These areas are gross, with allowance for growth, and necessitate compact arrangement. The amounts shown can be readily obtained in the vicinity of any of the suggested disposal localities, and are such that any of several tracts will fulfill requirements. At Squaw Island it is possible that the city may already have title to sufficient land above and below water to render unnecessary the acquisition of additional space, though the plans encroach upon private lands, with consequent provision for acquiring the same. At the Hertel Avenue site it may be that the cheapest land will include that portion containing the bed of Cornelius Creek, in which case a conduit or channel will be needed to handle the storm water from the Tonawanda Street overflow. The estimate includes allowance for an earth channel. At Squaw Island it will be advisable to reclaim certain areas by filling behind timber crib bulkheads, and to raise the general level of the land by the use of surplus materials of excavation. Further, in order to protect the tanks and sludge beds, it is advisable to inclose the treatment area by a dike on the canal side. These items have been included as part of the disposal cost. Elevations. The general requirement that the plants operate normally with the lake at elevation 575, and by increased pumping lift with the lake at elevation 578, has been met in the different plants by elevations as shown in Table 138. object has been to collect sufficient data for estimate purposes, and to present such detail as will permit of a review to check the conclusions. In the event of more critical tank studies as antecedent to construc tion, it is suggested that attention might well be devoted to any economics possibly resulting from the use of separate sludge digestion tanks. Such a plan would increase the masonry and acreage items, and would also require additional pumping of sludge, but would offer advantages in a decrease of excavation below water level. Rectangular tanks with party walls are more economic of material than are circular units, but there exists some question as to how far it is advisable to extend this economy through the lengthening of tanks and consequent increase in the sludge pocket area in any one unit. The designs made are for tanks 107 feet in length, with three digestion pits, wh ch are, however, interconnected. Of tanks built or building, those at Schenectady are the longest, being approxi mately 100 feet in length, and these have not as yet been in operation. The distribution system, also, is patterned upon the Schenectady layout, and will bear study of its likelihood to cause trouble through silting in the submerged influent channels. FINE SCREENS. Recourse to fine screens for the clarifying treatment affects the plant layout previously described by substi tuting screens and their mechanical adjuncts for the tanks; by the elimination of sludge beds, and by the addition of means for securing time contact between the screened sewage and the disinfecting chemical. These also result in a lessening of the area needed for treatment works. Although estimates have been made of the cost of screening and disinfecting, it has not been with entire satisfaction that the performance as to the removal of organic matter is entirely suited to the requirements Screens undoubtedly have a place in the lessening or elimination of nuisances due to sewage pollution, but the present instance is one affecting water sup plies, and while the bacteriological aspect has been met by the inclusion of disinfection as an integral part of the treatment, the effect of organic matter of sewage origin must be considered both in its influence on the disinfection process and in its direct action on the water supplies. The statements of screen performances are in con siderable variance, and the serious consideration of this type of treatment should be conditioned upon sub stantiation of the claims of high efficiency in the removal of suspended matter. 1 Consult the exposition of available data included in the paper b Kenneth Allen, Transactions American Society Civil Engineers, vol. 78 Upon the assumption that fine screening is in itself adequate for the removal of organic matter, there is no valid reason for subsequent sedimentation. If short time secondary sedimentation be needed to secure sufficient removal of solids, it is probable that the latter can be most economically attained from sedimentation alone as resulting from the longer periods of detention. Following these statements to their conclusion it is designed to disinfect the screen effluent in a long channel wherein the screened sewage will flow at a velocity to prevent deposition of the fine solids and for an average period of 30 minutes as the time required for contact. Such channels are cheaper to construct than tanks, are more readily cleaned when necessary, while the maintenance of carrying velocities obviates the continual handling and disposal of sludge. It should be noted that at South Buffalo the sewage, subsequent to treatment, is discharged into an outfall line of such length that some 50 minutes will be consumed in reaching the harbor. This fact renders it possible to eliminate the contact channels at this site, although pronounced and sudden rises in lake level would at times cause an influx of diluting lake water which it might be necessary to offset by the use of additional disinfectant. The South Buffalo fine-screen plan and estimate herewith presented are to be regarded as typical, inasmuch as they were prepared to serve as the basis Coarse screen... Excavation, 1,540 cubic yards... well. Mechanical Pumps and engines... plant. Disinfection Building, 34,000 cubic feet..... Subdivision of work. Total cost. Amount. Unit price. Cost. $2.35 $3,619 Concrete, 197 cubic yards.... Screen, 3.4 tons.. 12.25 2,413 16,000 Boilers and incinerators... Buildings and piping, 143,000 cubic feet. 13,500 .15 21,450 Imhoff tanks... Excavation, 99,000 cubic yards..... Tank, concrete, 530 cubic yards.. Floors, 1,290 square yards... Appurtenances... Total, 1 unit. Influent chan- Concrete, 380 cubic yards.. nels. Appurtenances....... Effluent chan- Excavation, 4,900 cubic yards... nels. Masonry, 1,530 cubic yards........... Sludge beds.... Excavation, 5,160 cubic yards.. Concrete, 680 cubic yards.... The variation in the unit costs is to be explained first by the difference in per capita sewage flow in the different districts, and second by the fact that in each district there is a certain but variable amount of sewage attributed to outside areas for which no popula tion allowances have been made. Contract prices for large plants are not available in any number. The following costs, however, in addition to showing the variation to be expected, offer a guide for judging the units used. Wisner, in his Chicago studies,' derived per capita costs as follows, the sewage flow being 200 gallons per head per day: Imhoff tanks, Atlanta design. Dortmund tanks, Gloversville.. 607,169.00 37,950.00 569,219.00 Straight-flow tanks, Columbus (6-hour). $1.44 84 15 E |