PLANKTON IN WATER SUPPLY MEIER 405 



the rate of filtration. Leaving these basins, the water flows into two 

 large settling basins (pi. 1) built in pairs so that cleaning may be 

 carried on without interruption of service. The water mixed with 

 the alum enters these large basins, each of about 4 million gallons 

 capacity. It requires about 3 hours to pass through, thus causing a 

 low velocity which gives an opportunity for the settling of the 

 aluminum floe formed when alum is added in properly regulated 

 amounts to river water. In the formation of the floe, which is a 

 chemical decomposition of the alum, all of the aluminum element is 

 removed from the sj^stem and does not harm the water either from 

 the point of view of potability or of hardness. When the water comes 

 into these two basins, it is muddy, but when it goes out, it is clear, 

 as it contains only a fine suspension of the floe. 



With about 10 percent of the floe in suspension, this partly clari- 

 fied water flows by gravity into 20 slow sand-filter units (pi. 2). The 

 remaining floe forms a filtering mat on the sand surfaces, which 

 strains out all suspended matter and entraps most of the bacteria. 



Chlorine plus ammonia (chloramine) are also added to the water 

 as it leaves the settling basins and goes into the filters. After the 

 water has passed through the filters and sand beds, more chlorine is 

 added. The amount of chlorine necessary to kill the small numbers 

 of harmful intestinal bacteria present in this water is small. 



Readjustment of the acidity is the final step at this plant. The 

 water when it enters the plant usually has a low pH (h3^drogen ion 

 concentration or acidity) value. However, when alum is added 

 to the water, hydrolysis occurs, for the aluminum sulfate is broken 

 up into aluminum hydroxide and sulfuric acid. The former is the 

 floe, which, settling, carries down the mud and the bacteria in the 

 water. The sulfuric acid combines immediately with the lime or 

 calcium carbonate normally present in river waters passing through 

 limestone country and forms lime or calcium carbonate and frees 

 carbon dioxide, which partially remains in solution and makes the 

 water slightly acid. To remove this carbon dioxide, sufficient lime 

 water is added to remove the gas but not enough to cause boiler- 

 scale trouble. 



The average turbidity of the water when it enters the plant is 

 140 parts per million; average hardness, 72; and average bacterial 

 count, 2,500 per cc. The pH or acidity value is 7.6. After filtra- 

 tion, the turbidity is ; hardness, 72 ; and the number of the bacteria 

 is reduced to an average of 2 per cc. The pH of the filtered 

 water is 8.0. 



Lauter, from observations made at Dalecarlia, believes the covered 

 reservoir is preferable to the open reservoir, since the covered type 

 has a much smaller plankton count than the open type. 



