COVERS FOR FILTERS.
The filters in England and Holland are built open, without protection from the weather. In Germany the filters first built were also open, but in the colder climates more or less difficulty was experienced in keeping the filters in operation in cold weather. An addition to the Berlin filters, built in 1874, was covered with masonry vaulting, over which several feet of earth were placed, affording a complete protection against frost. The filters at Magdeburg built two years later were covered in the same way, and since that time covered filters have been built at perhaps a dozen different places.
Interior View of Covered Filter, Ashland, Wis.
When in use the water rises nearly to the springing line of the arches.
Covered Filter in Course of Construction, showing Wooden Centers for Masonry Vaulting, Somersworth, N. H.
[To face page 12.]
It was found at Berlin that, owing to the difficulty of properly cleaning the open filters in winter, it was impossible to keep the usual proportion of the area in effective service, and as a result portions of the filters were greatly overtaxed during prolonged periods of cold weather. This resulted in greatly decreased bacterial efficiency, the bacteria in March, 1889, reaching 3000 to 4000 per cc. (with 100,000 in the raw water), although ordinarily the effluent contained less than 100. An epidemic of typhoid fever followed, and was confined to that part of the city supplied from the Stralau works, the wards supplied from the covered Tegel filters remaining free from fever. Open filters have since been abandoned in Berlin.
At Altona also, where the water is taken from an excessively polluted source, decreased bacterial efficiency has repeatedly resulted in winter, and the occasional epidemics of typhoid fever in that city, which have invariably come in winter, appear to have been directly due to the effect of cold upon the open filters. The city has just extended the open filters, and hopes with an increased reserve area to avoid the difficulty in future without resource to covered filters. (See Appendices II and VII.)
Brunswick, Lübeck, and Frankfort on Oder with cold winters have open filters, but draw their water-supplies from less polluted sources, and have thus far escaped the fate of Berlin and Altona. The new filters at Hamburg also are open. At Zürich, where open and covered filters were long used side by side, the covered filters were much more satisfactory, and the old open filters have recently been vaulted over.
Königsberg originally built open filters, but was afterward obliged to cover them, on account of the severe winters; and at Breslau, where open filters have long been used, the recent additions are vaulted over.
The fact that inferior efficiency of filtration results with open filters during prolonged and severe winter weather is generally admitted, although there is some doubt as to the exact way in which the disturbance is caused. In some works I am informed that in cutting the ice around the edges of the filter and repeatedly piling the loose pieces upon the floating cake, the latter eventually becomes so thickened at the sides that the projecting lower corners actually touch the sand, with the fluctuating levels which often prevail in these works, and that in this way the sediment layer upon the top of the sand is broken and the water rapidly passes without adequate purification at the points of disturbance.
This theory is, however, inadequate to account for many cases where such an accumulation of ice is not allowed. In these cases the poor work is not obtained until after the filters have been scraped. The sand apparently freezes slightly while the water is off, and when water is brought back and filtration resumed, normal results are for some reason not again obtained for a time.
In addition to the poorer work from open filters in cold weather, the cost of removing the ice adds materially to the operating expenses, and in very cold climates would in itself make covers advisable.
I have arranged the European filter plants, in regard to which I have sufficient information, in the table on page 15, in the order of the normal mean January temperatures of the respective places. This may not be an ideal criterion of the necessity of covering filters, but it is at least approximate, and in the absence of more detailed comparisons it will serve to give a good general idea of the case. I have not found a single case where covered filters are used where the January temperature is 32° F. or above. In some of these places some trouble is experienced in unusually cold weather, but I have not heard of any very serious difficulty or of any talk of covering filters at these places except at Rotterdam, where a project for covering was being discussed.
Those places having January temperatures below 30° experience a great deal of difficulty with open filters; so much so, that covered filters may be regarded as necessary for them, although it is possible to keep open filters running with decreased efficiency and increased expense by freely removing the ice, with January temperatures some degrees lower.
Where the mean January temperature is 30° to 32° F. there is room for doubt as to the necessity of covering filters, but, judging from the experience of Berlin and Altona, the covered filters are much safer at this temperature.
| TABLE OF PLACES HAVING OPEN AND COVERED FILTERS. | ||
|---|---|---|
| ARRANGED ACCORDING TO THE MEAN JANUARY TEMPERATURES. | ||
| Normal Mean January Temperature. Degrees F. | Place. | Kind of Filters and Results. |
| 37-40° | All English cities | Open filters only are used, and no greatdifficulty with ice is experienced. |
| 33-35° | Cities in Holland | All filters are open, and there is little serioustrouble with ice; but at Amsterdamand Rotterdam the bacteria ineffluents are said to be higher in winterthan at other times. |
| 32° | Bremen | Open filters. |
| 31° | Altona | Much difficulty with ice in open filters(see Appendices II and VII). |
| 31° | Brunswick | Open filters. |
| 31° | Hamburg | Open filters. |
| 31° | Lübeck | Open filters. |
| 31° | Berlin | Open filters were formerly used, but owingto decreased efficiency in cold weatherthey have been abandoned for covered ones. |
| 31° | Magdeburg | Covered filters, but a recent addition isnot covered. |
| 30° | Frankfort on Oder | Open filters. |
| 30° | Stuttgart | Part of the filters are covered. |
| 30° | Stettin | Part of the filters are covered. |
| 29° | Zürich | Covered filters were much the most satisfactory,and the open ones were coveredin 1894. The raw water has atemperature of 35°. |
| 29° | Liegnitz | Open filters. |
| 29° | Breslau | Open filters have been used, but recentadditions are covered. |
| 29° | Budapest | Covered filters only. |
| 29° | Posen | Covered filters only. |
| 26° | Königsberg | The original filters were open, but it wasfound necessary to cover them. |
| 24° | Warsaw | Covered filters only. |
| 16° | St. Petersburg | Covered filters only. |
In case the raw water was drawn from a lake at a depth where its minimum temperature was above 32°, which is the temperature which must ordinarily be expected in surface-waters in winter, open filters might be successfully used in slightly colder places.
The covers are usually of brick or concrete vaulting supported by pillars at distances of 11 to 15 feet in each direction, the whole being covered by 2 or 3 feet of earth; and the top can be laid out as a garden if desired. Small holes for the admission of air and light are usually left at intervals. The thickness of the masonry and the sizes of the pillars used in some of the earlier German vaultings are unnecessarily great, and some of the newer works are much lighter. For American use, vaulting like that used for the Newton, Mass., covered reservoir[3] should be amply strong.
Roofs have been used at Königsberg, Posen, and Budapest instead of the masonry vaulting. They are cheaper, but do not afford as good protection against frost, and even with great care some ice will form under them.
Provision must be made for entering the filters freely to introduce and remove sand. This is usually accomplished by raising one section of vaulting and building a permanent incline under it from the sand line to a door above the high-water line in the filter.
The cost of building covered filters is said to average fully one half more than open filters.
Among the incidental advantages of covered filters is that with the comparative darkness there is no tendency to algæ growths on the filters in summer, and the frequency of scraping is therefore somewhat reduced. At Zürich, in 1892, where both covered and open filters were in use side by side, the periods between scrapings averaged a third longer in the covered than in the open filters.
It has been supposed that covered filters kept the water cool in summer and warm in winter, but owing to the large volume of water passing, the change in temperature in any case is very slight; Frühling found that even in extreme cases a change of over 3° F. in either direction is rarely observed.
Removing Ice from a Filter, East London.
This represents the greatest accumulation of ice in the history of the works.
[To face page 16.]
At Berlin, where open and covered filters were used side by side at Stralau for twenty years, it was found that, bacterially, the open filters were, except in severe winter weather, more efficient. It was long supposed that this was caused by the sterilizing action of the sunlight upon the water in the open filters. This result, however, was not confirmed elsewhere, and it was finally discovered, in 1893, that the higher numbers were due to the existence of passages in corners on the columns of the vaulted roof and around the ventilators for the underdrains, through which, practically, unfiltered water found its way into the effluent. This at once removes the evidence in favor of the superior bacterial efficiency of open filters and suggests the necessity of preventing such passages. The construction of a ledge all around the walls and pillars four inches wide and a little above the gravel, as shown in the sketch, might be useful in this way, and the slight lateral movement of the water in the sand above would be of no consequence. The sand would evidently make a closer joint with the horizontal ledge than with the vertical wall.
Fig. 2.
In regard to the probable requirement or advisability of covers for filters in the United States, I judge, from the European experience, that places having January temperatures below the freezing-point will have considerable trouble from open filters, and would best have covered filters. Places having higher winter temperatures will be able to get along with the ice which may form on open filters, and the construction of covers would hardly be advisable except under exceptional local conditions, as, for instance, with a water with an unusual tendency to algæ growths.
I have drawn a line across a map of the United States on this basis (shown by the accompanying plate) and it would appear that places far north of the line would require covered filters, and that those south of it would not, while for the places in the immediate vicinity of the line (comparable to Hamburg and Altona) there is room for discussion.
In the United States covered filters have been constructed at St. Johnsbury, Vt., Somersworth, N. H., Albany, N. Y., Ashland, Wis., and Grand Forks, N. Dak., all of these places being considerably north of the above-mentioned line.
The filter at Lawrence, Mass., with a mean January temperature of about 25°, is not covered, but serious difficulty and expense have been experienced at times from the ice, so much so that it has been repeatedly recommended to cover it. Open filters have also been in use for many years at Hudson and Poughkeepsie, N. Y., with mean January temperatures about 24°; and although considerable difficulty has been experienced from ice at times, these filters, particularly the ones at Poughkeepsie, have been kept in very serviceable condition at all times, notwithstanding the ice.
At Mount Vernon, N. Y., with a mean January temperature of about 31°, and with a reservoir water, no serious difficulty has been experienced with ice; and at Far Rockaway, L. I., with a slightly higher temperature and well-water, no difficulty whatever has been experienced with open filters. Filters at Ilion, N. Y., with a mean January temperature of about 23°, are not covered, and are fed from a reservoir. No serious difficulty has been experienced with ice, which is probably due to the fact that the water applied to them is taken from near the bottom of the reservoir, and ordinarily has a temperature somewhat above the freezing-point throughout the winter.
Map showing
Normal Mean January Temperatures
in the United States
and the Area in which Filters should be covered
The cost of removing ice from filters depends, among other things, upon the amount of reserve filter area. When this reserve is small the filters must be kept constantly at work nearly up to their rated capacity; the ice must be removed promptly whenever the filters require cleaning, and under some conditions the expense of doing this may be considerable. If, on the other hand, there is a considerable reserve area, so that when a filter becomes clogged in severe weather, the work can be turned upon other filters and the clogged filter allowed to remain until more moderate weather, or until a thaw, the expense of ice removal may be kept at a materially lower figure.
In case open filters are built near or north of this line, I would suggest that plenty of space between and around the filters for piling up ice in case of necessity may be found advantageous, and that a greater reserve of filtering area for use in emergencies should be provided than would be considered necessary with vaulted filters or with open filters in a warmer climate.