8.6 How is the required filtration area determined?
The percolation test results can be correlated to the necessary infiltration surface areas for different classes of soils. A high safety coefficient must be applied because of (i) progressive clogging by soil and wastewater particles, depending on the soil characteristics, (ii) rainfalls, which add water to the daily flow of wastewater, and (iii) the duration of infiltration.
The formula to calculate the infiltration area is as follows:
The safety coefficient can be applied to consider variations in factors such as rainfall intensity and the quality of effluent. By incorporating this safety factor, the area will be able to handle a higher volume or rate of effluent reducing the risk of saturation or clogging of the infiltration area. The factor may range between 1.2 to 2 depending on the conditions. However, to ensure the long-term functioning of the infiltration process, the adequate treatment of effluent prior to infiltration and the limited use of chemicals that may reduce the hydraulic capacity of the soil is essential.
The following table shows the estimated required areas for infiltration trenches considering infiltration rates for different soil textures. The minimum area requirement for infiltration trenches is 20 m2 per m3 effluent.
| Soil Texture | Infiltration Rate (m/s) | Infiltration Rate (mm/h = l/m2.h) | Effluent Infiltration Rate (l/m2/day) | Area (m2) for 1000 litres/day |
|---|---|---|---|---|
| Gravel, Coarse Sand | > 4 . 10-4 | > 1440 | N.A. | |
| Sand | 4 . 10-4 – 1 . 10-4 | 1440 – 360 | 50 | 20 |
| Fine Sand | 1 . 10-4 – 8 . 10-5 | 360 – 288 | 42 | 24 |
| Silty Sand | 7 . 10-5 – 3 . 10-5 | 252 – 108 | 32 | 31 |
| Sandy Silt | 3 . 10-5 – 1 . 10-5 | 108 – 36 | 25 | 40 |
| Clayey Silt | 1 . 10-5 – 7 . 10-6 | 36 – 25 | 19 | 53 |
| Sandy Clay | 9 . 10-6 – 7 . 10-6 | 32 – 25 | 12.5 | 80 |
| Silty Clay | 7 . 10-6 – 4 . 10-6 | 25 – 14 | 8.5 | 118 |
| Clay | < 4 . 10-6 | < 14 | N.A. |
