A properly operated ion-exchange unit produces water with zero hardness, but with high corrosivity. Since a total hardness of 85 to 100 mg/l is the most desirable, treated water from the ion-exchange unit is generally blended with source water to raise hardness in the finished water. Blending is normally accomplished by metering both the effluent from the softener and added raw water. Meters are installed in both lines so that the operator can adjust and monitor the blend.