Sewage disposal . bear the weight of a man. Grass and vegetation sometimesflourish on top of the scum. An analysis of scum from a septictank at Worcester is cited in Table XLII. It was formerly believed that scum was essential to septic ac-tion. It has been shown, however, that scum is only incidentalto such action, and indeed may be distinctly detrimental to it,since the solids in the upper layers of the scum are removed fromthe sphere of most efficient septic decomposition. The scum isthickest in tanks receiving unscreened sewage, and particularly SEPTIC TANK SCUM 171 TABLE XLII COMPOSITION
Sewage disposal . bear the weight of a man. Grass and vegetation sometimesflourish on top of the scum. An analysis of scum from a septictank at Worcester is cited in Table XLII. It was formerly believed that scum was essential to septic ac-tion. It has been shown, however, that scum is only incidentalto such action, and indeed may be distinctly detrimental to it,since the solids in the upper layers of the scum are removed fromthe sphere of most efficient septic decomposition. The scum isthickest in tanks receiving unscreened sewage, and particularly SEPTIC TANK SCUM 171 TABLE XLII COMPOSITION OF SEPTIC SCUM AT WORCESTER Per cent. Silica, Si02 Iron sulphide, FeS 8-93 Iron, Fe (not united to sulphur) 1. 61 Sulphur, S, not as sulphide Alumina, A1203 Lime, CaO Carbon, C Hydrogen, H 6. 51 Nitrogen, N 5. 20 if the sewage contains a large proportion of street grain from horse droppings furnishes an excellent basis forthe growth of scum. In a given tank, the thickness of the. Fig. 48. Scum on Septic Tank at Worcester, Mass. scum varies with the seasons, and with the bacterial activity,although two tanks located in the same climate may have scumsdiffering widely in thickness. At Washington, Pa., the scum atthe upper end of a tank receiving unscreened sewage became sothick as to occupy the entire depth. When tanks receive well-screened sewage, or very dilute sewage, little or no scum is 172 PRELIMINARY TREATMENT BY SEPTIC PROCESS formed. This was the case with the experimental tanks atColumbus. Starting with a clean tank, the time required to establish septicaction varies, as does the intensity of the action, with the tem-perature, the action, of course, beginning sooner in the summermonths. The presence of certain kinds of bacteria is probably-necessary to start and to carry on septic action. These bac-teria, under ordinary conditions, exist in the sewage and soonestablish themselves in the tank. In certain cases, however,as discussed abov
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