~Table 31--Turbidity in Filtered Water at Different Temperatures Produced by Given Turbidity in Applied Water.~ ==========+================================================= Turbidity | of | ~Temperature, in Degrees, Fahrenheit.~ applied |---------+---------+---------+---------+--------- water. | 40 | 40 - 50 | 50 - 60 | 60 - 70 | 70 ----------+---------+---------+---------+---------+--------- 20 | 1.8 | 1.3 | 1.2 | 1.5 | 1.7 20-40 | 4.8 | 5.0 | 3.5 | 3.0 | 2.6 40-60 | 7.9 | 6.9 | 5.4 | ... | 3.7 60-80 | 10.7 | 7.7 | ... | ... | 5.4 80-100 | 11.3 | ... | ... | ... | ... 100 | ... | ... | ... | ... | 12.0[1] ==========+=========+=========+=========+=========+========= [Footnote 1: For an average turbidity = 150. approximately.] The influence of the temperature of the water on the turbidity of the effluent is very pronounced. For a temperature of less than 40 deg. Fahr. (actual average temperature about 35 deg.), the turbidity of the filtered water for a given turbidity of the applied water is practically twice as great as for a temperature greater than 70 deg. (actual average temperature about 75 deg.). This fact fits in very nicely with the influence of temperature on sedimentation. Referring again to this temperature relation, as set forth on a previous page, the hydraulic subsiding value of a particle in water, of a size so small that viscosity is the controlling factor in its downward velocity, is approximately twice as great at 75 deg. as at 35 degrees. We would then expect to find that, in order to obtain a given turbidity in the filtered water, a raw water may be applied at 75 deg., having twice the turbidity of the water applied at 35 deg., to produce the same turbidity; and further, as the turbidity of the filtered water, for a given temperature condition, varies quite directly in proportion to the turbidity in the applied water, it follows that an applied water of given turbidity will produce an effluent at 35 deg. with a turbidity twice as great as at 75 degrees. This is quite in accordance with the facts obtained in actual operation, as indicated on the diagram, Figure 15. _Preliminary Treatment of the Water._--The most striking features of the bacterial results given in Table 4 are, first, the uniformly low numbers of bacteria in the filtered water during perhaps 8 or 9 months of the year, and the increase in numbers each winter. This is shown clearly in the analysis of bacterial counts in Table 32.