Preventing Sewer Blowouts during High-Velocity Jet Cleaning Operations

Abstract

The flow capacity of municipal sewer lines often diminishes over time due to root intrusion and debris accumulation. As a consequence, many wastewater agencies use high-velocity jets to scour sewer pipes and restore the discharge capacity of the collection lines. Occasionally, jetting operations expel fluid from the sewer through connecting laterals to plumbing fixtures in adjacent homes and businesses. While rare, these hydraulic blowouts pose health problems, damage private property, and strain public relations. This paper investigates the conditions under which sewer line blowouts occur in order to predict and reduce their occurrence. Statistical analyses of field data gathered over a 10-year period by the Metropolitan Sewer District of Greater Cincinnati revealed that, among many competing factors, the incidence of sewer blowouts was highest in pipes with the smallest flow capacity. Based on this finding, a simple formula was developed to identify service zones with high blowout risks. Two computational flow models, FLUENT and SWMM, were used to simulate and corroborate the sewer blowout phenomenon.