Inoculum cultivation
A single bioreactor was operated, the bioreactor had a height of 150 cm and an internal diameter of 6.5 cm. The working liquid volume was 2.5 L and the headspace was 2.4 L. The reactor was maintained at 30 ± 1 ºC and the pH was controlled at 5.5 using 2 M NaOH (Biostat Stedim, Sartorius, the Netherlands). A 2% Antifoam C solution was added to the reactor 4 times slower than the base addition. Before intiating the different SRT a start up period was required to obtain granular sludge. The carbon and mineral medium used were identical as in (J. Tamis et al., 2015) using a glucose concentration of 10 g·L-1 in the influent (1.25 L). The reactor was seeded with anaerobic sludge digesting mixed primary and secondary sludge from the sewage treatment plant Harnaschpolder (Delft, The Netherlands). During the granulation process two general rules were applied determining the cycle length and settling time. The rules were that glucose should be fully consumed and all biomass should be retained in the reactor. Firstly, glucose was considered fully depleted once the base addition flattened out and CO2 and H2 were below 1% in the off-gas. Secondly, the settling time for biomass started at 3 hours and was manually shortened to 2 min, over a period of 20 d.