The flotation wastewater treatment is considered. The main stages and parameters of flotation process are pointed out. The multistage flotation model considering all main stages of flotation process is presented. However its main disadvantage is that for the definition of constant characterizing bubble-particle aggregate formation the constant value of superficial gas velocity is used, which is incorrect for horizontal flow flotation tank. The modified multistage flotation model with superficial gas velocity at the function of time is proposed. The experiments were carried out on a laboratory setup with phneumohydraulic system of aeration in order to verify proposed model. The comparison experimental and theoretical results proved the importance of superficial gas velocity change consideration.