Transactions of the American Society of Civil Engineers, vol. LXXII, June, 1911 / Water Purification Plant, Washington, D. C. Results of Operation.

by American Society of Civil Engineers; E. D. Hardy

The theory sometimes advanced, that the efficiency of filtration is controlled to a certain extent by gelatinous films, and that, as far as thus controlled, is less dependent on rate, would not seem to be borne out by these results. The Merrimac River water, carrying large amounts of organic matter, would certainly seem better adapted to the formation of such films than the clay‑bearing Potomac water, comparatively free from organic matter; but it is the Potomac water which seems to show the least influence of rate on efficiency.

The experiments show that turbidity passes more freely at the higher rates with the Potomac water, as has also been found to be the case with other clay‑bearing waters.

In the last lines of [Table 20] are given cost per million gallons for filtering at various rates. There is no discussion of these figures, and as they differ considerably from those which the writer has been accustomed to use, the calculation in [Table 23], made three years ago for a particular case, may be of interest.

Table 23—Relative Cost of Filtering at Different Rates.

	Nominal rate, in millions of gallons per acre daily:
	3	5	10	20
Percentage which average yield is of nominal rate	85	80	75	65
Average output per acre, in millions of gallons per day	2.55	4.00	7.5	13.0
Cost of that part of filters per acre dependent on rate	$12,000	$20,000	$40,000	$80,000
Cost of that part of filters per acre not dependent on rate	50,000	50,000	50,000	50,000
Total cost of filters per acre	60,000	70,000	90,000	130,000
Cost per million gallons of capacity	20,600	14,000	9,000	6,500
Cost per million gallons of average daily output	24,400	17,500	12,000	10,000
Capital charges and depreciation at 6% on cost per million gallons	4.00	2.87	1.97	1.64
Operating expenses, the same at all rates	1.00	1.00	1.00	1.00
Total cost of filtering, excluding pumping, storage, and all auxiliaries	5.00	3.87	2.97	2.64
Relative cost	1.29	1.00	0.77	0.68

When the costs of pumping, pure‑water reservoirs usually necessary, etc., are taken into account (which add equally to the cost at all rates), the cost of filtering will vary less with the rate than is indicated.

The effect of rate on cost, as calculated in [Table 23], and also the percentages of the bacteria of the raw water found in the effluents by the author and by Mr. Clark, are shown on [Figure 10].

Considering all these results together, and also all the other evidence known to the writer bearing on this point, it seems clear that filters are not as sensitive to changes in rate, within reasonable limits, as has been frequently assumed; but, on the other hand, there is usually a substantial increase in the percentage of bacteria passing through a filter with increased rate.

Filters furnish relative, not absolute, protection against infectious matter in the raw water. The higher the bacterial efficiency, the more complete is this relative protection.

The cost of filtering does not decrease in inverse ratio to the rate, but at a much slower rate. This is especially true with rates of more than 5,000,000 or 6,000,000 gal. per acre daily.

In general, a rate of filtration may rationally be selected at which the value of the possible danger resulting from an increase in rate is equal to the saving that may be made in cost by its use. This point must be a matter of individual judgment. The tendency of the last few years has been to use higher rates, or, in other words, to cheapen the process and to tolerate a larger proportion of bacteria in the effluent. The use of auxiliary processes has been favorable to this, especially the use of chloride of lime, in connection with either the raw water or the effluent.