SOME ASPECTS OF CHEMICAL TREATMENT AT ST. LOUIS WATER WORKS
The principal streams contributing to the water supply of the city of St. Louis are the Mississippi, Ilinois and Missouri rivers. The Illinois river enters the Mississipi 33 miles north of the intakes at the Chain of Rocks and thus an intimate mixture of the two waters is effected. The Missouri river enters the Mississippi five and one-half miles north of the intakes and causes a pressing of the Mississippi river water upon the east bank and in this way, as a rule, very little mixing of the two waters occurs. At times the turbidity of the water on the west side of the river is ten times as great as that on the east and at other times the color of the east water is twenty-five parts per million ereater than that of the west, showing the incompletions of the mixing. With a high stage in either river the mixing of the waters is more complete. The waters in each of these rivers have certain characteristics. The Mississippi river drainage area being covered with swamps, the water becomes highly colored at times of heavy run-off, while the Illinois river, carrying a large amount of sewage, contains colloidal organic matter which seems to act as a protective colloid on the turbidity carried. The water in the Missouri river, always turbid, becomes much more so at times of heavy run-off.
Water Enters Through Two Intakes.
The river water enters our plant through two intakes, one, the old or West, 1,500 feet east of the west bank of the river and connected to the wet well by a 7 foot circular, brick lined tunnel, 2,197 feet long. The other, or East intake and is connected to the wet well by an 8 foot circular, concrete lined, tunnel 2,749 feet long. The water drawn through the East intake is a mixture of the Missisippi and Illinois river waters, although at times the water at both intakes is practically the same. The East intake was in service only 97 days during the past year, whereas the West was used for 350 days. Because of the greater difficulty of treating the water from the East, this intake is not used unless low stages of the river or anchor ice, or both, are affecting the pumping. The water entering the tunnels flows by gravity to the wet well from whence it is pumped, against a dynamic head of 58.3 feet, into the delivery well and flows from there to the grit chamber where the average velocity of flow, at a rate of pumping of 150 million gallons per day, is only 0.33 foot per second. In this chamber the coarser and heavier part of the suspended matter is deposited. The efficiency of the grit chamber is shown in the fineness of the material removed, over 50 per cent of the matter deposited passing a 100 mesh sieve. The tons of matter removed by the grit chamber during the past year were 63,703 or 23 per cent, of the total suspended matter in the water.
The Mixing Chamber.
Leaving the grit chamber, the water flows through a short conduit to the mixing chamber where the milk of lime and the solution of sulphate of iron are added. These chemicals are prepared for adding to the water in the coagulant house and are pumped a distance of 900 feet to the mixing chamber. The lime is weighed out in automatic scales and is dumped into circular slaking tanks which are provided with revolving rakes. The temperature of the milk of lime in the slaking tank is kept at 200 degrees F. This is accomplished fiy keeping up the temperature of the fresh water supply by passing it through the coils of a heater tank into which the milk of lime at 200 degrees F. is drawn. From 4 to 4½ pounds of water per pound of lime are used in slaking. The water overflowing from the heater tank is run into a cooling and diluting box where the temperature is reduced to as low as 64 degrees F. in winter time to 108 degrees F. in summer. The strength of the milk of lime as pumped is 38,600 parts per million of CaO. A slaker tank is kept in service until the accumulated unslakeable material is great enough to impede the motion of the rakes. From 50 to 150 tons of lime are slaked before a tank is taken out of service, the amount depending upon the purity of the lime. The sulphate of iron is measured by passing through an adjustable orifice onto the surface of a cylindrical drum, revolving at a constant speed and is discharged in a continuous flow into a tank where it is dissolved without stirring, by water entering through a manifold at the bottom of the tank, the solution being drawn off through an overflow. The mixing conduit into which the chemicals are delivered is a reinforced concrete box, 2,392 feet long, 32 feet 1 inch wide and 12 feet 6 inches high, divided longitudinally into four compartments, each 7 feet wide and 11 feet high. The four compartments are supplied with a stop plank openings so that they may be thrown in parallel, used in series or withdrawn from service for cleaning. In normal operation the water enters the west channel and travels the full length four times, a total of 9,528 feet, having an average velocity of 3.3 feet per second when the rate of pumping is 150 million gallons a day. Provision is made so that the lime or iron may be added to either of the four compartments but the lime is added, for the greater part of the time, to the raw water as it enters tbe mixing conduit and the sulphate of iron as it leaves the conduit. The period ot mixing averages about one hour.
Value of Mixing Chamber Shown.
The value of the mixing chamber is shown by an occurrence of last year. A leak in the south end of the mixing conduit, due to the failure of the contractor to properly plug a drain, caused the conduit to be taken out of service. The water was passed from the delivery well direct to the first of the sedimentation basins, the sulphate of iron being added in the tunnel at the coagulant house and the milk of lime at the delivery well. The turbidity of the water in the last of the sedimentation basins was 20, the amounts of chemicals added being 6.25 grains of lime per gallon and 0.25 grains of sulphate of iron. After the mixing conduit was taken out of service, the sulphate of iron was increased to 2.50 grains per gallon, the lime remaining the same. In 40 hours the turbidity of the water, in the last of the sedimentation basins increased to 40, the turbidity of the river water remaining practically the same as on the preceding days. By adding ten times the amount of sulphate, the results were still inferior. The additional cost due to the use of a larger amount of sulphate of iron while the conduit was out of service, one and one-half days, was $390.
A Water Hard to Handle.
With high stages in the Mississippi and Illinois rivers and a low stage in the Missouri, we encounter our worst condition. The high color of the Mississippi together with the colloidal matter in the Illinois make a water hard to handle. The use of sulphate of iron, as a coagulant, at these times is accompanied by some difficulty. The coloring matter of the water combines with the iron, and instead of a diminution in color the color is increased. The suspended matter being really colloidal and some of the iron hydroxide remaining in the colloidal condition, the turbidity of the water after sedimentation is greater than that of the river. This water is much less amenable to treatment with sulphate of alumina. The amount required to give the required flocculation of the suspended matter is from 4 to 5 grains per gallon. With this large amount, the water passing the filters is clear but is still of high color, the iron content being 8 to 10 times as great as normally. At times no relief is experienced until the Missouri river run-off increases and thereby gives us a turbid water which offers enough suspended matter for the rapid subsidence of the floe of ferric hydroxide. The more turbid the water at our intakes, the less trouble we have with turbidity causing material remaining in suspension.
Six Sedimentation Basins.
After passing through the mixing conduit, the water enters the first of six sedimentation basins, each 400 feet long by 670 feet wide, 30 million gallons capacity. The first three division walls have five stop-plank openings and the last two four openings, all 4½ feet deep by 8 feet long. The time of sedimentation, based upon the capacity of the basins, varies from 30 to 43 hours, but the actual time is much less, the effects of a change in the amounts of chemicals added being noticeable in 12 to 15 hours in the last of the basins. About 30 per cent, of the suspended matter and bacteria are removed in the first basin and 9 per cent, in the remaining basins. The total amount of matter removed from the water during the past year, including the chemicals added and the dissolved solids removed, amounted to 326,775 tons or 434,111 cubic yards. Some of the mud was removed by opening the sewer gate for one-half hour at varying intervals but the greater part was removed from the basins by labor and teams. The teams are used to draw scrapers which cut off portions of the mass of mud and drag them to the central gutter, through which water is flowing. The men are provided with scrapers which are used as such and also as braces to keep small A shaped boxes in place, as the mud drawn by the horses and the water used to aid in removing the mud are drawn by the boxes. The cost of the removal of the mud from the sedimentation basins, not including the cost of the water, was 0.762 cents per cubic yard for the past year. The water leaving the sedimentation basins enters a collecting conduit and passes through two $ foot Venturi meters and into a small secondary coagulation basin, connected to the secondary sedimentation basins by stopplank openings. The solution of sulphate of alumina is added at the throat of the meters and is automatically controlled so that the quantity added per unit is constant for any setting, regardless of fluctuations in the flow through the meters. There are two secondary sedimentation basins, one east of the filter plant and one north, each of which is connected to the influent flumes of the filters. The water entering the filter plant is passed through 40 filters, each with a filtering area of 1,400 square feet, of 4 million gallons capacity. The filtering media consists of 30 inches of sand, above 12 inches of graded gravel. The effective size of the filter sand as placed in the filters was 0.341 m.m., with a uniformity co-efficient of 1.81.
Excerpts from a paper read before the St. Louis convention of tne American Water Works Association.
Liquid Chlorine Added.
Liquid chlorine, in the form of chlorine water, is added after filtration in a chamber, in which the filtered water, from the three connections to the effluent flume, is combined. Two conduits, one a 7 foot, one-half inch steel tube, the other a brick and masonry conduit 9 feet high, 11 feet wide, are connected to this chamber. These conduits convey the water to the pumping stations at Bissells Point and Baden. The reduction in bacteria in the water flowing through the steel line is always less than in the water in the brick conduit. Charges of chlorine great enough to give tests for free chlorine in the water in the brick conduit, three hours after treatment, give no test in the steel line three minutes after, The disappearance and ineffectiveness of the chlorine in the water entering the steel line is attributed to the steel of the line. The accompaning operation record for the year ending February 2Sth, 1918, shows the working of the purification system, the amounts of chemicals used and other details of operation:
Record of Chain of Rocks Filters, Year Ending February 28, 1918.
The chemicals used were as follows:
Lime, 30,147,933 pounds, an average of 5.371 grains per gallon. Sulphate of Iron, 4,294,689 pounds, an average of 0.765 grains per gallon. Sulphate of Alumina (Meters), 3,781,163 pounds, an average of 0.673 grains per gallon. Sulphate of Alumina (Influent), 29,560 pounds, an average of 0.005 grains per gallon. Sulphate of Alumina (Filters), 108,756 pounds, an average of 0.019 grains per gallon. Chlorine, 74,516 pounds, an average of 1.89 pounds per million gallons.
Number of Filters in Service, 40; Number of Filtering Hours. 343,743; Average Rate Filtration, M. G. D. per acre. 85.39; Rate of Wash G. P. M„ 21.000; Number of Washes. 8.931; Per Cent. Wash water used, 1.56; Average Gallons used per wash, 78,990; Run of Filter in hours. Maximum 206.75, Minimum 6.92. Average 50.2; Run of Filter in Million Gallons, Maximum 32.63, Minimum 0.876, Average 5.67; B. Coli per c. c. in effluent 0.04S5; B. Coli per c. c. tap water average 0.0121; Bacteria Agar. Tap water average 10; Bacteria Agar. Effluent, 16; Bacteria Gelatine Effluent, 1S5.