In upflow anaerobic sludge bed (UASB) reactors, biomass present as granules allows for long solids retention time. Here, granules from a process treating pulp and paper industrial wastewater were successfully applied as inoculum in UASB reactors treating pig manure supernatant, despite high particle content and high ammonium concentrations in the influent. We did a detailed characterization of archaeal and bacterial communities associated with the inoculum and with the aggregated and dispersed fractions of the influent and the reactors after one year of operation. The granular communities underwent major changes and adapted to the highly distinct conditions without disintegration of the granules. Although the granules persisted in the reactors, non-granular aggregates accumulated, and partly replaced the granules. Particles introduced to the reactors by the pig manure influent apparently contributed both as food and biofilm growth support. Archaeal communities in the dispersed reactor phase were similar to those dispersed in the influents, implying successful retention and little loss of archaeal biomass due to detachment or disintegration of granules and other aggregates. Unique bacterial communities developed in the dispersed fraction of the reactors despite of low hydraulic retention times. They probably consisted of fast growing organisms consuming readily degradable organic matter.