Membrane　fouling　significantly　limits　the　application　of　membrane　distillation　(MD)　in　wastewater　treatment,　and　organic　fouling　is　one　important　challenge,　especially　in　the　presence　of　inorganic　salts.　This　study　aimed　at　the　impact　of　various　concentrations　of　cations　including　Ca2+,　Mg2+　and　aluminum　on　organic　fouling　by　sodium　alginate　(SA)　and　humic　acid　(HA).　The　results　showed　that　the　most　severe　SA　fouling　with　Ca2+　was　observed　at　1　mmol/L　in　the　range　of　0.5-10　mmol/L,　while　the　normalized　flux　decreased　from　0.84　to　0.55　when　Mg2+　increased　from　0.5　to　10　mmol/L.　The　influence　of　aluminum　concentration　on　SA　fouling　was　similar　to　that　of　Ca2+,　with　the　maximum　flux　decline　rate　of　52.3　%　at　5.0　mmol/L　aluminum.　The　addition　of　NaCl　aggravated　SA　fouling　with　Ca2+　and　aluminum,　but　alleviated　that　with　Mg2+.　Different　SA　fouling　phenomena　under　Ca2+　and　Mg2+　were　attributed　to　the　different　mechanisms　of　forming　transparent　extracellular　polymeric　particles.　Regardless　of　cation　types,　aggravated　HA　fouling　was　observed,　and　the　higher　the　cation　(Ca2+,　Mg2+　or　aluminum)　concentrations,　the　more　severe　the　HA　fouling.　These　findings　were　further　confirmed　by　Fourier　infrared　spectroscopy,　the　change　of　hydrophobicity,　and　scanning　electron　microscopy　coupled　with　energy　dispersive　X-ray　spectrometer.