Which statement is true about the relative thickness of Type A bed layers?

Which statement is true about the relative thickness of Type A bed layers?

• A. The sand layer minimum thickness exceeds the stone layer minimum thickness by 100 mm
• B. The stone layer minimum thickness exceeds the sand by 100 mm
• C. They are the same thickness
• D. The sand layer minimum is thinner than the stone layer

Answer: (A)

In Type A bed designs, the sand layer is thicker than the stone layer by 100 mm. This arrangement places a thicker fine-filter media on top to provide adequate filtration and residence time as the effluent moves downward, while the coarser stone beneath offers drainage and structural support. Having the sand layer exceed the stone layer by 100 mm helps prevent fines from migrating into the stone layer and clogging the system, while still letting water drain efficiently through the lower layer into the soil. If the stone were thicker or the layers were the same thickness, filtration effectiveness and proper drainage could be compromised.

In Type A bed designs, the sand layer is thicker than the stone layer by 100 mm. This arrangement places a thicker fine-filter media on top to provide adequate filtration and residence time as the effluent moves downward, while the coarser stone beneath offers drainage and structural support. Having the sand layer exceed the stone layer by 100 mm helps prevent fines from migrating into the stone layer and clogging the system, while still letting water drain efficiently through the lower layer into the soil. If the stone were thicker or the layers were the same thickness, filtration effectiveness and proper drainage could be compromised.