Creating an Effective Wetland “Bog” Filter
Wetland Filters and Bog Filters…
In the pond and water gardening community you’ll often hear about “wetland filters” and “bog filters.” If you have no idea what these are, what they do in your pond, or how to build one then you are in the right place! These filters are an important part of maintaining the water quality and clarity of many Minnesota ponds. As you are looking at creating your own wetland or bog filter for your pond, it’s important to know what you are recreating so it’s most effective for it’s role in your water feature.
What are wetlands and bogs?
Wetlands and bogs are both types of ecosystems characterized by the presence of water, but significantly different in their conditions, types of vegetation, and functions.
Wetlands: Wetlands are areas where water covers the soil, or is present either at or near the surface of the soil. Types of wetlands include what we know as marshes, swamps, bogs, mudflats, mangroves, fens, and even along some riverbanks. Each type has distinct characteristics and can either be flowing or stagnant and can be rich in nutrients (eutrophic) or poor in nutrients (oligotrophic). What’s important about wetlands is their role in water purification, wastewater filtration, flood protection, carbon sink functions, and shoreline stabilization. Wetlands are among the most productive ecosystems in the world, similar to rainforests and coral reefs.*
Bogs: Bogs are a specific type of wetland ecosystem characterized by spongy peat deposits, acidic waters, and a floor dominated by a thick carpet of sphagnum moss. They are generally nutrient-poor and highly acidic with low levels of oxygen (anaerobic), which slows the decomposition process, leading to the accumulation of peat. The plants they support are adapted to the low nutrient conditions, including carnivorous plants like sundews and pitcher plants which derive some of their nutrients from capturing insects. They are poor draining areas that accumulate stagnant water. With the low decomposition rates under their anaerobic and acidic conditions, instead of decomposing fully, bogs allow organic matter to accumilate as peat.
In summary, while all bogs are considered wetlands, not all wetlands are bogs. Bogs represent a more specific and extreme type of wetland. Oftentimes when people are referring to bog filters, in reality they are describing wetlands in a more general sense than the specific low oxygen and stagnant waters of a bog. For this reason, we’ll continue referring to wetland filters / bog filters simply as wetlands.
Boosting Pond Clarity with Wetland Filtration
For those looking for a natural and incredibly efficient method to maintain clean and healthy pond water, a wetland filtration system presents one of the best solutions. This system, which recreates the natural filtration processes of a wetland, not only improves water quality but also enhances the aesthetic appeal of your pond setup. Let’s now take a look at basic and advanced wetland setups.
Basic Wetlands
The green hue of pond water often comes from countless tiny, single-celled plants known as algae. Like all plants, algae require sunlight, carbon dioxide, water, and nutrients to thrive. By eliminating just one of these essential elements, you can effectively inhibit its growth. Wetlands and “bog filters” are particularly effective in removing these crucial nutrients from pond water. By efficiently removing elements in the water, you minimize the need for chemical treatments and frequent cleaning.
This process is carried out by evenly distributing pond water through a bed of gravel through a network of perforated pipes. This gravel acts as a home for nitrifying bacteria to establish themselves. These bacteria then convert waste from fish and plants and even wastewater into a form of nourishment that the plants in the gravel can absorb. As the water circulates through this system and returns to the pond, it comes back virtually devoid of nutrients, effectively “starving" the algae and preventing its growth. This natural filtration method not only clears the water but enhances the overall health of the pond ecosystem. Here’s how you build one yourself.
Basic Wetland / “Bog Filter” Construction
When constructing a EPDM rubber liner pond that includes a wetland area, it's essential to set aside enough space for the wetland from the start. The size of the wetland should be based off the surface area of the pond. It’s usually best to plan on a wetland size roughly matching about 30% the size of your pond surface for an effective system. This will handle anything from a few small goldfish to a larger number of koi. Too small of a wetland won’t be able to keep up with the filtration needs of your water system.
Steps for Constructing the Basic Wetland Filter:
Prepare the Site:
Dig a 12-inch deep excavation, again roughly 30% of the size of your pond. Deeper isn’t better as the beneficial bacteria in your system need sunlight and oxygen to do their job.
Line the area with a 45 mil EPDM pond liner, allowing it to overlap the top of your excavation.
Install the Pump:
Position the pump on the opposite side of the pond from the wetland to ensure effective water circulation.
Choose a pump that can cycle the pond's volume every hour.
Set Up the Distribution Pipe:
Cut slots into the distribution pipe. The pump's outlet size will dictate the pipe size. For example, use a 1” pipe for pumps with a ¾” outlet.
Slot the PVC pipe a third of the way through, approximately 1” apart.
Attach a vertical capped stand pipe (clean out pipe) under the gravel, cutting it so it rises just above the gravel bed. You can camouflage this pipe rocks, driftwood, and plants.
Lay the Distribution Pipe:
Place the pipe on the pond liner in the designated wetland filter area, making sure the slots face up into the gravel.
For wider wetlands, use multiple lines of pipe spaced 2-3 feet apart, similar to a septic drain field setup.
Install a clean out pipe for each distribution line in larger wetlands. You want to be able to clean your system easily when that time comes.
Test the System:
Once you’re satisfied with the piping layout, glue everything together. (be sure to cap the ends of the perforated pipe so you’re not just shooting the water straight through instead of pushing water through the perforations)
Turn pump on to check if the water distributes evenly.
Add Gravel and Plants:
Shovel washed river gravel into the bog filter area, filling it halfway initially. (do not use pea gravel!)
Select and arrange your wetland plants in the half-filled gravel. Place taller plants towards the back and shorter ones in front for visual interest. Be careful with the plants and keep their root balls intact.
Gently add the remaining gravel, ensuring it covers the plant bases and that the final gravel level is above the water line to prevent standing water for insects to use as a breeding area.
Activate the Wetland Filter:
Turn on your pump to begin the filtration process. The filter is now working, promising years of clear water and enjoyment!
With these smaller, basic wetland systems, be sure to keep up on cleaning and maintenance to prevent clogging.
Advanced Wetlands
Here, the wetland filter media is made up of several grades of gravel, which is contained by the same EPDM rubber liner that can be custom designed according to your wants and needs. The concepts and philosophies are similar to the basic method, just more dedicated, more efficient, and easier to maintain. This is how we install wetlands for custom pond installations.
This wetland filter system that distributes the water, instead of being a standard PVC, is made of heavy-duty polyethylene, and comes in 6’ sections which can be snapped together (find it here). Each section has over 230 square inches of open area. This allows a flow rate of 1” per second at 3,000 gallons per hour. This slow, steady flow of water is critical for the functioning of a wetland filter. For each pump upgrade, you connect additional sections of the filter together to keep the flow rate equal to, or better than the level you need. At the opposite end of the filter (opposite the point where the water feeds into the filter) is the clean-out section. This is a tubular structure (snorkel) that extends from the bottom of the filter, all the way to the top with a cap on top that can be opened for filter cleaning. It’s easier to access than small PVC pipe cleanout pipes which makes maintenance more enjoyable. With this system, large ponds of any size size can easily be filtered. Not only is this design easier to install and more effective than DIY options, it’s much less expensive than custom building each filter like in the simple construction method above. It’s also designed to continue operating efficiently even when it’s full of filtered sludge.
Advanced Wetland / “Bog Filter” Construction
For each 6’ centipede section, there should be an excavated area of 10’ x 10’. If you’re building a larger wetland, and are connecting all four centipedes into the snorkel, the excavated area will need to be 20’ x 20’ and increase proportionally from there. Four centipedes are the maximum number that can be connected together in a single line. For the best location, the wetland outlet should be a stream or waterfall going back into the pond. This will restore the dissolved oxygen levels which have been lowered due to the heavy biological activity found within the filter. As an example, let’s install a simple wetland system with a 4,000 GPH pump.
Steps for Constructing the Advanced Wetland Filter:
1. Prepare the Site:
Start by digging a 10’ x 15’ area that will become the wetland area. Dig the pond to a depth of 18” around the perimeter, going to 2’ in the middle. The purpose of this is to allow solids and sludge to settle towards the middle during cleaning.
Slope the bottom 6” towards the middle.
Dig a trough down the center, approximately 18” wide and 18” deep for the centipede. Pitch the trough slightly from the side with the piping to the outside edge where the cleanout snorkel is located.
Place the underlayment and the liner into your excavation.
2. Install the System
Place the the snorkel and the centipede. Connect the piping to the centipede.
3. Add Gravel and Plants
Cover the entire bottom with a layer of 4” to 6” stones. The next layer is made up of 1½ to 2” gravel, 6” deep. • Finish filling with smaller ½” gravel.
Once the wetland filter is running, plant it with a variety of marginal aquatic plants which will remove nutrients from the water. Yearly harvesting of the aquatic plants will keep the nutrients from returning to the system. Regular bacterial additions are necessary to reduce organic build-up and assure optimal media colonization.
4. Enjoy the Wetland Filter
All that’s left now is to enjoy your clean and clear water feature!
Watch the video below for how this exact system works before reading about the best kinds of plants for your wetland filter below:**
Enhancing Pond Filtration with Aquatic Plants
No wetland is complete without plants. Plants are major building block to an effective system. Different species of plants are particularly effective at removing pollutants and are commonly used in constructed wetlands for this purpose. Aquatic plants like cattails, sedges, grasses, and rushes are not only extremely efficient at filtering pollutants but are also hardy and grow prolifically, making them ideal for both natural water purification and beauty enhancement in residential water gardens.
The Importance of Plant Variety
Incorporating a wide array of plant species into your pond is important for achieving balanced filtration. Different plants have roots that extend to varying depths, allowing them to access and remove nutrients from different levels of your pond's substrate. For instance, cattails typically have shorter roots, reaching 6 to 12 inches deep, but bulrushes can have roots extending up to 36 inches deep. This diversity allows for a network of roots under the gravel, which makes the most use of the nutrients and sediment that accumulate at the bottom of the pond.
A skilled landscape designer will select plants that not only complement each other aesthetically but also function effectively throughout our different seasons here in our land of 10,000 lakes. Some plants, like irises, offer excellent filtration capabilities and aesthetic appeal while in bloom early in the spring. It’s important to include plants that peak at different times to ensure continuous filtration and growth throughout the growing season. Tropical plants like taro, papyrus, and thalia extend their growth into the fall, maintaining the balance as other plants begin to recede. By thoughtfully selecting and planting a variety of plants, your pond will not only be more visually appealing but will also maintain a balanced ecosystem throughout the entire year. This strategic approach ensures your pond remains healthy, vibrant, and efficient in its natural filtration process
Plants to Try:
Arrowhead
Assorted Taros
Blue Carex
Blue Moneywort
Blue Rush
Bog Lily
Canna
Chinese Water Chestnut
Corkscrew Rush
Creeping Jenny
Dwarf Cattail
Dwarf Gold Sweetflag
Dwarf Horsetail
Dwarf Papyrus
Dwarf Red Spiderlily
Dwarf Variegated Sweetflag
Japanese Iris
Lemon Bacopa
Louisiana Iris
Melon Sword
Pickerel Rush
Rain Lilies
Ribbon Grass
Ruby Creeper
Siberian Iris
Spider Lily
Star Grass
Variegated Spider Lily
Variegated Water Celery
Water Hyacinth
Water Purslane
Building a wetland filter or “bog filter” is one of the best steps you can take on your journey to a clear and clean pond. Take the struggle out of your water garden maintenance by installing one, or start right with your upcoming backyard pond and build yourself a wetland at the same time. Get back to enjoying your water again. Doing things the right way will only give you more time to spend in your little slice of Minnesota nice paradise!
Additional reading resources:
