Denitrification

FI WRP DN-Whitney Flanagan-150x150At The Conservation Fund Freshwater Institute, we have a major interest in denitrification technologies that reduce nitrate pollution in agricultural and aquacultural waters. Denitrification is the microbially-mediated reduction of nitrate to dinitrogen gas; more simply, denitrification occurs when bacteria present in the environment convert nitrate in soil or water to atmospheric gas. The bacteria utilized in the process of denitrification are aptly named denitrifiers. These bacteria are naturally found in soil all over the world. They consume the carbon stored in soil and utilize naturally-occurring nitrates as part of their respiration (breathing) process. As such, the natural process of denitrification is a vital tool in our tool box for improving water quality.

Denitrification Page-David Donohue-488x39

The exchange involved in denitrification

The denitrification process used for industrial and municipal waste water treatment is typically heterotrophic, meaning that bacteria “eat” carbon in order to convert nitrate in the water to dinitrogen gas. However, sulfur-driven autotrophic denitrifiers also exist in the environment, and can perform a similar nitrate conversion. This sulfur-based autotrophic denitrification involves the use of reduced sulfur (e.g., hydrogen sulfide, elemental sulfur) as the electron donor and nitrate as the terminal electron acceptor in the bacteria’s electron transport chain. Similar to heterotrophic denitrifying bacteria, sulfur-based autotrophic denitrifiers require anoxic conditions, although the carbon source required for this chemo-autotrophic process is derived from inorganic carbon (e.g., carbon dioxide, bicarbonate). Despite the limitation of lower growth rates for autotrophs versus heterotrophs, autotrophic denitrification does not require an exogenous (outside) carbon source and produces less sludge waste, thus minimizing cost and labor versus a heterotrophic denitrification treatment.

Sulfur Denitrification-Kata Sharrer-512x512

The nitrogen cycle, involved in the process of denitrification

In order to de-nitrify wastewater without the use of bacteria, devices named biofilters are used. Ideally, a perfect biofilter “would[...]require little or no water pressure or maintenance, and require a small footprint”; however, each biofilter has its own unique advantages and disadvantages. Despite the high amount of solids and nutrients in the wastewaters of our institute, we have found that fluidized biofilters can effectively treat
these waters.

Biofilters-Scott Tsukuda-512x384

Fluidized biofilters (tall columns, far right) currently being tested at the Freshwater Institute; shown with solids settling cones (left) and radial flow settlers (close right)