How To Make A Bog Filter

What is a bog filter

A bog filter uses the natural cycle of the pond ecosystem to maintain balance keeping the water quality high. Ponds with bog filters rarely experience algae blooms. When set up correctly this type of filtration is high quality and low maintenance. Let mother nature do the heavy lifting as she has since the beginning. 

How does a bog filter work

I know the claims may sound too good to be true. Superior water quality and beautiful blooming plants. The best of both worlds in one. By creating a balanced ecosystem that is exactly what you get. 

The layout is nothing more than a grid work of pvc pipes that evenly delivers nutrient rich water to a gravel bed. The bacteria that colonizes in that gravel bed nitrify the nutrients (organic waste) in the water, turning it into plant food. This is then absorbed and used by the plants in the bog. What’s left is crystal clear water stripped of its nutrients and returned to the pond.

The most important thing to get right when setting up a bog filtration system is to size it properly. This is done by  based on the size of the pond. The bog should be 15 - 30 percent of the surface area of the pond to be filtered. 

Let's say we decide we need a bog to be 20 percent of the surface area of the pond based on our needs. A pond that is 10 feet wide by 15 feet long has a surface area of 150 square feet. This pond would require a bog that is 30 square feet, 6 feet by 5 feet by 12 inches deep. 

Oversizing the bog is ok if you have the room for it. This will only help for future needs and be more forgiving if the pond happens to get overstocked.

Clearing Your Pond Naturally

The gravel bog filtrations effectiveness is unmatched. No chemicals, special equipment and extremely low maintenance. 

One of the major things that plague every pond owner is algae. This comes in two forms, string algae and free floating algae. Both will take over your pond if not tended to. A UV light is typically installed to help combat this. This is not necessary with a bog filter.

Algae needs three things to live: sunlight, nutrients, and water. If one of these is missing it cannot survive. The bog filter strips the nutrients from the water before it returns to the pond. This in turn starves the algae. What’s left is crystal clear natural, chemically free water.

How To Set Up A Bog Filter

Two things need to be considered when designing and planning out your bog filter. Number one is water flow, and the second is size. Get these two things right and you are set up for years of low maintenance and high enjoyment.

Water Flow - The ideal water flow through the system is once per hour. If you have a 1500 gallon pond, then the ideal flow rate through your bog would require a pump capable of at least 1,500 gph (gallons per hour). This is a key to get right. 

Moving the water through the bog too quickly will not allow the bacteria to use up all the nutrients.  If the water flow is too slow, you run the risk of stagnation. You may wind up with pockets where the water doesn’t move. 

Size - The size of your bog filter will depend on its use, although it never hurts to make it bigger than you may initially need. If you are creating a water garden only, with plants and maybe a couple of goldfish, you can easily get away with a bog that is 10 to 15 percent of the size of the pond.

Stocking koi you will need to increase the size of the bog to 30 percent of the size of the pond.

Design

Now let’s get down to business and discuss exactly how to build one of these amazing, yet simple water clarifying systems. 

First excavate the area where the bog will be located to a depth of 12 inches (30.48cm). Follow the same steps for laying down underlayment and pond liner as you would when building a pond. Now we will be using a system of pvc pipes that connect to our waterfall pump to distribute the water evenly throughout the bog.

Use 1 ½” to 2” inch diameter pvc pipe to create a water distribution channel below the bog. To determine what pipe to use, look at the size of the output on your pump. Use the next size bigger pvc pipe for your system. For example if your pump has a 1 inch outlet, use 1 ½ inch pvc in your bog. Doing this will ensure proper flow through your system.

When we build a bog filter, instead of perforating the pvc pipe with holes like many others do, cut ⅛” wide slots using a circular saw set at a 3⁄4” depth. This gives a uniform outlet throughout the pipe. Space cuts about 3 inches apart for the length of the run. 

Each “leg” of pipe will cover a 2 - 3 foot width. For instance a bog that is 6 foot wide and 10 feet long will need 2 lengths of pipe to run the 10 foot length. The legs of the filter should all tie together. Place a vertical pipe at the opposite end of the inlet. The vertical pipe should come slightly above the surface of the bog. This will allow you to flush the system when necessary. This is know as the clean-out. See the figure below.

Once all your piping is in place, cover it with the ⅜"  pea gravel. Cover the pipes with about 3 - 4 inches of pea gravel. This will be the home of the bacteria that will turn waste into plant food. Now that the pipes are covered you are ready to plant your bog plants (skip to planting bog plants).

The Clean-Out

Do not neglect adding this to your water distribution pipes. The clean-out is simply the other end of the pvc “leg” that ties them all together. This must be capped off in order for water to flow properly. 

I suggest using a threaded plug to plug the clean-out to ensure it stays in place. The clean-out should raise up just above the surface of the bog. The pipe can easily be hidden with some dark brown or black spry paint.

Any leg that is not connected to the clean-out needs to be capped off. Water follows the path of least resistance, any pipe left uncapped will release the water on the end rather than through the slots in the pipe.

How Deep Should A Bog Filter Be

Your bog filter should be no deeper than 12 inches deep. Ideally the bog and the pond will be built using the same piece of liner to reduce the likelihood of leaks. Any deeper is a waste and unnecessary. 

Effectiveness Of A Bog Filter 

Your pond is always trying to reach a state of equilibrium. Just like water seeks its own level, the same holds true with your pond. Since it is a “man made” ecosystem and not naturally occurring, steps must be taken to ensure it can reach that state of balance. The point where, whatever goes in, equal parts must come out.

How much does it cost to set up a bog filter

The materials you will need to build your bog filter besides the standard pond kit items are as follows:

As you can see based on the materials, the cost of a bog filter is relatively inexpensive when compared to other “high end” filtration systems. The amount of pipe and gravel needed is determined by the size bog that is required. 

Space is the major expense, you need plenty of surface area for your bog to work properly. When it’s done right, no other system can top the bog filter.

How to incorporate a bog filter in your water feature

Whether you are planning on upgrading your current pond, or building a pond from scratch, keep these things in mind.

Pump Placement - The placement of your pond pump needs to be considered when using a bog filter. If you are building your pond from scratch you could opt to place your pump deep in the pond opposite of the bog. 

If building a pond using a pond kit the pump will be placed in the skimmer box opposite the bog.

Most pond skimmers come with a net to catch large debris, then the water passes through a pre-filter pad. We do recommend using a skimmer to collect debris as it enters the water surface. However, do not use the pre filter pad in the skimmer box. The pre-filter will not allow as many nutrients to pass into the bog and will starve your plants.

 There is more than one way to set up a bog filtration system. They seamlessly blend into the landscape because of the lush plants that are the result of the filtering.

Above the pond - The most typical way we build our bogs is above the pond. The water is filtered through the bog then returned to the pond through a spout, or a waterfall. This creates a wonderful sound and gives life to the water feature.

Beside the pond - The bog and the pond are separated by a porous retaining wall. Cinder blocks are perfect for this. Paint them black and cap them with natural stone. You will never know they are there.

Inside the pond - This is nothing more than a container that raises above the surface of the pond containing bog plants. keep their "feet" wet and they will clear your water. Perfect for smaller ponds on the deck or patio.

Planting Your Bog Filter

Before we go over what plants to start with in your bog filter, keep in mind that your bog is new. If you remove the dirt from your plants when you plant them they will starve.

Place all your plants where you want them to be planted, about one every square foot. Once you are satisfied with their look and location, remove them from their pots keeping the dirt around the roots. Don't worry about dirt getting into your pond. 

With all the plants in place fill the remainder of the space with more ⅜” pea gravel. Turn on your pump and let mother nature take over.

Be sure to use the right kinds of plants, as some are invasive. They can take over, choking out other plants and clogging your pvc pipes. Any varieties from the list below will suit you just fine.

There is no shortage of the variety of plants available to fill your new bog with. Quick design note: keep tall species in the back and smaller, shorter plants in the front.

Below is a short list of bog plants we recommend to get you started. These plants are not invasive and will be a great fit to any bog gravel filtration system.