78 SEWAGE AND ITS PURIFICATION 



elsewhere swarm with minute insects {Podura aquatica), which, 

 escaping in myriads, often form a thick layer like soot on the 

 surface of the effluent. In thus escaping these animals carry 

 with them some of the nitrogenous constituents of the sewage 

 which they have devoured, but as yet there has been no 

 estimate of the quantity so removed. There are also species of 

 worms always present in the bacteria beds in considerable 

 numbers, which no doubt also feed on the sewage."^ 



Infusoria and other minute animals assist in the work of 

 purification by acting as scavengers ; their presence in vigorous 

 activity is a proof of good aeration. Amoebae require scarcely 

 any oxygen ; I have even found them in small numbers in 

 the sediment of the Exeter septic tank, which is practically 

 anaerobic. They also must act usefully in attacking nitro- 

 genous matter. 



Algae and water-plants assist in the purification of an effluent 

 by the nascent oxygen which they disengage from their green 

 parts. They can also absorb by their roots and white parts 

 ammonia and putrescent nitrogenous matter ; they require, of 

 course, clearing out at intervals to prevent the decayed portions 

 from reversing the process. J. Konig^ has found that the 

 higher water-plants, Elodea canadensis (Anacharis), Potamogeton, 

 Myriophyllum, and Ceratophyllum, take up the nitrogen they 

 require from organic substances in the water, such as asparagin 

 or albumose, but not urea ; and that the same is probably true 

 for Salvinia and Azolla, All these plants, together with Lemna 

 minor and L. polyrhiza, assimilate carbon from organic carbon 

 compounds when the water is free from COg. 



Much trouble is sometimes caused by the clogging of 

 conduits and pipes by growths of Beggiatoa, Cladothrix, 

 Crenothrix, and other filamentous organisms allied to fungi, 

 and producing earthy, sulphuretted, and other odours. These 

 organisms are undesirable, and an indication of faults in the 

 management of the process. 



Beggiatoa (Fig. 13), however, although it has been called 

 the ''sewage fungus," seems, according to Winogradsky 

 (p. y^), to have really oxidizing functions. It lives in water 

 containing sulphuretted hydrogen, such as sulphur springs, as 

 well as in sewage. 



Crenothrix (Fig. 14), Pylobolus, and fresh-water sponges, are 



^ British Association Report, Glasgow, 1901, p. 601. 



2 Zeits.f. Untersiich. Nalir. u. Geniissniittcl, 1900, vi., 377. 



