While designing, initially suitable height of the reactor (about 6m) can be chosen, and superficial liquid upflow velocity is to be worked out as height/ HRT. It is recommended to adopt upflow velocity of 0.7 m/h at average flow and 1.0 m/h to 1.2m/h at peak flow. Accordingly, if the upflow velocity exceeds the maximum limits height of the reactor can be reduced in steps up to minimum of 4 to 4.5 m. If this is not possible in the applicable range of height, HRT shall be modified and fresh reactor volume and OLR shall be worked out. For low strength wastewater, the maximum liquid upflow velocity becomes limiting and for very high strength wastewater very low velocity (less than 0.1 m/h) is required while designing the UASB reactor. Under certain situations, the revised OLR may be less than the initial OLR recommended. It is advisable to allow lowering of OLR in such situations to control upflow velocity in the reactor for