Coalescence behaviour of bubble pairs markedly affected methane bioconversion because methane is usually pumped into a bacterial suspension through an aeration system, forming bubbles at capillary orifices. In the present work, a bioreactor was developed to observe the coalescence behaviour of bubble pairs forming at capillary orifices submerged in bacterial suspension via a visualization experiment. The results indicated that in a bacterial suspension the inrush bubble number decreased and the inrush time shortened, but the contact time was prolonged and the maximum contact area increased compared to those in pure water. Moreover, with an increase in the gas flow rate, the maximum contact area decreased and the contact time shortened, while more trailing bubbles merged into the leading bubble. Furthermore, the increase in the centre distance was conducive to increasing the maximum contact area and prolonging the contact time but the bubble inrush was observed at a centre distance of 0.94 mm. In addition, the larger centre distance enhanced the oscillation amplitude and frequency of the aspect ratio, which had not significantly changed at different gas flow rates. The results of this study will contribute to the optimal design of an aeration system that enhances the performance of the bioreactor.