. The sanitation of cities. 82 CHAPTER VIII DIFFICULT PROBLEMS FOUND INTREATING SEWAGE One of the greatest difficulties connected withsewage treatment is the disposal of sludge fromseptic tanks and settling tanks, as the sludge fromthese tanks is highly putrescible. This has led tothe invention of two forms of tanks, each of whichattempts to carry the septic treatment of thesludge to such a point that when discharged it willnot cause a nuisance. These tanks depend upon the assumption thatwhen the so-called septic action has been carriedto its extreme, the unstable organic matters whichare atta
. The sanitation of cities. 82 CHAPTER VIII DIFFICULT PROBLEMS FOUND INTREATING SEWAGE One of the greatest difficulties connected withsewage treatment is the disposal of sludge fromseptic tanks and settling tanks, as the sludge fromthese tanks is highly putrescible. This has led tothe invention of two forms of tanks, each of whichattempts to carry the septic treatment of thesludge to such a point that when discharged it willnot cause a nuisance. These tanks depend upon the assumption thatwhen the so-called septic action has been carriedto its extreme, the unstable organic matters whichare attacked by anaerobic bacteria, become in-capable of further putrescence and dissolve, orsplit up; part going into solution, part into inertmatters, and part remaining as organics, that donot readily putrefy, which are deposited with themineral materials present on the bottom of thetank. The greatest problem met by designers of tankshas been to design a tank in which the reten- 83. 84 Difficult ip^roblems founb in treating Sewage tion of these putrefying matters would not causethem to mix with the incoming sewage, andrender it highly septic, or putrescent, as it leftthe tank. This was the great difTiculty with theoriginal septic tank. The first two-stage, or two-story, tank was in-vented in England about fifteen years ago andwas known as the Travis tank. It is providedwith two upper, or flowing-through chambers, thebottoms of which are inclined at an angle of about45 degrees and are provided at the lowest pointwith ports, or openings, into the lower chamberwhich was placed between the two upper cham-bers, through which the settlings and sludge passby gravity. The lower, or digesting, chamber hasits bottom, which is the deepest part of the tank,formed in one or more hoppers, or inverted pyra-mids, provided with sludge outlet pipes. Theupper part of this chamber is at the same eleva-tion as the upper part of the flowing-throughchambers, and a portion of the sewage
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