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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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Pilot-scale microbial fuel cells produce electricity from wastewater

In wastewater treatment, aeration is an energy-intensive but necessary process to remove contaminants. Pumps blow air into the wastewater to supply the microbes in the treatment tank with oxygen. In return, these bacteria oxidize organic substances to CO2 and hence remove them from the wastewater. This process is the industrial standard and has proven itself for over a century. If the researchers at Washington State University and the University of Idaho have their way, that is changing now.

In their project, the researchers used a unique microbial fuel cell system they developed to replace aeration. Their novel wastewater treatment system cleans wastewater with the help of microorganisms that produce electricity. These microbes are called electrophiles.

The work should one day lead to less dependence on the energy-intensive treatment processes. Most of the energy in such processes is consumed in the activated sludge and its disposal. The energy consumption in water treatment produces around 4-5% of anthropogenic CO2 worldwide. to put that in perspective, according to the Air Transport Action Group in Geneva, international air transport produced 2.1% CO2 in 2019. The researchers published their work in the journal Bioelectrochemistry. In addition to cutting green house gas emissions, lowering the energy consumption of wastewater treatment would save billions in annual operation and maintenance costs.

Microbial fuel cells allow microbes to convert chemical energy into electricity, much like in a battery. In wastewater treatment, a microbial fuel cell can replace aeration while capturing electrons from wastewater organics. These electrons themselves are in turn a waste product of the microbial metabolism. All living organisms strive to discharge their excess electrons. This process is known as respiration or fermentation. The electricity generated the microbes can be used for useful applications in the wastewater treatment plant itself. The technology kills two birds with one stone. On the one hand, the treatment of the wastewater saves energy. On the other hand, it also generates electricity.

Up until now, microbial fuel cells have been used experimentally in wastewater treatment systems under ideal conditions, but under real and changing conditions they often fail. Microbial fuel cells lack regulation that controls the potential of anodes and cathodes and thus the cell potential. This can easily lead lead to a system failure. The entire cell must then be replaced.

To tackle this problem, the researchers added an additional reference electrode to the system that enables them to control their fuel cell. The system becomes more flexible. It can either work as a microbial fuel cell on its own and consume no energy, or it can be converted so that less energy is used for aeration while it purifies the wastewater more intensively. Frontis Energy uses a similar control system for its electrolysis reactors.

The system was operated for one year without major issues in the laboratory as well as a pilot in a wastewater treatment plant in Idaho. It removed contaminants at rates comparable to those in a classic aeration tanks. In addition, the microbial fuel cell could possibly be used completely independent of grid power. The researchers hope that one day it could be used in small wastewater treatment plants, such as cleaning livestock farms or in remote areas.

Despite the progress, there are still challenges to be overcome. They are complex systems that are difficult to build. At Frontis Energy we specialize in such systems and can help with piloting and commercialization.

(Photo: Wikipedia / National University of Singapore)

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Global wastewater resources estimated

In our last post on water quality in China, we pointed out a study that shows how improved wastewater treatment has a positive effect on the environment and ultimately on public health. However, wastewater treatment requires sophisticated and costly infrastructure. This is not available everywhere. However, extracting resources from wastewater can offset some of the costs incurred by plant construction and operation. The question is how much of a resource is wastewater.

A recent study published in the journal Natural Resources Forum tries to answer that question. It is the first to estimate how much wastewater all cities on Earth produce each year. The amount is enormous, as the authors say. There are currently 380 billion cubic meters of wastewater per year worldwide. The authors omitted only 5% of urban areas by population.

The most important resources in wastewater are energy, nutrients like nitrogen, potassium and phosphorus, and the water itself. In municipal wastewater treatment plants they come from human excretions. In industry and agriculture they are remnants of the production process. The team calculated how much of the nutrient resources in the municipal wastewater is likely to end up in the global wastewater stream. The researchers come to a total number of 26 million tons per year. That is almost eighty times the weight of the Empire State Building in New York.

If one would recover the entire nitrogen, phosphorus and potassium load, one could theoretically cover 13% of the global fertilizer requirement. The team assumed that the wastewater volume will likely continue to increase, because the world’s population, urbanization and living standards are also increasing. They further estimate that in 2050 there will be almost 50% more wastewater than in 2015. It will be necessary to treat as much as possible and to make greater use of the nutrients in that wastewater! As we pointed out in our previous post, wastewater is more and more causing environmental and public health problems.

There is also energy in wastewater. Wastewater treatment plants industrialized countries have been using them in the form of biogas for a long time. Most wastewater treatment plants ferment sewage sludge in large anaerobic digesters and use them to produce methane. As a result, some plants are now energy self-sufficient.

The authors calculated the energy potential that lies hidden in the wastewater of all cities worldwide. In principle, the energy is sufficient to supply 500 to 600 million average consumers with electricity. The only problems are: wastewater treatment and energy technology are expensive, and therefore hardly used in non-industrialized countries. According to the scientists, this will change. Occasionally, this is already happening.

Singapore is a prominent example. Wastewater is treated there so intensively that it is fed back into the normal water network. In Jordan, the wastewater from the cities of Amman and Zerqa goes to the municipal wastewater treatment plant by gravitation. There, small turbines are installed in the canals, which have been supplying energy ever since their construction. Such projects send out a signals that resource recovery is possible and make wastewater treatment more efficient and less costly.

The Frontis technology is based on microbial electrolysis which combines many of the steps in wastewater treatment plants in one single reactor, recovering nutrients as well as energy.

(Photo: Wikipedia)