8.6 How is the required filtration area determined? - Humanitarian Sanitation Hub

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 TextureInfiltration 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> 1440N.A.
Sand4 . 10-4 – 1 . 10-41440 – 3605020
Fine Sand1 . 10-4 – 8 . 10-5360 – 2884224
Silty Sand7 . 10-5 – 3 . 10-5252 – 1083231
Sandy Silt3 . 10-5 – 1 . 10-5108 – 362540
Clayey Silt1 . 10-5 – 7 . 10-636 – 251953
Sandy Clay9 . 10-6 – 7 . 10-632 – 2512.580
Silty Clay7 . 10-6 – 4 . 10-625 – 148.5118
Clay< 4 . 10-6< 14N.A.
Area Requirement for Infiltration Trenches Based on Infiltration Rate and Soil Texture
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