214 MICEO-ORGANISMS IN WATER 



' Some acid sewages contain a considerable amount 

 of iron in solution, and in these cases precipitation by 

 lime is really the rendering available of the copperas 

 already in the sewage, and so is properly classed as an 

 iron treatment rather than a lime treatment. In this 

 case the reaction with phenolphthalein shows the pre- 

 sence of enough lime. 



" In precipitation by ferric sulphate and crude alum 

 the addition of lime was found unnecessary, as ordinary 

 sewage contains enough alkali to decompose these salts. 

 Within reasonable limits the more of these precipitants 

 used the better is the result, but with very large quan- 

 tities the improvement does not compare with the in- 

 creased cost. 



' Using equal values of the different precipitants, 

 applied under the most favourable conditions for each, 

 upon the same sewage, the best results were obtained 

 with ferric sulphate. Nearly as good results were ob- 

 tained with copperas and lime used together, while lime 

 and alum each gave somewhat inferior effluents.-^ The 

 range of these results was, however, comparatively 

 narrow ; and it may be that, with sewage of a different 

 character, or with variations in the prices of the chemi- 

 cals, it would be advantageous to use copperas with 

 lime, or even alum. When lime is used there is always 

 so much lime left in solution that it is doubtful if its 

 use would ever be found satisfactory except in case of 

 acid sewage. 



' It is quite impossible to obtain effluents by chemical 

 precipitation which will compare in organic purity Avith 

 those obtained by intermittent filtration through sand. 



' It is possible to remove from one-half to two-thirds 



^ These remarks refer to the sohible organic matter removed by the 

 several chemicals, and it wiU be seen from the table on p.' 210 that these 

 precipitants do not by any means stand in the same order as regards their 

 relative power of removing bacteria. 



