Denitrifying bioreactors for water treatment

Contact information: Roger Herbert

In the research described below, denitrifying bioreactors have been developed for the removal of nitrogen from mining activities, rock quarrying, and tunnel excavation for roads.

Nitrogen release from undetonated ammonium-nitrate based explosives used in various industrial activities is an environmental issue that is just beginning to be recognized. Undetonated ammonium nitrate is readily soluble in water and quickly enters into the mine drainage. The nitrogen in these waters is primarily in the form of nitrate (NO3-) and ammonium (NH4+) with lesser amounts of nitrite (NO2-), which is eventually discharged to the environment. In the cold climate of northern circumpolar areas, aquatic ecosystems are often nutrient-limited, and even small nitrogen loadings could impact water bodies. In addition, a significant issue is the presence of nitrite and the generation of ammonia (NH3), both of which have toxic effects on aquatic ecosystems.

Algae growth in drainage from waste rock pile, demonstrating the availability of nitrogen nutrients in rock drainage
Algae growth in drainage from waste rock pile, demonstrating the availability of nitrogen nutrients in rock drainage.

What is a denitrifying bioreactor?

A denitrifying bioreactor uses the process of denitrification for the removal of nitrate from drainage waters. The process requires denitrifying bacteria in order to take place, and these microorganisms thrive in the oxygen-free environment that exists in the bioreactor’s organic substrate.

A denitrifying woodchip bioreactor is a simple construction (see figure below). In general, a bioreactor consists of a long excavation, about 40 meters long, 7 meters wide, and 2 meters deep. The excavation is covered with a geomembrane of HDPE-plastic, and then filled with woodchips. Pine woodchips have proven to be a good choice of wood since they degrade slowly.

Water flows into the left side of the bioreactor, through the woodchips, and out the other side through a perforated pipe. Treatment occurs when the water flows through the woodchips. Water flows with gravity through the bioreactor. But, depending on the field site, a pump may be necessary to lead water to the inlet.

Figure showing the simple construction of a denitrifying woodchip bioreactor.
Figure showing the simple construction of a denitrifying woodchip bioreactor.

Current research: NITREM

NITREM is a four-year innovation project (2018 – 2021) with funding from EIT RawMaterials, with the purpose of developing a service that combines a sustainable landscape design for waste rock piles with bioreactor technology that reduces nitrogen levels in leachate from waste rock piles.

Technology development has been driven by EUs Water Framework Directive and will enable the industry to meet current and future discharge requirements. The outcome is a low cost and low maintenance technology that is ready for market introduction and customer testing. The project consortium consists of Uppsala University, WSP Sverige AB, Cedervall Arkitekter, Agencia Estatal Consejo Superior de Investigaciones Cientificas, Boliden Mineral, LKAB, LTU Business and the Swedish University of Agricultural Sciences.

The bioreactor technology was previously evaluated in a pilot-scale system at LKAB’s Kiruna mine (see below), and is currently tested in full scale operation at the same mine. In the ”triangle area” at the Kiruna mine, a water collection system has been constructed that leads leachate from a waste rock pile to three bioreactors (see figure below). NITREM’s bioreactor technology removes nitrogen (in the form of nitrate) from the waste rock leachate.

Bioreactor 3 at the NITREM site.
Bioreactor 3 at the NITREM site.

Operations at the current bioreactor system in Kiruna started on 19 September 2019. The leachate from the waste rock pile is collected in a pumping chamber and then pumped to the bioreactors; at the start of operations, there was an initial concentration of 70 – 110 mg/L nitrate-nitrogen in the leachate which was pumped to the bioreactors at a flow of 0.5 L/s. The treatment temperature is low, with an average temperature in the bioreactors of ca. 3oC. Bioreactor performance is monitored through frequent sample collection and an online sensor system. In addition to nitrate, the discharge water from the bioreactor is also analyzed for nitrogen bi-products such as nitrous oxide, ammonium and nitrite. Concentrations of these compounds are generally low but the operation of the bioreactor system is being optimized in order to minimize the release of bi-products.

Porewater sampling at the NITREM site.
Porewater sampling at the NITREM site.

Previous research: MiNing project

Based on a series of laboratory experiments and a previous bioreactor study in Malmberget, a pilot-scale bioreactor was constructed at LKAB’s Kiruna iron ore mine in 2014. The aim of the study was to provide an improved understanding of the dominating biogeochemical processes in a bioreactor system, and how biofilms with high denitrification rates develop and are maintained.

These are the primary publications from this project:

Nordström, A., Hellman, M., Hallin, S., Herbert, R.B. (2021) Microbial Controls on Net Production of Nitrous Oxide in a Denitrifying Woodchip Bioreactor. J. Environ. Qual. 50, 228 – 240. 

Nordström, A. (2019) Biogeochemical Processes in Denitrifying Woodchip Bioreactors and their Application in the Mining Industry (Doctoral thesis). Uppsala University, Uppsala, Sweden.

Nordström, A., Herbert, R.B. (2019) Identification of the Temporal Control on Nitrate Removal Rate Variability in a Denitrifying Woodchip Bioreactor. Ecol. Engin. 127, 88 – 95.

Nordström, A., Herbert, R.B. (2018) Determination of major biogeochemical processes in a denitrifying woodchip bioreactor for treating mine drainage. Ecol. Engin. 110, 54-66.

Nordström, A., Herbert, R.B. (2017) Denitrification in a low-temperature bioreactor system at two different hydraulic residence times: laboratory column studies. Environ. Technol. 38, 1362 – 1375.

This project was part of the larger research project “MiNing - Reduction of nitrogen discharges in mining processes and mitigating its environmental impact” funded by VINNOVA, LKAB and Boliden AB over the period 2014 – 2018.

Pilot-scale bioreactor constructed in the MiNing project.
Pilot-scale bioreactor constructed in the MiNing project.

Last modified: 2021-08-10