Circular drop manhole residence time reduction by appropriate jet-breaker dimensions

Drop manholes are commonly employed to reduce slopes in sewer systems. Diverse particles can enter into these systems and gather in drop manholes where they can chain and clog the outflow. As their residence time in the manhole pool increases, the outlet blockage risk rises. The jet-breaker was used for improving the performance of drop manholes under Regime R2, in which the inlet jet collides with the manhole outlet. However, its effects on residence time were unknown, and its proper dimensions, in this regard, needed to be determined. Therefore, the effects of jet-breaker dimensions and the inlet pipe filling ratio on residence time were experimentally studied in this paper. Results showed residence time significantly reduces when the jet-breaker length and width are 2 and 1.4 times the inlet pipe diameter, respectively; jet-breaker sagitta is zero, jet-breaker angle is zero degrees, and the inlet pipe filling ratio is 80%. Under this optimum condition, residence time decreased 66% in comparison with the control test.