If the T-time in the soil underlying the proposed leaching bed is 55 minutes per centimeter, which of the following can you NOT do?

• A. Design a conventional leaching bed
• B. Design a mound bed
• C. Design a raised absorption trench
• D. Use a standard drain field design

Answer: (A)

A quick look at permeability is key. The T-time (time of travel) per centimeter tells you how fast water moves through the soil. A value of 55 minutes for each centimeter is very slow, meaning the soil beneath the proposed bed doesn’t accept effluent readily. Conventional leaching beds rely on the native soil to absorb and drain wastewater at a reasonable rate; with such slow infiltration, the bed would struggle to drain properly, leading to ponding, surface wetness, and potential system failure.

In soils like this, designers turn to alternatives that increase the effective absorption area or place the absorption above the natural grade, such as mound beds or raised absorption trenches. These designs provide more surface contact, use fill material to improve infiltration, and help ensure the effluent is treated and dispersed safely. Because of the sluggish soil, you would not choose a conventional leaching bed.