The development of A or B type granules is related to the average biomass retention time taking place in the start-up process. When the average biomass reten-tion time is too short, compact bacterial granules consisting almost exclusively of bacterial matter do not have the chance to be formed. This means that large conglomerates of bacteria can only be formed through attachment to inert support particles, which are heavy enough to be retained longer in the reactor (type B).Only if the average biomass retention time is big enough, compact bacterial granules (type A) can be formed.