對透(tou)(tou)(tou)(tou)(tou)過(guo)的(de)(de)(de)(de)(de)物質(zhi)(zhi)(zhi)(zhi)具(ju)有(you)選(xuan)擇性的(de)(de)(de)(de)(de)薄膜(mo)(mo)(mo)(mo)稱(cheng)為(wei)(wei)半(ban)透(tou)(tou)(tou)(tou)(tou)膜(mo)(mo)(mo)(mo)，一(yi)(yi)(yi)(yi)(yi)般(ban)將(jiang)(jiang)只能(neng)透(tou)(tou)(tou)(tou)(tou)過(guo)溶(rong)(rong)(rong)(rong)劑而(er)不(bu)能(neng)透(tou)(tou)(tou)(tou)(tou)過(guo)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)(zhi)的(de)(de)(de)(de)(de)薄膜(mo)(mo)(mo)(mo)稱(cheng)之為(wei)(wei)理想半(ban)透(tou)(tou)(tou)(tou)(tou)膜(mo)(mo)(mo)(mo)。當把相同體(ti)(ti)積(ji)的(de)(de)(de)(de)(de)稀溶(rong)(rong)(rong)(rong)液(例(li)(li)如(ru)淡水(shui))和濃(nong)(nong)(nong)(nong)溶(rong)(rong)(rong)(rong)液(例(li)(li)如(ru)鹽水(shui))分(fen)別置(zhi)于(yu)半(ban)透(tou)(tou)(tou)(tou)(tou)膜(mo)(mo)(mo)(mo)的(de)(de)(de)(de)(de)兩側(ce)(ce)時，稀溶(rong)(rong)(rong)(rong)液中的(de)(de)(de)(de)(de)溶(rong)(rong)(rong)(rong)劑將(jiang)(jiang)自然穿(chuan)過(guo)半(ban)透(tou)(tou)(tou)(tou)(tou)膜(mo)(mo)(mo)(mo)而(er)自發(fa)地向濃(nong)(nong)(nong)(nong)溶(rong)(rong)(rong)(rong)液一(yi)(yi)(yi)(yi)(yi)側(ce)(ce)流(liu)動(dong)，這一(yi)(yi)(yi)(yi)(yi)現象稱(cheng)為(wei)(wei)滲(shen)透(tou)(tou)(tou)(tou)(tou)。當滲(shen)透(tou)(tou)(tou)(tou)(tou)達到平衡時，濃(nong)(nong)(nong)(nong)溶(rong)(rong)(rong)(rong)液側(ce)(ce)的(de)(de)(de)(de)(de)液面會比稀溶(rong)(rong)(rong)(rong)液的(de)(de)(de)(de)(de)液面高出一(yi)(yi)(yi)(yi)(yi)定(ding)高度(du)，即形成(cheng)一(yi)(yi)(yi)(yi)(yi)個壓差，此壓差即為(wei)(wei)滲(shen)透(tou)(tou)(tou)(tou)(tou)壓。滲(shen)透(tou)(tou)(tou)(tou)(tou)壓的(de)(de)(de)(de)(de)大小取決于(yu)溶(rong)(rong)(rong)(rong)液的(de)(de)(de)(de)(de)固(gu)有(you)性質(zhi)(zhi)(zhi)(zhi)，即與(yu)(yu)濃(nong)(nong)(nong)(nong)溶(rong)(rong)(rong)(rong)液的(de)(de)(de)(de)(de)種(zhong)類(lei)、濃(nong)(nong)(nong)(nong)度(du)和溫度(du)有(you)關(guan)而(er)與(yu)(yu)半(ban)透(tou)(tou)(tou)(tou)(tou)膜(mo)(mo)(mo)(mo)的(de)(de)(de)(de)(de)性質(zhi)(zhi)(zhi)(zhi)無(wu)關(guan)。若在(zai)濃(nong)(nong)(nong)(nong)溶(rong)(rong)(rong)(rong)液一(yi)(yi)(yi)(yi)(yi)側(ce)(ce)施(shi)加一(yi)(yi)(yi)(yi)(yi)個大于(yu)滲(shen)透(tou)(tou)(tou)(tou)(tou)壓的(de)(de)(de)(de)(de)壓力(li)時，溶(rong)(rong)(rong)(rong)劑的(de)(de)(de)(de)(de)流(liu)動(dong)方(fang)(fang)向將(jiang)(jiang)與(yu)(yu)原(yuan)(yuan)來(lai)的(de)(de)(de)(de)(de)滲(shen)透(tou)(tou)(tou)(tou)(tou)方(fang)(fang)向相反(fan)，開始從(cong)濃(nong)(nong)(nong)(nong)溶(rong)(rong)(rong)(rong)液向稀溶(rong)(rong)(rong)(rong)液一(yi)(yi)(yi)(yi)(yi)側(ce)(ce)流(liu)動(dong)，這一(yi)(yi)(yi)(yi)(yi)過(guo)程(cheng)稱(cheng)為(wei)(wei)反(fan)滲(shen)透(tou)(tou)(tou)(tou)(tou)。 反(fan)滲(shen)透(tou)(tou)(tou)(tou)(tou)是滲(shen)透(tou)(tou)(tou)(tou)(tou)的(de)(de)(de)(de)(de)一(yi)(yi)(yi)(yi)(yi)種(zhong)反(fan)向遷移(yi)運(yun)動(dong)，是一(yi)(yi)(yi)(yi)(yi)種(zhong)在(zai)壓力(li)驅動(dong)下，借(jie)助于(yu)半(ban)透(tou)(tou)(tou)(tou)(tou)膜(mo)(mo)(mo)(mo)的(de)(de)(de)(de)(de)選(xuan)擇截(jie)留作用(yong)將(jiang)(jiang)溶(rong)(rong)(rong)(rong)液中的(de)(de)(de)(de)(de)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)(zhi)與(yu)(yu)溶(rong)(rong)(rong)(rong)劑分(fen)開的(de)(de)(de)(de)(de)分(fen)離(li)方(fang)(fang)法，它已廣泛應用(yong)于(yu)各種(zhong)液體(ti)(ti)的(de)(de)(de)(de)(de)提純(chun)(chun)與(yu)(yu)濃(nong)(nong)(nong)(nong)縮，其中最普遍的(de)(de)(de)(de)(de)應用(yong)實例(li)(li)便是在(zai)水(shui)處理工(gong)藝中，用(yong)反(fan)滲(shen)透(tou)(tou)(tou)(tou)(tou)技術將(jiang)(jiang)原(yuan)(yuan)水(shui)中的(de)(de)(de)(de)(de)無(wu)機離(li)子、細菌、病毒、有(you)機物及膠體(ti)(ti)等雜質(zhi)(zhi)(zhi)(zhi)去除，以獲得(de)高質(zhi)(zhi)(zhi)(zhi)量(liang)的(de)(de)(de)(de)(de)純(chun)(chun)凈(jing)水(shui)。