Yet another use for our beloved 55 gallon drums!

Koi hobbyists have been using several forms of sand and gravel filters for probably as long as they have been keeping Koi. These filters work great when fresh but with time they will pack, clog, and channel. Just standard back flushing just will not clean them up enough. A while back Kent Wallace of Living Water Solutions, also a contributor here at Koi Nations, came up with a design that I like and also uses an air blower to dislodge and flush all the trapped waste from the sand. This concept will keep your sand and gravel filter free from all the problems of the old style back wash filters.

How It Works

Water flows up through different layers of gravel that get progressively smaller as they get to the top then finally through a layer of sand. There is a 2in/5.0cm water inlet from the pump in the bottom of the barrel and two 3in/ 7.6cm outlets in the top of the barrel. One outlet takes clean filtered water to the pond. The other outlet takes dirty water to waste. The top sand layer traps the waste. There is an air manifold in the bottom of the barrel that blows air up through the layers of gravel and sand when cleaning. This dislodges all the waste, flushing it out a different exit. The progressively finer layers of gravel keep the sand from mixing with the gravel when flushing. This is perhaps the best mechanical fines filter design I have seen. It also provides some bio-filtration. Here is my version of Kent’s design.

Get Your Tools Out

Start by cutting the top out of your barrel. Try to make a nice round accurate cut as we will be using this as a template later. Next drill a 3.25in/8.25cm hole in the center bottom of your barrel. Use a hole saw to get a nice round hole (Image 1). This is for the 2in/5.0cm shower drain fitting for the water inlet. Now on one side of the barrel, measure down from the top 4in/10.1cm and drill a 3.5in/8.8cm hole there, using the 4in/10.1cm mark as the center of the hole. This will be the outlet to the pond (Image 2).

On the opposite side of the barrel, drill another 3.5in/8.8cm hole centered 8in/20.3cm down for the outlet to waste (Image 2). Now discard the gasket from your 2in/5.0cm shower drain and using 3M-5200 marine adhesive, install the shower drain in the bottom of the barrel. For the two outlets, I use 3in/7.6cm electrical male and female adapters. Electrical conduit is different from PVC as the threads are straight cut and will screw together tightly. Once again using 3M-5200 adhesive, screw together a male and female adapter in each of the two outlet holes (Image 3). This completes the modifications on the barrel. We also need a plug for the outlets. For this, cut a 4in/10.1cm piece of 3in/7.6cm PVC and glue a cap on one end.

The Air Manifold

Next, we will build the air manifold for flushing out our filter. Get four 1.25in/3.1cm electrical conduit long sweep 90’s and cut 1in/2.5cm off each end (Image 4). You will need four 4-way “T”s and one 5-way “T”, all 1.25in/3.1cm for the air manifold and 4 plugs. Glue the plugs into one of the 90 degree legs of the 4-way as pictured (Image 5). These plugged ends will act as legs to keep the air manifold off the bottom of the barrel. IMPORTANT, when gluing this manifold together remember the legs go down and the 5-way center goes up.

Now glue two of the electrical conduit 90’s into the 4-way “T”s as pictured. Next cut 4 spokes out of 1.25in/3.1cm PVC 7in/17.7cm long. Glue two of the 7in/17.7cm spokes in opposite sides of your center 5-way, and the last two 4-way “T”s on the ends of these spokes, remembering the center 5-way goes up and the plugged 4-ways go down. Glue the other two spokes in the sides with the electrical conduit 90’s. Lay the two assembled sides next to the center piece.

Glue the three pieces together. This can be a bit tricky, you have to act fast as your gluing three joints on each side and it will take some twisting and maybe force to get them together. Once I get the three joints started I pound the assembly on the ground and they go together tightly. If you did it right, you now have an assembled air manifold with the center hub facing up and the four plugged legs facing down (Image 6).

Now we need to drill a bunch of holes in the air blower manifold.

Use a 0.1875in/0.47625cm bit

for this. The holes are drilled at a 45 degree angle on the bottom of the manifold, pointing down and in toward the center. I use 1in/2.5cm spacing for these holes. Drill the holes in both sides of the spokes and all the way around the rim once again pointing down and into the center at a 45 degree angle (Image 7).

Getting To The Bottom Of It

Next we need a grating to keep the layers of gravel and sand off the bottom of the barrel. You can use the top of your barrel you cut out and drill a bunch of 0.5in/1.2cm holes in it. I prefer a poly, dog kennel, floor grating. It is very strong and comes in sheets large enough to make two of these filter grates. Take the top of the barrel you first cut out, lay it on the ground, and lay the dog kennel, floor grating over that. I sit on the grating to keep it from moving and using the barrel top as a template cut out the floor grating around it using a hand held jig saw (Image 8).

Now measure from edge to edge to get the center and cut a hole just big enough to fit over the air manifold center (Image 9). Glue in the vertical 1.25in/3.1cm pipe for the air blower, just long enough to reach the top of the barrel. This assembly can now go in your barrel (Image 10) and your filter is complete.

Test Your Work

One of the great things about this filter is it can go just about anywhere. Since it is pump fed, it can be as far away from the pump or pond as need be. The outlet can supply a waterfall or, with 12in/30.5cm of height above pond-level, supply TPR’s. Install the barrel where you want it, making sure it is level, hook up your piping and fire it up empty to make sure there are no leaks. Now that you know there are no leaks shut down the pump and start filling it.

Filter Material

I start out with 4in/10cm of 1.5in/3.9cm, drain field gravel. Next comes 4in/10cm of 0.75in/1.9cm gravel, followed by 4in/10cm of pea gravel. The final ingredient is the sand. Make each layer nice and level. There are several choices for sand. Kent uses #12 sand blasting sand. Many people like using crushed oyster shell chicken grit for its buffering properties. I use fine granite chicken grit. What ever you decide, fill the barrel the rest of the way almost to the bottom of the waste outlet. Do not worry about getting it too full. The first time you flush it if it is too full you will just flush the extra out the waste line.

What A Mess

Now the first time you start this up make sure your plug is in the pond side, blocking it and allowing the pump water to go out the waste side. Start you water pump, then your air blower. I recommend a one horsepower blower but some people use large shop vacs. The water will churn, bubble, and foam. It will take quite a bit of time and water for this first flushing. Keep the air blower going for a few minutes. There is so much water churning it is hard to tell if it is clean or not. When you think your getting close, shut off the air blower and keep flushing the filter to waste until the water flows clear. Then just pull your plug from the pond side, put it in the waste side, and your new filter is now in operation.

Operation

These filters are good for about 2,000 US gallons/7,570 liters/hour flow. If you get too much flow going through it, you will start to fluidize the sand and maybe even flow it out to your pond. If you see the sand start to fluidizing just pinch off your pump a little. If you need more flow just run two of them in parallel. I like to flush my sand and gravel filter every other week. Good luck, and have fun building!

** Steve Joneli lives in Central Oregon where he keeps Koi and owns High Desert Koi Ponds. Reach him at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .