COMBINED AND SEPARATE SYSTEMS OF SEWERAGE. 775 
required for a combined system of sewerage, a greater velocity could 
be obtained, and the sewers would be practically self-cleansing; and 
by the adoption of an efficient system of automatic flushing, the 
sewers would be kept free from all decomposed sewage, thereby in 
great measure preventing the generation of sewer gas, the escape of 
which is a very serious nuisance and the cause of much sickness. The 
large sewers required under the combined system of sewerage are 
frequently turned into immense sewer-gas retainers, and, in the* event 
of a sudden influx of large volumes of storm-water, permit the sewer 
gas to be displaced and to escape through the syphons, traps, and 
water-closets into the dwellings of the citizens. Under the separate 
system of sewerage this result is avoided by the rigorous exclusion of 
all rain-water. Again, in districts where the sewage is raised by 
pumping, the necessary machinery only for lifting the daily volume 
of sewage would be needed, and provision would not be made to lift 
the volume of storm-water, provision for such a contingency being 
necessary under a combined system of sewerage. 
The treatment of the sewage at the sewage disposal works would 
also be more economically carried out under the separate system, 
whether the treatment be one of irrigation or of chemical treatment. 
In either case if rainfall is admitted to the sewers the cost of treatment 
of large volumes of polluted rain-water would be much more costly, 
and the difficulty of effectual treatment would be augmented. In the 
case of the sewage being treated on sewage farms, the mammal 
properties of the sewage would be considerably reduced, and owing to 
the large volumes of sewage the effluent would not be of a satisfactory 
condition. 
The drains for discharge of storm-water falling within the 
district to be drained would be connected to the river or seaside, as 
the case may be, and the drains being laid with good gradients would 
discharge the storm-water with greater velocity, and consequently with 
smaller size of drain than would be the case if a combined system of 
sewerage were adopted with flat gradients, and the sewage disposal 
works some miles outside the drainage area. 
It has been contended by some writers that the dual system of 
drains required by private houses for the discharge of the sewage 
proper and the rain-water would be very expensive and a burden on 
the ratepayers ; but the rain-water drains might be connected to the 
street water-chanuelling, the water flowing down these and discharging 
into the storm-water drains by means of the street gullies. This 
system could in many instances be adopted with little additional cost 
to the householders beyond the cost of the sewage drain, and what 
the ratepayers paid for their own drain connections would be more 
than compensated for by the gain to the ratepayers in having the 
sewerage system carried out economically and effectually. 
The following general conclusion may fairly be arrived at : That 
where a town is situated upon a river or stream, or near the sea-coast, 
the drainage of the town would be more effectually and economically 
sewered under the separate system than under the combined system, 
when the exigencies of the particular case demand that the sewage 
should require special treatment, and where it is desirable that the 
river or stream should be kept free from pollution. 
