I 



INTRODUCTION 5 



dredged through a bar of sewage deposit near Port Huron, the 

 typhoid rate at Detroit, 60 miles below, showed a great in- 

 crease, the first death occurring fifty days later. Professor 

 G. S. Williams calculated that the probable periods required 

 for the flow and distribution of water, for the development of 

 the typhoid bacilli, and for the fever to terminate fatallj^, would 

 be ten, fourteen, and twenty-five days respectively, making a total 

 of forty-nine days. It was also learnt that every other case of 

 dredging in the St. Clair River, above the Detroit intake, had 

 been followed by a marked increase in the typhoid fever rate in 

 that city. 



Lortet^ showed that sewage mud-banks frequently contained 

 living pathogenic forms of bacteria. Therefore the fact that 

 thorough sedimentation may take place in a few miles' flow is 

 no guarantee of safety at points below, but may be even a 

 source of the greatest danger in times of flood. 



But towns and cities are not the only sources of pollution, 

 as the upper reaches of rivers and streams are contaminated 

 by the filth from farmyards, which is, as a rule, allowed to 

 gradually soak away in an unpaved and undrained yard, or 

 is washed by repeated rains into the nearest water-course or 

 pond. 



The substances that have to be dealt with in the purification 

 of sewage may conveniently be arranged under the heads of — 



{a) Excretory Substances. — i. Solid faeces consist of nitrogenous 

 partially digested matter, with vegetable non-nitrogenous resi- 

 dues of the food. The former are easily liquefied, but the 

 latter are slow in dissolving, being gradually attacked, chiefly 

 by anaerobic bacteria, and broken down into soluble compounds 

 of foetid odour and into black amorphous flocculi, which slowly 

 deposit as black sludge. 



2. Urine is the main source of ammonia, from fermentation 

 of the urea, the proportion of urine being approximately indi- 

 cated by the content of chlorine in excess of the content of 

 chlorine in the water-supply of the town. 



(6) Household Waste. — The larger solids pass to the ash-pits, 

 but the drainage of these, and sometimes their washings by 

 rain, are received into the sewers together with the discharges 

 from sinks. Vegetable refuse yields a liquid which is very foul 

 and fermenting, developing butyric odours and sulphuretted 



^ " Pathogenic Bacteria of the Mud of the Lake of Geneva," Centr. f. Bakteriol., 

 ix. 709. 



