About the Storm Sewer System

Q: What is a Municipal Storm Sewer System, or MS4?
A: An MS4 is a storm sewer system that is separate from the sanitary sewer and are generally operated by public entities (e.g. counties, cities, towns).

Q: What is the difference between a sanitary sewer and a storm sewer?

A: Sanitary sewers take household wastewater (such as water from sinks, toilets, washers, etc.) and carries it through a home's plumbing and into an underground sewer pipe.
This water then travels to a formal wastewater treatment plant where the water is cleaned to regulated standards.
Storm sewers take rainwater that falls on our communities travels via the storm drain system (e.g. MS4) which may include public streets, gutters, catch basins, storm pipes, retention basins, channels, washes, etc.
This water flows directly to your community's local parks, basins and waterways without receiving any formal treatment.

drain system

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Q: What is a catch basin?
A: A catch basin is a curbside receptacle that is connected to a storm sewer and whose sole function is to drain stormwater runoff from impervious surfaces (e.g. roads, parking lots, etc.).

catch basin

Q: Do catch basins and storm pipes get cleaned out?
A: Yes. Each respective jurisdiction within the Greater Phoenix Metropolitan Area has a program to clean the drainage system within their community.

Q: Can filters or screens be installed in front of catch basins?
A: It sounds like a good idea. But during a rainstorm, trash is quickly swept to the catch basin and any screen or filtration device placed in front of the catch basin would cause trash to accumulate and clog the grate, preventing proper drainage and end up flooding the street. With thousands of catch basins feeding miles of pipes and channels, there would be far too many blocked catch basins to have crews cleaning them as the rain falls.

Q: Why isn't a net/fence/barrier installed at the end of the storm drain channel to catch all of the trash?
A: Trash barrier nets and/or screens only catch the trash that floats in the channels or detention basins. However, most pollutants like pet waste, used oil, pesticides, fertilizers, etc. flow through the net and straight into our waterways.

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Q: What are detention/retention basins?
A: Detention basins hold large volumes of stormwater and are often drained manually after a large storm event. Retention basins tend to be more localized structures and infiltrate stormwater runoff into the subsurface soils recharging groundwater supplies. Both of these structures improve stormwater quality by keeping small discharges of pollutants from entering the storm drain system.

Q: Why don't communities build stormwater treatment facilities?
A: Treatment facilities are extremely expensive to build and maintain. For most of the year, the facility would have little to no flow to keep it operating properly. However, during a measurable rain event, the massive amount of water coming through the facility would easily overtax the system.

Q: What kind of pollutants are found in the storm drain system?
A: Paint thinner and paint products, motor oil, pesticides, sediment, styrofoam cups, paper, human and animal feces, antifreeze, golf balls, dirty diapers, and dead animals are but a few of the pollutants found in the system on a daily basis.

Q: How much water passes through the system?
A: The storm drain systems within each community in the Valley vary from one another. However, on a typical dry summer day, many communities have more than one million gallons of nuisance water flowing in the storm sewer system. This flow comes from sources such as over-watered lawns, fire hydrant pressure releases and residential car washing activities. In a heavy rainstorm, the combined flow in the Valley can increase to more than a billion gallons.

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