BACTERIAL PURIFICATION 263 



about ;f 800. Sewage averages 80,000 gals, per day, so that it 

 ordinarily flows through the tank in 20 hours. 



Independence, Missouri. Two sets of circular septic tanks, 

 each 50 ft. diam., divided by an annular wall of loosely laid 

 bricks. The flow can be reversed by valves to prevent clogging. 

 Two circular filters, to which the flow can be switched at 

 intervals, constructed on the Moncrieff system, with 4 coke 

 breeze trays separated by air spaces. 



Baltimore. Septic tank treatment approved by the Baltimore 

 Commission in 1901. The experimental works resulted in the 

 recommendation in 1906 of a scheme in which the sewage 

 (75 million gals.) remains 6 to 8 hours in the tanks, is then 

 sprayed on to sprinkling filters of broken stone about 9 ft. deep, 

 from nozzles 15 ft. apart, then stays in settling basins for 3 hrs. 

 (by which altogether about 95 per cent, of the bacteria are 

 removed), and is to be finally purified by sand filters in prefer- 

 ence to mechanical filters with a coagulant, which had also 

 been suggested. 



Holland, Michigan. Tank constructed of 3 in. tongued and 

 grooved pine planks in 1900, 100 ft. long by 14 ft. in width, 

 with a water depth of 6^ ft. Enclosed in wooden building, with 

 galvanised iron roof. Beyond the grit chamber the tank is 

 divided into three long compartments, each holding 18,850 

 gallons : total capacity, including the grit chamber, 63,375 

 gallons. Turning the sewage through only one compartment 

 gives a rest period of 40 per cent, of the total capacity, and 

 with two compartments 70 per cent. The three may be used 

 as one long tank, the sewage passing direct through the first, 

 back through the second, and out through the third. This 

 arrangement is thought to facilitate cleaning.^ 



La Grange, Illinois. Septic tank divided lengthways into three 

 compartments, each 100 ft. long, 10 ft. wide, and 10 ft. deep, with 

 a concrete roof 6 in. thick; in continuous operation since Novem- 

 ber, 1901. Sewage in each tank is 7 ft. deep, is stored from 

 12 to 18 hours, and flows from the tanks over three weirs in 

 thin sheets, dropping 18 ins. into a shallow basin, from which 

 it discharges back into the main sewer. Each compartment 

 has a regulating valve at the inlet, and baffle boards near the 

 outlet to prevent gas from escaping or scum from passing over 

 the weirs. The previous offensive sewage odours are not now 

 observed.^ 



^ Engineering Record, N.Y., March i6, 1901. 



- W. B. Ewing, Eng. Record, February 21, 1903. p. 200. 



