The following are the drainage areas of a number of European and American streams yielding more or less muddy waters at points where they are used for public water-supplies after filtration, with a few other American points for comparison. The results are obtained in most cases from measurements of the best available maps.
| Place. | River. | Drainage Area, Square Miles. |
|---|---|---|
| New Orleans, La. | Mississippi | 1,261,000 |
| St. Louis, Mo. | Mississippi | 700,000 |
| St. Petersburg | Neva | 108,000 |
| Louisville, Ky. | Ohio | 90,000 |
| Rock Island, Ill. | Mississippi | 88,000 |
| Budapest | Danube | 79,000 |
| Cincinnati, O. | Ohio | 75,700 |
| Dordrecht | Maas | 68,000 |
| Rotterdam | Maas | 68,000 |
| Schiedam | Maas | 68,000 |
| Altona | Elbe | 52,000 |
| Hamburg | Elbe | 52,000 |
| Stettin | Oder | 40,000 |
| Magdeburg | Elbe | 36,000 |
| Warsaw | Weichsel | 34,000 |
| Odessa | Dneister | 26,000 |
| Worms | Rhine | 25,000 |
| Grand Forks, N. Dak. | Red River of the North | 22,000 |
| Frankfort on Oder | Oder | 21,000 |
| Bremen | Weser | 15,000 |
| Suburbs of Paris | Seine | 12,000 |
| Poughkeepsie, N. Y. | Hudson | 11,600 |
| Pittsburg, Penn. | Allegheny | 11,400 |
| Posen | Wartha | 9,400 |
| Hudson, N. Y. | Hudson | 9,200 |
| Albany, N. Y. | Hudson | 8,200 |
| Breslau | Oder | 8,200 |
| Brieg | Oder | 7,500 |
| Lawrence, Mass. | Merrimac | 4,634 |
| Stuttgart | Neckar | 1,660 |
| Brunswick | Ocker | 650 |
| Somersworth, N. H. | Salmon | 171 |
PRELIMINARY PROCESSES TO REMOVE MUD.
With both sand and mechanical filtration the difficulty and expense of treatment of a water increase nearly in direct proportion to the turbidity of the water as applied to the filter; and it is thus highly important to secure a water for filtration with as little turbidity as possible, and thus to develop to their economical limits the preliminary processes for the removal of mud. One of the most important of these processes is the use of reservoirs.
Reservoirs serve two purposes in connection with waters drawn from streams: they allow sedimentation, and they afford storage. If a water having a turbidity of 1.00 is allowed to remain in a sedimentation-basin for 24 hours, its turbidity may be reduced by as much as 40 per cent, or to 0.60. If it is held a second day the additional reduction is much less.
If samples are taken of the water in the reservoir before and after settling and sent to the chemist for analysis, he will probably report that from 70 to 80 per cent of the suspended matters have been removed by the process. The suspended matters are removed in much larger ratio than the turbidity. This arises from the fact that there is a certain proportion of comparatively coarse material in the water as it is taken from the river. This coarse material increases the weight of the suspended matters without increasing the turbidity in a corresponding degree. In 24 hours the coarser materials are removed completely, and at the end of that time only the clayey or finer particles remain in suspension. It is these clayey particles, however, that constitute the turbidity, which are most objectionable in appearance, and which are most difficult of removal by filtration or otherwise.
Sedimentation thus removes the heavier matters from the water, but it does not remove the finer matters which principally affect the appearance of the water and are otherwise most troublesome. A sedimentation of 24 hours removes practically all of the coarser matters, and the clayey material remaining at the end of that time can hardly be removed by further sedimentation. The economic limit of sedimentation is about 24 hours.
Sedimentation has practically no effect upon the clearer waters between flood periods.
Let us consider the effect of a sedimentation-basin, or reservoir holding a 24-hours’ supply of water, into which water is constantly pumped at one end, and from which an equal quantity is constantly withdrawn from the other, upon the water of a stream of such size that the time of passage of water from the feeders to the intake is less than 24 hours. During the period between storms the water is comparatively clear and passes through the sedimentation basin without change. When a storm comes the water in the stream promptly becomes muddy, and muddy water is supplied to the reservoir; but owing to the time required for water to pass through it, the outflowing water remains clear for some hours. There is a gradual mixing, however, and long before the expiration of 24 hours somewhat muddy water appears at the outlet. The turbid-water period rarely lasts in streams of this size more than 24 hours, and at the expiration of that time the water in the sedimentation-basin is as muddy or muddier than the water flowing in the stream. After the height of the flood the stream clears itself by the flowing away of the turbid water much more rapidly than the water clears itself by sedimentation in the reservoir. That is to say, if at the time of maximum turbidity we take a certain quantity of water from the stream and put it aside to settle, at no time will the improvement by settling equal the improvement which has taken place in the stream from natural causes. Generally the improvement in the stream is several times as rapid as in the sedimentation-basin, and the water from it will at times have only a fraction of the turbidity of the water in the basin.
Let us now consider what the sedimentation has done to improve the water. During the period of clear water, that is for most of the time, it has done nothing. For the first day of each flood period very much clearer water has been obtained from it than was flowing in the stream. For the first days following floods the water in the sedimentation-basin has been more muddy than the water in the stream. The only time when the sedimentation-basin has been of use is during the first part of floods, that is, when the turbidity of the water in the stream is increasing. During this period it has been of service principally because of its storage capacity, yielding up water received from the stream previously, when it was less muddy. Such sedimentation as has been secured is merely incidental and generally not important in amount.