Exemplary embodiments describe apparatuses and related processes for improving mixing and sheer in a fixed film reactor. One process can include recycling at least a portion of a biogas product through at least one sparger below a fixed film zone in the fixed film reactor at conditions sufficient for mixing and sheering the film from an internal structure within the fixed film zone. Often, a cross-sectional area of the fixed film zone fills at least about 90% of a cross-sectional area of the fixed film reactor.