The hollows are filled with inert material like gravel, expanded clay aggregate, perlite, or vermiculite.
It is designed to let the water trickle over the longest possible treatment path along the length of the wall among the pebbles.
[citation needed] The water is brought in at the top, and percolates following a zigzag pattern inside the wall.
A film of beneficial bacteria grows over the pebbles, releasing the nutrients in the percolating greywater.
[citation needed] Greywater: The water feeding the plants in the wall must be free of heavy metals and/or unsafe pollutants, notably human waste.