Odor Control Study in a Deep Sewer System with Multiple Drop Structures

Abstract

A growing concern for many wastewater collection system operators is the release of volatile organic and odorous inorganic compounds from wastewater collection systems. For many years, the Metropolitan Council Environmental Services, which provides wastewater collection and treatment services for the greater Minneapolis-St. Paul metropolitan area, has experienced "sewer" odor complaints in the area around an 8-mile reach of interceptor sewer. The interceptor sewer, ranging in size from three to nine feet in diameter was originally tunneled through rock approximately 70 feet below the ground from downtown Minneapolis south through residential neighborhoods along the west bank of the Mississippi River.In the latter half of 1999, a study was conducted with the objectives being to• accurately quantify airflow dynamics within the interceptor sewer • identify the best method(s) to achieve ventilation less conducive to fugitive odors and system corrosion • identify sizing criteria and potential odor treatment alternatives.The study confirmed what has been suspected for some time; that the odor problems are the result of two factors.• drop structures of varying sizes and types which discharge wastewater flows from shallower sewers into the much deeper interceptor sewer • a downstream siphon which conveys wastewater flows under the Mississippi River Falling wastewater at the drop structures pulls air into the system pressurizing the interceptor sewer, and the siphon blocks the downstream flow of air further pressurizing the interceptor sewer. The resulting higher-pressure odorous air in the interceptor sewer releases to the atmosphere through access manholes and lower flow drop structures. This paper presents the following.• data from extended monitoring of pressures and hydrogen sulfide concentrations • the results of analyses of airflow dynamics in the interceptor sewer • alternatives to achieve sewer system ventilation and treatment in a manner less conducive to odors and corrosion.