26a SEWAGE AND ITS PURIFICATION 



the tank in dry weather occupies 24 hours or more, and in wet 

 weather may be reduced to 7 hours. But the more intractable 

 soHds remain much longer : they are entangled by the active 

 zooglcea scum on the surface, or may slowly sink to the bottom ; 

 and they accumulate in the tank until the rate of dissolution 

 is equal to the accretion of the fresh solids. When equilibrium 

 is established, the scum and the sediment, though showing 

 some fluctuation, should have an approximately constant 

 volume. A capacity equal to 24 hours' flow is generally 

 advisable : in America, with much weaker sewages, 8 hours has 

 been found sufficient. 



Possible survival of pathogenic organisms has been dealt with 

 in Chapter IV., p. 84. 



Notes on Septic Tank Plants. — Barrhead, near Glasgow (Septic 

 Tank Syndicate) Works, opened in i8gg, deal with a popula- 

 tion of 10,000 and a maximum daily flow of 400,000 gallons 

 including storm water, and have provision for larger amounts. 

 Two grit chambers, four septic tanks, and eight aerating 

 bacterial filters, the whole built in concrete. Each tank 100 ft. 

 long by 18 ft. wide and 7 ft. deep at low water, roofed by con- 

 crete arches on rolled steel joists with brick piers. Total dry- 

 weather capacity, 312,500 gallons, with storm allowance for a 

 rise of 18 inches, or an additional 70,000 gallons, the flow to 

 the filters maintained constant by two controlling modules. 

 Each filter is 55 ft. by 54 ft. with 4 ft. depth of broken furnace 

 clinker ; total filtering area 2,540 square yards. Two of the 

 filters are usually kept in reserve. Arrangements as at Exeter. 

 A report after 6 years' working^ states that, with periodical turn- 

 ing over on the surface, the filters have kept their capacity, and 

 produce without fail a good effluent, and that the septic tank 

 has never been interrupted nor cleaned out : the grit tanks are 

 emptied 2 or 3 times a year. Total cost of works and land 

 ;f6,776. 



Overbrook, New Jersey, U.S., on Exeter principle. Con- 

 crete tank 50x18x10 ft. deep, = 65,000 gallons: four filters 

 14 X 14 X 5 ft. deep, at first filled with sand as at Lawrence, 

 which rapidly clogged, afterwards with coke breeze. As a 

 nuisance occurred, two larger beds, 4,500 ft. area and 3 ft. deep, 

 were made, and gave a good effluent. The tank effluent lost 

 its odour after standing for three hours, and there was no sub- 

 sequent putrefaction, but a strong growth of algae. Total cost 



1 Ex-Provost Shanks, Glasgow Congress of Sanitary Institute, 1904. 



