If sedimentation were the predominating factor in the purification of the water, we would then expect to find the following phenomena in the operation of the filters: A more rapid deposition of a given amount of sediment under summer temperature conditions than under winter, as the water passes through the sand, and therefore, for the former condition of higher temperature: (a) A greater concentration of this turbidity-producing material in the top layer of sand, or, in other words, a thinner sand layer to be removed in scraping if all the dirty sand is removed; (b) Because of the greater concentration, a greater rate of Increase of the loss of head, and consequently shorter periods of service between scrapings; (c) A higher limit for turbidity in the water applied to the filter to produce a given turbidity in the effluent. ~Table 30--Service Periods and Scraping Depths for Runs Ending In Various Months; Covering Entire Period, October 1st, 1905, To March 1st, 1907.~ ==========+=========+===========+===============+============= | | Average | Average | Mean | Number | period of | depth of sand | temperature, Month. | of | service | removed, in | in degrees, | filters.| in days. | inches. | Fahrenheit. ----------+---------+-----------+---------------+------------- January | 13 | 75 | 2.09 | 39 February | 6 | 98 | 2.46 | 37 March | 5 | 130 | 2.66 | 41 April | 8 | 149 | 2.96 | 53 May | 7 | 130 | 2.80 | 67 June | 11 | 124 | 2.35 | 77 July | 17 | 70 | 2.12 | 81 August | 2 | 49 | 1.98 | 80 September | 5 | 73 | 2.48 | 76 October | 37 | 70 | 1.56 | 64 November | 20 | 42 | 0.81 | 49 December | 14 | 57 | 0.94 | 40 ==========+=========+===========+===============+=============