22 ON THE DISPOSAL OF SEWAGE. 



ig subject to great variations, so that storm overflows are 

 necessary to prevent flooding of the sewers and drains in times 

 of heavy and sudden rainfall. The disposal of the sewage and 

 rain water at the outfall is a matter of very much greater 

 difficulty than is the case of the Separate System, on account 

 of the variable nature of the rainfall. Where the sewage flows 

 directly into the sea, or into a tidal non-potable river, this 

 difficulty does not present itself ; but even in these cases a 

 serious objection against the Combined System lies in the 

 comparatively larger conduits which are required. These, in 

 times of dry weather, have only a small stream of sewage 

 flowing down them, and their size precluding adequate flushing, 

 sewer gas is generated, which, filling the sewers, may find its 

 way to the surface by means of imperfectly trapped sinks, 

 gullies, or manholes, and cause a serious danger to the public 

 health. But in the case of inland towns and others, where 

 discharge direct into the sea cannot be practised, the treatment 

 and disposal of the sewage at the outfall is much facilitated, 

 when the quantity of liquid to be treated is somewhat uniform. 

 This is the case, whether the sewage is treated chemically, by 

 downward intermittent filtration, broad irrigation, or by any two 

 or all of these methods combined — the more uniform the daily 

 fiow, the more easily and eftectively it can be purified. 



Purification of the sewage is the object aimed at, and this 

 should be attained economically, efficiently, and without causing 

 any nuisance. The Separate System, by insuring a degree of 

 uniformity in the flow and quantity of the sewage, affords 

 facilities for its treatment which cannot be aimed at by any 

 combined system. The relative cost of the two is in favour of 

 the Separate System in most cases, for the drains and sewers 

 required are much smaller, and are laid at a much less depth, 

 and when the sewage has to be raised from a lower to a higher 

 level the smaller quantity of liquid reduces very much the cost 

 of pumping. Wherever, practicable, therefore, it is advisable to 

 deal with the sewage separately from the rainfall — the latter 

 being carried oft" by surface grades as far as possible. The 

 system of sewerage introduced by Colonel Waring, of Newport, 

 R.I., is probably the best illustration of the Separate System 

 that we have. It was first tried at Memphis on the Mississippi, 



