Shale Water Desalination. From an economic point of view, the state of the art in seawater desalination is reverse osmosis (RO). However, RO is constrained to TDS lower than ~40,000 – 45,000 mg/L. For higher salinities it is necessary to use thermal desalination. Thermal desalination has been widely used in seawater desalination, however there are important differences: In seawater desalination the objective is to obtain fresh water, but in shale gas the wastewater is a byproduct and therefore the objective is treating the water for different reuses (internal reuse, irrigation, disposal) reducing at much as possible the final liquid waste. In seawater desalination, the rejected water must have low salinity (lower than 50,000 mg/L – 60,000 mg/L) and it is usually returned back to the sea. In the case of shale gas wastewater, the objective is to be as close as possible to Zero Liquid Discharge. In sea water desalination it is possible to use a large excess of water, for example cooling purposes and reject it directly to the sea, but in shale gas the water is scarce. In consequence, even though some thermal technologies have been adapted to the shale gas wastewater treatment, the adaptation is not straightforward. Among the available thermal (or hybrid membrane/thermal) alternatives we have deeply studied Multiple-effect evaporation with mechanical vapor recompression (MEE-MVR) and membrane distillation (MD).
