In selecting a method of sewage disposal, the conditions, surroundings, and requirements of the city should be carefully studied and analyzed, and judgment and discretion must be used. A matter of so much importance to the community should be placed in the hands of men qualified to make a proper solution of the problem. While in general several methods of purification may be applied to the requirements of the city, usually local conditions and considerations will narrow the choice to two plans or possibly to a single method.

The governing features of the dilution process are so distinct that it is not usually difficult to determine where this method is applicable, and likewise, the broad irrigation process has peculiar conditions. In both, the plans involve only matters of construction.

The other methods have more in common and the determination of their relative value is not so easy. As before stated, the recently developed biolytic processes promise to displace chemical precipitation. Where a suitable deposit of sand or gravel is conveniently located on cheap land which may be made without great expense into filtration areas and the sewage discharged upon them by gravity, intermittent downward filtration may be the most satisfactory, especially if a highly purified effluent is desired. The item of expense of attendance, and labor of maintenance must be considered in connection with the cost of this method. In the absence of such favorable conditions and especially where complete purification is not required the septic tank may be the most suitable. For higher purification the combination of the septic tank with filter beds, or contact beds run at comparatively high rates, makes a satisfactory purification and is applicable to a wide range of conditions. A bacteria bed may be substituted for the septic tank for the roughing process but its applicability in not so general.

In conclusion it may be said that if the next ten years gives as much development in sewage purification as has the last decade, some of the processes herein outlined will have been discarded and sanitary engineering will have achieved still greater triumphs.