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How does Tailwater's system differ from woodchip bioreactors?

March 23, 2019

Woodchip bioreactors and the Tailwater Nitrate Removal System rely on  bacteria to reduce nitrate (NO3) to nitrogen gas and carbon dioxide.  Both systems use a support for the bacteria.  The denitrifying bactera generate a biofilm that coats the supports and immobilizes the bacteria.  As water passes by and around the supports,  the nitrate molecule diffuses through the biofilm and enters into the bacteria where enzymes begin their work of breaking down the NO3 molecule.  Since the bacteria perform this function at the same rate, regardless of their location,  how can achieve very high rates of denitrification?

 

The answer is obvious- we need to a lot of surface area in a very small area to increase the number of bacteria.  Wood chip bioreactors use carefully prepare slivers of wood about 1 to 2 inches in diameter and less than 1/2 inch in thickness.   The term specific surface area (surface area /  volume) varies from < 1 to about 2 square inches/cubic inch.  Tailwater uses plastic biofilm carriers with a specific surface area (SSA) of 400 to 600.  This very high SSA packs a huge amount of bacteria into a small area.  The large surface area advantage enables our systems to be very much smaller (about 50X) than any woodchip bioreactor.  This number (50X) is based on side by side comparisons of our design vs a multiple woodchip bioreactors that we fabricated and tested.  It is also consistent with papers published by other authors in the technical/science journals.

 

The second area of difference is in the physical layout of the system.  Woodchip bioreactors (WBR) require an enormous area to operate.  They are generally all underground trenches dug into the land, filled with woodchips and covered with a tarp and over-burden.  In the WBR waste water flows horizontally through the system.  This means that the trench or ditch containing the woodchips must be graded to insure water flows. But this approach is fraught with problems.  The very low hydraulic flow rate (volume of flow / cross sectional area) means that any sediment carried by the waste will deposit near the entry of the wastewater and lead to fouling, channeling, and dead spots (no flow).  Deadspots can lead to the growth of sulfur reducing bacteria,  the generation of hydrogen sulfide (rotten egg smell), and numerous complaints from properties near the WBR.  

 

By contrast,  waste water flows up through our vertical bioreactor column  at a much higher flow rate.  Sediment is either carried to the top of the column and exists with the flow or deposits at the bottom of the column. This uniform flow leads to very stable nitrate reduction over time and rarely clogs or requires maintenance.  As the water flows,  the biofilm carriers (that are coated with bacteria),  roll and scrape against each other.  This "moving bed" concept increases the amount of time that the wastewater is in contact with the biofilm and also performs a self-cleaning function.  Excess biofilm is constantly sheared off and exits the bioreactor through the drain at the top.

 

In summary,  this comparison shows that Tailwaters nitrate removal will always outperform any woodchip bioreactor due it's superior design.

 

 

 

 

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