32 SEWAGE AND ITS PURIFICATION 



titrating one of them directly with standard acid, evaporating 

 the other to a low bulk (not to dryness, since the fixed alkali is 

 capable of decomposing many organic compounds and of 

 neutralizing itself) over the water-bath to drive off ammonia, 

 and titrating again. The first titration gives the total alkalinity, 

 the second the fixed; the difference is the volatile alkalinity. 

 The fixed alkalinity is ordinarily calculated into soda, the 

 volatile into ammonia. Each cubic centimetre of centinormal 

 standard acid solution required for neutralization corresponds 

 to 0*00031 gramme of sodium oxide, NagO ; 0*00040 of caustic 

 soda, NaOH ; 0*00053 of sodium carbonate, NagCOg ; and 

 0*00017 of ammonia, NH3. For a fairly clear liquid, methyl 

 orange may be used as an indicator of the end of the titration ; 

 for a thick or coloured one, delicate litmus-paper is the best. 

 Moderate alkalinity is favourable to the action of bacteria ; 

 therefore it is rarely necessary to make the above determination 

 in sewage, except where liquors from gasworks or chemical 

 factories are present. 



Occasionally the sewage is locally rendered acid by trade dis- 

 charges ; the degree of acidity must be determined by standard 

 alkali run in from a burette in the same way as in the determina- 

 tion of alkalinity. Acidity due to CO2 disappears on boiling the 

 liquid or on drying the litmus-paper. An acid sewage would be 

 unfavourable to bacterial action, but the acidity is usuall}' at 

 once neutralized by admixture with the larger volume of sewage. 

 (See later, Trade Effluents.) 



Oxidation in bacterial sewage treatment is accompanied by a 

 change of basic substances, like amines and ammonia, into acid 

 bodies such as carbonic, acetic, and nitric acids, which combine 

 with lime or soda. Therefore the effluent will become less 

 alkaline than the sewage, and the reduction of alkalinity has 

 been proposed as a measure of the degree of change, but the 

 figure will be greatly affected by the volatilization of ammonia, 

 and in some cases by the presence of added lime or other 

 chemicals. 



Solid Matter. — It is generally only necessary to ascertain (i) 

 the total solids, (2) the suspended matter. 



The total solids are determined by evaporating 50 or 100 c.c. 

 in a porcelain or platinum dish (or a thin glass basin if it has 

 not afterwards to be burnt), drying at 120° C, cooling in a 

 desiccator, and weighing. It is often difficult to obtain a 

 constant weight, owing to decomposition of the solids and to 



