There are a number of locations where a utility would want to be able to treat multiple sources of water with one treatment system. Those that the Bureau of Reclamation (Reclamation) has come across recently are:
The Texas Gulf Coast where brackish surface or groundwater is available for much of the year but only seawater is available during dry seasons;
South central California where the character of the irrigation drainage water changes with the intensity of irrigation;
Inland desert areas where the composition of brackish surface and groundwater is significantly different when augmented with storm water.
Reclamation assisted the US Office of Naval Research in development and demonstration of the Expeditionary Unit Water Purifier system which can produce 378 m3/day (100,000 gal/day) of potable water from any liquid source water up to 60 g/L TDS under 35°C. The system was tested under the Environmental Technology Validation (ETV) program overseen by NSF International for the US Environmental Protection Agency (EPA). Test water sources were seawater, turbid surface water, and tertiary wastewater. As a complete system, it performed well with each source. It was only capable of 50% recovery of water however. Because of this, the system has been turned down for potential emergency applications due to the excessive loss of water. This issue occurring at the same time as the three situations mentioned above prompted a proposal to design a flexible system that could be adapted to achieve high water recovery when the feed source allows while still maintaining the capability of recovering energy when the remaining concentrate pressure allows.