SEWAGE PURIFICATION 283 



of the sewage being quickly mixed with a large excess of water, 

 so that at all times the oxygen content is maintained at such 

 a point that ofiensive products cannot be produced. 



The extensive investigations of Letts and Adeney on the 

 pollution of estuaries and tidal waters have resulted in the 

 suggestion of various standards, in relation to the amount of 

 oxygen available in the mixed sewage and tidal water, to meet 

 the different conditions of discharge. 



In the majority of cases, even where the dilution method is 

 resorted to, and still more when it is a question of purification 

 by application to land or artificial filter beds, some form of 

 tank treatment is required, and the chemistry of this process 

 may now be considered. 



TANK TREATMENT OF SEWAGE 



When sewage passes through a tank of any description, 

 deposition of the heavier matters present will take place with 

 greater or less completeness according to the method of 

 construction of the tank, the rate at which the sewage passes 

 through and the addition, or otherwise, of chemical coagulants 

 to facihtate the deposition of the matters in emulsion. 



We may consider tank treatment, according as it is 

 directed, to effect one or other of the following results : — 



(a) Simple sedimentation ; 



(b) Anaerobic decomposition ; 



(c) Aerobic decomposition ; 



(d) Chemical clarification. 



(a) Simple Sedimentation.— In this case we shall expect 

 only the heavier matters in the sewage to be deposited, that is, 

 the mineral substances, sand and silt, etc., together with paper, 

 faeces, grease and soap. The character of the effluent will 

 depend on the dilution or strength of the original sewage, and 

 the distance between the sewage works and the source of the 



