Methods, systems, and techniques for removing ammonium from ammonia-containing water involve using a stack that has alternating product chambers and concentrate chambers for receiving ammonia-containing water and an acidic solution, respectively, with the chambers being bounded by alternating cation exchange membranes and proton permselective cation exchange membranes. Ammonium moves from the product chambers to the concentrate chambers across the CEMs and protons move from the concentrate chambers to the product chambers across the pCEMs when the stack is in use. An electrolyzer may also be used to convert the ammonium in the concentrate chambers into nitrogen.