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.
