There are several different methods for regenerating resins. The method chosen will largely determine how efficiently the ion exchange resins will operate and how complex the regeneration will be. The most commonly employed regeneration method is called coflow or cocurrent regeneration. The resin is regenerated in the same direction as the service flow. This method is used in almost all salt cycle exchangers, i.e., softening, dealkalization, etc. The usual cycle consists of a backwash to purge the resin bed of suspended solids and resin fines and fragments, followed by chemical injection of a solution of the regenerant salt, acid, or base through the resin bed, followed by a rinse cycle to flush the regenerant from the resin bed. Since the regenerant flow is in the same direction as the service flow, the ions at the top of the bed have to be pushed downward all the way through the resin bed before they can be purged. This makes the coflow method somewhat inefficient. It also leaves a portion of the exchanged ions remaining in the resin at the bottom of the bed, where they can cause leakage in subsequent service cycles. The leakage is highest at the beginning, and as the service cycle progresses, the leakage becomes less. The effect is more noticeable at low regenerant dose levels. This leakage phenomenon is very slight in softeners but very noticeable in nitrate removal salt exchange units. The nitrate leakage at the beginning of the cycle is significantly higher than...