There are many ways to treat arsenic but most rely on converting all of the arsenic to a valence of +5 (some will be +3) which is easier to remove. The technologies for removal include adsorption on alumina, reverse osmosis membrane separation, ion exchange, or lime softening. Alumina may have a problem with progressive loss of capacity and may be suitable for only a few cycles.
Standard treatments oxidize any trivalent arsenic with peroxide, permanganate, ultraviolet light, or other oxidizer followed by precipitation, filtration, etc. as above. UV light and titanium oxide have improved oxidization efficiencies for other compounds but I haven't seen any data on arsenic yet.