Main Drain trap

[ryandebny]

Is it true that code requires you NOT to trap the main sewage drain leading out to the street? Just trap floor, and fixture drains?

[jnaas2]

Code at one time required a main line trap to try and keep rats from being able to enter your house from the sewer line. They no longer require this, I think they found out rats can swim

[LazyPup]

It has absolutely nothing to do with rats..

Years ago when they were still dumping raw sewage into a local river or estuary the municipal sewers conveyed both sanitary waste and storm drainage. In those days the curb side storm drain openings also served as vents for the municipal sewers.

When the EPA was first activated one of the first pieces of legislation they mandated was that all municipal sewage had to be treated before the excess water could be released into the watershed. Each municipality was them forced to build sewage treatment plants. and in turn, the capacity of the sewage treatment plants was based upon the average daily discharge rate of the municipal sewage system plus a slight margin for future expansion. The problem with that was that storm drainage generates volumes of water that may be many times greater than the average daily discharge of the sanitary waste system. This resulted in the storage tanks at the treatment plants overflowing and once again raw sewage was released into the watershed.

The municipalities then installed a second parallel set of sewer mains dedicated to storm drainage. Since storm drainage is basically excess rain water they were permitted to discharge the storm drains directly into the watershed without treatment.

Problems began to occur because all structures had a main house trap, and since the municipal sewers no longer had the curb side storm drains to vent of the sewer gases the levels of combustible sewer gases built up to explosive levels. There were numerous explosions as a result of sewer gases, in fact, in one small city in Illinois a sewer line under the main street blew up, ripping a 4' wide trench down the main street for about 8 blocks, setting numerous buildings on fire and killing a half dozen people.

It was then determined that if they did away with the house traps the main vent for each structure connected to the sewer would also serve as a vent for the municipal main, thus as more structures are added to the main, they in turn add an additional amount of venting to compensate for the additional load.

While there still are some rare instances, especially in the inner city area of older cities where they still use the house traps under current national model codes house traps are prohibited.