DAYTONA SEWERAGE CONDITIONS
Studies of Sewage Screens by Florida State Board of Health—Dilution Ample—Chlorine Removes Eighty Per Cent of Bacteria—Fine Screens in Use
A study of sewerage conditions at Daytona. Fla., by the sanitary engineer of the Florida State Board of Health, George W. Simons, Jr., shows some interesting facts in connection with the use of Reinsch-Wurl sewage screens and the use of chlorine in the work of purification. The screens were installed by the city in 1916 and the performance studies by the State Board of Health show that although the percentage of removal of suspended matter is only 7, yet fine screening, followed by chlorination, gives results at low operating cost which meet local requirements of discharge into the Halifax River. The percentage removal of solids would be higher if pumping and storage did not break up the sewage considerably.
Daytona sewage is almost wholly of domestic origin. Originally strong, it is highly diluted by ground water on its way to the screens. Ejectors at seven stations lift the sewage from the area of the city over a ridge to the screening, disinfecting and main pumping station within the corporate limits of the city. For efficiency of pump operation, the sewage is stored in 560 ft. of 24-in trunk sewers and in nearby laterals for some five hours, and is then fed to the screens and at the maximum capacity of the pump in operation at the time. Regulation is effected by passing the sewage through a small slot, the size of which is automatically controlled by a float designed by G. A. Main, superintendent. As the most remote sewer connection is less than 7,000 ft. from the sewage works, the sewage would be comparatively fresh were it not for the storage just mentioned. The septic action, combined with the previous agitation of the ejectors and the subsequent passage of sewage through the regulating slot, comminutes the original solid matter to such an extent that much of it passes the screen in a finely suspended or colloidal state, as is shown by the analytical data and also by tests with Imhoff conical settling glasses.
The raw sewage, as delivered to the screen, shows upon evaporation (filter paper, etc., method) an average of 95.7 p. p. m. of suspended solids (800 lb. per 1,000,000 gal.) and 620 p. p. m. of dissolved solids—or 87 per cent, of the total solids content appears as dissolved solids or volatile matter. The same sewage showed 15 p. p. m. of nitrogen as free ammonia, 31 parts of total organic nitrogen and 142 parts of chlorine.
TABLE I—VOLUMES OF SEWAGE PASSING REINSCH-WURL SCREENS AND AMOUNTS OF CHLORINE USED AT DAYTONA
Sewage flow for 24 hours on May 23-24, 1917, with 335 house connections, was 280,000 gal., or 835 gal. per connection. Daily totals for Dec. 1-5, 1917, are shown by Table 1. The average for these five days was 470,000 gal., or 1,030 gal. a day for each of the 456 connections then in use. Some idea of the relative rates of flow during the 24 hours may be got from the records of intermittent pumping given in the table. On Dec. 6-7, when the pump was operated continuously for 24 hours, the maximum daily rate of flow was about 860,000 gal. and the minimum night rate was about 500,000 gal. All these .records fall outside the winter tourist season and in comparatively dry weather, although there was rain on Dec. 5-6 and again on Dec. 6-7. The ordinary night flow is apparently ground water, at a rate of 1,000,000 gal. or more a day. This dilution increases the volume of sewage to be pumped and treated, and it affects the operation of the screen.
The Screen and What It Accomplishes
The sewage screen is of the usual Reinsch-Wurl overhung type, 8 ft. in diameter, with 5/64 x 2-in. tapered rectangular slots. The screen is driven by a belt at a rate of approximately 0.5 r. p. m. The rotating brushes move around the vertical axis of the brush shaft at a speed of 7.5 r. p. m. and revolve around their own axis at a speed of 37.5 r. p. m. The screen is placed in a pit at the pump station, receiving the gravity flow from the trunk sewer.
According to the specifications, “the screen will be required to remove from the sewage all solids sedimentable and nonsedimentable above 0.10 in. in diameter.” The screen does this, but for reasons already stated a large part of this suspended matter passes the screen, the average suspended solids content of the screened sewage being 88-6 p. p. m., or 740 lb. per 1.000 gal. On Dec. 6, 1917, an increase appeared in the composite samples representing the sewage flow from 5 to 11 p. m., sample collections being made every 15 min.. Increases are not of regular occurrence, as decided reductions have also been made, as on Dec. 5, 1917. At this time composite samples representing the flow from 2:55 to 5:15 p. m., collected at 5-min. intervals, indicated a decided reduction.
Removal of Screenings Real Criterion
The removal of screenings is the one real criterion by which to measure screen performance. During the pumping period, 2:15 to 5:16 p. m. Dec. 5, 3.67 cu. ft. of wet screenings were collected from 195,000 gal. of sewage, representing a removal of 18.8 cu. ft. or approximately 1,080 lbs. per 1,000,000 gal. From the pumpage started at 11:20 p. m. on Dec. 5 and continuing for 2.5 hours and again resumed at 6 a. m. Dec. 6 and continuing for an hour, 15.3 cu. ft. of wet screenings were collected per 1,000,000 gal. of sewage, representing the night flow. During these two periods a total of 291,000 gal. of sewage were pumped ; 208,000 gal. from 11 :20 p. m. to 1 :50 a. m. following a heavy downpour of rain, and 83,000 gal. between 6 and 7 a. m. The influence of the rainfall can here be noted. The combined removal of wet screenings during the two periods representing the night flow was considerably less than during the period preceding, when there was no rainfall. The reduction of screenings following a heavy rain is made still more apparent in the following paragraph.
To get results with the screen operating continuously, a 24-hour test was instituted at 11:36 a. m.. Dec. 6, 1917. All sewage passing the screen was carefully gaged and samples were collected at frequent intervals. During this 24-hour run a heavy rain occurred which affected the flow curve decidedly, and 446,000 gal. of sewage passed the screen. From this total sewage flow only 4.7 cu. ft. of wet screenings were collected, representing about 10 cu. ft. per 1,000,000 gal. These wet screenings weighed 271 lb., or 608 lb. per 1,000,000 gal. of sewage. Unfortunately, no apparatus was available for making moisture determinations of the screenings. The screened sewage showed an average of 88.6 p. p. m. suspended solids, or 740 lb. per 1,000,000 gal. of dry matter.
As the sewage flows upon the screen the degree of comminution is easily noticeable, and the finely divided appearance of the screenings gives additional evidence of the disintegrating action. The sewage after screening is very turbid, high in suspended solids, but free of large particles which would have a tendency to float, despoiling banks and flats, and of a quality suitable for disposal into ample dilution water.
The wet screenings, which average 14.5 cu. ft. (2 1/2 cans) a week are removed from the screen station to a dumping ground a mile west of the city limit by a negro who is paid 37 1/2c. a can for this work.
Chlorination to Protect Oyster Beds
Chlorine is used to protect the oyster beds in the vicinity of the sewer outfall into the Halifax River. When the plant was first started the chlorine was supplied to the sewage, previous to the screening, at a rate of 20 lb. per 1,000,000 gal. of sewage treated. During the spring of 1917 the point of chlorine application was changed so that the greater part of the disinfectant was admitted to the chamber under the screen, a small portion still being admitted to the upper side of the screen. Bacteriological determinations made under the original plan (chlorine applied previous to screening) indicated a 75 per cent, reduction of all bacteria and a 90 per cent, removal of the colon type. At the present time, with the chlorine cut down to 5 lb. per 1,000,000 gal., there is an 80 per cent, removal of all bacteria.
For the year 1917 the cost of screen operation, including labor for removing and burying screenings and for other purposes and slight repairs, but with no allowance for power consumed, was $370. The sterilization cost for the year was $679.
Much has been said of late as to the relative merits of fine-screen and Imhoff-tank treatment. That there is a definite place for screen installations and a future for fine screens is unquestionable, but screens will hardly replace tank treatment where refined results are necessary. Out of curiosity the writer has collected a few suspended matter removals from Imhoff tanks in Florida, in plants treatment sewage closely resembling that at Daytona. These results are shown in Table II.
TABLE II.—COMPARISON OF SUSPENDED MATTER REMOVED BY IMHOFF TANKS AND R-W SCREENS
With the usual Florida conditions—where fresh sewage is to be handled, quantities are small, flow is regular and ample supplies of dilution waters are available—the screen is more to be preferred than the tank because a refined treatment is not essential.
In conducting investigations at Daytona the writer has received heartiest cooperation and assistance from George A. Main, superintendent of the sewerage department.