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Building a Simple Trickle Filter

At home we are working on a small research project for the treatment of organic solids and mineralization.  The trail is to determine the minimum amount of energy required for this process per kilo of wastes.  The trickle filter we have put together for this project is inexpensive and very simple to build.  Here is what we did. As you can see from the image to the left it is extremely simple trickle filter and at almost no cost.  In our filter we used 5 bread crates per filter which was limited by our bilge pump pumping head.  As we go higher from the surface area to the outlet from the pump, the less water flows.  So we split the size of the filter into two and set that up as you see it.  The pipe from the pump goes up through the crates and dumps water onto a distribution plate on the top.  We used some old plastic sheet we had laying about and drilled 8mm holes in it until the water did not sit on the surface of the filter.  You can use plastic tarp or even liner and “poke” holes required in it to distrubute the water over the area of the filter. Working out the size of a trickle filter can be complex requiring hydraulic surface area, media type, its height and feed loading. Designing the Filter for Biofiltration TAN Production: Work out how much TAN (Total Ammonia Nitrogen) is produced by feeding your fish.  This is easy.  The protein content in the feed (say 35%) multiplied by 10% multiplied by the amount of feed you will put in the system each day.  Let’s say we feed 500 grams per day (enough for around 40kg of fish at 1.2 %bw/day).  This gives us 0.5 kg * 0.35 * 0.10 = 0.0175 kg of TAN per day. Area of Filter:  Work out the area of media required to remove the TAN produced.  In our case with the trickle filter we want to use an Areal TAN removal rate.  This removal rate is very wide so we will use 1 gram per m2 per day, simple.  In this case we are assuming water temps around 20 to 25 degrees Celsius.  So, convert our 0.0175 kg of TAN to grams (17.5 grams TAN) divided by 1 gram/m2 removal gives us 17.5/1 = 17.5 m2 of media area required. Volume of Filter Media:  We calculate how much media we need, just like any other biofilter.  17.5 m2 of media area required divided by the surface area of our media say 100 m2/m3.  So 17.5/100 = 0.0175 m3 media or 17.5 liters required. Dimensions of Filter:  Now how big is this filter going to be, we have a few things to work out. Daily Flow Rate: For a home system this step is quite simple, for commercial there is a range of other steps required to calculate your fish tank required flow rate.  In our case we are going to suggest we need our tank exchanged a minimum of once per hour.  Our fish tank is 2000 liters.  Exchanged 1 times per hour by 24 hours in s a day.  This gives us 48 m3/day of water flow per day.  Which is the flow rate over the filter.  If you had more than one filter then divide that by the number of filters you plan to have.  This will be determined by the amount of space you have. Cross Sectional Area of Filter:  This is worked out from the hydraulic loading on the filter which can be from a minimum of 30m3/day/m2 to prevent scouring the biofilm off to 255m3/day/m2 at the maximum limit.  This can depend on the type of media you are using.  I suggest sticking around the 50m3/day/m2 for the home user.  So we divide the total flow rate for the system 48 m3/day by the hydraulic loading of 50 m3/day/m2 gives us an area of 0.96 m2. Height of filter:  This is simple geometry.  H=V/A or height equals the volume divided by the area.  In our case we have a volume (as per the amount of media needed for TAN removal) of 0.0175 m3 divided by the area of 0.96 m2 from the previous step giving us a filter height of 0.0182 meters or 18.2 millimeters. Now that size seems a little small for us, so let’s work it backward to check.  We have an area of 0.96 and a depth of 0.0182 (V=A*H) giving us a volume of 0.0175 m3 or 17.5 liters which is the amount of media needed for TAN removal previous calculated. What is important is the surface area (0.96 m2) of the filter to ensure at that flow rate or hydraulic loading rate all of the media is kept wet and the biofilm is not sheered off.  If the flow rate increases so does the area of the filter.  Remember the huge variation of hydraulic loading of 30 to 255 m3/m2/day so you can pick just about any number in there towards the higher end to modify the filter size. Making the Filter Area Smaller Let’s take a quick sidebar to determine the hydraulic loading for our bread crates in one stack to save us some space without dividing the filter into multiple units. With our bread crates having an area of 0.382 m2 to determine if our maximum hydraulic loading is exceeded we reverse the formula.  Daily flow rate of 48 m3 divided by the area of our vessel 0.382 will give us a hydraulic loading of 125 m3/m2/day which is well within the allowable range of 30 to 255 m3/m2/day! The Magical Bread Crates Because we are not packing a vessel full with media and using “shelves”, we have to know how much media we can fit.  The crates can hold up about the 50 mm mark of media which gives you about 2 liters per crate.  Remember we need say 18 liters of media in our filter so we will need at least 9 crates with media stashed in them.  You can fill the crates up to full (100 mm) but it does reduce the amount of air getting into them so pack them lightly if you can. So you can half the number of crates and say have five and double the amount of media in them and it will work like a charm. You can use any vessel and media you like.  Clay balls make for good filter media because they are light weight.  Gravel can get quite heavy and bow the bases in the crates.  Your vessel can be round, square, rectangle and even get adventurous with a triangle provided you follow the math example above and give yourself plenty of room for media, you and your fish will be fine. We have talked briefly before about using trickle filters for mineralization.  This a topic we will discuss soon.  It is a little more complex than the above but even that simple design will get your mineralization working really well for you.  Nothing like setting one up at home and having a go….

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