i62 SEWAGE AND ITS PURIFICATION 



period of maximum discharge, and this quantity may be roughly 

 estimated at one-seventh of the whole day's flow. 



In cases in which it is proposed to adapt tanks constructed 

 for chemical precipitation to the settling of sewage prior to 

 bacterial treatment their size must be augmented sufficiently to 

 allow time for the solution of the organic solids. Such tanks 

 are originally too large to act as grit chambers, and too small if 

 sludge is to be dissolved. 



The relation between occasional flushes and the steady 

 ordinary flow will vary with locality, and has to be specially 

 determined by gauging at intervals. A fairly constant average 

 from day to day will be found, with irregular interferences from 

 storms. On the combined system of sewerage these render j 

 necessary the large surplus capacity given in the Local Govern-j 

 ment Board regulations ; but even on the separate system they, 

 temporarily increase the volume. 



Forms of Settling Tanks, — These at first were simply earth or| 

 clay-lined reservoirs with flat bottoms, from which the settled J 

 liquid was drawn by siphon-pipes at a little distance from the 

 bottom, the soakage into the porous sides allowing great foul- 

 ness. Then iron tanks were constructed, with flat bottoms, 

 and outlet pipes placed generally at too Iowa level, the removal 

 of the sludge at intervals requiring emptying and drawing ofFj 

 with considerable labour. A further improvement was to make 

 the tank rectangular, four times as long as broad, with its lower 

 surface inclined i in 80 to i in 100 towards the inlet end. 

 Transverse walls, coming near to the surface, divided the tank, 

 so as to allow the sewage to deposit and flow over them, while 

 " scum plates " dipped from above, and intercepted any floating 

 matters. At the base of the transverse walls there were open- 

 ings allowing the sludge to gravitate, or be carefully swept 

 down, to a sump at the lowest point at the inlet end. At the 

 other end, the clarified liquid was drawn by a valve or a 

 floating arm. Santo Crimp gave as examples of the capacity 

 of settling tanks : Coventry, 42 per cent, of the day's flow ; 

 Birmingham and Burnley, 56 ; Leicester, 40 ; Wimbledon, 80 

 (designed for a large increase in population). In Germany it is 

 claimed that the best results are obtained from long, narrow 

 and shallow tanks. 



The quantity of sludge made is stated to average f ton per 

 day per 1,000 inhabitants, and the cost of filter-pressing to vary 

 from IS. lod. to 2s. 6d. per ton of sludge cake produced. The 



