EXPERIMENTS. 



At Croydon, as at South Norwood and at Leicester, 

 experiments were made in order to see how long the 

 purifying power of the soil would hold out under a 

 daily increasing dose of sewage. 



The soil and subsoil at Bed- 

 Croydorii dington are comparable with those 



at Nottingham and Cambridge, 

 though the Beddington subsoil is much the most 

 Btony of the three. The rate of filtration must also 

 be somewhat similar to the rates at Nottingham 

 and Cambridge, though no doubt appreciably faster. 

 The results obtained in the above short experiment 

 show very clearly (1) the high purifying power of 

 the Beddington soil and subsoil, and (2) the much 

 greater degree of purification that can be effected 

 when such a soil is used for filtration rather than 

 for surface irrigation. Experiments of this kind 

 present great practical difficulties, and it is impos- 

 sible, unless they are carried out during a prolonged 

 period, to draw, from a bacteriological point of 

 view, from them any conclusion of real value. The 

 only interpretation that can safely be put on the 

 bacteriological results is that land of the kind avail- 

 able at Beddington can purify, to a reasonable 

 extent, relatively large volumes of sewage. 



This experiment, though not 

 Leioester. affording a direct answer to the 

 question how much sewage an 

 acre of the Leioester farm will purify, is in some 

 ways instructive. First we have an effluent of 

 indifferent appearance and not really purified, 

 because of its having partly escaped into the drains 

 through cracks in the soil, but kept non-putrescent 

 in virtue of the accumulated nitrate which it has 

 washed out. After that there is an improvement in 

 purification up to No. 140, at which time it so 

 happened that just about the normal flow for the 

 Leicester farm was being treated. Then comes a 

 gradual falling off though with a few minor fluctua- 

 tions, until sample No. 147 is reached, when the 

 effluent is temporarily benefited by rain, only to 



relapse next day. In no case from No. 139 onwards 

 is there any appreciable nitrate present in proportion 

 to the total nitrogenous matter — that is to say, the 

 purification effected is insufficient. These few 

 observations thus emphasise the point that most 

 careful management is required on a clay sewage 

 farm. Perhaps the most practical conclusion to be 

 drawn from them is that any effluent from clay land 

 which is partly made up of liquor that has escaped 

 through cracks in the soil should be subsequently 

 passed /or certain over some uncracked ground — e.g., 

 ground in the condition that the above plots were in 

 shortly after the soil had been filled up with an 

 excessive flow. It will thus be seen that the results 

 in general were of an unsatisfactory nature. The 

 first two effluents were very unsatisfactory, later on 

 in the experiment the effluents improved in biologi- 

 cal quality ; but the results yielded by the different 

 tests varied so greatly as to render it impossible to 

 come to any but the general conclusion — namely, 

 that the results throughout were unsatisfactory. 

 Nevertheless, the percentage purification effected by 

 the process was considerable. Briefly, it may be 

 said that under the conditions of the experiment the 

 land failed to purify the sewage to a satisfactory 

 extent. To obtain definite results from experiments 

 of this kind the investigation would need to be 

 extended over a period of several months, or even 

 years, and this under the circumstances, of coarse, 

 was out of the question. 



As the conditions of this experi- 

 South Norwood, ment were decidedly abnormal, 

 both because of the large reserve 

 of nitrate in the soil and the slow way in which 

 this nitrate was given up, and also because of un- 

 cropped arable land only having been used, one 

 cannot safely draw any further conclusions from the 

 results with regard to the purification which the 

 South Norwood farm might ordinarily effect. As 

 regards the general bacteriological results, it appears 

 that the effluents were at first very bad, later they 

 improved somewhat, although sample 190 (one plot) 



6a 



