Where these pipes take a different angle (forming an elbow), or have a junction one with the other, inspection chambers should be provided, having air-tight covers if in a situation where open grates would be objectionable; though these manhole or inspection chambers, where required and practicable, should be utilised to get the necessary passage of pure air through the pipes, as Fig. 999, the lower part of course being properly trapped and securely ventilated from above, as shown; for the mere fact of carrying a soil pipe up full bore above the eaves of the roof will not cause circulation, unless an air inlet is also provided at a lower level.
Great care should be taken to provide the fresh-air inlet of an area equal to or slightly larger than that of the outlets, which is an important item, very frequently neglected; and it is best, where practicable, to admit fresh air separately to each branch at that point where it joins the main house sewer; and, if this is not practicable, the main inlet at the chamber next the sewer should be large enough to supply the various branches which it has to receive; and where fresh air cannot be supplied by these chambers, it should be let in by means of shafts (sometimes in a wall) provided with mica flap ventilators, to prevent the back pressure of foul air.
Cowls are sometimes fixed at the top of the outlet of the ventilating pipes, with the object of increasing the up current; but this is attended with such variable results that an open pipe end, protected from birds'-nests, etc., by a copper wire cage, is preferred by most sanitarians; and if in a position where snow is likely to block up the top in winter, slits should be cut longitudinally in the top of the pipe'to give the necessary outlet.
In cases where these ventilating pipes are fixed to trees, they should be constantly examined, or provided with expansion joints, as the growth or movement of the tree might disturb the jointing.
All ventilation pipes should be coated with non-corrosive substance and with molten lead joints. It has been discovered that uncoated pipes very soon rust and crumble, the scales of rust falling to the bottom of the pipe, where it is connected to the drain, and here it accumulates, until a heap is formed which entirely blocks the egress of foul air (see Fig. 1000).
To ensure thorough cleanliness in sewers, it is necessary that a means of regular flushing should be adopted; and this can be done in several ways, - either by collecting the rain water in an automatic flushing tank connected to the sewer (in such a manner that foul gas cannot get out) at the head of the system, or by providing a constant supply of water to a flushing tank that can be regulated at will to discharge a certain quantity of water, automatically, as circumstances require - more frequent in warm weather than in cold - though it is not advisable to have too large a discharge, as it is apt to disturb the seals of the various traps on the branch sections, from 20 to 30 gallons being a sufficient quantity for flushing an average house sewer system.
Oftentimes drainage systems can be properly flushed by the frequent use of the bath when placed at the head of the system, and the smaller the drain pipe the greater the chance of sending down a volume of water the full bore of the pipe, which is absolutely necessary to give confidence and security.
Where there is no public sewer for the proper disposal of sewage, as in the case of isolated mansions and country houses, the sewage has to be dealt with by cesspools, or by surface or sub-irrigation, the latter method being the most preferable.