Three biological methods of preliminary treatment of sewage are employed. (1) Mr. Scott Moncrieff passes the sewage slowly upwards through a filter 14 inches thick, consisting of successive layers of flint, coke, and gravel. This is called a “_cultivation tank_.” The solid sewage becomes liquefied in passing through this medium, the microbes in the filter dissolving the sewage. (2) In the “_septic tank method_,” introduced by Mr. Cameron at Exeter, a tank is employed which is covered in to exclude light, and to a large extent air. The tank is large enough to hold 24 hours’ flow of sewage. The microbes in the sewage under these conditions multiply rapidly, attack, and liquefy the sewage. As in the first process little or no sludge is left. The ultimate products of the decomposition are water, ammonia, and carbonic acid, and other gases. The effluent from the tank is comparatively clear and inoffensive. (3) _Aerobic biological filters_ are employed, as in Mr. Dibdin’s installation at Sutton, where the filtering material is coke. The sewage slowly passing through the filtering medium becomes liquefied, the solid matter being peptonised. This action is in part at least due to anaerobic microbes. The filtering beds are used intermittently to allow of aeration, and the liquefaction of solid organic particles entangled in the filter probably chiefly occurs at this stage. It is desirable to have small subsidence tanks, for the removal of large suspended matters and of road debris, etc., before the sewage is spread over the filtering beds. The material used in the filter varies. Most commonly coke-breeze has been employed, but coal slack and other material have also been utilised. After the preliminary treatment above described, the sewage requires to be passed over finer filtering beds, in which aerobic microbes complete the purification by changing the dissolved organic matter into inert inorganic compounds, by the process known as nitrification. The two processes run into one another, to some extent going on together. Hitherto the Local Government Board have required filtration of sewage through land before any sewage effluent is allowed to pass into a stream. In view of the successful results now obtainable by bacterial processes this requirement will be occasionally waived. It is unsafe to assume, however, that the clear effluent obtained is free from all disease-producing microbes; and drinking water should not be obtained from even a very large river below the point of discharge of such an effluent, without the most efficient sand filtration. CHAPTER XXIX. CONSERVANCY METHODS. The refuse to be removed from a house consists of fouled water, which is at least equal in quantity to the water-supply of the house; the excreta of the inhabitants; and “dust,” which contains, besides ashes, considerable kitchen refuse; consisting of both vegetable and animal matters.