By Markus Busch, Verónica García Molina, Yasushi Maeda, William E. Mickols, Ian Lomax and John Tonner
Not all membrane designs are ideal for seawater desalination, but Dow Water & Process Solutions has developed an approach that can reduce energy costs by up to 7%.
Desalination is essential to providing safe, ample supplies of drinking water to water-stressed coastal areas globally. Advances in pump efficiency, energy recovery devices and higher productivity membranes in desalination facilities have cut the energy costs associated with reverse osmosis (RO), a key function in seawater desalination.
Some sources predict the next substantial energy and cost savings in RO desalination plant operations will derive from membrane technology. Despite an array of products on the market, not all membrane designs are acceptable for seawater RO (SWRO) plants. For water with a fouling tendency, system design and operation will be defined by permeate flux through the membrane, system recovery, individual elements (ratio of permeate flow and feed flow) and the concentration polarization related to flux and recovery. To control fouling, engineering guidelines recommend maximum element production, recovery and/or concentration polarization. These guidelines limit acceptable design configurations and operating conditions.
Conventional plant design limits productivity of new membrane designs due to the high osmotic pressure of seawater feed, which effectively doubles in the brine stream to approach the maximum level of available operating pressure. This high pressure leads to an uneven distribution of flux across the pressure vessel of a seawater plant. Higher temperature, higher TDS feed waters and/or higher productivity membranes may also contribute to uneven flux distribution. One of the negative effects from uneven flux distribution is the potential for higher concentration polarization, which increases membrane fouling.
A novel membrane design concept developed by Dow helps to minimize uneven flux distribution across the pressure vessel using multiple element types in series. The approach was first described in 2004 and is called Internally Staged Design (ISD). The advantages of this approach include:
- Fouling reduction by lower flux on the lead element;
- Higher output per pressure vessel and per element, enabling potential reductions in capital requirements;
- Higher productivity at the same feed pressure and fouling rate for reduced energy consumption; and
- Higher recovery at the same feed pressure/fouling rate, enabling potential economic plant capacity enhancements.