Biological hydrogen production is the process of using microorganisms such as bacteria, algae and fungi to produce hydrogen gas as a by-product of their metabolic activities. This type of hydrogen production is a clean and renewable energy source that does not generate harmful emissions or pollutants. The microorganisms are typically supplied with organic substrates such as sugar or starch, and hydrogen gas is produced as a result of their metabolism. In addition, some microbial processes can also produce electricity while they produce hydrogen, making them a potentially cost-effective source of energy.