Analytical methods
Samples from the reactors were immediately filtered on 0.45 µm
polyvinylidene fluoride membranes (Millipore, USA) and stored at -20°C
until analysis. Volatile fatty acids (VFAs; formate to valerate),
lactate, succinate and glucose were analysed using high performance
liquid chromatograph (HPLC) method described previously (Rombouts et
al., 2019). Ethanol was analysed using a gas chromatography (GC) method
described previously (Rombouts et al., 2019). The off-gases were
monitored on-line for H2 and CO2 by a
connection to a NGA 2000 MLT 1 Multicomponent analyser (Rosemount,
Shakopee, Minnesota, USA). Data acquisition (base, H2,
CO2) was made using a BBI systems MFCS/win 2.1
(Sartorius, Göttingen, Germany).
Methane was measured manually using GC with a Varian CP 3800 (Varian
Medical Systems, Palo Alto, California, USA) equipped with a MolSieve
capillary column (1.2 m x 1 mm; 13 x 80/100 mesh, 50 °C) and a thermal
conductivity detector (200 °C) with N2 as a carrier gas
(2 mL min-1).
Biomass concentration was measured using a standard method which relies
on centrifugation of 150 mL to separate the cells from the medium,
drying these solids to obtain the total suspended solids (TSS) and
burning these solids at 550 °C to determine the amount of volatile
suspended solids (VSS) (APHA, 1998). This analysis was coupled to
absorbance measurement at 660 nm to establish a correlation. Absorbance
values were used to calculate the biomass concentration during the cycle
analysis and batch experiments.