TABLE OF CONTENTS The removal of metals in the soil is a complex process involving the mechanisms of adsorption, precipitation, ion exchange, and complexation. Adsorption of most trace elements occurs on the surfaces of clay minerals, metal oxides, and organic matter; as a result, fine-textured and organic soils have a greater adsorption capacity for trace elements than sandy soils have. The slow rate (SR) land treatment process is the most effective for metals removal because of the finer-textured soils and the greater opportunity for contact and adsorption. Rapid infiltration (RI) can also be quite effective, but a longer travel distance in the soil will be necessary owing to the higher hydraulic loadings and coarser-textured soils. Overland flow (OF) systems allow minimal contact with the soil and typically remove between 60 and 90 percent depending on the hydraulic loading and the particular metal.
In general, metals are present in typical municipal wastewaters in low concentrations. As shown in Table 3.8, the typical metals concentrations in raw sewage are below the requirements for drinking and irrigation waters.
| Element | Raw sewage, mg/L | Drinking water, mg/L | Irrigation water, mg/L | |
|---|---|---|---|---|
| 20 years* | Continuous | |||
| Cadmium | 0.004 0.14 | 0.01 | 0.05 | 0.005 |
| Chromium | 0.02 0.70 | 0.05 | 20 | 5.0 |
| Lead | 0.05 1.27 | 0.05 | 20 | 5.0 |
| Zinc | 0.05 1.27 | 0.05 | 20 | 5.0 |
| *For fine-textured soils only. Normal irrigation practice for 20 years. For any soil, normal irrigation practice, no time limit. |
Wastewater treatment by activated sludge and similar processes tends...
