- The new technique decreases carbon emissions from wastewater treatment systems and generates clean hydrogen energy.
- It uses purple photosynthetic bacteria and electric current to process the organic substance in human waste.
Typical wastewater plants require the dissipation of the contamination so that the treated water can be returned to the water cycle with minimal environmental impact. However, the high content of nutrients and organics in the domestic and industrial wastewater could be used to generate valuable energy.
Considering environmental and economic benefits, it has become kind of necessary to upgrade existing wastewater systems by implementing efficient technologies. At present, the carbon footprint is one of the biggest downsides of wastewater treatment plants.
Now, a team of Spanish researchers has discovered a method to decrease carbon emissions from wastewater treatment system and generate clean hydrogen energy at the same time. It uses purple photosynthetic bacteria to process organic materials in human waste.
Purple bacteria are widely distributed in nature. They can create their own food via photosynthesis, but unlike algae and plants, their metabolism (which is quite versatile) depends on infrared light. Since they are capable of performing a variety of metabolic reactions, they are present almost everywhere in nature, especially in lakes. Plus, they are often found in wastewater treatment systems.
How Do Bacteria Generate Hydrogen Gas?
Every single living organism on Earth has to maintain an equilibrium – a state of steady internal conditions (like fluid balance and body temperature) known as homeostasis. Purple photosynthetic bacteria contain excess electrons due to their unique metabolism.
They can release these electrons in two ways: either through CO2 fixation or as hydrogen gas. They use cyclic electron transport steered by a sequence of redox reactions. Also, they transfer electrons from external donors to cytochrome bc1.
Purple bacteria’s ability to generate hydrogen gas depends a lot on what’s in the waste. It won’t produce much hydrogen if the waste composition contains ammonium that mostly comes from proteins. Therefore, in order to generate hydrogen more efficiently, it’s necessary to completely eliminate ammonium prior to the process.
The internal electron recycling of purple bacteria is an important issue, and artificially adding electrons could result in unnecessary bioproducts. To solve this problem, researchers used microbial electrochemical technologies: they supplied electric current to microorganisms through electrodes. This step enhanced the activity of purple bacteria towards high value-added compounds.
Image credit: Wikimedia Commons
The modern wastewater systems are aiming for energy neutrality and for maintaining systems with no external energy. However, the new technique is far ahead: researchers are trying to transform the wastewater system into an actual biorefinery.
If implemented correctly in a mid-size wastewater treatment plant, the technique can yield enough energy to power 43-107 homes. Of course, a major portion of this energy would be consumed by the plant itself.
In this study, researchers were able to configure the purple bacteria’s metabolism to improve CO2 fixation, while keeping the hydrogen productivity same, which means no carbon footprint. They have patented the technology, and in the next step, they will try to convince wastewater industries and water management companies to implement their techniques.