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The following is a list of definitions of terms about combined sewers and more generally about wastewater treatment.


Average dry weather flow
The average non-storm flow over 24 hours during the dry months of the year (May through September). It is composed of the average sewage flow and the average dry weather inflow/infiltration.

Average wet weather flow
The average flow over 24 hours during the wet months of the year (October through April) on days when no rainfall occurred on that or the preceding day.

Base flow
Wastewater flow (including a reasonable amount of inflow and infiltration) originating from residential, commercial and industrial sources.

See CSO basin.

Baseline study
A study that documents the existing state of an environment to serve as a reference point against which future changes to that environment can be measured.

Best Management Practice (BMP)
A method, activity, or procedure for reducing the amount of pollution entering a water body.

The determination, checking, or rectifying of the graduation of any instrument giving quantitative measurements. With respect to a computer model, calibration is a process whereby data recorded during an actual event is compared with data derived from a computer simulation of that event in order to determine the accuracy of the simulation.

CATAD system
Computer Augmented Treatment and Disposal System, which monitors flows in the wastewater conveyance system and operates regulator and pump stations to gain maximum use of pipe capacities.

Clean Water Act (CWA)
Also known as the Federal Water Pollution Control Act (33 U.S.C. 1251 et seq.).

Collection main
In collection systems, this is a larger pipe in which smaller branch and submain sewers are connected. The collection main may also be called a main or trunk sewer.

Collection system
In a wastewater system, a collection system is a system of pipes which receives and conveys sewage and/or storm water.

Combined sewer overflows (CSOs)
Overflows, during wet weather, of combined wastewater and stormwater. CSOs happen when flows in the wastewater collection system exceed the capacity of that system. The term "CSO" is also sometimes used to denote a pipe that discharges those overflows.

Combined sewer system
A wastewater collection and treatment system where domestic and industrial wastewater is combined with storm runoff.

Combined sewers
A sewer that carries both sewage and stormwater runoff.

Cost-effective alternative
An alternative control or corrective method identified after analysis as being the best available in terms of reliability, performance, and costs.

CSO basin
A geographic area served by a common sewer and stormwater system.

CSO event
A period of rainfall during which an overflow was recorded and that was preceded by 24 hours with no overflow and followed by 24 hours.

CSO treatment plant
A plant designed to provide primary treatment of combined sanitary sewage and storm water for peak flows above the 2.25 times the average wet weather flow. Such plants operate only intermittently, unlike most wastewater treatment plants which operate continuously.


Janice Johnson
CSO Control Program