144 SEWAGE AND ITS PURIFICATION 



absorbs most of its carbon from the air and its nitrogen from 

 nitrates, and requires its food to be well prepared before it is 

 assimilated. Excess of ammonia acts very unfavourably. 

 S. Cloetz found that lo parts of ammonia in 100,000 (a strength 

 not uncommon in sewage) was injurious, Deherain showed 

 that ammonium salts were prejudicial, and that soils which had 

 received a dose of them un pen forte remained sterile for several 

 years.i 



In the ditches conveying sewage that used to be so common, 

 and even in the open drains from cottages, it is noticed that the 

 channel remains black and barren till the sludgy solids have 

 had time to deposit or become fermented, and the soil to 

 reassert its action, when the liquid clears and loses its odour, 

 and a copious growth of vegetation arises. Therefore, in 

 cottage gardens and allotments the sewage is not applied to 

 the ground till it has been dissolved and fermented in pits or 

 cesspools. Such a process, when scattered over a neighbour- 

 hood, is sure to create a nuisance, but carefully managed and 

 conducted collectively in special large areas it has proved to be 

 fairly successful, as shown by the late Dr. Poore."^ 



The main faults of irrigation with raw sewage are therefore — 



(a) Choking and felting of the surface by organic slime. 



{h) A surplus of unprepared organic matter and of ammonia 

 over the wants of the plants. 



{c) In consequence of the above, a deficiency of oxygen and 

 of healthy action in the body of the soil. 



{d) Great inconstancy owing to season, temperature, and 

 cultivation. 



The faults {h) and {c) are avoided to a certain extent by the 

 second system — that of 



B. Intermittent irrigation with copious under drainage, which is 

 really using the land as a partially-regulated bacterial tank and 

 filter. If properly arranged, the drains act also as aerators, so 

 that the soil is more thoroughly supplied with oxygen, allowing 

 nitrification to proceed more actively and to greater depths. 

 At Merthyr Tydvil in 1871 '* 20 acres of a porous soil, drained 

 from 5 to 7 feet deep, were arranged by Mr. Bailey Denton in 

 four series of beds ; and over each series in succession the 

 drainage water from 50,000 inhabitants, more than one-third 

 of whom were connected with the sewers, was poured for six 



^ Chimie Agricole, 1892. 



2 " The Earth in Relation to Contagia," 1902. 



