THE SEWAGE DISPOSAL PROBLEM 183 



principles underlying the treatment of sewage under varying 

 conditions, and we may look forward much more hopefully to 

 complete success in future. 



The chief methods of treatment which have survived the 

 severe tests of prolonged experience may be grouped as 

 follows : 



(1) Discharge into the sea or tidal estuary with or without 

 previous partial treatment. 



(2) Precipitation by means of chemicals, etc. 



(3) Biological purification : 



(a) Land treatment, with or without previous partial 



treatment. 



(b) Artificial bacterial methods. 



Besides these there are certain special processes which need 

 not be specifically referred to as they are so very little used. 



(1) Discharge into the Sea or Tidal Estuary. — Many authorities 

 still think that where practicable this is the simplest, cheapest, 

 and most expeditious method for the disposal of sewage. Neces- 

 sarily it must be limited to certain favourably situated towns. 

 Where the method is in actual use the sewage, generally in 

 a- crude condition, is discharged directly into the sea at ebb- 

 tide in order to carry it out to a great distance from the 

 shore, and to diffuse it well into the sea before the tide begins 

 to flow again. In some cases the sewage is partially or com- 

 pletely treated by precipitation before discharge into the sea. 

 Great care has to be exercised in deciding the proper time 

 for discharge. For this purpose careful observations have to 

 be made of surface tides and currents, rise and fall of tides, 

 configuration of coastline, etc. Storage tanks are sometimes 

 required in which the sewage is accumulated until the favour- 

 able moment for discharge into the sea. There is nothing 

 necessarily objectionable in this method of disposal if the 

 conditions are carefully studied and the town is favourably 

 situated. Sea-water certainly delays oxidation of organic 

 matter, and therefore, if the sewage is washed up on the 

 foreshore, one may find accumulations of offensive matter. 

 Furthermore, it has been shown that nuisances may arise in 

 respect of bathing grounds, or actual injury to health or popu- 

 lation, or spread of disease by shell-fish, etc. But broadly 

 speaking the method is unobjectionable in cases where the 

 volume of sea-water is large in proportion to the volume of 



