│—————│————— Total │ │ ───────────────────────────────────────────────────────────┴─────┴───── _Cost of Maintenance._—As to the cost of maintenance, very little that is definite can be said. Sub-surface irrigation plants should require no expenditure except for the occasional cleaning of the sedimentation tank. If this is emptied three times a year, the labor needed would amount to about a half-day’s time on each occasion for a family of ordinary size. For sand filters, either natural or artificial, the tank must be emptied as with sub-surface irrigation, and, in addition, the surface must be scraped occasionally, and at the approach of winter furrows must be dug. Perhaps two days’ time would be all that would be needed for a plant dealing with the sewage of a single family. A broken-stone bed requires no attention for seven or eight years, and then the stone has to be shovelled out, washed, and replaced. In none of the installations is this excessive in comparison with the benefits received, and it should not be considered a burden to expend this amount of time in maintaining so important a part of the household economy as the disposal of the household wastes in a sanitary manner. It must not be forgotten, however, that no sewage-disposal plant is exempt from occasional break-down or accidents, and that there must be a constant supervision exercised. This supervision should not require much more time than above suggested, but should be exercised for the purpose of correcting irregular flows or distribution before the value of the plant is utterly destroyed. INDEX Bacteria, action of, in reducing organic matter and sewage, 6 Baffle boards in settling tank, 30 Broad irrigation, 98 areas, maintenance of, 108 methods of applying sewage in, 104 sewage disposal by, area required, 102 when advisable, 110 Clay soils for sewage purification, 9 Cleaning settling tanks, 32 Composition of sewage, 4 Concrete for walls and floors of settling tanks, 24 walls, forms for constructing, 22 Contact beds, 87 alternating flow to, 89 general features of construction of, 89 principles involved, 88 table for use in constructing, 92 underdrainage of, 89 Cost items of sewage-disposal plant, table on which to base estimates, 125 Cost of broken stone, 124 of concrete, 119 of dosing devices or siphons, 122 of excavating and refilling, 115 of finishing and cleaning up, 124 of forms for concrete walls, 121 of laying sewers and drains, 113 of maintaining sewage-disposal plants, 127 of material, 113 for filter beds, 123 of rock excavation, 117 of sand, 114 of sewage-disposal plants, 112 of valves, 122 Crops, yield of, with and without sewage irrigation, 103 Disease, transmission of, by insects, 1