What consequence can occur if rapid sand filters are not properly washed during backwash?

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Backwashing keeps the filter media bed clean and restores the spaces between sand grains for water to flow through. If the wash is not done properly, the fines and captured solids can remain and even bind the sand grains together, forming a cemented layer. This bonding reduces the bed’s porosity and permeability, making it harder for water to pass and leading to degraded filtration performance and higher headloss. That cementing of sand grains is the likely consequence of inadequate backwashing. The other options imply the bed becomes more permeable or unchanged, which isn’t the case when washing is ineffective.