Peak Flow Management of an RDI/I Affected Wastewater Utility

Abstract

Nearly every large wastewater utility in the world is unique with different factors affecting each. Some have significant topographic changes while others are flat. Some collection systems are well above the groundwater table while others must operate submerged. One factor that affects most systems is rain and groundwater infiltration/inflow (I/I) into the gravity wastewater collection systems. During storm events, this I/I can have high peaks which are difficult to measure or reduce. To prevent illegal discharges of wastewater prior to treatment, the utility must transmit and treat all flow in the system, or somehow reduce it. This paper will investigate means of evaluating a relatively flat, submerged wastewater collection system to reduce peak flows during storm events and minimize sanitary sewer overflows (SSOs) which can threaten public health and the environment. The specific focus of the paper will be the Miami-Dade Water and Sewer Department's Collection System in Miami-Dade County, Florida, and efforts to quantify and reduce peak flows. These flows can cause tremendous strain on the treatment facilities and operation of the County's 926 pump stations, and lead to SSOs. Since 1993, Miami-Dade County has been undergoing a $1 billion upgrade to its wastewater facilities with the main focus on reducing SSOs. The major portions of this plan include reducing I/I and increasing the transmission capacity of the pump stations and force mains plus increasing treatment and disposal capacity. Early upgrades to pump stations and force mains were based on average flows with some extra capacity for peaks; however, actual peak flow data was not available for design. The estimated flows were based on annual domestic water usage and night flows taken twice a year (once in the wet season and once in the dry season). In addition, designs incorporated reduced I/I from the ongoing I/I Rehabilitation Program. Unfortunately, it has been found that the common techniques used to reduce normal I/I is having little effect on reducing peak flows caused by RDI/I. New technologies have been developed since the program inception to better quantify the peak flows and prioritize rehabilitation. In addition, several alternatives have been developed to address peak flows whether it be at the source or at the treatment plants (e.g., service lateral RDI/I reduction versus chemical treatment processes). This paper will investigate each of these subjects and suggest a means of proceeding toward the County's goal of reducing SSOs.