Replace PVC Cap?

Right behind our house we have two capped PVC pipes like in the attached photo. I noticed one of the caps looks like it was damaged maybe from a mower or something like that.

Should that cap be replaced and is that something easy to do or should a plumber be called? I didn't know if it just screwed on and/or what would happen if I screwed it off.

I didn't know if when it rained or when we water our yard if water could get into the sewer line. We are on an aerobic septic system.

Thank you!

It's probably a cleanout to the septic system. It is cracked and no longer sealed, but there's probably not much of anything going in. If it doesn't smell, you might just leave it.

To repair it, you need a new threaded fitting. Hassle is, when PVC is attached, it is solvent welded - the two pieces become one part (not the cap, that can be removed - it isn't glued in).

You might be able to saw through the fitting in a few places, then peel it off. It will likely take some of the pipe with it, but maybe not enough to ruin it so you can't glue on a new one. If it does, then you'd need to cut it off, add a coupling, then a short stub of pipe and a new fitting.

While annoying, you could raise it so you wouldn't be likely to chip it with the lawn mower. If you put a threaded fitting on the pipe, you could probably find a brass cap and cleanout that might be a little more robust, or just put a fake stone or some other thing around it. A sprinkler burried control box might be a good idea, too. They come in green, and wouldn't stick out as much. If you cut the existing fitting off, then installed the box, you could make it essentially flush, rather than sticking up, too.

I often tell customers that I'll install it flush and all they do is put a planter pot on top of it...

Makes it easy to find in the winter...

Re:

How can I know what size cleanout it is?

I measured the cleanout from side to side and it measured 5". It seems online the most common size is 4." Does the 4" indicate the size of the pvc pipe the cleanout slides onto?

You have yourself there a 4" female adapter with a 4" plug.

The ID of the pipe is the size...
4" is my guess.

cleanout

Cut the pipe below the adapter fitting, and then install a new one. It does not even have to glued on, just step on it to push it all the way onto the pipe.

Re

I appreciate everyone's help. I went to Home Depot and got the replacement part this morning.

Thank you!