High-rate sedimentation for wastewater treatment processes

High-rate sedimentation is an inexpensive way to increase the capacity of normal settling tanks. It produces an effluent of more uniform quality for a very wide range of hydraulic and solids loads. These features can be attractive for developing countries. In this study, the influence on sedimentation efficiency is analyzed for different geometric configurations such as the use of a square tank (since this implies a reduction in construction costs of the order of 25%), different types of inlets (square, baffled, triangular and square with reduction), different types of modules (plates or tubes), and positioning of the high-rate module (high or low). The results indicate that the use of a square sedimentation tank does not hamper the process's efficiency. Regarding the type of inlet, a square one leads to increased efficiency and a more uniform effluent for a hydraulic load range from 27 to 108 m(3) m(-2) d(-1). Concerning the type of modules the square tube module operates at a higher efficiency (96% for a solids load of 15 kg m(-2) d(-1)). There are no significant differences in using the module in either high or low position, but the efficiency increases from 42 to 85% when the module is used as opposed to when it is not used. The use of a flocculant (alum) helped to obtain greater removal rates (>95%). The optimal inclination of the tubes was 42 degrees for rotating biological contactor flocs, 35 degrees for trickling filler flocs and 45 degrees for activated sludge flocs.