Sedimentation tanks were simply tanks in which the sewage was left for a period of time during which some of the solids settled out. Plain sedimentation had been used with the early sewers in the nineteenth century to reduce the nuisance caused from sewage going into streams, but because the sludge was sometimes not removed allowing it to build up and occupy most of the space in the tanks, it was not considered a satisfactory method and was seldom seriously considered before the Royal Commission. It was thought to be "a process midway between chemical precipitation and septic tank treatment, but having the advantages of neither". Plain sedimentation won out for municipal sewerage works because it was good enough, not because it was technically superior, achieved a better effluent or even because it was considered a satisfactory treatment on its own, that is without a second stage of treatment. Sedimentation was simpler and more easily controlled. In many places, particularly at ocean outfalls, one stage processes were installed and sedimentation was definitely cheapest if that was all you were installing. Moreover, even where two stages were planned, the first stage was often built some time in advance and the tendency was to go for the cheapest solution with respect to short-term costs.
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