i8 4 SCIENCE PROGRESS 



sewage, and still more so where the sewage has been treated 

 beforehand by precipitation or by some other method. 



(2) Precipitation. — If sewage is allowed to settle in tanks a 

 portion of the suspended matter will subside, and a more or 

 less clarified liquid can be decanted from the top. When 

 certain chemical substances are added to the sewage before 

 it enters the settling tanks, a still greater degree of pre- 

 cipitation will result. For this purpose very many chemicals 

 have been tried — e.g. lime as milk of lime, sulphate of alumina, 

 iron salts, etc. The most commonly used is probably lime, 

 either alone or in combination with sulphate of alumina or 

 iron. These chemical substances rapidly subside in the 

 settling tanks, and in doing so carry down with them most 

 of the suspended matters of the sewage, forming " sludge " 

 at the bottom of the tank. The quantity of lime used is 

 generally about twelve grains per gallon of sewage, or, if 

 lime and sulphate of alumina are used together, about five 

 grains of each. The chemicals not only produce more or less 

 complete deposition of suspended matters of the sewage, but 

 also remove to some extent the sewage odour. The organic 

 matter in solution is only partially reduced. The clarified 

 sewage is then in some cases discharged direct into a quickly 

 running stream or river, and so long as the volume of water 

 is sufficiently large, and the river below that point is not 

 used to provide drinking water, this may be allowable. 

 But difficulties do arise in many ways — e.g. during periods 

 of prolonged drought — and therefore it has been found 

 advisable in most cases to follow up precipitation by further 

 treatment on filter beds or on land. 



The sludge coming from the settling tanks contains some 

 ninety per cent, or more of water, and its disposal is one of 

 the most difficult problems in connection with this method of 

 sewage treatment. The general opinion appears to be that it 

 is practically worthless, and must be regarded as an inevitable 

 source of trouble and expense. It has been stated that " a 

 town of 100,000 inhabitants will produce thirty tons of pressed 

 sludge daily," and from this fact alone one can easily realise 

 the seriousness of the question. Various plans have been 

 adopted at different places for dealing with it — e.g. it may 

 be conducted into a well, from there pumped out in a semi- 

 liquid condition, and distributed over a specially prepared 



