When selecting aeration equipment to use for a specific application, issues to address include reliability, serviceability, capital cost, system appurtenances, and cost of operation and maintenance. Another important consideration is oxygen transfer rate (OTR). Horizontal floating aerators have relatively high transfer rates and have been used successfully in aerated lagoons and oxidation ditches. Using equipment with high OTR values reduces the horsepower input that is required and subsequently lowers capital cost and O&M costs. Mixing of wastewater in lagoons and oxidation ditches is also important to maintain solids in suspension. While oxygen transfer (instead of mixing) may be the controlling factor in design of facultative aerated lagoons, mixing often is the controlling design constraint in an oxidation ditch reactor. Obviously, it is important to adequately mix the wastewater with the biomass to ensure effective pollutant removal. Mixing is promoted by ensuring that wastewater velocities in the reactor are sufficient to create turbulence which keeps the biomass and solids in the mixed liquor suspended. Generally, average sustained velocities of about 1 fps are considered sufficient for this task. TNRCC also has a requirement of 100 HP/Mgal as a secondary requirement to ensure adequate mixing. Data on dissolved oxygen levels and channel velocities in the Lindale, Texas, Oxidation Ditch will be presented in this paper. In addition, channel velocities were determined in the Mertzon, Texas, Oxidation Ditch before and after the aeration basin was converted from a fixed rotor design to a floating horizontal aerator design. In February, 2000, the City of Hattiesburg, Mississippi, installed floating horizontal aerators in its two aerated lagoons. The floating horizontal aerators replaced aspirating aerators to improve treatment process performance. Dissolved oxygen levels and plant performance (BOD and TSS removal efficiencies) before and after brush aerator installation will be presented.