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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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Biochar from waste removes pharmaceuticals from wastewater

Biochar is a coal-like substance that is mainly made from agricultural waste products. It can remove contaminants such as pharmaceuticals from treated wastewater. This is the result of research carried out by scientists of the Pennsylvania State University and the Arid Lands Agricultural Research Center in Arizona. The biochar was made from two agricultural residues common in the US: cotton and guayule.

To test the ability of biochar to adsorb pharmaceuticals from treated wastewater, the scientists compared three common compounds. During adsorption, a material like a pharmaceutical adheres to the surface of solid biochar particles. In the case of absorption, in turn, one material is taken up into another, such as in a sponge.

The shrub guayule grows in the dry southwestern US and its waste was used for the biochar tested. Among bonatics, it is also called Parthenium argentatum. The shrub is cultivated as a source of rubber and latex. The plant is chopped to the ground and its branches crushed to extract the latex. The dry, mushy, fibrous residue that remains after the stalks are chopped up to extract the latex is called bagasse.

The results are important as they demonstrate the potential of biochar made from abundant agricultural waste. If it wasn’t re-used, this waste would have to be disposed at a cost. The production of biochar is an inexpensive additional processing step to reduce contamination in treated wastewater used for irrigation.

At the same time, most wastewater treatment plants are currently not equipped to remove emerging contaminants such as pharmaceuticals. If these toxic compounds were removed by biochar, the wastewater could be reprocessed in irrigation systems. This re-use is crucial in regions where water scarcity is a constraint for agricultural production.

The pharmaceutical compounds used in the study were: sulfapyridine, an antibacterial drug commonly used in veterinary medicine; docusate, a widely used laxative and stool softener, and erythromycin, an antibiotic used to treat infections and acne.

The results, published in the journal Biochar, suggest that biochar can effectively adsorb agricultural waste. The biochar obtained from cotton processing waste was a lot more efficient. It adsorbed 98% of the docusate, 74% of the erythromycin and 70% of the sulfapyridine from aqueous solutions. In comparison, the biochar obtained from guayule residues bagasse adsorbed 50% of the docusate, 50% of the erythromycin and only 5% of the sulfapyridine.

Research found that a temperature rise from about 340°C to about 700°C in the oxygen-free pyrolysis process used to convert agricultural waste materials to biochar resulted in a improved capacity for adsorption.

To date, there have been no studies on the use of guayule bagasse to make biochar and remove contaminants, nor are there any for cotton processing waste. Some research has been carried out into the possible removal of other contaminants. However, this is the first study to use cotton gin waste specifically to remove pharmaceuticals from water.

The research is more than theoretical. At Frontis Energy we hope that the technology will soon be available on industrial scale. With cotton gin waste being widespread even in the poorest regions, we believe this source of biochar holds great promise for decontaminating water. The next step would be to develop a mixture of biochar material to adsorb a wider variety of contaminants from water.

(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)