Collecting solids from water in a swirl filter

[120ThingsIn20Years]

I'm playing around with separating solid fish waste from water in my backyard aquaponics system. (Fish as livestock, grown for the plate, feeding vegetables via their waste in a closed loop as far as water goes. Inputs are fish feed pellets, air, sunlight, and a bit of water)

 

Normally your fish waste goes into an inert growing media where you plant your veggies, but another method called Nutrient Film Technique (NFT) is one used a lot in hydroponics to grow lettuce etc in plastic gutters with nutrient rich water flowing 1 or 2 mm deep through the tubes. The lettuce grow in holes cut into the top of the tubes and are either suspended bare rooted, or in an inert media like gravel. If you have bought lettuce with roots still attached, that's probably how they were produced.

 

The problem with using this system in aquaponics, is that the nutrient rich water also has solids in it. These solids collect in the roots, and block the NFT tubes.

 

The method I'm playing with to separate them out is to use a swirl filter. Water is pumped into a container so that it creates a gentle whirl pool. Solids collect toward the centre. Then, in my case they are removed with a bell siphon*, and redirected back to a gravel filled grow bed, and the cleaner water is pulled from the top of the filter, at the top of the bell siphon's cycle, and for 10 seconds or so of every minute, sent to the NFT tubes. Other systems might expel the solids from the system to waste. (fouling rivers in the process)

 

My question is, what's going on in my swirl filter?

 

Is it just a case of the water being calmer in the centre, and allowing the heavy particles to settle out, or is there something else going on?

 

Watching it, I get the feeling there is a change in the pressure at the outside vrs the centre, changing the relative weights of the poop and the water. It reminds me of the way a helium balloon behaves in a car. ie: moving to the right in a right hand corner, and rushing to the back under brakes.

 

But I dont think water compresses enough for this to be the case.

 

Anyone?

 

All idle speculation, answers, or derisive laughter welcome.

 

I have no formal science, so English rather than equations would be preferred, but Ill take what I can get

 

 

 

* Bell siphon - click if it's not animated

 

 

[edit from the future - what's a quastions? it seems I cant edit titles]

Edited August 29, 2011 by 120ThingsIn20Years

[120ThingsIn20Years]

pretty please

[Hal.]

I watched a lot of Elvis films when I was younger , the TV channel must have got them for free . Anyway , Elvis , I think it may have been a stuntman , memory fades , had a motorbike and used to travel around in a closed container with sidewalls that was cylindrical in the fairground . The faster he went the more he and the motorbike were forced outwards to the side of the container . Once he mained a minimum speed he was able to keep at the same height and the reason he was there was centrifugal force . If he slowed down the centrifugal force got less and he moved lower and nearer to the centre .

 

When you have a container of a mixture of solids and liquids if you spin/stir it from the centre the centrifugal forces will have a bigger effect on the lighter particles , pushing them further outwards , than they will on the heavier particles which stay nearer to the centre . There are also containers which use centrifugal forces to throw the heavier particles further away and the lighter particles stay near to the centre in a reverse separation technique .

 

 

 

[JustinW]

If you can put any kind of pressure to the water before you run it through your system there is an answer. Although you would probably have to build one yourself because of the small scale and amount your using at a time. I work at sand plant making frac sand for gas wells. We seperate sand from water using cyclones. High volume water/sand mixture enters a funnel that rotates the water counterclockwise. The funnel pushes the water through a discharge on top while dropping the solids through a discharge on the bottom. We usually only have about 10-20lbs of pressure so I don't think it will be too hard. You may just have to run your water through smaller pipe to increase pressure, then after solids are seperated return to normal. I'm not sure if the volume of water matters. There is a lot of push behind the water we run that being about 4000 gpm. Don't know if it'll help but I hope it works out.