84 SEWAGE AND ITS PURIFICATION 



Among aquatic animal forms that have caused serious 

 nuisance in their growth are the protozoa Uroglcsna and Symira, 

 the former fishy and oily, the latter the origin of the so-called 

 *' cucumber" smell at Boston, U.S.A., in 1881 and 1892 ; other 

 protozoa are Bursaria gastris (strong seaweed smell), Crypto- 

 monas (sweetish), Mallomonas (fishy). Fresh-water sponges and 

 Bryozoa have also given trouble, mainly in their decay. 



Whipple gives a classification of organisms and odours.^ 

 Of 1,404 waters examined in Massachusetts, only 275 were 

 inodorous ; the smell of 100 was reported as offensive, 92 dis- 

 agreeable, 458 vegetable or sweetish, 202 grassy, 84 mouldy, 

 47 fishy, and 146 aromatic. 



In an investigation as to the origin of the smell in Chichester 

 Harbour in 1901, that of the scum w^as undoubtedly due to 

 diatoms. The mud contained an abundance of confervoid 

 algae and of diatoms belonging to species known to be objection- 

 able in this sense, and the very marked odours noticed in the 

 samples I collected at different points were certainly produced 

 by these organisms, which, as we have seen, grow independently 

 of sewage, and some of them preferably in clean waters. 

 Diatoms flourish best in shallows with a muddy bottom, and 

 the periods of stagnation and aeration characteristic of such 

 mud-banks are distinctly favourable to these kinds of life. 



The Cladothrix so frequently noticed in mud samples is a 

 fungoid organism, often present in large numbers both in fresh 

 and brackish waters, whether running or stagnant, and in its 

 growth it generates a mouldy smell, which was well marked in 

 many of these specimens. 



The city sewage in the above case had been precipitated b}' 

 aluminoferric and lime, settled in tanks, then passed over land 

 into a tidal basin, and stored till the state of the tide admitted 

 of its discharge into the creek. It contained only 3 to 7 parts 

 per 100,000 of suspended matter, and the smells and deposition 

 in the harbour seemed to be derived from other sources. 



Survival of Pathogenic Organisms. 



This important point was raised at the Exeter Local Govern- 

 ment Board Inquiry, referring to the pathogenicity of the 

 product after anaerobic treatment, since it has been suggested 

 that, whilst cultivating the bacteria necessary for the destruc- 

 tion of the organic matter in sewage, the pathogenic organisms 



^ " Microscopy of Drinking- Water," p. 125. 



