Page 388 -

12.46 CHAPTER TWELVE

the backwash, regeneration, and rinse cycles. The concentration ratio is found by divid-
ing the throughput capacity in gallons per cubic foot of resin by the regeneration volume.
Multiply the concentration ratio by the concentration of the contaminant in the raw wa-
ter level to determine the average concentration to be discharged.
For example, since each cubic foot of softener resin requires about 80 gal for regen-
eration, let's assume for a specific installation that the throughput is 2,400 gal/ft 3 of resin
during the service cycle. The concentration factor is 2,400 divided by 80, or 30. This
means that the hardness level in the regenerant waste will be 30 times as high as in the
influent water. In most cases as of 1995, most regenerant wastes are discharged into the
local sanitary sewer. Many softeners are already in use on individual homes and are dis-
charging such wastes even in areas that contain objectionable substances such as barium
or radium. In the future, it is likely that these discharges will be restricted or prohibited.

Monitoring and Validating System Performance

The ion exchange process is quite predictable provided that the feedwater conditions do
not change and that the condition of the resin bed remains reasonably constant, free of
fouling and degradation. For this reason many ion exchange systems are designed with
nothing more than throughput monitoring and regeneration initiation based on through-
put. In the case of home water softeners, time demand regeneration based on an average
expected usage is often used. For treatment systems such as home softeners, the only con-
sequence of overrunning the ion exchange unit is hard water getting into the downstream
piping and perhaps affecting the ability of soap to suds in a shower. When a contaminant
is potentially harmful, failure to monitor could be unsafe. Centralized treatment systems
should have periodic monitoring of objectionable constituents to verify that the effluent
water quality from the ion exchange system remains within the desired limits. In certain
cases there is the possibility of an objectionable contaminant being dumped by the resin
if the system were to be overrun past exhaustion. In these cases, the need for periodic
monitoring becomes much more significant as there is the potential for the contaminant
level to reach significantly higher levels in the effluent than its level in the raw water.
Treated water reservoirs can store enough water to significantly reduce the impact of
dumping. However, continuous overruns can result in complete discharge and therefore
total negation of the treatment process, i.e., zero average removal.
Certain ion exchange processes depend on the substance being exchanged to be in a
certain ionic form before it can be successfully removed by ion exchange. Such is the
case of ions like barium, silicate (silica), arsenate (arsenic), selenate (selenium), and oth-
ers. In such cases, it is necessary to validate the effluent quality and monitor the feed-
water to be certain that the impurities are in the proper ionic state for removal. If pre-
treatment equipment is provided to alter the ionic form, it too must be monitored and
validated. Beyond these requirements, other parameters that should be monitored depend
on the availability of workforce, size of the system, and intent of the user to operate the
system in the most economical possible fashion. For instance, chemical concentration and
dosage used during regeneration are very necessary for determining the economics of the
operation and may be of little value for any other purpose.

Resin Replacement

The usual rate of resin deterioration in properly pretreated clean water systems is quite
slow and is somewhat predictable. Nevertheless, it is helpful to periodically monitor the
condition of the resin bed itself to verify the condition of the resin. Such monitoring en-