Sampling and analytical methods
The bioreactor was monitored online using measurements of the pH, and base addition. The biogas was measured qualitatively for CO2, H2 and CH4 in-line with a mass spectrometry system (Omnistar, Pfeifer vacuum). Offline measurements were done for SVI, soluble chemical oxygen demand (CODSol), glucose concentration, organic acids concentration, total suspended solids (TSS) and volatile suspsended solids (VSS). The VSS content of the reactor was determined by taking a VSS sample at the end of the reaction phase, when the reactor was completely mixed. The reactor was sampled in more detail at the end of the enrichment elucidating the conversions during a cycle. To this end the reactor was sampled every 5 min during the presence of glucose and once every 30 min after glucose depletion. Samples taken for CODSol glucose, and organic acid analysis were filtered directly after sampling using a 0.45 µM pore size filter (PVDF membrane, Millipore, Ireland). The CODSol was measured using commercially available spectrophotometrically test-kit from Hach-Lange. The organic acid composition and quantity were measured using a high-performance liquid chromatography with a BioRadAminex HPX-87H column and a UV/RI detector (Waters 2489). As a mobile phase 1.5 mM H3PO4 in Milli-Q water was used with a flow rate of 0.6 mL·min-1 and a temperature of 60°C. The TSS content of a sample was obtained by first centrifuging a sample at 2500 g after decanting the supernatant the sample was oven dried at 105 ºC. The VSS content was obtained according to (Clesceri et al. 1999). The SVI60 was obtained by taking the apparent sludge volume (SV) at the end of the settling phase in the bioreactor and dividing this by the measured TSS from the mixed reactor.