8.5.3 Infiltration Mounds
Infiltration mounds are a practical solution for sites where traditional leach fields (infiltration trenches) are not feasible due to soil limitations. Natural soil limitations may imply poor drainage, shallow depth to bedrock, or high-water tables.
An infiltration mound, also known as a raised bed system or an elevated sand mound, is constructed above the natural ground surface and acts as a barrier to prevent the effluent from reaching the natural soil. The mound is made of sand or other fill material. The treated wastewater effluent is evenly distributed over the surface of the mound, usually done by dosing devices or distribution pipes. The effluent percolates through the sand or fill material, removing impurities and pathogens that may still exist. Once the treated effluent has passed through the sand or fill material, it is allowed to percolate into the natural soil beneath the mound for further treatment and final disposal.
The following table summarises the recommended design criteria for infiltration mounds.
| Design Criteria | Recommendations |
|---|---|
| Height of the mound | 1 m |
| Maximum Slope for Surface | 10% |
| Thickness of filter sand layer | 0.3 – 0.6 m |
| Distance between mounds | >2 m |
| Distance to water source | >30 m |
| Level above groundwater table | >0.6 m (absolute minimum even when water table rises is 0.3 m) |
