24 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



amoebic cysts may survive in sterile normal saline, distilled water, river 

 water, or raw sewage for a few days to almost 3 months depending on 

 the temperature of the suspending medium. With a cyst concentration 

 of about 1 million per ml of fluid, the survival time may be expressed by 

 the equation: t = 87.2 x 10—*'°^^'', where t is the time in days and T tem- 

 perature in °C. The survival time is essentially the same whether the 

 suspending fluid is sterile normal saline, river water, or raw sewage. 



C. Sedimentation of Cysts of E. histolytica in Water or Sewage. As 

 stated before, the specific gravity of amoebic cysts was found to be about 

 1.06. Assuming that the modal diameter of amoebic cysts is 15 microns 

 (except in the small race which may have an average diameter of 10 

 microns), the settling rate in perfectly quiet, pure water is about 1 foot 

 in 16 hours at 10° C and in 10 hours at 25° C. Under natural conditions, 

 the settling of cysts in polluted water is bound to be much slower, since the 

 water is usually flowing and has convection currents and a higher specific 

 gravity than pure water. This, of course, refers to the isolated cysts only. 

 For those that are embedded in solid matter, the settling rate would be much 

 faster. Hence, while the embedded cysts may be settled out in polluted 

 water, one cannot depend on storage of water or sewage for removal of 

 cysts unless a long period of storage is used. 



D. Removal of Amoebic Cysts by Filtration, Flocculation and Sedi- 

 mentation, or by Both Methods. Spector, Baylis and Gullans (1934) have 

 shown the effectiveness of the rapid sand filter in removing cysts from 

 water. In the preparation of purified cyst suspensions for cysticidal 

 studies, the author (1944) has used a small sand filter composed of a 2- 

 inch layer of sand of size between 100 and 140 mesh to separate cysts 

 from starch grains and found that such a sand filter removes practically 

 all cysts from the suspension if the sand is fresh and undisturbed and if 

 no pressure is exerted on the filter. Brady and Black (1945) reported that 

 the portable pressure filters used by the Army removed 88% of the cysts 

 from the water filtered at a rate of 15 gallons per minute. The cyst 

 removal was increased to 99% if flocculation was applied to the water 

 before filtration. These authors also reported that the diatomaceous earth 

 filter developed by the Army removed almost all the cysts from the treated 

 water. 



Brady and Black (1945) reported that flocculation with good floe 

 formation followed by sedimentation for 2 hours removed over 99% of the 

 cysts from the supernatant. In a few tests made by the author (unpub- 

 lished data), it was found that good flocculation of a river water by the 

 use of alum followed by sedimentation for 2 hours removed practically all 

 the cysts (in water containing 100 cysts per ml) from the supernatant 

 of 10 cm in depth, and that the settled floes contained cysts in lumps and 

 unharmed. However, with poor flocculation, only 90% to 95% of the 

 cysts were removed. Hence, it seems that this method of water purifi- 

 cation, if properly done, may remove all the cysts in water. 



E. Resistance of Cysts of E. histolytica to Acids and Bases and Various 



