Constructed wetlands have grown in popularity over the last few years, mainly because of their amazing filtration capacity, natural look, and aesthetic appeal. But what exactly is a wetland and just how does it function?
In our experience, there is no biological filter that outperforms a constructed wetland. We have constructed ponds that are well over a metre deep, and the water stays so crystal clear that you can count the individual pebbles on the pond’s floor! When torrential rain or our general meddling causes cloudiness in a pond’s water, the wetland filter has it clear again in just a few short hours.
A wetland filter works as a biological filter by creating an area in your pond that is thick with naturally-filtering pebbles and plants, providing a surface for bacterial colonisation – nature’s own filters. The plants, rocks, and pebbles act as the filtration media, similar to what you see in nature.
One of the greatest things about wetland filtration is that it can be scaled in size to suit small ponds through to large lake systems.
What is an Aquascape Wetland Filter?
A constructed Aquascape wetland filter in a water feature application is a simple concept. It is an area created as part of your pond or water feature where plants and beneficial bacteria work in harmony with Mother Nature’s “soup” of microorganisms to remove nutrients and fine sediments from a pond’s water. We like to say that a wetland filter polishes our water, much like reeds and aquatic plants do in naturally occurring marshes and wetlands.
Water is pumped into the base of the wetland from the skimmer or intake bay of the pond and into the bottom of the wetland through the Centipede module, where it disperses and slows the water velocity. The design allows the water to move slowly enough at this point that some sediment will fall out of suspension and collect in the bottom of the Centipede Module, where it can be removed through the cleaning process if the digestion is not as rapid as the accumulation.
Once the water has dispersed and slowed down within the Centipede module, it then flows up into the void created by the AquaBlox blocks, where it spreads out evenly across the entire floor of the wetland, slowing the flow down even more and allowing for further sedimentation. This process also prevents the channelling effect through the pebbles that other systems that use only slotted pipe will face. The water then flows gently upward through the gradients of pebble layered on top of the AquaBlox. We use a 15-20cm layer of 80-150mm pebble, then a 15-20cm layer of 40-80mm pebble, followed by a 15-20cm layer of 10-20mm pebble on top. This formula has allowed us to create enough substrate for proper filtration and sedimentation, while still keeping the filter relatively easy to clean. Here, growing prolifically in your pebbles and plant roots are nature’s army of microorganisms, readily waiting to collect and digest the fine particles and nutrients still suspended in the water. This is where the biological filtration happens.
Extreme situations or specific design requirements may call for more aggregate depth, but the above formula works well for us in most applications. Routine additions of Beneficial Bacteria are also added to the system to ensure optimal populations to deliver the desired water quality results.
The reason we use graduated levels of pebbles in the wetland is to prevent smaller pebbles getting packed into each other, which can limit the amount of water in the wetland. Using a range of gravel sizes as described creates the best balance between maximum space for housing bacteria and maximum space for water storage. Layering the gravel serves an important function as well. One reason for this layering 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.
We do not recommend using scoria or sharp gravel, as this type of stone can compact and lock in tight together allowing sediment to get trapped in between the gaps and stopping the water from flowing freely. (unlike rounded river pebbles of various sizes).
The important thing to remember when considering a constructed wetland filter is that, similar to your biological filter’s placement, the wetland filter should be flowing down to your pond for optimal efficiency. The wetland must sit a little higher to allow for the oxygen-depleted water from the filter to flow back into the pond in the form of a waterfall or swift-moving stream. This allows the water to re-oxygenate as it enters the pond.
Which Plants Are Best?
Plant selection varies by region, but our suggestion is to use a variety of plants. We like to use plants that are more clump-forming and manageable. Some use of tropical plants can be a good choice in regions where they die off, because the root mass starts new each season. Use plants that are different heights to get roots down at different levels in the pebble mass. Variety will work for you, and experimentation will show you what’s best in your area.
The cleaning of the constructed wetland is simple. We remove excess foliage, dead plant material and any sludge we can pick up from the surface. The pump feeding the wetland is shut off and the siphon is stopped. We place a clean-out pump down into the bottom of the Snorkel, and pump out all the water. This initial flush removes accumulated solids from the Snorkel and Centipede, and we then use a second clean-out pump positioned in the pond with an attached 40mm or larger hose to begin flushing the surface of the wetland with clean pond water. This forces any sediment build-up downwards for removal by the pump in the Snorkel, which is continually removing the dirty water. It’s here that our constructed wetland beats the competition’s designs. If you bury pipes or hoses in a huge gravel bed, it is extremely difficult - if not impossible - to back-flush or clean the filter in the future. Each time you clean, it becomes more compacted until it finally blocks and fails.
With the design we use, blasting water volume down through the gravel reverse-flushes debris into the Snorkel and Centipede in the base of the wetland. We flush until the clear water turns murky, like chocolate milk, as it moves through the pebble layers while continually being pumped out, until the water remains clear. The pond gets a water change as we clean. For smaller ponds, this process may need to be done over several cleans so as not to empty the entire pond. Remember, we are not trying to sanitise the system; we just want to remove the solids. After this clean, the feature is ready to turn back on. We do this cleaning whenever it is needed, generally once a year. It can be performed at any time of the year, without disturbing the pond, the fish or the functionality of the biological filter.
All of the above reasons are why we think that a constructed wetland is the perfect biological filter. As for sizing your wetland, we recommend sizing the wetland anywhere from 10-30% or your pond’s surface area, based on the use of the pond. Fish ponds can have as little as 10% of the surface area, whereas a Recreation Pond used for swimming is recommended to be a minimum 25%. The pump size will also be determined by the size of your filter.
Compare a wetland filter to any biological filter on the market and how it is rated, in litres. For example, think of the volume of media in a Biofalls filter and how much water it can filter, versus the volume of media in a wetland and how much water it can filter. Every pond is different. Every fish load, plant load, water depth, the amount of sun exposure, and the runoff factor will all play a role in affecting the filter’s effectiveness, but all constructed wetlands seem to be the same — awesome! Undersized pond filters need to be cleaned more often than oversized filters. Experiment; you will not be disappointed in the quality of your results!
Each of the Aquascape product elements play a key role in the success of a working wetland. Without any one of these components the system would not work as designed. One common issue that can occur with systems missing any or all of these components is the channelling of water up through the pebble substrate, which majorly reduces the filters efficiency as the water is not in contact with the majority of the pebble media. Another common issue is sediment blocking up a filter and inferior components as the systems havae not allowed for sediment removal in the intake or in the filter set up with no way of physically cleaning the filter it can become anaerobic and fail in as little as 3-5 years.
The bottom line is that constructed wetlands work, and they make any pond look natural and beautiful. Aesthetically it’s like adding an aquatic garden to your landscape, and you can even grow edible plants in them much the same way as in an aquaponic system!
With today’s growing tendency toward creating sustainable landscape solutions, you can see why wetland filtration is an obvious choice.
- Natural biological system
- No other filter outperforms a wetland filter
- Created with strong, durable components that are built to last. Will not fail under tonnes of pebbles
- Upflow system – down flow systems will block up and fail over time
- Can be easily cleaned and maintained
- Without all of the key Aquascape product components the system can block up with sediment and fail in 3-5 years.
- Is scalable to suit any size pond or lake
- A proven system – used in thousands of projects all over the world
- 20 plus years of trials and development to create the most efficient biological filter on the market