Granular sludge is the key factor for an efficient operation of an upflow anaerobic sludge blanket (UASB) reactor. However in some cases in an UASB reactor we lost granules for many reasons.
there are several factors for loss of granules. from biological to physical phenomenas involved. However, i would suggest you to calculate settling velocity, density of your granules, you can optimize in superficial velocity accordingly. do you have homogeneous granules in the system ? The attached document can help you understand optimizing your UASB.
In UASB, mentaining shear force is difficult and need proper optimization, One can use neutral gas bubbling to enhance granulation and then optimise required superficial velocity to stabilize granules.