Chapter 5

When the water was comparatively clear, either one of the three systems, or slow sand filtration alone, was entirely satisfactory. At times of high turbidity, however, Filter Plant No. 3 was the only one which could be depended on to produce a clear effluent.

A fair comparison between the results of the three systems when treating turbid water in January, 1908, is given in Table 16.

Table 16 shows very clearly that neither Filter Plant No. 1 nor No. 2 would prove at all satisfactory when treating turbid water, while No. 3 could be depended on under all conditions. The results of operation are shown in detail in Tables 17, 18, and 19. It will be noticed in Table 17, that on March 10th, 1908, Filter Plant No. 1 was put out of service and a Puech system of preliminary filters was substituted for it.

The Puech preliminary filters consisted of five units containing gravel of varying sizes through which the water was filtered successively before it was finally applied to the final slow sand filter. A general idea of this system may be obtained by referring to Figure 8.

~Table 16--Turbidity Results with Experimental Filters, During Period of High Turbidity, January, 1908.~

Columns: A - Effluent preliminary filter. B - Effluent sand filter. C - Effluent preliminary filter. D - Effluent sand filter. E - Effluent coagulant basin. F - Effluent sand filter.

============+======+=============+=============+============== | Raw |Filter No. 1.|Filter No. 2.|Filter No. 3. Date. |water.|------+------+------+------+------+------- | | A | B | C | D | E | F ------------+------+------+------+------+------+------+------- January 12th| 40 | 10 | 1 | 12 | 1 | 2 | 0 January 13th| 110 | 45 | 2 | 51 | 2 | 2 | 0 January 14th| 210 | 95 | 3 | 113 | 4 | 2 | 0 January 15th| 325 | 190 | 12 | 222 | 15 | 3 | 0 January 16th| 360 | 210 | 37 | 247 | 42 | 5 | 0 January 17th| 242 | 122 | 24 | 147 | 26 | 6 | 0 January 18th| 137 | ... | ... | 73 | 7 | 6 | 0 January 19th| 117 | 40 | 12 |clean-| ... | 5 | 0 | | | | ing | | | January 20th| 72 | 31 | 6 | sand | ... |clean-| 0 | | | |filter| | ing | January 21st| 55 | 20 | 4 | 25 | 4 | sand | ... January 22d | 49 | 17 | 3 | 21 | 4 |filter| ... January 23d | 40 | 12 | 3 | 15 | 3 | 3 | 0 January 24th| 40 | 11 | 3 | 13 | 3 | 3 | 0 ============+======+======+======+======+======+======+=======

It is unfortunate that this system was not in operation in January, 1908, when the water was cold and turbid. The results, however, indicate that it would be no more successful than either Filter Plant No. 1 or No. 2.

_Experimental Rate Studies._--In September, 1908, an experimental plant consisting of six small filters was put in operation. The object of these experiments was to study the relative efficiencies and cost for the operation of slow sand filters when operated at different rates.

The units of the plant consisted of cylindrical galvanized-iron tanks 4 ft. in diameter and 9 ft. high. The filter sand in these tanks was taken from the supply for the main filters. It was supported on gravel layers and supplied with under-drains of suitable sizes for the proposed rate of flow in each case.

The units of the experimental plant were designated as Nos. 1, 2, 3, 4, 5, and 6, and it was the original intention to operate them at rates of 1,000,000, 3,000,000, 6,000,000, 10,000,000, 30,000,000, and 100,000,000 gal. per acre daily, respectively.

This schedule of rates was carried out in a general way with all the filters, with the exception of Nos. 5 and 6. For these, the rates were found to be higher than could be maintained for any great length of time, owing to the deeper penetration of the mud in the filter sand, which caused high initial losses of head, short runs, and deep scrapings. A rate of about 30,000,000 gal. was maintained in the case of Filter No. 5 from the time it was started on September 9th, 1908, until November 8th, 1909, when it was reduced to about 17,000,000 gal., which rate was maintained thereafter until the filter was shut down in February, 1910.

In the case of Filter No. 6, it was found impossible to maintain a rate of 100,000,000 gal. for more than a very few days at a time. It was started at about this rate, however, at the beginning of each run, and kept as high as possible for the remainder of the time during the first seven runs. At the end of the seventh run, on October 17th, 1908, the filter was given a very deep scraping and re-sanded.

The layer of clean sand restored the original capacity, and the filter was operated as before, but with gradually decreasing rates until December, 1908, when the rate was reduced to about 40,000,000 gal. Even this lower rate was too high to be maintained without removing and replacing a large part of the sand. The rates, therefore, gradually decreased to about 23,000,000 gal. on March 13th, 1909, when the filter was again re-sanded. After this re-sanding the rate was reduced to about 20,000,000 gal., and the filter was operated at approximately that rate until it was again re-sanded on November 13th, 1909, when the rate was again reduced to about 14,000,000 gal., which was maintained until the filter was put out of service on February 28th, 1910.

This experimental plant was in service from September, 1908, to the latter part of February, 1910, or for about 1-1/2 years, and the leading results are summarized in Table 20.

~Table 17--Record of Experimental Filter Plant No. 1.~

Columns: A - Rate, millions of gallons per acre daily. B - Loss of head. C - Rate, millions of gallons per acre daily. D - Loss of head. E - Applied water. F - Effluent preliminary filter. G - Effluent final filter. H - Applied water. I - Effluent preliminary filter. J - Effluent final filter.