Downflow vs Upflow Brining Explained

An ion exchange softener can be regenerated by either downflow or upflow brining.  In downflow brining,  the brine flows through the softener from top to bottom, in the same direction that the service flow occurs (co-current).

In upflow brining, the brine flows through the softener from the bottom to the top, in the opposite direction of service flow (counter-current).  In general, upflow brining softener systems are more brine efficient than downflow systems.  For example, they are generally run at lower brining flow rates which increases contact time as discussed previously.  Another reason is the efficiency gains from co-current to counter-current regeneration.  Water efficiency is also generally better in upflow systems as these systems will have, on average, more capacity between regenerations with similar water use per regeneration.

At the beginning of a regeneration cycle, the resin highest in hardness is at the top of the bed, and the resin with the least amount of hardness is at the bottom of the bed.  In downflow (co-current) regeneration, hardness that is exchanged off of the resin during brining is pushed down the bed, where it continues to exchange on and off the resin until its pushed from the bed.  The freshest brine is being used on the most depleted portion of the bed.

In upflow (counter-current) regeneration, the hardness that is exchanged off of the resin during brining is pushed back up the bed, exiting the system at the top of the bed.  The freshest brine is being used to regenerated the least depleted portion of the bed.  This highly regenerated portion of resin acts as a polisher which decreases hardness leakage.  This allows an upflow regenerated softener to use a lower salt dose while achieving the same quality water (i.e. low hardness leakage).

Care must be taken when choosing upflow versus downflow regeneration as upflow regeneration is not suitable for all installations.  In particular, brine and slow rinse rates must be maintainable at a level low enough to prevent fluidizing of the resin bed.  It is most suitable in cases where feed water has low concentrations of iron and particulates.