Extend your overflow pickup tube all the way to the bottom of the fish tank. Include a TEE above the water line to break a siphon if the pump should quit. The poo will eventually be sucked up and out to the beds, but if you place more fish in the tank you may find a need to pre-filter the poo with a radial filter so as not to clog up the media beds.
Try to create some rotation of the return water in the FT. Adjust your inflow pipes at 45 degrees to the side walls of the FT. Extend the drain pipe to the center bottom of the FT. You may place a 90 degree elbow, facing the bottom of the FT, with cut spaces where the elbow makes contact with the base of the FT, similar to the construction of a bell siphon pipe. The idea is to develop a vortex in the FT which drives all the waste to the center(or close enough to it) and the suction placed at the center bottom should vacuum most if not all of the solids.
My experience has shown that the fish will continually stir up the debris that settles on the bottom and all of it will eventually be removed by the pickup tube. Creating a rotation might be nice for the fish to swim against but it's not at all necessary for working the poo over to a collection site.
I've seen a tote with this type of configuration, and while its not as effective as the circular tanks that I use, it works very well.The waste however, should go to the GB's for processing rather than filtering them out with a mechanical filter. The hydroton in the beds serves that purpose of both filtering and releasing nutrient.
I would agree with Harold if you beds are large enough that they do not become clogged. Red Wigglers can help keep that from happening. I like to use a filter to keep the sludge from building up. Even worms do not keep up with the amount that accumulates, but to be fair my gravel media bed is probably too small.
Even so I see no benefit to keeping the slug. I've tested it and found no more nitrates in the slug than the water. It possibly it contains some micro-nutrients, but removal does not seem to cause deficiencies.
If someone were very generous they could have a test done to compare the micro-nutrients of muck to the water at a lab and post the results. For me I think it's a good idea to filter as much of the solids out before letting it enter the media even if the media is up to standard.
Radial filters are so simple and inexpensive I don't understand your resistance to using them.
Oh no resistance whatsoever. The operator can design/utilize his/her AP to their preference. The original AP GB system is simple in design and has been proven to work over time. For newer people my choice is to recommend basic AP. In this way they can learn the basic principles and with knowledge and then ideas overtime move to adjust/modify their systems if they choose. Filtering waste before a GB media system is not necessary, adds an extra layer of work and adds cost as well, but if the operator chooses, then that's fine
Although I have not come across any conclusive studies on the subject, I tend to suspect that solids processed over time in the GB has a correlation to system maturity/plant health and growth.
Personally, I paid very good money for that sludge, and I want to keep and mineralize every last bit of it (or as much as is possible). There is a great deal more than just some "micro-nutrients" in there http://uesc.br/cursos/pos_graduacao/mestrado/animal/bibliografia201...
Throwing out the 'sludge' seems like a horribly wasteful and inefficient thing to do. A lot of us moan about how expensive AP is (I know I have), but then we go and make design and operating decisions based on either out dated, misrepresented, or only half understood ideas that are themselves taken out of their original context where they tend to make even less sense...(i.e ones decision making process can vary greatly according to whether it is grant money, or commercial profits that are paying the bills)...and grossly over-stock our tanks (and do other stuff) to "make up" for these inefficiencies...Which only add to our mechanical filtration woes and expenses.
Seems like we should be working on how to better mineralize 'the sludge' and free up it's plant essential element content instead of just removing it. Fish create a good deal of ammonia nitrogen just by breathing, but the sludge is pretty much where most of the other 13 essential elements needed for plant growth are at...why toss it all? Be creative...I've had good luck mineralizing phosphates and chelating metal cations just by using very specifically targeted bacteria (isolated from soil, cultured then added to my system[s]) that are up to the task...That's just one of probably many solutions...but it's resulting in less fish food needed to grow the same amount of plants, which means less money and less solids to deal with.
Amy, try looking up SLO (Solids Lift Overflow) to get some of those solids off the bottom of the FT and into the GB's...
Thanks for the study, can't believe what I'm seeing from a just a quick read through! Very interested in the bacterial process to recover those phosphates and the chelation as well.
Thanks Vlad that's more information about sludge than I had hoped to see. I got from this report that the sludge contains an excess of nutrients. I know that the standard basic system would not separate any of that sludge, but even before my fish out grew my system I experienced too much sludge building up on top of the gravel. At first I filtered it with filter pads, but the pads became clogged too quickly. The radial filter does not remove all of the sludge, but it keeps my gavel clean enough with very little maintenance It only requires I open a valve for about 60 seconds once or twice a week and the total cost was about $20. I feel confident that plenty of micro-nutrients are still available. But I doubt that the iron is soluble as I still need to add it.