Membrane　wetting　and　fouling　were　studied　using　hollow-fiber　polyvinylidene　fluoride　(PVDF)　membranes　to　concentrate　reverse　osmosis　(RO)　brine　in　a　direct　contact　membrane　distillation　(DCMD)　process.　The　effect　of　the　operating　conditions,　such　as　the　feed　temperature,　the　flow　velocity　and　the　feed　solution　concentration,　was　investigated.　Membrane　wetting　was　more　significant　at　high　feed　temperatures,　and　the　salts　in　the　feed　promoted　membrane　wetting;　membrane　wetting　resulted　in　a　decrease　in　the　flux　and　salt　rejection.　A　theoretical　model　was　developed　to　simulate　the　DCMD　process,　and　the　results　of　the　model　were　analyzed.　The　concentration　factor　significantly　affected　the　flux　and　the　electrical　conductivity　of　the　distillate.　When　the　concentration　factor　(CF)　was　less　than　3.5,　CaSO4　did　not　crystallize　because　of　high　ionic　strength　and　higher　solubility　of　CaSO4　at　the　membrane　surface　than　in　the　bulk　solution;　beyond　CF　of　3.5,　membrane　fouling　was　exacerbated　because　of　CaSO4　crystallization.　Membrane　fouling　was　more　significant　at　the　higher　of　the　two　temperatures　investigated　for　long-time　DCMD　operation:　square　CaSO4　crystals　formed　at　77　degrees　C,　whereas　snowflakes　formed　at　55　degrees　C.　The　permeate　was　lower　in　the　divalent　cations,　Mg2+　and　Ca2+,　than　in　the　monovalent　cations,　Na+　and　K+.　(C)　2014　Elsevier　B.V.　All　rights　reserved.