Tag: anammox

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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

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Ammonia energy storage #2

Recently, we reported on plans by Australian entrepreneurs and their government to use ammonia (NH3) to store excess wind energy. We proposed converting ammonia and CO2 from wastewater into methane gas (CH4), because it is more stable and easier to transport. The procedure follows the chemical equation:

8 NH3 + 3 CO2 → 4 N2 + 3 CH4 + 6 H2O

Now we have published a scientific article in the online magazine Frontiers in Energy Research where we show that the process is thermodynamically possible and does indeed occur. Methanogenic microbes in anaerobic digester sludge remove the hydrogen (H2) formed by electrolysis from the reaction equilibrium. As a result, the redox potentials of the oxidative (N2/NH3) and the reductive (CO2/CH4) half-reactions come so close that the process becomes spontaneous. It requires a catalyst in the form of wastewater microbes.

Pourbaix diagram of ammonium oxidation, hydrogen formation and CO2 reduction. At pH 7 and higher, the oxidation of ammonium coupled to methanogenesis becomes thermodynamically possible.

To prove our idea, we first searched for the right microbes that could carry out ammonia oxidation. For our experiments in microbial electrolysis cells we used microorganisms from sediments of the Atlantic Ocean off Namibia as starter cultures. Marine sediments are particularly suitable because they are relatively rich in ammonia, free from oxygen (O2) and contain less organic carbon than other ammonia-rich environments. Excluding oxygen is important because it used by ammonia-oxidizing microbes in a process called nitrification:

2 NH3+ + 3 O2 → 2 NO2− + 2 H+ + 2 H2O

Nitrification would have caused an electrochemical short circuit, as the electrons are transferred from the ammonia directly to the oxygen. This would have bypassed the anode (the positive electron accepting electrode) and stored the energy of the ammonia in the water − where it is useless. This is because, anodic water oxidation consumes much more energy than the oxidation of ammonia. In addition, precious metals are often necessary for water oxidation. Without producing oxygen at the anode, we were able to show that the oxidation of ammonium (the dissolved form of ammonia) is coupled to the production of hydrogen.

Oxidation of ammonium to nitrogen gas is coupled to hydrogen production in microbial electrolysis reactors. The applied potentials are +550 mV to +150 mV

It was important that the electrochemical potential at the anode was more negative than the +820 mV required for water oxidation. For this purpose, we used a potentiostat that kept the electrochemical potential constant between +550 mV and +150 mV. At all these potentials, N2 was produced at the anode and H2 at the cathode. Since the only source of electrons in the anode compartment was ammonium, the electrons for hydrogen production could come only from the ammonium oxidation. In addition, ammonium was also the only nitrogen source for the production of N2. As a result, the processes would be coupled.

In the next step, we wanted to show that this process also has a useful application. Nitrogen compounds are often found in wastewater. These compounds consist predominantly of ammonium. Among them are also drugs and their degradation products. At the same time, 1-2% of the energy produced worldwide is consumed in the Haber-Bosch process. In the Haber-Bosch process N2 is extracted from the air to produce nitrogen fertilizer. Another 3% of our energy is then used to remove the same nitrogen from our wastewater. This senseless waste of energy emits 5% of our greenhouse gases. In contrast, wastewater treatment plants could be net energy generators. In fact, a small part of the energy of wastewater has been recovered as biogas for more than a century. During biogas production, organic material from anaerobic digester sludge is decomposed by microbial communities and converted into methane:

H3C−COO− + H+ + H2O → CH4 + HCO3− + H+; ∆G°’ = −31 kJ/mol (CH4)

The reaction produces CO2 and methane at a ratio of 1:1. Unfortunately, the CO2 in the biogas makes it almost worthless. As a result, biogas is often flared off, especially in places where natural gas is cheap. The removal of CO2 would greatly enhance the product and can be achieved using CO2 scrubbers. Even more reduced carbon sources can shift the ratio of CO2 to CH4. Nevertheless, CO2 would remain in biogas. Adding hydrogen to anaerobic digesters solves this problem technically. The process is called biogas upgrading. Hydrogen could be produced by electrolysis:

2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)

Electrolysis of water, however, is expensive and requires higher energy input. The reason is that the electrolysis of water takes place at a relatively high voltage of 1.23 V. One way to get around this is to replace the water by ammonium:

2 NH4+ → N2 + 2 H+ + 3 H2; ∆G°’ = +40 kJ/mol (H2)

With ammonium, the reaction takes place at only 136 mV, which saves the respective amount of energy. Thus, and with suitable catalysts, ammonium could serve as a reducing agent for hydrogen production. Microorganisms in the wastewater could be such catalysts. Moreover, without oxygen, methanogens become active in the wastewater and consume the produced hydrogen:

4 H2 + HCO3– + H+ → CH4 + 3 H2O; ∆G°’ = –34 kJ/mol (H2)

The methanogenic reaction keeps the hydrogen concentration so low (usually below 10 Pa) that the ammonium oxidation proceeds spontaneously, i.e. with energy gain:

8 NH4+ + 3 HCO3– → 4 N2 + 3 CH4 + 5 H+ + 9 H2O; ∆G°’ = −30 kJ/mol (CH4)

This is exactly the reaction described above. Bioelectrical methanogens grow at cathodes and belong to the genus Methanobacterium. Members of this genus thrive at low H2 concentrations.

The low energy gain is due to the small potential difference of ΔEh = +33 mV of CO2 reduction compared to the ammonium oxidation (see Pourbaix diagram above). The energy captured is barely sufficient for ADP phosphorylation (ΔG°’ = +31 kJ/mol). In addition, the nitrogen bond energy is innately high, which requires strong oxidants such as O2 (nitrification) or nitrite (anammox) to break them.

Instead of strong oxidizing agents, an anode may provide the activation energy for the ammonium oxidation, for example when poised at +500 mV. However, such positive redox potentials do not occur naturally in anaerobic environments. Therefore, we tested whether the ammonium oxidation can be coupled to the hydrogenotrophic methanogenesis by offering a positive electrode potential without O2. Indeed, we demonstrated this in our article and have filed a patent application. With our method one could, for example, profitably remove ammonia from industrial wastewater. It is also suitable for energy storage when e.g. Ammonia synthesized using excess wind energy.