General Conclusions. 



Propor- 

 tions— 

 Working 

 and 



Resting 

 Areas. 



tion, or subsidence in settling tanks, or in a septic 

 tank, is assumed. 



"Apart from the question of the quality of the 

 effluents it is obvious that, generally speaking, a 

 larger volume of sewage can be treated by artificial 

 bacteria-bed processes than by land." (Part I., 

 pages 113 and 114.) " Effluents from land usually 

 yield more variable results than effluents from bac- 

 teria-bed processes of sewage treatment." (Part III., 

 page 19, footnote.) 



" It is impossible to lay down any hard-and-fast 

 rule as regards the proper proportions between the 

 area actually being irrigated at one time and the 



should be 5,000 gallons per acre per twenty-four 

 hours for an irrigation farm and 30,000 gallons for 

 a filtration farm. Allowing 40* gallons of sewage 

 per head per day, this works out to 125 and 750 

 persons per acre respectively. On the basis of 

 population, other witnesses gave 100 to 1,000 persons 

 per acre as the working limits, the number varying 

 with the process of treatment adopted, the nature of 

 the soil and other factors. Converted into volumes 

 of sewage per head per twenty-four hours (at 

 40 gallons per head), this gives 4,000 — 40,000 gallons 

 per acre. Others, again, suggested the following 

 figures for population per acre : — 



surplus (' resting ') irrigable area. Each case must Again assuming 40 gallons of sewage per head per 

 be judged on its own merits, but broadly speaking day, the above figures may be expressed thus : — 



witnesses' 

 FlKurss. 



the tentative assertion that at least (a) four-fifths 

 of a surface irrigation farm and (6) two-thirds of a 

 filtration farm respectively should be at rest is 

 probably not remote from the truth, it being 

 assumed that (a) one-fifth and (6) one-third are 

 capable of effectively purifying the sewage until 

 such time as the next one-fifth and one-third respec- 

 tively of the surplus irrigable area, due in rotation, 

 have been sufficiently rested. The above is subject 

 to the reservation that the whole question of inter- 

 mittency is but little understood, and, as we have 

 said before, whether or not rapid or slow alterna- 

 tions of working and rest are advisable we are 

 unable to say." (Part I., page 17.) 



" A divergence also showed itself in the opinions 

 of witnesses with respect to the question of popula- 

 tion and of volume of sewage allowable per acre. 

 Some of them considered that the maximum limits 



Broadly speaking, therefore, the various witnesses 

 examined placed the extremes at 50 persons or 2,000f 

 gallons per acre per twenty-four hours, and 1,000 

 persons or 40,000 gallons per acre per twenty-four 

 hours." (Part I., page 111.) 



"Without making any definite or final statement Chamioai 

 on the subject at present, we think it would prob- standard*. 

 ably be found that any effluent which did not within 

 the twenty-four hours after drawing take up more 

 than about 3 c.c. to 4 c.c. of oxygen per litre (when 

 kept in a full bottle at, say, 18 deg. 0. or 65 deg. 

 Pahr.) would be found to be chemically satisfactory. 



* " This figure of 40 gallons is taken because it was the average volume 

 at the eight principal sewage farms kept under observation. According 

 to some statistics collected recently by our colleague. Mr. Colin 0. Prye, it 

 is probably not far from the mean for towns and cities with a population 

 of 10,000 to 100,000 and over, though much too high for small towns and 

 villages. It is brought forward here for comparative calculations only . " 



t " It is difficult to see how this small volume could be satisfactorily 

 spread over an acre of ground.'* 



73 



