Chlorination of the Drain, OR Public Water Supply
The town of Drain, Oregon (pop. 1,170; see attached map) takes its public water supply from
nearby Elk Creek. Because the town has a population fewer than 10,000, the system was not
previously required to disinfect its water. Now, however, the Federal Long Term 2 Enhanced
Surface Water Treatment Rule (“LT-2″), December 15, 2005 and re-approved in 2016,
strengthens microbial controls for small systems, i.e., those systems serving fewer than 10,000
people.
The new rules require Drain to disinfect its water, and include provisions specifically to address
Giardia cysts. (With a name like “Elk Creek” the EPA knew the supply could like be
contaminated by the wastes of large mammals! Beaver are also very common in the watershed
and are particularly likely to harbor this pathogen.) As to the new rule, the EPA summary states:
Among its provisions, the rule requires that a surface water system have sufficient treatment to reduce the
source water concentration of Giardia lamblia and viruses by at least 99.9 percent (3-log) and 99.99
percent (4-log), respectively.
In addition, 99%, 2-log removal of Cryptosporidium is required but that will be taken care of
later in a filtration unit.
The town system will serve no industries and only a few shops and restaurants, so the total water
use can be estimated from the U.S. national average for domestic water use which is 105 gpcd.
At this level of supply, it may be cost effective to chlorinate with either HTH or Cl2 gas.
Prepare a report to be submitted to Mr. Sal Monella, the Chief Health Officer for Drain. Your
report should summarize issues raised in the questions below.
Elk Creek River Data:
Average annual minimum temperature = 5˚C
Average annual maximum temperature = 24˚C
pH = 6.0
A chlorine demand test on the raw river water gives results as follows:
Elk Creek Chlorine Residuals
Cl Residual
Cl Dose, mg/L after 10 min, mg/L
0.4 0.23
0.8 0.57
1.2 0.86
1.6 1.15
2.0 1.47
2.4 1.71
2.8 1.92
3.2 1.76
3.6 1.40
4.2 0.93
4.6 0.74
5.0 0.98
5.4 1.50
5.8 1.96
6.2 2.35
6.6 2.74
The water system will pump the water to a contact tank that allows enough detention time to
meet the Ct standard. The water will be chlorinated immediately after the pump. A pipe
connecting the chlorinator to the detention tank will have a residence time at average flow of
about 10 minutes. This is enough time for all the chlorine demand reactions to go to completion
by the time the water enters the detention tank, so you can use the demand curve data provided.
It is recommended that the contact concentration be maintained at 2.0 mg/L because Giardia is
common in surface waters in this area. Preliminary tests indicate that the residual Cl
concentration is fairly stable after the 10 minutes spent in the supply pipes, but to be on the safe
side, is proposed that there be residual chlorine concentration of 2.2 mg/L in the water when it
enters the tank (i.e., after 10 min of contact in the pipe).

1. Sketch the chlorine demand curve as shown in the text and in the class handout (overhead
   slide). What is the breakpoint dosage?
2. Find the dose of chlorine (mg/L) that must be used to achieve a (post-breakthrough)
   residual chlorine concentration of 2.2 mg/L.
3. Determine many pounds of liquified chlorine gas per day must be used to treat the
   average daily flow in the system.
4. Determine how many pounds of HTH per day must be used to treat the average flow of
   water through the system. Note that the HTH is described as having “70% chlorine
   content as Cl2″. This means that 1.0 lb of HTH produces the same amount of hypochlorite

Sample Solution