An NH3/CO2 forward osmosis (FO) membrane brine concentrator (MBC) pilot was tested in the desalination of frac flowback and produced waters from natural gas extraction operations in the Marcellus shale region. The average concentration of these waters was 73,000 +/- 4200 mg/L total dissolved solids (TDS), with an average hardness of 17,000 +/- 3000 mg/L as CaCO3. Pretreatment included chemical softening, media filtration, activated carbon, and cartridge filtration. Average pilot performance characteristics were: system recovery of 64 +/- 2.2%, nominal water flux of 2.6 +/- 0.12 L/m(2)-h, concentrated brine concentration of 180,000 +/- 19,000 mg/L TDS, and product water with 300 +/- 115 mg/L TDS. The thermal energy required by the FO MBC pilot, when operated within the efficient flow specification of the draw solution recycling system, averaged 275 +/- 12 kWh(th)/m(3) of product water, approximately 57% less thermal energy input than that estimated for a conventional evaporator operated in a comparable single stage, non-mechanical vapor compression (MVC) configuration. In an MVC configuration, which uses electrical rather than thermal energy, modeling indicates that the FO MBC process will require 42% less electrical energy than a conventional forced circulation MVC evaporator. (C) 2012 Elsevier B.V. All rights reserved.