One of the greatest equipment upgrades for home owners was the creation of tankless water heaters. Owners can now have access to hot water on demand without the large holding tank that spent too much energy keeping that standing water hot. Can this revolutionary technology be applied to another familiar home water system that takes up a lot of space with holding tanks, such as a reverse osmosis purification system?
Every RO membrane is built to produce a specific amount of water within a 24 hour period. For example, a 100 GPD membrane will not produce more than .069 gallons a minute (100/1440), no matter how high your feed water flow rate or pressure is. However there are feed water considerations that will cause the membrane to produce less than 100 gallons, such as pressure, temperature, and water quality. In this case we will assume the best water conditions to achieve the full 100 gallons per day.
Without a separate storage tank, the 100 GPD RO membrane will trickle water at 0.069 gallons per minute. To fill an 8oz glass of water at this flow, it would take approximately 60 seconds to fill. In comparison with a full pressurized storage tank providing just 3 gallon per minute, it would take approximately 8 seconds to fill that same glass.
So what if we used a higher producing membrane? If we go up to a 500 GPD membrane that's approximately 0.34 gallons per minute, 5 times the amount of water compared to the 100 GPD membrane. That means the 8oz glass will fill 5 times faster in approximately 12 seconds. This is a much better flow and its almost comparable to the flow a storage tank can provide. This flow is not pressurized through, so you will still get a soft stream of water in just a slightly higher volume output.
For a single-tap drinking water RO system with a 500+ GPD membrane, the flow rate is tolerable to use without a storage tank. However a larger membrane means more space and higher costs. The RO system has to be built to accommodate the larger membrane in both dimension and equipment, typically including a booster pump to push water through the large membrane.
For a whole house RO system to be able to accommodate the average flow rate of a home, the membrane size needs to be over 14,000 gallons per day (10gpm output). Besides the incredible price tag of this industrial-sized RO system, this is still a non-pressurized stream of water which would make a shower feel like standing in a heavy rainstorm. Water pressure plays an important role in household use and without the RO storage tank, you will not have pressurized water.
When counting the downfalls of a tankless RO system, a separate storage tank doesn't sounds as bad. A tankless system will still require more space, higher cost, and provide a below-average water supply. In most cases the RO system with the storage tank will be cheaper, require a similar footprint, and actually provide you with the water flow and pressure you're used to using without the unnecessarily large membrane.