The "Science" of Wetland Filters

OR EVERYTHING YOU ALWAYS WANTED TO KNOW ABOUT WETLAND FILTERS, BUT WERE AFRAID TO ASK!

​When a fellow pond builder asked Ed Beaulieu (inventor/designer of the Aquascape Constructed Wetland Filter) for the explanation of how our wetland filters work, he provided the following in-depth response.  Ed, Chief Science Officer for Aquascape Inc., was in Bogota, Colombia at the time and crafted this response on his phone!  Ed LOVES science -but is hardly a science nerd- and as you can see by the last paragraph, has a sense of humor!
​ (Ed's response, in its entirety, follows verbatim)
​"You may want to grab a beverage of your choice before reading..... This should answer his questions: Thanks for questioning the function of this filter, you clearly understand the importance of a well designed and built Biofilter.
The Constructed Wetland Filtration System is a very simple yet effective Filter.
It’s a combination of controlled water flow rates combined with a sedimentation chamber and specific grades of River rock that will provide the necessary surface area for microbial colonization.
But before I describe the functions of the Wetland I need to stress the importance of a Mechanical Prefilter such as a Skimmer or Intake Bay both of which will remove large organic compounds from the water before it reaches the Wetland.
If this simple device is not in place the Wetland Filter will not function properly for two reasons.
1st the leaf debris, uneaten fish food, etc.... will overload the sedimentation chamber over time.
2nd reason is due to dissolved oxygen which we all know is critical for a healthy aquatic ecosystem.
In this particular instance the skimmer or Intake Bay draws the surface water from the surface of the pond during operation.
The surface water will have the highest dissolved oxygen level because the water is in direct contact with the atmosphere.
A healthy Wetland Filter consumes oxygen through a variety of biological processes, the greater the fish load the more critical this becomes, the mixture of sediments, fish waste, dissolved nutrients and oxygen will allow flow into the bottom of the filter. With that out of the way we can discuss the actual Wetland Filter in detail:
Let’s start at the bottom of the filter where the water is discharged into the wetland from the skimmer/Prefilter.
The bottom of the filter uses a patented water distribution system(Snorkel vault and Centipede modules) this modular design allows us to customize the filter according to the goals of the project.
Example:
High fish loads,
Existing pond with high benthic nutrient deposits(eutrophic System)
Swimming pond,
Excessive runoff from the area surrounding the pond, etc..... We have an initial 93% reduction in water velocity as the water enters the Centipede module, this allows the larger sediments(sand and small gravel) to settle out in this first chamber.
The water then flows up into the main sedimentation chamber which is comprised of our Aquablox water matrix units. When the water enters this chamber we have an additional drop in water velocity creating a hydraulic residence time of 10-15 minutes within the sedimentation chamber depending on the actual flow rates of the filter(Total Dynamic Head) this allows the finer sediments(fine organic compounds, fish waste, silty clays, etc....)with the necessary hydraulic residence time to accumulate within this zone.
The Aquablox have a series of internal baffles within them which eliminates laminar water flow, these baffles create a gentle mixing of the water which allows the fine suspended sediments and particles to literally bump into each other, these particles have a weak electric charge so when they bump into each other they’ll stick together in larger clumps when the flow rate permits.
The larger clumps have a slightly higher specific gravity than the water so they sink to the bottom of the sedimentation chamber.
This process is very important because if the fine sediments are not removed(precipitated out)they’ll adhere to the river rocks which will change the surface area for the beneficial bacterial colonies causing a shift in the specific species that can live in the filter.
Our goal is to establish the colonies directly on gravel not on sediments stuck to the gravel..... From here the water flows up through a minimum of 18” of River rock comprised of three distinct sizes/layers of stone.
The bottom layer has the largest gravel made with 3”-5” River rock, 8” thick.
The second layer is 2”-3” gravel 8” thick and the final layer located at the top of the filter is 3/4” gravel 8” thick.
The top layer of 3/4” gravel is perfect for planting aquatic plants.
The declining nature of the gravel diameters will keep them separated because the interstitial spaces will not permit the migration of the rocks down into the lower levels, does this make sense???
3”-5” rock will have void spaces of 1”-1.5” maximum this will not allow the 2”-3” gravel to migrate.
The other reason for the different grades of gravel is because they each have different surface areas(large rock has a lower
Surface area to volume versus small gravel which has a higher surface area for its volume) this allows for different levels of colonization to occur within the different river rock layers.
As the water continues it’s path to the surface/top layer of the filter the water flow will split into smaller and smaller pathways(interstitial spaces)by doing this we expose the flowing water to the diverse colonies of bacteria, fungi, biofilms and countless aquatic species of copepods, rotifers, tardigrades, etc....
Every living organism on our Planet is competing against other similar organisms for food, a home, sunlight, etc.....
This competition is fierce!
You wouldn’t realize this because it takes place on all trophic levels
From large vertebrates all the way down to the microscopic/nano level.
Bacteria and fungi compete for space within the filter bed.
The reason that I bring this up is that this filter only functions properly when it’s “seeded” with the right strains of bacteria, we use a specific formula that has been developed to consume nitrogenous wastes generated by fish and other aquatic organisms along with allocthonous organic compounds such as leaf debris and lawn clippings.
Without the proper mix of bacteria added to the pond your leaving it up to chance that the right beneficial microbes will just happen to show up?!?!
That’s not to say that it can’t happen because it can but if you want consistent results then this is the only way to insure it!
I apologize for the long explanation but I felt that it was relevant to explain some of the details of the system before answering your actual questions. How do you clean it, very easily, Once per year we recommend the Wetland Filter to be thoroughly cleaned.
This is accomplished by placing a solids handling sump pump into the snorkel vault which goes to the lowest elevation within the filter(sump) turn the filter pumps off and turn the solids handling pump on.
This pump should have 50’+ of piping/tubing connected to it to allow the sediment laden water to be discharged into the surrounding garden beds or turf grass.
The dirty filter water is filled with organic compounds and is a great source nutrients for the plants.
Once the filter is completely drained place a second pump along with the associated piping into the pond, discharge the pond water on to the top of the filter bed.
Move the water around to actively back-flush the gravel bed sending the fine sediments and water down through the entire filter.
Continue pumping the dirty water out of the snorkel until the water being discharged into the Landscaping is sufficiently clean(it doesn’t have to be perfect!)
The top of the wetland is typically planted with a wide variety of marginal aquatic plants which will further aid in the removal of Nitrogen from the system.
These plants should be cut back at this time and thinned to encourage new plant growth.
More plant growth=greater nutrient removal. Aeromonas and Pseudomonas are not a problem if the filter is installed and maintained properly this is due to the efficient water distribution system combined with high dissolved oxygen levels throughout the filter bed; this is not the desired conditions for those organisms.
This combined with the highly competitive nature of the beneficial microbes that are added to the system will literally starve them out because they will not be able to get a foothold in the filter(everyone competing for food and a home) Fish waste is kept in suspension within the pond due to Bioturbations from the fish(constantly feeding on the bottom of the pond) along with underwater jets that will push the waste towards the Prefilter.
Large waste will be removed in the Prefilter and the finer compounds will pass through the pump where they’re agitated and broken into finer particles to be distributed through the Centipede modules and Sedimentation chamber, this is similar to the Solids Chamber on a Septic System if your familiar with that technology?
The remainder of the fish waste is ammonia secreted from the gills during respiration.
This dissolved form of nitrogen will be subjected to the nitrifying bacteria living within the filter bed. As you can see there are several factors that need to be accounted for outside of the actual filter that allows the system to function properly, if one of the other steps is not done it will have an impact in the water quality.
I hope my response helps you to more fully understand the inner workings of this Revolutionary Filter System that was created by my Mentor: Ed Beaulieu the Grandmaster of Pond and Water Feature design and construction.
Without his knowledge and understanding of aquatic ecosystems this would not be possible.....
I look forward to hearing back
from you!"