While this mythconception was debunked a long time ago, it still crops up from time to time. As recently as 2011, a book was published in which the author states…..
“A 12 inch (30cm) deep bed rarely or never needs to be cleaned out because the robust eco-system enabled by a deep grow bed breaks down the solid waste and takes care of this for you.”
A bit further on, she says…..
“If a media system is constructed, stocked and operated as instructed in this book, the only solids removal that will need to be done is a monthly shot of high-pressure water through your pipes and pump to knock lose (sic) any solid waste buildup inside. Otherwise there is no regular cleanout of the grow beds or fish tank. Ever.”
That the author is among the basic flood and drain fraternity (most of whom refute the need for dedicated mechanical and biological filtration) makes the position all the more precarious.
Solid wastes in an aquaponics system have two sources – the fish and uneaten food. Even when fully oxidised, some sludge remains. It has to go somewhere, so some of it remains in suspension (not good for fish) and some settles in the grow beds. Over time, the spent sludge builds up.
This is borne out by Dr Wilson Lennard who – in his fact sheet “Media Beds and Sizing” – describes it thus……
“The outcome is that it seems apparent that there may always be a percentage of the fish waste solids that are unable to be completely mineralised in aerated or aerobic aquatic systems. This means that if fish waste solids are directed to media beds for mineralisation, then at some stage, the bed will eventually start to clog because of the presence of these materials that do not appear to completely mineralise.”
Our recommendation is that media grow beds in basic flood and drain systems (sans dedicated filtration) should be cleaned at 3 – 6 monthly intervals.
Of course, if solid wastes are captured and removed from the system (for external processing) then it’s highly unlikely that clogging (and its associated problems) will ever occur.