You're probably talking about [Potlatch 2][1], which is the editor that you get by default on the "edit" tab in the site.
The reason why the number of markers in the "simple" view in there is limited is only partially because "no-one has added them yet" - having too many options may be confusing to new users, and the limitations of screen size would make the finding of one marker among many more cumbersome.
Just in case you're not aware, in Potlatch there is an "advanced" view (just click "advanced" at the bottom left when you have something selected) that allows you to [add any tags you like][2].
It may be, of course, that waste water treatment plants and pipelines are important enough to feature in the simple view. If so, the place to make that case is [trac][3] (the bug-tracking system for most of OSM), against the component "potlatch2". In fact, one was added for "pipeline" a couple of months ago - [Ticket #4365][4]. There's also a [mailing list][5] - all "potlatch2" trac entries and comments pop up there.
Finally, you may want to have a look at the "Get Involved" section at the bottom of the [Potlatch 2 wiki page][6] which describes how it's developed, and how you can get involved.
[1]: http://wiki.openstreetmap.org/wiki/Potlatch
http://wiki.openstreetmap.org/wiki/Potlatch_2
[2]: http://wiki.openstreetmap.org/wiki/Any_tags_you_like
[3]: http://trac.openstreetmap.org/
[4]: https://trac.openstreetmap.org/ticket/4365
[5]: http://lists.openstreetmap.org/listinfo/potlatch-dev
[6]: http://wiki.openstreetmap.org/wiki/Potlatch_2
