228 SEWAGE AND ITS PURIFICATION 



durability is unquestioned, as shown in brick. But when treated 

 in the cheaper way employed for roads and railway embank- 

 ments — that of piling the clay in alternate layers with small 

 coal and refuse, firing in places and allowing to burn slowly 

 till the mass has the usual brick-red colour — the burning 

 is apt to be irregular and insufficient, so that much of the 

 material on wetting becomes soft and crumbly. Burnt ballast 

 appears to be almost unknown in America, since the Massa- 

 chusetts Board did not include it in their investigations of 

 materials, and Rudolph Hering alludes to it in September, 

 igoo,^ as " a material made out of clay which is used in 

 England because they have very little sand." He says it is 

 not permanent, but crumbles, and adds that unless very hard, 

 coke behaves in the same way, observing that it is a question 

 for calculation whether it is cheaper to get a more expensive 

 permanent material, such as gravel or quartz sand, which will 

 last longer, though it may not purify so much sewage at first, 

 or to occasionally renew the material. 



Burnt ballast seems to have been first tried for this purpose 

 by Dibdin at Barking in 1891 (see p. 210), and was used after- 

 wards at Sutton and elsewhere for coarse primary beds (the 

 fine secondary ones being generally coke breeze, p. 216). At 

 Dorking and some other places besides Sutton, it has since 

 been found faulty, and has been replaced by clinker or other 

 material. That it is capable, if well made, of considerable 

 permanence is shown by an examination of a coarse bed at 

 Harrow, after two and a halt years' working. Washed samples 

 from different depths were " clean and red, and apparently as 

 hard as when it was laid down." In the washings, however, 

 *' the heavy particles consisted almost entirely of small sharp 

 particles of ballast " — such would occur with any material, 

 whatever its hardness, that had not been thoroughly washed at 

 first — with " not more than the slightest suspicion of ballast- 

 mud." The material lost on washing 4*44 per cent, of its 

 weight, made up of ballast dust and sand i'i4, raw clay 2'6o, 

 organic matter 07 per cent. The capacity of the beds was 

 considerably less than at first, but this was due, not to dis- 

 integration of the ballast, but to clay that had come in from 

 the top. At Belfast broken brick is used for the coarse beds.'- 

 At Friern Barnet Sewage Works, Middlesex, it was found that 



^ y. New England W. Works Assoc, xv. 2. The meaning is not given in 

 Webster's large dictionary, 1890, nor in Nuttall. 

 2 City Surveyor's Report, 1900. 



