Examination of the storage function of intercepting sewers using long-term flow monitoring data

Abstract

This study proposes a method for examining storage function of intercepting sewers, following sewerage rehabilitation works to convert combined-type to separate-type systems. The selected case study sewage treatment area recently completed a large-scale rehabilitation work; the area incorporates six sub-areas (SAs) and one wastewater treatment plant (WWTP), the latter having treatment capacity of 11,000 m(3)/d. Sewage flow generated in dry weather was domestic sewage flow only. Wet weather wastewaters consist of the dry weather sewage flow, in addition to rainfall-derived infiltration/inflow. In order to calculate wet weather wastewater flows of the six SAs, an advanced regression model was used. This was calibrated and verified using long-term monitoring flow data of 816 and 204 h, respectively; the model was then used to predict wastewater flows for 168 h. Hydraulic simulations of intercepting sewers were conducted using a conventional pipe hydrodynamic model (i.e. Saint-Venant equations). By assuming different inflow conditions to the WWTP (multiples of daily peak flows, Q(d)), storage function tests were conducted, based on water balance calculations between sewage flows generated from the six SAs and inflow measured at the WWTP. The existing intercepting sewer of the case study area appears to have 70, 11, and 3 h flow storage functions, for inflow controls of 1.5Q(d), 2Q(d), and 2.5Q(d), respectively. Under 3Q(d) inflow conditions, almost all wastewater flowed to the WWTP. The storage function is thus expected to be effective for wet weather operation of the WWTP. Such storage function would also be achievable in other areas conducting large-scale rehabilitation works. The method proposed in this study will be useful for decision-making concerning the removal of existing intercepting sewers.