8.2 What conditions need to be in place for safe infiltration?
When planning for safe infiltration, it is important to ensure that the following conditions are met:
Setback distances
The location of infiltration systems must maintain a minimum distance from other existing structures, infrastructures, or natural elements. Local regulations may specify the requirements. The table below summarises recommendations for the minimum distances that must be maintained. Adhering to these distances ensures a sufficient buffer to prevent any potential contamination or health risks.
| Reference Point | Horizontal Distance to Reference Point (m) |
|---|---|
| Well or spring used for water supply | 30 – 35 |
| Lake or stream, marsh, or pond | 15 |
| Consumption water line | 3 |
| Property boundary | 3 |
| Residence | 5 |
| Drain | 5 |
| Top of an embankment | 3 |
| Tree | 2 |
Soil suitability
The soil where infiltration is planned should have suitable properties for effective percolation and filtration. These properties may include soil texture, structure, permeability, and depth to the water table (the bottom of the infiltration system is recommended to be at least 1.5 m above groundwater level). Soil testing and analysis are often conducted to assess soil suitability. A percolation test is considered a must prior to the implementation of an infiltration system.
Adequate pre-treatment
Pre-treatment (primary and secondary) of wastewater before infiltration is critical to remove solids and contaminants that could harm the infiltration system or the receiving soil. This may include using biodigesters, anaerobic baffled reactors, trickling filters, or other treatment units. The effluent should be tested to guarantee that it is suitable for discharge into an infiltration system. The recommended BOD and TSS levels for effluent infiltration into the soil depend on soil texture and hydraulic loading.
Scheduling and loading
The rate at which effluent or wastewater is introduced into the infiltration system must be carefully controlled to avoid overloading and potential environmental harm. The system should include a dosing system with proper scheduling to ensure adequate loads and time between loads. Also, it is essential to ensure that an even distribution of wastewater across the infiltration system is realised.
While local regulations should always be considered, the following table summarizes some typical characteristics of infiltration systems and applications that should be avoided.
| Characteristic | Typical Application | Application to Avoid |
|---|---|---|
| Type of wastewater | Domestic and commercial | Facilities with non-sanitary and/or industrial wastewater |
| Minimum pre-treatment | Biodigester, septic tank | Discharge of raw wastewater |
| Landscape position | Ridge lines, hilltops, shoulder/side slopes | Depressions, foot slopes, concave slopes, floodplains |
| Topography | Planar, mildly undulating slopes <20% grade | Complex slopes of >30% |
| Soil texture | Sands to clay loams | Very fine sands, heavy clays, expandable clays |
| Soil structure | Granular, blocky | Platy, prismatic, or massive soils |
| Drainage | Moderately drained or well-drained sites | Extremely well, somewhat poor, or very poorly drained sites |
