“While English filters are able to remove satisfactorily on an average about 125 parts of silt and clay of the unsubsided water, actual experience shows that they can regularly handle suspended clay in subsided water in amounts ranging only as high as from 30 to 70 parts (depending upon the amount of the clay stored in the sand layer), and averaging about 50 parts per million. But it is true that for two or three days on short rises in the river, or at the beginning of long freshets, the retentive capacity of the sand layer allows of satisfactory results with the clay in the applied water considerably in excess of 70 parts. If this capacity is greatly overtaxed, however, the advantage is merely temporary, as the stored clay is washed out later, producing markedly turbid effluents.”
Translating Mr. Fuller’s results into terms of turbidity, the 125 parts per million of suspended matters in the raw water represent a turbidity of about 0.40, and the 30 to 70 parts of suspended matters in the settled water represent turbidities from 0.20 to 0.40, the average of 50 parts of suspended matters corresponding to a turbidity of about 0.30.
Upon this basis, then, sand filters are capable of treating raw waters with average turbidities up to 0.40, or settled waters with average turbidities up to 0.30, but waters more turbid than this are incapable of being successfully treated without the use of coagulants or other aids to the process. These results are in general accordance with the results of the experiments at Pittsburg, and demonstrate that while sand filters as generally used in Europe are adequate for the clarification of many, if not most, river waters in the United States, there are other waters carrying mud in such quantities as to make the process inapplicable to them.
EFFECT OF MUD UPON BACTERIAL EFFICIENCY OF FILTERS.
The question is naturally raised as to whether or not the presence of large quantities of mud in the raw water will not seriously interfere with the bacterial efficiency of filters. Experiments at Cincinnati and Pittsburg have given most conclusive and satisfactory information upon this point. Up to the point where the effluents become quite turbid, the mud in the raw water has no influence upon the bacterial efficiency; and even somewhat beyond this point, with effluents so turbid that they would hardly be suitable for the purpose of a public water-supply, the bacterial efficiency remains substantially equal to that obtained with the clearest waters. Only in the case of excessive quantities of mud, where, for other reasons, sand filters can hardly be considered applicable, is there a moderate reduction in bacterial efficiency. As mentioned above, particles constituting turbidity are often much smaller than the bacteria, and in addition, the bacteria probably have an adhesive power far in excess of that of the clay particles. For these reasons clay particles are able to pass filters under conditions which almost entirely prevent the passage of bacteria.
On the other hand, it does not necessarily follow that the removal of turbidity is accompanied by high bacterial efficiency. Although this is often the case, there are marked exceptions, particularly in connection with the use of coagulants, where very good clarification is obtained, and notwithstanding this, effluents are produced containing comparatively large numbers of bacteria.
LIMITS TO THE USE OF SUBSIDENCE FOR THE PRELIMINARY TREATMENT OF MUDDY WATERS.
When water is too muddy to be applied directly to filters, the most obvious treatment is to remove as much of the sediment as possible by sedimentation. Sedimentation-basins are considered as essential parts of filtration plants for the treatment of muddy waters. The effect of sedimentation, as noted above, is to remove principally the larger particles in the raw water. By doing this the deposit upon the surface of the filters and the cost of operation are greatly reduced.
These larger particles are mainly removed by a comparatively short period of sedimentation, and the improvement effected after the first 24 hours is comparatively slight. The particles remaining in suspension at the end of this time consist almost entirely of very fine clay, and the rate of their settlement through the water is extremely slow; and currents in the basin, due to temperature changes, winds, etc., almost entirely offset the natural tendency of the sediment to fall to the bottom.
There is thus a practical limit to the effect of sedimentation which is soon reached, and it has not been found feasible to extend the process so as to allow much more turbid waters to be brought within the range which can be economically treated by sand filtration.