Hydrogen has higher energy density than natural gas, ethanol, and methane. To produce hydrogen, we could use natural gas, coal and biomass as feedstock. However, these are finite resources and they emit greenhouse gases. Also, they are not sufficient to meet rising demands for energy.
Recently, researchers from IICT, Hyderabad reported a more sustainable raw material to produce hydrogen: food waste.
They collected and processed food waste from the IICT canteen. The mechanically masticated waste was filtered to remove coarse particles. A gravity separation system removed oil. And the oil-free filtrate was used as substrate in a fermentation plant.
For fermentation, the team collected sludge from a wastewater treatment plant in Hyderabad and used the bacterial consortium present in the sludge. In such dark-fermentation, methanogenic bacteria, in the mixture of microbes used, might produce methane. To suppress the methanogens, the team subjected the mixed culture to heat shock – above 80 degrees Centigrade for about two hours – before culturing in synthetic media.
Microbes use different pathways to produce hydrogen. Yield from the acidogenic route was reported to be better and so the team decided to use that. To keep the pH in the desired range, they had to have an acid-base holding tank.
Thus, the pilot plant, designed and constructed by the researchers, had a waste food shredder, and wastewater feeder. The plant also had tanks for inoculum preparation, acid–base holding, acidogenic reaction, gas collection and water storage.
“Heat-inactivation of inoculum suppressed the methanogens only partially. So, every 12 hours, we bubbled air into the bioreactor to reduce the anaerobic production of methane’, says Omprakash Sarkar, IICT, Hyderabad.
“One mole of glucose in food waste can produce four moles of hydrogen in this process. And about 57% of food waste was converted to hydrogen,” says Ranaprathap Katakojwala, IICT, Hyderabad.
Most of the remaining food was converted to volatile fatty acids by acidogenic fermentation. These by products, if wasted, will affect the environment. However, if recovered, volatile fatty acids are useful as building blocks to produce polymers, preservatives, and flavoring agents. So, Venkata Mohan, IICT, Hyderabad thought of integrating a biorefinery facility into the pilot plant to recover these byproducts.
Integrating the biorefinery into the pilot plant improved cost effectiveness and resource recovery efficiency. But then, there were other byproducts such as methane, oxygen, algal biomass and acetic, butyric and propionic acids. The team used these as feedstock for algal production.
They set up an ecologically engineered system consisting of submerged plants, emergent plants and filter feeders, along with aquatic algae, connected in series. This integration promoted the growth of microalgae and converted atmospheric carbon dioxide to biomass as well as oxygen.
The treated wastewater was re-circulated back into the water storage tank for reuse, reducing the amount of water needed to run the system sustainably.
Using lifecycle analysis, the researchers compared the costs for producing biohydrogen and other byproducts and the impact on the environment in standalone fermentation, in fermentation integrated with the biorefinery and in the complete setup, including the ecologically engineered processes.
“Standalone fermentation requires lower energy input than the integrated biorefinery,” says Venkata Mohan, IICT, Hyderabad. “But the end products of standalone fermentation can cause aquatic toxicity”.
Integrating a biorefinery process helped recover resources, reduced environmental impact and increased economic returns. Adding an artificial wetland system to recycle water made it a circular economy model for hydrogen production.
Currently, only a small amount of hydrogen is produced from water by electrochemical breakdown. But the process is limited by fresh water and electricity requirements to split water molecules. In contrast, food waste is a cheap resource.
According to the Food Waste Index Report 2021, an average of nearly 50 kilograms of food waste is thrown away by Indian households, accounting for nearly seven crore tonnes of food waste every year!
Food waste accumulates in landfills, polluting water as degradation products leach into soil. Degradation also emits methane, a greenhouse gas. However, if food waste is used for producing hydrogen, the environmental disaster is turned into an opportunity for producing energy. In the pilot plant, food waste per litre with 36 kilograms of chemical oxygen demand can produce one kilogram of hydrogen. And it costs only 175 Rupees.
“The production cost can be reduced further by increasing plant capacity,” points out Venkata Mohan, IICT, Hyderabad.
By establishing similar plants across major canteens, hotels and bigger plants in municipalities, water contamination and air pollution can be reduced. And we get clean hydrogen fuel to drive the carbon-neutral agenda.
Green Chem., 23, 561-574 (2021);
G Sharath Chandra
Indian Institute of Horticultural Research
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