The Relative Applicability of Current Methods for the Determination of Putrescibility in Sewage Effluents.

Less attention was formerly paid to the question of the putrescibility of sewage effluents than is the case at the present time. For the most part the purification processes then embraced either broad irrigation fields and intermittent sand filtration, which normally yield effluents of a high degree of purity, or chemical precipitations which remove only about one-half of the total organic matter. Today there are many sewage problems in which purification requireto be carried apparently only to the extent of obtaining a non-putrescible effluent. For this purpose coarse-grain filters have a wide field of usefulness, and they also are a factor to be considered as an intermediate treatment in those sections where porous, sandy soil is not available naturally, and where thorough purification is needed. Contact filters, or sprinkling filters of broken stone, do not, of course, effect as high a degree of purification at their best as do filters of fine-grain material, and when unskilfully operated may yield effluents containing sufficient unstable organic matter to render them putrescible. To obtain the most satisfactory results from such processes, frequent data are required regarding the putrescibility of the effluent. The great need, therefore, as is universally recognized, is for a test, whereby a determination of the character of the effluent, so far as relates to its putrescibility, may be speedily made, and thus enable the results to serve as a direct guide in operating the plant. The putrescibility tests now in general use, from a practical standpoint, possess a common weakness in that nearly all, if indeed not all, are based upon a method whereby the samples of the effluent require incubation for several days before the result can be definitely known. So far as they are of assistance to the operators of