Systems and methods for purifying produced water comprise a closed loop cation exchange unit, a closed loop anion exchange unit, and an intermediate degasifier. The processes for purification of produced water, involves: providing a closed loop cation exchange unit and a closed loop anion exchange unit, the cation exchange unit and the anion exchange unit connected in series through an intermediate degasifier. The produced water may comprise elevated levels of total dissolved solids, sodium, carbonate and chloride ions. A cation exchange unit may comprise a strong acid cation resin bed, and the anion exchange unit may comprise a weak base anion resin bed. Each of the exchange units may further comprise a plurality of treatment zones, the treatment zones comprising at least an adsorption zone, a rinse zone, a regeneration zone, a pulsing zone and a backwash zone.