We derive a macroscopic model for biofilm formation in a porous medium reactor to investigate the role of suspended bacteria, mesoscopic attachment, and growth kinetics. The starting point is the mesoscopic one-dimensional Wanner-Gujer biofilm model. Mesoscopic processes included are: hydrodynamics and transport of substrate in the reactor, biofilm and suspended bacteria growth in the pore space, attachment of suspended cells to the biofilm, detachment of biofilm cells, and cell death. The mesoscopic equations are up-scaled from the biofilm scale to the reactor scale yielding a stiff system of balance laws, which we study numerically. We find overall reactor performance is influenced by suspended bacteria and microbial growth kinetics while attachment only affects biofilm dynamics.