The filtration of public water-supplies / Third edition, revised and enlarged.

by Allen Hazen

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.

INFLUENCE OF AMOUNT OF SULPHATE OF ALUMINA ON BACTERIAL EFFICIENCY OF MECHANICAL FILTERS.

The number of bacteria passing a mechanical filter is dependent principally upon the amount of sulphate of alumina used; and by using a larger quantity of sulphate of alumina than was actually used in the experiments the bacterial efficiency could be considerably increased. To investigate this point, the results obtained each day with each of the mechanical filters were arranged in the order of the sulphate of alumina quantities used, and averaged by classes. In this and the following tables a few abnormal results were omitted.[44] A summary of the results is as follows:

SUMMARY OF RESULTS WITH WARREN MECHANICAL FILTER, ARRANGED ACCORDING TOSULPHATE OF ALUMINA QUANTITIES.
Number of Days Represented.	Turbidity.	Bacteria.		Per cent remaining.	Per cent removed.	Sulphate of Alumina used Grains per Gallon.
		Raw Water.	Effluent.
7	0.05	4,773	1713	35.89	64.11	0.00
2	0.08	2,785	850	30.52	69.48	0.12
4	0.10	5,109	726	14.21	85.79	0.26
2	0.20	8,713	214	2.45	97.55	0.36
8	0.06	3,224	112	3.47	96.53	0.44
19	0.06	3,488	123	3.53	96.47	0.55
11	0.06	5,673	154	2.71	97.29	0.64
10	0.10	6,100	112	1.84	98.16	0.74
8	0.09	8,647	148	1.71	98.29	0.85
5	0.16	5,645	142	2.52	97.48	0.93
13	0.12	10,397	200	1.92	98.08	1.07
10	0.08	12,778	121	0.95	99.05	1.13
13	0.14	13,397	164	1.22	98.78	1.25
19	0.13	10,462	160	1.53	98.47	1.34
10	0.12	12,851	107	0.83	99.17	1.46
4	0.27	16,015	77	0.48	99.52	1.57
7	0.53	12,262	191	1.18	98.82	1.64
4	0.58	26,950	347	1.29	98.71	1.74
5	0.29	14,570	86	0.59	99.41	1.84
3	0.23	13,833	153	1.11	98.89	1.92
19	0.40	18,222	92	0.50	99.50	2.48
5	0.45	29,300	1119	3.82	96.18	3.37
5	1.06	33,030	535	1.62	98.38	8.06
SUMMARY OF RESULTS WITH JEWELL MECHANICAL FILTER, ARRANGED ACCORDING TO SULPHATE OF ALUMINA QUANTITIES.
Number of Days Represented.	Turbidity.	Bacteria.		Per cent remaining.	Per cent removed.	Sulphate of Alumina used Grains per Gallon.
		Raw Water.	Effluent.
6	0.03	14,037	6217	44.29	55.71	0.00
5	0.07	4,267	680	15.93	84.07	0.24
14	0.06	2,613	170	6.50	93.50	0.35
10	0.06	2,446	113	4.62	95.38	0.44
9	0.11	7,303	234	3.20	96.80	0.55
20	0.09	6,979	220	3.15	96.85	0.65
9	0.08	5,191	130	2.50	97.50	0.75
16	0.12	8,504	242	2.84	97.16	0.83
22	0.16	8,506	99	1.16	98.84	0.96
12	0.11	11,998	246	2.05	97.95	1.05
14	0.18	18,982	423	2.23	97.77	1.16
5	0.14	13,981	224	1.60	98.40	1.23
9	0.27	19,806	325	1.64	98.36	1.34
14	0.27	16,549	324	1.96	98.04	1.45
9	0.29	12,194	96	0.79	99.21	1.54
6	0.25	13,483	51	0.38	99.62	1.65
7	0.53	24,243	220	0.91	99.09	1.72
3	0.90	20,953	602	2.88	97.12	1.90
5	0.43	25,958	307	1.19	98.81	2.19
4	0.84	21,017	228	1.09	98.91	3.71

These results are shown graphically by Fig. 21.

Fig. 21.—Bacterial Efficiencies of Mechanical Filters.