Fertilizing elements in sewage. Value of sewage for irrigation. 98–111 Area required for sewage irrigation. Methods of applying the sewage. Maintenance of irrigated areas. CHAPTER VII. ESTIMATES OF COST Cost of material: of laying sewers and drains; of sand; of 112–128 excavating and refilling; of rock excavation; of concrete work; of valves; of dosing devices; of filling material for beds; of finishing and cleaning up. Table to show items to be considered in estimate of cost. LIST OF FIGURES A sub-surface irrigation sewage-disposal plant _Frontispiece_ FIGURE PAGE 1. Plan of settling tank 15 2. Longitudinal section of settling tank 17 3. Sketch of settling tank with longitudinal 19 partition wall 4. Forms used for building side walls for concrete 23 tank 5. View of settling tank, showing baffles, sludge 26 pipe, drain pipe, and inlet and outlet pipes 6. Section showing tank with concrete roof and form 28 for constructing roof 7. Form for manhole opening 30 8. Plan and longitudinal section of modified Imhoff 33 tank 9. Vertical cross-section of modified Imhoff tank 34 10. Sludge valve for floor of tank 38 11. Sludge valve for side wall of tank 39 12. Sluice gate valve made by Coffin Valve Co 40 13. Ordinary gate valve 40 14. English slide valve with wedge-lock handle 41 15. Flap valve attached to length of sewer pipe 42 16. Flap valve with metallic seat attached 43 17. Flap valve with loose-link hinges 44 18. Intermittent dosing apparatus made by Ansonia 45 Manufacturing Co. 19. Simplest form of automatic siphon 46 20. Van Vranken automatic siphon 47 21. Miller automatic siphon 48 22. Double alternating siphons of the “Merritt” type 49 23. Triple alternating siphons of the Miller type 50 24. Single “Merritt” automatic siphon 51 25. Air-lock siphon for admitting and releasing 52 sewage from each one of four beds in regular order 26. Plan and section of sub-surface irrigation system 61 27. Plan and section of a portion of a sub-surface 62 irrigation system