SEWAGE DISPOSAL 161 



The former may be defined as " the concentration of sewage at short 

 intervals, on an area of specially-chosen porous ground as small as 

 will absorb and cleanse it, not excluding vegetation, but making the 

 product of secondary importance" (Metropolitan Sewage Commis- 

 sion). The intermittency is essential, and the process is partly 

 mechanical and partly bacterial, that is to say, due in part to the 

 nitrification set up by the bacteria in the superficial layers of soil. 

 For successful filtration a porous soil is requisite, a proper inclination 

 of the land to allow of distribution, and a division into areas, in 

 order that each part may receive sewage for, say, six hours, and then 

 have eighteen hours' rest. Soil pipes carry off the effluent. Broad 

 irrigation (sewage-forms) is the " distribution of sewage over a large 

 surface of ordinary agricultural ground, having in view a maximum 

 growth of vegetation (consistently with due purification) for the 

 amount of sewage supplied/' To ensure success, the area must be 

 large (say, about one acre to every 100 of the population), the sewage 

 passed on intermittently to allow of aeration of the soil, and the soil 

 itself must be light and porous. Like the former, there is a bacterial 

 influence at work here. Both of these methods are much to be 

 preferred to chemical treatment (and were recommended by the 

 Sewage Commission of 1865) ; yet, on account of space and manage- 

 ment, as well as on account of the tendency of the land to clog or 

 become, as it is termed, " sewage sick," their success has not been 

 all that could be desired. 



In 1868, a Commission was appointed to inquire into the best 

 means of preventing the pollution of rivers. They made several 

 reports, the fifth and last being made in 1874. The opinion of this 

 Commission on the comparative merits of the three classes of pro- 

 cesses for the treatment of sewage, viz : chemical precipitation, 

 intermittent filtration, and broad irrigation, may be stated thus : (1) 

 All these processes are to a great extent successful in removing pollut- 

 ing organic matter in suspension. But intermittent filtration is best, 

 broad irrigation ranks next, and the chemical precipitation processes 

 are less efficient. (2) But for removing organic matters in solution the 

 processes of downward intermittent filtration and broad irrigation are 

 greatly superior to upward filtration and chemical processes. 



The last Commission was appointed in 1882. They were directed 

 to inquire into and report upon the system under which sewage was 

 discharged into the Thames by the Metropolitan Board of Works, 

 whether any evil effects resulted therefrom, and if so, what measures 

 could be applied for remedying or preventing the same. In 

 November 1884 they issued their final Eeport. They found that 

 evils did exist " imperatively demanding a prompt remedy," and that 

 by chemical precipitation a certain part of the organic matter of the 

 sewage would be removed. They reported, however, " that the liquid 



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