Dec. 30. u^86] 



NA TURE 



195 



spelling Podidpcs, for the ungrammatical, senseless, and , 

 misleading Podiceps, thereby removing a reproach \vh ic ! 

 every literary man could successfully cast at a zoologist. | 

 Exemplum sequejiditm ' I 



INTERMITTENT DOWNWARD FILTRATION 

 Ten years' Experience {now Fourteen Years) in Works of 

 Intermittent Downward Filtration. By T. Bailey- 

 Denton. Second Edition. (London : E. and F. N. 

 Spon, 1S85.) 



THE treatment of sewage by intermittent downward 

 filtration on specially prepared areas of land is now 

 generally recognised as the most efficient method for the 

 purification of the sewage of towns. Mr. Bailey-Denton 

 is one of the ablest exponents of this system, and one 

 who has had large experience in its practical application. 

 He is also well known as being the joint author, with 

 Col. Jones, of a well-devised scheme for treating the 

 sewage of the metropolis on Canvey Island at the mouth 

 of the Thames — a scheme, however, which has not been 

 received with any sort of approval by the Metropolitan 

 Board of Works. The Royal Commission on Metro- 

 politan Sewage Discharge considered very fully the 

 merits and demerits of the system, and expressed their 

 opinion — " (i) That the process has great scientific merit, 

 and offers valuable practical advantages for the disposal 

 of sewage in situations where broad irrigation is imprac- 

 ticable, and where land suitable for filtration can be 

 obtained. (2) That, however, it appears desirable, when 

 the area of land is considerably reduced, that the sewage 

 should be previously treated by some efficient process for 

 removing the sludge. (3) That an arrangement of this 

 kind would be applicable to the metropolis. . . .'' Broad 

 irrigation was defined by the Royal Commission to mean 

 " 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," whereas filtration 

 means " 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 produce of secondary importance." On 

 a suitable soil — a sandy loam with a small proportion of 

 gritty gravel to quicken percolation is the best — specially 

 prepared by surface levelling and deep under-drainage, 

 one acre is capable of effectually purifying the sewage — 

 without any preliminary treatment — of 1000 people, pro- 

 vided that the sewage is free from any large proportion 

 of trade or manufacturing refuse, and that storm and 

 surface waters are kept out of the sewers. The obliga- 

 tion to treat storm waters, which come down in the 

 sewers in times of heavy rain, is one of the greatest 

 obstacles in the path of any system of sewage purification, 

 and will continue to be until all towns are supplied with 

 a dual system of drains and sewers. One inch of rain, 

 thrown off 100 acres, equals 2,262,200 gallons ; " and if,'' 

 says Mr. Bailey-Denton," one-tenth of this quantity sud- 

 denly reaches the outfall — say, in half an hour — no mode 

 of treatment yet devised can deal with such a quantity 

 without injury or defect." As a rule, at the present time, 

 despite prospective penalties for river pollution, the mixed 

 sewage and storm water is allowed to pass into the rivers 



without any sort of treatment. Mr. Bailey-Denton recom- 

 mends that the storm overflow be connected with osier 

 beds. " The beds are formed in horizontal areas which 

 serve to check the rapidity of flow of suddenly discharged 

 rainfall. This check causes the deposit of the floating 

 solid matters in the furrows, while the flood-water rises 

 and overflows the ridges and the osiers growing on them. 

 These beds are not under-drained in any way ; their 

 simple purpose being to clarify those excess-waters 

 which, without the check afforded by them, would be 

 impetuously discharged, together with everything floating 

 in them, into the natural streams of the watershed." Mr. 

 Bailey- Denton does not think it necessary or even desir- 

 able, in most cases, to precipitate the sludge— the minute 

 suspended particles, organic and inorganic, of sewage — 

 by chemical processes or depositing tanks, before the 

 sewage is applied to the filtration beds. He does not 

 believe that the sludge, unless mixed with solid trade 

 refuse, under proper treatment is capable of clogging the 

 pores of the land or of injuring vegetation. He recom- 

 mends the filtration beds to be laid out in ridges and 

 furrows — the sewage only flowing into the latter, and not 

 being allowed to flood the ridges on which plants and 

 vegetables are growing. The plants cannot then be 

 injured by the deposit of the solid ingredients of the 

 sewage on their stalks and leaves. "As soon as the 

 deposit of sludge on the sides of the furrows is sufficient 

 to prevent infiltration in any great degree, the sewage is 

 withheld from the areas so affected. The sludge is then 

 allowed to dry (partially) in the furrows, and when in a 

 fit condition it is lifted and dug into the ridges,— as can 

 be seen practised at Gennevilliers (Paris). The slimy 

 matter which had appeared so considerable, and which 

 puddled the bottom of the farrows, when in a wet state, 

 shrinks to a skin of very insignificant thickness when 

 dry, and is readily broken up and mixed with the soil." 

 Still Mr. Bailey-Denton admits that the extraction of the 

 sludge has one great advantage, viz. that " the same land 

 will filter double the quantity of clarified sewage liquid 

 that it would cleanse sewage of which the finer particles 

 have not been removed ; " a very important point to 

 towns where the area of land at disposal for sewage 

 purposes is strictly limited. 



The intermittency of the application of the sewage to 

 the filter beds is a sine qua non. Each bed should have 

 18 hours' rest out of the 24, to allow air to follow the 

 sewage as it percolates through the pores of the land, 

 thereby renewing the oxidising properties of the soil — 

 properties largely dependent, no doubt, on the life and 

 growth of certain Bacterial organisms resident in the 

 superficial layers of the soil, which have been shown by 

 Warington and other observers to be the principal agents 

 in the nitrification and purification of the nitrogenous 

 organic matters of sewage. The assimilative power of 

 growing plants is doubtless also a great aid in the puri- 

 fication of sewage, and the plan of ridges and furrows 

 adopted by Mr. Bailey-Denton, in enabling him to raise 

 large crops on filtration areas, has taken away from the 

 system the reproach that it was utterly unremunerative. 

 There can, however, be no doubt that it is in combination 

 with surface or broad irrigation that intermittent filtration 

 is likely to have its most useful application. In a valuable 

 chapter on sewage farming, Mr. Bailey- Denton points 



