From algal blooms to dead zones, we cause considerable damage by allowing nitrogen to be released into our waterways.
Since the activated sludge process commonly used at water treatment plants does not effectively remove nitrogen and phosphorus, post treatment processes are needed. The traditional method for removing those nutrients involves adding a carbon source.
Unfortunately, there are risks with excess carbon causing the water effluent to be highly flammable. The current method using carbon is also often unavailable to poorer and rural communities. With the changing attitudes towards sustainability, researchers are looking for nitrogen removal methods that are low maintenance, low cost, and doesn’t use virgin material.
Ditches As a Line of Defense
One of the simplest methods for nitrogen removal might already exist along our roads and agriculture fields. Published in Biogeosciences, Tatariw et al. looked at the lowly ditch.
They took soil samples from 96 ditches in three different watersheds in Alabama. Ditches in forested, urban, and agricultural areas were compared to see if there were differences in their ability to remove nitrogen.
Tatariw et al. added nitrogen to a slurry made using the soil samples. They were then able to measure how much nitrogen was reduced by the microbes and extrapolate to calculate the nitrogen removal potential of the ditch.
The study concluded that ditches have the ability to remove over 89% of the nitrate. Ditches are essentially human-made lowlands that act as wetlands. Wetlands are already proven to be highly effective at removing nitrogen.
In forested areas the denitrification potential rates of ditches were half the rates in urban and agricultural ditches. Soil characteristics and microbial diversity were similar for al three land use types, but urban and agricultural ditches contained Nitrososphaeraceae, Nitrosomonadaceae, Gaiellales, and Myxococcales. This is believed to be the reason for the difference in nitrogen removal rates.
Vermifiltration, the use of worms to treat wastewater, has gained traction as a viable method to remove organics. This low-tech solution has been used in Africa and Asia and in communities where access to power and money is limited.
Researchers at the University of Nebraska and the University of Mississippi looked at using vermifiltration as a method to remove nitrogen. Since previous studies have focused on the removal of organics, the effect of worms on nitrogen isn’t well known.
This lack of understanding of how well vermifiltration removes nitrogen has been a factor in the slow adoption of vermifiltration. Nitrogen removal is important, and communities don’t want to use vermifiltration unless it removes nitrogen as well.
Previous studies found that both ingestion of soil and tunneling, earthworms two main activities, are effective at enhancing nitrification/denitrification. The earthworm’s gut contains the Nitrospira bacteria, which plays a part in removing and fixating nitrogen. Earthworm’s tunneling increases the amount of oxygen in the system – aiding in nitrogen removal too.
As expected, several factors enhance or impede the removal of nitrogen: pH, temperature, hydraulic retention time, bed material, worm species, and bed depth.
The study concluded that increasing the number of earthworms and increasing the hydraulic retention time can significantly increase the amount of nitrogen removed. Some other factors like temperature and worm species can also be optimized. Where the data is lagging is in the impact of pH on vermifiltration efforts to remove nitrogen. The researchers suggested further studies to understand the impact of pH.