Comparative Cost of Treatment of Sewage by Land TABLE VI. Land Treatment. Class of Soil and Subsoil, and Method of Working. * Cost per head of population draining to works. Class i. Sub-class a. Filtration with cropping Sub-class c. Surface irrigation with Class ii. Surface irrigation with cropping TABLE VII. Contact Beds: Methods of Treatment. Contact Beds. *Cost per head of population draining to works. £ s. d. Chemical precipitation, quiescent settlement, followed by single contact beds Chemical precipitation, continuous flow settlement, Quiescent settlement, followed by double contact beds contact beds Septic tanks, followed by double contact beds 0 I 4 O I 61/ O I 7 O I 9 *This is calculated on a population of 30,000, and on a flow of 33 gallons of sewage per head per day, and assuming that really suitable land can be purchased at £ico per acre. The relative cost must depend on a variety of circumstances, which will differ in almost every case. CHAPTER XXIV. Percolating Filters. INTRODUCTORY. With this class of filtration the tank effluent flowing on to the filtration beds passes down through them, and out by similar drains to contact beds, and does not remain in contact for any period, beyond the time of passage through the filter. Percolating or trickling filters give a greater rate of filtration than contact beds. The rate of filtration in the case of the former may, generally speaking, be double, or nearly double, that of the latter. Coarse material should be employed if the tank effluent contains much matter in suspension. Where the effluent is weak, fine material on shallow filters will generally suffice for satisfactory results. Local conditions and the nature of the sewage, in conjunction with the above features, will determine the circumstances under which filters should be used. Distributing Tank Effluents over Percolating Filters. There may be considered to be five chief methods of effecting this: (1) A springler which moves, either automatically, or by applied power, dropping or spraying the liquid as it rotates or travels backwards and forwards over an area of filtering material (as Figs. 17-20).* (2) A stationary form of distributor, in which the liquid to be distributed is carried over the filter area by means of pipes and forced under head of water through a large number of orifices, so that it is broken up into fine jets, and thus sprays the effluent over the media. (3) A form of distribution in which definite volumes of liquid are automatically delivered in flushes. (4) Dripping trays, a stationary form of distributor invented by Mr. F. Wallis Stoddart, by which the *See pp. 89-92. G sewage liquid is carried over the filtering area in perforated trays of corrugated iron, from which it falls upon the filtering material in a succession of drips (Figs. 15-16a).* * (5) Tipping troughs, a stationary form of distributor, consisting of troughs balanced on pivots in such a way that, when filled, they tip and discharge the sewage on to the filter bed, and then return to their original position (Figs. 21-22a).† This method is, generally speaking, more suited to small and private installations. Construction of Filter Base. If the materials employed are coarse in character, large quantities of the solids in suspension in the tank liquor will make their way through and pass away with the "finished effluent. In this way aeration is practically prevented, owing to choking of the bottom layers in the material which arrests some of this suspended matter. From time to time the upper part of the filter material should be washed or riddled, to remove the matters in suspension therefrom. The layers of material in the lower portions should be such as to permit free passage of air through the bed. The filters themselves need little attention, the uppermost layers requiring only occasional raking; the lower ones are not touched. Some Filters and Apparatus Described.‡ Stoddart's Continuous Filter. At Horfield (Bristol) Stoddart's Continuous Sewage Filter is used (Fig. 15). For his system the inventor claims that it satisfies the following requirements : (1) Utilisation of the whole available "fall." (2) Instantaneous adaptability, without adjustment, to all rates of flow, however varied. (3) Treatment of sewage and storm-water on the same area. (4) Total absence of moving parts. See pp. 83, 84 and 86. † See pp. 128-131. In outlining some of the features of one or two varieties of filtering apparatus, it is by no means the intention of the writer to infer that other kinds do not possess valuable features. B C At Horfield, gas works clinker, graded as nearly as possible to 3 inches in diameter, and a depth of 7 feet 5 inches of filtering material, is used. The grading of this This filter can, if desired, be entirely closed, so that the sewage is never in a position to diffuse odour. H Fig. 16. Key to parts of the Stoddart Sewage Filter. |