24. REPORT— 1895. 



means of purification, entailed difficulties in thickly settled districts, owing 

 to the extent of land required. 



The cliemical treatment of sewage produced an effluent harmless only 

 after having been passed over land, or if turned into a large and rapid 

 stream, or into a tidal estuary ; and it left behind a large amount of 

 sludge to be dealt with. 



Hence it was long contended that the simplest plan in favourable 

 localities was to turn the sewage into the sea, and that the consequent 

 loss to the land of the manurial value in the sewage would be recouped by 

 the increase in fish-life. 



It was not till the chemist called to his aid the biologist, and came to 

 the help of the engineer, that a scientific system of sewage purification 

 Avas evolved. 



Dr. Frankland many years ago suggested the intermittent filtration of 

 sewage ; and Mr. Bailey Denton and Mr. Baldwin Latham were the first 

 engineers to adopt it. But the valuable experiments made in recent 

 years by the State Board of Health in Massachusetts have more clearly 

 explained to us how by this system we may utilise micro-organisms to con- 

 vert organic impurity in sewage into food fitted for higher forms of life. 



To effect this we require, in the first place, a filter about five feet thick 

 of sand and gravel, or, indeed, of any material which affords numerous 

 surfaces or open pores. Secondly, that after a volume of sewage has 

 passed through the filter, an interval of time be allowed, in which the air 

 necessary to support the life of the micro-organisms is enabled to enter 

 the pores of the filter. Thus this system is dependent upon oxygen and 

 time. Under such conditions the organisms necessary for purification are 

 sure to establish themselves in the filter before it has been long in use. 

 Temperature is a secondary consideration. 



Imperfect purification can invariably be traced either to a lack of 

 oxygen in the pores of the filter, or to the sewage passing through so 

 quickly that there is not sufficient time for the necessary processes to 

 take place. And the power of any material to purify either sewage or 

 water depends almost entirely upon its ability to hold a sufficient propor- 

 tion of either sewage or water in contact with a proper amount of air. 



Smoke Abatement. 



Whilst the sanitary engineer has done much to improve the surface 

 conditions of our towns, to furnish clean water, and to remove our 

 sewage, he has as yet done little to purify town air. Fog is caused by the 

 floating particles of matter in the air becoming weighted with aqueous 

 vapour ; some particles, such as salts of ammonia or chloride of sodium, 

 have a greater affinity for moisture than others. You will suffer from 

 fog so long as you keep refuse stored in your towns to furnish ammonia, 

 or so long as you allow your street surfaces to supply dust, of which much 

 consists of powdered horse manure, or so long as you send the products of 



