1908.] Manurial Value of Sewage Sludge. 691 



referred to the subject* in connection with the land purification 

 of sewage and they have also devoted a good deal of attention 

 to the question of the possible value of sewage sludge as a 

 manure. 



Manurial Constituents in Sewage. — The most important 

 manurial constituent of sewage is the ammonia produced 

 by the fermentation of the urea of the urine, but sewage 

 also contains organic nitrogen compounds in smaller quantity,, 

 together with phosphates and salts of potash. In the process 

 of sewage purification by artificial filtration, varying quantities 

 of the nitrogen of the ammonia and other compounds disappear, 

 partly, no doubt, from conversion into gaseous nitrogen, and 

 partly from being assimilated by vegetable growths, worms, 

 flies, &c. ; the remainder of the nitrogen is converted (in a well 

 purified effluent) into nitrate. The amount of nitrogen which 

 disappears during the treatment of sewage and sewage liquors 

 upon contact beds and percolating filters seems to be from 

 about 40 to 50 per cent. In the case of the filtration and irriga- 

 tion of sewage on land, the loss is about 60 per cent., but here 

 much of the nitrogen which has vanished has been used up by 

 growing crops. 



It is in the form of nitrate that nitrogen is taken up by most 

 plants. Much has been written about the loss to the country 

 arising from the non-utilisation of the nitrogen of sewage which 

 has been purified by artificial filtration, and the Commissioners 

 observe that it is a loss which is certainly to be deplored. It is, 

 however, frequently forgotten that when water-borne sewage 

 is purified on land, upon which grass or other crops are grown, 

 loss of nitrogen is, to a large extent, unavoidable in the colder 

 seasons of the year, since plants can only assimilate nitrate 

 very slowly in cold weather. As regards the production of 

 nitrate in the soil, it is well known that this is largely influenced 

 by temperature, coming practically to a standstill when the 

 temperature is very low, but it is probable that in this country 

 the temperature would very rarely be sufficiently low for a long 

 period, to arrest nitrification. 



There is, unfortunately, no economical method of extracting 



* Fifth Report of Royal Commission on Sewage Disposal [Cd. 4278. Price, 

 2s. gd.]. Appendix viii to Fifth Report : Manurial value of Sewage Sludge 

 [Cd. 4286. Price, 3a 7 .] 



