Poly(3-hydroxybutyrate) (PHB) is a thermoplastic polyester that can be synthesised by many bacteria under unfavourable growth conditions. Among all its properties, biodegradability is the most attractive since it makes PHB a potential alternative to non-biodegradable plastics. However, the current market penetration of this polymer is limited by the high production costs, which accounts for approximately 40% of the total price, and the complexity in scaling-up the biological process. In this work, an innovative approach is presented, simulating the production of PHB through a double stage aerobic fermentation in which a Methylocystis strain grows and accumulates PHB granules. The effects of the reactor geometry on PHB yields were investigated in terms of aspect ratio (A), which was varied in the range 5-16 both at constant volume and variable volume: in the first case, the working volume was kept constant at 100 L and the diameter of the reactor was varied in the range 0.2-0.3 m; in the second case, the working volume was varied between 100 and 340L, keeping constant the diameter at 0.3 m. The effect of the geometry on transition parameters was evaluated as well. Results show that a maximum transition superficial gas velocity (Ut) of 0.05 ms-1 was obtained at A=16; while the productivity of PHB ranged between 0.12 and 0.25 kgm-3d-1.