20 SEWAGE AND ITS PURIFICATION 



Some forty-six towns in England report in 1900 that they 

 are able by their situation to discharge their sewage direct 

 into the sea or into strong tidal rivers. Several have for 

 various reasons inaugurated or are experimenting upon bacterial 

 treatment. 



The application of the law has varied even in places close 

 together. As an instance, the Rathmines suburbs of Dublin, 

 with a population of 60,000, have been permitted for more 

 than twenty years to discharge their sewage direct into Dublin 

 Harbour during the first five hours of the ebb-tide, without any 

 previous treatment except an imperfect sedimentation, whereas 

 the city itself has had to lay down works for purifying its sewage 

 by chemical precipitation. The alleged nuisance from the 

 Rathmines outfall has been disputed,^ and from Dr. Adeney's 

 analyses of the sewages and sea waters, especially with reference 

 to the dissolved N, O, and COo,^ it was argued that the pollu- 

 tion was not serious. Parry and Adeney drew from their 

 results the following principles as to the discharge of sewage 

 into a tidal estuary similar to that of the Liffey : 



" I. The discharge should be made into a definite deep water 

 channel with a current setting seaward. 



*' 2. The width of the water in the deep channel at low tide 

 must be sufficient for the adequate dilution of the sewage. 



" 3. The whole of the sewage should reach open water before 

 the turn of the tide. 



"4. The heavy solids and the floating matter should be 

 separated from the sewage before its discharge. This can be 

 done efficiently by simple subsidence and screening." 



But it was precisely because these conditions had been only 

 partly fulfilled that great dissatisfaction arose, leading to much 

 litigation and to an inquiry by a Committee of the House 

 of Commons in 1905, which has not yet led to a decision. 



Sewage is lighter than sea water, therefore the discharge 

 should always be below the surface, in order that mixture may 

 occur by the agitation.^ 



It is necessary by careful float experiments to determine the 

 directions of the currents round the shore, and these tests should 

 be repeated at intervals, since changes are likely to occur. That 



^ Parry and Adeney on " The Discharge of Sewage into a Tidal Estuary," 

 Proceedings of the Institute of Civil Engineers, vol. cxlvii., part i., 1902. 



2 See Chapter III. 



2 "On the Hydraulic Conditions of Discharge," r/Wc G. B. Lath^n, Sanitary 

 Record, September 14, 1905. 



