Metals removal from acid mine drainage (Tinto River, SW Spain) by water gap and air gap membrane distillation

Water pollution by metals and, in particular, acid mine drainage (AMD) is one of the most severe environmental problems to be faced by society. In this sense, treatment by membrane processes is a promising alternative, highlighting membrane distillation (MD) features to overcome the limitations of conventional processes. In this work, water gap MD (WGMD) and air gap MD (AGMD) were tested under different feed temperatures and organic matter concentrations, using 0.45 mu m polytetrafluoroethylene (PTFE) membranes and real AMD from Tinto River, which is located in SW Spain. For both MD modes, high permeate fluxes (up to 16.80 L h(-1) m(-2) for WGMD and 10.16 L h(-1) m(-2) for AGMD) were reached and over 99% rejection was obtained for electrical conductivity, ions and metals (Na+, K+, Ca2+, Mg2+, F-, Cl-, NO3-, SO42-, Cu, Fe, Zn, Cd, Co, Mn, Ni). In addition, specific thermal energy consumption (STEC) per unit of permeate water was calculated to obtain MD heat requirements. Furthermore, MD efficiency in terms of heat recovery was estimated by gained output ratio (GOR) between useful and total heat of the system. On that basis, MD and specifically the WGMD mode, showed great potential for the sustainable treatment of AMD.