The report is divided in two parts, reflecting the chronology of the research work performed. The first part presents an extensive study of the most important hydrodynamic characteristics of fairly large-scale bubble plumes, created in a cylindrical vessel having a diameter of 2 m with water depth of 1.5-2 m. The bubbles were produced near the bottom of the vessel by a multi-needle circular injector having a diameter of 0.3 m. Several measurement techniques and a variety of tools were used to measure and to analyze the data. Particle Image Velocimetry (PIV), double-tip Optical Probes (OP), photographic techniques and three-dimensional Electro-Magnetic Probes (EMP) were extensively applied to measure bubble and liquid velocities, void-fraction, interfacial area concentration, bubble size and liquid re-circulation rates. PIV measurements in a vertical plane crossing the centre of the injector provided the instantaneous velocity fields for both phases, including selected hydrodynamic parameters, such as the movement of the plume axis and its instantaneous cross-sectional width. Statistical studies were performed using image processing to determine the diameter and the instantaneous centreline position distributions and their fluctuations in time. An important finding was that there is not much instantaneous spreading of the plume. The stress tensor distributions obtained from the instantaneous data indicate that for the continuous phase, these stresses scale linearly with the local void-fraction in the range of 0.5%< a