Chlorine Monitoring and Dechlorination Techniques Handbook

6.1.1 Steps
Chlorine Monitoring and Dechlorinating Techniques Handbook
Step 1 - Prepare for the situation. Ensure there is excess dechlorinating agent on-site.
Step 2 - Measure the TRC. Use a field testing kit to do so. In this case, we will assume
that the field testing kit indicated a TRC of 0.5 mg/L.
Step 3 - Record all of the activities during this situation. Use the record of activities
sheet attached, or keep a record book.
Step 4 - Prepare the stock solution. Refer to Table 3-3. To prepare a 2% stock solution
of sodium thiosulphate pentahydrate, one will need 20.3 grams of
dechlorinating agent per litre of water or 92.4 grams per gallon of water,
therefore to calculate how much would be needed to make a 45 gallon drum:
92.4 grams/gallon x 45 gallons = 4,158 grams or 4.2 kg,
where there are 1,000 grams in a kilogram.
Step 5 - Determine the flow rate of water. Using Table 3-4 and assuming a flow
velocity of 1.8 m/s, one notes that an 8" or 20 cm pipe has a typical flow rate
of 59 LPS (litres per second). To determine the volume of water that would
be discharged after 30 minutes, apply the following:
59 L/s * 60 s/minute * 30 minutes total = 106,200 L or 23,340 gallons 7
Step 6 - Determine the dosing rate. Using Table 3-5 for a 2% solution of sodium
thiosulphate pentahydrate, and cross-referencing the TRC at 0.5 mg/L, one
would need 354 mL of stock solution for every 10,000 litres of discharge
water. Since the water is flowing at a rate of 59 LPS, the dosing rate would
be:
(354 mL/10,000 L) * 59 L/s = 2.09 mL of dechlorinating agent per second or,
2.09 mL/second * 60 seconds/minute = 125.3 mL/minute
Step 7 - Add the dechlorinating agent by using a chemical metering pump. The
chemical metering pump can be attached to a 45 gallon/204 litre drum. An
example of this set-up is shown in Figure 4-1. As in Figure 4-1, the
7
Remember that 1 litre = 4.55 gallons.
Page 40