BACTERIAL PURIFICATION 243 



removed by a final small settling tank. The aeration of the 

 effluent was then satisfactory, and was maintained for some 

 days — sufficient, in fact, to allow, in most cases, the effluent to 

 reach the sea or come in contact with a large volume of natural 

 fully-aerated river water. 



Various Ways of Constructing the Sides of Bacteria 



Beds. 



1. Digging out the soil and replacing it with another material 

 such as ballast, clinker or coke (p. 216). 



2. Impervious walls constructed as tanks. These usually are 

 previous chemical-precipitation or sedimentation plants which 

 have been utilized for present practice. In the Manchester 

 experiments they were made with sloping sides (p. 274). 



3. Perforated walls, constructed in different ways : — 



(a) Col. Ducat's system of sloping drain pipes (p. 241). 



(b) Honey-comb or pigeon-hole brickwork, as in the 

 Whittaker-Bryant (p. 242). 



(c) Loose large pieces of clinker or stone, built up as a 

 wall, and filled in with filtering material. This form has been 

 used at Leeds and Guildford. Winslow speaks approvingly of 

 a filter of the kind at West Allis, near Milwaukee, " cinders and 

 clinker, from J to | in. diam., held together by large clinkers 

 on the outside, piled with a slight batter"; depth 8 ft., top 

 area 30 x 54 ft., concrete floor 40 x 60 ft. : covered by a roof 

 supported by brick piers, sides left open in summer and 

 protected by planks in winter.^ 



(d) PaUsading of tarred wood, braced and strengthened with 

 hoop-iron ; or " expanded metal " lattice work, galvanized or 

 tarred, to support the material of the bed. 



4. Sides consisting simply of the filtering material itself, 

 forming its own slope, as in Stoddart's filter (p. 247), and 

 Corbett's (p. 246). 



Method I is only possible on the " holding-up " system in a 



strong clay soil or with puddling. The lower part of the bed is 



of coarse material, and ventilating pipes run up the sides to 



above the upper surface. Methods i and 2 are not advisable 



for sprinkler beds, because they do not allow lateral aeration. 



In the so-called " streaming " bed, however, in which the liquid 



is distributed on the surface in periodic flushes, as from a 



1 J. Assoc. Eng. Soc, U.S., xxxiv. 6, 1905, p. 348. 



16 — 2 



