TRADE EFFLUENTS 343 



etc., disappear on keeping, which indicates that the satis- 

 factory purification of sewage containing this class of refuse 

 is not beyond the range of possibility.^ 



On the whole, the effect of trade liquors generally has been 

 greatly exaggerated. In the case of small settlements collected 

 round factories, the domestic products may be only in small 

 proportions, and the effluent must be treated specially by 

 chemical methods and not as a sewage proper. In large towns 

 these discharges are usually so largely diluted that they cannot 

 interfere with a bacterial process when rightly carried out. 



It has been said that the antiseptic action of some chemicals 

 would arrest the bacterial changes. But by actual cultures it 

 has been shown that the amount of disinfectant required to 

 kill or even inhibit the organisms is far in excess of what can 

 be present in the mixed sewage. For example, at Yeovil, 

 where arsenic as sulpharsenite of calcium is derived from the 

 refuse of glove-making, I found that the maximum quantity of 

 orpiment, AS2S3, that could enter the sewers per week, if the 

 whole amount escaped, was 2 cwt., equal in 120,000 gallons of 

 sewage daily to 3*9 parts of AS2O3 per 100,000, or '0039 per 

 cent., whereas Miquel observed that 0*6 per cent., or 600 parts 

 per 100,000 of AS2O3 was required to prevent bacterial growth, 

 and Frankland and Ward assert that it has little effect on 

 lower forms of life. 



In December, 1899, ^ examined the waste liquors from two 

 of these factories, and found : — 



Arsenic (As) parts per 100,000 ... 

 Equal to arsenious acid AsgOg ... 

 Total bacteria per c.c. 

 Rapidly liquefying ditto 

 Spores 



Therefore, although arsenic in this quantity has an inhibitory 

 eifect on some organisms, the liquid still contains a large 

 number, including those of a rapidly liquefying character, and 

 spores, so that the bacterial work would not be arrested, even 

 if the liquid reached the tanks undiluted with sewage or storm- 

 water. In comparative trials with the sewage alone, and mixed 

 with yV of waste liquor, I found that both denitrifying and 

 nitrifying changes proceeded similarly with either. As a matter 

 of fact, the total volume of trade liquors in the Yeovil sewage 

 on any one day does not exceed one-fortieth of the estimated 

 dry-weather flow. 



1 Kershaw, Association of Sewage Disposal Managers, 1905. 



