fitted with FID and an autosampler. The samples collected from the digestors were centrifuged at 10,000 rpm to remove the solids and extracted with MIBK as an extraction solvent. Methyl-isobutyl-ketone (MIBK) was used as an extraction solvent after painstaking examinations of various solvents and the recoveries of individual VFAs in the selected solvents. The pH of the aqueous sample is lowered using formic acid, which is favored over inorganic acids like phosphoric acid to prevent column damage at the tips. The pH of the sample is lowered to promote the diffusion of VFAs from the aqueous to solvent phase. Sodium sulfate is added to the aqueous sample during the liquid-liquid extraction process. The addition of the salt causes a reduction in the solubility of the VFAs in the raffinate or aqueous phase and increases in the extract or solvent phase. Also, sodium sulfate acts as a drying agent for the extract or solvent phase.  
The concentrations of VFAs (acetic acid, propionic acid, iso-butyric acid, n-butyric acid, iso-valeric acid, n-valeric acid, iso-capric acid, n-capric acid, and heptanoic acid) were analyzed, and pentanoic acid was used as an internal standard. A DB-FFAP (nitro tetrephthalic-acid-modified polyethylene glycol (PEG)) column of high polarity for VFAs was used for the analysis. A column of 25 m in length and 0.22 mm diameter with a film thickness of 25 \(\upmu\)m was procured from SGE for the same. The analysis method used a nitrogen flow of 3 mL\(\cdot\)min\({}^{-1}\) as a carrier. The oven was programmed from 85 \({}^{\circ}\)C for 1 min to 180 \({}^{\circ}\)C at a rate of 30 \({}^{\circ}\)C min\({}^{-1}\). The temperature was finally raised to 220 \({}^{\circ}\)C at 40 \({}^{\circ}\)C/min. The injector and detector temperatures were set at 230 and 250 \({}^{\circ}\)C, respectively.  
The biogas generated in the digester was quantified using a gas chromatograph (Agilent 7890 B, Santa Clara, CA, USA), equipped with a TCD and FPD detector. The permanent gases were analyzed using TCD, and sulfur gases were analyzed in FPD. The gas chromatographic conditions used for gas analysis were as follows. A sample loop size of 0.25 mL was used for gas sampling. The nitrogen (N\({}_{2}\)) gas (5 mL/min) was used asthe carrier gas for FPD set at 300 \({}^{\circ}\)C. Argon (30 mL/min) was used as a carrier gas for TCD maintained at 200 \({}^{\circ}\)C with a split ratio of 25:1. Gas sampling was used for sampling analysis. The valves were placed in a valve box maintained at 120 \({}^{\circ}\)C. A molecular sieve column (1/8 in diameter and feet long) was used to separate N\({}_{2}\), O\({}_{2}\), H\({}_{2}\), and CH\({}_{4}\). A Porapak column (1/8 in diameter and feet long) was used for CO\({}_{2}\) analysis. A backflush function was provided in the GC to prevent CO\({}_{2}\) poisoning of the molecular sieve column. The oven program was initiated at 45 \({}^{\circ}\)C for 6 min, then increased to 180 \({}^{\circ}\)C over a period of 2.25 min at a rate of 20 \({}^{\circ}\)C/min. The gas generated in the biogas digesers was connected to the GC, and the gas composition was analyzed. The amount of biogas generated from the reactors was calculated using the ideal gas law after accounting for the pressure generated due to water evaporation at STP [61; 62].