Group....... _A_ _B_ _C_ _D_ Maximum..... 204 178 189 206 Minimum..... 61 45 62 57 Average..... 120 107 104 155 The following is a summary of the turbidity results for a similar period: Group....... _A_ _B_ _C_ _D_ Maximum..... 10.8 11.7 8.7 9.3 Minimum..... 6.7 4.7 6.2 5.7 Average..... 8.7 8.3 7.2 7.9 These numbers, though high, do not show any significant differences. All the averages for each group are less than the lowest maximum, and all are greater than the highest minimum, and therefore vary less than do the individual filters, from other causes, within the different groups. _Future Capacity of the Filters._--An indication of the dangers which might affect the future capacity of the filters was shown in the above discussion of the present capacity. A more effective way of showing this was obtained by a study of the initial resistances or losses of head in the four groups. A filter kept in ideal condition would show no increase in this initial loss of head from one run to the next. If there is such an increase, it means that at some future time measures more heroic than ordinarily used would be necessary to restore the proper capacity. The average initial losses of head for the different groups are plotted on the diagram, Figure 13. Group _A_ shows an initial loss of head, increasing gradually but slightly during more than two years of service. In Group _B_ the initial loss of head increased in a manner similar to that in Group _A,_ up to the time of the beginning of these experiments; after which the increase becomes more rapid. Groups _C_ and _D_ show conditions generally similar to Group _B_, with some variations which are self-explanatory. _Conclusions._--The quality of the effluents from all four groups was satisfactory, and no consistent difference was apparent in favor of one or another method of treatment. The method pursued with Group _C_ was entirely insufficient to maintain the capacity indefinitely. The methods pursued in Groups _B_ and _D_ were both insufficient, but would have been more effective if a greater depth of sand had been removed. The costs of treatment of Groups _B_ and _D_ were less than for Group _A_. It appears, then, that a treatment which would be more economical than the old method of Group _A_, and would still maintain the proper capacity, would be one similar to that of Groups _B_ or _D_, with the removal of a quantity of sand greater than was done in the case of these two groups, but less than in the old method. [Illustration: ~Figure 13--Average Initial Losses of Head for Groups _A_, _B_, _C_, and _D_ for Successive Runs.~]