Tag: wastewater treatment

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Poly-electrolyte layers determine the efficiency of desalination membranes

Increasing water scarcity and pollution with micropollutants are responsible for the increasing cost of drinking water. Desalination of sea water and better wastewater treatment are necessary to counter this trend. Membranes can desalinate and remove most wastewater pollution. However, a lot of energy is required. Therefore, modern membranes must be more efficient in order to achieve satisfactory results.

Nano-filtration membranes consisting of poly-electrolytic layers are a promising approach to treat water more efficiently. Accordingly, the composition of poly-electrolytic layers has stirred up much interest in the production of nano-filtration membranes. Such membranes are manufactured layer by layer, which enables a good tuning of membrane properties for different purposes.

Commercially available nano-filtration membranes are usually a trade-off between high water permeability and good salt retention (desalination). This trade-off impacts either the quality or the volume of the cleaned water. Nano-filtration membranes that are produced in layer by layer can have a positive impact on this trade-off balance due to the formation of nano scaled layers. It is important to know which component plays a crucial role in the layer forming process.

A research group of the Technical University Eindhoven in the Netherlands had therefore undertaken the task of examining these layer components more closely. They specifically investigated the poly-electrolyte concentration. It is known that a higher poly-electrolyte concentration produces thicker layers. However, their impact on the membrane performance has so far been unknown. They now published their work, in which the researchers used two well-known strong poly-electrolytes: PDADMAC and PSS (polydiallyldimethylammonium chloride and poly(sodium-4-styrene sulfonate)). The membrane output was examined with regard to water permeability, the molecular weight cutoff and salt retention.

In the first double layer, the membranes made with a 50 mM saline solution showed a lower water permeability and molecular weight cutoff, as well as better salt retention (magnesium sulfate) due to the higher poly-electrolyt concentration. After a certain number of double layers, the molecular weight cutoff and the salt retention efficiency for all poly-electrolyte concentration leveled off. The higher the poly-electrolyte concentration, the sooner the plateau value was reached.

The membranes prepared with a 1 M salt concentration had a lower or comparable salt retention efficiency with one exception. The scientists concluded that the poly-electrolyte concentration significantly changed the membrane properties. A plateau was reached with seven or more double layers. The thicker layers showed a lower water permeability than those that were coated with poly-electrolyte solutions using a 50 mM salt concentration. Due to the reduced swelling of these membranes, they all had better salt retention efficiency, with the exception of magnesium chloride.

The results showed that increasing the poly-electrolyte concentration also increased the amount of poly-electrolyte adsorption. Due to a higher coating thickness, this led to lower permeability with pure water. However, this did not lead to a lower molecular weight cutoff or salt retention. The additional poly-electrolyte adsorption resulted in fewer links between the individual layers. The higher diffusivity of PDADMAC compared to PSS resulted in highly positively charged membranes, which in turn led to a better salt retention of magnesium and sodium chloride.

Overall, increased poly-electrolyte concentration and the salt concentration influenced the membrane charge exclusion significantly due to a higher charged surface, which led to better salt retention. However, the membrane size exclusion has not changed, which led to the same plateau values. The study presented here will allow chemists to produce better tuned desalination membranes, which will reduce the energy requirement and raw material requirements during production.

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Decentralized waste energy systems produce biogas where it is needed

Among others, the current European energy crisis was caused by a surge in demand after the pandemic, the embargo on Russia, the reluctance of investors to finance fossil energy projects and the throttling of production by the OPEC countries. In this complex situation, European countries are forced to develop alternatives and renewable energy sources. At the same time, however, natural gas is difficult to replace in many industries. One exception is the food and beverage industry, which sits enormous untapped resources of biogas in their wastewater.

Wastewater is a resource of which 380 billion m³ are produced worldwide. It contains valuable nutrients and energy. Global production is projected to increase by 51% by 2050. Wastewater treatment consumes about 3-4% of the energy generated globally. The full reeovery of the energy that is contained in this wastewater would completely offset the energy consumption of its treatment and in many cases even produce a surplus. In addition, the entire global water treatment is estimated to account for up to 5% of man-made CO2 production. Unfortunately, many businesses and municipalities do not invest in complex and expensive wastewater treatment technologies and continue to waste this valuable resource. The European Biogas Association estimates that by 2050, a maximum of 65% of gas requirements (~167 billion m³) could be covered by biogas.

Europe is the largest cheese maker in the world. More than 9 million tons of cheese are produced annually. With every ton of cheese, 9 m³ of cheese whey remain. Despite its high nutritional value, whey is often treated like wastewater for various reasons. Yet, the very high organic load in the whey makes it difficult to treat. Wasted whey can also be used for biogas production. In addition to whey, regular wastewater is also produced by cheese makers. For example, a medium-sized cheese factory pays 1.5 million euros a year for its waste water. Reducing these costs by producing biogas would turn dairy industry wastewater into a valuable resource.

This situation is similar in many other food and beverage sectors such as breweries, distilleries, winemakers, bakeries etc. All of these sectors have high energy requirements. Renewable electrical energy cannot meet this need. The market for wastewater treatment in Europe and the US is around 12 billion euros.

Traditional wastewater treatment is a cascaded process including aeration and anaerobic sludge digestion followed by incineration. These methods often consume more than 70% of the energy in a wastewater treatment plant. If contaminants such as high-energy total organic carbon or ammonia were converted into biogas before the process, at least 80% of the energy needed for wastewater treatment could be saved. It is absurd that this energy is removed from the wastewater using even more energy.

An ever-increasing number of sewage treatment plants already recover the resources contained in their wastewater, apart from the water itself. The oldest recivered products are biogas and fertilizers made from sewage sludge. Due to its heavy metal content such as copper and mercury, sewage sludge is no longer used as fertilizer but incinerated.

Biogas is particularly popular in Europe as the produced volumes and prices are high enough to compete with natural gas. Biogas is also a green alternative to natural gas as no additional CO2 is emitted. (Hence, it is often called Renewable Natural Gas in North America.) A disadvantage of classic biogas is the CO2 and sulfide content. Another disadvantage is that anaerobic digestion is the terminal treatment step, wasting valuable wastewater resources in the preceding treatment. Finally, the size and complexity of current digestion requires significant commitment from users when it comes to capital expenditures. Most food manufacturers prefer to focus on making food rather than cleaning their wastewater.

Novel high-performance biogas reactors solve these problems through miniaturization. A 20-fold size reduction is achieved compared to conventional systems. The new technology used was developed in Japan in the early 1990s and is called microbial electrolysis. The electrolysis of wastewater is catalyzed by electroactive microorganisms on the anode (the positive electrode). The reaction products are CO2 (from organic matter) and nitrogen gas (N2 from ammonia).

Principle of a microbial electrolysis reactor. On the left anode, the organic material is oxidized to CO2. The free electrons are absorbed by the anode and transported to the cathode. Hydrogen gas (H2) is released there. CO2 and hydrogen form methane, the final microbial reaction product.

At the same time, hydrogen gas (H2) is generated at the cathode (the negative electrode). This hydrogen reacts with CO2 to form methane. The final methanation step completes the biocatalytic treatment of the wastewater. Gas grid injection is one possible use. But for cheese makers, the gas would be used on site to generate electricity and/or heat.

The reaction is accelerated using an applied voltage and is based on the laws of thermodynamics. As a result, the reactor volume can be reduced. The size reduction has several advantages. First, it makes biogas accessible in markets where it was previously not possible due to the high investment costs. Second, it enables higher throughput at a lower cost. Smaller units are mobile and can be shared, moved or rented. After all, food manufacturers want to do what they do best, which is to make food.

 

Image: Pixabay

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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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Lead removal from water using shock electrodialysis

Lead was widely used in water pipes during the industrial revolution that triggered urbanization and exponential growth of the population in metropolitan centers. The reason for its popularity was the plasticity of the material used in service lines near the end user. The negative health effects have been known since the 1920s, but infrastructure modernization in industrialized countries remains an enormous economic challenge. Lead service lines therefore continue to circulate water in our supply systems. The city of Flint in the northwest of Detroit, for example, received much press attention due to its long struggle with lead poisoning (e.g. Flint Water Crisis). Dissolved lead is highly toxic in a very small concentration and accumulates in body tissues.

The biggest challenge when removing lead from the water cycle is that it is usually dissolved in very low concentrations. Other compounds “mask” the dissolved lead, which makes its removal difficult. Sodium, for instance, is concentrated ten thousand times higher than lead. While nowadays lead can be removed from water by reverse osmosis or distillation, these processes are not selective and thus ineffective. They consume a lot of energy, which in turn is an environmental issue in itself. High energy consumption makes water treatment also very expensive. At the same time, other minerals dissolved ion water are beneficial and therefore desired ingredients that should not be removed.

MIT engineers have developed a much more energy-efficient method to selectively remove lead from water and published their results in the journal ACS EST Water. The new system can remove lead from water in private households or industrial plants and hence from the water cycle. Through its efficiency, it is economically attractive and offers its users the clear advantage of not being poisoned.

The method is the most recent of a number of development steps. The researchers started with desalination systems and later developed it into radioactive decontamination method. With lead the engineers have found an attractive market. It is the first system that is also suitable for private households. The new approach uses a process that was named shock electrodialysis by the MIT engineers. It is essentially very similar to electrodialysis as we know it, as charged ions migrate into an electric field through the electrolyte. As a result, ions are enriched on one side while being depleted on the other.

The difference of the new method is that the electric field moves as a sort of shock wave through the electrolyte and drags dissolved ions along. The shock wave traverses from one side to the other is the voltage increases. The process leads to a lead reduction of 95%. Today, similar methods are also used to clean up aquifers or soil contaminated by solvents. In principle, the shock wave makes the process much cheaper than existing processes because the electrical energy is targeted to remove specifically lead while leaving other minerals in the water. Hence, a lot less energy is consumed.

As usual for bench top prototypes, shock electrodialysis is still too ineffective to be economically viable. Its up-scaling will take time. But the strong interest of potential users will certainly accelerate its industrialization. For a household whose water supply is contaminated by lead, the system could be placed in the basement and slowly clean the water carried by the supply pipes because high rates occur only during peak hours. For this purpose, a water reservoir is necessary, keeping a stock of purified water. This can be a fast and cheap solution for communities such as Flint.

The process could also be adapted for some industrial purposes. The mining and oil industries produce much heavily contaminated wastewater. One imagine to reclaim dissolved metals and sell them to the market. This would create economic an incentives for wastewater treatment. However, a direct comparison with currently existing methods is difficult because the longevity of the developed system is yet to be demonstrated.

At Frontis Energy we are thrilled by the idea of ​​creating economic incentives to help implementing environmentally friendly processes and are already looking forward to a commercial product.

(Photo: Wikipedia)

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Self-cleaning membranes for biofouling control and prevention in water treatment

Membrane-based water treatment is critical for obtaining potable water, for example through wastewater treatment and seawater desalination. However, membrane fouling remains a common undesirable phenomenon affecting all membrane-based separation processes. Various efforts have been made to either directly control biofouling or to prevent it.

Ceramic membranes have better thermal and chemical stability along with higher fouling resistance and longer lifetimes when compared to polymeric membranes. These properties render ceramic membranes superior to polymers.

During the filtration process, the amount of water that can pass through a membrane is known as membrane flux. Due to membrane fouling, this flux is reduced and the affected membrane needs to be refurbished. Different membrane cleaning strategies have been researched including self-cleaning conductive polymeric membrane and electrically-assisted filtration but neither of them has shown a satisfactory flux recovery behavior.

Previous researches have suggested the use of ‘nano zeolite’ and carbon nanostructures for water treatment and desalination applications.

  • Zeolites are crystalline aluminosilicates possessing a well-defined inorganic structure, whose microporous 3-D channels and pores act as filters.
  • Carbon nanostructures consist of highly entangled carbon nanotubes which are made through a standardized chemical vapor deposition method.

To investigate the use of ceramic membranes made from nano zeolite and carbon nanostructures, a group of researchers at the New York University Abu Dhabi, United Arab Emirates, developed a new electro-ceramic membrane and evaluated its antifouling performance. Their research findings were published in the Chemical Engineering Journal.

Research Approach:

Zeolite / CNS membrane preparation:

Nano zeolite-Y (nano-Y) membranes were prepared by dispersing the desired amounts of nano-Y, carbon nanostructures, and polyvinylidene fluoride (PVDF) binder in a water-alcohol solution.

The suspension was vacuum filtered through a microfiltration membrane filter and the membrane was peeled off from it before drying it at room temperature.

Three different ratios of zeolite and carbon nanostructures were prepared initially, with 60, 70, and 80 wt% zeolite. The carbon nanostructures and the binder were prepared at a ratio of 1:1.

Membrane characterization:

The electrical conductivity and mechanical properties of the dried membranes were investigated.

The surface morphology of the zeolite carbon nanostructure membrane was studied through scanning electron microscopy and transmission electron microscopy.

Other tests including the membrane contact angle test were also performed on the different labeled membranes.

Membrane cleaning setup and antibacterial assessment:

Two foulants, yeast (200 mg / L) and sodium alginate (30 mg / L) were used as biofoulants.

A custom-made cell was designed and a fresh membrane was used for each electrochemical measurement performed using linear sweep voltammetry.

Antibacterial properties of the nano-Y carbon nanostructure membranes were determined by the disk diffusion method. Different bacteria were cultured overnight at 37°C in a shaking incubator at 100 rpm.

Results:

Membrane cross-sections showed a uniform distribution of nano-zeolite particles with the carbon nanostructure. Decreasing tensile strength was seen interpreted as successful nano zeolite incorporation. These values changed from 3.3 MPa to 2.1, 1.1 or 0.3 MPa, respectively for 60, 70 and 80 weight% nano-Y. In addition, a decrease in water contact angle from 84.7±2° to 18±4° was demonstrated within 4 min.

The composite membrane demonstrated enhanced electrocatalytic activity for hydrogen evolution in two foulants; yeast and sodium alginate.
These MF electro-ceramic self-cleaning, anti-bacterial membranes seem promising for various separation processes such as in wastewater treatment, dye separation and oil / water separation where fouling and bacterial growth are a major concern.

(Photo: WET GmbH, Attribution, via Wikimedia Commons)

Reference: https://doi.org/10.1016/j.cej.2020.128395 Electro-ceramic self-cleaning membranes for biofouling control and prevention in water treatment, Chemical Engineering Journal, Volume 415, 2021

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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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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)

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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)

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China has improved inland surface water quality

During the last decades, China has achieved rapid development in technology and economics, however at a huge environmental cost. The deterioration of inland surface water quality is considered one of the most serious environmental threats to ecosystem and ultimately public health.

Since 2001, China made major efforts to tighten the application of environmental rules in order to stop water pollution emitted by cities, farm and industry. According to the government’s “10th National Five-Year Plan”, large investments were made for pollution control and wastewater discharge regulation systems.

Small research studies showed that with this campaign, Chinese’s lakes and rivers got cleaner. Since then water quality has improved significantly − however, other parts of country still have problems with polluted water.

Now, a team of researchers of the at the Chinese Academy of Sciences in Beijing, has published one of the most comprehensive national investigation of China’s surface water quality in the renown journal Science. The researchers investigated all regions of the country to learn how surface water responds to multiple driving forces over time and space. Their report covers the assessment of water quality by means of three parameters: dissolved oxygen level (DO), chemical oxygen demand (COD) and ammonium nitrogen (N) in inland surface waters. They performed monthly site-level measurements at major Chinese rivers and lakes across the country between 2003 and 2017.

Due to regional variations in China’s inland water quality as well as the dynamics in multiple anthropogenic pollution sources, such studies are crucially important to identify the necessary regulation measures and water quality improvement policies adapted to ecosystem sustainability at all diverse country regions.

The results show that during the past 15 years, annual mean pollution concentration has declined across the country at significant linear rates or was maintained at acceptable levels. Consequently, the annual percentage of water quality have increased by 1.77% for COD, 1.83% for N and 1.45% for DO per year. While China has not yet implemented environmental water standards, the study shows that China’s water quality is improving nonetheless.

The best news is that the notoriously high pollution levels have declined as cities and industry have worked to clean up and reduce their discharges. According to the authors, the most visible alleviation was noticed in northern China, while in the western region of the country water quality remained at their low pollution level throughout the observation period. The reason is likely that pollution is caused by human activity, of which there is less in those parts of the country.

Despite large efforts toward decreased pollution discharges, urban areas are still considered as the major pollution centers. These areas face additional pressure due to the constant migration and fast urbanization of the rural regions. Especially in northern China, with high-density human activity and exploding urbanization, achieving and maintaining a clean environment is a permanent struggle.

To further reduce pollution and improve water quality, the authors recommend that future activities focus on water management systems and the water pollution control. For both, the central government issued guidelines to control and improve water use and pollution discharge at regional and national levels for 2020 and 2030.

At Frontis Energy, we certainly support activities in China that help improving the countries water quality and public health. The Frontis technology gives its user an incentive to to clean wastewater before discharge by extracting its energy. Our patent pending solutions are based on microbial electrolysis which helps to extract energy from wastewater and apply in particular to China.

Mima Varničić, 2020

(Photo: Gil Dekel / 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.