Wastewater. Energy resource?

Hello wonderful community of steemit. In today's article I want to talk about the use of wastewater as an energy resource; this post is an English adaptation of a publication that I made some time ago in spanish.

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As we know, wastewater management is a great debt that our society has with the environment. In my country, as in many others in Latin America, investment in wastewater treatment plants does not keep pace with urban development, and in many cases the treatment systems are far from what the environment requires, why? Conventional treatment plants, besides having an expensive installation, have high operating costs, not only require the consumption of chemical products but also energy. To get an idea, according to the Water Environment Research Foundation in the US, the wastewater sector consumes 22 terawatt hours of electrical energy every year.

Images source: Wastewater, CC0 , dollar, CC0

Talking with some friends about the problem, they asked me if these plants cannot be self-sustaining, since in many cases biogas is produced. Therefore, I gave myself the task of investigating, and I tell you, it is possible!Through this publication, I want to tell you about some technologies in use or development, which have called my attention for referring to the subject. Mainly, because they do not suppose another option for the treatment of residual water but ask for a change of paradigm, “stop seeing the residual waters as an expense and begin to consider it as an energy resource”.

GENERATION OF GAS

The Co-digestion

The concept is not really new, in many treatment plants operate units for the stabilization of sludge that come from the separation units of the solid fractions, this process involves anaerobic biodigestors, where by a biological treatment decomposes the organic matter, in absence of oxygen, producing gas with a high content of methane that can be used as fuel. The problem with this technology is that it has a very low performance; the system is sometimes difficult to operate due to the different reaction rates of the bacteria and very high residence periods.

However, since the 1980s, the processing alternative known as co-digestion has been considered since it has gained special relevance in recent years due to the results obtained. The process consists in treating the stabilized sludge of the treatment plants with other substrates that compensate the nutrients of these, improving the performance of the digestion. The plants currently operating with this procedure have registered large increases in the production of biogas.

General scheme of co-digestion. Own image

To mention some examples. In Graz, Austria, with three digesters of 4000 m³, increases in the production of biogas higher than 90% have been obtained by accepting the floating sludge from the effluents of a chicken slaughterhouse as a substrate. In other interesting applications of codigestion, in Wisconsin given the availability of whey, the Madison MSD (Metropolitan Sewerage District) has accepted cheesemilk from a local dairy for years and fed it directly to the biosolids digesters, the MSD of Milwaukee has been handling the spent de-icing fluid from Mitchell International Airport at its South Shore treatment plant. But even more interesting is what the company ACCIONA has achieved, it got the Copero treatment plant in Seville, through the codigestion system, to stop consuming 2190000kWh annually, which has allowed this plant to be self-sustaining from the point of Energetic view.

LIFE SAVING-E Project

On the other hand, a research group of the Autonomous University of Barcelona is developing the LIFE SAVING E project, which contemplates the modification of the treatment process, so that instead of consuming energy it will generate it.

The idea is to use all the organic carbon content contained in the wastewater for the production of biogas. As I already mentioned, biogas is produced during anaerobic digestion from the organic matter present in the wastewater, the problem is that a part of the organic carbon is consumed during the elimination of nitrogen, so not everything is recovered for the production of biogas. The innovative step of the project is to generate the elimination of nitrogen without consuming organic carbon, using a group of bacteria called Anammox, which grow in an autotrophic manner.

Nitrogen enters the treatment plants in the form of ammoniacalnitrogen, conventionally it is first oxidized to nitrate and then reduced to gaseous nitrogen, which is released into the atmosphere. What bacteria do is to transform ammonium and nitrite into nitrogen, without consuming organic carbon. In addition, to achieve this, part of the ammonium in nitrite must be reduced first, and this conversion consumes up to 50% less energy than the current process. With this change in the treatment scheme, not only is it expected to save energy but also to produce it, if everything goes as planned in the project it is expected to produce an energy surplus equivalent to 9 kWh per habitant each year. Nothing bad!

GENERATION OF FUEL

There are already technological alternatives that contemplate the generation of a useful fuel from the wastewater treatment.

One of these technologies, which in recent years has emerged as an alternative to the expensive conventional systems for the elimination of phosphorus and nitrogen, is the microalgae purification known as photobiotreatment. The wastewater contains high concentrations of nitrogen and phosphorus that must be eliminated before they are discharged to avoid eutrophication problems in the receiving environment, the conventional systems used for this, besides being expensive, have a high energy consumption (between 60 and 80% of the total treatment process).

The principle is simple, even mimics the normal functioning of nature. It consists of accelerating the growth of photosynthetic microorganisms, especially microalgae selected for their characteristics and ability to generate other useful compounds, introducing them into the wastewater under light and aeration conditions.

The species are selected not only for their ability to grow and adapt to the environment, but also for their lipid content. In addition to the obvious advantage of eliminating nutrients with a less complicated process than the current ones, the potential for using the biomass generated for the production of biodisel, giving it an added value compared to the usual systems of oil production for its use as fuels.

In Veracruz, Mexican scientists in cooperation with scientists from Spain and Italy, since 2012 have installed a bio-refinery for the production of biogas and biodiesel from an aquatic plant called water lettuce, which in addition to efficiently remove pollutants of water are able to accumulate lipids in their cells, reaching to produce oil yields 100 to 200 times higher than the soybean oil derivative.

GENERATION OF ELECTRICITY

Yes, the options do not end with the generation of gas and the possibility of generating fuel. Bioelectrogenesis reaches the treatment plants to take advantage of the chemical energy contained in the wastewater and convert it into electrical energy.

A new field, bacterial electrochemistry, develops the ability of certain bacteria to produce electricity. Through this process, certain bacteria can oxidize organic matter and directly transfer the generated electrons to a conductive solid surface such as graphite .

The operation seems simple, on paper certainly, strains of bacteria are incorporated into the wastewater using so-called microbial fuel cells, where the selected bacteria use the organic matter present in the water as if it were fuel, oxidizing it to generate CO₂ and protons, and emitting electrons that can be recovered by an electrode.

diagram of a Microbial Fuel Cell. Image source: MFCGuy2010, CC-BY-SA-3.0

Thus, bioelectrogenesis becomes a promising process for the treatment of wastewater, allowing self-consumption of energy. There are already some companies immersed in this line of work, preparing solutions based on the generation of energy from wastewater.

Each technology has many more details than I can tell but I did not wanted to make the publication too long. I just want to highlight the importance of changing the paradigm of wastewater management, let us see them as an energetic resourceinstead of considering them as another waste problem to solve, with this I believe that we can really advance in the solution of the problem.

Thank you so much for reading this article, I hope you enjoyed the information.