PLANKTON m WATER SUPPLY MEIER 407 



come through, the algae entirely disappear and treatment can be 

 omitted until the muddy waters subside. 



JELLYFISH IN THE WATER SUPPLY 



Occasionally, rare forms of plankton appear without apparent 

 cause or warning in the filter beds only to disappear as quickly. In 

 July 1927, when one of the filter beds was opened at Dalecarlia, 

 thousands of the tiny jellyfish, Craspedacusta sowerbii (Lankester), 

 came crowding to the light from all directions. After 2 weeks all 

 had disappeared. No jellyfish were found in any of the reservoirs. 

 The filter beds were in total darkness and there is very little circu- 

 lation of air in them. At that time, their temperature varied from 

 71" to 75° F. During the time that the jellyfish were found in the 

 filtration plant, the water 8 miles above this point was not being 

 treated with alum and their sudden disappearance coincided with the 

 resumption of the alum treatment. Scrapings of the filter beds re- 

 vealed no trace of the hydroid generation, and their sudden visita- 

 tion remains a mystery to this day. 



CONTROL OF ALGAE 



When there is difficulty with your water supply and you make a 

 complaint to your city water department, each complaint is recorded 

 and investigated. Four types of examinations are included in this 

 investigation. The physical examination concerns the temperature, 

 turbidity, color, and odor of both the hot and cold water. The micro- 

 scopical examination relates to the quantity of microscopic organisms 

 per cc, as well as to the amount of inorganic and amorphous matter. 

 The bacteriological examination has to do with the number of bac- 

 teria per cc. and the presence of Bacillus coli and other bacteria 

 associated with pollution. Finally, the chemical examination may 

 include tests to show the total residue on evaporation, loss on ignition, 

 fixed solids, alkalinity, hardness, incrustants, chlorine, iron, nitrogen 

 as free ammonia, nitrogen as nitrites, total organic nitrogen, oxygen 

 consumption, dissolved oxygen, and free carbonic acid, depending 

 on the nature of the complaint. 



Excessive growth of algae in reservoirs is usually controlled by 

 the application of coagulants or algicides to the water. A coagulant 

 acts indirectly by causing a chemical change in the water by which 

 a flocculent or floe, a gelatinous material, is formed. This acts as 

 a net entangling the algae, and then the whole mass sinks by its own 

 weight to the bottom. Coagulants are generally those compounds 

 that at contact with water form hydroxides of iron or aluminum in 

 colloidal form. The compound used and the resultant reaction varies 

 according to the chemical qualities of the water. 



