EXCRETA DISPOSAL (CONTINUED) 8. Purification processes are designed to treat the effluent from the various types of sedimentation basins to secure a stable effluent, that is, one which is not subject to further decomposition changes. Where satisfactory most of the nitrogen will have been changed to nitrates. All depend upon active oxidation, which is secured by microbial activities. The following methods are employed to secure this result: (a) Subsurface Irrigation.—The effluent is intermittently dis charged through one or more strings of open joint tile just under the surface of the ground. The liquid is thus discharged into the layer of soil where bacterial activities are greatest. The method is only adapted to open or porous soils, particularly under rural conditions where the volume of liquid is small. (Figs. 26 and 27). The soil must not be permitted to become water logged, otherwise aerobic decomposition will be super seded by anaerobic, and a nuisance result.
(b) Broad Irrigation.—The sewage effluent is discharged into irrigation furrows on cultivated land and permitted to percolate downward through the soil. This method is only adapted to open or porous soils, preferably in arid regions, where water for purposes of irrigation is at a premium. Under these circumstances it may pay for itself. It has not been a success in this country.
(c) Intermittent Sand Filtration.—The effluent is applied in termittently, soaks quickly away and most of the time the filter is exposed to the air. With proper operation very good results are secured. On the other hand, if too large quantities of effluent are applied, or applied too frequently, the filter be comes water logged and oxidation is prevented. Its use is practically limited to areas of sandy soil. These should be ridged for winter operation to prevent freezing (Fig. i8).
(d) Contact Beds.—These are similar in construction to roughing filters, but are operated int ermittently on the fill and draw method. The suspended matter settles on the crushed stone. When the tanks or beds are full the action is septic or anaerobic, when empty the matter on the stone is subjected to aerobic or oxidative action. The beds are usually operated
in series (Fig. 19).
(e) Trickling Filters.—These are concrete vats, on the floor of which are open tile drains, and above which are several feet of crushed rock, over which the sewage effluent is sprinkled or sprayed. The interstices of the rock are exposed to the air at all times when properly operated. As a consequence corn plete aeration is secured and the effluent is stable. They are well adapted to areas where sandy soil is not available (Fig. 20).
9. Finishing the first three methods de scribed above, the filtered effluent joins the ground water. In the latter two it is discharged from the beds into some water course. In order to destroy any pathogenic bacteria which have managed to pass through the treatment beds, the present practice is to sterilize the effluent by the employment of chlorine, either as compressed chlorine or bleach. The methods of application are similar to those used in water disinfection where they will be discussed in more detail. The quantity applied varies with the condition of the effluent. It is also applied to raw or unsettled sewage where other treatment is not used.
1o. Domestic Excreta will now consider the methods of excreta disposal available for isolated dwellings, for example those in unsewered portions of cities and towns, unsewered villages, farm homes, summer resorts, and camps.
It is essential that individuals under these circumstances be sufficiently educated in personal hygiene to realize the impor tance (r) of having some place about the dwelling where the excreta of all members of the household is concentrated, (2) that at the place of concentration the excreta is so cared for that it is not a further menace to their health, and (3) that modesty, decency, and cleanliness, are cultivated. Only where these are observed can soil pollution or contamination by ex creta be prevented (Figs. 21, 22), and domestic water supplies be safeguarded.