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  • Jim G.

Natural Filtration

Today I want to go into more detail of the filtration system I built for our natural pond. There will be pictures on the website. Very inexpensive and simple to build. So, tune in and learn how to build a filtration system.

An effective filtration system requires an understanding of the biology and the physics of water filtration. I mentioned some of this in the episode on a natural pool. I discovered there are beneficial bacteria that form in a couple weeks that form nitrites then form nitrates. In Europe they use planted zones to help with this process. This can produce a beautiful pool but takes up a lot of area.

A quote I got from a company that builds natural pools all over the country and was shocked by the price. But I did learn they use expanded shale to filter the water instead of 50% planted area. Expanded shale is porous and the bacteria builds up in the pores. The key is to pump water across the media to expose it to the bacteria.

Natural Pool Water Flow
Natural Pool Water Flow

As I mentioned before we had a ½ acre pond built. The deep part should be 9 to 10 foot deep. When full it will hold a million gallons of water. To put that in perspective an Olympic sized pool holds 660,000 gallons. An Olympic pool is 164’ long and 82’ wide and 6’ deep. That footprint is larger than most houses. They are crazy big. I had my pond dug for $10,000. Many natural pools call for a liner. My understanding was to prevent water leakage.

We have a ton of clay in our soil and clay is a known water sealer. One of the first remedies to a leaky pond is to add sodium bentonite clay. The part I missed is clay is a tiny particle. 2 microns in diameter. A human hair is 70 microns in diameter. So, we ended up with a lot of suspended clay in the water. Natural pools have crystal clear water. The best way to filter out the clay is to use a sediment filter. A simple method is to run the water across gravel. As it moves around the gravel the clay settles out.

I still need to add a planted area. But I designed and built a filtration system to provide the bacteria to keep the algae out and to filter the clay out. As I relate the sheer size of our swim pond as we call it I know I need to add more filtration. The pond is not full yet and the sides of the island and bank around that area are still just clay dirt. So when it rains a decent amount we get clay run off. It takes about four weeks to get the water back to clear. And a few more to be clear to the bottom.

Right after we had the pond dug the rain and even snow seemed to stop. In fact, the following year the paper reported the driest year in county history. After three years it has become a family joke. Yesterday and today were forecast for 80% chance of rain. Nothing came close yesterday. This morning a good-sized storm was moving up from the south and as you watched the map it evaporated about 20 miles south of us. Some of it skirted around to the east and left us in an open semicircle shape of no rain.

This seems to have happened a lot. The whole family is blown away. My sons in a town north and west of us 40 miles away seem to get every storm. This is bad for them since they hate to mow. To be fair we have been dealing with a leak. I bought a product called damit from an Australian company that is doing a fantastic job sealing the pond. The water level has only dropped an inch and a half in three weeks. Some is evaporation but the leak area is wet on the back side of the damn. We have high hopes with more product on the way to finally seal it up.

The pond is 2 feet below the max. At that level all of the exposed dirt will be submerged. So, I expect very little dirt to run off into the pond. The filter system I will detail here keeps the water clear down to 18 inches. And with no rain eventually you can see the bottom of the pond. I have gravel all around the dock and once the pond is full there will be nowhere you can stand and touch the dirt bottom.

We always have a party weekend on the fourth of July and I expect the water will be several feet deep of clear water and a foot or so deeper than right now. Hoping for fall and winter moisture to finish filling the pond for next year. The undersized filtration simply takes longer to clarify the water. But once full it should be easier to maintain.

So onto the details of the system. I will have several pictures on the website. I was not able to find the assembly pictures but it is simple enough to explain. Step one I went and bought three 55-gallon plastic barrels with screw on lids. You need the screw on lids to seal the system since it will be under pressure. Then I bought a several feet of pvc and fittings. I found a plastic disc that is ½” thick with a bunce of 3/8” holes drilled in it. This will be the screen to hold the filter media up.

I then took a pvc tee and cut lengths of pipe with 90 degree elbows on the end the right length to span the bottom of the barrel. Then I cut pvc legs to go into the 90 degree elbows 5” long. This created a stand in the bottom of the barrel. This will hold the perforated disc up off the bottom of the barrel.

Tank Plumbing

I then bought some 2” pvc bulkhead fittings. You drill a 2” hole in the barrel where you want it then the fittings clamp the side of the barrel through the hole with gaskets to seal. The have an internal thread for pipe fittings. I put one 3” up off the bottom of the barrel and one on the opposite side 3” from the top of the barrel. You can see this in the pics on the website. Then each barrel got another 1-1/4” bulkhead on the bottom.

As you will see this is used as a drain when I back flush the barrels. The bottom 2” bulkhead is where the water will come into each barrel and the top one is where the water will leave each barrel. By pushing the water up when it encounters the gravel gravity helps to settle out the clay. I then built three stands for the barrels. I used treated lumber and built them with 2x6 tops for strength. Each stand puts the next barrel so the top is below the discharge on the previous barrel.

Tank Drain

This takes advantage of how water always finds level. If you take a clear hose and fill it partially full of water then hold the ends up the water will be level across from one end to the other. It has been used to level things before there was a bubble level. The outlet on the first highest barrel runs to near bottom of the second barrel. Then the top of the second barrel is ran to the near bottom of the final barrel. So if there was no media in the barrels I could simply pour water into he first barrel and gravity would fill the lower two barrels since the discharge is higher than the top of the next lower barrel.

You will see it in the pics. Once I got the barrels on the stands and orientated properly used 90 degree elbow where on side is threaded to thread into he bulkhead fitting and the other is a female glue joint for the piping. You will see pics of the set up. The bottom bulkheads got ball valves. Then I placed the four legged pvc stand in the bottom and put the plastic perforated disc in place. In the first barrel I have a picture of a coarse gravel I put in. I bought three bags of this at home depot for around $5 a bag. Then I got three bags of smaller gravel to layer on top of that. Then three bags of pea gravel. The idea is the coarse gravel on the bottom filters out bigger stuff and as the water level goes up smaller particles are filtered out.

Going around all of that gravel slows the water down quite a bit and the clay gets captured in the gravel. I some pictures of what it looks like after a week of running. The I added some ¾” thick filter media sheets to help capture more clay. This is the main process to capture the clay.

The second and third barrels are filled with lava rock. I use the same design pvc stand and perforated disc to hold the gravel off the bottom of the barrel. A lot of dirt settles in the bottom of the barrel as I have found. The lava rock barrels are designed to hold the beneficial bacteria to keep out algae and also help clear the water. Lava rock works as well as expanded shale but is much easier to obtain and cheaper. I also bought a 50lb box of activated charcoal and some nylon bags. I put those in the last barrel on top to help with any potential odors. To keep out the lake smell really.

Then I bought a submersible fountain pump that was rated for continuous running. It is important to get a pump that can run continuously. Not all pumps are built for this. The one I have pumps 9,000 gallons per hour of 150 gallons a minute. It has a threaded 2” discharge. I set it under the dock on the deep-water side. The piped it up to the bottom of the dock and over to the shore. Then it goes into the bottom of the first barrel. I have a ball valve there so that when I shut the pump off all the water in the barrel does not run back into the pond.

The water fills the barrel from the bottom up. And at the top is the discharge pipe, almost like an over flow that runs down to the bottom of the next barrel. The process repeats. And the second barrel feeds the bottom of the third barrel. The discharge at the top of this barrel runs into the pond. The water coming out is crystal clear. I unexpectedly had to add a second line off the top of the first barrel because the pressure build up to run through just the three barrels was more than the plastic lids could handle and it was leaking profusely around the lids.

So, I ran the second discharge 30 feet down shore then the ten feet to be over the water. I have a 90 degree elbow on the end pointed up. It makes a short fountain. I have a ball valve in this line and closed it off the maximum I could to push as much water through the lava rock as possible. But the gravel does a great job of cleaning the water. The fountain is super clear water as well. But 150 gallons a minute into what is around 700,000 gallons of water takes some time to clear up.

The discharge lines will eventually be valved to create a waterfall. Besides an aesthetic value the waterfall also works to cool the water. In July and August, the top 4 or 5 inches of water is bath water warm. The experts suggest having at least a six-foot-deep part for cool water. This worked well last year. It was warm enough to just walk into the pond. But the deep areas felt very refreshing.

Once a week I back flush the system. I unplug the fountain pump then close the valve to the first barrel. Then I open the valve at the bottom of the barrel and drain all the water out into a bucket. There are pictures on the website of how dirty that water is. Generally, it looks like chocolate milk. Special dark chocolate. I have since plumbed the first two barrel drains 30 feet away into the field. Saves a ton of time filling buckets and opening and closing the valve.

I remove the tops of each barrel and use a garden hose sprayer to wash down the gravel. The water runs opposite of normal from the top to the bottom of the barrel basically acting as a back flush. There is so much clay suspended in the pond that I have to back flush the lava rock barrels as well. Last fall when you could easily see the bottom very little got into the lava barrels. With all the materials including the wood barrel stand and the $180 pump I have under $700 in the whole system. A set up like this on a more traditional sized back yard pool of say 20,000 gallons would filter the entire water volume in two and a half hours. I have one more expansion planned for the dock and I will add a second filter system like this to it.

That system will be a waterfall over a grotto. And we plan to create a planted area that will work like the diagram picture on the website. Just to add more filtration but also there are some very pretty water plants you can use. I will talk more about the dock in a future podcast. But one section is covered by a gazebo. There is a water slide. And room to layout in the sun. We easily have under $25,000 in the entire project. We put down heavy plastic sheeting and large rocks on the back side of the dam. The sides are steep and it is about 10 above the ground behind it. A nightmare to mow.

I compare this to the fiberglass 12 x 24 5,100-gallon fiberglass pool I looked at where four companies quoted me $80,000 to install with no concrete included. I encourage you to look up natural pools online and see all the amazing pictures. They can be installed anywhere. Another bonus for me is a half-acre less to mow! The water is as clear as any pool and with no chemicals. No work to close the pool or open it each year. Simply stop swimming when it gets too cold. Jump back in when it is warm enough.

It is a fantastic place to entertain. The grandkids bug their parents daily to come swim! Feel free to email me with any question you have about a natural pool or the filtration system. Tune into the podcast to hear more.

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