17th July 2012 at 2:00 pm #801
Which video do you mean?
Particles that are not broken down by chemical and biological processes will accumulate within the first few centimetres of sand (if the sand has been sized correctly) – periodic maintenance will be required using methods such as wet harrowing, but how regular this maintenance is will depend on the type of raw water and nature of particles I guess. See http://biosandfilter.org/biosandfilter/index.php/item/299 for details about maintenance.