124 SEWAGE AND ITS PURIFICATION 



quantity of a fertile garden soil. The general conditions of 

 nitrification may be recapitulated : 



(a) In every case the formation of ammonia by some other 

 organisms precedes the appearance of nitrous or nitric acid 

 (p. loi). 



(b) Some fixed base must be present to combine with the 

 acid formed. Therefore, in a sewage farm, if the soil is devoid 

 of lime, it must be added. Ordinary sewage contains fixed 

 alkali derived from washing soda, and any acid discharges are 

 generally neutralized by this and by the free ammonia. 

 E. Chuard^ observed that nitrification may occur in an acid 

 medium, but that it was very slow. Hence in strong manufac- 

 turing effluents a treatment with lime may be necessary before 

 nitrification will take place. 



(c) The solution must not be too strong nor too alkaline. 

 According to Warington, a 12 per cent, solution of urine is 

 the highest strength nitrifiable, and the highest alkalinity not 

 prohibitive corresponds to 44'6 parts per 100,000 NH3. Such 

 a strength only under such special circumstances would be ap- 

 proached in sewage, and in runnings from urinals, stables, etc., 

 dilution would be necessary. Letts' experiments tend to show that 

 large quantities of chlorides retard nitrification.^ Winogradsky 

 and Omeliansky^ found that sodium carbonate is essential to 

 nitric and nitrous organisms, and that the oxidation of the 

 nitrite and the growth of the former microbe are inseparable. 

 As to the products of hydrolysis, '* peptone in excessive amount 

 cannot alter the specific function of the microbe, but destroys 

 or completely checks it under certain conditions "; asparagin 

 (p. 107) is injurious ; urea is inert below 0*05 per cent. ; infusion 

 of hay (14 per cent.) is beneficial ; broth (8 per cent.) had no 

 effect. Two per cent, of urine increased the time required for 

 oxidation five times. Iron salts assist the process. " The 

 nitrite is much more sensitive than the nitrate bacterium to 

 nitrogenous substances such as peptone and asparagin. The 

 more complex, unstable, and for most microbes the more 

 assimilable the substance, the more injurious is its effect on 

 nitric organisms." 



{d) Darkness and free admission of air. 



In natural soil Warington proved that nitrification rapidly 



1 Comptes rendus, cxiv., i8i. 



2 Royal Commission on Sewage, 1902, vol. ii., pp. 481, 483. 



•^ Chem. Centr., ii., 132, 217, 264. The use of soap-water in plant cultivation is 

 well known. 



