## 2 Materials and Methods
### Analytical Methods  
Sludge samples were digested with the modified method adapted from [PERSON] et al. [21]. Briefly, collected sludge samples were placed in 100 mL crucibles and dried overnight at 105 \({}^{\circ}\)C in an oven until constant weight was observed and then ground and homogenised in an agate mortar. A total of 25 mg of the ground sludge sample was digested (4 mL 65% of HNO\({}_{3}\), 200 \(\mu\)L 40% of HF and 4 mL of H\({}_{2}\)O) in a microwave digester (Multiwave 3000, [PERSON], North Ryde, Australia). The digested solution was diluted with 5% of HNO\({}_{3}\) in a volumetric flask. The sodium concentration was measured in a microwave plasma-atomic emission spectrometer (MP-AES 4100, Agilent, Santa Clara, CA, USA).  
The total chemical oxygen demand (TCOD) and VSS were measured in accordance with standard methods [22], and the VFA measurement was described previously [23]. For the biogas measurement, a manometer (Dwyer Series 475 Mark III) was utilized [24]. At the 6 th h, the pressure inside the serum bottle was zeroed to atmospheric pressure with the manometer and nitrogen gas. At the 48 th h, the pressure of the cumulated biogas was measured again with the manometer. The determination of biogas production was based on the ideal gas equation of state.  
The composition of the biogas was determined with gas chromatography (GC) and a thermal conductivity detector (TCD) (Agilent Technologies 7890A GC system, US). The temperature for the injector, detector and oven were controlled at 120, 150 and 115 \({}^{\circ}\)C, respectively. Five columns were utilized for the analyses of methane (CH\({}_{4}\)), carbon dioxide (CO\({}_{2}\)), hydrogen (H\({}_{2}\)), nitrogen (N\({}_{2}\)) and oxygen (O\({}_{2}\)).  
Helium (He) was used as a carrier gas for the determination of CH\({}_{4}\) and CO\({}_{2}\) with the three packed columns (Agilent HayeSep R 0.9 m \(\times\) 1/8\" \(\times\) 2.0 mm, Agilent HayeSep C 3.0 m \(\times\) 1/8\" \(\times\) 2.0 mm and Agilent MolSieve 5A 3.0 m \(\times\) 1/8\" \(\times\) 2.0 mm). Argon (Ar)was used as the carrier gas for the determination of H\({}_{2}\) with another two columns (Agilent HayeSep Q 0.9 m \(\times\) 1/8\({}^{\prime\prime}\)\(\times\) 2.0 mm and Agilent MolSieve 13 \(\times\) 3.0 m \(\times\) 1/8\({}^{\prime\prime}\)\(\times\) 2.0 mm).  
For EPS extraction, the sludge sample was centrifuged (4114\(\times\)\(g\), 30 min) [9], and the remaining pellet was resuspended with an equivalent volume of 1 \(\times\) PBS buffer (pH 7.4) [25]. EPS was extracted from this solution with the sodium hydroxide-formaldehyde method [26]. The carbohydrate and protein contents of EPS were measured within 24 h by the colorimetric method [27] and Lowry method [28], respectively.  
ATP measurement was used as a rapid and simple method for evaluating methane potential and total biomass activity [29]. Recently, ATP was proposed as a reflection for the loss of cell activity by sodium inhibition [30]. The amount of ATP present in the biomass would be proportional to the luminescence released from the reaction of biomass with the reagent BacTiter-GloTM (Promega, Madison, WI, USA).  
Each sludge sample was diluted with 1 \(\times\) PBS buffer (pH 7.4). The specific commercial reagent (100 \(\mu\)L) and standard or sludge samples (both of 100 \(\mu\)L) were incubated in an opaque-walled 96-well plate and mixed on an orbital shaker at room temperature for 5 min. The luminescence data was recorded by a plate reader (Infinite M200 Pro Tecan, SciMed, New York, NY, USA). The specific ATP concentration was normalized against VSS concentration [31]. Triplicates were used for each test in this study.