The　present　work　focused　on　the　reduction　of　the　calcium　and　sulfate　ions　in　a　membrane　permeate　generated　from　a　newly　developed　two-stage　of　tight　ultrafiltration　(TUF)　for　recovering　humic　substance　from　leachate　nanofiltration　(NF)　concentrate　by　a　precipitation　process　of　calcium　sulfate　dihydrate　(gypsum).　The　operating　parameters　(temperature　and　pH),　mechanism　of　precipitating　behaviors,　and　kinetics　of　this　process　in　the　presence　of　fulvic　acid　(FA)　and　magnesium　ion　simultaneously　were　investigated.　It　was　found　that　Ca2+　removal　increased　with　increasing　temperature.　The　decline　in　pH,　especially　towards　low　pH　range　of　2-3　around　the　point　of　zero　charge　of　gypsum,　showed　a　significant　effect　on　gypsum　precipitation.　Adsorption　of　FA　onto　gypsum　surface　had　an　important　effect　on　the　precipitating　behavior　of　gypsum　whose　mechanism　might　be　mainly　weak　interactions　of　hydrogen　bonding　and　hydrophobic　effect　at　low　pH,　while　electrostatic　interactions　became　dominant　at　neutral　pH.　In　contrast,　retardation　effect　of　Mg2+　ion　on　gypsum　precipitation　was　negligible　at　pH　2.5.　The　soluble　calcium　removal　during　precipitation　process　corresponded　with　first　order　rate　equation.　The　results　indicated　that　the　optimal　operating　parameters　were　found　at　70　degrees　C　and　around　pH　2.5,　which　can　be　applied　to　the　scale-up　and　industrial　design　of　the　precipitation　process　of　gypsum　for　removing　the　calcium　and　sulfate　ions　in　the　presence　of　FA　and　magnesium　ion.