Practical preventive medicine . icularlyunder rural conditions where the volume of liquid is small.(Figs. 26 and 27). The soil must not be permitted to becomewater logged, otherwise aerobic decomposition will be super-seded by anaerobic, and a nuisance result. 1141 EXCRETA DISPOSAL 5 (b) Broad Irrigation.—The sewage effluent is dischargedinto irrigation furrows on cultivated land and permitted topercolate downward through the soil. This method is onlyadapted to open or porous soils, preferably in arid regions,where water for purposes of irrigation is at a premium. Underthese circumstances it m
Practical preventive medicine . icularlyunder rural conditions where the volume of liquid is small.(Figs. 26 and 27). The soil must not be permitted to becomewater logged, otherwise aerobic decomposition will be super-seded by anaerobic, and a nuisance result. 1141 EXCRETA DISPOSAL 5 (b) Broad Irrigation.—The sewage effluent is dischargedinto irrigation furrows on cultivated land and permitted topercolate downward through the soil. This method is onlyadapted to open or porous soils, preferably in arid regions,where water for purposes of irrigation is at a premium. Underthese circumstances it may pay for itself. It has not been asuccess in this country. (c) Intermittent Sand Filtration.—The effluent is applied in-termittently, soaks quickly away and most of the time thefilter is exposed to the air. With proper operation very goodresults are secured. On the other hand, if too large quantitiesof effluent are applied, or applied too frequently, the filter be-comes water logged and oxidation is prevented. Its use is,. Fig. 19.—Double contact beds for purification of sewage. Photograph of amodel in the American Museum. {Am. Mus. Nat. Hist. Guide Leaflet 33.) practically limited to areas of sandy soil. These should, beridged for winter operation to prevent freezing (Fig. 18). (d) Contact Beds.—These are similar in construction toroughing niters, but are operated int ermittently on the fill anddraw method. The suspended matter settles on the crushedstone. When the tanks or beds are full the action is septic oranaerobic, when empty the matter on the stone is subjectedto aerobic or oxidative action. The beds are usually operatedin series (Fig. 19). (e) Trickling Filters.—These are concrete vats, on the floorof which are open tile drains, and above which are several feetof crushed rock, over which the sewage effluent is sprinkled orsprayed. The interstices of the rock are exposed to the airat all times when properly operated. As a consequence com- n6 PRACTICAL PREVEN
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