How long till it clogs

[Jeremiah Robinson]

I'm trying to figure out how to quantify and estimate the media cleaning schedule for an all-in-one flood-drain media bed system.

It seems like your cleaning schedule would depend on the following factors:

• Fish feed rate
• Media size
• Grow bed horizontal surface area
• Grow bed depth
• Uniformity of solids application
• Solids removal (if applicable)

Dr. Lennard in his calculator recommends every 3-6 months, which is a shorter maintenance cycle than I've heard of others using on this forum and elsewhere.

Does anyone know how to quantify this, or have practical/anecdotal experiences as to how often you have to clean your media?

[George]

not once in three years.  six beds.  no solids removal.  lots of feed.  lots of worms.

[Mike]

I hope mine does the same. That is fantastic. 
George said:

not once in three years.  six beds.  no solids removal.  lots of feed.  lots of worms.

[Jeremiah Robinson]

Hey George.

That's great!  I've heard of others having the same experience, but also of some for whom it didn't last nearly that long.

Can I ask what size media you use?

[George]

.5 inch Stalite -  Flood and Drain media system - AKA Aquaponics Easy.  Sometimes a bed lays fallow for a while and that may help.  I have a bed absolutely loaded from top to bottom with basil and bean roots and it drains just fine.  The beds are 100 gallons and two feet deep so possibly that's a factor.

Jeremiah Robinson said:

Hey George.

That's great!  I've heard of others having the same experience, but also of some for whom it didn't last nearly that long.

Can I ask what size media you use?

[Rob Nash]

havent cleaned media in 6-7 years.. before adding red wrigglers, we cleaned every 3-6 mos.

this is with 12" deep media, with 1" granite on the bottom 6" and 1/2" granite on the top 6" (i try not to mix the two layers)

stocked heavy on fish... 3/4 - 1lb of fish / gallon   

here is the formula I use for media bed to water ratio...  

Gallons times .266 = sft of grow beds at 12" deep (cubic ft)

ie- 100 gallons needs 26.6 sft   ,   (or roughly one 4ft x 8ft box per 150 gallons)

1000 gallons needs 266   (I have four 5ft x 12ft on 1000 gallons (olnly 240sft))

you have some wiggle room here... but its roughly 4:1 gallons to media

[George]

Rob, I recall that you drain from beds to rafts.  

Do you remove any solids, other than in your beds?  

Some recommend removal of solids in such a situation but I've never understood why they think it is necessary.

Rob Nash said:

havent cleaned media in 6-7 years.. before adding red wrigglers, we cleaned every 3-6 mos.

[Jeremiah Robinson]

...you have some wiggle room here... but its roughly 4:1 gallons to media

So you're at a ratio of approximately 1.5 lb fish to 1 ft3 of grow bed space.  At 1% body weight/day, and presuming that 1/3 of feed is converted to solid waste, that's 0.5 lb fish waste per ft3 every 100 days.  

I'm not an expert on worms but have raised them in compost and seen that their castings take up space approximately 1/3 the size of the original feed.  Your experience suggests either that the castings take up less than 1/10th of the space that the fish waste does (since it takes more than 10x as long to clog) or that they are in some way resistant to becoming anaerobic.  Do you have any idea how this works?  Have you taken apart any of your beds to see how much space is taken up by worm castings?

Here is a long quote from the instruction manual associated with Dr. Lennard's calculator, which it sounds like your experiences disagree with:

There is little available scientific data on the volume reduction which occurs when fish waste solids are consumed by earth worms. By this I mean that if you add 1L of fish waste solids to a worm farm (or aquaponic media bed) how many litres of worm castings are produced? Because I cannot find comprehensive scientific data on this subject I have assumed a 1:1 ratio; meaning that 1L of fish waste will become 1L of worm castings (this approach has been followed as a “safe” approximation that covers all situations). Therefore, whilst it may be argued correctly that the addition of earth worms to a media bed will assist fish solid wastes breakdown, it cannot be assumed that: 

1. The worms consume all the fish wastes.
2. The worms convert fish wastes solids and produce NO solid wastes of their own.
3. The solid wastes produced by the worms (worm castings) breakdown any faster via bacterially mediated mineralisation than fish waste solids do.

[George]

I've always assumed they dissolve.

Jeremiah Robinson said:

Do you have any idea how this works?  Have you taken apart any of your beds to see how much space is taken up by worm castings?

[George]

Another thing to consider about solids in a media bed is that they settle to the bottom of the bed - in my beds, there is a 2 inch layer which doesn't drain.

[Jeremiah Robinson]

That's interesting.  I would guess that that 2" layer is anaerobic, unless worm castings have some kind of special consistency that anaerobic bacteria can't live off them.  Or perhaps the worms eat that bacteria.  

Do you have any idea if that layer has grown over time?  If it's 2" after 3 years, it would presumably be 4" after 6 years.  In 24" beds that may not be a problem.  In 12" beds it seems like it might be.

[Jeff S]

If I'm reading this correctly your stocking density is 1lb fish per gallon of water? Are you using other filtration besides the GBs?
Rob Nash said:

havent cleaned media in 6-7 years.. before adding red wrigglers, we cleaned every 3-6 mos.

this is with 12" deep media, with 1" granite on the bottom 6" and 1/2" granite on the top 6" (i try not to mix the two layers)

stocked heavy on fish... 3/4 - 1lb of fish / gallon   

here is the formula I use for media bed to water ratio...  

Gallons times .266 = sft of grow beds at 12" deep (cubic ft)

ie- 100 gallons needs 26.6 sft   ,   (or roughly one 4ft x 8ft box per 150 gallons)

1000 gallons needs 266   (I have four 5ft x 12ft on 1000 gallons (olnly 240sft))

you have some wiggle room here... but its roughly 4:1 gallons to media

[Jeff S]

I wonder if you had a tapered bottom on a GB with the siphon at the bottom, would the solids be more prone to drain from the tank?
 
Jeremiah Robinson said:

That's interesting.  I would guess that that 2" layer is anaerobic, unless worm castings have some kind of special consistency that anaerobic bacteria can't live off them.  Or perhaps the worms eat that bacteria.  

Do you have any idea if that layer has grown over time?  If it's 2" after 3 years, it would presumably be 4" after 6 years.  In 24" beds that may not be a problem.  In 12" beds it seems like it might be.