Tag: nitrogen removal

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Advancing wastewater sustainability: Nafion-powered ammonium recovery

Wastewater treatment plant Bern

DOI: 10.13140/RG.2.2.25254.59202

With global population growth and the resulting increase in environmental stress, the need for sustainable wastewater treatment is becoming ever more urgent. Traditional methods focus on removing pollutants but often overlook the opportunity to recover valuable resources. One such resource is ammonium. This nitrogen-containing molecule promotes growth and is a key component of fertilizers. When mishandled, such as through over-fertilization, ammonium becomes one of the main contributors to nitrogen pollution.

A promising solution lies in bioelectrical systems. This umbrella term refers to innovative technologies that not only purify wastewater but also recover resources like ammonium. At the same time, bioelectrical systems generate clean energy such as electricity or biogas. The technology is based on galvanic cells, where the two cell chambers are often separated by a membrane. High-performance cation exchange membranes enable precise ion transport and system stability. The premium product among cation exchange membranes is Nafion, such as our Nafion 115 membrane.

At Frontis Energy, we have demonstrated that bioelectrochemical systems can remove ammonium from wastewater, offering an energy-efficient alternative to the energy-intensive Haber-Bosch process. To validate this concept, we developed microbiological electrolysis cells populated with microorganisms from oxygen-deprived marine sediments off the coast of Namibia. These sediments are naturally rich in ammonia and low in organic carbon, ideal conditions for microbes capable of anaerobic ammonium oxidation. For comparison, we also used conventional municipal wastewater to populate the electrodes.

Maintaining anoxic conditions was crucial to avoid nitrification, a process that transfers electrons directly to oxygen, bypassing the anode and resulting in energy loss and reduced hydrogen production. Instead, we regulated the anode potential between +150 mV and +550 mV, well below the redox potential required for water oxidation (+820 mV). This configuration enabled the oxidation of ammonium to nitrogen gas (N₂) at the anode, while hydrogen (H₂) or methane gas was produced at the cathode.

Central to this process is Nafion 115, a membrane made of perfluorosulfonic acid polymers (PFSA polymers). Its exceptional proton conductivity, chemical resistance, and mechanical robustness make it ideal for demanding wastewater environments. Nafion 115 acts like a selective gate, allowing ammonium ions (NH₄⁺) to migrate from the anode to the cathode while blocking competing ions and maintaining anoxic conditions. This selective transport, driven by electric field gradients and concentration differences, ensures efficient nutrient recovery and stable performance of the bioelectrical system.

A practical validation of this technology comes from our earlier report, in which researchers developed a two-chamber, anoxic bioelectrical reactor that continuously removed ammonium at a rate of about 5 g/m³/day. Their system converted over 97% of the ammonium directly into nitrogen gas. This transformation occurred without the formation of harmful byproducts like nitrite or NOx gases. Particularly impressive was the energy consumption, just 0.13 kWh per kilogram of nitrogen removed. That is a 35-fold reduction compared to conventional aeration, which typically requires around 5 kWh/kg.

These results highlight the transformative potential of bioelectrical systems. As mentioned earlier, significant energy is used to remove nitrogen from wastewater, only to make it available again via the Haber-Bosch process, accounting for 1–2% of global energy consumption. Bioelectrical systems offer a circular alternative: by coupling ammonium oxidation with hydrogen production, wastewater treatment plants could become net energy producers. The generated hydrogen and biogas can be used directly for electricity generation and ultimately to reduce greenhouse gas emissions.

With the right biofilms, well-controlled electrode potentials, and robust membranes like Nafion 115, ammonium can serve as a clean, resource-efficient alternative to water electrolysis. This underscores the potential of bioelectrical systems to build a circular water economy, where waste is treated as a resource.

This technology reflects Frontis Energy’s commitment to promoting clean, efficient, and circular solutions that turn ecological challenges into sustainable opportunities.

Siegert and Tan, 2019, Electric stimulation of ammonotrophic methanogenesis, Frontiers in Energy Research 7:17, DOI: 10.3389/fenrg.2019.00017

Image: Wastewater treatment plant Bern

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Bio-electrical system removes nitrogen from the wastewater

Hazardous compound removal from sewage such as organic matter and nitrogen makes wastewater treatment an energy intensive process. For example, treating activated sludge requires blowing oxygen or air into raw, unsettled sewage. This aeration significantly increases the cost of the wastewater treatment. About 5 kWh per kilogram nitrogen are required for aeration depending on the plant. The cost associated with energy consumption makes uof approximately EUR 500,000 per year in an average European wastewater treatment plant. This is up to one-third of the total operational costs of WWTP. It is therefore obvious that nitrogen removal from wastewater must become more economical.

Alternative approach: Microbial electrochemical technology

The conventional way of removing nitrogen is a cascade of nitrification and denitrification reactions. Nitrification that is, aerobic ammonium oxidation to nitrite and nitrate is carried out by ammonia-oxidizing bacteria. Subsequent denitrification is the reduction of nitrate to nitrogen gas (N2). In addition to the costly aeration process, the remaining intermediate products as nitrite and nitrate require further effluent treatment.

Instead of expensive pumping of oxygen into the wastewater, bioelectrical systems could accomplish the same result at a much lower cost. In such systems, an electron accepting anode is used as electron acceptor for microbial ammonium oxidation instead of oxygen, making aeration obsolete.

Complete conversion of ammonium to nitrogen gas

We previously reported the use of such an bio-electrical system to remove ammonia from wastewater in fed-batch reactors. Now, researchers of the University of Girona reported proof-of-concept on a novel technology. Their bioelectrical system is a complete anoxic reactor that oxidizes ammonium to nitrogen gas in continuous mode. The dual-chamber reactor nitrifies and denitrifies and ultimately removes nitrogen from the system.

The electricity-driven ammonium removal was demonstrated in continuously operated one-liter reactor at a rate of ~5 g / m3 / day. A complex microbial community was identified with nitrifying bacteria like Nitrosomonas as key organism involved anoxic ammonium oxidation.

From an application perspective, comparison between bioelectrical systems and aeration in terms of performance and costs is necessary. The researchers reported that the same removal range and treatment of the similar amounts of nitrogen was achieved but that their bioelectrical system converted almost all ammonium to dinitrogen gas (>97%) without accumulation of intermediates. Their system required about 0.13 kWh per kilogram nitrogen energy at a flow rate of 0.5 L / day. Using a bioelectrical system consumes 35 times less energy compared with classic aeration (~5 kWh per kilogram). At the same time, no hazardous intermediates like nitrite or NOx gases are formed.

Unveiling microbial-electricity driven ammonium removal

The new article also indicated potential clues for microbial degradation pathway that may lead to better understanding of the underlying processes of anoxic ammonium removal in bioelectrical systems.

The proposed nitrogen removal pathway was the bioelectrical oxidation of ammonia to nitrogen monoxide, possibly carried out by a microbe named Achromobacter. That was supposedly followed by the reduction of the nitrogen monoxide to nitrogen gas, a reaction that could have been performed by Denitrasisoma. Alternatively, three other secondary routes were considered: bioelectrical oxidation followed by anammox, or without nitrogen monoxide directly to N2. Some sort of electro-anammox may also be possible.

At Frontis Energy, we believe that the direct conversion of ammonium to nitrogen gas through the reversal of nitrogen fixation is a possibility as nitrogen fixation genes are ubiquitous in the microbial world and it would generate the universal bio-currency ATP rather than consuming it.

It was shown that Achromobacter sp. was the most abundant microbe (up to 60%, according to sequence reads) in the mixed community. However, anammox species (Candidatus Kuenenia and Candidatus Anammoximicrobium) and denitrifying bacteria (Denitratisoma sp.) have been also detected in the reactor.

Two possible electroactive reactions were identified: hydroxylamine and nitrite oxidation, reinforcing the role of the anode as the electron acceptor for ammonium oxidation. Data obtained from nitrite and nitrate tests suggested that both, denitrification and anammox based reactions could take place in the system to close the conversion.

As a result, ammonium was fully oxidized to nitrogen gas without accumulated intermediates. Taking it all together, it has been shown that ammonium can be removed in bioelectrical system operated in continuous flow. However, further reactor and process engineering combined with better understanding of the underlying microbial and electrochemical mechanisms will be needed for process scale up.

Experimental system set-up

  • The inoculum consisted of a 1:1 mix of biomass obtained from nitritation reactor and an aerobic nitrification reactor of an urban treatment plant
  • The reactor design was constructed of two 1 L rectangular chambers comprising an anode and cathode compartment
  • The separator, an anion exchange membrane,  was used to minimize the diffusion of ammonium to the cathode compartment
  • The anode and cathode chambers were filled with granular graphite as electrode support
  • Ag/AgCl reference electrode was used in the anode compartment
  • Two graphite rods were placed as current collectors in each chamber
  • The system was operated in batch and semi-continuous mode

Image: 5056468 / Pixabay