● 8040 reverse osmosis membrane: Due to its larger diameter, the surface area of ​​the membrane is usually between 365-400 square feet. ● 4040 reverse osmosis membrane: Due to its smaller diameter, the surface area of ​​the membrane is between 85-100 square feet.

In the water treatment industry, RO membranes are usually named in a combination of numbers and letters, which include the size and characteristics of the membrane element. For the 4040 RO membrane, "4040" represents the size of the membrane element