196 SEWAGE AND ITS PURIFICATION 



mixed with " the necessary quantity of sulphuric acid for the 

 purpose of fixing the ammonia," heated in tubular boilers to 

 120° C, evaporated till semi-solid, and reduced in a rotary 

 chamber to a dry powder, which is said to be worth £"] to £'8 

 per ton. J. A. Jones, sanitary engineer of Madras, has reported 

 in favour of the use of the system in India. It has been in use 

 at Stansted, Essex, since 1902. 



In large towns evaporation would be impossible ; as an 

 alternative, a bacterial treatment was proposed, with sterilization 

 of the sludge by acid and heat and reduction to manure. The 

 process used at Cassel, Germany, for treatment of sludge by 

 sulphuric acid, combined with the extraction of grease, is 

 described later, in Chapter XIV. See also English patent 21,856 

 of 1901. 



Town Refuse. 



The solid matters included under the general name of " dust," 

 as removed by carts, have of late years been destroyed by heat 

 in place of the former insanitary methods of shoots, sorting 

 yards, and " made ground," especially since a chance has 

 appeared of utilizing the energy derived from the burning. 

 The methods of disposal have included : 



I. Carting and Tipping on Waste Land. — Street dust is recom- 

 mended as manure by Strabo, Pliny, and Columella, and was 

 highly valued during the Middle Ages : its use is still continued 

 in many places where there is plenty of land.^ By-laws in 

 London and other localities enact that no land on which refuse 

 has been deposited can be built on until it has remained 

 untouched for at least seven years. Organic matter in such 

 made ground disappears very slowly. At Leicester^ some heaps 

 of refuse after one year retained 30 per cent, of organic matter, 

 and after nine years 27 per cent. Hering records a case which 

 indicated that garbage can remain in a decomposing condition 

 for hundreds of years, as evidenced by some excavations in the 

 city of Rome.^ We may conclude that " made ground " of this 

 kind remains unhealthy until it has been purified by frequent 

 ploughing and planting. An attempt has lately been made to 



1 For analyses, and an advocacy of the practice, see Casali, Staz. Sper. Ital 

 agrar., 1898, xxxi., 377. 



'^ Leicester Meeting of Cleansing Superintendents, 1899. 



3 Transactions of the American Public Health Association, xx., 196; also see a 

 paper by Dr. Savage on " The Self-purification of 'Made Soil.' " giving chemical 

 and bacteriological analyses; Journal of the Royal Sanitary Institute, vol. xxiv., 

 p. 442, 1903. 



