SEWAGE OUTFALLS AND DISCHARGE 317 



(a) Sewers and drains opening directly into the nearest body of 

 water. This primitive method is still common, although it is 

 not even excusable in remote districts, as it is now known that 

 small and simple installations with tank and filters can be easily 

 constructed and work well. 



(6) Tunnels or pipe lines to convey the sewage to a long distance. 

 The Manchester " Culvert Scheme " of 1899 proposed to convey 

 the effluent to the tidal river Mersey at Randall's sluices, by a 

 circular tunnel 6| ft. diam., with a fall of i in 2,100, and a 

 full discharging capacity of about 67,000,000 gallons a day. 

 The scheme was not adopted, as considerable nuisance was 

 anticipated. Eventually a tunnel-syphon 5 ft. diam., carrying 

 bacteria-bed effluent under the river Mersey, with a shaft at each 

 end, was constructed. On the further side of the river the tunnel 

 communicates with a supply channel 13 feet wide, following 

 the contour of the high land, from which the effluent radiates 

 over the land in subsidiary carriers 18 inches wide, with disc 

 valves on the upper ends in the carrier wall, so that any length 

 can be opened or shut at will. The main drain lies midway 

 between these carriers, communicating with an open channel 

 delivering into the Ship Canal at a point over a tumbling bay, 

 placed sufficiently far back from the canal bank to minimise 

 any current prejudicial to navigation. 



In Chap. I., p. 19, I have already discussed sewage discharge 

 into the sea. Letts and his colleagues have studied the effect 

 of the admixture of sewage with a body of sea water,^ and 

 conclude that (i) the first effect is chiefly absorption of O and 

 production of almost the equivalent amount of COg ; (2) com- 

 parisons between the " oxygen-consumed " test and the O 

 actually absorbed as determined by analyses of the dissolved 

 gases prove that *' a more energetic oxidation is induced by 

 micro-organisms and free oxygen than by the nascent oxygen 

 of the permanganate solution " ; (3) the nitrifying organism 

 can grow in sea water, but a large quantity of free ammonia 

 remains for a long time unnitrified ; (4) under no circumstances 

 is a nuisance likely to arise from sea water contaminated by 

 sewage up to i per cent. ; but the changes in the nitrogenous 

 constituents as measured by the free and albuminoid ammonia 

 and the nitrates are very slow. (See further Chap. I., p. 19.) 



Purvis and Coleman in a series of experiments just published^ 



1 Proc. R. Dublin Soc, July 27, 1900. 



- y. R. San. Inst., xxviii,, No. 8, 1906, p. 433. 



