A clean out gives access to a plumber or sewer and drain cleaner in order to relieve a clogged sewer from a blockage.
The sewer cleaner will need to be sure the sewer is not holding sewage at the cleanout before he opens it. When it is determined the sewer, at the location of the cleanout is not going to allow sewage to escape when the cleanout cover is removed, the sewer and drain cleaning technician will set up his equipment in that area.
Prior to removing the cover, precautions should be made to be certain no sewage escapes. Have a bucket placed under the cleanout. Maybe even a tarp on the floor, depending on the work space. Occasionally, buildup will be found in the mouth of a horizontal cleanout. Rather than send this down the sewer, pull as much of this out of the sewer as possible in order to prevent a sludge blockage that is difficult to clear unless high pressure jetting is used.
If the plumber or sewer and drain cleaning service can not locate a cleanout inside the building, where sewage is not going to escape the sewer system, the sewer and drain cleaning service will need to "bleed" off the sewage into a bucket and transfer it back into the sewer after the sewer is able to conduct sewage. The sewer and drain cleaning technician should be prepared to bleed off several gallons of sewage before the cleanout cover can be completely removed.
As shown in the picture above, cleaning a sewer can not always be done with a sewer snake or main line sewer cleaning machine. If the cleanout is inside the home, jetting may not be possible from this location, without risking a tremendous amount of back wash into the home or place of business. In the photo above, the jetting was done through a cleanout located in the garage floor.
Location of a cleanout is also a very important consideration. Suppose you have a cleanout in the garage floor or driveway. If vehicles are constantly running over the cleanout, what could happen, is the vehicle will force its weight down onto the cleanout. The cleanout will be forced to bare the vehicle's weight or cause the vertical riser beneath the cleanout to sink into the plumbing below. This would create a blockage that could not be relieved without excavating the broken pipe fittings below.
Here is an image of a cleanout located in a driveway, where vehicles stand a good chance of running over the cleanout.