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Builidng Simple Swirl Filter

Making your own simple swirl filter for collecting those fish solids from your backyard aquaculture or aquaponic system is very simple and can be inexpensive.  A simple swirl filter will do the job of removing the heavy solids like uneaten fish food and fish poop before they get back to your fish in an aquaculture or even before the solids get into your grow bed in an aquaponic system. [heading] Building a Simple Swirl Filter [/heading] Often many people get confused between a settling filter which are large and have a very slow rotational velocity, and a simple swirl filter which has a rotational velocity (the way the water spins in the filter) like when you pull the plug on a small sink. Other types of settling filters made at home have some form of media in them which interferes with the flow of the water and effectively are not a settling or swirl filter but a static packed media filter.  While the differences between these types of filters is small, the effectiveness of each relies on them being designed correctly.  Most of the time, combining different filters in one to make a “new” type of filter does not work. With that in mind, here are a few things to think about when building one of these simple swirl filters yourself posted below. Retention Time The retention time refers to the amount of time a volume of water will be in the filter.  For swirl type filters a very small retention time of a few minutes will get the filter working very well. If you go over this (so the water is moving much slower in the filter), you will not really be making a swirl filter but a settling filter so you will be looking for that much larger drum to suit a retention time of 20 minutes or more. What we want to do is create a strong vortex (like pulling the plug on the sink) in the filter and force the solids out of suspension rather than letting them soak and settle out. The slower settling filters work well enough in any application but we want to keep a small foot print so we are looking for a much smaller drum to run our filter in. In our case in my fish farm, we have 3000 litres of water and we are exchanging that at 4000 litres per hour.  We work out the required flow rate by multiplying the area of the top of the drum by 200. Our filter drum has a 0,3 meter radius with an area of 0.2827m2 (PI x radius^2).  So 0.2827 x 200 = 56.54 litres per minute or 3392 litres per hour.  Looks like we are on the money with this drum diameter.  But what about the volume? We also need to check the volume of the drum (200 litres) against the flow rate of the system (4000 litres).  If you divide the flow rate by the volume of your drum ours gives us 20 exchanges on the drum every hour or a retention time of 3 minutes.  Not too bad.  I would like it a bit quicker or closer to 30 seconds or a minute.  If we wanted to improve the filter the drum volume would only need to be about 40 litres.  Pretty small huh? The picture below you will see one a commercial swirl filter attached to a 2000 litre fish tank.  It is about 22 litres which gives you about a 40 second retention time at one exchange per hour.  You will love the pipe work on that… Pipe Size We used on a 40mm inlet and outlet on this simple swirl filter but I would have liked to upsize the outlet to at least 50mm to be sure the skimmer tee is not submerged and skims.  Because I did not do this I have to slow the flow rate into the solids filter a little  so it does not back up.  It never overflows but the outlet is submerged under the water.  Another way around it would be to make the skimmer out of larger diameter pipe to get the little wedge cuts higher in the water. Inlet Placement You want to place the inlet at a point in the drum were the water coming in will not resuspend any solids at the bottom of the drum and not be too high that the solids don’t get a chance to get trapped. As a general rule half way down will do you but to correctly place the inlet, but to have it 1/3 of the diameter of the tank top submerged under the water level. This is part of the reason why it is handy not to glue the fittings inside the drum.  The inlet pipe can be extended or cut to suit the drum you are using as a filter to get it working as best as possible. Other than that, these filters are very simple swirl filter and inexpensive to build and they are very effective at trapping solids. Parts list as requested A drum that is a suitable size for the volume of water and flow rate of your system.  This one is 200 litres but would work better if it was about 50 to 100 litres, Inlet and outlet Uniseal.  This will depend on the size of your system again and water flow.  This one uses 40mm pipes so 2 x 40mm uniseals are used. Inlet pipe  work is 40mm and uses a short peice through the inlet uniseal at the bottom, then a 90deg elbow and another short piece of pipe vertical and another 90deg bend. Outlet screen has 2 x 40mm push on caps, 1 x 40mm tee, 1 x 40mm socket and some short pieces of pipe to join it all together. Waste dump at the bottom is 25mm and has 2 x 45deg bends, a 25mm uniseal and a 25mm ball valve. Thats about it. If you plan to build this type we always hearing from our members and checking out your photos of your projects. Regards Paul…

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