CHEMICAL PRECIPITATION 



155 



Chemical Clarification or Precipitation. 



Lime, since the success of Clark's process for treating waters, 

 has been very widely used for sewage, either alone or as an 

 accompaniment to other precipitants.^ The Rivers Pollution 

 Commission of 1868 made their first experiments on the precipi- 

 tation of sewage with milk of lime alone, and pronounced it to 

 be a failure, as, although the liquid was rendered clear, it was 

 not sterilized, was rendered alkaline, ammonia was developed, 

 and the whole rapidly became foul. Where Local Boards 

 have used lime and sedimentation alone before discharge into 

 rivers, a prosecution for nuisance has almost invariably 

 followed. 



At Birmingham, for example, systematic sewerage of the 

 borough was commenced in 1852, liquids and solids being dis- 

 charged direct into the river Tame. In 1858 tanks were con- 

 structed, and various methods of purification by precipitation, 

 sedimentation, and filtration tried. Treatment with about 

 12 grains per gallon of lime, with subsequent sedimentation, 

 was adopted in 1872 and continued till igoo. The sludge was 

 collected in three large tanks of a total capacity of 440,400 cubic 

 feet, with sixteen smaller tanks of 729,000 cubic feet capacity. 

 The effluent and sludge were both treated separately on land, 

 the area of which increased from a few acres round the tanks 

 in 1870 to 2,800 acres in 1902.^ 



The results of the lime treatment led to a series of legal pro- 

 ceedings and an injunction, on account of the great pollution 

 of the Tame. In some cases J grain per gallon of chloride 

 of lime added with the lime used for precipitation was said to 

 have beneficial results and to prevent the growth of fungus, 

 total cost being stated at eightpence per head of population 

 per annum. But the precipitant rendered the effluent alkaline, 

 and its discharge into rivers gave rise to nuisance, and was 

 destructive to fish. Afterwards a bacterial process was adopted, 

 and septic tank effluent is being treated on land with success. 



^Clark's patent, 1841. In 1846 Higgs patented the use of lime for sewage, 

 which seems to have been the earliest chemical method adopted on a large scale. 

 H. Stothert introduced the alumina process in 1852, and the use of salts of iron, 

 first known as Dover's process, was brought out in 1851. For further historical 

 details, see Baldwin Latham's address to the Association of Managers of Sewage 

 Disposal Works, April 15, 1905. 



2 O'Shaughnessy on Birmingham Sewage, Journal of the Society of Chemical 

 Industry, May 31, 1902, p. 665. 



