What is the minimum thickness of the sand layer of a Type A dispersal bed?

• A. 100 mm
• B. 200 mm
• C. 300 mm
• D. 400 mm

Answer: (C)

The sand layer acts as a porous, protective medium that supports even distribution of effluent and provides filtration before the wastewater reaches the soil below. For a Type A dispersal bed, this layer must be thick enough to slow the flow, trap solids, and prevent clogging of the underlying soil while allowing water to pass through uniformly. The minimum thickness used in design standards is 300 mm (about 12 inches), which ensures adequate filtration capacity and distribution across the bed. Using less than 300 mm risks rapid clogging and uneven distribution, while a thicker layer (such as 400 mm) can be used but is not required by the minimum specification.