Dechlorination of Highly Chlorinated Water - Disinfection of Pipelines and Storage Facilities Field Guide - page 97

be mixed directly within the container prior to use, rather than
using premixed dechlorinating solutions provided by suppliers.
As part of the dechlorination procedure, samples can be collected
downstream of the feed point, analyzed for pH, dissolved oxygen,
and residual chlorine, and if necessary, the chemical feed rate
can be adjusted to ensure a nondetectable residual chlorine con-
centration prior to discharge.
Gravity feed systems are simple to operate, have minimal
equipment requirements, have been used effectively by various
utilities, and are inexpensive (low-density polyethylene carboys
equipped with spigots). Unless adjusted, chemical feed rates can
be expected to decrease slightly over time, however, as the avail-
able head within the carboy decreases during use. A disadvan-
tage of using this technique is that it involves field testing and
calculations for flow rates and water quality parameters to
adjust chemical feed rate. Field maintenance crews sometimes
prefer a method that does not involve field calculations.
Chemical metering pumps. This method for injecting a
dechlorinating solution is similar to the gravity feed method
except that a chemical metering pump is used to inject the
dechlorinating solution from a container into the water flow.
Chemical metering pumps are capable of delivering chemical
solutions over a wide range of flows, the flow rates (Figure 7-1)
are adjustable, and the pumps provide a constant chemical feed
rate.
Relative to a gravity feed system, this type of system requires
more equipment (e.g., storage container, pump, energy source,
and tubing), and costs are significantly higher. Although chemical
metering pumps may provide field personnel with a more repro-
ducible method, this feature involves much higher cost and
greater operator ability and attention during dechlorination.
Venturi injector systems. Venturi injectors are differential
pressure injection devices that allow for the injection of liquids
(e.g., dechlorinating solutions from hydrants, etc.) into a pressur-
ized water stream. This method is well suited for pressurized
water releases such as those through hydrants.