Both Mr. Pitkethly and Mr. Hammond refer to the system of liquefying tanks used at Monterrey preparatory to turning the sewage on the irrigation lands, and both express doubts as to their efficiency. The writer is now separated from his library and notes by many thousands of miles, and cannot quote "chapter and verse" as accurately as he would like, in order to support his views that the system adopted was adequate for dealing with a system such as that at Monterrey. It must be pointed out that not only was it intended to prevent the sewage from becoming a nuisance, but that the sewage flow plus a large quantity of surplus water was intended to be used profitably for irrigation purposes. With that object, the Company--or rather its allied Company, the Monterrey Railway, Light, and Power Company--obtained the control of 2,246 acres of the very finest arable land, with almost perfect natural drainage conditions, so that this land could be utilized to create a profitable revenue from the use of the sewage. The outfall sewer was accordingly designed to carry sufficient water and sewage to irrigate about 2,500 acres of land, which area could be considerably extended if necessary at any future time. Most authorities now agree that before turning sewage upon land, a preliminary treatment is required to remove as much as possible of the suspended matter, and then reduce the latter by subsidence in liquefying or septic tanks, so that the quantity remaining in the effluent is so small and finely divided that it may be readily decomposed and oxidized by bacterial action without risk of clogging the surface or interstices of the land upon which it may discharge.[12] [12] See Raikes, "Sewage Disposal Works," pages 97-98. Mr. Pitkethly quotes Messrs. Watson and O'Shaughnessy as saying, in their evidence before the Royal Commission on Sewage Disposal, that not more than 10% of the solids are digested in septic tanks, but it must be remembered that in many other places evidence was given before the same Commission showing that from 25 to 30% was actually obtained. Mr. J. D. Watson, in his paper, "Birmingham Sewage-Disposal Works,"[13] read in March, 1910, points out that: [13] _Minutes of Proceedings_, Inst. C. E., Vol. CLXXXI, p. 259. "The much-maligned sewage-farm still may be allowed (where the conditions are favourable) to rank as one of the best methods of sewage-disposal. Diverse opinions may be held as to what are favourable conditions, particularly as conditions are sure to vary widely with locality; but it may be assumed that where there is 1 acre of suitable land per 100 persons, as in Berlin and several other important cities, the efficiently-worked sewage-farm, when judged solely by the standard of the effluent produced, is still in the front rank. Effluents from such a farm are remarkable for their paucity of micro-organisms, their low albuminoid ammonia, and their unvarying character."