REDUCING BACTERIA 363 



Cellulose fermentation, next to protein hydrolysis, is the most 

 important work of bacteria in sewage purification. Paper, cotton 

 fabric, wood and other cellulose-containing substances are rapidly 

 attacked by various organisms with the production of soluble 

 substances starches, sugars, acids and finally carbon dioxid, 

 methane and hydrogen. 



Probably fewer organisms possess the power of saponifying fat 

 than of liquefying proteins or hydrolyzing cellulose. For this 

 reason and also due to the fact that the fat tends to rise to the 

 surface out of the sphere of bacterial action, there is a great ten- 

 dency for the fat to accumulate. At times this may accumulate 

 around some solid and give rise to "grease balls" which cause 

 clogging of pipes. The fat which is acted upon by bacteria is 

 broken into fatty acids and glycerin. The fatty acids are quite 

 resistant to further bacterial activity, but the glycerin is rapidly 

 broken into simpler products. 



Oxidizing Bacteria. The complex microflora of the sewage must 

 have energy. This they get in a great degree from the oxidation 

 of the comparatively simple products yielded through the hydrolysis 

 of the proteins, carbohydrates and fats. These are changed prob- 

 ably similarly to- the acetic acid fermentation with the production 

 of acids and finally carbon dioxid and water. 



The ammonia liberated through the deaminization of the amino- 

 acids is oxidized by the Nitrosomonas to nitrous acid and by the 

 Nitromonas to nitric acid. 



Reducing Bacteria. The nitrites and nitrates formed by the 

 nitrifying bacteria are in a great measure reduced to free nitro- 

 gen through denitrification. The sulphur in the protein molecule is 

 liberated as sulphates, sulphur dioxid and hydrogen sulphid. The 

 sulphate formed is reduced to hydrogen sulphid. This reacts with 

 the small amounts of iron and other metals present with the result- 

 ing black residue of metallic sulphids always found on the bottoms 

 of tanks and streams in which sewage is decomposing. 



Each of these processes is going on simultaneously in sewage 

 and the one is dependent upon the other, there being a true bio- 

 logical cycle, as is pointed out by Whipple. 



"The decomposition and oxidation of the organic matter in 

 sewage are brought about by bacteria, and the bacteria serve as 

 food for protozoa and other forms of microscopic animal life. The 

 dissolved organic matter in sewage serves as food for algse. These 

 algse and protozoa are, in turn, consumed by rotifers and Crustacea, 

 while the latter form the basis of food supply for various aquatic 

 animals and fishes. Thus, there is a continuous biological cycle. 

 Again, animal forms require oxygen and produce carbonic acid, 

 while plants consume carbonic acid and produce oxygen. Where 

 these processes occur normally and with a proper equilibrium main- 



