WASTING EFFLUENT AFTER WASHING FILTERS.
After washing a mechanical filter the effluent for the first few minutes is often inferior in quality to that obtained at other times, and if samples are taken at these times and averaged with other samples taken during the run, an apparent efficiency may be obtained inferior to the true efficiency. To guard against this source of error, whenever samples have been taken at such times, the average work for the day has been taken, not as the numerical average of the results, but each sample has been given weight in proportion to the amount of time which it could be taken as representing; so that the results represent as nearly as possible the average number of bacteria in the effluent for the whole run. As a matter of fact, however, comparatively few samples were taken during these periods of reduced efficiency, and thus most of the results represent the normal efficiency exclusive of this period. A study has been made, however, of the results of examinations of samples taken directly after washing, somewhat in detail. The following is a tabular statement of the average results obtained from each filter by months, including only the results obtained on those days when samples were taken within twenty minutes after washing, the results of other days being excluded.
| AVERAGE NUMBER OF BACTERIA IN EFFLUENT. | ||||
|---|---|---|---|---|
| Shown by Record Sheets. | Within Ten Minutes after Washing. | 11 to 20 Minutes after Washing. | More than Twenty Minutes after Washing. | |
| WARREN FILTER. | ||||
| February | 115 | 118 | 114 | |
| March | 316 | 50 | 515 | 301 |
| April | 79 | 417 | 207 | 75 |
| May | (Special experiments, omitted.) | |||
| June | 197 | 493 | 272 | 170 |
| July | 300 | 546 | 207 | |
| August | 174 | 356 | 601 | 223 |
| JEWELL FILTER. | ||||
| February | 2453 | 2425 | 2099 | |
| March | 455 | 657 | 958 | 354 |
| April | 99 | 665 | 462 | 165 |
| May | 144 | 998 | 346 | 127 |
| June | (Special experiments, omitted.) | |||
| July | 279 | 1330 | 272 | 274 |
| August | 344 | 612 | 323 | 376 |
The time of inferior work very rarely exceeded twenty minutes. It will be seen from the tables that the results as shown by the record sheets are never very much higher, and are occasionally lower than the results of samples taken on corresponding days more than twenty minutes after washing; and thus while a decrease in bacterial efficiency was noted after washing, no material increase in the average bacterial efficiency of the mechanical filters would have been obtained if these results had been excluded. The results for the whole time would be affected much less than is indicated by the table, because the table includes only results of those days when samples were taken just after washing, while the much larger number of days when no such samples were taken would show no change whatever.
It has been suggested that these inferior effluents after washing should be wasted. Such a procedure would mean wasting probably on an average two per cent of the water filtered, and a corresponding increase in the cost of filtering. Mr. Fuller[43] in his Louisville report comes to the conclusion that with adequate washing and coagulation it is unnecessary to waste any effluent, and that inferior results after washing usually indicate incomplete washing. While our experiments certainly indicate a reduction in efficiency after washing so regular and persistent as to make it doubtful whether incomplete washing can be the cause of it, it may be questioned whether or not wasting the effluent would be necessary or desirable in actual operation. At any rate the results as given in this report are not materially influenced by this factor.