based, the water consumption records for the summer of 1913 would have served as well.

Owing to the many values that must be assigned to filter capacity in order to obtain a comprehensive picture of the correlation between

MASS DIAGRAM FOR DETERMINATION OF FILTERED WATER STORAGE SPACE NECESSARY, WITH VARIOUS TOTAL FILTER CAPACITIES, TO MEET AN ANNUAL AVERAGE DAILY DRAFT OF 1,000,000 GALLONS (On Basis of Winter Period of 1917-1918)

this element and filtered water storage, the calculations are tedious. It is apparent that the same end may be reached, as in an impounding reservoir study, through the medium of a mass diagram. Accumu

TABLE 15

Relation between total filter capacity and required filtered water storage Annual average daily capacity of works 1,000,000 gallons

REQUIRED FILTERED WATER STORAGE

December 9, 1917 to
February 27, 1918

(3)

lated daily drafts take the place of stream flows and a series of lines representing accumulated filter outputs replace the draft lines of the

ordinary mass diagram. The maximum ordinate between the curve of accumulated draft and each line denoting an assumed filter output gives the corresponding amount of filtered water storage required. Figure 10 is the mass diagram for the winter period of 1917-1918. Necessity of compact reproduction caused extension of the diagram only far enough to include the maximum ordinates between draft and output lines.

Table 15 shows the relation between total filter capacity and required filtered water storage, on the basis of an annual average daily works capacity of 1,000,000 gallons, as calculated from the records of the summer of 1913 and the winter of 1917-1918. This tabulation is derived either from table 14, extended to include all of the filter capacities shown, or better from the mass diagram. From table 15 there may be found the amount of filtered water storage necessary to supply any annual average daily demand for water, when total filter capacities range between 1 and 1.515 times the average daily demand. If the average daily water consumption is 1 million gallons, for instance, and the filters cannot deliver in excess of this quantity, there must be filtered water storage space of at least 20.27 million gallons; but if the filters can supply 1.25 million gallons a day, the filtered water reservoir need have a capacity of but 2.53 million gallons to meet the excess draft of the period. Similarly, with a daily average water consumption of 20 million gallons and a filtering capacity equal to 21 million gallons (20 X 1.05 21), the filtered water reservoir should have available storage space for at least 20 X 13.31, or 266.2 million gallons of water. If the filters can normally supply 27 million gallons daily (20 X 1.35 = 27), however, an annual average daily demand of 20 million gallons can be met with a filtered water storage reservoir of 5.4 million gallons (20 X 0.27 = 5.4).

It will be noted that capacities of filtered water reservoirs for purification works, designed to meet the particular water consumption fluctuations illustrated, would be governed by the requirements of the summer period if the filters have a reserve capacity of less than approximately 8 per cent of the annual average daily draft. Winter water consumption controls the size of filtered water storage. space, under the conditions illustrated, if the total filter capacity is proportionately larger than this figure.

Projected water filtration works or additions to existing works

Relation between net daily capacity of works, total daily filter capacity, and filtered water storage capacity

Summer, 1913 Winter, 1917-18 Summer, 1913 Winter, 1917-18

should be designed with the assistance of a tabulation of the same character as table 15, with values based upon local conditions. In order to decide intelligently upon the average daily additional output to be sought in designing an enlargement of existing works, however, it becomes necessary first to determine the net daily capacity of the plant as it stands. This can be done by the use of table 16, which gives a transformation of the figures contained in table 15.

Based upon the water consumption records under discussion, only those works possessing very large filtered water storage facilities, in comparison with total daily filter capacities, have their net capacities dependent upon the summer period of high draft.

Curves showing graphically the results found in tables 15 and 16 will be found convenient for use. Such curves are given on figure 11. From them it is a simple matter to adjust the balance between total daily filter capacity and required filtered water storage space,