The Mushroom cap can be used as a replacement cap to any OdorHog filter or it can be installed directly on any 3" or 4" plastic PVC/ABS pipe as a rain cap, or to keep debris from falling down into the vent pipe. This Mushroom cap is sold separately and does not come with a filter. The cap attaches to a plastic vent pipe with 3 sheet metal screws that screw into and through the vent pipe to secure it to the pipe. This cap fits 3" or 4" plastic pipes only. If you have questions, please email info@odorhog.com.