This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Low concentrations of caustic soda (sodium hydroxide, NaOH) are more efficient than high doses for preparing organic waste for biogas production. This finding increases methane production, reduces reagent consumption and, consequently, diminishes process costs and minimizes the risk of inhibiting the microorganisms responsible for the process.
The raw material for the study that reached such a conclusion was the banana—its peels and parts unfit for sale collected from wholesale markets. Despite their high organic matter content, the energy potential of these waste materials is "locked" within a kind of "plant-made armor" composed primarily of substances like cellulose and lignin. This hinders microorganisms' access to fermentable sugars and limits the efficiency of anaerobic digestion—the decomposition of organic matter by microorganisms that live in the absence of oxygen.
Throughout the process, complex molecules are broken down into simpler compounds until methane (CH₄), the main energy component of biogas, is formed. "To increase the efficiency of biogas production, a pretreatment is necessary. We opted for thermo-alkaline pretreatment [which combines heat and chemical products].
The biomass was heated in the presence of sodium hydroxide at different concentrations, ranging from 0.2% to 3%. The goal was to partially break down the lignin barrier, making the carbohydrates more accessible to microorganisms," reports Sandra Imaculada Maintinguer, a member of the Bioenergy Research Institute (IPBEN) at São Paulo State University (UNESP) who coordinated the study. The result contradicted the intuitive expectation that more reagent would yield better performance.
Maximum efficiency was achieved with the lowest concentrations tested. Specifically, a concentration of 0.2% NaOH promoted the greatest solubilization of carbohydrates, reaching 12,110 milligrams of methane per liter (mg/L), compared with the 4,785 mg/L observed in the untreated material. A slight increase in concentration to 0.4% yielded the highest cumulative methane production; however, the 0.2% concentration exhibited the highest rate of gas production.
"This is important because, in addition to increasing production costs, high use of sodium hydroxide inhibits the activity of the microorganisms responsible for methane production and can have a greater environmental impact," highlights Larissa Ayumi Yamamoto, a graduate at IPBEN-UNESP and first author of the article that describes the finding, published in May in the journal BioEnergy Research. In small quantities, caustic soda partially breaks down plant structure and releases sugars that feed microorganisms. However, in excess, it begins to attack the microorganisms themselves, reducing or even halting the process.
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