Page 343 - Water and wastewater engineering

8-12 WATER AND WASTEWATER ENGINEERING

• Fast rinse. This is a final rinse step. The fast rinse flows at the same flow rate as the
service flow rate to remove any remaining regenerating solution.
• Return to service. The column is put back in use.

Countercurrent Operation
In this mode of operation the regenerant is passed though the resin in the opposite direction to
that of the water being treated. Generally, the mode of operation is raw water flowing downward
and regenerant flow upward. In most cases, countercurrent operation will result in lower leak-
age and higher chemical efficiency than cocurrent operation. However, countercurrent opera-
tion is a more expensive design and is more complicated to operate. Countercurrent operation is
used where (1) high purity water is required, (2) chemical consumption must be minimized, or
(3) waste volume must be minimized.

Bypass
As noted previously, there will be some leakage of hardness through the column because the
passage of the saturation wave through the column is spread out, as shown in Figure 8-2 , and
because the high concentration of regenerant being released from the upper levels of the column
will “regenerate” lower portions of the column where polyvalent ions were not completely removed
in the regeneration cycle. The amount of leakage is usually less then 5 mg/L as CaCO 3 (Clifford,
1999). Thus, the treated water is softened far more than is necessary for normal consumer use.
Thus, passing the entire flow to satisfy demand through the column results in a larger column than
is necessary as well as consuming larger amounts of regeneration chemicals. In addition, very soft
water is often corrosive.
To improve the stability of the water and make it less corrosive while reducing costs, a por-
tion of the flow is bypassed around the column and blended with the treated water to achieve the
design hardness. The bypass flow is calculated by solving the mass balance for hardness at the
point where blending takes place. The mass balance of hardness is

Qtreated treated Qbypass bypass Qblended bleended (8-17)
C
C
C

and the flow balance is
Qtreated Qbypass Qblended (8-18)

3
where Q treated flow rate of raw water entering column for treatment, m /d
3
Q bypass flow rate of water that is not treated, m /d
Q blended total design flow rate
C treated concentration of hardness in the treated water, mg/L as CaCO 3
C bypass hardness of the raw water, mg/L as CaCO 3
C blended design final hardness, mg/L as CaCO 3

The bypass flow rate is determined by simultaneous solution of these two equations.