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

• A. 100 mm
• B. 150 mm
• C. 200 mm
• D. 250 mm

Answer: (C)

The main idea is that the stone layer in a Type A dispersal bed must be thick enough to provide proper drainage, uniform distribution of effluent, and sufficient pore space for infiltration. Two hundred millimeters is the minimum that meets these needs: it gives enough media to spread wastewater evenly over the bed and to maintain good hydraulic conductivity, while avoiding excessive compaction or piping issues. A thinner layer, like 100 or 150 mm, can lead to uneven distribution and quicker clogging of the soil beneath. A thicker layer, such as 250 mm, isn’t necessary under standard design and just adds cost and height without improving performance.