You're leading a field camp this summer and the last thing you want to do is take care of a bunch of college kids with giardia. You have the GravityWorks filtration system, but could do with a bit more capacity, so you've grabbed the Platypus GravityWorks 4. 0L Replacement Reservoir Kit. Your previous dirty reservoir was getting a bit grungy, so you get to start over with a brand-spanking-new bag, and now you can store an additional four liters of fresh clean waters for your thirsty undergrads. While this kit does not include the actual filter and filtration tube, these replacement reservoirs come in handy when you need additional capacity or a couple backups—just in case.