590 TROPICAL HYGIENE, SANITATION, ETC. 



have been adopted, the most recent being the " Tonne Vidangeuse 

 Pneumatique," but it is a costly method. 



No town with a population of over 15,000 can deal efficiently with 

 its excreta other than by a pipe-water system. 



The Disposal of Excreta. 



Apart from what has already been said generally on this subject 

 one may include the following remarks: — 



(i) Excreta may be collected into cesspools and pits and then used 

 for agricultural purposes on farms as previously stated. 



(2) It may be carried to an incinerator and destroyed completely, 



but such is regarded generally as being a needless waste. 



(3) It can be carried by a water-carriage system in the case of larger 



towns and discharged into large rivers, or by a pipe out to 

 sea. The failings of the latter system are waste, offensive 

 odours and troubles from the ebb tide. 



(4) After a water-carriage system it can be treated by precipitation 



.with or without filtration, intermittent filtration through land, 

 broad irrigation, or bacterial treatment. Special works should 

 be consulted for technical details. The principles have been 

 dealt with under Water Purification. 



(5) Chemical processes are used apart from the one mentioned 



above proposed by Coplane and Menzies. 



The " Poudrette " method is fairly well known. 



After fermentation by urea, volatile ammonium carbonate is 

 formed which is usually fixed with calcium sulphate. The 

 sewage is then desiccated by evaporation in numerous shallow 

 pans. 



The resulting powder is known as " Poudrette," and is sold 

 to farmers for manure. The process is costly and the demand 

 is usually small. Two tons of excreta yield about two 

 hundredweight of poudrette. The system is unsuitable where 

 there is much rain. There are various modifications of this 

 system. 



(6) Trenching and cultivation are perhaps the methods most com- 



monly employed in hot climates. The trenching ground must 

 be high, well drained, with a loamy soil, to the leeward of 

 the village or town, and well screened from the public view. 

 The area should be large enough to deal with all excreta from 

 the population and divided into twelve monthly plots for 

 trenching. Tiie trenches should be o'6 metre wide and 

 o'4 metre deep. They should be 0*3 metre apart. Each 

 trench should receive o'2 metre of excreta. The area required 

 for a population of 1,000 persons would be 14 square metres 



