Using Wetland Filtration on Your Backyard Pond
Does your pond have sparkling clear water? If so, does it require a lot of effort to keep it that way? Thanks to the benefits of a properly-built wetland filter, it doesn’t have to!
What is a wetland filter?
A wetlands filter is an advanced form of natural filtration. Think of it as a big hungry sponge that feeds on the waste from fish and plants. Like a sponge, it is porous, allowing water to slowly pass through. Like a sea-sponge, it is alive, and it literally has an appetite for the stuff that clouds your pond water. Unlike a sponge, a wetlands is a complete ecosystem, populated by of trillions of individual bacteria, and usually, a handful of plants and invertebrates. These organisms colonize on the bottom of the pond and feed on particulates suspended in the water.
When you look at an established wetland in a backyard pond, it just looks like a shallow area with slow-moving water over fine gravel. Maybe there’s some grasses or thalia plants growing there. What you won’t see, however, is what’s going on beneath the surface that allows it to clean your water so efficiently.
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To understand how a wetland filter functions, it helps to first learn how they are constructed. The picture above shows the basic setup. It all starts with a hole. Typically, we’ll dig down about four to five feet and set the underlayment and liner in place. Then we run a pipe from a pump in the pond and feed it into something called a Centipede® Module. That serves to slow the water down and spread out the distribution of pond-water into the wetland. Next, we line the bottom with AquaBlox®, a heavy-duty latticed block that allows for maximum water storage in the basin. On top of the AquaBlox goes three layers of gravel. The first layer is roughly softball-sized river rock, then a layer of golf ball-sized river rock, then a layer of pebbles. Each layer is roughly 8“-12“ deep, making for a total of 2‘-3‘ of gravel atop of the AquaBlox. This creates a stable foundation that allows for maximum water distribution throughout the wetland. The water moves up from the bottom so slowly that suspended sediments are able to sink to the bottom and accumulate in the Snorkel® Vault, where they can be easily pumped out during maintenance.
Now that you are familiar with the design of a wetland filter, you might be wondering how a bunch of rocks are going to effectively filter the water. It’s all about the unsung heroes of the wetland: billions and billions of bacteria and microbes. These simple microorganisms are the most important part of the equation. They feed on the phosphorus and nitrogen that accumulates in the water as a result of decaying organic matter such as fish waste and dead leaves. The surface of submerged rocks are where they live and colonize, so the more surface area there is on those rocks, the more of these tiny helpers that can fit in the wetland and feed on the stuff you don’t want in the water.
The reason we use graduated levels of gravel in the wetland is because while the smaller stones have more surface area for bacteria, they get packed in much more closely to each other, limiting the amount of water in the wetland. Using a range of gravel sizes as described creates the best balance between maximum space for our 'little helpers' and maximum space for water storage. Layering the gravel as described serves an important function, as well. One reason is that the larger stones atop the Aquablox prevent the smaller stones from passing through the gaps and accumulating within the Aquablox, where they would reduce the water capacity of the wetland. Another reason is that the smallest gravel at the surface is much better than the larger stones at catching any leaves and debris that falls or blows into the wetland. We can easily pick any debris off the top of a wetland covered with medium-fine gravel. Larger stones would allow the debris to slip thru and get caught deep within the wetland, where they would decay, ultimately increasing the filter's burden and reducing its efficiency.
There is one more element of a wetland that supports the filtration: Plants!
While microorganisms do the majority of the work, plants offer additional support and work in harmony with them to provide the best filtration possible. To get the best results out of your wetland filter, we recommend using a variety of water-loving plants, especially ones that are native to your area. Variety is important; not just for aesthetics, but for performance. The roots of these plants will penetrate the gravel layer to different depths, supporting the filtration at multiple depths of the wetland.
Our favorite plant for using in wetlands is thalia. This is a tall plant with large leaves and small purple flowers. With its huge appetite for nutrients, thalia is a superhero when it comes to biological filtration. We have seen drastic improvements in water quality by merely adding thalia to a pond, it outperforms many more-common alternatives. Another excellent plant to use in wetland filters is the narrowleaf or dwarf cattail. With their extreme height-to-width ratio, a large volume of these plants can fit into a relatively small area. They are very effective at removing toxicity from water, and they are easy to grow. There can be complications with cattails, however, due to their strong roots that are able to penetrate a rubber pond liner and create leaks. That is much more likely when planting cattails in shallow areas of the pond, but it is worth mentioning. Other plants we like to use include creeping jenny, rain lilies, day lilies, spiral rush, iris, and sweet flag. There are many good candidates for plants to include in your wetland, too many to mention them all in this article. Just about any small plant you find in your area growing near water will probably be happy in your wetland; experiment with them and discover what you like best.
The most important thing when selecting plants for your wetland is to consider how they will fill in over the years. Some plants, thalia and cattail included, will completely take over the space provided if you allow them to spread uncontrolled. We advise an annual maintenance to trim back these prolific growers. Some people like to include tropicals in their wetland. Those have the benefit of dying back each year, preventing the need to prune them back as you would with perennials. If you like switching it up every year, then tropicals might be a great choice for you.
Perhaps the worst candidate for planting in a wetland filter is a large tree. We have heard horror stories of tree roots completely filling every bit of space in a wetland, creating leaks and causing the filter to fail. Unfortunately, by the time such problems are identified, the tree has had years to grow and become a focal point in the landscape. Nobody wants to call an expert with this problem and learn that their favorite tree has damaged their pond to the point that it's going to require an expensive overhaul or a new liner to save the pond. Adding insult to injury, if they want to fix the pond, the tree has to go. Avoid that scenario by choosing compatible plants for your pond. If you aren't sure if your selection is recommended, give us a call!
The Evolution of Wetland Filter Design
We have had the benefit of pooling our experience with dozens of pond builders from around the country, and we have learned some great tricks for creating the best wetland filters. One of these tricks comes from John Adams of Modern Design Aquascaping. He suggests that for a more efficient wetland filter, the line from the pump can be split, with the second line feeding a waterfall at the top of the wetland. This has the benefit of providing additional aeration, and it slows the water entering the bottom of the wetland, increasing the time the water spends being cleansed within the wetlands. Another benefit of having a waterfall flowing into the wetland is that it creates a vent in the plumbing, preventing the back-flow of dirty water from siphoning down to the pond when the pump is off.
To get a better idea of why we love wetland filters so much, it helps to compare them to alternative methods of filtration. In the past, the popular thinking was that to maintain a crystal-clear pond you needed to have an external filter setup similar to an in-ground pool. We’ve seen a lot of these setups when we’re called out to diagnose a problem pond. Often they resemble a giant mechanical octopus with twisting arms of PVC... not an easy apparatus to wrap your mind around, and especially troublesome when it comes time to perform maintenance or repairs. This style of filter evolved out of the pool industry, where lifeless plastic sterility is the name of the game. That kind of approach becomes a relentless battle against nature; a time-consuming, expensive, and most-likely frustrating battle.
Calculating the proper dimensions for your wetland filter
This is a very important consideration when designing your wetland. Every pond is unique, and there is no one-size-fits-all solution. A good rule of thumb is to have the area of the wetland be 10% the total area of the pond, although it certainly doesn't hurt to go bigger. In fact, in extreme conditions, the wetland might need to be nearly half the size of the pond! Factors that would warrant a bigger wetland include fertilizer runoff entering the pond, a heavy fish-load, visits from ducks or other waterfowl that foul the water, age of the pond, extreme algae problems, and also the desired goals for that pond, -maybe you want it to be as self-sufficient as possible during extended absences.
It can be difficult to read about different types of filters and know which best fits your space and budget. There are a lot of competing filters on the market, some are shiny and sophisticated, some are cheap and utilitarian, and there's a whole lot that fall somewhere in between. With all those options to review and consider, it would be easy to overlook the simplest, most elegant solution.
This subject can get complicated. If you have any questions that weren't addressed here, let us know so we can improve this article for everyone!
Landvista Aquascapes provides Pond & Water Feature Design, Installation & Maintenance -Repair services for South New Jersey Homeowners