What percolation time provides the best treatment of sewage in the soil?

• A. Less than 5 min/cm
• B. Between 10 and 20 min/cm
• C. Between 25 and 35 min/cm
• D. More than 50 min/cm

Answer: (B)

The main idea is that treatment in soil depends on how long the effluent stays in contact with soil and its microbes. If the soil drains very quickly, the liquid moves away too fast for meaningful filtration, adsorption, and microbial breakdown of contaminants, so treatment is limited. If drainage is very slow, water ponds or pools, increasing the chance of surface issues and reducing effective treatment time, and can create conditions that don’t optimally support the beneficial processes in the soil.

A moderate percolation time provides enough residence time for physical filtration, adsorption of organics, and microbial activity to reduce pathogens and pollutants, without causing drainage problems or surface pooling. The 10 to 20 minutes per centimeter range represents this balanced rate, making it the best for treating sewage in soil.