Biological Nitrate Reduction in Water

Nitrate nitrogen is a major pollutant in groundwater, surface water, and wastewater. Its excessive presence poses health risks (e.g., methemoglobinemia in infants) and drives eutrophication. Conventional physicochemical methods such as ion exchange, reverse osmosis, or chemical denitrification are effective but often costly and generate secondary waste. 

Biological nitrate reduction, which harnesses microbial metabolism to convert nitrate into harmless nitrogen gas or biomass, offers a sustainable, cost‑effective alternative. Several biological strategies are available, each suited to different water conditions.

The Conventional Workhorse

Heterotrophic denitrification is the most widely applied biological process. Under anoxic conditions, denitrifying bacteria use nitrate as an electron acceptor and organic carbon as both electron donor and energy source, sequentially reducing nitrate to nitrite, nitric oxide, nitrous oxide, and finally to dinitrogen gas. 

This process requires a sufficient carbon‑to‑nitrogen ratio (C/N). When wastewater lacks native organic carbon, external carbon sources such as methanol, acetate, or glucose must be added. Recent advances include slow‑release carbon materials (e.g., biodegradable polymers) that supply carbon steadily, avoiding overdosing. Optimized heterotrophic systems can achieve over 95% nitrate removal, making them the standard for municipal and industrial wastewater treatment.

For Low‑Carbon Waters

In groundwater or certain industrial effluents with limited organic carbon, heterotrophic denitrification becomes impractical. Autotrophic denitrifying bacteria use inorganic electron donors—such as hydrogen gas, reduced sulfur compounds, or ferrous iron—and fix inorganic carbon (CO₂ or bicarbonate) as their carbon source. 

Sulfur‑based autotrophic denitrification uses elemental sulfur or thiosulfate, producing sulfate as a by‑product, but requires alkalinity addition. Hydrogen‑based denitrification, often implemented in membrane biofilm reactors, uses electrolytically generated hydrogen, yielding clean water without residual organics. Iron‑driven autotrophic denitrification, which couples nitrate reduction to ferrous iron oxidation, is emerging as a promising option for nitrate removal in iron‑rich aquifers.

Anaerobic Ammonium Oxidation (Anammox) Coupled with Partial Denitrification

While anammox primarily removes ammonium using nitrite, its combination with partial denitrification offers a novel route for nitrate removal. In this process, a fraction of nitrate is reduced to nitrite (instead of all the way to nitrogen gas), and the resulting nitrite is then consumed by anammox bacteria together with ammonium. 

This coupled pathway (partial denitrification‑anammox) drastically reduces the demand for organic carbon, minimises excess sludge production, and lowers greenhouse gas emissions. It is particularly attractive for treating low‑C/N wastewaters that contain both nitrate and ammonium.

Assimilation and Plant Uptake

Beyond respiratory denitrification, microorganisms and algae can assimilate nitrate into cellular biomass (proteins, nucleic acids) under adequate carbon and nutrient conditions. In aquaculture or constructed wetlands, promoting heterotrophic assimilation by adding organic carbon effectively removes nitrate from the water column. 

Macrophytes such as reeds and cattails also take up nitrate directly through their roots, providing an additional, nature‑based polishing step.