For many years, I have had an RO system in my kitchen sink that I use for drinking water, coffee and brewing. I employ a bladder vessel "pressure tank" because I can fill a drinking glass in a few seconds and several gallons in a minute or two. If you're looking to collect 15-20 gallons you may notice a small improvement in water production without the bladder tank. It's important to note that in most systems, the bladder inside the "pressure tank" only requires 6-9 lbs. of compressed air before you start up the system. So if you leave your faucet/valve fully open (collecting water) until the bladder tank is empty, the city water pressure (60-100 lbs) is only pushing against 6-9 lbs. of pressure inside the bladder. I can't believe there's a significant loss of production. Plus, as Alton Brown would say "the only "unitasker" in the kitchen should be a fire extinguisher".