| 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 | — | — |
- KEY: ×1 = Single
- ×2 = Double
- ×3 = Triple
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.
- 10,000 gallons × 2·40 = 24,000 gallons
- = 3,840 cubic feet
- = total cubic capacity of filters
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.
- 3,000 cubic metres × 2·24 = 6,720 cubic metres
- = total cubic capacity of beds
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:—