Depth exposed membrane for water extraction (DEMWAX) for seawater desalination

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Courtesy of EconoPure Water Systems, LLC

Introduction
The Depth Exposed Membrane for Water Extraction (DEMWAX) is a novel approach to both fresh surface water treatment and seawater desalination that deploys a system directly within the source water body. The DEMWAXTM system is configured to take advantage of the natural (free) pressure of the source water body to drive a membrane process. This configuration has many advantages over the incumbent technologies. This paper provides detail on the DEMWAX system and its advantages.

Basic Theory
The idea behind the DEMWAX system is harnessing natural water pressure to drive a reverse osmosis process in lieu of artificially creating this pressure. The system deploys membranes where natural pressure already exists and it takes advantage of natural water movement, eliminating the need to mechanically handle the corrosive feedwater. The DEMWAXTM system has many applications (see discussion below) but the two primary applications are the desalination of seawater and the treatment (potable) of fresh surface water.

For both primary applications, there are five basic premises or design traits behind the technology.

  1. Natural water pressure in a water body: This pressure is free, constant, and abundant. The use of this free pressure means that with the DEMWAX system, only the treated water is pumped.
  2. Atmospheric pressure communication: Communication of atmospheric pressure to the product water side of the membranes allows the natural creation of the pressure differential needed for the process.
  3. Natural water movement: All membrane processes require the movement of feed water to the membrane surface and removal of the concentrate or brine. Membrane sheets in the DEMWAX system are designed so that gravity removes the concentrate and brings more water to the surface of the membranes. The space between the membrane elements is nearly 10 times that used in spiral wound configurations allowing for the natural flow of water between them and avoiding surface tension “locking” the source water in place.
  4. 4. Low recovery: The water bodies can be considered an endless supply of high pressure source water, similar to the concept of a ‘heat sink’. As such, there is no need for high recovery, which just raises the osmotic pressure requirement. This allows the design pressure to be just greater than osmotic pressure for the source water (osmotic + transmembrane or driving pressure).
  5. 5. Low flux: Membrane flux (produced water per unit of membrane area) is often associated with system efficiency in traditional systems. That is, higher flux means less pre-treatment costs, less membrane, fewer pressure vessels, etc. However, higher flux also means higher transmembrane pressure and higher velocities into the membrane face. Low flux, on the other hand, reduces the driving pressure requirement and lowers the velocity of the water into the system. Low flux also reduces stress on membranes increasing the effective life and reduces particulate fouling.

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