Chlorine Monitoring and Dechlorination Techniques Handbook

3.1 Types of Responses
Chlorine Monitoring and Dechlorinating Techniques Handbook
APPROACH TO DECHLORINATION
The methods and equipment used to dechlorinate a water release will be dependent on the
type of water release encountered. For ease of discussion, three types of water releases have
been defined, as follows:
Emergency
Planned Short-term
Permanent
During an emergency water release, such as a water main break, the location and volume of
the water release are not known prior to its occurrence. Therefore, equipment, chemicals and
supplies needed for dechlorination must be assembled prior to the event, and they must be
ready to deploy in a very short time. Furthermore, as the conditions of the event are generally
not known until a crew arrives at the site, the dechlorination method used must be relatively
easy and quick to deploy in a variety of situations.
As an example, the GVWD has emergency dechlorination kits ready to deploy at all times
and are kept in their vehicles. Their response to an emergency water release is to place fibre
mesh bags containing granular sodium thiosulphate in the flow path of the released water
(the bags allow water to easily contact the chemical). These bags and ancillary equipment
needed to deploy the bags are made up in advance and are immediately available for a
response.
Where the location and volume of water to be released are known and can be controlled, this
is referred to as a planned release. Planned releases include events such as water main
flushing or concrete curing at a construction site. It should be noted that water originating
from a concrete curing operation likely has a high (basic) pH which can also be lethal to fish.
For a planned release, time is not as critical as an emergency event. Time is available for
preparation at the site. In these situations, the GVWD typically uses a solution of sodium
thiosulphate for dechlorination instead of the solid form. This solution is metered into the
flow of the released water. Using a solution of sodium thiosulphate is much more cost
effective than using bags of solid sodium thiosulphate as much less dechlorinating agent is
used in the dechlorination process.
Where chlorinated water will be released on a long term or permanent basis, such as in
cooling water systems, a more permanent approach to dechlorination is required. In these
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