Although this separate section is devoted to the question of calculating the capacities of percolating filters and contact beds, it is mainly for the purpose of stating that it is impossible to formulate any rules which admit of general application. It may reasonably be pointed out that this last statement is a truism, and affords no assistance to those in search of information on the subject. There is, however, so great a tendency in some quarters, to rely upon results obtained in one place under certain conditions as a guide in designing a scheme in another place under possibly totally different conditions, that it is impossible to repeat the statement in question too often.
In the first place, long and practical experience is necessary to enable an engineer to come to a decision as to what are the conditions under which any particular scheme is to be carried out, and which of them will have a bearing upon the methods to be adopted in the design of the works. A careful study of the fifth report of the Royal Commission on Sewage Disposal, will show that they assume over 70 different sets of conditions under which percolating filters and contact beds may be adopted. The capacity of the filters required to produce the desired results will depend upon the strength of the sewage to be treated, the type of tank adopted for the preliminary process of sedimentation, the grade of material to be used, the amount of fall available, the final destination of the effluent, and other factors, all of which again may be affected by other circumstances, which must of necessity be taken into consideration. As the basis for calculating the capacity of filters may vary between 15 and 200 gallons of the daily dry-weather flow per cubic yard of material, it is evident that there is a wide margin for error, and the only safe course to adopt is to allow for the worst possible conditions and thus provide a large margin of safety. Although the suggestions made in the fifth report of the Royal Commission with regard to the provision to be made under various sets of conditions may be taken as a guide, to some extent, it should be borne in mind that the figures given represent the minimum which should be allowed in each case, and the only really safe guide in these matters is long practical experience of a large number of works under the greatest possible variety of conditions.
In order, however, to provide a rough guide for the purpose of making preliminary estimates, it may be stated here that, under ordinary conditions, with sewage of average strength, a properly designed preliminary process, suitable material of medium grade, and not less than 4 feet deep for percolating filters, it should be possible to produce an effluent which will not create a nuisance by providing—
(a) Percolating filters, at the rate of one cubic yard for every 84 gallons of the daily dry-weather volume, or
(b) Contact beds, at the rate of one cubic yard in each series for every 56 gallons of the daily dry-weather volume.
In other words, the ratio of the cubic capacity of filter material to the daily dry-weather volume of the sewage for all ordinary purposes may be taken as—
(a) 2 to 1 for percolating filters.
(b) 3 to 1 for contact beds.
TABLE, Giving the Ratio of the Total Cubic Capacity of Percolating Filters and Contact Beds to the Daily Dry-Weather Volume of the Sewage under Varying Conditions.
| Preliminary Process. (See pages 23, 29). |
Percolating Filters | |||||
| Strength of Sewage | ||||||
| Strong | Average | Weak | ||||
| Grade of Material | ||||||
| Coarse or medium | Fine | Coarse or medium | Fine | Coarse or medium | Fine | |
| Detritus tanks | 11·20 | — | 6·72 | — | 4·20 | — |
| Septic tanks | 3·73 | — | 2·40 | — | 1·68 | 1·68 |
| Continuous flow settlement | 3·73 | — | 2·40 | — | 1·68 | 1·68 |
| Quiescent settlement | 3·36 | 6·72 | 1·68 | 2·40 | 1·29 | 1·29 |
| Continuous flow chemical | ||||||
| precipitation | 2·58 | 3·36 | 1·68 | 2·10 | 1·12 | 0·96 |
| Quiescent chemical | ||||||
| precipitation | 1·68 | 2·58 | 1·29 | 1·29 | 0·98 | 0·84 |
| Preliminary Process. (See pages 23, 29). |
Contact Beds | ||||||||
| Strength of Sewage | |||||||||
| Strong | Average | Weak | |||||||
| Number of Series | |||||||||
| ×1 | ×2 | ×3 | ×1 | ×2 | ×3 | ×1 | ×2 | ×3 | |
| Detritus tanks | — | — | 6·72 | — | 6·72 | — | — | 4·42 | — |
| Septic tanks | — | — | 5·09 | — | 4·42 | — | 2·24 | 2·54 | — |
| Continuous flow settlement | — | — | 5·09 | — | 4·42 | — | 2·24 | 2·54 | — |
| Quiescent settlement | — | — | 3·81 | — | 3·36 | — | 1·68 | — | — |
| Continuous flow chemical | |||||||||
| precipitation | — | 5·09 | — | — | 3·36 | — | 1·26 | — | — |
| Quiescent chemical | |||||||||
| precipitation | — | 3·90 | — | — | 2·54 | — | 1·26 | — | — |
In the case of contact beds these figures give the cubic capacity of each series, and they must be doubled or trebled respectively for double and triple contact. Both percolating filters and contact beds, constructed on this basis, would be capable of treating up to three times the dry-weather flow in times of storm.
Having given the above method of calculation, in a form not usually adopted in connection with sewage disposal, and bearing in mind the misunderstandings which frequently arise in comparing the various methods in use at the present time in different countries, it may be useful to set out in this form the figures which, it is understood, have been adopted by the Local Government Board, on the basis of the fifth report of the Royal Commission on Sewage Disposal, as the minimum which they consider suitable under varying conditions. In the Table opposite, the figures given represent the ratio which the cubic capacity of the filters bears to the daily dry-weather volume of the sewage, whether it be in gallons, cubic feet, cubic metres, vedros, or any other term of measurement.
Examples.—1. A daily dry-weather volume of 10,000 gallons of sewage, of average strength, is to be treated upon percolating filters of medium sized material, after preliminary treatment in septic tanks.
2. A daily dry-weather volume of 3,000 cubic metres of weak sewage is to be treated upon single contact beds, after preliminary treatment in continuous flow settlement tanks.
For the purpose of the above Table, the strength of the sewage is estimated according to the amount of oxygen absorbed from permanganate of potash in four hours, as indicated in the fifth report of the Royal Commission as follows:—
| Parts per 100,000 | |||
| “Strong” sewage = | oxygen | absorbed | 17 to 25 |
| “Average” ” = | ” | ” | 10 to 12 |
| “Weak” ” = | ” | ” | 7 to 8 |
A quick method of converting gallons into cubic feet is to multiply the gallons by the reciprocal 0·16. This can be done rapidly (frequently by mental calculation) by multiplying the gallons by 4, and the product again by 4, and inserting the decimal point between the second and third figures from the right-hand side, thus:—
This is useful in calculating the capacity of tanks, and for all similar purposes.