THE LAWRENCE FILTER.

The filter consists of a single bed 2¹⁄₂ acres in area, the bottom of which is 7 feet below low water in the river, and filled with gravel and sand to an average depth of 4¹⁄₂ feet. The filter is all in a single bed instead of being divided into the three or four sections which would probably have been used for a continuous filter of this size. The water-tight bottom also was dispensed with, and the gravel was prevented from sinking into the silt by thin intermediate layers of graded materials. The saving in cost was considerable; but, on the other hand, a considerable quantity of ground-water comes up through the bottom and increases the hardness of the water from 1.5 to 2.6 parts of calcium carbonate in 100,000; and while the water when compared with many other waters is still extremely soft, the addition cannot be regarded as desirable. The ground-water also contains iron, which increases the color of the water above what it would otherwise be.

The underdrains have a frictional resistance ten times as great as would be desirable for a continuous filter, the idea being to check extreme rates of filtration in case of unequal flooding, and also to limit the quantity of water which could be gotten through the filter to that corresponding to a moderate rate of filtration.

The sand, instead of being all of the same-sized grain, is of two grades, with effective sizes respectively 0.25 and 0.30 mm., the coarser sand being placed farthest away from the underdrains, where its greater distance is intended to balance its reduced frictional resistance and make all parts filter at an equal rate.

The surface instead of being level is waved, that is, there are ridges thirty feet apart, sloping evenly to the valleys one foot deep half way between them, to allow water to be brought on rapidly without disturbing the sand surface. For the same reason, as well as to secure equality of distribution, a system of concrete carriers for the raw water goes to all parts of the filter, reducing the effective filtering area by 4 or 5 per cent. The filter is scraped as necessary in sections, the work being performed when the filter is having its daily rest and aeration. Owing to the difference in frictional resistance before and after scraping, and to the fact that it is impossible to scrape the entire area in one day, considerable variations in the rate of filtration in different parts of the filter must occur. The heavy frictional resistance of the underdrains when more than the proper quantity of water passes them tends to correct this tendency especially for the more remote parts of the filter, but perhaps at the expense of those near to the main drain.

The filter is not covered as the suggestions in Chapter II would require, but this is hardly on account of its being an intermittent filter.

The annual report of the Massachusetts State Board of Health for 1893 states that during the first half of December, 1893, the surface remained covered, that is, it was used continuously, and after December 16th it was so used when the temperature was below 24°, and was drained only when the temperature was 24° or above. The days on which the filter was drained during the remainder of December are not given, but during January and February, 1894, the filter remained covered 29 days and was drained 30 days. Bacterial samples were taken on 44 of these days, 22 days when it was drained and 22 when it was not. The average number of bacteria on the days when it was not drained was 137 and on those days when it was drained 252 per cubic centimeter.

From February 24th to March 12th the number of bacteria were unusually high, averaging 492 per cubic centimeter, or 5.28 per cent of the 9308 applied. During this period the filter was used intermittently; there was ice upon it, and parts of the surface were scraped under the ice, and high rates of filtration undoubtedly resulted on the scraped areas. After March 12th the ice had disappeared and very much better results were obtained.

While there may be some question as to the direct cause of this decreased efficiency with continued cold weather and ice, the results certainly are not such as to show the advisability of building open filters in the Lawrence climate.

The cost of building the filter in comparison with European filters was extraordinarily low—only $67,000, or $27,000 per acre of filter surface. To have constructed open continuous filters of the same area with water-tight bottoms, divided into sections with separate drains and regulating apparatus, with the necessary piping, would have cost at least half as much more, and with the masonry cover which I regard as most desirable in the Lawrence climate the cost would have been two or three times the expenditure actually required.

It was no easy matter to secure the consent of the city government to the expenditure of even the sum used; there was much skepticism as to the process of filtration in general, and it was said that mechanical filters could be put in for about the same cost. Insisting upon the more complete and expensive form might have resulted either in an indefinite postponement of action, or in the adoption of an inferior and entirely inadequate process. Still I feel strongly that in the end the greater expense would have proved an excellent investment in securing softer water and in the greater facility and security of operating the filter in winter.

In regard to the effect of the Lawrence filter upon the health of the city, I can best quote from Mr. Mills’ paper in the Report of the Massachusetts State Board of Health for 1893, and also published in the Journal of the New England Water-works Association. Mr. Mills says: “In the following diagram [Fig. 15] the average number of deaths from typhoid fever at Lawrence for each month from October to May, in the preceding five years, are given by the heavy dotted line; and the number during the past eight months are given by the heavy full line.

“The total number for eight months in past years has been forty-three, and in the present year seventeen, making a saving of twenty-six. Of the seventeen who died nine were operatives in the mills, each of whom was known to have drunk unfiltered canal water, which is used in the factories at the sinks for washing.

Fig. 15.—Typhoid Fever in Lawrence.

“The finer full line shows the number of those who died month after month who are not known to have used the poisoned canal water. The whole number in the eight months is eight.

“It is evident from the previous diagram [not reproduced] that the numbers above the fine full line, here, follow after those at Lowell in the usual time, and were undoubtedly caused by the sickness at Lowell; but we have satisfactory reason to conclude that the disease was not propagated through the filter but that the germs were conveyed directly into the canals and to those who drank of the unfiltered canal water. Among the operatives of one of the large corporations not using the canal water there was not a case of typhoid fever during this period. Warnings have been placed in the mills where canal water is used to prevent the operatives from drinking it.

“We find, then, that the mortality from typhoid fever has, during the use of the filter, been reduced to 40 per cent of the former mortality, and that the cases forming nearly one half of this 40 per cent were undoubtedly due to the continued use of unfiltered river water drawn from the canals.”

The records of typhoid fever in Lawrence before and after the introduction of filters are as follows:

DEATHS FROM TYPHOID FEVER IN LAWRENCE, 1888-98.
Years.	Total Number of Deaths.	Deaths per 10,000 of Population.	Persons who are known to have been exposed to infection.
Years.	Total Number of Deaths.	Deaths per 10,000 of Population.	By drinking Canal Water.	While living out of town just before falling sick in Lawrence.
1888	48	11.36
1889	55	12.66
1890	60	13.44
1891	55	11.94
1892	50	10.52
1893	39	7.96
1894	24	4.75	12
1895	16	3.07	9	2
1896	10	1.86	2	4
1897	9	1.62
1898	8	1.39	1
Filter put in operation September, 1893.
Average rate before the introduction of filtered water (1888-92)				11.31
Average rate afterward (1894-98)				2.54

These results show a striking reduction in the deaths from typhoid fever with the introduction of filtered water, which has been most gratifying in every way.

The more recent history of the underdrains of the Lawrence filter is particularly instructive. Owing to the absence of a water-tight bottom to the filter, and its low position, a certain amount of water constantly entered the filter from the ground below. This water contained iron in solution as ferrous carbonate. When this water came in contact with the filtered water in the gravel and underdrains, the iron was oxidized by the dissolved oxygen carried in the filtered water and precipitated. This was accompanied by a growth of crenothrix in the gravel and underdrains, which gradually reduced their carrying capacity. This reduction in carrying capacity first became apparent in cold weather when the yield from the filter was less free than formerly. There was difficulty in maintaining the supply during the winter of 1896-7 and more difficulty in the following winter.

Fig. 16.—Typhoid Fever in Lawrence, 1888 to 1898.

The sand of the filter was as capable of filtering the full supply of water as it ever had been, and the efficiency was as good; but the underdrains were no longer able to collect the filtered water and deliver it. As the filtering area was ample for the supply, it was desired to avoid construction of additional filtering area. The underdrains were dug up and cleaned during the periods when the filter was drained. As the filter is all in one bed, the times when the filter could be allowed to remain drained, and when the work could proceed, were limited. Great care was taken to leave the work in good condition, and free from passages, at the end of each day’s work, but the numbers of bacteria in the effluent nevertheless increased somewhat. Some weeks afterward the number of cases of typhoid fever in the city increased. The numbers did not become as high as they had been prior to the introduction of filtered water, but they were much higher than they had been since that time, and they pointed strongly to the disturbance of the underdrains as the cause of the increase.

The numbers of bacteria in the applied water and in the effluent from the Lawrence filter by months, from the time the filter was put in operation, compiled from the reports of the State Board of Health, as far as available, are as follows:

BACTERIA IN WATER APPLIED TO AND EFFLUENT FROM LAWRENCE FILTER.
RAW WATER.
	1893.	1894.	1895.	1896.	1897.	1898.
January		7,700	18,700	7,500	13,314	6,519
February		7,600	15,040	12,600	13,113	4,653
March		6,500	20,770	5,900	12,055	3,748
April		11,200	8,420	3,800	6,904	2,320
May		6,000	7,000	9,600	4,625	2,050
June		8,300	9,000	6,400	4,650	6,775
July		2,400	10,000	3,900	6,240	2,840
August		3,100	5,000	2,700	10,700	8,575
September	57,500	6,500	5,000	12,300	27,300	6,100
October	22,200	25,300	19,000	5,300	13,200	5,120
November	10,800	16,600	8,700	5,600	6,644	4,310
December	8,100	23,800	6,700	9,695	5,581	5,200
Average	24,650	10,417	11,111	7,108	10,360	4,850
EFFLUENT.
January		129	206	166	91	39
February		244	283	315	79	45
March		455	405	133	67	34
April		281	84	40	47	21
May		134	68	56	35	48
June		110	68	22	56	50
July		25	50	39	106	22
August		36	38	146	72	28
September	6,850	42	40	37	98	67
October	1,216	116	60	30	33	28
November	161	175	64	37	27	122
December	111	364	84	67	24
Average	2,084	176	121	91	61	46
Average efficiency	91.55	98.31	98.91	98.72	99.41	98.95