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Sanitary Sewer Overflow

Overflow of sanitary sewage, mixed with stormwater, to a stream, from the separate sanitary sewer system in times of wet weather due to the entry of extraneous water into the System. These wet weather overflows occurs when the sewers become hydraulically overloaded. By-passes are usually designed and are used to prevent back-ups into basements. Dry weather overflows occur form maintenance-related reasons, such as a blockage or pipe failure.

Sanitary Sewer System

The sewer system that caries liquid and wastewater from residences, commercial buildings, industrial plants and institutions, together with minor quantities of ground, storm and surface waters that are not admitted intentionally.

Secondary Waste Treatment

A wastewater treatment process used to convert dissolved or suspended materials into a form more readily separated from the water being treated. Usually the process follows primary treatment by sedimentation. The process commonly is a type of biological treatment process followed by secondary clarifiers that allow the solids to settle out from the water being treated.


Supplemental Environmental Project; Under the terms of the Consent Decree, MSD implemented a Sewer Connection & Septic Tank Closure Program for low income residents. Funding for the program is now completed. 

Separate Sewers

Sewers that carry only wastewater or only carry stormwater. The separate sanitary sewers are ultimately connected to a treatment plant. The separate storm sewers ultimately discharge directly to a stream.

Service Area

All of St. Louis City and 90% of St. Louis County, with Highway 109 as general western boundary and the three major rivers: Missouri, Mississippi, and Meramec as boundaries.


A pipe or conduit that carries wastewater or drainage water. Also referred to as a collection line.

Sewer Connection & Septic Tank Closure Program

A supplemental environmental project required by the Consent Decree to connect low income residents’ homes that are on septic tanks to the MSD sewer system.

Sewer Main

A sewer pipe to which building laterals are connected. Also called a collection main.


A comprehensive term that includes facilities collecting, pumping, treating and disposing of wastewater.

Sheet Flow

Water flow over a surface at a uniform depth.

Also known as Overland Flow.


The solids removed from sewage during wastewater treatment.

Sludge Management

The purposeful, systematic control of the generation, storage, collection, transport, processing, and disposal of sludge.


Surface runoff produced from melting snow.

Soil Amendments

Include any material added to soil to improve its physical, chemical, biological, or structural properties or to provide enhanced plant growth. Improved permeability, infiltration, drainage, structure, or nutrient availability may all be goals of using soil amendments.


Sewer system evaluation survey

Storm Sewer

A separate pipe, conduit or conveyance that carries stormwater runoff and discharges it into streams.


Any water resulting from precipitation that may or may not be mixed with an accumulation of dirt, soil, and other debris or substances collected from the surface on which such precipitation falls or flows.

Stormwater Control

The management of stormwater on- or off-site, including detention, retention, infiltration, or use.

Stormwater Runoff

The portion of rainfall, melted snow or irrigation water that flows across ground surfaces and eventually is returned to streams. Runoff can pick up pollutants from the air or land and carry them to the receiving waters.

Stormwater Service Charge

The user charge to generate the revenue to operate and maintain the stormwater system.

Sump Pump

A pump used to remove water that has accumulated in a water collecting sump basin, commonly found in the basement of homes.


The additional charge for the treatment of wastes containing suspended solids, biochemical oxygen demand or chemical oxygen demand exceeding normal wastewater strengths, for commercial customers.


A broad, shallow, gently sloped channel for conveying stormwater runoff. It may be lined with vegetation, compost, riprap, or other material and is designed to slow runoff velocity, trap particulates, and promote infiltration. Vegetated swales are often referred to as bioswales, enhanced swales, or water quality swales.

  • Dry swale (bioswale)—incorporates additional elements in the design. Water treatment is aided by a soil bed (natural or amended) with an underdrain system typically composed of a perforated pipe surrounded by gravel. 
  • Wet swale—temporarily retains stormwater runoff, but unlike the dry swale, lacks an underdrain system. The wet swale is marsh-like and supports wetland vegetation that is adaptable to variable moisture conditions.

The entire sewer and drainage system owned and operated by the District for the collection, storage, handling, and treatment of wastewater, for the collection, storage, handling and treatment of stormwater, and combined sewers for the collection, storage, treatment and handling of wastewater and stormwater to serve the needs of the District and its customers.