2. Methods
2.1. Inoculum and substrates for anaerobic co-digestion
Seed micro-organisms and raw sludge were collected from the anaerobic digestion tank of the municipal sewage treatment plant in Busan, South Korea. Food waste leachate was taken from a waste processing plant in Busan, South Korea (Supplementary Fig. 1a). The microalgal biomass, Chlorella sp., was generously provided by
Daesang Co. (South Korea), and concentrated microalgae were refrigerated at 4 C prior to use (Supplementary Fig. 1b). Table 1 shows the physicochemical characteristics and elemental composition of the raw sludge, food waste leachate, and algal biomass.
2.2. Experimental set-up
The batch anaerobic co-digestion experiments were performed in 500 mL amber-glass bottles with a working volume of 400 mL (Supplementary Fig. 1c). Initially, each digester was sparged with 65% nitrogen gas for 2 min. Then, the bottle was filled with 200 mL of seed microorganisms and 200 mL of substrate. After an
additional 10 min of flushing with 65% nitrogen (to ensure anaerobic conditions) the digester was capped with a rubber septum, and then placed in a shaking incubator (35 ± 2 C and 120 rpm) (Supplementary Fig. 1d). Over a 42-day period, biogas was collected with gas-tight plastic syringes; liquid samples were collected in plastic syringes for volatile fatty acid (VFA) analyses. The effect of different ratios of the 3 substrates on methane production was tested (Table 2). All experiments were run in triplicate and results are presented as means ± standard deviations.