Dissolved oxygen refers to the mass of oxygen that is contained in water. The concentration of dissolved oxygen is an important indicator of the environments water quality. The presence of oxygen in water is desirable therefore it is a positive sign; whereas the absence of oxygen is a sign of severe pollution. An adequate supply of dissolved oxygen is important for waste water treatment processes. Many naturally occurring biological and chemical processes use oxygen, thereby diminishing the dissolved oxygen concentration in the water. The physical process of oxygen transfer or oxygen absorption from the atmosphere acts to replenish the used oxygen. This process has been termed aeration. Aeration is the primary requirement of the biological treatment of water and wastewater treatment. As reported in the literature, the aeration process consumes as much as 60-80% of total power requirements in wastewater treatment plants. Therefore, it is necessary that the design and operation of aeration process should be economized in terms of their energy efficiency. The performance of surface aeration systems is rated in terms of their oxygen transfer rate; hence the choice of a particular surface aeration system depends on its performance and efficiency of oxygen transfer rates. Oxygen transfer rate and the corresponding power requirement to rotate the rotor are very vital parameters for the design and scale-up of surface aerators. Basically two types of operation are in use for surface aeration systems – batch operation and continuous operation. Batch operation involves a single vessel which is filled, aerated then completely emptied. Continuous operation method of operating a biological treatment plant is characterized by a steady input stream (in terms of chemical and biological composition and flow rate, when flow and concentration equalization is practiced), steady process conditions during the treatment steps and by a fairly consistent flow of treated material with only little variation in its composition. The work presented in this thesis consists of two parts. The first one deals with the experimental investigations on the three types of batch surface aeration tanks. A comprehensive design analysis has been worked out and presented on these types of surface aeration systems. In the second category, experimental investigations have been carried out extensively on continuous flow surface aeration systems of different sizes. Analysis has led to the formulation of optimal geometric dimension and the simulation criteria for the design purposes. As far as the first category of investigations is concerned, a substantial work has been reported on batch surface aerators on various issues, during the past several years. Still, a general methodology to scale up or scale down the process phenomena is lacking. In the present work, experiments were done on different shaped batch surface aeration system for generalizing or devising the scale up and scale down criteria for oxygen transfer coefficient and power consumption. Present work through experimental observations established that unbaffled circular tanks are more energy efficient than baffled when used as surface aerator. Power consumption in surface aeration systems is characterized by a fundamental non-dimensional parameter named power number. This number relates drag force to the inertial force in fluid flow system. Power number scaling up of unbaffled surface aerators of square, circular and rectangular shaped tanks is one of the most important contributions of the present work. Design charts have been developed for all the three shape of tanks for the installation as the batch surface aeration systems. Based on the experimental analyses in the present work, it was found that circular shape is the most efficient than any other shape and it is also established that generally a number of smaller sized tanks were more economic and efficient than using a single big tank while aerating the same volume of water. Based on the energy economy analysis, present work suggests the optimal speed range of batch systems of different shaped surface aeration tanks. Different sized rectangular aeration tanks with different aspect ratios (that is length to width ratio) were tested along with a series of square and circular tanks for comparing their relative performances. Present work by doing experiments answered this fact and found that square tank (aspect ratio =1) was more efficient than any other aspect ratio rectangular tanks. Vortexes are inherently present in any type of unbaffled tanks. Present work analyzed the vortex behavior of unbaffled surface aeration systems to determine the critical impeller speed in unbaffled batch surface aeration systems at which oxygen transfer rates are more. The second part of the present work establishes the optimal geometrical parameters of a continuous flow surface aeration systems. These types of operations were found to be least reported in the available literature and there appears to be of no report in the literature on optimal geometrical parameters. Extensive experimental work is reported in the present thesis on the establishment of the optimal geometrical parameters of continuous flow surface aeration systems. From there, simulation criteria are established by maintaining optimal geometrical similarity in different sized continuous flow surface aeration tanks; so that the scale up or scale down criteria can be applied to predict oxygen transfer rates and power number.