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Examination of the Effectiveness of Filter Candles (Ceramic shells with gravity and whole house filters)


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Report Prepared for: 

American Water Filter


1) Two Filter cases. These were constructed of aluminium and comprised two parts: 

a) Receiving vessel with top. 
b) Raw water vessel. This fitted on top of the receiving vessel and it was covered with a lid. There was a hole in the base into which the filter candle is fitted. When assembled correctly the raw water passes through the candle into the receiving vessel. 

2) Four bacteriological "BelKraft" Filter candles. 

Test Organisms: 

Salmonella typhi NCTC 786. 

Shigella dysenteriae Type 2. NCTC 2966. 

Vibrio cholerae Inaba NCTC 10732. 


1. Preliminary 

Two complete assemblies were tested for efficiency of filtration using Escherichia coli. During the course of these tests we found that two of the filter candles provided had hair-line cracks which allowed organisms through the filter. These were discarded and the two assemblies finally used gave complete retention of E. coli. There were no leaks thorough the seals. 

2. Test 

Between test runs the the Raw Water vessel was filled with 1% formalin which was allowed to run through the filter. This was followed by two lots of tap water. This treatment effectively killed off pathogenic test organisms and the washing effectively removed inhibitory traces of formalin. The following test sequence was followed: 

a) The raw water vessel was filled with tap water to a level of 3 cm above the top of the filter candle. 

b) Inoculum of test organisms (in broth culture) was added to give approximate contamination level of 100,000 per ml. This was mixed and a sample taken to confirm inoculum level. 

c) When the whole volume had passed through the filter samples were taken for detection of contaminating organisms and the absence of inhibitory levels of formaldehyde. 

Two filtrations were run for each test organism. 


OrganismTest organisms per ml 
Before Filtration
Test organism per 10ml 
After Filtration
Shigella dysenteriae390,000 
Salmonella typhi71,000 
Vibrio cholerae340,000 



1. Providing the physical integrity of the filter candles is intact these can be used to filter sterilise water. 

2. Success of the treatment is dependent on the apparatus being assembled correctly with no leaks and on the absence of cross contamination. 

Signed: N.J. Butler 
M.Sc., F.I.Biol., A.I.F.S.T

Salmonella typhi