This procedure is probably the most perfect which has been used for the removal of disease-producing qualities from highly polluted waters; and the cost of the process may not be as much greater than that of simple filtration as would at first appear, because the cost of cleaning the after filters is merely nominal, and the attendance, pumping, etc., are practically common to both sets of filters, and are not materially greater than they would be for a single set.
For very bad waters the first filters might appropriately be intermittent, while the after filters should be continuous. This was the procedure originally intended for Lawrence, but the intermittent filter first constructed yielded such very good results that it has not been considered necessary to complete the plant as originally projected.
At Bremen and at Altona a different procedure has been adopted. The filters are all upon the same level, and of the same construction. When a filter is put in service the effluent from it, instead of being taken to the pure-water reservoir, is taken to another filter which has already been some time in service. After the first filter has been in operation for some time its effluent is taken to the pure-water reservoir, and in turn it is supplied with the effluent from a filter more recently cleaned. The loss of head of water passing a freshly cleaned filter is comparatively slight, and the water of the second filter is allowed to fall a few inches below the high-water mark, at which level it will take the effluent from the other filter. The connections between the filters are made by siphons of large pipe, the summits of which are considerably above the high-water line. These siphons are filled by exhausting the air, and when opened to the air there is no possibility of a flow of water through them. The process has given extremely good results in practice, yielding effluents of the very greatest purity and at a quite moderate cost of operation.
An objection to the method is the possible filling of a siphon some time when the water standing upon the after-filter is higher than that in the pure-water well of the fore-filter, and while the fore-filter is connected with the pure-water reservoir. Such a connection would send unfiltered water into the pure-water reservoir direct. I do not know that any trouble of this kind has ever been experienced at Bremen or at Altona; and the objection to this system is perhaps not well founded where the management is careful and conscientious. The fact that an unscrupulous attendant can make the connection at any time to help out a deficiency of supply, or simply through carelessness, is certainly objectionable.
For the treatment of river-waters and lake-waters containing only a small quantity of sediment, and where the removal of bacteria or disease-producing qualities is the most important object of filtration, sand filters can be used. Where the rivers are subject to floods and moderate amounts of muddy water, sedimentation-basins or storage reservoirs for raw water will often be found advantageous.
For the treatment of extremely muddy waters, and waters which are continuously muddy for long periods of time, and for the removal of color from very highly colored waters, resource must be had to coagulants. The coagulants which are necessary in each special case and which can be used without injury to the water must be determined by most careful investigation of the raw water.
For the filtration of these waters after coagulation either sand or mechanical filters can be employed. As the principal work in this case is done by the coagulant, the kind of filtration employed is of less consequence than where filtration alone is relied upon, and the cheapest form of filter will naturally be employed. Under present conditions mechanical filters will usually be cheaper than sand filters for use in this way; but where waters, in addition to the mud, carry bacteria in such large numbers as to make high bacterial efficiency a matter of importance, sand filters may be selected, as the bacterial efficiency obtained with them is not dependent upon the use of coagulant; and is therefore less subject to interruptions from the failure to apply coagulant in the right proportion.
Mechanical filters have also been used for the treatment of comparatively clear waters where bacterial efficiency was the principal object of filtration. For this purpose the efficiencies obtained with them are usually inferior to those obtained with sand filters, while the cost of coagulants is so great as to make their use often more expensive than that of sand filters.
In the case of many streams which are comparatively clear for a part of the year, but occasionally are quite turbid, the use of sand filters has this advantage, that the use of coagulants can be stopped and the cost of operation reduced whenever the water is clear enough to allow of satisfactory treatment by them; and that coagulant can be employed on those days when otherwise insufficient clarification would be obtained.
In this case the high bacterial efficiency is secured at all times, while the cost of coagulant is saved during the greater part of the time. In such cases, also, the preliminary process of sedimentation and storage should be developed as far as possible.