Biofilters rely on natural plant filtration mechanisms
Plants provide fresh oxygen to breathe and aesthetics that can take our breath away, but many people do not think about the role plants can play in storm water treatment. The principal notion of low impact development (LID) is founded upon mimicking pre-development conditions—not just by matching peak flow rates, but also by matching the pollutant load discharging from the drainage area. What better way to follow LID principles to meet these pre-development loads than to imitate nature’s biological removal mechanisms in pre-development conditions?
Biofilters often are specified to do just that. “Bio” implies that living organisms are a component of the treatment process, working in concert with other pollutant removal mechanisms. Without plants as a component of the synergistic community of living organisms in these systems, biofilters are not as effective.
Plants also are key to sustainability in these systems and help maintain an assimilative capacity over and above systems void of vegetation. Biofilters capture pollutants through various mechanisms, which are further broken down by microorganisms into forms available for plant uptake via phytoremediation. As the plant grows and increases in biomass, so does the system’s capacity to capture and process more pollutants. Plants regenerate media pollutant removal capacity by making the media adsorption sites available for the next storm event.
Sustainability is not just defined by maintaining high pollutant removal, but also by maintaining design hydraulic flow rates over time. Plants help maintain hydraulic flow rate through their expanding root system. Plant roots continuously penetrate filter media as the plant grows, and the roots themselves die and regrow, forming micro channels. This prevents media compaction and increases porosity, ultimately maintaining aeration and hydraulic rates. Plant roots and associated microbiological growth provide exudates, which build and maintain soil structure. This increases macropore development for maintaining infiltration rates.
Most of the biological processes occur underground in the rhizosphere, the habitat where the plant root system interacts with the soil environment via root secretions and soil microorganisms. Millions of diverse microorganisms exist in the rhizosphere and exist to this magnitude only because of the plant. There are 10 times more microorganisms in the rhizosphere than outside of it.
Just as with plants in a natural, pre-development landscape, microorganisms degrade and transfer pollutants into less toxic forms through chelation for plant uptake, and sequestration of pollutants through carbon and nutrient assimilation. Microorganisms alter the soil chemistry in the rhizosphere to enhance pollutant removal efficiency. Plants increase organic matter in the soil through decomposition of biomass, including the roots themselves, known as cell sloughing, which provides a carbon source to the microorganisms in the media.
Additionally, mycorrhizae fungi create a symbiotic relationship with plant roots. Mycorrhizae fungi increase the surface area of plant roots which ultimately enhances absorption of phosphorus, nitrogen and metals, which are all macro and micro plant nutrients that are vital for plant growth and reproduction.
So next time you are considering a biofiltration practice, remember to mimic pre-development removal mechanisms and do not forget the plants.