BACKGROUND: A novel procedure that involved regeneration and recycling of ammonia and sulfuric acid from monosodium glutamate isoelectric supernatant with bipolar membrane electrodialysis (BMED) was proposed. As the performance of the membranes deteriorated during the batch runs, fouling of the cation-exchange membrane (CEM) in contact with the base cell was studied. RESULTS: During ten consecutive batches of BMED, some operating parameters deteriorated gradually. Using scanning electron microscopy observations, fouling deposits were found on the CEM surface on the base cell side. Using Fourier transform infrared spectroscopy and reversed-phase high-performance liquid chromatography (RP-HPLC), the organic fouling fraction of the CEM foulants was found to contain eight amino acids. Using X-ray energy-dispersive analysis, the mineral fouling fraction was shown to be mainly O and Ca elements, and a little Mg. Using X-ray diffraction, the inorganic foulant was identified as CaCO(3), mainly in the form of calcite and a little aragonite. CONCLUSION: The CEM was subject to membrane fouling consisting of an organic fouling fraction and a mineral fraction. The organic fraction occurred as ions with some positive charges from the isoelectric supernatant and probably existed in the form of amino acids or their peptides. The mineral fraction was mainly CaCO3 calcite and aragonite, and probably a little amorphous Ca and Mg hydroxides. (C) 2011 Society of Chemical Industry