Land Treatment of Sewagre. 



meant, of course, whether the alternate periods of 

 working and resting the land should occur at short 

 or long intervals. It is obvious that if the alterna- 

 tions be too frequent the ' resting ' area may not have 

 time to recover its fall purifying capacity before it 

 is again sewaged, and, on the other hand, if the 

 alternations be too infrequent the ' working ' area 

 may be rendered sewage - sick ; meanwhile the 

 ' resting ' area would be lying idle, long after it 

 had recovered its purifying ability. A competent 

 manager finds out by experience the best way of 

 working the farm under his care ; but it is desirable 

 that the matter should be investigated experiment- 

 ally, so as to enable certain general rules to be laid 

 down for the preliminary guidance of persons re- 

 sponsible for the management of sewage farms." 

 (Part I., page 109.) 



The following remarks are made with regard 

 to filtration : — 



Filtration. " (1) Excellent results can be obtained from light 

 loamy soil overlying a porous subsoil. (2) A sandy 

 soil and subsoil are also capable of yielding good 

 results. (3) The same may be said of a partially 

 peaty soil overlying gravelly sand. (4) Peat pure 

 and simple is not well adapted for sewage purifica- 

 tion. (5) With regard to chalk we can hardly speak 

 with confidence, more data being required. The 

 few (artificially obtained) effluents which we exam- 

 ined from a chalk farm had percolated in about a 

 minute and a half through approximately 3 ft. of 

 chalk (from which the surface soil had been re- 

 moved) ; in so doing they had undergone compara- 

 tively little purification organically, although the 

 liquid in its passage dissolved out a large quantity 

 of nitrate — the product, no doubt, of the oxidation of 

 sewage matter* run on at a previous time. The 

 purification would, of course, have been better had 

 the surface soil also taken its part. But the above 

 very rapid rate of filtration of sewage through the 

 fissured chalk emphasises the necessity for care- 

 fully considering any possible connection that may 

 exist between a sewage farm on chalk and a 

 water supply. 



Surfaoe " Heavy loam and clay soils, althousrh not so well 



Irrlgratlon ■. ■, t -n ^- 



and suited for sewage purification purposes, may, in our 



Oomblned -iji..! ■,/« 



Surface experience, yield fairly good effluents if the volume 



and of sewage treated per acre is relatively small. 



Filtration. ., r^^^^^ almost any kind of soil can be used for the 



purposes of sewage purification, provided, of course, 



that the volume is proportionate to the purifying 



capacity of the soil in question. In certain cases, no 



doubt, this volume would be so small as to render 



• " Whether this nitrification had gone on solely in the surface soil or 

 alio in the chalk itself we cannot say." 



the particular method of treatment impracticable, 

 but where the line should be drawn it is difficult to 

 say. The price of land and other local conditions 

 must needs influence this question of practicability. 

 We are far from advocating the treatment of sewage 

 upon land which is, practically speaking, not well 

 suited for the purpose ; but this does not invalidate 

 the truth of the proposition — that the matter is 

 nearly always one of degree of suitability, and seldom 

 one of intrinsic disability. In reference to this we 

 would lay stress on the good quality of the best efflu- 

 ents obtained from the eight sewage farms kept under 

 detailed observation." (Part I., pages 110 and 111.) 



" To summarise all our results within the limits Areas 

 of a few sentences is impossible, but we may say in 

 conclusion, and speaking in general terms, that we 

 doubt whether even the most suitable kind of soil 

 worked as a filtration farm should be called upon to 

 treat more than 30,000 to 60,000 gallons per acre 

 per twenty-four hours at a given time (750 to 1,500 

 persons per acre), or more than 10,000 to 20,000 gal- 

 lons per acre per twenty- four hours, calculated on the 

 total irrigable area (250 to 500 persons per acre). 

 Further, that soil not well suited for purification 

 purposes, worked as a surface irrigation or as a com- 

 bined surface irrigation and filtration farm, should 

 not be called upon to treat more than 5,000 to 

 10,000 gallons per acre per twenty-four hours at a 

 given time (125 to 250 persons per acre), or more 

 than 1,000 to 2,000 gallons per acre per twenty-four 

 hours, calculated on the total irrigable area (twenty- 

 five to fifty persons per acre). It is doubtful if the 

 very worst kinds of soil are capable of dealing quite 

 satisfactorily even with this relatively small volume 

 of sewage. The population per acre is calculated on 

 40 gallons of sewage per head per day. It is here 

 assumed that the sewage is of medium strength, and 

 is mechanically settled before going on to the land. 



" Comparing the above figures with the volume of 

 sewage capable of being treated by artificial pro- 

 cesses, we note that the witnesses examined by the Com- 

 mission generally expressed the following opinion : — 



" 750,000 gallons per acre per twenty-four hours. Contact 

 allowing for periods of rest, but not for secondary 

 treatment. Allowing 1 acre of secondary bed for 

 every 2 acres of primary bed, about 500,000 gallons 

 per acre per twenty-four hours could, according to 

 this view, be finally treated. It is assumed always 

 that the sewage has been previously treated, either 

 by chemical precipitation or by subsidence in settling 

 tanks, or in a septic tank. 



"About 484,000 to 2,904,000 (4,840,000 according Oontmuou. 

 to one witness) gallons per acre per twenty-four 

 hours. Previous treatment by chemical precipita- 



72 



