You will need to add those types (sludge pits, UV filters, etc)to the ssNetworkStructureType domain if you want to represent them.
If you do not want to modify domains you can represent them as ssNetworkStructures, and use the type "Other". You can then utilize the "Name" field to indicate device type, and list which treatment plant it is located at in the "LOCDESC" field.
I am sure there are better ways but that's what popped into my head.
Have a great day,
Anthony Milden
Wastewater Technical Support
Collier County Government, FL