156 SEWAGE AND ITS PURIFICATION 



The land is laid out in various ways, partly as intermittent 

 filters, and the greater part in broad irrigation.^ 



Whatever be the cheapness of lime, therefore, it has not been 

 found to be successful alone, but as an adjunct to other pro- 

 cesses it is frequently of great use, and may be absolutely 

 necessary in some cases where the sewage is strongly acid from 

 trade effluents. 



A good quality of lime is slaked, and then ground with a 

 portion of the sewage or other water to an even cream. The 

 quantity to be added must be regulated by the content of actual 

 free lime ; this should be determined at intervals by diluting a 

 measured sample (5 c.c.) of the well-mixed cream with recently 

 boiled distilled water to 250 c.c. in a stoppered flask, well 

 agitating, allowing to settle, withdrawing an aliquot portion of 

 the clear liquid with a pipette, and testing the alkalinity by ttt 

 hydrochloric acid and phenolphthalein. Quicklime sometimes 

 contains large quantities of impurities, and in all forms it loses 

 strength by absorption of carbonic acid if exposed to air; 

 therefore bins, vats, or tanks for storage require to be carefully 

 covered. The usual dose of lime, when used alone, has been 

 one ton to each million gallons, or 15*68 grains per gallon. The 

 following conditions must be observed : 



1. Sufficient must be used, in the case of acid or trade 

 effluents, for neutralization and precipitation. 



2. In ordinary cases enough must be added to combine with 

 the free carbonic acid and half of that combined as bicarbonate, 

 as in ordinary water softening ; the precipitated carbonate of 

 lime carries down much organic matter. 



3. A slight excess is generally needed to precipitate organic 

 acids and colouring matters of a humous character. 



4. Best results are obtained when the lime is in solution ; if 

 only suspended it is less active as a chemical precipitant, while 

 all the insoluble impurities are added to the sludge. 



5. The effluent must not be rendered more than faintly alka- 

 line, and determinations of the alkalinity of raw sewage and 

 effluent must be made. 



6. The amount used will vary according to the quality of the 

 sewage and of the lime. 



Dibdin has drawn attention to the solvent action of lime on 

 many of the suspended matters in sewage, so that "the addition 

 of an excessive quantity of lime, while affording a rapid settle- 

 1 Report of the Royal Commission on Sewai^c, 1902, vol. li., p. 536. 



