67 
Exercise 33. The Purification of Water 
MATERIALS: 
Sterile Berkefeld filter for faucet 
0.003 per cent of solution of chloride of lime 
1 sterile test tube 
5 sterile Petri dishes 
5 tubes of sodium caseinate agar 
Case of sterile 1 cc. pipettes 
Many cities must use water which is subject to contamination with harm¬ 
ful types of bacteria. "Such water must be rendered safe. To accomplish this, 
the water may be filtered slowly through sand filters; or by the addition of 
lime and an aluminum salt, a gelatinous precipitate can be produced in the 
water, and the bacteria will be held in the precipitate which can be removed 
by coarse filters. Chemicals which destroy bacteria and at the same time are 
not injurious to the consumer, are sometimes used to purify drinking water. 
Calcium hypochlorite is frequently used for this purpose. 
1. Collect some tap water in a sterile test tube and prepare a plate culture 
with 1 cc of the water. 
2. Add 1 cc. of a 0.003 per cent solution of chloride of lime to the 9 cc. of tap 
water in a sterile test tube and shake. This gives a solution of 1 part of chlorine 
in 1,000,000 parts of water. Permit the tube to stand 15 minutes and pre¬ 
pare two 1 cc. plate cultures from it. 
3. Collect some tap water as it passes through a sterile Berkefeld filter. 
Prepare two 1 cc. plate cultures from it. 
4. Incubate the cultures at room temperature for one week. 
5. Examine the 5 culture plates; count and record the number of bacteria 
in the water before and after treatment. 
(a) What was the effect of filtering water and of treating it with 
hypochlorite? 
( b ) Which method was the more efficient? 
(c) Which method is the more practicable for water supplies? 
( d ) What are the objections to unglazed porcelain filters? 
(e) What is the principle upon which hypochlorite is supposed to 
work in destroying bacteria? 
(/) What is the practical objection to the hypochlorite treatment 
of water supplies? 
