Figure 12—Average Number of Days of Service of Successive Runs for Groups A, B, C, and D.
Quality of the Effluent.—The averages given in [Table 29] show but little difference in the bacterial contents of the effluents from the four groups of filters. All are entirely satisfactory, and the differences in favor of one method or another are small. In looking for possible differences in the quality of the effluents from the four groups, it was thought that such differences might be most apparent at a time when the entire plant was working under the most adverse conditions. The bacterial counts, therefore, were summarized for the period from December 23d, 1907, to January 6th, 1908, inclusive, following a period of high turbidity and high bacteria in the raw water, with results as follows:
| 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.