DOWEX Ion Exchange Resins WATER CONDITIONING MANUAL

Ion Exchange Resin Operational Information
down into the bed, water can begin to channel through the resin bed, causing a portion of the bed to be
unused, ultimately resulting in a lower bed capacity. For this reason, the resin bed is backwashed prior to
regeneration to remove the filtered solids and reclassify the bed.
The backwash flow rate must be sufficient to expand the resin bed and remove the particulate without
losing resin from the top of bed. When the backwash flow is controlled at the backwash inlet, pressure is
sometimes released within the resin bed, causing the evolution of dissolved gases in and around the
resin particles. These gases tend to adhere to the resin particles and float them from the column. This
problem is alleviated by keeping the column under pressure during backwash.
In locations where water temperature fluctuates seasonally, water viscosity will change with temperature,
increasing as the temperature drops, thus affecting the expansion of the resin bed during the backwash
operation. Failure to adjust flow rates of the backwash operation with temperature fluctuations can result
in either insufficient backwash or loss of resin. Backwash flow rates as a function of temperature are
given in the individual datasheets and engineering brochures for DOWEX resins.
The percent expansion for an ion exchange vessel can be calculated using the formula below:
Backwash recommendations are as follows:
Fluidized Height
% Expansion =
× 100
SettledBedDepth
1. Expand the resin bed with a uniform flow of water sufficient to wash out particulates:
• 80–100% bed expansion for conventional polydispersed resins
• 60–80% bed expansion for uniform particle sized resins
2. Continue flow for two to three displacements of the unit freeboard volume.
3. Position the backwash expansion outlet distributor at the recommended expansion point plus 12 in.
(30 cm).
4. Maintain constant backwash water temperature year round, or adjust the backwash flow rate
according to the fluctuating water temperature.
5. Use degassed backwash water or operate the backwash under pressure.
6. For mixed beds, the backwash flow should be sufficient to expand the cation resin a minimum of 20%
for proper removal of particulate from the mixed bed.
Freeboard
IX bed depth
Underdrain
distributor
Figure 22. Diagram of backwash procedure.
IX Resin
Reference Document: Tech Facts 177-01753
Backwash
collector
Regenerant
inlet
Backwash outlet
Expanded
IX Resin
Backwash inlet
30 cm
DOWEX Ion Exchange Resins 51 Water Conditioning Manual