SBRs are wastewater treatment reactors based on an activated sludge process where all the different stages of nitrification/denitrification, settling and separation take place in the same reactor but dislocated in time. The treatment consists of a cycle of five stages: fill, react, settle, draw and idle. The reactor containing active biomass is first charged with influent, closed and than aerated to satisfy the oxygen demand of the wastewater. During this phase, bacteria oxidize the organic matter as in activated sludge systems. Then, aeration is finally stopped to allow sludge settling. In the next step, the water and the sludge are separated by decantation and the clear layer (supernatant) is discharged from the reaction chamber. Depending on the rate of sludge production, some sludge may also be purged. After a phase of idle the tank is filled with a new batch of wastewater. At least two tanks are needed for the batch mode of operation as continuous influent needs to be stored during the operation phase. SBRs are suited to smaller flows, because the size of each tank is determined by the volume of wastewater produced during the treatment period in the other tank and the need to be seeded for starting up reactors.