PROPOSAL FOR NEW CRITERIA AND NEW EQUATIONS TO GUARANTEE THE SELF-CLEANING OF SEWAGE PIPES (Atena Editora)

Abstract

The sizing methodology for sanitary sewage collection systems uses fixed criteria to avoid sediment deposits at the bottom of free conduits at low flow rates, and these self-cleaning criteria have values that generally do not take into account the synergy of physical and hydraulic parameters in real operational situations, especially in partial separator, unitary and dry weather collection systems. This work therefore details new self-cleaning concepts that take into account the characteristics of the conduits, fluids and sediments, as well as the type of sediment transport in real operational situations. In addition, existing standards and practiced criteria were duly analyzed in order to mathematically model 829 existing and representative data extracted from specific laboratory and field tests for self-cleaning in the operation of sewage pipelines around the world, with different diameters, situations and materials. After statistical analysis and validation of the proposed model, new tensile stress values and new self-cleaning equations for design criteria are presented, together with a new methodology, in order to hydraulically dimension the sanitary sewers in initial flow situations. In this case, the minimum tractive stress values proposed are 1.0Pa for separator-type sewage systems, 1.5Pa for systems with time collection (collectors or interceptors) and 2.0Pa for unit or rainwater systems. Finally, it is recommended to use a new general self-cleaning equation containing the Froude number (Fr*) of the sediment, in order to then use both the minimum self-cleaning velocity (vn) and the minimum self-cleaning tensile stress (τn), under the actual operating conditions of the sanitary system.