2i6 SEWAGE AND ITS PURIFICATION 



In 1894, following the success at Barking, experiments on 

 the same principle were started at Sutton, Surrey. The filters 

 were of different materials, but again showed coke breeze to be 

 the best, with burnt ballast as a good second, the latter being 

 very simply constructed by digging out the clay to form a pit 

 about 3 feet deep, and filling it up with the same clay after 

 burning, the cost of a filter of this kind, having an area of 

 rather more than one-tenth of an acre being given as less than 

 £"100, including all charges. It will be remembered that the 

 cost of the Barking i-acre coke-filter was stated as ^2,000. 



The Sutton sewage (500,000 gallons) was at that time treated 

 with g grains of lime and 2 grains of ferrous sulphate per gallon, 

 and the settled liquid passed on to land. The soil, London clay, 

 acting unsatisfactorily, in 1895-96, on the advice of Mr. Dibdin, 

 two "fine -grain bacteria filters" were constructed for the 

 treatment of the precipitated effluent. The sludge from the 

 precipitation was pressed into cake at a cost of £"] per week, 

 but there was no demand for the product, and "the nuisance 

 which is inseparably connected with the process was highly 

 offensive." 



In November, 1896, chemical precipitation was definitely 

 abandoned, but an important feature of mechanical aid was 

 still retained, since the raw sewage was " screened from grosser 

 solids" by a revolving wire drum (p. 151). From 2 to 3 tons 

 of solid matter per 1,000,000 gallons of sewage thus escape 

 bacterial action.^ 



From the screen the liquid passed on to the top of pits filled 

 with coarse burnt ballast called variously " bacteria tanks " or 

 " coarse filters," analogous to the " roughing filters " of former 

 systems, but differing from them in the intention not only to 

 remove solid matter, but to alter it bacterially. The effluent, 

 though greatly improved, was liable to secondar}^ putrefaction, 

 therefore it was distributed by channels over fine beds of coke 

 breeze, whence it issued at intervals as a liquid usually clear 

 and deprived of offensive character. 



The coarse beds were constructed by filling the chemical pre- 

 cipitation tanks with burnt clay ballast 3J feet deep, the bottom 

 having a 6-inch main drain with screw-down outlet valve, and 

 3-inch branch drains 6 feet apart. The bed is charged to 



1 Th-udichum, Society of Engineers, December 5, 1898. In 1899 the rotary 

 screen was abandoned in favour of a detritus tank, which has now, in its turn, 

 been converted into a septic tank by trapping both inlet and outlet. 



