Bacterial Treatment of Sewage. 383 



(2) The air supplied to intermittent filters may be more than 

 five times the volume of the sewage treated, therefore more highly 

 'Oxidised effluents are attainable. 



(3) The filter does not deteriorate if properly made, the only 

 plugging which takes place being on the surface. 



It will be seen that, on the whole, the advantages rest with 

 percolating filters ; there are, however, circumstances in which con- 

 tact beds may be preferred ; this may be the case where the subsoil 

 is so stiff a clay that the beds may be watertight without any 

 retaining walls, and where there is little fall. 



The advantages of the bacterial treatment of sewage is that the 

 smallest populations can be as effectually treated as the largest towns, 

 the area of ground required being small in comparison with land 

 treatment. 



In private houses the sewage varies greatly in strength and 

 quantity ; sudden flushes of water from baths causes too quick a flow 

 through the tank. This has to be provided against by ponding the 

 sewage in the tank, allowing only a small flow to pass through an 

 aperture, provision being made to pass any further increase or 

 excessive volume by an overflow pipe. 



Grease must be rigidly kept out of the tank. This is done by 

 providing efficient grease traps to catch the slop water from the 

 kitchen and pantry. The soapy water from laundries must also be 

 treated in the same manner, and the hot water should be cooled off 

 by having the traps of sufficient size to permit of this. These traps 

 require to be cleaned of the grease frequently. 



The great difficulty in all these small installations is to overcome 

 the smell which emanates from the effluent of the Septic Tank when 

 being distributed on the filter beds. In many cases it has been neces- 

 sary to construct these installations close to the dwellings, sometimes 

 within 40 feet, and, of course, if any smell, it is highly disagreeable 

 to the occupiers. As it is impossible to get rid of the smell entirely, 

 I find it best to enclose the filter bed in entirely on the top, and to 

 have a fresh air inlet and an outlet, which can be carried up, say, 

 12ft., or fixed against some adjoining wall or building, when any 

 smell will be carried up this vent pipe and distributed without 

 any annoyance. This is only necessary when the installation has to 

 be put down in a confined space. 



Percolating filters are, I think, more suitable for small installa- 

 tions, as an effluent is obtained by one bed quite equal to that 

 obtained from double contact, and the cost is less in construction. 



The best distribution is obtained by Stoddart's trays. The 

 liquid is brought into a gutter, overflows the margins, and, on reaching 

 the under surface, meets with a series of drip points, from which it 

 drops upon the filter. There are no mechanical parts about it, so 

 it is impossible to get out of order from this cause, and the only 

 attention required is the occasional brushing out of the troughs. 

 Another good point is that it is completely unaffected by variations 

 of flow, whether great or small. 



