. The sanitation of cities. 70 ^bc Sewaoe tlrcatment problem dissolved oxygen from the water, or where theformation of banks of deposited sewage soHdsmight cause a nuisance, even if the water for dilu-tion were sufficient, preliminary or preparatorysewage treatment is necessary. Also, where thedisposal is on land, or by means of sand filters, inorder that the polluting effect of the sewage maybe so far diminished that a much greater quantitycan be disposed of per acre by these methods, pre-paratory treatment is required for the samereason. 71. 72 CHAPTER VII THE BIOLOGICAL TREATMENTOF SEWAGE S
. The sanitation of cities. 70 ^bc Sewaoe tlrcatment problem dissolved oxygen from the water, or where theformation of banks of deposited sewage soHdsmight cause a nuisance, even if the water for dilu-tion were sufficient, preliminary or preparatorysewage treatment is necessary. Also, where thedisposal is on land, or by means of sand filters, inorder that the polluting effect of the sewage maybe so far diminished that a much greater quantitycan be disposed of per acre by these methods, pre-paratory treatment is required for the samereason. 71. 72 CHAPTER VII THE BIOLOGICAL TREATMENTOF SEWAGE Sewage treatment is seldom undertaken at asewage disposal plant, regardless of the methodemployed, with the object of producing actualsewage purification. This would indeed be a verycostly matter, and fortunately is required onlyunder exceptional conditions, and is practicallyrestricted to the method known as sand is, however, the end or result whichevery method has in view. As this may seem aparadoxical statement unless further explained,it should be pointed out that the purification ofpolluted waters observed in nature is strictly abiological process; and that considerable time isrequired for its complete action, whether by fil-ters, on land, or by dilution. The object sought by sewage treatment is theremoval to a greater or lesser extent, as condi-tions require, of the floating matters, settling mat-ters, non-settling putrescible matters, and ob-jectionable bacteria. Trade wastes, which are 73
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